Activated sludge process is turbid. A microscopic exam shows 60% rotifers (old sludge), 30% nematodes (old sludge), 10% stalked colonizers. What action should be taken?

• A. Increase aeration
• B. Increase WAS
• C. Decrease aeration
• D. Decrease WAS

Answer: (B)

When the activated sludge shows a high proportion of larger, slow-growing organisms like rotifers and nematodes, it signals a long solids retention time and an aging biomass. An older sludge tends to be less effective at rapidly settling and can keep the mixture turbid because the community has matured beyond the most responsive, fast-growing bacteria.

Increasing wasted activated sludge reduces the solids retention time, refreshing the biomass with younger cells. This helps restore better floc formation and settling, which reduces turbidity. By removing a portion of the old sludge, you shift the microbial community toward organisms that settle more readily and respond quickly to available substrate, improving overall clarifier performance.

Decreasing aeration or decreasing WAS would either not address the aging biomass or would worsen it, and increasing aeration alone doesn’t directly fix the aging sludge issue. Decreasing aeration could reduce oxygen supply and harm treatment, while decreasing WAS would lengthen sludge age further, exacerbating the turbidity problem.