Fecal sludge treatment in a pilot-scale vertical-flow constructed wetland for the removal of helminth eggs

Fecal sludge (FS) is produced in on-site sanitation technologies (e.g., pit latrines and septic tanks), and the result of the collection, storage or treatment of excreta and blackwater (Stenström et al. 2011; Tilley et al. 2014; Strande et al. 2018). The direct discharge of raw FS into the environment was found to be a major environmental and public health concern in many studies (Soh Kengne et al. 2014; Letah Nzouebet et al. 2016; Strande et al. 2018; Velkushanova et al. 2021; Tanoh et al. 2022). Recent outbreaks of waterborne diseases have raised public concerns regarding the safety of water resources (Prüss-Ustün et al. 2019). One of the major barriers to FS reuse is concerns regarding the health risk associated with chemical and microbiological contaminants (Stenström et al. 2011; Letah Nzouebet et al. 2016).

Annually, FS discharge is increasing in Cameroon as many cities operate latrine-based sanitation systems and face continuous population growth (Berteigne 2012; Soh Kengne et al. 2014; Douanla Maffo et al. 2019). Nowadays, vertical-flow constructed wetlands (VFCWs) have become a popular option for FS treatment and have been recognized as attractive alternatives to conventional FS treatment methods (Kengne et al. 2011; Ronteltap et al. 2014; Kouawa et al. 2015; Seck et al. 2015). This is due to their high removal efficiency, easy operation and maintenance, low-energy requirements, high rates of water recycling and potential for providing significant wildlife (Kadlec & Wallace 2008). The lack of knowledge about this eco-technology may be one of the main factors leading to the uncontrolled discharge of untreated FS into the environment (Kengne et al. 2011). One of the threats associated with FS is soil-transmitted helminthiasis (STH), a neglected tropical disease (Yen-Phi et al. 2010; Appiah-Effah et al. 2015; Seck et al. 2015). Ascariasis is the most common STH and endemic in Africa (Jimenez 2007). Even though the mortality rate is low, most of the people infected are children under 15 years with problems of faltering growth and/or decreased physical fitness. To stop or interrupt the spread of helminthic diseases, the treatment of FS is a necessity as it provides a barrier and prevents STH from entering the environment.

Several operational variables and design factors have been found to affect the reduction of helminth eggs in VFCW. Indeed, Sonko et al. (2015) pointed out the reduction of helminth eggs in FS to be affected by hydraulic loading. Manga et al. (2016) revealed the effect of sand filter media thickness on the removal performance of helminth eggs in FS drying beds. The objective of the present study was to evaluate the performance of the system implementing different operational strategies (free flow, 7 days of retention time (RT) and 14 days of RT) as well as the influence of planted unit in terms of helminth eggs removal.

This study was conducted with a pilot-scale VFCW at the University of Yaounde I, Cameroon. The field site is located 760 m above sea level at 3 °45 N and 11 °32 E and has a typical equatorial Guinean climate characterized by two rainy seasons (September–mid-November and mid-March–June) and two dry seasons (mid-November–mid-March and July–August). The annual average rainfall is 1,600 mm and the average daily temperature is 22–35 °C.

The experiment ran for 9 months corresponding to one vegetative cycle of Echinochloa pyramidalis including 1 month of plant acclimatization. All planted containers were planted using nine stems of E. pyramidalis per container. After planting, they were submerged in tap water and kept flooded for a month. After this period, the containers were washed with tap water until the analysis of leachate confirmed that no constituents of concern (e.g., nutrients) were leaching out, as well as that there were not any other differences found among the replicated containers. The triplicated different containers (excluding the unplanted unit) were distinguished on the field with the numbers 1, 2 and 3. The letters A, B and C represented, respectively, free flow, 7 days and 14 days of RT. For any given treatment combination, the containers were fed in a batch mode in order to allow sufficient oxygenation, based on the findings of Kaiser & Fuchs (2015).

