Integrated Treatment Technology for Domestic Sewage in Rural Areas

Abstract

A²/O, as a classic sewage treatment process, has been extensively applied in rural sewage treatment. The A²/O process is superior in efficiently removing organic matters, total nitrogen (TN) and total phosphorus (TP), but defective in its unstable effect of denitrification and phosphorus removal, which is not applicable for the treatment of sewage with low C/N and C/P. The short residence time of denitrification and long residence time of nitrification is the cause of unstable TN in water treated by the A²/O process. Moreover, the nitrate nitrogen of reflux sludge affects the anaerobic environment, leading to a poor effect of biological anaerobic phosphorus release effect (Chen et al. in Environ Protect Eng 37(3):218–219, 234, 2019a; Technol Water Treat 45(5):126–128,134, 2009b). Also, under the influence of living habits, C/N and C/P are low in most rural sewage, which may result in unsatisfactory nitrogen and phosphorus removing efficiency by the A²/O process (Qin et al. in China Sci Technol Inf (5):13, 85–86, 2018). Therefore, the A³/O process is developed to further improve the efficiency of nitrogen and phosphorus removal by adding a pre-denitrification zone before the anaerobic zone. It returns the sludge to the pre-denitrification zone, thus removing nitrates while maintaining a perfect anaerobic environment in the back-end anaerobic zone and overcoming the defects related to the A²/O process.

3.1 A³/O-MBBR Process

3.1.1 R&D Background

A²/O, as a classic sewage treatment process, has been extensively applied in rural sewage treatment. The A²/O process is superior in efficiently removing organic matters, total nitrogen (TN) and total phosphorus (TP), but defective in its unstable effect of denitrification and phosphorus removal, which is not applicable for the treatment of sewage with low C/N and C/P. The short residence time of denitrification and long residence time of nitrification is the cause of unstable TN in water treated by the A²/O process. Moreover, the nitrate nitrogen of reflux sludge affects the anaerobic environment, leading to a poor effect of biological anaerobic phosphorus release effect (Chen et al., 2019a, 2019b). Also, under the influence of living habits, C/N and C/P are low in most rural sewage, which may result in unsatisfactory nitrogen and phosphorus removing efficiency by the A²/O process (Qin et al., 2018). Therefore, the A³/O process is developed to further improve the efficiency of nitrogen and phosphorus removal by adding a pre-denitrification zone before the anaerobic zone. It returns the sludge to the pre-denitrification zone, thus removing nitrates while maintaining a perfect anaerobic environment in the back-end anaerobic zone and overcoming the defects related to the A²/O process.

Moving Bed Biofilm Reactor (MBBR) is a derivative process of the biological contact oxidation process. It achieves efficient removal of pollutants through the carrier-attached biofilm by adding suspended carrier to the reactor (Yang et al., 2017). The carrier is suspended in water as its density approaches to that of water, providing a place for developing the attachment of microorganisms, as shown in Fig. 3.1. It is conducive to increasing the number and types of microorganisms in the system, offering a more favorable selection for functional microorganisms, and providing enrichment sites for long-generation microorganisms such as nitrifying bacteria. The carrier has a re-cutting effect on air bubbles, which can improve the utilization of oxygen in the system. The fluidized state of the carrier should be achieved via aeration or stirring in most cases to intensively mix the carrier with sewage.

Fig. 3.1

MBBR process flow (Zhou et al., 2021)

A³/O-MBBR integrated treatment process is the “pre-denitrification-anaerobic-anoxia- aerobic (MBBR)-precipitation-soft carrier filtration” combination process. It is characterized by the improvement in the anaerobic environment at the back end through adding the pre-denitrification zone in front of the anaerobic zone, and the enhanced biomass and species of the system as well as the shortened residence time of the aerobic zone through adding the suspended carriers to the aerobic zone. Further, SS can be efficiently removed through combing partial precipitation and soft fixed carrier filtration. And A³/O-MBBR can also be flexibly designed as a mud-film composite process or a full-scale biofilm process as per different water inlet conditions.

3.1.2 Process Flow

The flow of the A³/O-MBBR (mud-film composite) integrated treatment process is shown in Fig. 3.2. Domestic sewage collected by the pipeline flows through the pre-denitrification zone, the anaerobic zone, the anaerobic tank and the aerobic zone successively for biochemical treatment. Microorganisms undergo a denitrification reaction with nitrate nitrogen in the return sludge in the pre-denitrification zone using organic matters in raw water. Hydrolysis of organic matters and biological phosphorus release is achieved in the aerobic zone. Nitrogen removal by denitrification can be achieved by denitrifying bacteria in the anaerobic tank where the dissolved oxygen concentration is extremely low. Meanwhile, partial alkalinity provided by denitrification can create favorable conditions for the subsequent nitrification reaction in the aerobic zone. The preferred biomass-increasing sponge carrier in the aerobic zone has the advantages of high sludge concentration (with MLSS reaching up to 10,000–20,000 mg/L), high volume load, strong resistance to impact load, low sludge yield and long service life. Microorganisms in activated sludge and carrier can decompose organic matter under aerobic conditions. Also, organic nitrogen and ammonia nitrogen are gradually converted into nitrite and nitrate via the nitrification reaction, and nitrification liquid is refluxed from the aerobic zone to the anaerobic tank. At the same time, high-concentration phosphorus-containing sludge is produced with excessive phosphorus uptake in phosphorus accumulating bacteria. Finally, mud and water are separated in the sedimentation zone, and the supernatant is filtered and sterilized by soft fixed carriers to meet the standard for discharging or recycling. The excess sludge in the sludge hopper of the secondary secondary sedimentation tank is discharged into the sludge thickening tank for concentrating and drying before transportation for treatment.

Fig. 3.2

Process flow of A³/O-MBBR (mud-film composite)

The A³/O-MBBR (full-scale biofilm) integrated treatment process has main functional areas including the anoxic zones 1, 2, and 3, the aerobic zone, secondary sedimentation tanks and soft carrier filtration, as shown in Fig. 3.3. High-efficiency fixed carriers are set in the anoxic zone, with the preferred biomass-increasing sponge carrier added to the aerobic zone.

