Overview of Rural Domestic Sewage Treatment Technology

Abstract

Collection, as the premise of treatment, is one of the important factors affecting the efficiency of sewage treatment. According to the experience of China, Japan, Europe, the United States, and other countries, decentralized collection, centralized collection, and nanotube collection are three basic modes for collecting rural domestic sewage. Various factors such as local population distribution, sewage quantity, economic development level, environmental characteristics, climatic conditions, topography, local drainage system, and current status of drainage pipeline network should be comprehensively considered before selecting a sewage collection mode. Moreover, the collection system should be planned based on local conditions and the distribution of villages and farmer households. Long-distance drainage pipes should be avoided.

2.1 Collection of Domestic Sewage in Rural Areas

2.1.1 Collection Modes

Collection, as the premise of treatment, is one of the important factors affecting the efficiency of sewage treatment. According to the experience of China, Japan, Europe, the United States, and other countries, decentralized collection, centralized collection, and nanotube collection are three basic modes for collecting rural domestic sewage. Various factors such as local population distribution, sewage quantity, economic development level, environmental characteristics, climatic conditions, topography, local drainage system, and current status of drainage pipeline network should be comprehensively considered before selecting a sewage collection mode. Moreover, the collection system should be planned based on local conditions and the distribution of villages and farmer households. Long-distance drainage pipes should be avoided.

To be concrete, decentralized collection characterized by saving the investment in pipe network as well as simple and fast construction, is applicable for areas not suitable for pipe networks due to complex terrain, scattered village distribution, and low population density. Decentralized collection can be divided into decentralized collection and treatment as well as decentralized collection and centralized treatment, according to different treatment modes (Table 2.1 and Fig. 2.1). More precisely, indoor sewage collection pipes and single household/connected household integrated sewage treatment equipment are prerequisites for decentralized collection and treatment. Indoor sewage collection pipes, septic tanks, and collection tankers for periodic clearing are required for decentralized collection and centralized treatment with less investment due to the simple structure and low cost of the septic tanks. But it is worth noting that the accumulation rate and clearing frequency of toilet sewage in septic tanks are high in areas with a high penetration rate of flush toilets, which might be beyond the capacity of the collection and transportation system (Wang, 2021a).

Table 2.1 Comparison of collection of domestic sewage in rural areas

Fig. 2.1

Collection of domestic sewage in rural areas (Li et al., 2021)

Centralized collection refers to the collection of sewage by the pipe network to the nearby sewage treatment plants from farmer households within a certain range or a village, applicable for areas away from towns with gentle terrain, compact settlement, or large resident population. Where possible, a pipe network diversion system should be performed. In such a model, sewage collection, treatment, and discharge are conducted according to the principle of proximity. In other words, treated sewage can be discharged into the nearest river, or directly leveraged as farmland irrigation. The difficulty in engineering construction of centralized collection is low, but collection pipe networks and lifting pump stations are needed (Wang, 2021a, 2021b).

Nanotube collection refers to the collection of domestic sewage from towns and surrounding villages by branch sewers and directly incorporated into urban main sewers, and finally treated in municipal sewage treatment plants. It features convenient management, low investment, quick return, high collection efficiency, and significant treatment effect. Its application is limited by geographical restrictions with collection pipe networks and lifting pump stations required (Li et al., 2021).

2.1.2 Collection Process

Gravity collection is the main approach leveraged for collecting rural domestic sewage. Gravity collection is a major process used in collecting rural domestic sewage because of its superiority in low investment, low energy consumption, and simple operation and maintenance (Table 2.2 and Fig. 2.2). But gravity collection may result in problems such as difficult construction, pipelines susceptible to plugging, and sewage leakage in complex terrain. In that case, the vacuum negative pressure processing method can be adopted to collect domestic sewage in areas with difficulty in laying gravity pipe networks. It is a process that creates a vacuum in the drainage pipeline via the vacuum equipment and provides the power to convey liquid using the air pressure difference, as shown in Table 2.2 and Fig. 2.2 (Ye et al., 2021). Featuring small pipe diameter, shallow buried depth, incline capacity, and non-clogging, the above process is applicable for collecting domestic sewage under complicated geographical conditions. Although it has been applied at home and abroad, it is rarely applied in rural areas as a whole for the large investment in the vacuum collection system and the difficulty in operation and maintenance (Islam, 2017).

Table 2.2 Comparison of gravity collection and vacuum collection

Fig. 2.2

Gravity collection and vacuum collection (Li et al., 2021)

2.2 Domestic Sewage Treatment in Rural Areas

Compared with cities, villages and towns feature dispersed population distribution, complex terrain, and a lack of complete pipe networks, making it more difficult for sewage collection and treatment (Deng & Wheatley, 2016).

The quality and quantity of domestic sewage differ due to great differences in economic and social conditions, population factors, and customs under a diversity of geographical, climatic, and ecological conditions in rural areas worldwide. Moreover, different treatment processes selected as the sewage treatment terminal may lead to a big difference in infrastructure cost, treatment effect, and the complexity of operation and maintenance management. Evidently, a reliable and safe sewage treatment process with favorable treatment effect, low energy consumption, simple operation and maintenance management, and up-to-standard discharge should be adopted with full consideration of local conditions rather than using a single sewage treatment process.

In this section, common processes in rural sewage treatment are discussed from the aspects of physical and chemical processes, biofilm process, activated sludge process, and natural biological treatment process. Moreover, septic-tank and MBR are commonly used in rural sewage treatment. The septic tank and MBR are studied separately for them, strictly, are neither part of the biofilm process nor the activated sludge process. Then, the rural sewage treatment selected for B&R is discussed based on the experience of rural sewage treatment in China.

2.2.1 Physical and Chemical Processes

Grid, grit chambers, equalization tanks, chemical phosphorus removal, and disinfection are common physical and chemical processes in rural domestic sewage treatment.

• a. Grid

Grid is a simple filter device typically used as the first session for sewage treatment in the pretreatment process, which is set in front of sewage treatment plants or pumping stations.

Grid is leveraged to efficiently block and collect coarse suspended solids or floating solids such as leaves, entanglement, and solid waste in sewage, which can prevent the blocking of subsequent pipeline valves or water pumps and lay a foundation for subsequent biochemical treatment processes (Zhang, 2015).

There are artificial grids and mechanical grids, which are selected as per the required slag amount to be removed. The grid can be split into the coarse grid, fine grid, and refined grid as per the gap of the grid bar. The coarse grid with a gap of 26–40 mm is typically selected when grid slag is removed manually; and when grid slag is removed mechanically, the gaps of 15–25 mm, 3–10 mm, and 1–2 mm will be selected for the coarse grids, fine grids and refined grids, respectively. A fine grid, in general, is set before MBR (Membrane Bio-Reactor) process to prevent hair from wrapping film components.

• b. Equalization Tank

Great changes in the water quality and quantity are one of the characteristics of rural sewage, which might seriously affect the biochemical processing at the back end with a significant change in the pollutant load of sewage. To this end, a equalization tank is normally set up at the front end of the sewage treatment plant to homogenize the quantity and quality of raw water. In this way, it ensures that biochemical treatment can be conducted at the back end under stable inflow conditions, degrade some of the organic matter, and improve the shock resistance and processing effect of the whole system (Liu, 2019). Besides, a liquid level controller is generally set in the equalization tank to adjust the water volume by controlling the start and stop of the lifting pump. At the same time, a stirring apparatus can be also installed in the equalization tank to adjust water quality.

The effective volume of the equalization tank should be determined based on factors such as the treatment scale, and the fluctuation of water quantity and quality, and the hydraulic retention time shall not be less than 12 h. Liner should be performed on the wall and bottom of the tank together with odor-resistant and explosion-proof measures. The sewage treatment plant should conform to the peak flow requirements without overflow in cases where equalization tank is not applicable.

• c. Chamber

The grit chamber can be set up as required to remove solid particles with a high density such as mud and sand in a simple and effective manner (Zhang, 2015). In general, it is set before the pumping station and secondary secondary sedimentation tank to reduce the wear of the water pump, prevent the blockage of the pipeline, increase the content of organic components of the excess sludge produced by the sewage treatment plant, and improve the value of sludge as a fertilizer for resource utilization.

