Promoting the application of anaerobic digestion technology: microbial electrolysis cell (MEC) integration and struvite precipitation

Date

2023-6-16

Author

Kutlar, Feride Ece

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Two novel technologies were studied to promote the application of anaerobic digestion (AD) in this thesis. AD was reconfigured with the microbial electrolysis cell (MEC). This thesis compared two methods for AD-MEC integrated system start up: via the use of bioelectrodes (pre-biofilm formed in a controlled environment) vs. bare electrodes (without biofilm). Among all reactors, graphite-stainless steel bioelectrode AD-MECs fed with cattle manure at -0.95 V (vs. Ag/AgCl) cathode potential produced the highest CH4 yield per gram volatile solids (VS), corresponding to 105% increase compared to conventional AD. Graphite cathode AD-MECs fed with cattle manure generated similar CH4 productions in bioelectrode and bare electrode reactors resulting in 8-10% increase at -0.9 V (vs. Ag/AgCl) cathode potential and 45-51% increase at -1 V (vs. Ag/AgCl) cathode potential. When reactors were fed with wastewater biosolids, Bioelectrode, and pre-biofilm formed granular activated carbon (GAC) combination increased CH4 production by 16% in comparison to the control. Overall, the methane production kinetics were enhanced with the implementation of AD-MECs. The results showed that the reactor enhancements mainly depended on the cathode material and cathode potential. Archaeal community analysis confirmed the dominance of hydrogenotrophic methanogens on the cathode of graphite – graphite AD-MECs fed with cattle manure. The comparison of AD-MEC reactor performances indicated that pre cultivation of biofilm on the electrodes in a controlled environment is not necessary and the use of bioelectrodes does not provide a significant advantage in the performance over bare electrodes. As a post-treatment of AD effluents, struvite precipitation with sustainable additives was investigated in this thesis. Bone meal and waste magnesite powder were used as a replacement for pure P and Mg additives for struvite precipitation from cattle manure digestate. The important parameters and recovery efficiencies were determined via the Box-Behnken model. Under optimal conditions (pH = 9.0, Mg:N = 2.2, and P:N = 1.8), 97.8 ± 0.1% NH4-N, 96.6 ± 0.31% PO4-P, and 84.4 ± 0.9% Mg2+ were recovered. When the precipitated product was examined, X-ray diffraction showed only struvite crystals. The product purity was around 49%, which is relatively high when the use of wastes is considered. Heavy metals in the product were below regulatory limits for fertilizer application. In summary, both technologies promoted the application of AD by providing selective benefits. The proposed processes have the potential to lower the impacts of climate change, reduce nutrient depletion impacts, and help achieve sustainable development goals. Next-generation biogas technology research should focus on commercializing these processes.

Subject Keywords

Microbial electrolysis cells, Bioelectrodes, AD-MEC integration, Industrial symbiosis, Struvite precipitation

URI

https://hdl.handle.net/11511/104805

Collections

Graduate School of Natural and Applied Sciences, Thesis