Fig. 3.3

Process flow of A³/O-MBBR (full-scale biofilm)

The full-scale biofilm process is adopted for treating domestic sewage with low concentration without adding suspended active sludge. When treating conventional or high-concentration sewage, activated sludge in a suspended state can be added to improve the efficiency. The full-scale biofilm process is superior in terms of strong resistance to impact load, few excess sludges as well as simple operation and maintenance, but should be complemented with other means to address its defects in low phosphorous removing efficiency.

3.1.3 Design Parameters

Design water volume: 30–500 m³/d.

Design parameters of the A³/O-MBBR (mud-film composite) process: The residence time of the biochemical section: 10.4 h; The gas–water ratio: 17:1; the reflux ratio of sludge: 100%; the reflux ratio of nitrifying liquid: 200%; MLSS: 2000–3500 mg/L; the dissolved oxygen in the aerobic zone: 2–4 mg/L.

Design parameters of the A³/O-MBBR (full-scale biofilm) process.

The residence time of the biochemical section: 12.6 h; the gas–water ratio: 50:1–70:1; the reflux ratio of sludge: 100%; the reflux ratio of nitrifying liquid: 200–400%; MLSS: less than 1000 mg/L; the dissolved oxygen of the multi-stage aerobic zone: greater than 5 mg/L.

Design parameters for the quality of domestic sewage and treated water are shown in Table 3.1.

Table 3.1 Design parameters for the quality of domestic sewage and treated water

3.1.4 Features

• a. Prominent effect of removing nitrogen and phosphorus with stable effluent.

• b. Minimized carbon source is added in the anoxic zone, with low operating cost.

• c. The residence time in the aerobic zone can be significantly reduced, saving land occupation and reducing energy consumption.

• d. Filtration with soft fixed carriers can lower energy consumption without the need for filter pumps and backwash pumps.

• e. The risk of sludge bulking can be effectively avoided.

• f. A complete food chain is formed in the system, effectively reducing sludge volume.

• g. The carrier has strong resistance to impact load and a long service life.

3.1.5 Scope of Application

A³/O-MBBR (mud-film composite) process can be extensively applied in rural sewage treatment, domestic sewage treatment in scenic spots without municipal pipe networks, decentralized domestic sewage treatment in schools, hospitals, and inns as well as the source control and sewage interception for black and odorous water. It is especially applicable for areas with strict nitrogen and phosphorus discharge standards, and sewage treatment with low C/N and C/P. The A³/O-MBBR (full-scale biofilm) process is mainly applicable for low-concentration domestic sewage treatment.

3.1.6 Tips for Operation and Maintenance

• a. TN and TP removal rates can be enhanced by 5–10% by distributing part (70–95%) of the domestic sewage into the anaerobic zone in the A³/O-MBBR (mud-film composite) process.

• b. When operating A³/O-MBBR (mud-film composite) process in winter, the MLSS in the system can be appropriately increased.

• c. Short flow in the anoxic zone should be avoided in the A³/O-MBBR (full-scale biofilm) process.

• d. Intermittent aeration should be established in the anoxic area/anaerobic zone to prevent sludge settling and excessive biofilm thickness, with regular inspection of its normal operation.

• e. The carrier in the aerobic zone should be checked for no accumulation, no damage, and no non-functional biological overload.

• f. The biomass-increasing sponge carrier selected requires no replacement within 5 years. After that, 5% of the carriers should be supplemented every two years.

• g. The carrier block at the end of the aerobic zone should be unobstructed, and cleaned once every 15–30 days if necessary.

• h. The clean water tank should be cleaned at unregulated intervals, normally 30–60 days.

3.2 Improved Bardenpho-MBBR Process

3.2.1 R&D Background

The Bardenpho process is a typical process for efficiently conducting simultaneous denitrification, as shown in Fig. 3.4. The process is formed by adding an anoxic zone and an aerobic zone to the A/O process, so it is also known as a four-zone enhanced denitrification process. Efficient denitrification is the main strength of the Bardenpho process. With abundant carbon sources, domestic sewage in the first anaerobic tank has a high denitrification efficiency. Denitrifying bacteria mainly denitrify with intracellular carbon sources through endogenous respiration to improve the denitrification effect in the second anaerobic tank. It is defective in poor phosphorus removing effect. Hence, an anaerobic zone is added in front of the first anaerobic tank to improve its phosphorus removal efficiency, forming an improved Bardenpho process, as shown in Fig. 3.5.

Fig. 3.4

Flow of Bardenpho process (Wang, 2019)

Fig. 3.5

Process flow of improved Bardenpho-MBBR integrated treatment

The improved Bardenpho-MBBR integrated treatment process is a combined process of “anaerobism-anoxia-MBBR-anaerobism-anoxia-sediment-filtration with soft carriers”. This process, combining advantages of the modified Bardenpho process and the MBBR process, is characterized by strong impact resistance, high TN loads, outstanding comprehensive treatment performance, and favorable treatment water quality (meeting the environmental quality standard of surface water).

3.2.2 Process Flow

The improved Bardenpho-MBBR integrated treatment process is shown in Fig. 3.5. Domestic sewage collected by the pipeline flows through the anaerobic zone, the anoxic zone 1, the aerobic zone 1, the anoxic zone 2, and the aerobic zone 2 for biochemical treatment. On this basis, mud and water are separated in the sedimentation zone, and the supernatant is filtered by soft fixed carriers and sterilized to meet the standard for discharging or recycling. The excess sludge in the sludge hopper of the secondary secondary sedimentation tank is discharged into the sludge thickening tank for concentrating and drying before transportation for treatment.

Main mechanisms of different biochemical treatment zones:

• a. The anaerobic zone is functioned as decomposing organic matter and releasing phosphorus with the help of phosphorus accumulating bacteria. The efficiency of phosphorus release with phosphorus accumulating bacteria is greatly improved in the strictly anaerobic environment. Also, the enhanced biodegradability of sewage is conducive to the subsequent aerobic treatment.