• d. Secondary Sedimentation Tank

The main function of the secondary sedimentation tank is to achieve solid–liquid separation by removing suspended solids in sewage with the help of gravity precipitation. It was used independently in sewage treatment at the early stage, presenting limited treatment effect, and now is used in combination with biological treatment processes. The secondary sedimentation tank is composed of primary and secondary sedimentation tank, with the former rarely applied in rural sewage treatment.

The primary secondary sedimentation tank is set before the biological treatment unit and mainly utilized to remove the suspended solids (SS) dominated by organic matters in the raw water, and, at the same time, improve the operating conditions of the biological treatment unit in the later stage and reduce the pollutant load. The secondary sedimentation tank is set behind the biological treatment unit to precipitate and separate activated sludge or biofilm exfoliation, and achieve certain sludge concentration. It is an integral part of the biological treatment system.

The SS removal efficiency of the secondary sedimentation tank is proportional to its size and the SS sedimentation rate, and is inversely proportional to the daily treated water volume. Hence, the surface hydraulic load (daily treated water volume/surface area) is critical in the design of the secondary sedimentation tank.

• e. Phosphorous Removal

The water quality discharged from the rural domestic sewage treatment plant is generally determined by environmental sensitivity and environmental requirements of receiving water. When the effluent quality standard limits TP discharge, the biological phosphorus removing technology will be adopted. And the chemical phosphorus removal technology will be supplemented to further treat TP when the discharge requirements cannot be satisfied by the biological phosphorus removal. Chemical phosphorus removal technology can be divided into chemical process and physical–chemical process, as shown in Table 2.3. The former includes the precipitation process and crystallization process, and the latter includes the adsorption process (Wu et al., 2019).

Table 2.3 Comparison of chemical phosphorus removal processes

The precipitation process is to remove phosphorus in sewage through precipitation generated from the reaction of metal ions and phosphate radicals by adding metal salts (such as aluminum salts, iron salts, and calcium salts). This is a common process of phosphorus removal in sewage. Al³⁺ salts (such as polyaluminum chloride PAC), instead of calcium hydroxide (slaked lime), are frequently used in the treatment of rural sewage with the precipitation process. The supplement of chemical phosphorus removal in the biochemical treatment by adding chemicals can be considered in the centralized treatment of rural sewage.

As there is a risk of depleted phosphorus resources in the next few decades, phosphorus recovery and utilization are one of popular topics in the field of sewage treatment. Recovery and resource utilization of phosphorus in sewage can be achieved by removing phosphorus with the crystallization process. The representative MAP process (Magnesium Ammonium Phosphate hexahydrate), taking advantage of the reaction of magnesium salts with ammonia ions and phosphoric acid in sewage, can generate ammonium magnesium phosphate precipitation while removing nitrogen and phosphorus. Although the crystallization process is rarely used in rural sewage treatment, it will have a broad application prospect with the introduction of relevant policies on the utilization of rural sewage resources (Liu et al., 2013).

Rural domestic sewage treatment plants are distributed dispersedly with a lack of specialized operation and maintenance personnel. The electrolytic dephosphorization process has been widely used in recent years to ease the difficulty of operation and maintenance of sewage treatment plants (Sato, 2013). In electrolytic dephosphorization, the anode and cathode generate metal ions and hydrogen, respectively, with metals such as iron and aluminum as electrode plates during electrolysis; on this basis, phosphorus in sewage can be removed with the precipitation generated from the reaction of metal ions and phosphate groups and the air flotation effect of hydrogen. The operation and maintenance period can be extended to 2–3 months when double iron plate electrodes are utilized and the positive and negative electrodes are switched regularly.

Phosphorus removal by the adsorption process (such as activated carbon adsorption, oyster shell, and other biomass adsorption, iron oxide, and hydrotalcite adsorption) is to remove phosphorus through the bonding effect between the active groups of the removal agent and the phosphorus in sewage. It features fast adsorption rate, and environmentally friendly removal. But it also presents defects in low saturated adsorption capacity and complicated operation and maintenance (Wu et al., 2019). As some suggested, recently, that chemical phosphorus removal should not be used in rural domestic sewage treatment plants with a treatment scale of less than 20 m³/d, the adsorption process has certain application prospects in rural sewage treatment.

• f. Disinfection

Disinfection should be performed before effluent when the bacterial index is limited in the discharge standard of the sewage treatment plants. Chlorination disinfection and ultraviolet disinfection are commonly used disinfection processes in rural domestic sewage treatment under the limitation of factors such as operating costs, complexity of operation and maintenance management (Zeng et al., 2021).

Sodium hypochlorite disinfection can destroy proteins, nucleic acids, and other components in bacteria and viruses relying on the strong oxidizing properties of its hydrolyzed product hypochlorous acid (HClO). Note that a strong disinfection effect can only be achieved after reaching the chlorine breaking point when reducing substances and ammonia nitrogen in sewage are all consumed and free chlorines (HClO and hypochlorite ion) are accumulated. Sodium hypochlorite solution with a concentration of more than 10% is usually added to the clear water tanks through a metering pump for disinfection in rural sewage treatment (Zeng et al., 2021).

Chlorine dioxide (ClO₂) with a strong oxidizing property is an efficient and environmentally friendly disinfectant that can directly oxidize microbial proteins, nucleic acids, and other components in molecular form after dissolving in water (Ren, 2005). ClO₂ effervescent tablets can be selected in rural sewage treatment with effective ClO₂ content of roughly 10%, featuring convenient storage and use. A certain amount of effervescent tablets are dissolved in water, and then delivered to the clear water pond by a diaphragm metering pump for disinfection (Zeng et al., 2021).

Ultraviolet disinfection is a physical disinfection process, and the best disinfection effect can be achieved at the band near 253.7 nm. It realizes disinfection by destroying and changing the structure of DNA and RNA in microorganisms (Shiohara et al., 2005). Ultraviolet disinfection with the strengths of simplicity, convenience, high efficiency, no secondary pollution, easy management, and automation is also susceptible to the turbidity of sewage, SS, and inorganic ions (such as Fe³⁺).

The disinfection in rural sewage treatment has received extensive attention after the worldwide outbreak of COVID-19 pandemic in 2020. Zeng et al. (2021) indicate that integrated equipment should be adopted in rural sewage treatment plants in response to the pandemic, as its independent and closed treatment unit can reduce the contact between sewage and management personnel. Detailed requirements covering the disinfection of septic tanks, tailwater, and sludge are proposed in the “Technical Regulations for the Operation and Maintenance of Village Domestic sewage Treatment plants” (T/CCPITCUDC-003-2021) for the operation and maintenance of sewage treatment plants during the pandemic. As pandemic prevention and control have become the new normal, rural sewage treatment plants should have the ability to conduct emergency disinfection and detection, meeting the emergency supervision requirements at special times.

2.2.2 Septic Tank

Septic tank, is the simplest sewage treatment device with the lowest investment, with its concept dating back to the 19th century. Mouras from France designed a new type of septic tank in which both the inlet and outlet pipes were deeply inserted under the liquid surface to form a water seal (Zhang et al., 2021) in 1860. “Mouras pond” is the origin of modern septic tanks and is also seen as the beginning of anaerobic biological treatment technology (Fan et al., 2017). Cameron and Cummins in England named the modified “Mouras tank” as “Septic tank” and patented it in 1895. A two-grid “Imhoff tank” was proposed by Imhoff from German based on the septic tank in 1905 to strengthen the separation of solid matter and effluents, and has been extensively used in the primary treatment of urban drainage worldwide.

Static separation and anaerobic fermentation are the treatment principles of septic tanks (Kamel & Hgazy, 2006). The manure is formed into the scum layer, the liquid layer, and the sediment layer under the combined action of gravity and buoyancy after entering the septic tank, as shown in Fig. 2.3. Preliminary treatment of sewage can be achieved through decomposing organic matters into methane, carbon dioxide, hydrogen sulfide, and ammonia via anaerobic bacteria in the tank during the long retention time. Meanwhile, high ammonia nitrogen and high pH environment in the anaerobic digestion process can kill pathogens such as parasite eggs, realizing the harmless treatment of effluent and slag (Fan et al., 2017). After sufficient stabilization, cleaned solids can be used as fertilizers, and the liquid in the middle layer can be used for farmland irrigation or directly discharged under low environmental requirements, otherwise, further treatment is needed before discharging.

