1 Introduction

With the increase in population and industry, the amount of biodegradable waste continues to increase rapidly. The generated biomass can be converted into useful bioproducts using various bioconversion methods. Many studies have been conducted on obtaining bioproducts through different bioconversion methods [1,2,3]. Some industrial wastes rich in organic matter and nutrients can be utilized to transform into beneficial bioproducts such as biopesticides, biohydrogen, biofuels, enzymes, and bioplastics through bioconversion methods [4].

Managing biodegradable waste produced in significant amounts, such as treatment sludge, animal manure, and agricultural waste, through bioconversion methods and controlling these wastes is an important issue. Treatment sludge and animal manure cause many environmental problems if they are not disposed of properly. The large amount of domestic treatment sludge from treatment facilities is still being disposed of in landfills in Turkey, causing operational problems. In recent years, the disposal of treatment sludge by incineration has also risen. However, these methods cause different ambients to be adversely affected. If treatment sludge is not properly managed, it can create problems such as odor, toxic gases, vector attraction, and esthetic issues. Also, the leachate formed due to high moisture content contaminates soil and groundwater with high organic matter, pathogens, and heavy metals [5, 6]. Due to their high moisture content, transporting them to disposal sites or burning them in incinerators without pre-treatment also poses a problem. Animal manure is also a biodegradable waste rich in organic substance content. The application of animal manure to the soil positively affects the microorganism population and provides significant macro- and micronutrients to the soil [7]. In our country, animal manures are dried only in rural areas and used in the soil for the purpose of growing vegetables and fruits. In most settlements, they are left to nature in an uncontrolled manner, posing environmental risks. Inappropriate disposal of animal manure causes various problems such as odor, leaching of pollutants like nitrogen and phosphorus into groundwater, and eutrophication in lakes [8, 9]. Additionally, 30–60% of antibiotics used in animal farming are released through animal manure, posing risks to the environment and human health [10]. Considering the waste management hierarchy, waste reduction and recovery are among the priority waste management alternatives. Due to the richness of organic matter and nutrient content, treatment sludge and animal manures can be recovered by using them in agriculture [11]. However, these wastes have harmful properties such as pathogens and synthetic organic pollutants. For this reason, the composting process, which is a clean and sustainable strategy, is widely used to remove the harmful properties of this type of waste [12, 13]. The composting process is a complex biological process in which wastes are decomposed by various microorganisms into stable end products under controlled conditions in aerobic surroundings [14,15,16,17]. Through the composting process, the odor problem caused by treatment sludge and animal manure is eliminated; their moisture content and quantities are reduced; unstable forms of carbon, nitrogen, and phosphorus are converted into stable forms beneficial to the soil; and antibiotics contained in animal manure are reduced. Additionally, hygiene is ensured by eliminating the pathogens they contain [18,19,20,21].

Due to the low porosity properties caused by high moisture content and very small particle size, treatment sludge and animal manure cannot be composted alone and must be mixed with bulking agents [22]. Bulking agents added to the compost mix provide benefits such as moisture, optimization of the C/N ratio, and porosity balancing [23]. Due to the substantial quantities of treatment sludge and animal manures, the appropriate selection and use of bulking agents are crucial for the quality and cost-effectiveness of the resulting compost. Many researchers have worked on the composting of high moisture organic wastes with different bulking agents. In these studies, brush clippings, sawdust, green waste, broken wooden pallets, mushroom waste, acacia shavings, food processing wastes, office paper, newsprint, and tree branches were used as bulking agents [24,25,26,27].

There are many studies in which treatment sludge [22, 28, 29] and animal manures [30,31,32] are composted separately. Although the number of studies in which these wastes are composted together is more limited, they are available in the literature [33,34,35,36,37]. Jalili et al. [34] aimed to achieve the appropriate C/N ratio by creating two different mixture recipes and carried out a 60-day laboratory-scale study. Pistachio shell waste, cattle manure, sewage sludge, and sawdust were used as raw materials. In a study by Arias et al. [33], they created four mixtures by using green forest waste, pig manure, food waste, and sludge from chipboard production. At the end of the study, they concluded that the final product could be used for soil fertilization due to its rich nutrient content and low heavy metal and electrical conductivity (EC) content. The forms and amounts of nutrients contained in different animal manures vary. In a study by Li et al. [38], phosphorus characterization in dairy, swine, and broiler manures was compared, and total phosphorus was determined to be 9, 34, and 29% in these manures, respectively.

