Speaker: Anna Kogler
Flexible electrochemical stripping (FECS) was demonstrated to effectively remove and recover ammonia nitrogen as acidic ammonium sulfate fertilizer and alkaline ammonia (a disinfectant or feedstock for other chemical synthesis). When treating fecal sludge treatment plant (FSTP) effluent in batch mode, the process achieved high removal and recovery efficiencies (100% removal and 97% total recovery with 13% recovery as ammonium sulfate and 84% recovery as ammonium hydroxide). Tests further demonstrated the ability to select between acidic and alkaline ammonia products by simple process control, which facilitates tailoring product distribution to local and dynamic markets to maximize revenue.
Nitrogen in fecal sludge can harm human health and the environment because fecal sludge treatment plants (FSTPs) generally do not perform nitrogen removal. Moreover, 4.5 billion people worldwide lack access to adequate sanitation. Recovering nitrogen for beneficial reuse in a circular economy can reduce detrimental environmental impacts and generate revenue that could finance the expansion of sanitation systems.
Anaerobic treatment of fecal sludge produces an ammonia-rich liquid stream – a potential source of valuable ammonia-based products: acidic ammonium sulfate (fertilizer) and alkaline ammonia (disinfectant or feedstock for synthesis of other chemicals). Electrochemical stripping was previously demonstrated to effectively recover ammonia nitrogen from urine (Tarpeh et al., 2018), hydrothermal liquefaction effluent (Li et al., 2018), and other complex influents as ammonium sulfate (Figure 1). This study demonstrated customizable acidic ammonium sulfate and alkaline ammonia recovery using novel flexible electrochemical stripping (FECS) to treat FSTP effluent.
Materials and Methods
Experiments were carried out in a four-chamber electrochemical reactor. Electrodialysis and membrane stripping recovered ammonia nitrogen from influent fed into the first chamber (Figure 2). Total ammonia nitrogen (TAN) concentration was measured in each chamber at regular time intervals during continuous and batch experiments via ion chromatography to determine removal and recovery efficiency.
Results and Discussion
FECS was first operated a concurrent mode with electrical potential applied to all chambers. During batch treatment of FSTP effluent, complete removal of nitrogen from chamber 1 occurred within 1 hour (Figure 3A, B). Within 9 hours, a total recovery efficiency of 97% was achieved with 13% recovery as acidic ammonium (chamber 3) and 84% as alkaline ammonia (chamber 4). Similar results were observed during treatment of ammonium phosphate solution (Figure 3B), suggesting that the complex FSTP effluent matrix did not significantly interfere with nitrogen removal and recovery.
FECS was also operated in batch mode with staggered operation (Figure 3C): electrical potential was applied to chambers 1 and 2 for the first 5 hours, and for the next 4 hours electrical potential was applied to all chambers. Complete removal was achieved within 1.5 to 2 hours. After 5 hours, total recovery efficiency reached 80% with 75% recovered as acidic ammonium and 5% recovered as alkaline ammonia. After the next 4 hours, total recovery efficiency increased to 90% with 65% recovered as alkaline ammonia and 25% recovered as acidic ammonium. Staggered operation of FECS reduced overall recovery slightly compared to concurrent mode but allowed selection of a specific product: mainly acidic ammonium when electrical potential was applied to only the first two chambers and mainly alkaline ammonia when electrical potential was applied to all chambers.
Our partner, Delvic Sanitation Initiatives (DSI), currently operates nine FSTPs throughout Senegal and emphasizes fecal sludge valorization. DSI has recovered multiple products from the solids portion of fecal sludge: fertilizer, biochar, and compost used to produce crops (e.g., papayas, eggplants). The liquid portion of fecal sludge is discharged or sent to wastewater treatment plants for further treatment. Diversifying DSI’s product portfolio to include both acidic and alkaline ammonia liquid products would not only reduce environmental impacts of FSTP discharge but also increase the profitability of FSTPs by providing local sources of fertilizer, disinfectant, and a precursor to a variety of nitrogenous compounds, such as nitric acid, amino acids, and polymers.
In partnership with DSI, long-term field trials at FSTPs in Dakar will inform scale-up of FECS and operation and maintenance requirements. Product characterization and market research on fertilizers and disinfectants in Senegal will inform business models for recovered products and support implementation of FECS at DSI’s facilities. We expect FECS to effectively and selectively recover ammonia nitrogen from versatile influents as electrochemical stripping does. We further anticipate translation of FECS to fecal sludge management in broader global contexts, facilitating resource recovery tailored to meet local product demands.
Li Y. Tarpeh W.A. Nelson K.L. and Strathmann T.J. (2018) Quantitative Evaluation of an Integrated System for Valorization of Wastewater Algae as Bio-oil Fuel Gas and Fertilizer Products. Environ. Science Technol. (52) 12717-12727
Tarpeh W.A. Barazesh J.M. Cath T.Y. and Nelson K.L. (2018) Electrochemical Stripping to Recover Nitrogen from Source-Separated Urine. Environ. Science Technol. (52) 1453-1460