Fig. 2.3

Septic tank

The three-grid septic tank (Fig. 2.4) widely used today was developed in China, with one grid added for storing the cured manure. In this way, the necessary hydraulic retention time is not affected while feces can be easily taken out (Zhang et al., 2021). The three-grid septic tank featuring a simple structure and satisfactory treatment effect has the same treatment principle as the single-grid septic tank. Feces are precipitated in the first grid and degraded by fermentation, the fecal liquid is further fermented upon flowing into the second grid, and harmless fecal fluid is stored in the third grid (Cheng et al., 2018; Zheng et al., 2022).

Fig. 2.4

Three grid septic tank

The 3:1:1 structure ratio proposed in “Code for Design of Water Supply and Drainage in Buildings” (GB50015-2019), the 2:1:1 structure ratio proposed in “Technical Specifications for Town and Village Drainage Engineering” (CJJ 124-2008), and the 2:1:3 structure ratio proposed in “Rural Household Toilet Hygiene Standard” (GB 19379-2012) are three mainstream structures of three-grid septic tanks. Among them, the 3:1:1 type septic tank with a retention time of 12–24 h is used for removing suspended solids in domestic sewage. It serves as the primary transitional domestic sewage treatment structure before the discharge into the urban downcorner network. The 2:1:1 type septic tank with a retention time of 24–36 h, has the same function as that of the 3:1:1 structure, and the 2:1:3 septic tank with a retention time of no less than 60 days is suitable for the harmless treatment of feces in rural household toilets.

Liner should be performed on B&Rck or reinforced concrete septic tanks as well as prefaB&Rcated septic tanks using glass fiber reinforced plastics or polyethylene to prevent the pollution of groundwater and the surrounding environment. Meanwhile, odor-resistant and explosion-proof measures should be also taken. The septic tank is more applicable for treating toilet sewage. Domestic grey water should not be discharged into the septic tank, otherwise, its treatment effect under the design retention time cannot be guaranteed, and the feces diluted by the domestic miscellaneous drainage may have less value of fertilizers after treatment (Zheng et al., 2022). Moreover, the septic tank should be cleaned in time at an interval of 3–12 months.

Featuring a simple structure, simple operation, energy-saving, and low investment cost, septic tank is superior to other domestic sewage treatment processes. But its limited treatment capacity should be also concerned. Its discharge should be first considered as resource utilization. If not, further treatment should be performed before discharge to reach the discharge standard.

2.2.3 Biofilm Process

The biofilm process is to remove pollutants in sewage using the dense biofilm attached to the carrier surface, a sewage treatment technology juxtaposed with the activated sludge process. Biofilms are formed by microorganisms wrapped by extracellular macromolecular polymers (EPS). EPS is composed of polysaccharides, proteins, and other matters, which can immobilize microorganisms on the carrier like glue to enable their growth and reproduction (Zhang, 2015). Organic pollutants of sewage enter the biofilm through diffusion and are extracted and decomposed by microorganisms as nutrients. Then, the sewage is purified. The multiplication of microorganisms, the growth, thickening, and shedding of biofilms are coupled with the process.

The biofilm process is applicable for rural domestic sewage treatment with small and medium water volumes as it has characteristics that the activated sludge process does not have, such as strong adaptability to changes in water quality and quantity, convenient management, long sludge age, diversified species, and few excess sludges.

The biofilm process frequently used in rural sewage treatment is composed of an anaerobic biofilm tank, biological filter, biological contact oxidation tank, and rotating biological disc.

• i. Anaerobic Biofilm Tank

Limited by economic conditions in rural areas, the anaerobic sewage treatment technology with no energy consumed is more advantageous than the anoxic technology. Moreover, the biofilm process can better meet the actual needs of rural domestic sewage treatment with its features of easy control, strong shock load resistance, difficulty in biomass loss, and less dependence on the control of process operating conditions (Cui et al., 2021). The anaerobic biofilm tank is suitable for the treatment of rural domestic sewage as it combines anaerobic technology and biofilm process.

The anaerobic biofilm tank is an anaerobic reactor with carriers. Anaerobic microorganisms grow on the surface of the carrier in the form of biofilm. When the sewage flows through, its organic matters are absorbed by the biofilm and removed under the combined action of biofilm adsorption, microbial metabolism, and carrier interception. Biochemical reaction in the anaerobic biofilm tank is an anaerobic digestion process that degrades organic matter into gases (primarily methane and carbon dioxide). This process can be divided into the hydrolysis stage (microorganism degrading the high molecular organic matter into low molecular soluble organic matter via extracellular enzyme), acidification stage (low molecular organic matter being decomposed into volatile fatty acids, ethanol, and lactic acid, etc.), acetogenic stage (product in the acidification stage being further decomposed into acetic acid, H₂, CO₂, etc.), and the methanogenic stage (generating methane and new cells) (Henze et al., 2008). This technology can effectively remove organic matters in sewage, and explosion-proof measures should be taken.

The selection of carrier is essential for the anaerobic biofilm tank technology, which has a direct impact on the treatment effect of sewage (Zhang, 2021). Hence, the carrier should be selected with consideration of its surface properties, biochemical stability, roughness, biotoxicity, and economical applicability. Ceramics, zeolites, plastics, and activated carbon are commonly used carriers in practical application engineering (Karadag et al., 2015). The anaerobic biofilm tank technology is employed in the anaerobic tank part of the Japanese combined treatment type Johkasou with commonly used carriers of plastic materials such as polyethylene (PE) and polypropylene (PP) in a variety of shapes, such as grid-based cylinder, flat, grid-based flat, grid-based cylinder, and sphere, as shown in Table 2.4.

Table 2.4 Common anaerobic carriers in Johkasou

The SS removal effects of various shapes and materials of carriers are compared in a combined treatment Johkasou. The findings showed that grid-based plate (the first anaerobic tank) and spherical (the second anaerobic tank) carrier are the carrier combination with the highest SS removal rate (Ogawa & Iwahori, 2002).

The reaction rate and sludge production rate of the anaerobic treatment process are far lower than that of the oxic treatment process, removing up to 90% of the excess sludge production (Henze et al., 2008). The hydraulic retention time of the anaerobic biofilm tank, in general, is taken as two days to five days in practical engineering applications with the sludge discharge interval of three months to 12 months.

To sum up, the low removal rate of TN and TP of the anaerobic biofilm tank should be noted although it has many advantages. Apart from being used for farmland irrigation, the discharge from the treatment plant can be further treated for the removal of TN and TP with other processes. Moreover, the long biofilm domestication time of the anaerobic biofilm technology and the difficulty in the control of old biofilms shedding are critical for its application (Escudié et al., 2011).

• ii. Biological Filter

Biological filter is a typical biofilm technology developed from the practice of sewage irrigation according to the principle of soil self-purification (Liu, 2019). It is one of the earliest and most widely applied processes in the field of sewage treatment, with a history of more than 100 years. It can be divided into various process types such as common biological filter (trickling filter), high-load biological filter, tower biological filter, and aerated biological filter. Common biological filters are most frequently used in rural sewage treatment.

The biological filter is filled with fixed bed carriers. The sewage flows through the carrier layer from top to bottom in the form of dropwise spraying and is purified by contacting with the biofilm on the carriers. It uses natural ventilation for oxygen supply, and is characterized by simple operation, low operating cost, low sludge production, and stable operation. Its basic process flow is shown in Fig. 2.5. The sewage enters the biological filter for treatment upon the removal of suspended solids and other pollutants that may block the carrier, and then passes through the secondary secondary sedimentation tank to retain the biofilm that falls off the biological filter, ensuring water quality. In addition, a equalization tank should be set when the water quality and quantity fluctuate greatly.

