Treatment of high strength acidic wastewater using passive pH control

A passive pH control system to process high strength (COD ∼ 20,000 mg l−1) acidic wastewater without alkali addition was developed. The low-energy wastewater treatment system consisted of a packed bed up-flow anaerobic reactor followed by an aerobic trickling filter, both of which used biochar as the biofilm support media. The passive pH control process relied on controlled recycle of the effluent leaving the anaerobic stage to adjust the pH of the acidic feedstock. The anaerobic reactor was able to achieve steady state reductions in total chemical oxygen demand (TCODred) above 91% and the combined system (anaerobic-aerobic) achieved COD reductions above 97% without alkali addition. Methane production was 0.311 m3 per kg COD reduced. The system was also able to produce an effluent that met EPA regulations in terms of BOD5 and TSS, possessed a near neutral pH, and was highly nitrified.