Metrics for stabilization of fecal sludge and relation to dewatering metrics

The sanitation needs of 46% of the world's population are served by non-sewered sanitation (WHO/ UNICEF 2021), resulting in the accumulation of fecal sludge (FS) that requires treatment. FS is stored wastewater that is comprised of varying fractions of readily degradable organic matter and is typically 80–95% water (Strande et al. 2014; Gold et al. 2016; Semiyaga et al. 2017). FS requires stabilization and dewatering at treatment to ensure protection of public and environmental health (Strande et al. 2014; Gold et al. 2018). Dewatering entails solid–liquid separation, while stabilization involves the breakdown of readily biodegradable organic matter into smaller molecules that exert less oxygen demand (Strande et al. 2014; Semiyaga et al. 2017). However, characteristics of FS are highly variable due to a number of factors (e.g. differences in containment type, inputs, management practices, frequency of emptying, and environmental conditions) (Semiyaga et al. 2017; Ward et al. 2019). FS stabilization processes occurring inside containments during storage are not well understood (Shaw & Dorea 2021; Ward et al. 2023), and hence, dewatering performance is inconsistent due to this variability (Semiyaga et al. 2017; Gold et al. 2018), which complicates the prediction and performance of both treatment processes.

Recent studies have hypothesized that a potential linkage exists between FS stabilization and dewatering, as more stabilized FS has been observed to dewater faster than fresh FS (Cofie et al. 2006; Semiyaga et al. 2017; Ward et al. 2019; Ward et al. 2021). Understanding this link could improve predicting dewatering performance and optimizing treatment design. However, there are no agreed-upon metrics for measuring or defining degrees of biological stabilization of FS, which are needed to understand reported linkages to dewatering. Gradation scales of stabilization (or stability indices) would enable comparisons among different substrates (e.g. FS) or degradation processes (e.g. changes during storage) (Barrena et al. 2009). Metrics for measuring stabilization of organic substrates such as municipal wastewater, landfill leachate, and composts exist (Bernal et al. 1998; Samson & Ekama 2000; Benito et al. 2005; Ferrer 2006; Mangkoedihardjo 2006; Borglin et al. 2012; Cokgor et al. 2012; Bożym & Siemiątkowski 2020). However, since characteristics of FS (e.g. total solids (TS), volatile solids (VS), chemical oxygen demand (COD)) can be two orders of magnitude higher than other organic substrates (e.g. municipal wastewater), direct application of the metrics of stabilization from these sectors is not necessarily feasible (Niwagaba et al. 2014; Gold et al. 2018; Tayler 2018). Currently, descriptive and qualitative parameters such as color and odor of FS are used (Ward et al. 2019; Ward et al. 2021), or various metrics such as VS/TS, biological oxygen demand (BOD), and biomethane potential (BMP) that are not comparable between studies. Hence, a need exists to develop reliable metrics of FS stabilization to aid in predicting dewatering performance and comparing biological stability, and for treatment process control.

In order to fill this need, the objectives of this study were: (1) to evaluate metrics of stabilization that are most relevant and meaningful for FS and (2) to determine whether there is a relationship between level of stabilization and dewatering performance in FS.

A systematic literature review identified 14 metrics of stabilization of organic substrates that are used with wastewater sludge and compost. All 14 metrics were scored according to 11 criteria: selectivity, availability of a stability index (a continuous scale with a known point where biological activity slows significantly, indicating stabilization), robustness, demonstrated ability to measure stabilization, clear laboratory protocols, simplicity of laboratory techniques, required effort, working range cost, feasibility for local application in Zambia, reproducibility of results, and application in low-income setting. Each criterion was assigned a weighting and was scored on a scale from 0 to 10. The score was multiplied by the assigned weighting for each criterion and the product divided by the maximum possible score to obtain a weighted score. The complete list of criteria and scores (points) are provided in the Supplemental Information (Table S1). The five metrics with the highest scores were selected for analysis.

