Use of pH as Control Parameter for Aerobic/Anoxic Sludge Digestion

Abstract

Alternating aerobic/anoxic (A/A) digestion of waste activated sludge offers many advantages over traditional (aerobic or anaerobic digestion) methods. However, one of the main concerns in A/A digestion is the development of an on-line control strategy for alternating aerobic and anoxic cycles. Computerized pH monitoring has revealed two well-defined control points that correspond to complete nitrification (ammonia valley) and denitrification (nitrate apex). The pH control points were consistent with those previously identified using the oxidation reduction potential. Based on these findings, two real-time control strategies were developed and tested. First, pH was allowed to increase (anoxic) and decrease (aerobic) within preset limits to control A/A cycling. A second approach detected the points of complete nitrification and denitrification using the differential of the pH signal. Comparison of the A/A digestion results with aerobic digesters indicated similar mixed-liquor volatile suspended solids and fecal coliform destruction with aerobic digestion but with better total nitrogen removal and superior dewatering characteristics.