The application of FS began after the acclimatization of the young shoots. FS was applied every 14 days for 32 weeks following the nominal load of 200 kg TS m⁻² year⁻¹ (Kengne et al. 2009). Among the FS samples received at the experimental station in vacuum trucks (n = 16), eight samples came from septic tanks (50%), three from traditional pit latrines (18.75%) and five from public toilets (31.25%). Before the application of FS on the bed units, some physicochemical parameters such as nitrogen ammonia (NH₄-N), dry matter (DM) and pH known to affect the helminth eggs survival in FS samples were analyzed (Pecson et al. 2007). Raw FS and leachates were collected and analyzed for physicochemical parameters as outlined in the standard methods for the examination of water and wastewater (APHA/AWWA/WEF 2005). Helminth egg reduction was determined in the incoming sludge and outlet leachates as a function of the retention times tested (free flow, 7 days and 14 days). The ‘free flow’ or free-drainage wetland means without hydraulic retention compared to other operational units operating with hydraulic retention (7 days of hydraulic retention and 14 days of hydraulic retention). Helminth eggs were analyzed according to Schwartzbrod & Banas (2003). At the end of the experiment, three samples of biosolids accumulated on the surface of each bed unit were randomly sampled using the drilling method. Samples were air-dried and sieved before further analysis. Analysis for helminth eggs was performed on approximately 25 g of biosolid mixtures collected in each bed unit according to the retention times tested following the protocol of Schwartzbrod & Banas (2003).

The growth response of E. pyramidalis was measured every 2 weeks during the study by counting the plant density for each entire bed unit. To assess the effect of vegetation on the removal efficiency of the systems, the quality of the outlet leachates of each planted bed unit was compared to the quality of that from the unplanted bed unit (Standard). Effects of RT on the performance of the system were assessed taking into consideration three retention times: free flow, 7 days and 14 days. The ratio between the applied sludge (inlet) and the collected leachates (outlet) was evaluated depending on the hydraulic RT tested. The statistical comparison between the results was done in order to see the effect of the hydraulic RT and vegetation on the performance of the system.

Analysis of variance (ANOVA) at p = 0.05 was carried out to compare the operational behavior data of the system. The evaluated factors were RT and vegetation, while the variables were helminth eggs, pH, NH₄-N and DM. Sixteen measurements were collected (two per month) per variable for each incoming sludge and leachates. Before the ANOVA test, normality and variance homogeneity were tested using the Levene tests. Helminth egg detection frequencies as well as the distribution of their diversity in samples were examined. Descriptive statistics of samples were undertaken to express the parameters' dispersion. Due to the non-normal distribution in the prevalence and diversity of helminth eggs in this study, Kendall's Tau and Spearman's rank tests were computed to assess the relationship between the total helminth eggs in samples as well as the description of the relationship between the helminth eggs and physicochemical parameters of the sludge (NH₄-N, DM, pH and temperature).

The FS originating from septic tanks, public latrines and traditional pit latrines showed a very high variation between the various campaigns as shown in Table 1. The recorded mean concentrations were in the order of 10.30% DM, 0.25 g/L NH₄-N, 31.14 °C temperature and a pH of 7.89. Generally, FS samples in this study were found to contain lower concentrations of the analyzed parameters when compared to manually direct sampled FS from on-site sanitation systems. An explanation for the difference is the dilution factor because usually, water is added to FS during mechanical emptying. Several studies pointed out the influence of emptying modes as factors responsible for the quality of FS (Kengne et al. 2008; Bassan et al. 2013; Eliyan et al. 2021). FS samples from traditional pit latrines showed higher concentrations when compared to samples originating from other on-site sanitation systems analyzed in this study. Indeed, Koné & Strauss (2004) working on the FS treatment options in developing countries, and Letah Nzouebet et al. (2016) have shown that the FS from the traditional pit latrines is very concentrated compared to those from septic tanks.

The helminth egg analysis of the untreated FS revealed high concentrations (Table 2). Even though the number of samples was statistically insufficient to represent the whole city, a total number of 62.51 helminth eggs/g DM was obtained. Among these, seven predominant species of helminth eggs were identified. These species included Ascaris lumbricoides (28.95 eggs/g DM), Enterobius vermicularis (18.06 eggs/g DM), Strongyloides stercolaris (22.68 eggs/g DM), Trichuris trichiura (16.18 eggs/g DM), Schistosoma mansoni (9.96 eggs/g DM), Hymenolepis nana (9.59 eggs/g DM) and Fasciola hepatica (35.88 eggs/g DM).