From May to August 2021, 27 samples were collected in situ from 27 onsite containments in Lusaka, Zambia. Sample collection included 15 samples from partially lined pit latrines (defined here as sides lined with concrete blocks and open at the bottom); eight samples from fully lined pit latrines (defined here as fully sealed tanks with concrete blocks and a concrete bottom), one of which was a vertical vault latrine (VVL), an improved toilet being promoted under the Lusaka Sanitation Program; and four samples from septic tanks (double chambered tanks with baffles and overflow to a soak-away). Onsite containments were selected to represent variations in the design, usage and source in order to obtain a diverse range of samples with varying characteristics and levels of stabilization. Pit latrine samples were taken with a prefabricated elongated scooper that is used for manual emptying. Three samples were taken (approximately 4 L) during emptying: one from the first 60-L barrel at the start of the emptying job, one from the middle barrel, and one from the final barrel. Samples were then combined and homogenized by stirring, and a 2-L composite sample was taken for analysis. Septic tank samples were taken with a 3 m long core sampler based on Koottatep et al. (2021). The core sampler was emptied into a bucket, homogenized, and a 2-L sample taken for analysis. All samples were transported to the laboratory in a cooler box and stored in a fridge at 4 °C until analysis. Prior to analysis, samples were well mixed by vigorous shaking and stirring, and divided into two 500 mL portions. One was homogenized using a blender for physical–chemical characterization, and the other kept unblended for stabilization and dewatering tests (for these tests, blending was avoided so as not to disrupt aggregates and dewatering behavior). In addition, one fresh sample was obtained. The fresh FS sample was prepared by mixing urine, feces and water in a blender (250 mL combination of urine and feces to 500 mL of tap water). To rapidly evaluate the stabilization metrics, six samples were selected based on representing a variety of storage times in containment: one sample with time since last emptying event of zero (the fresh FS), one sample with the last emptying event 6 months ago (the VVL) and four samples where the last emptying event was 3 years ago (three pit latrines and one septic tank). The time since last emptying event was obtained from the emptiers prior to desludging. The remaining 22 samples were used to assess the relationship between the metrics of stabilization and the dewatering performance of FS.

TS, VS, pH, and electrical conductivity (EC) were analyzed according to standard methods (APHA 2017). COD was quantified with the closed reflux photometric method (APHA 2017) with medium range COD test cells from Merck and quantified with a Hydrotest Photometer HT1000. BOD was measured using the 5-day BOD and membrane electrode method (APHA 2017). Capillary suction time (CST) is a measure of the rate of water release from sludge (Velkushanova et al. 2021). CST was quantified as a metric of dewatering performance with a Triton 319 Multi-CST apparatus with 18 mm funnels, according to Method 2710 G (APHA 2017) as adapted in Velkushanova et al. (2021). CST values were standardized by subtracting the CST of tap water and normalized by TS concentration.

Five metrics of stabilization were evaluated for suitability: BOD/COD, VS/TS, Dehydrogenase activity (DHA), BMP, and Specific Oxygen Uptake Rate (SOUR). BMP was setup based on Holliger et al. (2016) and Filer et al. (2019). 175 mL mixture of the inoculum and FS at a ratio of 1:4 (based on VS concentration) was placed in a 200 mL serum bottle, which was sealed with a butyl rubber stopper, capped with an aluminum crimp seal, and incubated in a water bath at 35 °C. The inoculum was collected from a bio-digester at Manchinchi wastewater treatment plant in Lusaka. Biogas was measured intermittently with the liquid replacement system described in Pham et al. (2013). DHA was measured using a modified method adapted from Chung & Neethling (1989) and Ghaly & Mahmoud (2006). Both DHA and SOUR are mainly designed for samples with low solids content and high solids concentrations could limit oxygen transfer rate. To use the methods on FS, all samples in this study were diluted to a VS between 2.5 and 5 g/L. The DHA modification also included a dissolved oxygen (DO) concentration check (DO<1g/L). This was done to avoid high microbial activity, which would give a DHA value above the method detection limit and oxygen interference as an electron acceptor. A 0.2% (v/w) tetrazolium chloride (TTC) solution was used as an oxidant or electron acceptor. Triphenyl formazan (TF) was extracted by centrifugation at 3,500 g for 6 min and dissolved in 5 mL of ethanol. DHA was recorded as optical density of the TF solution with a spectrophotometer at 485 nm. SOUR is directly related to DHA, as dehydrogenase enzymes participate in the transport of electrons during aerobic stabilization (Lopez et al. 1986; Chung & Neethling 1989; Sánchez et al. 2006). SOUR was measured using a modified method adapted from USEPA (2001). Similar to DHA, the modification included sample dilution. Because the FS samples were collected from a mainly anaerobic environment, they were aerated for 1 h prior to conducting the SOUR test using a diaphragm vacuum pump. The laboratory protocols for the DHA and SOUR modified methods can be found in the Supplemental Information (Table S2). For AD experiments, 500-mL batch reactors were setup with nine pit latrine and one septic tank FS samples as feedstock, placed in a water bath at 35 °C for 60 days and manually shaken once daily. A needle attached to a plastic syringe was used to periodically degas the reactors. A sample was collected from the reactors at day 14, day 28, day 48 and day 60 and analyzed for changes in stabilization and dewatering characteristics. Due to sample volume limitation, SOUR was only measured on the first and last day of the experiment. To ascertain associations between stabilization and dewatering performance, Spearman correlation analysis was conducted in RStudio.