Fig. 2.5

Basic process flow of biological filter

The common biological filter is comprised of the filter, carriers, water distribution device, and drainage system. To be specific, plastics and gravels such as polyethylene, polystyrene, and polyamide are common carriers, and materials with large specific surface area and high porosity such as corrugated plates, porous screened plates, and plastic honeycombs are plastic carriers. Either fixed or mobile water distribution device can be adopted to evenly distribute sewage on the surface of the filter tank. The drainage system, consisting of a seepage device, water collection ditch, and general drainage ditch, is set at the bottom of the filter for drainage and ventilation. The hydraulic load of the ordinary biological filter should be ranged from 0.1 to 0.5 m³/m²/h (Administration and for Market Regulation, 2019).

The biological filter is applicable for centralized treatment of rural domestic sewage due to its characteristics of simple operation and low operating cost. But it also faces problems such as low processing load, large coverage, susceptible to plugging, backwash, large water head loss, and filter flies attracted by stinking (Yue et al., 2013).

• iii. Biological Contact Oxidation Tank

First proposed by Wring at the end of the nineteenth century (Wang, 2015), the concept of biological contact oxidation was developed on the basis of biological filters and has grown into a proven aerobic biofilm sewage treatment process widely used in the treatment of rural sewage, urban sewage, and industrial sewage.

The carrier of the biological contact oxidation tank is completely immersed in the sewage and should be supplied with oxygen through air blow and aeration, so it is also known as the “submerged biological filter” or “contact aeration process”. The biological contact oxidation system consists of fixed bed carriers and supports, an aeration system, a water inlet and outlet device, a sludge discharge pipeline and a tank. The sewage is fully contacted with the biofilm fixed on the surface of the carrier under aerobic conditions. Organic matters and nutritive salts in sewage can be removed through microbial degradation. After that, the sewage is purified. Sufficient dissolved oxygen in the system, abundant species of microorganisms, and old biofilm shedding by the aeration are beneficial to maintaining the biofilm activity. The technology is characterized by high biomass (with the biofilm amount reaching 8000–14,000 mg VSS/L), strong removal ability of organic matters, strong adaptability to impact load, less sludge production, simple operation, and easy to manage (Zhang, 2015).

Carriers such as suspended carriers and fixed carriers can be used in the contact oxidation sewage treatment (HJ 2009–2011) with flexible settings, as shown in Fig. 2.6. Common carrier materials are plastics, glass fiber reinforced plastics, and fiber in the shapes of honeycomb, cylinder, corrugated plate, and bundle. The contact oxidation process includes the primary-stage contact oxidation process and multi-stage contact oxidation process, and the secondary-stage or multi-stage contact oxidation process can be used for organic matters of high concentration, as shown in Fig. 2.7. The BOD₅ of the biological contact oxidation tank is ranged from 0.15 to 0.18 kg/m³/d when the treatment capacity is less than 5 m³/d, and should be ranged from 0.15 to 0.2 kg/m³/d when the treatment capacity is more than 5 m³/d according to GB/T51347-2019 (Administration and for Market Regulation, 2019).

Fig. 2.6

Carriers setting process (left: full aeration; middle: central aeration; right: side aeration) (Matsuo, 2015)

Fig. 2.7

Flow chart of contact oxidation process

The biological contact oxidation tank is used for household sewage treatment and centralized treatment of rural domestic sewage. The combined process of anaerobic tank and aerobic tank should be used when denitrification is required. With a poor effect in biological removing of phosphorus, chemical phosphorus removal is normally required in response to the requirements of phosphorus removal. Note that the treatment effect can be affected by carrier blockage, and uneven water and gas distribution under improper design or operation.

• iv. Rotating Biological Disc Reactor

The rotating biological disc reactor, born in Germany in the 1920s (Chan & Stenstrom, 1979), is a typical biofilm treatment technology developed from biological filters, and has been widely used in the treatments of rural sewage, urban sewage, and industrial sewage.

Consisting of a disc, a contact reaction tank, a rotating shaft, and a driving device, it performs sewage treatment based on the rotation of the disc covered with biofilm. In general, multiple sets of discs are strung on the rotating shaft, with 35–45% of the rotating disc area immersed in sewage. With the rotation of the shaft and the disc, the biofilm on the disc contacts with organic matters, ammonia nitrogen, and other pollutants while being rotated to the sewage, and contacts with oxygen while being rotated to the air. In circulation, pollutants are discomposed by microorganisms to purify the sewage (Chan & Stenstrom, 1979). The matured biofilm is peeled under the action of the shear force of water flow and sedimented in the secondary secondary sedimentation tank, as shown in Figs. 2.8 and 2.9.

Fig. 2.8

Sewage treatment mode of rotating biological disc reactor (Zhang, 2015)

Fig. 2.9

Process flow of rotating biological disc reactor

The rotating biological disc reactor has the advantages of high efficiency, simple structure, easy to main, and low power consumption, together with a strong resistance to water shock load (treat the sewage with a BOD₅ value of 10–10,000 mg/L) and high microbial concentration (reaching up to 50,000–60,000 mg VSS/L), multiple species of microorganisms, long sludge age, low sludge production rate (accounting for roughly 1/2 of the activated sludge process), no need of aeration and sludge return (Gao and Li, 2018).

Discs in this process are mostly circular and regular polygonal mesh plates or corrugated plates made of polyethylene or polyester fiberglass, featuring light material, high strength, corrosion resistance, and large specific surface area. In general, the disc diameter is ranged from 2 to 4 m with the disc space between 10 and 35 mm and the rotational linear velocity ranging from 15 to 18 m/min (Zhang, 2015). Uniaxial multi-stage rotating biological discs should be used for centralized sewage treatment in rural areas with no less than three stages, as shown in Fig. 2.10 (Administration and for Market Regulation, 2019).

Fig. 2.10

Single-axis multi-stage rotating biological disc reactor

The rotating biological disc reactor can be used for the centralized treatment of rural domestic sewage. Note that there is a risk of reduced pH value in the contact reaction tank with the nitrification reaction. Moreover, the tiny SS that is difficult to remove may lead to low visibility of the treated water, causing problems such as odor and breeding of flies and mosquitoes (Matsuo, 2005).

2.2.4 Activated Sludge Process

The activated sludge process developed based on the principle of water self-purification has a history of more than 100 years and is a biological water treatment technology most extensively used today. It can purify the sewage using the free-flowing tiny “sludge particles” (a flocculating constituent formed by a large number of microorganisms and EPS) suspended in the sewage, which is different from the biofilm process that depends on the biofilm attached to the carrier for purification. The activated sludge flocculating constituent flows freely in the sewage and adsorbs organic matters with the action of aeration or mechanical stirring. The microorganisms in the flocs carry out aerobic metabolism with oxygen in the oxic environment, through which, organic matters are decomposed into carbon dioxide (CO₂) and water (H₂O) to purify sewage. During this, microorganisms obtain nutrients and energy for multiplication and are finally removed in the secondary sedimentation tank via solid–liquid separation. As the activated sludge can be mobilized freely in sewage, it has a higher chance of contacting pollutants and oxygen. In that case, a higher pollutant removal efficiency with a more flexible process can be realized compared with the biofilm process.

The activated sludge process has experienced continued innovation and development in terms of theory, technology, and process, and the derived technologies can efficiently degrade organic matter and remove nitrogen and phosphorus using microorganisms at the same time. Currently, the traditional activated sludge process and its modified process have been widely used in rural sewage treatment.

• i. Traditional Activated Sludge Process

B&Rtish scientists Ardern and Lockett announced the birth of the activated sludge process at the B&Rtish Chemical Society and defined its basic principle that the solid suspended matter generated in sewage should be recycled and returned to sewage for accumulation for its purifying effect upon the introduction of air into the sewage, rather than being removed (Henze et al., 2008).

Upon the introduction of air into the sewage (aeration), organisms such as bacteria, protozoa, and metazoans can be multiplied with oxygen and organic matters in sewage and aggregated to form gelatin yellow–brown flocculating constituent or activated sludge. The suspended activated sludge will be sedimented rapidly as soon as the aeration is stopped, with clear water obtained upon the separation of muddy water. Note that the sludge in the anaerobic treatment process cannot be called activated sludge.

From the perspective of microbial engineering, the activated sludge process refers to the rapid adsorption of colloidal and granular macromolecular organic matters dissolved in sewage upon contact with activated sludge and converting organic matter into small molecules finally taken into cells for metabolism by extracellular enzymes of microorganisms (Matsuo, 2005). Specifically, part of organic matter (roughly 50%) is metabolized to be newly-synthesized bacterial cells while the other is catabolized to generate CO₂, H₂O, and energy used to maintain life activities and anabolism of microorganisms, as shown in Fig. 2.11.