In this study, four different animal manures (cattle (CAM), pig (PM), horse (HM), and chicken (CHM)) were composted together with food industry treatment sludge (TS) and sunflower stalk (SS) as bulking agent. The objective of this study was to determine the effect of these four different animal manures on the composting of treatment sludge. In the literature, there is no study comparing the effects of the animal manures used in this study on the composting of treatment sludge. In the study, the physical and chemical changes occurring during the composting of different animal manures with treatment sludge were monitored, and the effects of different manures on the composting of treatment sludge were determined.

2 Materials and methods

Treatment sludge originating from the food industry was composted alone and together with four different types of animal manure (cattle, pig, horse, and chicken) used as additional substrate materials in five separate aerobic compost reactors (M1, M2, M3, M4, and M5). Sunflower stalks were used as a bulking agent in the composting process. The overall process flow diagram is provided in Fig. 1.

Fig. 1
figure 1

The overall process flow diagram

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2.1 Raw materials

In this study, treatment sludge was taken from the filter press outlet of a canned food factory in Bursa, and animal manures (cattle, pig, horse, chicken) were taken from the Farm of the Veterinary Faculty of Bursa Uludag University. Sunflower stalks were obtained from the Farm of the Agriculture Faculty of Bursa Uludag University. The characterization of the raw materials (TS, SS, CAM, PM, HM, and CHM) is presented in Table 1. As illustrated in Table 1, the OM content of animal manure is higher than that of treatment sludge, the moisture content of treatment sludge is higher than that of animal manure, and the electrical conductivity of chicken manure is higher than that of the other compost materials used.

Table 1 General characteristics of the raw materials
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2.2 Laboratory-scale composters and method

In the composting experiments, aerobic composting reactors made of stainless-steel material and with a volume of 30 l were used. All reactors were insulated with 50 mm of glass wool and aluminum sheet. A perforated grid was placed 5 cm above the bottom of the reactor to ensure the homogeneous distribution of air within the compost mixture and to allow the leachate formed to drain from the compost mass. Air was supplied to the reactors by means of an aquarium pump, and the airflow rates were measured with a rotameter and recorded online. Aeration was carried out to give an airflow rate of 600–700 ml/min for 15 min every hour. The temperature data in the reactors were measured by means of temperature sensors (thermocouples, TCR-M06-L180-K04.J). The reactors were covered with insulation material in order to maintain the temperature of the waste mass and to prevent heat exchange with the environment.

The composting process was performed as an active composting phase for 21 days in five different reactors. In the compost experiments, the treatment sludge and animal manures were mixed moistly without any pre-treatment. The sunflower stalks used in the experiments were ground to sizes of 0.5–1 cm. The M1 reactor was prepared by mixing only treatment sludge and sunflower stalk without adding animal manure. The other reactors were prepared by adding treatment sludge and sunflower stalk, and respectively, cattle manure to the M2 reactor, pig manure to the M3 reactor, horse manure to the M4 reactor, and chicken manure to the M5 reactor. The mixture ratios of all reactors are given in Table 2. The reactors were mixed manually every other day and on the sampling days for aeration purposes. The temperature and airflow values were recorded with SCADA software for 21 days.

Table 2 Mixture ratios of compost reactors (wet weight basis)
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2.3 Laboratory analysis

Samples were taken from the reactors on the 0th, 2nd, 7th, 14th, and 21st days, and experimental studies were carried out. All analytical tests were conducted in triplicate. The dry matter (DM) content of the samples was calculated by measuring the weight loss of samples dried at 105 °C, taking into account the volatile matter content determined based on combustion loss at 550 °C [39]. Moisture content was calculated by subtracting the DM amount from 100. The amount of volatile matter was accepted as organic matter (OM) [40, 41]. Easily oxidizable organic carbon concentrations of the raw materials and the compost mixtures used in the composting were calculated based on the OM content of the samples using a factor of 1.83 (%C = (100 − %ash)/1.83) [42, 43]. The pH and electrical conductivity (EC) values were determined in 1:10 (w/v) distilled water extract [44, 45]. To determine the total Kjeldahl nitrogen (TKN) contents, samples were burned using the Kjeldahl method, and the TKN concentration was determined by water vapor distillation [46]. Free air space (FAS) was determined using the method suggested by Madejón et al. [47]. Total phosphorus concentrations were determined by the ascorbic acid method [39].