Esterase activity (EA) and adenosine triphosphate (ATP), despite being inexpensive and simple, have not been widely applied in stabilization studies. The static and dynamic respiratory index (SRI and DRI) are both solid-state methods and require equipment that is expensive and not as readily available. Humification, cellulose and lignin content metrics lacked well-defined stability indices. One of the most important criteria was local availability of materials. Although DHA had one of the highest scores, tests could not be completed, as it was not possible to obtain the calibration standard for the spectrophotometer within the study period. In the case of Lusaka, the assumption that these standards would be easily obtainable was not correct due to supply chain issues.

Summarized in Table 1 are characteristics of all of the FS samples in this study, compared with mean literature values for pit latrine and septic tank FS. As shown by the standard deviation, the values are quite variable, which is consistent with the literature (Semiyaga et al. 2017; Tembo 2019; Ward et al. 2023). The physical–chemical and dewatering characteristics of FS were in the range of values reported by other studies for septic tank and pit latrines (Bassan et al. 2013; Gold et al. 2018; Tembo 2019; Awere et al. 2020; Ward et al. 2021). Generally, partially lined pit latrines recorded the highest COD, BOD, TS, and CST values followed by fully lined pit latrines and then septic tanks with an exception of pH and VS, which were almost the same across all the types of containment categories. On the contrary, fully lined pit latrines had the highest EC, followed by partially lined pit latrines and then septic tanks. This could be due to the NH4-N accumulation from urine in fully lined pit latrines as compared to partially lined pit latrines where liquid percolation happens and septic tanks where there is an overflow. The COD and VS values of the fresh FS sample were slightly higher than the results reported by other studies (Cofie et al. 2006; Sam et al. 2022) with an exception of pH. Further, the BOD and VS values for fresh FS were higher than the pit latrine and septic tank FS samples, which could show a reflection of reduced readily degradable organic matter through microbial degradation of FS during storage in containment. The fresh FS sample is slightly above the range observed in literature, but since this was only one sample, no general conclusions can be drawn.

In comparison to literature stability indices, all of the metrics of stabilization showed that the FS samples were not stabilized, except for VS/TS with samples PL1 and PL3. The contradictory results of VS/TS are an indication that the literature stability index, or VS/TS itself as a metric of stabilization, is not relevant to FS and requires further investigation.

With all metrics, fresh FS was clearly the least stabilized. The VVL (a fully lined pit latrine), which was 0.5 years since it was last emptied, was in general more stabilized than the fresh FS, and less than the other samples. The differences in level of stabilization between the 0.5-year- and the 3-year-old samples were minimal for BOD/COD and SOUR, and more significant with BMP and VS/TS. These results indicate the usefulness of stability metrics for fresh FS versus FS that has been stored in containment, but a higher level of ambiguity for comparing FS of various ages. These results confirm that time since last emptied for FS that has been stored in containments is not a clear predictor of stabilization (Ward et al. 2023). Passive onsite storage is not analogous to process-controlled AD, and during onsite storage there are many variables that could affect degradation, such as variable inputs to the containment, quality of construction, and soil characteristics (Shaw & Dorea 2021; Ward et al. 2023).

Positive statistically significant correlations (p < 0.05) were observed among BOD/COD, SOUR, and BMP, but not for VS/TS. Positive correlations between methods indicate consistency as they are measuring the same thing. This is a preliminary indication that all the metrics, other than VS/TS, give consistent stabilization predictions of FS, although further research is needed to understand the variability. It is promising that both aerobic (SOUR, BOD/COD) and anaerobic methods (BMP) correlate. Positive correlations between aerobic (e.g. SOUR, respiratory index) and anaerobic metrics (BMP) for sewage sludge and composts have also been reported in literature (Sánchez et al. 2006; Ponsá et al. 2008). However, if rapid results are needed for decision making in practice (e.g. process control at treatment), SOUR, which takes 2 h, and BOD/COD ratio, which takes 5 days, are more recommendable in terms of time than BMP, which takes 3–4 weeks.

On the contrary, for BOD/COD the majority of samples did not change substantially with AD and increased in some samples to values above the stability index. Borglin et al. (2012) also reported an increase in BOD/COD in anaerobically digested municipal solid waste leachate, which was attributed to initial COD concentrations that are more than fourfold higher than BOD concentration, which is similar in our FS samples. As mentioned previously, this is likely due to the influence of COD measuring many pools of organic matter with varying biodegradability (Maqbool et al. 2024), and its relationship with dewatering (both before and after AD) was ambiguous. There is potentially a positive trend if the outlier at 0.6 is not considered. However, the trend would still not be clear enough to draw conclusions from.