Fig. 2.11

Metabolic reaction of heterodoxic microorganisms

Figure 2.12 shows the traditional activated sludge process flow with a secondary sedimentation tank, oxygen supply device, and reflux equipment. After removing larger particles of suspended solids by the primary secondary sedimentation tank, the sewage is mixed with the reflux sludge flowing out from the bottom of the secondary sedimentation tank. Then the sewage flows into the aerobic tank which provides the oxygen required by microorganisms for aerobic metabolism, and plays the role of suspending the activated sludge to make it fully mixed with the sewage to improve the efficiency of pollutant removal. At last, mud and water are separated in the secondary sedimentation tank. The clear water in the upper layer is discharged as treated water, and part of the sedimented sludge is mixed with the new influx sewage before returning to the aerobic tank as return sludge, while the rest is discharged from the equipment for disposal as excess sludge. Then, the whole process of sewage treatment is done. The volume load of the activated sludge process is set at 0.1 kg BOD₅ (m³/d) in rural sewage treatment.

Fig. 2.12

Process flow of traditional activated sludge process

• ii. A/O Activated Sludge Process

The activated sludge process has become a mainstream process of sewage treatment with its features of high efficiency, flexible operation mode, and low daily operating costs. Based on this, a variety of variant processes has been evolved to remove nitrogen (N) and phosphorus (P) elements in sewage, so as to prevent the eutrophication of received water. Specifically, the anoxic/oxic activated sludge process (A/O) is a typical biological denitrification process.

Nitrogen in sewage mainly exists in the form of organic nitrogen (protein, amino acid, etc.) and NH₃–N, which originated from the human body’s metabolism of ingested protein. Biological denitrification achieves the purpose of denitrification by converting organic nitrogen and NH₃–N into nitrogen gas (N₂) based on some obligate bacteria (ammonifying bacteria, nitrifying bacteria, and denitrifying bacteria) (Henze et al., 2008). Among them, organic nitrogen is decomposed into NH₃–N via the effect of ammonifying bacteria. After that, NH₃–N is oxidized to nitrite (NO₂⁻) by nitrite bacteria under the aerobic state (Table 2.5), and then further oxidized to nitrate (NO₃⁻) by nitrate bacteria. These two reaction processes are nitrification reactions, and nitrite bacteria and nitrate bacteria are nitrifying bacteria. At last, NO₃⁻ is reduced to N₂ by denitrifying bacteria using organic matters (electron donor) in the anoxic environment (Table 2.5) before discharged into the air (denitrification).

Table 2.5 Definitions of anaerobic, anoxic, and oxic environments and their biochemical reactions

The above nitrification and denitrification processes, together with nitrogen fixation, constitute the traditional nitrogen cycle process, NO₃⁻ can be excluded from the circulation to shorten the reaction process and save the input of oxygen and organic matters from the point of the technical process. Some microorganisms can generate nitrous oxide (N₂O) under micro-aerobic conditions using NO₃⁻ and NH₂OH and convert NO₂ into NO gas under anoxic conditions, according to studies. Nitrogen can be removed from the liquid phase using these two processes. But the toxicity of NO and the strong greenhouse effect of NO₂ (approximately 310 times that of CO₂) should be highly concerned. By contrast, Anammox bacteria can generate N₂ using NH₄⁺ and NO₂⁻ under anaerobic conditions, which is considered the next-generation biological denitrification technology as it can shorten the nitrogen cycle without generating harmful by-products (Henze et al., 2008).

The traditional nitrogen cycling route is still the mainstream biological denitrification process. More precisely, A/O activated sludge process (Fig. 2.13) is constituted by adding an anoxic pond (A tank) and a nitrification solution return system based on the O tank of the traditional activated sludge process. The nitrifying liquid reflux system functions as refluxing the NO₃⁻ in the outlet of the aerobic tank to the anaerobic tank for denitrification. The anaerobic tank can also be set behind the aerobic tank. But setting the anaerobic tank in the front with the nitrification reflux (Fig. 2.13) can fully utilize organic matters in the raw water for denitrification reaction since the concentration of organic matters in the outlet of the aerobic tank is low, and the participation of organic matters is required in the denitrification reaction. Moreover, the intermittent aeration operation mode can be also adopted in the traditional activated sludge process for denitrification in the rural sewage treatment since the oxygen content (dissolved oxygen) in the oxic pond is reduced to reach the anoxic state after the aeration stops, as shown in Table 2.5. By doing so, it can provide conditions for the denitrification of denitrifying bacteria.

Fig. 2.13

A/O process flow

• iii. Enhanced Biological Phosphorus Removal (EBPR) and A²/O Activated Sludge Process

Phosphorus, as an essential inorganic element of the vital activity of the organism, plays an important role in cell composition, metabolic reaction, and material transformation. The phosphorus in domestic sewage mostly come from urine and can be removed by the chemical process. In other words, phosphorus can be removed by the chemical precipitation of external chemical substances and phosphorus. But biological phosphorus removal remains the most economical and effective process for phosphorus removal.

As phosphorus is the inorganic element with the largest demand for microorganisms, 15–25% of phosphorus can be removed from domestic sewage via the anabolism of microorganisms in the traditional activated sludge process. The quantity of phosphorus absorbed by PAOs in the system can exceed the amount required for normal microbial anabolism, thereby achieving enhanced biological phosphorus removal (EBPR), as shown in Fig. 2.14. The phosphorus content in PAOs can reach as high as roughly 0.38 mgP/mgVSS, and the overall phosphorus content in the sludge in the EBPR system can reach approximately 0.06–0.15 mgP/mgVSS (Henze et al., 2008), far higher than 0.02–0.03 mgP/mgVSS of traditional activated sludge process (Matsuo, 2005).

Fig. 2.14

Process flow of EBPR system

The environmental condition of first being anaerobic (Table 2.5) and then oxic is required for concentrating phosphorus with PAOs. The metabolic process is shown in Fig. 2.15. PAOs ingest organic matters (mainly volatile fatty acids such as acetic acid) using the energy (ATP) generated by the hydrolysis of Poly-P, and finally store them in cells in the form of PHA in the anaerobic environment. The tricarboxylic acid (TCA) cycle or the degradation of glycogen (Mino et al., 1998) contributes to the reducing power required for the synthesis of Poly-P as a reduction product. Specifically, the former is known as the Comeau-Wentzel model (Comeau et al., 1986), while the latter is known as the Mino model (Mino & Matsuo, 1984). Besides, the release of PO₄³⁻ is accompanied by the hydrolysis of Poly-P, as shown in Figs. 2.15 and 2.16.

Fig. 2.15

Metabolism of PAOs

Fig. 2.16

Dynamics of organic matters and phosphorus in EBPR system

The PHA stored in the PAOs cells during anaerobism is used for catabolism and anabolism after entering the aerobic environment. The energy obtained is adopted for the synthesis of Poly-P or glycogen, so as to prepare for entering the next anaerobic environment. Specifically, PO₄³⁻ should be obtained for the synthesis of Poly-P, as shown in Figs. 2.15 and 2.16. PAOs can metabolize normally even if there is no intake of organic matter in an oxic environment since it stores PHA in an anaerobic environment. In this way, dominant bacteria will be formed by PAOs in the anaerobic-oxic environment.

The anaerobic/anoxic/oxic activated sludge process (A²/O) can be obtained by adding a first-stage anaerobic tank (A) to the A/O process, as shown in Fig. 2.17. The P in sewage is absorbed by PAOs in the sludge and enriched in the sludge after the activated sludge flows through the anaerobic and oxic ponds in turn. Finally, the separation of muddy water and the discharge of excess sludge are performed to remove P, achieving the purpose of simultaneous denitrification and phosphorus removal.