• b. High-efficiency denitrification of nitrate nitrogen in the backflow nitrification solution can be achieved in the anoxic zone 1 by microorganisms with the help of organic matters in water. Meanwhile, partial alkalinity offered by denitrification can provide a favorable condition for the subsequent nitrification in the aerobic zone.

• c. The preferred biomass-increasing sponge carrier in the aerobic zone 1 has the advantages of high MLSS (up to 10,000–20,000 mg/L), high volume load, strong resistance to impact load, low sludge yield, and long service life. The microorganisms in activated sludge and carrier decompose organic matter under aerobic conditions. Also, organic nitrogen and ammonia nitrogen are gradually converted into nitrite and nitrate via the nitrification reaction, and nitrification liquid is refluxed from the aerobic zone to the anaerobic tank. At the same time, phosphorus accumulating bacteria complete excessive phosphorus uptake, producing high-concentration phosphorus-containing sludge.

• d. The anoxic zone 2 uses the externally added carbon source or the carbon source provided by the diverted inlet to conduct a denitrification reaction on nitrate nitrogen contained in the treated water of the anoxic zone 1, thus improving the denitrification efficiency of the system.

• e. The aerobic zone 2 removes the residual organic matter, ammonia nitrogen, and fine air bubbles attached to the sludge flocs in the treated water of the anoxic zone 2.

3.2.3 Design Parameters

Design water volume: 50–500 m³/d.

Design parameters: The residence time of the biochemical section: 12.6 h.

The gas-water ratio: 25:1–30:1; the reflux ratio of nitrifying liquid: 150–200%; the reflux ratio of sludge: 50–100%.

MLSS: 2500–5000 mg/L.

Design parameters for the quality of domestic sewage and treated water are shown in Table 3.2.

Table 3.2 Design parameters for the quality of domestic sewage and treated water

3.2.4 Features

• a. There is a prominent effect of comprehensive water treatment with high TN loads, with stable and high-quality effluents.

• b. Minimized carbon source is added in the anoxic zone according to the step feed, reducing operating cost.

• c. The slashed residence time in the aerobic zone can save land occupation and reduce energy consumption.

• d. Filtration with soft fixed carriers can lower energy consumption without the need for filter pumps and backwash pumps.

• e. The risk of sludge bulking can be effectively avoided.

• f. A complete food chain is formed in the system, effectively reducing sludge volume.

• g. The carrier has strong resistance to impact load, and a long service life.

3.2.5 Scope of Application

The improved Bardenpho-MBBR integrated process can be applied in rural sewage treatment, domestic sewage treatment in scenic spots without municipal pipe networks, decentralized domestic sewage treatment in schools, hospitals, and inns as well as the source control and sewage interception for black and odorous water. It is especially applicable for sewage treatment with a high nitrogen load (TN ≤ 70 mg/L).

3.2.6 Tips for Operation and Maintenance

• a. Part of the raw water should be distributed into the anoxic zone 2, which can lower the amount of external carbon sources, saving operation and maintenance costs.

• b. The MLSS in the system should be appropriately enhanced during operation in winter.

• c. Intermittent aeration can be set in the anaerobic zone/anoxic zone to prevent sludge sedimentation; also, regular inspection should be performed to ensure normal operation.

• d. The carrier in the aerobic zone should be checked for no accumulation, no damage, and no non-functional biological overload.

• e. The carrier block at the end of the aerobic zone 1 should be unobstructed, which can be cleaned once every 15–30 days if necessary.

• f. The biomass-increasing sponge carrier selected requires no replacement within 5 years. After that, 5% of the carriers should be supplemented every two years.

• g. When the carbon source is added to the anoxic zone 2, the sludge growth of the system will be accelerated with the rising demand for daily sludge discharged.

• h. The clean water tank should be cleaned at unregulated intervals, normally 30–60 days.

3.3 Multi-level and Multi-stage A/O-MBBR Process

3.3.1 R&D Background

The denitrification efficiencies of traditional A/O and A²/O processes are limited by the reflux ratio of nitrification solution, and their theoretical removal rate of TN can only reach up to 70%, making them not suitable for rural domestic sewage treatment with a high TN concentration. The step-feed multi-level multi-stage A/O process (referred to as SMMAO process) is a derivative process of the single-level and single-stage A/O activated sludge process. Two segments of A/O processes are added to the typical A/O process (Liu et al., 2012), with the theoretical removal rate of TN improved up to 78% (on the premise of being free from reflux of nitrification solution). The As SMMAO process can effectively improve the TN removal rate of the system, therefore it is appropriate for the treatment of rural domestic sewage with high TN concentration. But it still has a large room for improvement in the nitrification rate, tank capacity, and SS concentration of treated water. SMMAO-MBBR integrated process is the combined process of “anoxic zone 1-aerobic zone 1 (MBBR)-anoxic zone 2-aerobic zone 2 (MBBR)-anoxic zone 3-aerobic zone 3 (MBBR)-precipitation-soft carrier filtration”. Its feature is that the preferred biomass-increasing sponge carrier is added to each aerobic zone on the basis of the SMMAO process, and based on the advantages of the MBBR process, it can strengthen the nitrification and denitrification efficiency and improve the comprehensive water treatment efficiency of the system.

3.3.2 Process Flow

The integrated process flow of SMMAO-MBBR is shown in Fig. 3.6. Domestic sewage collected by the pipeline flows through the anoxic zone 1, the aerobic zone 1, the anoxic zone 2, the aerobic zone 2, the anoxic zone 3, and the aerobic zone 3 in segments. On this basis, mud and water are separated in the sedimentation zone, and the supernatant is filtered by soft fixed carriers and sterilized to meet the standard for discharging or recycling. The excess sludge in the sludge hopper of the secondary secondary sedimentation tank is discharged into the sludge thickening tank for concentrating and drying before transportation for treatment.

