A review of modified and hybrid anaerobic baffled reactors for municipal wastewater treatment with a focus on emerging contaminants

This review discusses municipal wastewater treatment using anaerobic baffled reactors (ABRs) and modified ABRs. Conventional ABRs can convert organic carbon to renewable energy in the form of biogas. ABRs can achieve more than 90% COD removal at HRT as low as 8 hours at mesophilic temperatures, while COD removal in the range of 70-90% is typical at uncontrolled temperatures. However, effluents from ABRs do not meet discharge criteria and must be polished. Several techniques have been applied to improve the effluent quality including: pre-screening of raw wastewater using a mesh or sedimentation tank, inoculation with acclimatized sludge, effluent recirculation, electrocoagulation, microbial electrodes for improved VFA degradation, COD degradation and methane production, packing materials, carriers or meshes in individual compartments, polymeric membranes in the final compartment or external to the ABR, constructed wetlands and aerobic bioreactors. Recently, much research has focused on concurrent carbon and nitrogen removal in modified ABRs using novel strategies including microaeration, membrane aerated biofilms, an ABR followed by an aerobic membrane bioreactor with sludge recycling, anammox bacteria and nitrite/nitrate-dependent anaerobic methane oxidation bacteria. For P removal, promising chemical techniques include electrocoagulation and biological P removal includes denitrifying phosphate accumulating microorganisms. Some of these techniques applied in independent studies resulted in effluents containing <20 mg L-1 BOD, <1 mg L-1 TN and 0.2 mg L-1 TP, indicating the feasibility of mainstream anaerobic treatment of municipal wastewater, but pilot scale studies on biogas production and C, N and P removal are still lacking. Furthermore, ABRs have also been found to degrade concurrently emerging contaminants in municipal wastewater such as perchlorate, nitrophenols, and antibiotics with no effect on COD removal at typical concentrations found in municipal wastewater, but for some complex organics, an aerobic step is required for the complete oxidation.<br />