Fig. 2.17

A²/O activated sludge process flow

• iv. Oxidation Ditch

As a kind of activated sludge process of extended aeration, oxidation ditch is also known as the continuous circulation reactor, which was originally a process for treating dairy farmer’s sewage on a simple facility that emerged in the Dutch countryside and then formally put into engineering application in 1954 (Thakre et al., 2019). It is widely used in the treatment of rural and municipal sewage nowadays. Obral oxidation ditch, Carrousel oxidation ditch, and single-ditch oxidation ditch are the widely used oxidation ditches, and the Carrousel oxidation ditch with the longest history is the most widely used one worldwide.

The simplest continuous oxidation ditch process is shown in Fig. 2.18. The annular reaction tank is equipped with a secondary secondary sedimentation tank, and it has developed into a reinforced concrete structure from the originally simple excavated ditch. The mechanical aeration device is leveraged for oxygen supply, which can provide the required oxygen for activated sludge treatment and mix the activated sludge and sewage, driving the circulation of the mixture at a certain flow rate, and forming a unique flow pattern between the complete mixture and plug flow. The activated sludge experience aerobic and anoxic states during the circulation in the annular ditch, and organic matters and TN are removed at the same time.

Fig. 2.18

Process flow chart of single-ditch oxidation ditch

An oxidation ditch can be seen as a completely-mixed aeration tank. Raw water has minor effect on the concentration of pollutants in the ditch, as it can be diluted by dozens or hundreds of times of flow upon entering the tank. The ditch is normally operated at a low sludge load of 0.03–0.10 kgBOD₅ (kgMLSS/d) (Ai and Cui, 2018; Administration and for Market Regulation, 2019) with the ability to resist the impact load of water quality and quantity. Moreover, a certain processing capacity can be maintained at a low temperature (roughly 5 °C). The nitrification reaction can be performed easily with the oxidation ditch technology for its small sludge load and long retention time. Hence, the continuous water intake and intermittent aeration operation mode can be adopted in the single-ditch oxidation ditch for denitrification, and the removal rate of TN can reach roughly 70% by setting the anoxic area in the annular waterway.

To sum up, the traditional activated sludge process, A/O activated sludge process, A²/O activated sludge process, and oxidation ditch are all applicable for centralized treatment of rural domestic sewage. By comparison, traditional activated sludge process, A/O activated sludge process, and A²/O activated sludge process, with higher requirements for operation, are called “operation-dependent” processes (Wang, 2018), frequently requiring special persons for process inspection or maintenance. Hence, they face great difficulty in implementation in rural areas. Moreover, there are also problems such as high energy consumption (blowers, reflux systems, etc.), poor resistance to shock loads, susceptibility to sludge bulking, and unstable biological phosphorus removal (for instance, after mixed with rainwater) (Yoshida et al., 2005). The oxidation ditch process featuring simple treatment flow, stable and reliable process, simple operation and maintenance, and low investment, can be used for the centralized treatment of rural domestic sewage. Note that the oxidation ditch requires a large land area for its long retention time and shallow tank.

2.2.5 Membrane Bioreactor

Membrane bioreactor (MBR) is a sewage treatment process organically combined by membrane separation and biological treatment. The concept of MBR originated in the United States. Dorr-Oliver built the world’s first MBR process sewage treatment plant (14 m³/d) in the 1960s. MBR experienced rapid growth with the development of material technologies and the reduction of energy consumption costs in the 21st century after a long R&D development stage (Hao et al., 2018). The technology is extensively applied in various fields, such as domestic sewage treatment and industrial sewage treatment (Fig. 2.19).

Fig. 2.19

Separation range of different membranes (Patsios & Karabelas, 2010)

MBR has replaced the secondary sedimentation tank of the traditional activated sludge process by introducing membrane separation technology. While retaining the advantages of the activated sludge process, it has greatly improved the solid–liquid separation efficiency, reduced land use, and improved the quality of effluents. The effluents can be directly used for compensating landscape water and recycling reclaimed water (Henze et al., 2008). Microfiltration membranes and ultrafiltration membranes are adopted in MBR membrane modules in various forms such as hollow fiber, flat and tubular types. There are also various types of membrane materials, including PVDF, PP, PVC, PE, PES, and ceramic membranes (Zhang et al., 2020). MBR is comprised of a bioreactor, membrane module, and smallwater pump (Henze et al., 2008). MBR can be classified into immersion membrane bio-reactor (IMBR) and separate membrane bio-reactor (SMBR) by processing technology, as shown in Fig. 2.20. IMBR consumes less energy and has broader applications than SMBR (Lin, 2015). Furthermore, MBR can be classified into anaerobic MBR, facultative MBR, and aerobic MBR processes according to the DO concentration level in the reaction tank.

Fig. 2.20

IMBR (upper) and SMBR (lower)

MBR is superior to the traditional sewage treatment process in the following aspects. First, a compact structure (small floor space). As no secondary sedimentation tank is needed, it can decrease 1/5 to 1/3 of land use in the traditional activated sludge process. Second, high quality of effluents. The membrane module can efficiently intercept SS, turbidity, bacteria, and viruses. Third, the MLSS can reach up to 10–20 g/L with a large volume load, strong resistance to the impact of water quality and quantity, and less output of excess sludge. Fourth, HRT and SRT of the reactor are completely separated, and the HRT can be adjusted according to the treatment effect, with more flexible operation control. Long SRT can enrich the long-generation microorganisms such as nitrifying bacteria to promote the removal of ammonia nitrogen and refractory organic matters. Finally, there is no risk of sludge bulking (Jia, 2016). The compact structure (that is, small coverage) and good outlet quality are the absolute advantages of MBR.

Prominent disadvantages such as high operating costs and membrane pollution can be found in MBR. Expensive membrane modules, various accessory facilities, and high energy consumption are the causes of high operating costs. Relevant studies revealed that the investment cost of MBR is between 2000 and 5000 yuan/m³/d (roughly 3800 yuan/m³/d on average), 10–30% higher than the traditional activated sludge process plus three-stage filtration combined process (Krzeminski et al., 2012). Regarding energy consumption, maintaining membrane flux via pressurization (membrane pollution leads to a decrease in flux) and higher aeration rate (the aeration rate should be increased to relieve the membrane pollution) cause the energy consumption of MBR as high as roughly 2 kW h/m³, 60–900% higher than the traditional activated sludge process (approximately 0.3 kW h/m³) (Sun et al., 2016). In addition, although remarkable scientific research results and technologies have been made to address the problem of membrane pollution as a hot spot of MBR research, the problem is not solved fundamentally (Meng et al., 2009). Online cleaning for maintenance (once in 1–3 months) and offline chemical cleaning for restoration (once in half a year to 1 year) are required, resulting in the reduced service life of the membrane and increased operating costs. What’s more, the high degree of automated operation poses higher requirements for the operation and maintenance personnel.

To sum up, MBR is a process with outstanding advantages and disadvantages and is applicable for domestic sewage treatment in rural areas where land space is strictly limited with high discharge standards, a need for reclaimed water reuse, and economic conditions allowed.

2.2.6 Natural Biological Treatment

Natural biological treatment is a process technology that purifies sewage using natural ecological or artificial ecological functions. Sewage is purified through the physical and chemical effects of soil or artificial carriers, the purifying function of natural organisms, and the interception and absorption of aquatic plants. It has been widely applied with its features of high purification efficiency, low operation and maintenance costs, and low management level required. Constructed wetlands and stabilization ponds are two commonly used processes in rural sewage treatment.

• i. Constructed Wetlands

Constructed wetland is an artificially designed and constructed complex consisting of substrates (fine sand, gravel, etc.), wetland plants, organisms, microorganisms, and water bodies by simulating the natural wetland (Li et al., 2018). It has been widely used in domestic sewage treatment since the world’s first constructed wetland for sewage treatment was built more than 100 years ago in Yorkshire, England in 1903. Specifically, the constructed wetland can purify sewage through a series of filtration, adsorption, co-precipitation, ion exchange, plant absorption, and microbial decomposition combing physical, chemical, and biological treatment processes (Kadlec & Wallace, 2008). It can be subdivided into surface flow, horizontal subsurface flow, and vertical subsurface flow as per the characteristics of water flow, as shown in Fig. 2.21.