Fig. 3.6

Step-feed multi-level and multi-stage A/O-MBBR process flow

Main mechanisms of different biochemical treatment zones:

• a. High-efficiency denitrification of nitrate nitrogen in the backflow nitrification solution can be achieved by the microorganisms in the anoxic zone 1 with the help of organic matters in water. At the same time, partial alkalinity offered by denitrification can provide a favorable condition for the subsequent nitrification in the aerobic zone.

• b. The nitrate nitrogen generated in the treated water of the aerobic zone 1 is denitrified in the anoxic zone 2 using the carbon source provided by the step feed domestic sewage.

• c. The nitrate nitrogen generated in the treated water of the aerobic zone 2 is denitrified in the anoxic zone 3 using the carbon source provided by the step feed domestic sewage.

• d. The microorganisms in the preferred biomass-increasing sponge carrier and the activated sludge can decompose organic matters under aerobic condition in the aerobic zones 1, 2, and 3. Organic nitrogen and ammonia nitrogen are converted into nitrite and nitrate via nitrification reaction.

3.3.3 Design Parameters

Design water volume: 50–500 m³/d.

Design parameters: The residence time of the biochemical section: 20.4 h; The gas-water ratio: 20:1–30:1; the reflux ratio of nitrifying liquid: 50–100%; the reflux ratio of sludge: 50–100%; MLSS: 2500–5000 mg/L.

Design parameters for the quality of domestic sewage and treated water are shown in Table 3.3.

Table 3.3 Design parameters of sewage inlet and treated water under SMMAO-MBBR process

3.3.4 Features

• a. A high TN removing efficiency, and stable and high-quality effluents.

• b. The system has a small tank volume, reducing roughly 25% of the occupied area compared with the multi-level single-stage A/O system.

• c. Minimized carbon source is added in the anoxic zone by step feeding with equal inlet distribution.

• d. The slashed residence time in the aerobic zone can save land occupation and reduce energy consumption.

• e. The TN removal rate can be improved by up to 90% with the reflux of nitrifying liquid in each A/O zone.

• f. Filtration with soft fixed carriers can lower energy consumption without the need for filter pumps and backwash pumps.

• g. The risk of sludge bulking can be effectively avoided.

• h. A complete food chain is formed in the system, effectively reducing sludge volume.

• i. The carrier has strong resistance to impact load and a long service life.

3.3.5 Scope of Application

The SMMAO-MBBR process can be used for the treatment of rural domestic sewage with high TN concentration (TN ≤ 100 mg/L) in domestic sewage as well as decentralized sewage treatment in areas such as expressway service areas, some public toilets, and scenic spots.

3.3.6 Tips for Operation and Maintenance

• a. The reflux ratio (0–100%) of nitrifying liquid should be flexibly adjusted according to the domestic sewage, saving operation energy consumption and maintenance costs.

• b. When C/N is insufficient, carbon sources should be added to control C/N within 4:1 and 6:1.

• c. The MLSS in the system can be appropriately enhanced during operation in winter.

• d. Intermittent aeration should be established in the anoxic zone to prevent sludge settling, with regular inspection of its normal operation.

• e. The carrier in the aerobic zone should be checked for no accumulation, no damage, and no non-functional biological overload.

• f. The carrier block at the end of the aerobic zone should be unobstructed, which can be cleaned once every 15–30 days if necessary.

• g. The biomass-increasing sponge carrier selected requires no replacement within 5 years. After that, 5% of the carriers should be supplemented every two years.

• h. The clean water tank should be cleaned at unregulated intervals, normally 30–60 days.

3.4 Multi-stage A/O Biological Contact Oxidation Process

3.4.1 R&D Background

The A/O activated sludge process has become one of the mainstream sewage treatment processes because of its simple process, simultaneous denitrification and removal of carbon, and high treatment efficiency. But it is defective in poor resistance to water quality and impact load, and high excess sludge production (Du et al., 2017), with the lack of professional operation and maintenance management in rural domestic sewage treatment. To deal with the non-acclimatization of the A/O process in rural domestic sewage treatment, a multi-stage A/O process (multi-stage A/O biological contact oxidation process) was developed based on the biofilm method.

The process is to carry out subdivision on the A and O segments of the traditional A/O process forming multi-stage anoxic zones (A1, A2, A3) and aerobic zones (O1, O2, O3), which also adds anoxic and aerobic carriers to the anoxic zone and the aerobic zone, respectively, based on the theory of biotic population in ecology (Xue, 2020). To be specific, the multi-stage anoxic zone adopts the form of a fixed bed, while the multi-stage aerobic zone adopts a fixed bed or the fluidized bed (MBBR) process. Based on the process, the dominant bacteria in each zone are more prominent and the system biomass increases through the biofilm on the carrier, enhancing the system’s resistance to water quality and water impact.

3.4.2 Process Flow

The process flow of multi-stage A/O biological contact oxidation integrated treatment is shown in Fig. 3.7. Domestic sewage collected flows into the pretreatment unit (solid–liquid separation tank), the multi-stage anoxic zone, the multi-stage aerobic zone, and the soft carrier filtration zone successively before meeting the standard for discharging.

Fig. 3.7

Process flow of multi-stage A/O biological contact oxidation integrated treatment

Main mechanisms of various biochemical treatment zones:

• a. The solid-liquid separation zone has the comprehensive effect of regulating water volume, solid-liquid separation, and reduced dissolved oxygen (deoxidizing).

• b. The multi-stage anoxic zone contributes to SS interception, hydrolysis of organic matters, and denitrification, with varied effects in each stage. The anoxic zone 1 functions as interception, precipitation, and deoxidation, and the anoxic zones 2 and 3 are responsible for the hydrolysis and denitrification of organic matters.

• c. The multi-stage aerobic zone functions as the degradation and nitrification of organic matter, among which the aerobic zone 1 degrades organic matters, and the aerobic zones 2 and 3 realize the nitrification of ammonia nitrogen and organic nitrogen.

3.4.3 Design Parameters

Design water volume: 0.6–50 m³/d.

Design parameters: The residence time of the biochemical section: 26–32 h.

