In Part 1 of this series, we discussed the role of Biochemical Oxygen Demand (BOD) loading on biological activity in aerated stabilization basins (ASBs). In this article, we will discuss pH in the context of a biological growth pressure.
Growth Pressure #2: pH
The pH of the environment has a profound effect on the rate of microbial growth. As with temperature, pH affects the function of metabolic enzymes. Acid conditions (low pH) or basic conditions (high pH) alter the three-dimensional structure of the enzyme and can stop microbial growth. Most microorganisms do well within a pH range of 6.5 to 8.5. As long as bulk water pH remains in the range of ~5.5 – 9.5, biological activity will continue and generally buffer the system toward neutral. However, if bulk water pH goes below 4.5 or above 10.2, significant biological inhibition will occur and BOD conversion will suffer until acceptable pH levels return.
However, as with temperature, some enzyme systems can tolerate extreme pH and thrive in acidic or basic environments. Fungi, for example, do well in acidic conditions. Most bacteria and protozoa, however, grow best in neutral pH conditions. Abnormal pH in biological treatment processes can lead to a significant decrease in the removal rate of organic compounds and result in a predominance of undesirable types of organisms.
Many mills must control the pH of their effluent within an acceptable range before it is mixed with biomass in the ASB. Even short-term exposure (less than one minute) to extreme pH causes significant microbial destruction. Some mill effluents are slightly on the basic side (pH 9 – 10.5). Because bacteria generate CO2 (an acidic gas) as a by-product of metabolism, they will self-regulate the pH to some extent, as long as the pH is not so severe it completely stops metabolism.
Flow pattern and aeration basin configuration play a role in the importance of pH control. In systems with complete-mix characteristics in the early part of the ASB, a short-term spike of high or low pH influent will be diluted or neutralized by the bulk water in the aeration basin itself. In a plug-flow situation, there is less potential for mixing of extreme pH influent with the bulk water in the aeration basin.
While all eight growth pressures are important, pH is one of the most critical. If bulk water pH is not maintained within the acceptable range, other growth pressures such as dissolved oxygen or nutrients become irrelevant.
This article is adapted from “Aerated Stabilization Basins in the Pulp and Paper Industry” by Paul Klopping and Mike Foster. Copyright 2003. Callan & Brooks. For information on purchasing a copy of this manual, contact EBS.