The leachate samples had a pH ranging from 6.48 to pH ≤ 7.51 (Table 3). Only a slight variation of the pH in leachates from the planted beds was observed independent of the hydraulic retention times tested: 7.65 ± 0.52 (free flow), 7.68 ± 0.57 (7 days of RT) and 7.67 ± 0.51 (14 days of RT). Only the unplanted drying bed showed a slightly lower pH of 7.59 ± 0.46. On an average basis, a significant reduction of the outflow concentrations of DM and NH₄-N was noted in each bed unit irrespective of the hydraulic retention times and growth of plants (p < 0.05). The concentration of NH₄-N decreased from 0.63 ± 0.57 to 0.08 ± 0.06 g/L for free flow, to 0.09 ± 0.06 g/L for 7 days of RT, 0.08 ± 0.06 g/L for 14 days of RT and to 0.09 ± 0.05 for unplanted drying beds (p < 0.05). The DM concentration decreased from 10.53 ± 9.80% to 0.76 ± 0.30% for free flow, to 0.68 ± 0.28% for 7 days of RT, to 0.52 ± 0.22% for 14 days of RT and to 0.93 ± 0.38% for unplanted drying bed (control test). This study revealed that an increase in RT was coupled with the reduction of FS physicochemical parameters. The DM removal is the result of a complex set of internal processes, including the sedimentation and production of transportable solids by wetland biota (Kadlec & Wallace 2008; Vymazal & Kröpfelová 2009). After the treatment in the VFCW, the NH₄-N concentration in the leachate has not met the requirements of the Cameroonian guideline for safe water irrigation in agriculture (<30 mg/L) (MINEPDED 2008).

In this study, sedimentation on the filter media (layer) seems to be the predominant removal mechanism, because the differences between free flow and retention times for longer periods are low, especially for the parameter pH. However, the reduction of NH₄-N in the constructed wetlands is due to many reactions such as chemical reactions and plant uptake (Brix & Arias 2005; Dan et al. 2011).

As shown in Tables 4 and 7, the FS dewatering process on the drying beds was not efficient enough to remove all the helminth eggs. Similar results for helminth egg removal efficiency were obtained in unplanted drying beds (3.36 eggs/g DM) and planted drying beds operated as free flow (3.3 eggs/g DM) (p < 0.05). An increase in helminth egg reduction comes with an increase in the RT. With 7 days of RT, concentrations in the leachate were 2.7 eggs/g DM, and with 14 days of RT, only 2.25 eggs/g DM could be detected. However, the total helminth eggs recorded in leachate samples were observed during the first four campaigns of this study. Kadlec & Wallace (2008) pointed out the decrease observed in the reduction of the total helminth eggs to be the result of the predation, dehydration and RT which are the main mechanisms in planted drying beds. In addition, sedimentation and filtration are also common mechanisms for the elimination of helminth eggs in FS. Tchobanoglous et al. (2003) examined slow sand filtration and reported that helminth eggs and DM are trapped on the surface of the filter. Ingallinella et al. (2002) summarized various reports and showed that the treatment of FS in a planted drying bed reduced the concentration of helminth eggs between 600 and 6,000 helminth eggs/L of FS to 170 eggs/g DM.

The removal efficiencies of physicochemical parameters based on the difference in input and output fluxes showed a great variation within a same pilot bed and between campaigns (Tables 5 and 6). This removal efficiency varies between campaigns of the study. Removal efficiency ranged from 75 to 97% (DM) and from 79 to 96% (NH₄-N). About NH₄-N, the removal efficiencies recorded in this study were greater than those observed with vertical constructed wetlands operating in Europe for wastewater treatment (average of 40%) (Meyer et al. 2012; Kadlec & Wallace 2008). This reduction of nitrogen is due to plant uptake, adsorption, ammonia volatilization and denitrification (Kadlec & Wallace 2008). Looking at the DM, sedimentation could be the main removal mechanism. In tropical regions, it is possible to achieve DM percentages of at least 30% by treating FS on planted drying beds (Kengne et al. 2009).

