Short-term effects of low pH on the microfauna of an activated sludge wastewater treatment system

Abstract

Optimum pH for biological (e.g., activated sludge) wastewater treatment is stated to lie between pH 6.5 and 8.0; however, the pH of processed effluent from thermomechanical pulp mills is closer to 4.5 and 5.5. Consequently, pH adjustment of effluent is required with associated costs. The ability of the microfaunal community (protozoa and metazoa) of activated sludge to survive at pH levels below 6.5 was evaluated with samples collected from Corner Brook Pulp and Paper Ltd. (Newfoundland, Canada). Effect of pH was examined at "high pH" (4.5, 5.5 and 6.5 control) and "low pH" (2.5, 3.5 and 6.5 control) under "summer" temperatures of 30°C and "winter" temperatures of 15°C, with impacts assessed after 1 h and 24 h exposure. Effect of pH was found to be temperature-dependent: pH levels down to 4.5 appeared to have little impact on microfaunal abundances at 30°C, but a number of microfauna were negatively affected at 15°C. Low pH levels of 2.5 and 3.5 were detrimental to the population densities of most microfauna. Adverse pH effects were more marked with increased exposure in some cases. An acid-neutralizing ability may be inherent in the activated sludge, as treatment pH increased over 24 h.