3 Results and discussion

3.1 The initial properties of the compost mixtures

The initial properties of the compost mixtures are shown in Table 3. Compost mixtures subjected to the composting process must have a certain amount of moisture, C/N, and FAS in order for the process to run efficiently. For efficient composting processes, it is recommended that the C/N ratio be within the range of 20–40, and the FAS content be between 20–30% [48, 49]. In this study, initial C/N ratios ranged from 18.3 to 37.8 in the reactors. C/N ratios exceeding the optimal range may result in insufficient nitrogen for microbial growth and development, thereby requiring a longer composting period. Conversely, if the C/N ratio remains below the optimal range, nitrogen loss as ammonia gas can occur through evaporation [50]. Wu et al. [51] stated that an initial C/N ratio of 25 in their composting trials with pig manure, rice straw, and corn stalks was suitable for the formation of high-quality compost.

Table 3 Initial characteristics of the reactors
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The moisture content of raw materials used for composting provides a suitable transport environment for the nutrients required by microorganisms. In compost mixtures with low moisture content, microorganisms undergo dehydration as they cannot obtain the necessary nutrients for their metabolism. Conversely, in mixtures with high moisture content, water accumulation within compost mass leads to anaerobic conditions during the process [52, 53]. Li et al. [54] determined the optimum moisture content under different moisture levels (45–61%) in their 45-day composting trials using chicken manure. Their research found that a moisture content of 53% achieved the highest composting temperature and longest thermophilic period. In this study, the treatment sludge and 4 different animal manures (CAM, PM, HM, and CHM) to be composted have high moisture content (85.2%, 81.6%, 78.7%, 71.2%, and 77.0%, respectively). Bulking agent was used to increase the moisture value to the required values for composting [55, 56]. For this purpose, sunflower stalk, which has a low moisture (10.3%) value, was added to the compost mixtures. The initial moisture values in the compost reactors ranged between 61.1 and 70.2% (Table 3). The initial pH values in the reactors ranged between 6.7 to 7.6. Ji et al. [57] also suggested that the initial pH values should range from 6.5 to 8.5 in their composting study with animal manures. The initial OM content of the compost reactors increased in consequence of mixing SS, which has a high OM content (91.6%), with animal manures and treatment sludge (56.1%), which have a lower OM content. The organic matter content in all prepared compost mixtures was over 75%.