Contrary to removal efficiencies of the physicochemical parameters, the helminth eggs’ removal efficiencies are mainly due to sedimentation processes. The removal efficiencies of helminth eggs ranged from 82 to 100% independent of the RT-tested and unplanted drying bed unit (Table 7). After the application of the FS on the bed units, helminth eggs are trapped on the surface of the filter beds. Table 7 shows that from campaign 8 onwards, the removal efficiencies for all four different trials are approximately 100%. This result indicates that the RT had an effect on helminth egg reduction (p < 0.05). Indeed, predation, sedimentation and dehydration are the main mechanisms in planted drying beds that result in pathogen reduction in the FS. For Koné et al. (2010) working on the inactivation of helminth eggs within tropical drying beds, the insufficient RT may affect the removal processes (filtration and sedimentation) of helminth eggs. The findings of Sonko et al. (2015) working on the effect of plant species on FS drying and the fate of pollutants in planted drying beds showed that pathogens are mainly trapped inside the sludge cake accumulated on the surface of planted filters. The helminth egg reduction observed for the 7 days and 14 days of RTs could be explained by the long contact time between the helminth eggs adsorbed on the FS surface and the constituents of the system (vegetation and filter media).

Feachem et al. (1983), working on the excreta reuse, report that helminth eggs in comparison with other pathogens are larger in size and are mainly eliminated by the natural processes in planted drying beds. Basically, to remove helminth eggs from FS, it suffices to realize that they are in fact particles forming a fraction of DM contents. This is why helminth eggs’ content is related to the DM content in wastewater, specifically to the number of particles measuring 20–80 μm (Seck et al. 2015). As helminth eggs are particles, mechanisms used to remove suspended solids are also useful for removing helminth eggs from FS.

The presence of plants in the bed units could affect the removal efficiency of NH₄-N and DM (p < 0.05), as the assessment of the performance between the campaigns of this study revealed a very great variation of the removal efficiency in comparison to the unplanted bed (standard). The root system of E. pyramidalis allows retention of particles present in the FS contrary to the unplanted drying bed unit. Furthermore, aquatic macrophytes can affect the redox status of drying bed sediments by releasing oxygen from their roots into the rhizosphere and thereby stimulating aerobic decomposition (Brix & Arias 2005). In this study, the plant density has an indirect effect on the removal of helminth eggs and DM as it is known to increase the clogging phenomena via the development of its root system.

This study constitutes a pilot study in central Africa on the removal of helminth eggs in FS via constructed wetlands. It evaluates the performance of the system implementing different operational strategies (free flow, 7 days of RT and 14 days of RT) as well as the influence of planted unit in terms of FS helminth eggs removal. FS applied on the beds revealed the presence of high parasitic concentrations with seven helminth egg species recorded. The presence of plants has strongly affected the removal of nitrogen ammonia, DM and pathogens (p < 0.05) as compared to the unplanted bed unit (standard). Looking at the helminth egg diversity, a higher prevalence of A. lumbricoides was recorded in samples. The VFCW was not efficient enough to remove all the helminth eggs at the beginning of the experimentation. The biosolids removed from the beds revealed a high concentration of helminth eggs which are not in accordance with the WHO standards guidelines (<1 egg/g of DM) for safe agricultural practices. Biosolids removed from the bed units should undergo the stabilization process (e.g., composting) before agricultural reuse. In order to better master the reduction of helminth eggs in FS via VFCW, other operational parameters such as seasons, substrate, hydraulic load and feeding frequency could be studied for further investigations in large-scale projects.

This research was supported by the German Academic Exchange Service (DAAD) under the program ‘Bi-nationally supervised doctoral degree’, the Swiss National Centre of Competence in Research Programme North–South: Research Partnerships for Mitigating Syndrome of Global Change (NCCR N-S RP08: Productive Sanitation) co-funded by the Swiss National Science Foundation (SNSF), the Swiss Agency for Development and Cooperation (SDC) and the Department of Sanitation of the Swiss Federal Institute for Science and Technology (EAWAG/SANDEC).

We dedicate this article to Prof. Ives Magloire Kengne Noumsi, who passed away on 14 June 2020, for his valuable contribution to fecal sludge management (FSM) in Cameroon and abroad.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.