Anaerobic Digestion Pasteurization Latrine – Self-sustaining onsite fecal sludge treatment for developing countries

Despite significant advances in public health and engineering over the last 100 years, diarrheal disease remains one of the highest global burdens of disease, particularly for children under 5 years of age. Access to clean water, sanitation, and hygiene greatly reduces this risk, but access to improved sanitation remains a challenge for a large percentage of the world. Due to the lack of access to safely managed sanitation and rapid urbanization, sustainable onsite fecal sludge treatment systems need to be developed and deployed to reduce the burden of diarrheal disease.

The Anaerobic Digestion Pasteurization Latrine (ADPL) is a concept that was developed by Professor Marc Deshusses to meet this need. The goal of the ADPL was to produce a pathogen-free effluent through pasteurization powered by biogas produced from anaerobic digestion of fecal sludge. The concept was supported through laboratory studies on the anaerobic digestion of a simulant fecal sludge and inactivation of E. coli in a pasteurization system using a heater maintained at 65-75 °C and a tube-in-shell counter-flow heat exchanger for heat recovery.

The goal of this research was to build upon initial laboratory-based research on the ADPL and demonstrate the feasibility of the ADPL concept at full-scale in field conditions, simulate improved digester designs to increase digestion efficiency, evaluate digester effluent post-treatment for residual organic and nutrient removal, and develop a remote data acquisition and controls system to improve system understanding and operation of the pasteurization system. The desired outcome of this work is a complete, self-sustaining system that efficiently digests fecal sludge for maximum biogas production, produces a polished effluent that can be reused, and pasteurizes the effluent efficiently and reliably, all while being low-cost with minimal operation and maintenance requirements.

Two ADPL systems were installed on residential plots with 15-35 residents in a peri-urban area outside of Eldoret, Kenya. Each system was comprised of 3 toilets built above a floating dome digester and heat pasteurization system. The ADPLs are simple systems, with no moving parts and relying on gravity-induced flows. Adoption at two sites was successful, and residents reported that the systems had little to no odor or flies, and the residents were interested in the possibility of excess biogas and effluent reuse. The ADPLs were monitored daily for biogas production and temperatures in the pasteurization system. The ADPL serving 35 residents produced on average 350 Lbiogas d-1, and the temperature in the heating tank was greater than 65 °C on 87% of sampling days. The treated effluent was analyzed periodically for chemical oxygen demand (COD), biochemical oxygen demand (BOD), total ammonia nitrogen (TAN), and pH. On average, the effluent contained 4,500-5,600 mg COD L-1 (an 87-89% reduction of the estimated input), 2,000-3,900 mg BOD L-1, 2,400-4,800 mg NH3-N, and had a pH of 7.4-7.7. Results from this field study show that anaerobic digestion of minimally diluted fecal sludge can provide enough energy to pasteurize the effluent, and that the ADPL can be a suitable option for onsite fecal sludge treatment.

Three variations of a 2 m3 anaerobic digester were simulated with a flow of 120 Lwater d-1 – a reactor with no internal baffle walls (CSTR), a reactor with baffle walls that forced flow to wind in the xy-direction (HABR), and a reactor with baffle walls that forced flow to wind in the xz-direction (ABR). Results showed that increasing the number of baffle walls significantly improved the hydraulic performance of the reactor in terms of residence time, dead space, and Morrill Index. Adding angled portions to the end of baffle walls and adjusting the D:U ratio in the ABR had minimal impact while a variable inflow had a moderate impact on performance. Overall, these results suggest that adding 3-5 baffle walls inside of an anaerobic digester would greatly improve the digester’s hydraulic efficiency and better utilize the reactor volume. These adjustments would thus cause enhanced solids removal and digestion efficiency, resulting in higher biogas production and a cleaner effluent. However, simulation work including solids and biological reactions would be beneficial to future reactor design considerations.

The biological filter study analyzed the treatment of high-strength anaerobic digester effluent using trickling filters for nitrification and then submerged attached growth filters for denitrification. Five media types were tested in the trickling filters (8 L volume): biochar, granular activated carbon (GAC), zeolite (clinoptilolite), Pall rings, and gravel. Five columns were tested for denitrifying filters (4 L volume) using sand, bamboo wood chips, eucalyptus wood chips, bamboo with sand, and eucalyptus with sand. Wood chips were used in denitrifying filters as a supplemental carbon source for denitrification. From six months of operation, biochar, GAC, zeolite, Pall rings, and gravel media had turbidity removal efficiencies of 90, 91, 77, 74, and 74%, respectively, and NH3-N removal efficiencies of 83, 87, 85, 30, and 80%, respectively. The primary mechanism for ammonia removal was nitrification to nitrate, but some adsorption was seen in biochar, GAC, and zeolite filters. From four months of operation, sand, bamboo, bamboo with sand, eucalyptus, and eucalyptus with sand filters had NO3-N removal efficiencies of 30, 59, 51, 31, and 30%, respectively, and turbidity removal efficiencies of 88, 89, 84, 89, and 88%, respectively. Bamboo had the greatest NO3-N removal rate at 0.054 kg N m-3 d-1 and released more COD than eucalyptus (0.076-0.120 gCOD gbamboo-1 compared to 0.012-0.043 gCOD geucalyptus-1). Biochar and bamboo were selected as the best media types from this study for the nitrification and denitrification filters, respectively, due to their low-cost and sustainable supply. Based on an average initial influent of 600 mg NH3-N L-1 and 980 NTU, the biochar filter’s expected effluent would be 97 mg NH3-N L-1, 450 mg NO3-N L-1, and 120 NTU. The bamboo filter would then produce an effluent of 82 mg NH3-N L-1, 180 mg NO3-N L-1, and 13 NTU. This theoretical combined performance would thus result in 56% removal of total N and 98.7% removal of turbidity. Based on nitrate removal rate, full denitrification could be achieved by doubling reactor volume. Total nitrogen removal efficiency of 80-90% could thus be achievable. These filter media were successful in treating high-strength digester effluent and present an alternative for sustainable, low-cost, and low-maintenance post-treatment options for nitrogen management.

A low-cost data acquisition and controls system with remote, real-time data access was developed using the Particle Electron. This device records temperature and liquid flow data while controlling a gas valve and igniter as part of pasteurization system. The device was tested in lab and field conditions. The power consumption is low, 34 Wh per day, and data acquisition matched the results of standard laboratory devices. The field deployment (Eldoret, Kenya) successfully operated the pasteurization system in its target range while reporting real-time data. This low-cost and low-power device has improved the operation of the onsite pasteurization system, and adaptations of the device would be valuable in many other onsite fecal sludge treatment systems.

Together, these objectives have demonstrated the ADPL concept works in field conditions, digester performance can be improved with simple modifications, digester effluent can be further treated to encourage reuse or for safe disposal with biological filters using sustainable media that have low operational requirements, and low-cost controls can improve the pasteurization system efficiency and reliability while generating more data to expand understanding of the system.