3.2 Temperature

The temperatures in the compost reactors and ambient temperatures are given in Fig. 2. In general, temperatures increased to the thermophilic phase (> 40 °C) in all reactors. This temperature increase in the compost masses is due to microorganisms breaking down and consuming the easily degradable components and, meanwhile, generating heat during the first days [58]. Maximum temperature was measured as 48 °C in reactor M1, which was prepared only with treatment sludge without adding animal manure, 62.7 °C in reactor M2 with cattle manure added to treatment sludge, 51.5 °C in reactor M3 with pig manure added, 60.2 °C in reactor M4 with horse manure added, and 62.2 °C in reactor M5 with chicken manure added. An increase of temperature indicates the degradation rate of organic matter and can be related to microorganisms present in the environment [59]. Temperature is the main factor used to evaluate the change of pathogenic bacteria in the composting process. High temperature can effectively reduce pathogenic bacteria during the composting process [60]. According to the U.S. Environmental Protection Agency (EPA), for significant pathogen control, the wastes must be kept in the reactor at 40 °C for at least 5 days, during which the temperature must exceed 55 °C for 4 h. Compliance with the EPA’s pathogen reduction criteria is essential for producing safe and effective compost. The impact of effective composting on public health is significant. It provides numerous benefits such as preventing diseases, protecting the environment, improving soil health, and ensuring safe agricultural practices [61]. According to Yin et al. [62], temperatures need to exceed 55 °C to eliminate pathogenic microorganisms. In reactor M1, the temperature remained at 40 °C and above for 3 days, but did not reach 55 °C; in reactor M2, it remained above 40 °C for 8 days and above 55 °C for 2 days; while in reactor M3, it remained above 40 °C for 8 days, but did not reach 55 °C. The temperature in the M4 reactor stayed above 40 °C for 7 days and remained above 55 °C for 1.5 days during this period. The temperature in the M5 reactor, on the other hand, rose above 40 °C for 4 days and remained above 55 °C for 2 days during this period. Considering the EPA criteria in terms of the temperature values, an adequate level of pathogen control could not be achieved in M1 with only sludge, and in reactor M3 with pig manure added. In reactors M2, M4, and M5, however, it is thought that pathogens were significantly reduced and that microbial stabilization was achieved to a large extent. Abouelwafa et al. [63] stated that in a well-functioning composting process, temperatures can reach 50–65 °C with microbial activity. In the study conducted by Zhan et al. [64], the aim was to examine the changes in pathogenic bacteria during the co-composting of sewage sludge, food waste, and corn stalks. At the beginning of the study, they identified 50 pathogenic bacteria in the raw mixture, while after composting, they found 27 pathogenic bacteria. A significant reduction in most bacteria was observed after transitioning to the thermophilic phase. In this study, a significant decrease in the concentration of pathogens was detected at temperatures above 50 °C. Tan et al. [65] stated that during the composting of sewage sludge, the compost piles remained in the thermophilic phase (> 55 °C) for 21 days, with the temperature rising to 78 °C during this period. They indicated that during the thermophilic phase, where high temperatures were reached during composting, temperature-sensitive potential pathogenic bacteria were likely to be affected.

Fig. 2
figure 2

Reactor and ambient temperatures during composting

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It can be stated that in the conducted composting trials, a more efficient process was achieved in the reactors in which cattle, horse, and chicken manure were added as substrate (M2, M4, and M5, respectively), compared to the composting of treatment sludge alone, and that a significant amount of pathogen removal and hygienization was able to be achieved.

3.3 pH and electrical conductivity

The pH values for the reactors are shown in Fig. 3a. The pH values in the reactors ranged between 6.5 and 8.5 throughout the process. The pH of compost mass can affect microorganisms and therefore the temperature of the compost and evaporation of ammonia [66]. Moreover, pH is affected by many reactions (decomposition, mineralization, etc.) during composting [67]. pH values increased during the first days due to intense microorganism activities in all reactors except M1 and M3. According to Jamroz et al. [68] and Hanc et al. [28], these increases in pH values can be explained by the degradation of organic substances, and the release of ammonium ions or volatile ammonia. From another perspective, Negi et al. [69] stated that throughout the composting, an increase in pH values was observed in parallel with temperature increase, and that the rate of degradation increased as pH increased. In the M1 and M3 reactors, however, a significant change did not occur during the first days. It can be said that, initially, no significant microbial activity or mineralization took place in these two reactors. In the following days, the pH value increased in both these reactors. During the process, active microorganisms need different pH values in order to secrete their enzymes. This environment was provided for microorganisms at pH ranges varying from 6.7 to 8.5 throughout the process. In all reactors, pH values at the end of the process were in the range of 7.8–8.3. During successful and fully developed composting, the pH value usually rises to around 8–9 [70].

Fig. 3
figure 3

The variation of pH (a) and EC (b) throughout the composting

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The EC value is a parameter that reflects the content of soluble salts found in the mixture. In other words, it can also be defined as the salinity indicator, which shows the salt toxicity levels during composting [71]. The EC values for the reactors are shown in Fig. 3b. Values in all reactors ranged between 3.85 and 6.85. The increase in EC values during the process may be due to the decomposition of organic matter which is not reduced by binding to stable organic complexes [72]. In this study, it was determined that the highest increase in EC was in the M2, M4, and M5 reactors, in which the temperature rose above 60 °C and the decomposition of organic matter was the highest. If the compost formed at the end of the process is to be used for plants, the EC values must be below 4 mS/cm for the plants to tolerate it [73, 74]. It can be said that as a result of the formation of more stable forms in the mature compost obtained at the end of the maturation phase following the active composting phase, the EC values may decrease to more favorable values for plants.