The gas-water ratio: 60:1–70:1; the reflux ratio of the mixture: 200–400%; the filling ratio of the multi-stage anoxic zone: 50–70%; the filling ratio of the multi-stage aerobic zone: 30%; the dissolved oxygen of the multi-stage aerobic zone: 6–8 mg/L.

Design parameters for the quality of domestic sewage and treated water are shown in Table 3.4.

Table 3.4 Design parameters of sewage inlet and treated water under multi-stage A/O integrated treatment

3.4.4 Features

• a. The system uses biofilm to efficiently remove pollutants and has a strong resistance to impact load.

• b. A solid-liquid separation zone may not be set as per the process requirements.

• c. Anoxic zone 1 can effectively eliminate oxygen and provide anoxic conditions beneficial to denitrification for the anoxic zones 2 and 3.

• d. The carbon source addition is minimized with improved utilization of organic matters in the multi-stage anoxic zone, saving the operating cost.

• e. The low concentration of organic matter in the aerobic zones 2 and 3 are conducive to the growth of nitrifying bacteria on the biofilm, improving nitrification rate.

• f. Low sludge production (2‰), low sludge treatment and disposal cost, and long operation and maintenance period.

• g. The system is simple with low requirements for operation and maintenance technology, saving the operation energy consumption and maintenance costs.

• h. A precipitation zone should be set when the fixed bed process is adopted.

• i. When the fluidized bed process is adopted, the MLSS can be reduced to lower than 100 mg/L, and no separate secondary secondary sedimentation tank is required in the system, reducing land occupation and simplifying operation and maintenance.

3.4.5 Scope of Application

The multi-stage A/O biological contact oxidation integrated treatment process is extensively applied in rural domestic sewage treatment, especially for domestic sewage and tail water treatment in scenes such as villages, inns, farmhouses, villas, and scenic spots.

3.4.6 Tips for Operation and Maintenance

• a. This process should be supplemented with phosphorus removal measures to achieve effective phosphorus removal.

• b. The appropriate water temperature is 10–25 ℃; insulation can be performed through burying or adding a thermal insulation layer if necessary.

• c. The carrier in the multi-stage anoxic zone requires no replacement and supplement normally.

• d. There requires no replacement of carriers when the fixed bed process is adopted in the multi-stage aerobic zone. And updating and maintenance can be considered after five years of adopting the fluidized bed process.

• e. The anoxic zone should be cleaned and dug periodically. In general, the top layer floats should be cleaned first, and then the sludge at the bottom is pumped.

• f. The carrier block in the multi-stage aerobic zone should be unobstructed for selecting the fluidized bed process or should be cleaned if necessary.

• g. When the carrier in the multi-stage anoxic zone is blocked, it can be dredged using aeration.

• h. Carbon sources can be supplemented to make the TN compliance rate higher when the C/N ratio of domestic sewage is low.

• i. The dissolved oxygen in the reflux liquid can be appropriately lowered by adjusting the aeration parameters to enhance the denitrification efficiency.

3.5 SND-Type Biological Contact Oxidation

3.5.1 R&D Background

Simultaneous Nitrification and Denitrification (SND) refers to the biological denitrification process that is synchronized in time and space of nitrification and denitrification reactions under the conditions of no significant anoxic and aerobic partitions in space and no anoxic/aerobic alternation in time (Yang et al., 2003). The process has outstanding advantages such as a simple process, short residence time, and small floor space, but it is defective in its limited denitrification efficiency (Run et al., 2012).

The SND-type biological contact oxidation process is a process to achieve simultaneous nitrification and denitrification based on special carriers of certain volume and biofilms grown on the carriers. Its specific implementation is shown in Fig. 3.8. A certain number of preferred porous sponge carriers placed in the SND zone may generate a DO gradient inside and outside of the carrier due to the limitation of oxygen diffusion. The outer surface of the carrier is dominated by aerobic nitrifying bacteria with high dissolved oxygen, and deep inside the carrier, denitrifying bacteria are dominant since an oxygen-deficient zone is generated due to the obstruction of oxygen transfer. In that case, the microenvironment generated in the carrier contributes to achieving simultaneous nitrification and denitrification.

Fig. 3.8

SND-type biological contact oxidation model

3.5.2 Process Flow

The process flow of SND-type biological contact oxidation is shown in Fig. 3.9. The domestic sewage collected flows into the system and reaches the SND zone after the pretreatment of the solid–liquid separation zone 1 and zone 2 (with the effect of regulating water volume), which then passes through the SND1-SND3 zones in turn. Then, efficient biomass increasing sponge carrier for SND is placed in the tank to realize the simultaneous degradation of organic matters, ammonia nitrogen and TN under the synergistic action of various microorganisms on the carrier surface and inside the carrier. Mud-water separation can be achieved upon treated water in the SND3 zone flows into the sedimentation zone. Of which, the lower inorganic mud is air-lifted and refluxed to the solid–liquid separation zone according to the set reflux ratio, and the supernatant is discharged or recycled after being sterilized.

Fig. 3.9

Process flow of SND-type biological contact oxidation

3.5.3 Design Parameters

Design water volume: 0.6–50 m³/d.

Design parameters: The residence time of the biochemical section: 14.4–16 h; the gas-water ratio: 50:1–60:1; the reflux ratio of the mixture: 200–400%; The filling ratio of the biomass incremental sponge carrier specific for SND: 30%; the dissolved oxygen in the SND zone: 3–6 mg/L.

Design parameters for the quality of domestic sewage and treated water are shown in Table 3.5.

Table 3.5 Design parameters for the quality of domestic sewage and treated water

3.5.4 Features

• a. SND process can be achieved, avoiding the inhibition of NO₃⁻ accumulation on the nitrification reaction, accelerating the rate of the nitrification reaction.

• b. The alkalinity released by the denitrification reaction can partially compensate for the alkali consumption of the nitrification reaction, making the pH value of the system stable.

• c. The process is simple without the reflux of nitrifying liquid, reducing cost and difficulty in operation and maintenance.

• d. The floor space is small, the tank capacity can be reduced by 20–30%.

