What causes low pH in an aeration tank?
A couple/few things to consider:
-what is the influent pH?
-What is the influent ammonia concentration? The nitrification process (conversion of ammonia to nitrate) can be alkalinity intensive and impact pH.
-what does your denitfication efficiency look like? The denitrification process (converting nitrates to nitrogen gas) can recover some of the alkalinity spent in the nitrification process and assist in stabilizing pH.
-Are you adding and chemicals? Some chemical (e.g. ferric chloride) can chew up alkalinity and impact pH stability.
-What is your influent and/or process basin alkalinity?
You want to look for and treat the problem at its source.
The current characteristics of the industrial inlet at the plant might account for the lowpH—and probably other parameters at the wastewater inlet.
I think you should get in touch with the industrial plant and share your concerns with them. They need to keep you informed of any activities underway such as maintenance, emptying, cleaning, and so forth, especially any operations undertaken recently.
What does this plant do?
Also look into whether there has been a disruption in the injection of any reagents used to eliminate phosphorus.
Apologies for the lack of specifics.The plant in question is in the agri-food business, specifically potato processing.
Upstream of the aeration tank, there is a UASB-type methane reactor, which is working just fine (pH 7.5).
It also collects water from the centrifuge (pH 6.5 but usual)
Three weeks ago, we had a problem with one of the treatment steps in the facility: really dirty water flowed directly into the aeration tank. Might that be the source of the problem, even after all the time elapsed?
I will take your advice and perform a physical follow-up as well as discuss the matter with the industrial plant in case he has forgotten to tell me something.
Nitrification releases H+ ions, which increase the acid content.
The presence of carbonates provides a buffer against pH changes and, if necessary, an alkaline compound can be added to counter the consumption of calcium carbonate that occurs with oxidized NH4 +
My guess is the inflow with à high loaded water accounts for the drop in pH.
Problems also arise in agri-food effluents during tank washing when strong acidic or basic solutions are used to flush out fats and grease; sometimes this is done several times a day. In addition, digestate and supernatant produced through methanogenesis can also contain acidic compounds, and if your effluent is too poorly buffered, nitrification will only push pH lower.
At pH = 6, nitrification is completely inhibited–hence the problem—if the need is urgent, might be better to inject sodium bicarbonate (NaHCO3) to raise the pH above 7.