3.4 Organic matter (OM) and C/N ratio

The changes in OM for the reactors are shown in Fig. 4a. OM concentrations decreased in all reactors during the process. Organic matter can be decomposed through the biochemical reactions of various microorganisms, including bacteria, fungi, plants, and algae. These microorganisms have the ability to transform pollutants into stable and harmless substances [75]. According to Li et al. [76], these decreases occur due to the fact that OM is a source of carbon and energy for microorganisms. Similarly, Zhong et al. [77] mentioned that many types of microorganisms are involved in composting and that some specific bacteria are effective in the degradation and transformation of organic materials during the process. When the OM losses occurring in the reactors were determined, they were found to be 46.2%, 64.8%, 51.2%, 58.3%, and 60.3% for reactors M1, M2, M3, M4, and M5, respectively. It was determined that the highest loss of OM was in the M2 reactor in which cattle manure and treatment sludge were used, and in which the temperature was measured as 62.7 °C. In the composting study conducted with dairy manure and sugar cane leaves by Jiang et al. [78], high temperatures (68 °C) were attained, and similar to this study, a highest OM loss rate of 61.8% was obtained. In the M1 reactor, in which there was the lowest OM degradation, the temperature (48 °C) was unable to reach the desired levels, and the microorganisms were unable to achieve adequate mineralization.

Fig. 4
figure 4

The variation of OM (a) and C/N ratio (b) throughout the composting

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The C/N ratio of the organic material to be used for composting is important, since it has an effect on the microbial compost community, the product quality in terms of degree of stabilization, and the ultimately available nutrients [79]. While carbon is used as a primary source of energy and to form microbial cells, nitrogen is required for microbial growth and reproduction via protein synthesis [53, 80]. The changes in the C/N ratios are given in Fig. 4b. In general, the C/N ratios vary according to the raw materials used. For composting, C/N ratios of 25:1 and 30:1 are optimal, but values of 20:1 and 40:1 are also acceptable. Since higher C/N values will cause problems in microbial growth, the composting process will be prolonged [49]. At low values, however, the C/N ratio indicates that there will be a need for a higher N loss through evaporation, and longer maturation periods associated with the short duration of the thermophilic phase [51, 81]. Since the C/N ratio is widely used as a parameter of compost maturity, it is usually evaluated at the beginning and end of the composting process [82]. As can be seen in Fig. 4b, the initial C/N ratios in this study correspond with the values in the literature. The decrease in the C/N ratio due to the simultaneous consumption of carbonaceous and nitrogenous organic substrates indicates an effective composting process [83, 84]. Among the reactors, the highest reduction in the C/N ratio (40%) occurred in the M2 reactor. However, the lowest decrease in the C/N ratio (3.3%) was observed in the M1 reactor, which was set up with the addition of treatment sludge and sunflower stalk, and in which microbial activity remained at the lower limits and the temperature increase (48 °C) was not great. In the M3, M4, and M5 reactors, decreases in the C/N ratio of 26.7%, 35.7%, and 27.4% occurred, respectively.

3.5 Total Kjeldahl nitrogen (TKN) and total phosphorus (TP)

The changes in TKN during the composting process are given in Fig. 5a. The initial TKN values in all reactors ranged between 1.2 and 2.3%. Since nitrogen concentrations, which play an important role in the functioning of the process, directly affect the C/N ratio, they can cause unsuitable conditions if they are found in high values in the mixture. Throughout the process, increases and decreases were observed in TKN values, which are affected by many factors, especially microorganism activities, temperature changes, and dry matter losses. Moreover, increases and decreases in TKN values could be observed throughout the process depending on the amount of waste and the ventilation rate [85]. TKN values increased at the end of the process in all reactors except for the M1 reactor, which was set up with the addition of 80% treatment sludge and 20% sunflower stalk. The reason for this was that since the temperature increase (48 °C) in the M1 reactor was not at the desired level, sufficient mineralization did not occur in the waste mass, and organic materials could not be broken down. The co-composting of various organic wastes results in a more efficient composting process because it influences the biochemical processes through a greater number of microorganisms and minimizes nutrient losses during the composting process [86, 87].