• e. The denitrification efficiency of SND is approximately 50%, lower than that of the traditional A/O process.

• f. The fluidized bed or fixed bed SND process can be flexibly selected.

• g. With a small amount of mud produced, it has a long maintenance period.

3.5.5 Scope of Application

The SND-type biological contact oxidation process is applicable for household/combined domestic sewage treatment and small centralized domestic sewage treatment in rural areas with low limits (≤30 mg/L) or no limits for TN discharge.

3.5.6 Tips for Operation and Maintenance

• a. This process should be supplemented with phosphorus removal measures to achieve effective phosphorus removal.

• b. The appropriate water temperature is 10–25 ℃; insulation can be performed through burying or adding a thermal insulation layer if necessary.

• c. The selected biomass increasing sponge carrier for SND requires no replacement within eight years. Maintenance is required after eight years.

• d. The inorganic sludge in zone 1 of solid-liquid separation should be cleaned on a regular basis (ranging from 3 to 6 months).

• e. The carrier block in the SND zone should be unobstructed for selecting the fluidized bed process, which should be cleaned once every 15–30 days if necessary.

3.6 A/O-MBR Process

3.6.1 R&D Background

The A/O activated sludge process is advantageous in the efficient removal of organic matters and TN. To ensure a sufficient nitrification reaction, a longer residence time should be set in the aerobic zone, with a risk of poor treated water quality caused by sludge bulking. The organic combination of the process and membrane bioreactor (MBR) is widely used in the field of sewage treatment (Huang et al., 2020), which can intercept microorganisms and other suspended solids (incl. bacteria, viruses, and insect eggs.) through setting a hollow fiber membrane or flat membrane (membrane pore size 0.01–1 μm) in the biochemical reaction tank and replace the secondary secondary sedimentation tank in the activated sludge process to achieve solid–liquid separation (Hong, 2022).

The A/O-MBR integrated treatment process is the combined process of “pretreatment zone—anoxic zone—aerobic zone (MBR)—treated water zone”. It is characterized by the short process flow, short residence time, no risk of sludge bulking, good quality of treated water, and direct recycling of treated water (Li, 2007, 2016).

3.6.2 Process Flow

The A/O-MBR integrated processing process flow is shown in Fig. 3.10. The sewage is intercepted by a fine grid machine for removing hair and coarse particles to achieve the pretreatment effect before flowing through the anoxic zone and the aerobic zone for biochemical treatment. After that, it enters the treated water zone for discharging or recycling. Denitrification and degradation of some organic matters are performed in the anoxic zone. High MLSS in the aerobic zone can effectively degrade organic matters and carry out nitrification reactions, and the mixed liquid is returned to the anoxic zone; the membrane module can filter and intercept refractory organic matters and sludge. The process can regulate the volume ratio of the anoxic zone to the aerobic zone, therefore effectively cutting the biochemical residence time, reducing the equipment size, and lowering investment costs.

Fig. 3.10

A/O-MBR process flow

3.6.3 Design Parameters

Design water volume: 30–500 m³/d.

Design parameters: The residence time of the biochemical section: 8 h.

The gas-water ratio: 50:1–60:1; the reflux ratio of the mixture: 200–400%; MLSS: 8000–12,000 mg/L.

Dissolved oxygen in the aerobic zone: 2–4 mg/L.

Of which, the size of the gas-water ratio is associated with the membrane material, type, production design and the process flow. The membrane air-sweeping water ratio can reach 3:1–3.5:1 with the advances of technology. And the biochemical tank can be separated from the membrane tank, with the gas-water ratio of the process reaching from 10:1 to 12:1, or even smaller.

Design parameters for the quality of domestic sewage and treated water are shown in Table 3.6.

Table 3.6 Design parameters for the quality of domestic sewage and treated water

3.6.4 Features

• a. There is a prominent effect of SS treatment with stable and high-quality water production, and recycling.

• b. The process is simple with the short flow, short residence time and small occupation of land.

• c. With high MLSS, the system has a strong resistance to impact load.

• d. With a small amount of mud produced, the treatment cost is low.

• e. No risk in sludge bulking.

• f. Membrane modules are expensive with high operation and maintenance costs.

• g. The process is complex with high technical requirements for operation and maintenance, and is normally combined with intelligent control.

• h. The process can be flexibly designed as per the working conditions, and the aerobic zone can be separated from the membrane zone.

3.6.5 Scope of Application

The A/O-MBR process is applicable for rural domestic sewage treatment in areas with developed economies, land constraints, and water resources shortages as well as treatment of decentralized domestic sewage from scenic spots, schools, hospitals, and inns in environmentally sensitive areas without municipal pipeline networks.

3.6.6 Tips for Operation and Maintenance

• a. The appropriate water temperature is 10–25 ℃; insulation can be performed through burying or adding a thermal insulation layer if necessary.

• b. When C/N is insufficient, carbon sources should be added to control C/N within 6:1 and 8:1.

• c. The MLSS in the system can be appropriately enhanced during operation in winter.

• d. Intermittent aeration should be established in the anoxic zone to prevent sludge settling, with regular inspection of its normal operation.

• e. It shall regularly check the pressure gauge, flow meter and turbidity meter of the effluent system, as well as any damage to the membrane module.

• f. Membrane cleaning is the highlight of the operation and maintenance of the process, including daily backwashing with clean water and regular online cleaning of chemicals. Daily backwashing with treated water (for example, backwash once for 1–2 min every 6 h or 24 h) is performed according to the quality of raw water and the situation of produced water. When the membrane pressure difference is high (the maximum limit of the hollow fibrous membrane and the flat membrane is 60 kPa and 20 kPa, respectively), the cleaning agent such as NaClO with the concentration of 1–3‰ should be used for online cleaning (the membrane biological process water treatment engineering technical specification for sewage treatment HJ 2010–2011). In general, the hollow fiber membranes should be cleaned no less than once a month, and the flat membranes can be cleaned once every 2–3 months.

