Wastewater treatment often consists of removing heavy metals and/or organic compounds from effluent streams. pH adjustment by addition of acidic/basic chemicals is an important part of any wastewater treatment system as it allows dissolved waste to be separated from water during the treatment process.
The pH of water can be used to kill off bacteria in wastewater in addition to the treatment. Most organic matter and bacteria we are familiar with and contact daily are best suited to a neutral or slightly basic environment. At an acidic pH the excess hydrogen ions begin to form bonds with and break down the cell, slowing their growth or killing them outright.
After a wastewater treatment cycle the pH must be raised back to neutral by use of additional chemicals or it will continue to damage any living cell it contacts.