Processing Magazine

Activated-sludge troubleshooting guidelines

Someone recently asked, “What are some troubleshooting guidelines or procedures for overall process control of activated sludge?”

June 18, 2014

Be assured of one thing, for any given observation, challenge or problem, probable causes exist, as well as suggested remedies and corrective measures.

To solve a problem using guidelines and procedures: 1) identify the problem, 2) determine the most likely cause and 3) consider suggested remedies or corrective measures.

In identifying a problem, visual observation of the treatment process is essential and settle-ability tests are vital. Settling-test observations lead to appropriate remedies and corrective measures. Clues that there may be a problem include a cloudy effluent, pin floc or stragglers in effluent, ash on clarifier surface or floating solids after extended settling time.

For any conventional activated-sludge process, the probable causes of cloudy effluent include that mixed-liquor suspended solid (MLSS) in the aeration tank is low due to process start-up, organic-loading increase, toxic-shock loading or over-aeration, causing mixed liquor floc to shear or improper DO levels maintained in aeration tank.

Abounding remedies

Excess organic load can be remedied by reducing the waste-activated sludge (WAS) rate by an amount less than 10% per day, to return to proper loading parameters and increase the returned activated sludge (RAS) rates. About a 30 percent level of settled solids in the clarifier should be established and maintained.

The probable causes of pin floc or stragglers in effluent include the onset of de-nitrification or excessive grease amounts in the mixed-liquor suspended solids (MLSS). An industrial waste monitoring and enforcement program can at least minimize grease going into the aeration system.

The cause of ash on clarifier surfaces may be the aeration tank approaching under-loaded conditions by operating with high MLSS because of old sludge in treatment process.

Remedies for ash on clarifier surfaces, caused by aeration tank foaming, include adjusting RAS rates to maintain settled-solids levels in the clarifier of approximately 30 percent; or adjust MLSS and RAS concentrations, as well as dissolved oxygen (DO).

The probable causes for any floating solids evident after an extended settling time of one or two hours include that de-nitrification or septicity are happening in the clarifier.

Floating-sludge clumping due to septicity may be remedied by maintaining DO at a minimum level of 1.0 Mg/L, along with making sure adequate mixing is occurring in the aeration tank. Alternatively, adjust RAS rate to maintain a level of settled solids depth in clarifier of approximately 30 pecent.

Extended aeration

The below is particular to extended-aeration activated-sludge processes.

1. Some plants, due to clarifier and sludge-return system design won’t allow maintenance at 30 percent of settled solids.

2. A small amount of shiny, dark-tan foam is acceptable on extended-aeration plant aeration basins.  This is primarily true in the winter, when increased solids concentrations are often required.

3. Nitrification, which is the conversion of influent ammonia to nitrites and nitrates, is normal and expected in extended aeration plants, most especially in summer.  If sludge is rising and clumping in the secondary clarifier, increase return sludge rates and or lower aeration rates a little.  Lowering sludge age can also help, if it is a little too high.

4. Don't forget that slow-settling sludge can be caused by both old and young sludge. Perform a diluted settle-ability test of 50 percent MLSS and 50 percent clarifier supernatant to determine whether more or less wasting is appropriate.

5. A small amount of pin floc on the final clarifier surface often accompanies the old sludge that is normal for the extended-aeration process. If amounts become excessive and cover more than 25 percent of the clarifier surface, an increase in wasting of a small percentage may be helpful.

Equipment issues

The probable causes of equipment issues often include leaks in the aeration-system piping, plugged diffusers, air discharging from diffuser-header blow-off pipes, causing local boiling to occur on surfaces near the diffuser-header pipe or insufficient or inadequate oxygen transfer.

Leaks in the aeration system piping may be remedied by tightening flange bolts or replacing the flange gaskets. Insufficient or inadequate oxygen transfer may be remedied by improving aeration-system performance by adding or replacing more effective diffusers or mechanical aerators.

If you have specific issues related to controlling activated sludge or other wastewater queries, please submit a question.