Fig. 5
figure 5

The variation of TKN (a) and TP (b) throughout the composting

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Phosphorus is an essential macronutrient for all living organisms. Animal manure also contains 70–80% of the phosphorus consumed by the animal [88]. In their study in China, Li et al. [38] stated that the TP concentrations of feeds given to poultry and livestock were higher than the specified feed standards, and that, therefore, they encountered high phosphorus concentrations in excreted manure. For this reason, the aerobic composting of animal manure is a long-term and sustainable way to increase phosphorus production for use in the agricultural sector [89]. By composting animal manures, treatment sludge, and agricultural wastes separately or together, a stabilized soil amendment is obtained that can provide higher amounts of available phosphorus compared to inorganic fertilizers [35, 90, 91]. The changes in TP for the reactors are given in Fig. 5b. The initial phosphorus concentrations in all reactors ranged between 1.04 and 1.38%. Except for the M1 reactor, increases in phosphorus concentrations were observed due to the increase in temperatures, decomposition of OM, decline in the C/N ratio, and rise in pH in the reactors. The increase in TP concentration can be termed as the “concentration effect” due to the losses that occur with the outgassing as CO2, H2O, and NH3 [92]. Another reason for the increase in total phosphorus values was the loss of mass occurring in the reactors. In their study, Rehman et al. [35] co-composted sewage sludge and farm manure, and they noted that while sewage sludge contained a significant amount of phosphorus, the addition of farm manure contributed to an increase in TP.

3.6 Free air space (FAS)

Free air spaces allow air to diffuse in the environment and provide the oxygen needed by microorganisms. For the biodegradation of many materials by mixing and for providing adequate aerobic conditions, a FAS ratio of 20–30% is recommended [48]. At very high FAS values, thermophilic temperatures cannot be maintained during the composting process, and the biodegradation rate of volatile solids decreases [93, 94]. The FAS values for the reactors are shown in Fig. 6. In the study, the lowest FAS value (22.8%) was determined in the mixture formed with 80% treatment sludge (M1), while the highest (38.5%) was determined in the mixture formed with the addition of horse manure and treatment sludge (M4). Except for the M4 reactor, the FAS values were between or very close to the values in the literature. Since bulking agents directly affect the FAS values, these values can be found within the recommended ranges provided that the required amounts of bulking agents are used.

Fig. 6
figure 6

Free air space (FAS) values for the reactors

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4 Conclusions

Co-composting animal manure with treatment sludge can provide a more sustainable solution for managing treatment sludge. In this study, the compostability of treatment sludge with the addition of different types of animal manures (cattle, pig, horse, and chicken) and sunflower stalk was investigated. Adding animal manure to the treatment sludge resulted in greater microbial activity, leading to higher temperatures in the reactors and greater organic matter losses. The maximum temperatures reached in the reactors were 62.7 °C (cattle), 62.2 °C (chicken), 60.2 °C (horse), 51.5 °C (pig), and 48 °C (only treatment sludge). Similarly, the highest organic matter loss occurred in the M2 reactor with cattle manure, while the lowest organic matter loss occurred in the M1 reactor without animal manure. When treatment sludge was composted alone, the composting efficiency was lower due to fewer microbial communities and less substrate. On the other hand, it was determined that better results were achieved in composting experiments with different types of animal manures as an additional substrate and inoculum added to the treatment sludge. The most successful composting process occurred in the reactor with added cattle manure (M2), followed by the reactors with added chicken (M5), horse (M4), and pig (M3) manure, respectively. The addition of animal manure increased the population of microorganisms responsible for carrying out the composting process and the substrate quantity, thereby enhancing composting efficiency. It might be possible to achieve even better results with different bulking agent, different animal manures, and different composting recipes, or after the maturation phase.