3.7 Membrane-Aerated Biofilm Reactor

3.7.1 R&D Background

The biofilm process is a common sewage treatment process featuring strong adaptability to changes in water quality and quantity, and convenient management. It removes pollutants in sewage by the dense biofilm attached to the carrier surface. COD, NH₃–N and other pollutants in sewage can enter the biofilm through diffusion, and are then decomposed and removed by a variety of aerobic bacteria, anaerobic bacteria and other microorganisms in the biofilm (Henze et al., 2008). Normally, oxygen, COD, and NH₃–N are diffused and transferred from the outside to the inside of biofilm in the same direction (Semmens et al., 2003). Therefore, autotrophic nitrifying bacteria are normally disadvantageous in competition with heterotrophic aerobic bacteria on the biofilm surface, resulting in limited treatment efficiency of NH₃–N and TN. On the other hand, the biological treatment process, in general, supplies oxygen using aeration with large power consumption and large cost accounting for 60–80% of the total operating cost (Sun, 2015). At the same time, the short residence time of bubble generated by traditional aeration in water results in low oxygen utilization efficiency, less than 20% (Ahmed & Semmens, 1992). Membrane Aerated Biofilm Reactor (MABR) technology has been highly concerned for its superiority over the above two aspects. MABR is a bubble-free aeration technology that treats sewage using a breathable membrane and attached biofilm. In general, hollow fiber microporous membranes or dense silicone rubber membranes with hydrophobic properties are adopted as the breathable membrane in MABR (Sun, 2015). During aeration, air enters the water body in the form of dissolved diffusion or extremely tiny bubbles, thus a high oxygen utilization rate can be obtained with the aeration power efficiency reaching 10 kgO₂/kWh (Sun, 2015). In the meantime, oxygen and pollutants diffuse in the biofilm in opposite directions, which are different from the forms of diffusing oxygen and pollutants in traditional biofilms, as shown in Fig. 3.11. Nitrifying bacteria have an advantage in the aerobic layer since ammonia nitrogen with small molecules is easier to diffuse compared with organic matter. On this basis, the removal of NH₃–N and TN in sewage can be enhanced by the simultaneous nitrification and denitrification (SND) together with the denitrifying bacteria in the anoxic layer. MABR technology is less applied in municipal sewage treatment worldwide with few cases for rural sewage treatment (Chen et al., 2019a, 2019b).

Fig. 3.11

Diffusion patterns of oxygen and organic matter in traditional biofilms and MABR biofilms

3.7.2 Process Flow

The process flow is shown in Fig. 3.12. Sewage enters the anaerobic-MABR reaction zone for biochemical treatment via pretreatment units such as fine grids, and then undergoes the separation of mud and water in the sedimentation zone. The supernatant in the sedimentation zone can remove SS via the sand filtration system, and then it is sterilized by sodium hypochlorite before discharging. With the concrete structure as its main body, the system features an integrated layout and a compact layout as a whole.

Fig. 3.12

MABR process flow

3.7.3 Design Parameters

Design water volume: 20–100 m³/d.

Design parameters: The residence time in the MABR reaction zone: 0.38–18 h; the air flow rate: 1.4–5.3 L/m² h (Lu et al., 2021), and the mixture reflux ratio: 50–100%.

Design parameters for the quality of domestic sewage and treated water are shown in Table 3.7.

Table 3.7 Design parameters for the quality of domestic sewage and treated water

3.7.4 Features

Bubble-free aeration, heterogeneous mass transfer and layered structure are the main features of MABR (Wei, 2012). MABR supplies oxygen in the form of membrane aeration. Oxygen enters the water body through the polymer membrane in the bubble-free form and is then utilized by microorganisms attached to the surface of the aeration membrane. Theoretically, the oxygen utilization can reach 100%. Moreover, oxygen and pollutants diffuse into the biofilm in opposite directions, which is beneficial to the formation of the aerobic layer, anoxic layer, and anaerobic layer in the biofilm as well as the simultaneous nitrification and denitrification reactions, realizing a high denitrification efficiency. MABR process is characterized by:

• a. With the prominent oxygen utilization, roughly 50–80% of the aeration volume can be saved, lowering energy consumption.

• b. Outstanding BOD and TN treatment effects.

• c. The system can achieve simultaneous nitrification and denitrification.

• d. The alkalinity released by the denitrification reaction can partially compensate for the alkali consumption of the nitrification reaction, making the pH value of the system stable.

• e. The floor space is small, the tank capacity can be reduced by 20–30%.

• f. The process is simple, which can turn off the reflux of nitrifying liquid according to the conditions of domestic sewage and treated water, reducing costs and difficulty in operation and maintenance.

• g. With a small amount of mud produced, the treatment cost is low.

• h. The membrane module is expensive, with high construction costs as well as operation and maintenance costs.

• i. Low Noise and no Odor.

3.7.5 Scope of Application

Membrane modules of MABR are expensive. In general, the price of a set of membrane boxes is as high as several hundred thousand yuan. Hence, the high cost will be the main factor limiting its large-scale application. For this reason, MABR is suitable for rural domestic sewage treatment in areas with a developed economy, high land occupation requirements, and high TN concentration of domestic sewage.

3.7.6 Tips for Operation and Maintenance

The removal efficiency of ammonia nitrogen shows a downward trend with the rising biofilm thickness, according to relevant research findings (JIEI Innovation Laboratory). Hence, reasonably controlling biofilm thickness is a focus of operation and maintenance as per the concentration of domestic sewage and the standard of treated water.

• a. The membrane module should be flushed periodically through aeration to reasonably control the biofilm thickness.

• b. The normal function of the pretreatment unit should be checked periodically, to prevent hair from entering the system and damaging the membrane components.

• c. Check any damage to the breathable membrane regularly.

• d. The appropriate water temperature is 10–25 ℃; insulation can be performed by burying or adding a thermal insulation layer if necessary.

• e. This process should be supplemented with phosphorus removal measures to achieve effective phosphorus removal.

• f. The inorganic sludge in the sedimentation zone should be cleaned regularly (ranging from 3 to 6 months).