All samples recorded a lower CST (i.e. better dewatering performance) after AD. On average, CST decreased by 64%, which was more than the 20% reduction observed for AD of FS by Ward et al. (2023). Sam et al. (2022) also reported an improvement in dewatering performance with AD of FS. Among all the metrics, SOUR seemed to be the best predictor of dewatering performance. However, further analysis is required, and the relationship was most strongly reflected in undigested FS samples (Figures 3(a) and 4(a)). SOUR also appears to be the most robust to the wide range of stabilization levels seen in FS. This supports the assertion by Ward et al. (2023) that the empirical relationships between stabilization and dewatering seem to be most useful in predicting the dewatering performance of extremes (e.g. very fresh versus very stabilized FS), and not different levels of stabilization. These results support the current understanding that more stabilized FS is easier to dewater than less stabilized FS (Cofie et al. 2006; Semiyaga et al. 2017; Ward et al. 2019; Ward et al. 2021).

In this study, VS/TS was selected as a metric of stabilization due to its widespread usage with other streams of organic waste management (e.g. compost, municipal wastewater). However, due to its conflicting results and lack of relation to dewatering performance, we do not recommend its usage as an indicator of stabilization for FS. The results indicate that the metrics that are based on microbial activity (e.g. SOUR) are more representative for FS than VS/TS. However, the determination of grit and sand content might improve the use of VS/TS as a metric.

Physical–chemical metrics of stabilization such as VS/TS could be useful when comparing similar matrices during a stabilization process (e.g. monitoring a composting process with time), or within one defined matrix, such as wastewater sludges that are well defined and relatively similar (e.g. the defined use of primary sludge, waste activated sludge, and digested primary sludge in conventional wastewater terminology (Metcalf & Eddy 2003)). For this reason, the use of stability indices (e.g. VS/TS < 0.5) is also most useful within the same matrix, but not for FS. VS is composed of all fractions of organic matter, from readily degradable to recalcitrant matter, and it is therefore not a reliable indicator with highly variable streams such as FS. FS has a wide range of unstabilized to fully stabilized organic matter, and TS can include highly variable fractions of inert matter from soil intrusion or non-biodegradable wastes (i.e. grit and rubbish) (Gudda et al. 2017; Ahmed et al. 2018). In addition, the results also showed that the other commonly used metric BOD/COD was not a useful indicator of stabilization for FS in this study due to the possible influence of pools of organic matter within total COD. Thus, we do not recommend the usage of BOD/COD as a metric of FS stabilization and dewaterability without further investigations, including quantifying soluble or dissolved COD in FS as a metric of more bioavailable fractions. Based on the success of the SOUR method in this study, the DHA method which is also based on microbial activity and is inexpensive and simple to perform, is a potential target for future evaluation, but supply chain issues need further investigation.

Based on these findings, conclusions of this study include the following:

• Among all the methods evaluated within this study, SOUR seemed to be the most promising metric for measuring FS stabilization and its relation to dewatering performance. Based on these findings and the fact that it is an inexpensive and rapid metric (requires about 2 h) it appears to be suitable for application in FS dewatering and stabilization process control at treatment facilities. However, because of the considerations associated with applying the method to FS (as outlined in the Methods section), it is recommended to use the adapted protocol as provided in the Supplemental Information (including 1 h of aeration and dilution).

• Based on these results, it is clear that VS/TS is not an accurate metric of FS stabilization. BOD/COD was inconsistent, and requires further investigation (e.g. soluble or dissolved fractions). Although VS/TS and BOD/COD still provide some information on stabilization, we conclude that microbial activity metrics (e.g. SOUR) will be more representative than those based on physical–chemical characteristics.

• Although the relationship between stabilization and dewatering performance was not consistent among all the evaluated stabilization metrics in this study, dewatering was generally related to the level of FS stabilization and improved with AD. This is corroborated with what practitioners have been observing in the field.

The present study is part of the ongoing research on fecal sludge stabilization and dewatering funded by the Swiss Development Corporation (SDC) through the Swiss Federal Institute of Aquatic Science and Technology (Eawag). The authors would like to acknowledge the technicians and other personnel who assisted with the field and laboratory work. These are the Environmental Engineering Laboratory at UNZA: Enock M. Mutati, Dan Mkandawire, Derrick Muwowo, Gomezyo Mhone and Lwiindi Kozozo Phiri, the Water Quality Testing Laboratory for Lusaka Water Supply and Sanitation Company (LWSC): Derrick Katele, the Pit Emptiers from Chazanga Water Trust, and Lunem Enterprises: Samuel Kanyanta.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.