In aerobic composting, maintaining proper air supply to the composting material is essential to facilitate the production of gases through biodegradation and provide oxygen for decomposer microorganisms. Adequate ventilation and regular turning of the compost material are critical for ensuring optimal oxygenation28. However, ventilation is often hindered by compaction, necessitating thorough mixing of materials to enhance accessibility and susceptibility to microbial attack. This promotes the restart of the biological process, leading to a rise in temperature, which explains the observed temperature fluctuations during different phases of composting29. In our experiments, each unit was treated with varying amounts of biological decomposer solution. Subsequently, air renewal and turning of the compost were conducted every 5th day to assess pH, temperature, and humidity levels within the composting units. These parameters were crucial in evaluating their impact on the reduction of EPG, total coliform bacteria, and faecal coliform bacteria in the slaughterhouse sludge before using as fertilizer.
During first 10 days of composting, a rise in pH was noted, attributed to metabolic degradation of organic matter containing nitrogen, such as proteins and amino acids, leading to the formation of amines and ammonia salts through the mineralization of organic nitrogen, a process supported by previous studies30. This pH increase might also be due to the decomposition of organic acids, releasing alkali and alkali earth cations previously bound by organic matter, as suggested by Smith and Hughes31 and Mupondi et al.32. Similar pH elevation during composting has been reported in various studies30,33. The subsequent decline in pH during the later stages of composting could be attributed to the nitrification process, releasing H + ions, a phenomenon supported by the significant increase in NO3− observed during these stages. The pH levels ranged from 6.0 to 8.0 during composting, indicating a successful and fully developed process, and aligning with the literature33. Importantly, pH values attained in all treatments at the end of the experiment fell within the acceptable range for organic fertilizer, as recommended by Tognetti et al.34.
The stabilization of pH especially in Unit 3 showed the presence of a suitable environment for the microbial process which is important for the decomposition of organic matter. The thermophile microorganisms responsible for the decomposition of highly complex organic matter thrive at the relative pH of 6.5 and 8.5 which promotes their activity hence improves the rate of decomposition35. Additionally, stable pH affords inactivation of pathogens that include parasitic eggs and disease-causing bacteria such as coliforms as they are less likely to survive in such conditions. This stability eventually enhances the composting process and pathogenic reduction resulting in production of better quality and safe compost36. Earlier works also support the statement that pH stability is necessary for battling pathogens and boosting microbial efficiency during composting37.
The lack of pathogen (parasite eggs, coliforms) reduction during the last days of our study may relate to the stabilization of pH and the temperature returning to normal levels. High temperature (thermophilic phase) and high pH emerging during the initial phase of composting process assist in pathogen reduction. As the thermophilic temperatures range is reached and temperatures below thermophilic range (< 45 °C) are attained, pathogen degrading microorganisms may be slowed down tremendously38. It is well known that some parasitic eggs and coliform bacteria show special traits allowing them to survive changes in the environment temperature and pH among others. One line of bacteria parasites, for instance, is coliform that can tolerate neutral pH so it becomes impossible to completely eradicate them at the advanced stages of composting. In the same way, some impeded eggs contain tough shells which protect them from attacks due to the changes in environmental conditions and they will remain dormant unless there is adequate heat for a given period time38,39.
There was an initial rapid increase in temperature during the first few days of composting, followed by a gradual decline over time, eventually reaching ambient temperature. These temperature fluctuations marked the thermophilic, mesophilic, and maturation phases of the composting process, respectively. The swift transition from the initial mesophilic phase to the thermophilic phase in the treated units indicated a high proportion of readily degradable substances and the waste’s self-insulating capacity33. This temperature pattern aligns with other composting studies30,40. The rise in temperature within the composting mass occurred due to the accumulation of heat generated from the respiration and decomposition of sugar, starch, and protein by the microbial population, outpacing dissipation to the surrounding environment41.
Variations in temperature among the composting treatments might have been influenced by factors such as the amount of material, initial moisture content, and aeration. Differences in the temperature profiles of the units, despite similar volume and aeration (air circulation), can be attributed to varying moisture content in the compost. Regular turning of the compost mass every 5th day in all three treatments likely facilitated air circulation, enhancing microbial activity in the oxidation process and thereby raising the temperature. Units 3 and 2, with a combination of aeration and relatively high moisture content, maintained a higher temperature for a more extended period than Unit 1. This finding is supported by Finstein et al.42, who demonstrated a linear relationship between oxygen consumption and heat production during aerobic metabolism, corroborating the results of this study.
