Many wastewater operators consider themselves “zoologists”. However, unlike at zoos, where the species being looked after are lions, tigers, and bears (oh, my!), wastewater operators are responsible for maintaining the stability and growth of “critters” they can’t even see – microorganisms. But, oh, what amazing life and activity that can be seen when examining a wastewater sample under a microscope! It’s a whole different world!
The processes involved in recovering clean water and valuable carbon, nitrogen, and phosphorous by-products from wastewater are actually very elegant. In the traditional, conventional approach, as wastewater enters a facility, the biggest contaminants are removed using grit filters and screens, followed by primary clarifiers where the heaviest of the suspended solids are allowed to settle to the bottom as “primary sludge”, allowing the remaining soluble and suspended contaminants to travel onward with the water. In the next stage, hungry microorganisms consume the remaining suspended organic contaminants in the wastewater through aerobic and anoxic processes, and in so doing multiply to tremendous numbers – called “activated sludge”. In a properly designed secondary treatment process, the activated sludge so effectively removes the wastewater contaminants that the bacteria can actually become cannibalistic toward each other. Upon recycling this activated sludge to the front end of the process, the extremely hungry bacteria are ready to consume the fresh wastewater contaminants entering the system, and the whole cycle repeats itself – with extremely clean water being the end result.
Over time, the volume of activated sludge grows so large that some need to be removed from the system. Therefore, a percentage of the activated sludge is separated from the secondary system and joined with the primary sludge from the beginning of the process. These two sludges are typically fed into another microbiological system called an anaerobic digester, to be reduced in number and converted into methane–biogas, “green” energy.
How wastewater microorganisms can lower the pH
All microorganisms display certain types of metabolic activity, meaning that they have the ability to chemically react with certain contaminants in the wastewater to reduce the contaminant into a smaller substrate. These chemical reactions driven by wastewater microorganisms can have an impact on the pH of the overall water system. For instance, the microbiological activities involved in removing nitrogen from wastewater are called nitrification and denitrification. In nitrification, bacteria convert ammonia and similar nitrogen compounds into nitrite (NO2–) and nitrate (NO3–). These reactions consume alkalinity, which can result in a drop in pH. If not countered by feeding an alkaline additive into the wastewater, the pH can fall to a level that is below that for the bacteria to continue to perform their functions. If the pH falls too far, it can actually lead to microorganism death.
As was mentioned previously, the industry standard for treating low pH is to feed caustic soda (NaOH), which is hazardous for the employees needing to store and feed it. It is also hazardous for the microorganisms, as NaOH kills any bacteria it encounters upon entering the wastewater stream until it becomes diluted to the desired pH. At that point, its job has been completed and it has no lingering buffering ability. Lastly, the increased sodium ion (Na+) concentration can become a detriment to microorganism activity and can interfere with the formation of beneficial flocs for solids settling and dewatering.