Novel Applications of Waste Treatment Technologies to Generate Energy and Treat Water

Abstract

This research sought to maximize energy generation and optimize water treatment from a combined waste stream of food waste and blackwater (FW-BW) and monitored a farm-scale anaerobic digestion (AD) incorporating co-digestion and composting. The FW-BW substrate was gravity separated to obtain a liquid fraction (~90% of the volume) and a solid fraction (~10%). Solids and liquids were pretreated with hydrodynamic cavitation (HDC) to investigate changes in water treatment and energy generation from this pretreatment strategy. Energy generation from the solid fraction was assessed using biochemical methane potential tests (BMP) that quantified methane (CH4) production from AD and integrated a combined anaerobic digestion-microbial electrolysis cells (AD-MEC) design, which operated at various voltages (0.5, 0.9, and 1.2 V). Electrocoagulation (EC) at various voltages (10-25 V) and timeframes (15-90 minutes) was assessed for contaminant removal from the liquid fraction. AD at mesophilic conditions (35 °C) of solids after HDC (post-HDC solids) generated 81.2% of the cumulative CH4 (348 mL CH4/g volatile solids (VS) in 10 days. At longer digestion times (30 days), post-HDC solids generated significantly more CH4 (63%, 429 mL CH4/g VS) than solids before HDC (no-HDC solids) due to increased substrate availability and degradability. Energy generation from AD-MEC at 1.2 V was not significantly different from AD, with only 12.7% more CH4 (292 mL CH4/g VS) generated from post-HDC solids compared to no-HDC solids (259 mL CH4/g VS) after 10 days. Electrocoagulation conducted at 15 V for 90 minutes removed 96.2% of chemical oxygen demand (COD) and 100% of total suspended solids (TSS) from post-HDC liquids. Increasing EC conductivity with electrolytes decreased the timeframe (15 minutes) and voltage (10 V) needed for COD removal (66%) via increased contaminant flocculation. The performance of a farm-scale AD system co-digesting FW and dairy manure (DM) was monitored and verified by analyzing the substrates for nutrient and solids content. A combined heat and power generator produced electricity from the biogas. The system parameters were monitored with an online data-logging system that collected data every 15 minutes for biogas, temperature, hydrogen sulfide (H2S), energy generation, and CH4 content. A life cycle assessment (LCA) was applied to explore the environmental impacts of the current condition compared to four alternative scenarios. The LCA suggested the co-digestion system as currently operated had the largest reductions in environmental impact in 8 out of the 10 impact categories compared to a baseline scenario (no digestion), generating substantial reductions in global warming (81%), eutrophication (442%), and acidification (321%). Using AD can successfully convert various wastes into energy, with HDC increasing overall energy production. Moreover, farm-scale AD showed substantial reduction in global warming.