Fluctuations in humidity played a significant role in shaping the composting process. Higher humidity levels at the beginning likely facilitated initial decomposition stages by creating a favorable environment for microbial activity. Makan et al.43 indicated that humidity ranging from 70 to 75% was associated with the most significant degradation. Huerta-Pujol et al.44 noted that moisture content significantly influences biological activity, with a range of 40 − 60% recommended for effective composting. In a study conducted by Jain et al.45, vegetable wastes with an initial moisture content of 89% were adjusted to 57% by adding bulking agents to maintain a suitable composting environment. However, as composting progressed, excessive humidity could have restricted aeration, creating anaerobic conditions. This might have slowed organic matter decomposition, impacting overall composting efficiency. Conversely, excessively low humidity levels in later stages could have hindered microbial activity, affecting organic material breakdown46. Furthermore, variations in humidity among units likely influenced the composition and quality of the final compost. Composting units with stable and optimal humidity likely produced compost with superior nutrient content and microbial diversity, essential for effective soil amendment. Therefore, regulating humidity levels during composting is a crucial factor for optimizing the process and ensuring production of high-quality compost.
When humidity levels changes, the behavior of the microorganisms present in the compost may alter, which may delay the rate of the composting process, or leading to death of pathogens. Excessive humidity may lead to compaction and reduced aeration, hindering aerobic conditions, while insufficient moisture may restrict the activities of micro-organisms. It has been reported that ranges of 50–60% humidity content in the mass are favorable for the effective composting38,47. In future studies, other approaches should be adopted such as increasing ventilation, turning the compost regularly and humidity control to avoid compaction and promote even aeration in order to improve the process of composting.
Our findings clearly showed that eggs of pathogenic parasites in faecal sludge are more effectively sanitized by the high temperature created in the thermophilic phase of composting. According to Koné et al.48, high temperatures may make egg shells more permeable, facilitating movement of hazardous chemicals while also speeding up desiccation. Although many authors claimed that parasitic eggs were completely eliminated under thermophilic conditions49, this was not the case in the current study. Helminth eggs were still present and viable despite the thermophilic condition (45 °C) being maintained for approximately 15 days in Units 1 and 2 and for approximately 20 days in Unit 3. The complete death of eggs may not be guaranteed because it is possible that the lethal temperature is not distributed uniformly throughout the compost biomass. Due to their exposure to the open air near the top of the compost, the substrates there may have been slightly cooler than those inside.
Diversity in variables may be a reason for EPG reduction after day 30, including processes like a decline in microbial activity and stabilization of certain environmental parameters such as temperature and pH. The thermophilic phase of composting (days 5 to 25) is often characterized by high temperature and active microorganisms that can significantly reduce pathogen loads, including the eggs of certain parasites. However up to this point, once the compost has warmed and shifted towards mesophilic conditions, this is followed by reduced pathogen destruction abilities because of drop in microbial activities. Furthermore, neutral or near neutral pH conditions probably does not favor the erosive degradation of parasite eggs anymore and therefore no further reduction of pathogens was noticed35,36.
According to Strauch50, composting assures material hygienization as long as all biomass is exposed to a high enough temperature (55 °C for 14 days). According to the temperature readings obtained in this study, Unit 3 only had a temperature > 55 °C for an average of 10 days, during which time the compost was only rotated once, allowing it to reach that temperature. This would imply that if the compost feedstock had been turned more frequently, perhaps every two or three days, the biomass could have experienced the lethal high temperature evenly and for longer periods of time, leading to higher efficiency of helminth egg removal. This reasoning obviously conflicts with results by Koné et al.51, who showed that turning frequency had no discernible impact on the effectiveness of inactivation of helminth eggs. It has been stated, however, that the amount of heat produced and the amount of time the thermophilic phase is sustained during composting depends on the volume of the compost feedstock. The amount of heat produced by a compost increases with volume, stays in the compost for longer, and can be turned less frequently due to the extended duration of the thermophilic phase. Smaller composts only experience a brief thermophilic phase; thus’ unless they were stirred often, the biomass outside would not have the opportunity to experience the fatal high temperature. In the US, compost is considered hygienically acceptable if a temperature of > 55 °C is maintained in windrows for no less than 15 days with an average of 5 turnings during the high temperature phase52.
The cessation of the egg viability reduction between day 25 and day 60 can suggest that there are certain constraints in the composting mechanism, most likely limited microbial activity and/or microclimatic conditions e, g., temperature dropping below thermophilic range causing killing of the pathogens are hindering the process. Some eggs of the parasites may be more tough and may not be destroyed easily unless exposed to higher temperature continuously afterwards8,35,38. Prolonging the thermophilic phase or finding other means to enhance the effectiveness of egg viability reduction can increase the efficiency of the composting process in the longer-term.
