Extended Aeration Activated Sludge Problem
Relatively new operator here just took over as lead from all the old operators retiring and looking for some outside opinions from people with more extended aeration activated sludge experience.
We run a small extended aeration plant rated around 400,000 USG/day, and I inherited what appears to have been a heavily over-wasted system from our previous head operator. Since spring flows increased, it feels like we’ve been stuck walking a tightrope between very young sludge and hydraulic overload conditions.
What I’m consistently seeing:
Persistent pin floc in the clarifier
Fine floc carryover during higher flow periods
Poor settleability once MLSS starts climbing
Difficulty retaining solids long enough for the biomass to mature
As soon as MLSS gets into the low 2000s, clarifier performance seems to deteriorate again. The last few years as when the plant was running well, we would operate with an MLSS as high as 6000mg/L successfully. Now we keep washing out and have been as low as 1600.
Aeration setup is three aspirating aerators plus one splashing aerator. Lately I’ve been trying to run gentler on the floc by operating two aspirators at night and three during warmer daytime conditions while leaving the splasher off entirely. The aeration tank is also fairly foamy right now, which seems to support the idea that the sludge is still very young.
Our D.O is in a healthy range between 1.6-2.6m and the PH is maintain between 7.1-7.5.
One challenge is our RAS control is limited — no VFDs, so operationally it’s basically either one RAS pump or two pumps on, with not much fine adjustment available between those points.
Peak spring flows are pushing the plant hydraulically pretty hard at times, and despite trying multiple approaches to retain solids and slowly rebuild the biomass, the floc just doesn’t seem to stabilize before we start losing it again.
At this point I’m trying to determine whether:
1.)The primary issue is still under-aged sludge from historical over-wasting
2.)Hydraulic loading is preventing proper floc development and settling
3.)Or both issues are feeding into each other and keeping the plant unstable
I do have limited ability to use our digester temporarily as an aerated holding tank when absolutely necessary to avoid losing solids, but otherwise options are fairly limited.
Has anyone dealt with a similar situation where the plant can’t seem to “break through” into a stable sludge age? Especially where MLSS begins to recover, then settleability falls apart again before the biomass can mature?
I have attached a picture of the fine solids loss that we start getting at low flow, which leads to a rolling loss later in the day as flow increases. Any thoughts or suggestions would be appreciated.