3.8 Improved Anaerobic Biofilm Process

3.8.1 R&D Background

Water resources on earth are approximately 1.4 billion km³, of which roughly 97.5% are seawater, with only 0.01% of the freshwater existing in the form of rivers and lakes (easy utilization), showing the scarcity of freshwater resources on earth (Ministry of Land, Infrastructure, Transport and Tourism, 2021). Studies suggest that two-thirds of the world’s population may face water scarcity by 2025, and roughly half of the world’s population may suffer from high water stress by 2030 with global population growth, expansion of industrial and agricultural activities, and global warming (Scheierling et al., 2011).

Reclaimed water is an unconventional water resource (resource utilization of sewage), its utilization is one of the effective solutions to alleviate water shortage (Almuktar et al., 2018). Given that about 70% of the world’s water is used for irrigation (Pedrero et al., 2010), farmland irrigation becomes an important direction for sewage resource utilization. In China, for instance, the total agricultural water consumption was 368.23 billion m³ in 2019, accounting for roughly 61.2% of the national water consumption. 100 billion m³ of sewage produced nationwide can greatly ease the pressure on farmland irrigation if they are recycled (Ministry of Water Resources, 2019).

Rural domestic sewage is relatively simple in composition because of no mixture of industrial sewage, which can meet the requirements of farmland irrigation upon simple biological treatment. For the decentralized collection and decentralized treatment of rural domestic sewage, as shown in Table 2.1, septic tanks are normally adopted to simply treat black water for resource utilization. But the amount of water flowing into septic tanks has risen sharply with the popularization of flushing toilets, resulting in the residence time of feces being lower than the design value and poor quality of effluent. To enhance the treatment capacity of septic tanks, up-flow anaerobic sludge bed reactor (UASB) septic tanks and carrier-based septic tanks have been developed (Fan et al., 2017). The UASB septic tank with an upward flow inlet can enhance the removal rate of SS and dissolved organic matters, but it faces the problem of poor resistance to impact load. The carrier septic tank, in general, is filled with carriers such as ceramsite, elastic three-dimensional carrier, and gravels, the impact load resistance and removal of organic matters are improved in accordance with the anaerobic biofilm principle, but there is a problem of easy blockage.

The improved anaerobic biofilm integrated treatment process combines the strengths of the UASB septic tank and carrier septic tank, can enhance the removal efficiency of pollutants by setting the upward flow of the water inlet through the diversion tube. Meanwhile, the resistance to impact load of the system is strengthened using the anaerobic biofilm process after filling high-performance carriers. Further, the risk of carrier blocking can be lowered by setting the equipment at the back of the traditional septic tank.

3.8.2 Process Flow

The process flow of the improved anaerobic biofilm integrated treatment for sewage is shown in Fig. 3.13. Domestic sewage collected enters the septic tank to remove large solid particles and garbage, and then flows through the anaerobic zones 1 and 2 in the form of gravity flow for biochemical treatment. Finally, the treated water is utilized as a resource locally. The upward flow of domestic sewage is adopted in the anaerobic zone 1. To be specific, the sewage enters the lower part of the anaerobic zone through the guide tube and then flows through the preferred carrier layer during upward flow. Also, SS in sewage is intercepted efficiently with the help of the physical interception of carrier and the adsorption of biofilm. Meanwhile, organic matter is decomposed by the anaerobic biofilm. In addition, no carrier is filled at the bottom and upper part of the zone for it is reserved for storing the deposited SS, the fallen biofilm, and the scum accumulated in the upper part. The anaerobic zone 2 adopts downward inflow water, that is, domestic sewage flows through the carrier layer from top to bottom, through which, organic matters are decomposed via anaerobic biofilm. Among them, carriers in the anaerobic zones 1 and 2 are of different sizes and shapes, which simultaneously possess large specific surface area, high bioburden, large porosity, and high-durability plastic material carrier, contributing that the anaerobic zones 1 and 2 perform their respective functions.

Fig. 3.13

Process flow of improved anaerobic biofilm

3.8.3 Design Parameters

Design water volume: 0.2–5 m³/d.

Design parameters: The residence time of the biochemical section: 2–5 days, the filling rate of anaerobic zone 1: 20–50%.

Filling rate of anaerobic zone 2: 40–70%.

Design parameters for the quality of domestic sewage and treated water are shown in Table 3.8.

Table 3.8 Design parameters for domestic sewage and treated water of improved anaerobic biofilm process

3.8.4 Features

• a. The treated water can be used as a resource locally.

• b. The system has a strong resistance to impact load with the efficient removal of pollutants using the biofilm.

• c. No energy consumption with low operating cost.

• d. Simple operation and management as well as a long maintenance period.

• e. Low sludge production, low sludge treatment and disposal cost as well as low operation and maintenance costs.

• f. The equipment is free of secondary pollution such as noise.

• g. Secondary secondary sedimentation tank is not required, occupying small floor space.

• h. The low efficiency in the removal of nitrogen and phosphorus can retain the nitrogen and phosphorus nutrients in the sewage to the maximum extent.

3.8.5 Scope of Application

The water treated by the improved anaerobic biofilm process can meet water using standards for the paddy field and dryland crop in the “Water Quality Standard for Farmland Irrigation” (GB5084-2021). And it is applicable for the resource utilization of domestic sewage with decentralized collection and treatment in rural areas. The treated water can be used for vegetable gardens, small orchards, and small gardens on private plots or residential lands.

3.8.6 Tips for Operation and Maintenance

• a. With a long acclimation period of anaerobic biofilm, it is normal to have high BOD in treated water within 150–200 days after the machine startup.

• b. The sludge deposited at the bottom of the equipment and the scum accumulated on the top should be regularly discharged.

• c. The inlet and outlet pipes of the anaerobic zones 1 and 2 are in the form of a tee to prevent a high concentration of methane and hydrogen sulfide in the equipment.

• d. When the equipment is blocked and there is a large amount of floating mud on the upper, the blocked part should be checked with the blockage removed in time.

• e. Smoking and open flames should be kept away from the equipment.