Fig. 2.21

Three types of constructed wetland

Surface flow constructed wetlands are open waters, with the appearance most approaching natural wetlands. They are planted with floating plants, emergent aquatic plants, or phreatophytes. The sewage flows horizontally in the surface layer with a shallow water level between 0.1 and 0.3 m, presenting a favorable oxygenation effect. Organic matters are removed by sedimentation, filtration, oxidation, reduction, and adsorption during the flow. The surface flow wetland is similar to the natural wetland in terms of structure, and can serve as the habitat of insects, fish, and birds, presenting satisfactory ecological benefits and landscape effects. But there are also problems such as low pollutant loads, susceptibility to mosquito breeding in summer, and significant impact by temperature (Wu, 2014).

The horizontal subsurface flow constructed wetland is the most popular constructed wetland sewage treatment system worldwide with aquatic plants as surface vegetation, surface soil and lower gravel as matrix carriers, and water propelling in horizontal infiltration from the inside of the matrix bed. Unlike the surface flow constructed wetland, the role of its matrix carriers is fully exerted. The flowing sewage is purified via the function of biofilm on the surface of the matrix and the interception and adsorption of the matrix and the huge plant root system. Besides, plant roots have favorable oxygen releasing capacity to form an oxic, anoxic, and anaerobic microenvironment near the root system, which is conducive to the removal of organic matters and TN, according to Kiehuth’s “root zone theory” (Hu, 2011). The horizontal subsurface constructed wetland is characterized by high pollutant load, good thermal insulation, and no mosquitoes breeding as the sewage is not exposed to the air. But it has the disadvantages of high investment as well as complicated construction and management.

The vertical flow constructed wetland is a process developed from the horizontal subsurface flow constructed wetland, which has the characteristics of both the horizontal subsurface flow and the surface flow constructed wetlands. Sewage is purified through passing by wetland plants and matrix carriers in vertical flow from top to bottom. The system adopts intermittent operation with oxygen accessing to the wetland through atmospheric diffusion and plant rhizome transportation, which has a better oxygen recovery than the horizontal subsurface constructed wetland. It has the strengths of high pollutant load and small floor space, but is defective in high investment, complex construction and management, and susceptible to plugging and mosquito breeding (Wang, 2019).

Constructed wetland technology also beautifies the landscape with a large buffer capacity and simple process. Note that constructed wetlands are normally used for advanced treatment, in other words, biological treatment technology should be adopted to reduce the concentration of pollutants before sewage enters the constructed wetland. As they usually require large land occupation, they are suitable for rural areas with rich land resources. The constructed wetland is designed according to the calculation of pollutant load and hydraulic load, as shown in Table 2.6, and desludging should be performed regularly during operation and maintenance.

Table 2.6 Main design parameters of constructed wetlands (Administration and for Market Regulation, 2019)

• ii. Stabilization Pond

Stabilizing pond, also known as oxidation pond or biological pond, is a biological treatment technology set with a causeway and anti-seepage layer to treat sewage using the natural biological purification function (Zhang, 2015). The first stabilization pond system was built in Texas, USA, in 1901 (Sopper & Kardos, 1973). With more than 100 years of proven practice, it can be effectively used for the treatment of domestic sewage. The purification process of the stabilization pond is similar to the self-purification process of the river. By flowing slowly after entering the stabilization pond, the sewage is diluted, precipitated, and purified by the combined effects of microorganisms, micro-fauna (protozoa, metazoa), algae, and aquatic plants in the long retention process (Cao et al., 2004). It can be divided into oxic stabilization pond, facultative stabilization pond, anaerobic stabilization pond, and aeration stabilization pond by the type of microbial dominant groups and the concentration of dissolved oxygen (Zhang, 2015).

Oxic ponds are shallow with roughly 0.5 m in depth, as shown in Fig. 2.22, which can be permeated by sunlight. The oxygen required for the metabolism of aerobic microorganisms in the pond is provided by the photosynthesis of algae and the atmospheric reoxygenation of the water surface. It can be seen that the system is essentially an algal–bacterial symbiotic system. More specifically, the algae in the pond release oxygen through photosynthesis under sunlight; oxic microorganisms produce CO₂ after oxic metabolism is performed on organic matters; and algae can absorb CO₂ and nutritive salts such as NH₄⁺ and PO₄³⁻ in sewage (Han, 2011).

Fig. 2.22

Oxic stabilization pond, facultative stabilization pond, and anaerobic stabilization pond

Facultative pond is the most commonly seen stabilization pond (Figs. 2.22 and 2.23). With a depth of 1–2 m, the pond has an oxic zone (the upper layer of the pond surface, with significant algae photosynthesis and sufficient dissolved oxygen), an anaerobic zone (at the pone bottom, anaerobic fermentation is performed on sediment by anaerobic microorganisms), and an in-between anoxic zone (Mahapatra et al., 2022). Pollutants such as organic matter, NH₄⁺ and PO₄³⁻ are removed in the algal–bacterial symbiotic system in the oxic zone after the sewage flows into the pond. Denitrification is carried out in the anoxic zone. Refractory organic matters in the sewage and the dead algae are precipitated in the anaerobic zone at the pond bottom, which can be converted into organic acids by anaerobic fermentation and then partially diffused to the aerobic zone and anoxic zone for decomposing, while some organic acids are decomposed via anaerobism into methane (CH₄) and CO₂. In that case, the purification reaction in the facultative pond involves different aspects, and the biological phase in the system is also more abundant.

Fig. 2.23

Materials conversion in a facultative pond (Tchobanoglous & Schroeder, 1985)

The anaerobic pond is deep, over two meters in general, and is in an anaerobic state as a whole, according to Fig. 2.22. Its BOD₅ surface load is remarkably higher than that of aerobic and facultative ponds, reaching up to 20–40 g/(m² d). Anaerobic fermentation is carried out in the pond, leading to a slow purifying rate and long sewage retention time.

The aerobic zone is normally more than two meters in depth. Oxygen is provided by surface aerators, with high oxygen supply efficiency. But the growth and photosynthesis of algae are inhibited under the aeration condition. Compared with the anaerobic ponds with similar depths, the aerobic zone has lower organic loads, which is more conducive to the effective removal of NH₃–N due to sufficient oxygen.

The stabilization pond is suitable for rural sewage treatment with sufficient land resources with its advantages of simple engineering, low infrastructure investment, low operation and maintenance costs, simple control, effective removal of organic matters in sewage, removal of nutrients to a certain extent, and not requiring sludge treatment. But it should be noted that problems such as low organic load (Table 2.7), large land use, poor environmental conditions, and the treatment effect susceptible to temperature and light can be observed. Hence, it is normally leveraged for advanced treatment of domestic sewage. Besides, liner is required at the bottom and surrounding of the stabilization pond to prevent groundwater pollution.

Table 2.7 Main design parameters of stabilization pond (Administration and for Market Regulation, 2019)

2.3 Sludge Treatment and Disposal in Rural Areas

2.3.1 Importance of Sludge Treatment and Disposal

The scum, sediment, and excess sludge produced by the primary secondary secondary sedimentation tank, secondary secondary sedimentation tank, and other process units are called sludge. As a by-product of sewage treatment, sludge is easily overlooked (Hang et al., 2004). Sludge treatment is dependent on sewage treatment. To be specific, sewage treatment is a process in which suspended, colloidal, and dissolved pollutants are converted into sludge by the adsorption and metabolism of microorganisms, and then precipitated from the liquid phase. Sewage treatment is merely an enrichment or transformation of pollutants in sewage. Closed-loop sewage treatment is not completed until the treatment and disposal of sludge are done. Therefore, sludge treatment should be seen as a part of sewage treatment.

There are large amounts of organic matter, nutrients, pathogenic bacteria, parasite eggs, heavy metals, and some toxic and harmful refractory organic substances in sludge besides microorganisms, showing high moisture content (above 99%), large volume, putrefactive, and generation of stink. They might easily cause secondary damage to groundwater and soil, posing threats to environmental safety and public health if not properly treated and disposed.

Moreover, approximately 55% of COD, 30–45% of N, and 85–95% of P are left in the sludge during sewage treatment (China Urban Water Supply and Drainage Association, 2021). CH₄, H₂, and other fuels with higher heat value can be obtained by rich organic matters via the anaerobic treatment. Also, resource utilization of energy and materials in disposal and treatment can be achieved using the stabilized product upon treatment as agricultural fertilizer (Dai, 2020).