In a study conducted by Topal et al.53, total coliforms were decreased to 78.2 − 99.9%, whereas faecal coliforms were decreased to 72.5 − 99.9% following the thermophilic stage. They used six different aeration rates during in-vessel aerobic composting of vegetable and fruit wastes to eliminate total and faecal coliform bacteria. In addition to high temperatures, the decline in organic matter led to a reduction in coliforms. Because they cannot multiply on complicated compounds like lignin and humic materials, coliforms typically use highly degradable materials. When there are sufficient nutrients, coliforms can multiply. However, they are killed when the nutrients are depleted and inappropriate circumstances arise.
Hassan et al.54 used municipal solid waste in a semi-industrial pilot plant and applied a moderate aeration during the composting process, to eliminate the coliform bacteria. Similar to our findings, they found that throughout the thermophilic phase, the average number of faecal coliforms declined significantly from 2.5 × 107 bacteria/g waste at the start of the procedure to 7.9 × 107 bacteria/g waste. The high temperature (60–65 °C) and the adverse conditions created during the thermophilic phase are likely to be responsible for this drop. High coliform eradication efficiencies have been reported previously, consistent with our findings. Pathogen inactivation in four compost piles was researched by Pereira-Neto et al.55 using the aerated static pile technique. Using common indicator organisms, they assessed bacterial inactivation. Similar to our findings, E. coli and faecal streptococci decreased from around 107 org/g to less than 102 org/g. Similarly, Larney et al.56 found that > 99.9% of total coliforms and E. coli were removed in the first 7 days during open-air windrow composting of cattle feedlot manure. Salmonella, faecal coliforms (E. coli), and faecal streptococci were found in samples collected after 1, 30, 60, and 90 days of composting in the study by Banegas et al.57. Similar to our findings, they observed that the majority of bacteria were eliminated at the high temperatures (57–61 °C) reached during composting, making the resulting composts safer for agricultural use.
In contrast to our findings, Cekmecelioglu et al.58 composted food waste along with cow dung and bulking materials during a 12-day period and reported a faecal coliform decrease of 59.3% with a maximum temperature of 56.6 °C. It is generally accepted that the high temperatures (> 55 °C) created by microbial activity along with aeration rates, feedstock composition, and composting methods are principally responsible for inactivating bacteria during composting. A huge surface-to-volume ratio of the mass, however, could prevent it from reaching such lethal temperatures because a significant amount of the heat produced is lost to the environment. Additionally, microbial communities require the right ratios of carbon and nitrogen to develop and produce an adequate amount of heat. A substantial amount of heat may also be lost from the composted substrate to the environment at low ambient temperatures36. By comparing the results with other studies on coliform bacteria reduction, it is evident that variable findings might be reported presumably due to variety of composting materials, methods, and other microclimatic factors. The studies of Topal et al.53, Hassan et al.54, and Larney et al.56 stressed that even though high temperatures are fundamental in achieving pathogen destruction, other factors like oxygen supply, feedstock addition, and composting methods are also important for the decrease of coliforms. It should be done in future research regarding these variables in more detail to get the most efficient composting deformation on pathogenic eradication.
Under the limited budget of the project, standardization of methods relative to lowering the burden of parasites and coliforms, were used as significant measures for risk assessment of the pollution monitoring and public health threat. Although factors such as heavy metals and pharmaceuticals contaminants are of significances in further studies, this is not an area to be tackled in this specific case. This study made use of the membrane filter method, however, we are of the opinion that application of more sophisticated techniques in subsequent studies will improve the quality and breadth of studies of the microbial communities. This aspect of additional completion will be accomplished in future works urgent for those additional aspects. It is true that core temperatures (> 55 °C) are essential for the inactivation of pathogens, nevertheless, we recognized that thermophilic phase did not overtly last in all the units of this study particularly regarding the more tolerant helminths’ eggs sanitization. Reliance on temperature alone is not optimal, to say the least. Optimizing turning frequency in future studies to achieve better heat distribution and isolating the bio heat for longer periods could be fruitful in extending the thermophilic phase thus potentially improving pathogen kill within the entire compost mass.
The compost derived from slaughterhouse sludge can serve as an organic soil modifier to upgrade soil quality and fertility. This compost can also be practiced in the agricultural fields in order to minimize the use of synthetic fertilizers and enhance soil characteristics. In addition, the parameters of the composting process can be modified to include biological decomposer solutions, adding feedstock, adjusting moisture contents, and turning frequencies to ensure maximum pathogen kill and nutrient retention. Implementing these measures would also improve the waste management and decreas environmental pollution fostering healthier environmental practices.