Microbial electrochemical cells (MxCs) are a novel technology that use anode-respiring bacteria (ARB) to bioremediate wastewaters and respire an electrical current, which can then be used directly to produce value-added products like hydrogen peroxide (H2O2). Ninety-five percent of the world’s H2O2 is currently produced using the anthraquinone process, whose production requires expensive and potentially carcinogenic catalysts and high amounts of electricity. However, the amount of H2O2 that can be produced from these microbial peroxide-producing cells (MPPCs) has not been thoroughly investigated. Predicting potential H2O2 production in MxCs is further complicated by a lack of mathematical models to predict performance utilizing complex waste streams like primary sludge (PS).

A reactor design methodology was developed for MPPCs to systematically optimize H2O2 production with minimal energy consumption. H2O2 stability was evaluated with different catholytes, membranes, and catalysts materials, and the findings used to design and operate long-term a dual-chamber, flat-plate MPPC using different catholytes, ferrochelating stabilizers, and hydraulic retention times (HRT). Up to 3.1 ± 0.37 g H2O2 L-1 was produced at a 4-h HRT in an MPPC with as little as 1.13 W-h g-1 H2O2 power input using NaCl catholytes. Attempts to improve H2O2 production by using weak acid buffers as catholytes or ferrochelating stabilizers failed for different reasons.

A non-steady-state mathematical model, MYAnode, was developed combinging existing wastewater treatment, anode biofilm, and chemical speciation models to predict MxC performance utilizing complex substrates. The model simulated the large-scale trends observed when operating an MPPC with PS substrate. At HRTs ≥ 12-d, the model demonstrated up to 20% Coulombic recovery. At these conditions, ARB required additional alkalinity production by ≥ 100 mgVSS/L of acetoclastic methanogens to prevent pH inhibition when little influent alkalinity is available. At lower HRTs, methanogens are unable to produce the alkalinity required to prevent ARB inhibition due to washout and rapid acidification of the system during fermentation. At ≥ 100 mgVSS/L of methanogens, increasing the diffusion layer thickness from 500 to 1000 μm improved Coulombic efficiency by 13.9%, while increasing particulate COD hydrolysis rates to 0.25/d only improved Coulombic efficiency by 3.9%.

To address sustainability issues in wastewater treatment (WWT), Siemens Water Technologies (SWT) has designed a "hybrid" process that couples common activated sludge (AS) and anaerobic digestion (AD) technologies with the novel concepts of AD sludge recycle and biosorption. At least 85% of the hybrid's AD sludge is recycled to the AS process, providing additional sorbent for influent particulate chemical oxygen demand (PCOD) biosorption in contact tanks. Biosorbed PCOD is transported to the AD, where it is converted to methane. The aim of this study is to provide mass balance and microbial community analysis (MCA) of SWT's two hybrid and one conventional pilot plant trains and mathematical modeling of the hybrid process including a novel model of biosorption. A detailed mass balance was performed on each tank and the overall system. The mass balance data supports the hybrid process is more sustainable: It produces 1.5 to 5.5x more methane and 50 to 83% less sludge than the conventional train. The hybrid's superior performance is driven by 4 to 8 times longer solid retention times (SRTs) as compared to conventional trains. However, the conversion of influent COD to methane was low at 15 to 22%, and neither train exhibited significant nitrification or denitrification. Data were inconclusive as to the role of biosorption in the processes. MCA indicated the presence of Archaea and nitrifiers throughout both systems. However, it is inconclusive as to how active Archaea and nitrifiers are under anoxic, aerobic, and anaerobic conditions. Mathematical modeling confirms the hybrid process produces 4 to 20 times more methane and 20 to 83% less sludge than the conventional train under various operating conditions. Neither process removes more than 25% of the influent nitrogen or converts more that 13% to nitrogen gas due to biomass washout in the contact tank and short SRTs in the stabilization tank. In addition, a mathematical relationship was developed to describe PCOD biosorption through adsorption to biomass and floc entrapment. Ultimately, process performance is more heavily influenced by the higher AD SRTs attained when sludge is recycled through the system and less influenced by the inclusion of biosorption kinetics.