F/M and microfauna proliferation - Water and Wastewater.com Help Forum

Question by Purewaterplus

the sludge concentration in a SBR was increasing because no sludge was removed, and accordingly the sludge loading rate(F/M) decreased from 1.5 to 0.2 gCOD/gMLVSS, I observed that abundant protozoan and metazoan microfauna proliferated when the F/M was in the range of 0.7-0.4. but, when the F/M continued to decrease, nematodes and metazoan fauna disappeared. Could you please give an explanation why higher level fauna disappeared when lower F/M applied?

Answer by Victor Santa Cruz
There are several parameters (environmental factors) that are important for determining which organisms will be present and they are (but not limited to) F/M, detention time (HRT,SRT,MCRT, sludge age), type of food available, and growth rate. F/M and detention time are interrelated. In your case, as decreased your F/M ratio, meaning that there was less food per microorganism, you invariably affected the chances that an organism will statistically come into contact with food. This said, when there is a lot of food available (high F/M) SRT's in the 2-3 days range, the organisms that have a short reproduction span and can take in large amounts of food within a short period of time will proliferate--this includes single celled bacteria such as coliforms, flagellates, and amoebas. Growth rate for these organisms is very high and some will reproduce within an hours time. If you are constantly wasting solids to preserve this F/M ratio you will waste any organisms that cannot reproduce within 2-3 SRT days. As you decrease your F/M, there is less food available but your detention time has increased. This increase in detention time of the solids allows other organisms to not only grow but gives them the time to multiply. Organisms that require longer detention times to mature and become sexually active include the higher life forms such as the metazoans (bristtle worm, other nematodes, rotifers...). When you come to an F/M where there is very little food available per given organism, only those organisms that have very small growth rates (and this includes some notorious filamentous organisms) you will have the type of environment where overoxidation occurs. The mixed liquor will look gray to black in coloration and it will settle fast but leave a the supernatant turbid. When there is very little food available, organisms such as rotifers and nematodes will not have sufficient food to reproduce and will quickly die off. And as Grrun has consistently preached, sludge age is perhaps one of the most important parameter in wastewater treatment but perhaps one of the least understood. Lastly, type of food available to the organisms is most important to those industries that create/process products such as sugars, alcohols, wood. In these industries, nutrient ratios are important for the organism's ability to grow and multiply.

Further comment by Victor Santa Cruz
The answer is very simplistic and it does not take into consideration temperature and kinetic effects though.