Hence, attention should be given to the treatment and disposal of sludge in rural sewage treatment based on the attributes of “pollution” and “resources” of sludge.

2.3.2 Main Principles and Technical Options for Sludge Treatment and Disposal

Sludge treatment and disposal are two different concepts. Sludge treatment, in general, refers to the stabilized, reduced, and harmless treatment of sludge, including concentration (conditioning), dehydration, anaerobic digestion, oxic digestion, drying, and composting (aerobic fermentation). While, sludge disposal refers to the final consumption of sludge, including land use (agricultural use), landfill, and use and incineration of building materials.

Treatment and disposal of sludge require skilled technicians to operate the facilities, generating high construction and maintenance costs. Thus, the sludge treatment and disposal of rural sewage treatment should be performed in coordination with the urban sludge treatment and disposal system, otherwise a new sludge treatment plant can be built for centralized treatment. Furthermore, the treatment technology and process with low sludge yield should be prioritized in the treatment of rural domestic sewage in view of the scattered distribution of rural domestic sewage as well as the high cost of sludge collection and transfer.

The treatment and disposal of sludge in rural sewage treatment plants should be set under the principles of harmlessness, reduction, and stabilization, as well as the principle of resource utilization to achieve sustainable development of sewage treatment. Particularly, the sludge produced by rural sewage treatment is quite different from that produced by urban sludge, in terms of low content of heavy metals and great potential for land use (incl. agricultural use, landscaping, and soil improvement).

As economic development, treatment scale, residents’ living habits, and natural climatic conditions are varied in the rural areas of B&R countries, sludge treatment and disposal technologies should be based on regional characteristics, comprehensive consideration of industrial structure, mud characteristics, processing scale, environmental conditions, economic factors such as the level of economic and social development and the final disposal process, so as to achieve the purpose of protecting the overall environment at a high level.

In general, gravity concentration and natural drying should be preferred for sludge concentration and dehydration, respectively, in the sludge treatment in rural areas. Farmland use or land use should be prioritized at the final disposal of sludge. Also, anaerobic digestion or oxic fermentation (composting) should be employed for stabilization and harmless treatment when the sludge is treated for landscaping and farmland utilization.

Since sludge and its leachate contain high concentrations of pollutants and pathogenic bacteria, spillage, leakage, and seepage should be avoided during storage and transportation. Otherwise, it will cause harm to the surrounding environment, surface water, groundwater, and soil. Meanwhile, the leachate should be also properly handled.

2.4 Technical Selections for Rural Sewage Treatment in B&R Countries

2.4.1 Main Principles for Selecting Rural Sewage Treatment Process

China has signed over 200 cooperation agreements under B&R with 147 countries and 32 international organizations since the initiative was proposed in 2014. With different climates, landforms, cultures, living habits, economic conditions, and the quality of rural domestic sewage in different B&R countries, the following principles should be followed in their rural sewage treatment:

• i. Select the simplest process with strong resistance to shock load and stable compliance. The daily sewage volume in rural areas is small, and the water volume is mainly distributed in the morning, at noon, and in the evening, with a daily variation coefficient of 5–10 (Wang, 2018). Processes with strong anti-shock load capacity should be prioritized in the treatment of rural sewage, according to the characteristics of large changes in water quantity and quality.

• ii. Select an energy-saving process with no power or less power consumption and low operating cost. The high operating costs of rural sewage treatment plants, small in scale and high in energy consumption per unit, are not affordable for rural areas with weak economic conditions (Zheng et al., 2020), making these facilities prone to failure in operation at a later stage due to the lack of funds.

• iii Select a process with simple and less operation and maintenance. There are various rural sewage stations widely distributed in rural areas, lacking sewage treatment professionals for operation and maintenance. Therefore, facilities that are simple in operation and maintenance without frequent process testing or adjustment are essential in rural sewage treatment (Wang, 2021a, 2021b).

• iv. Select a process that is conducive to resource utilization. Sewage resource utilization means the sewage meets specific water quality standards upon harmless treatment and can be used as reclaimed water for residents’ living, ecological water supply, and agricultural irrigation in replacement of conventional water resources, or other resources and energy can be extracted from sewage. It is of great significance to increase the supply of water resources, alleviate the shortage of irrigation water, and ensure the safety of water ecology.

• v. Adjustment should be made as per local conditions. Field investigation should be conducted before the selection of the rural sewage treatment process. Based on the functional requirements of the receiving water, the treatment process suitable for the local area can be determined with consideration of the economic conditions of rural areas, the complete status of infrastructure, natural environment, rural water-using habits, water consumption, the permanent population, climatic conditions as well as sewage situation and the final drainage destination of surrounding factories and aquaculture farms.

2.4.2 Thought of Technical Selection for Rural Sewage Treatment

Based on the above technology selection principle, and the actual needs and conditions of rural domestic sewage treatment, the following technical processes should be adopted (Administration and for Market Regulation, 2019):

• i. With the purpose of removing organic matters, sewage can be discharged or recovered as resources upon being treated by bio-contact oxidation units.

• ii. With the purpose of removing organic matters, sewage can be discharged or recovered as resources upon passing by the anaerobic organism membrane unit and biological treatment unit in the areas qualified for setting biological units.

• iii. With the purpose of removing TN, sewage can be discharged or recovered as resource after being treated by anoxic and oxic biological units.

• iv. With the purpose of removing TN and TP, sewage can be discharged or recovered as resources by phosphorus removal units after being treated by anoxic and oxic biological units.

2.4.3 Integrated Sewage Treatment

Based on the main principles and selection thought for the rural sewage treatment process discussed above, sewage treatment plants must be set and constructed to meet the requirements of low investment, high quality, and convenient operation and maintenance due to the shortage of funds for rural infrastructure construction and the low technical level and management level of employed person in specific practices. More than that, the setting of independent and closed process units should be considered since the outbreak of the COVID-19 pandemic in 2020, reducing the contact between sewage and management personnel. The integrated sewage treatment process becomes mainstream in the implementation of the current process based on the above characteristics.

Based on biochemical reactions, the integrated sewage treatment plant is a sewage treatment assembly formed in the factory through the organic combination of various functional units such as pretreatment, biochemical, sedimentation, disinfection, and sludge return with electrical components, instrument components, pipelines, automatic control systems, and equipment rooms (Wen, 2016). Biofilm processes such as biological contact oxidation, and biological filter as well as activated sludge processes such as A/O, and A²/O, or a combination of two or more processes can be adopted as the core biochemical unit. The integrated sewage treatment plant with standardized production conforms to the principles of complete sets, modularization, and automation. Its process, structural and appearance design, test processes, and type inspection are regulated by relevant standards to ensure the equipment quality.

The integrated process and structural design can save civil construction costs with a compact facility structure and small floor area. Also, standard production can ensure reliable operation, and simple operation and maintenance, so that operation and maintenance personnel can carry out the daily operation after simple training. Moreover, as the integrated facility is assembled in the factory and transported as a whole, with simple on-site civil construction, it can be automatically operated after installation and commissioning, boasting the advantages of short construction time and low construction cost. Operation and maintenance are performed more conveniently using intelligent control and online monitoring technology, effectively saving labor costs. These features make the integrated sewage treatment plant particularly suitable for rural domestic sewage treatment, and it has been extensively promoted in rural sewage treatment, and grown into a mainstream treatment process.

A Japanese Johkasou is a typical integrated sewage treatment plant. The combined process of anaerobic filter-contact oxidation is adopted as the mainstream process of the combined treatment Johkasou in Japan (Ministry of the Environment Protection, 2011). China introduced Johkasous from Japan by the end of the 20th century and has carried out a series of localized technology and equipment research and development based on the characteristics of rural domestic sewage (Wang, 2021a). The integrated treatment processes and facilities of varying scales have been developed for different single households, connected households, villages, and towns, which have been extensively applied and achieved satisfactory results.

Moreover, the integrated sewage treatment process and facilities have been continuously innovated and developed from numerous applications and practices. Meanwhile, progress has been made in the improvement of the main process, the optimized combination of the process flow, and the improvement of the carrier performance in recent years. The integrated sewage treatment plant will play a crucial role in the fields of rural sewage treatment and decentralized sewage treatment in the future.