Wasting is the most powerful control an activated sludge operator has. It sets the solids retention time (SRT), which decides what organisms grow, how much oxygen you use, whether you nitrify, and how the sludge settles. It is also the control most likely to cause trouble when it is off, because the effects show up slowly, often days or weeks after the change.
This article covers the warning signs of wasting too much (and too little), how to check your numbers, and a simple method for setting wasting that keeps the process steady.
What wasting does
Every day, the biomass in your aeration basin grows by eating BOD. If you did not waste, the MLSS would keep climbing until the clarifiers could not hold it and solids went over the weirs. Wasting removes the daily growth, and the rate you waste at decides the average age of the sludge:
SRT (days) = solids under aeration (lb) / solids leaving the system (lb/d)
Solids leaving = WAS + effluent TSS. Waste more, and SRT goes down. Waste less, and SRT goes up.
Signs you are wasting too much
1. MLSS is falling day after day. If MLSS drops steadily while flows and loads are stable, more solids are leaving than growing. This is the most direct sign.
2. Effluent ammonia is rising. Nitrifiers are the slowest growing organisms you care about. When SRT drops below what they need for the temperature, they wash out first. A rise in ammonia a week or two after an increase in wasting is a classic pattern. See Why Is My Effluent Ammonia High?.
3. Straggler floc and cloudy supernatant. Young sludge forms weak, fluffy floc. The settleometer shows light particles floating in the supernatant, and effluent TSS creeps up.
4. White, billowing foam. Crisp white foam on the aeration basin often means young sludge. See the white foam guide.
5. F/M is climbing. With less biomass to eat the same food, F/M rises.
6. Microscope shows a young sludge community. Many flagellates and free-swimming ciliates, few stalked ciliates and rotifers, and small, loose floc.
7. Oxygen uptake rate per gram is high. SOUR above your normal range often means a young, very active biomass.
Signs you are wasting too little
1. MLSS is climbing. And so is the clarifier blanket.
2. Pin floc and hazy supernatant. Old sludge forms small, dense floc that settles fast but leaves fines behind. See the pin floc guide.
3. Dark brown foam and long SRT filaments. Nocardioforms and M. parvicella favor long SRT.
4. Higher oxygen demand. Endogenous respiration uses more air. Blowers work harder for the same load.
5. Clarifier blankets rising. More mass sent to clarifiers means more blanket.
6. Microscope shows an old sludge community. Many rotifers, nematodes, and testate amoebae; dense, compact floc.
Check the numbers before you act
Many "over-wasting" problems are measurement problems. Before you change wasting, confirm:
The WAS flow meter. Small WAS lines and magnetic meters at low velocity are common sources of error. Do a drawdown test on the WAS wet well or a timed fill of a known volume. A meter reading 20% low means you are wasting 20% more than you think.
The WAS concentration. If you waste from the RAS line, RAS concentration changes with RAS rate and time of day. A single grab can be far off. Use composites or several grabs.
The MLSS. Sample location and time matter. A sample taken at the end of the basin at peak flow will differ from one at the start at low flow.
The effluent TSS. Include it. During storms it can be the largest solids loss.
The basin volume in service. If a train is out of service, use only the volume in service.
Worked example
White River WRRF (fictional) design data: aeration volume 3.6 MG, MLSS 3,000 mg/L, effluent 8.0 MGD at 8 mg/L TSS, WAS at 9,000 mg/L. Target SRT: 11.6 days.
Inventory = 3.6 x 3,000 x 8.34 = 90,072 lb
Solids that must leave each day = 90,072 / 11.6 = 7,765 lb/d
Effluent solids = 8.0 x 8 x 8.34 = 534 lb/d
WAS needed = 7,765 minus 534 = 7,231 lb/d
WAS flow = 7,231 / (9,000 x 8.34) = 0.0963 MGD = 96,300 gpd
Now suppose the operator wastes 120,000 gpd for a week because the WAS meter reads 20% low. Actual WAS = 0.12 x 9,000 x 8.34 = 9,007 lb/d. Actual SRT = 90,072 / (9,007 + 534) = 9.4 days, and falling as MLSS drops. In summer that may be fine. In winter, with a target of 12 to 15 days, it could cost nitrification within two weeks.
Setting wasting: a simple method
- Pick an SRT target for the season based on temperature and nitrification needs, with a safety factor.
- Calculate WAS each day from the 7 day average inventory and effluent solids using the WAS rate calculator.
- Limit changes to about 10 to 15% per day.
- Watch the response: MLSS trend, SVI, settleometer, effluent ammonia and TSS, microscope.
- Adjust the SRT target slowly if the process tells you to (for example, pin floc means it may be too long; ammonia bleed means too short).
Waste continuously if you can
Wasting a little all day is better than a big batch once a day. Batch wasting swings the inventory and can pull thin or thick sludge depending on the time of day. If you must batch waste, do it at the same time each day and keep the volume steady.
What about wasting to an MLSS target?
Some plants waste to hold a fixed MLSS. That works when loads are steady, but when the load drops (for example in a dry season), holding MLSS constant raises SRT, and when the load rises it lowers SRT. SRT control is more stable for the biology. MLSS still matters, mostly as a check on clarifier capacity.
Sludge handling limits
Sometimes the problem is not process control but solids handling. If the thickeners, digesters, or dewatering cannot keep up, operators are tempted to waste less, and MLSS climbs. That is a capacity problem to raise with management, not something to solve by letting the process drift. Track solids handling capacity against WAS needs so the conflict is visible early.
Wasting after a storm
Storms are a special case. When peak flows push solids over the weirs, the effluent becomes a large, uncontrolled waste stream. At White River during the September 13 storm, effluent TSS on September 14 was 28 mg/L at elevated flow, several times the normal effluent solids loss, on top of the normal WAS. The operators cut WAS temporarily, which was the right call. Their SRT for that week had already dropped because of the solids lost over the weirs.
After a washout:
- Calculate the actual solids lost (effluent flow x TSS x 8.34) and include it in SRT.
- Reduce or stop wasting until MLSS and SRT recover.
- Watch ammonia closely for one to two weeks.
- Resume normal wasting gradually, using the SRT method, not a fixed volume.
Summary
- Over-wasting: falling MLSS, rising ammonia, straggler floc, white foam, young organisms.
- Under-wasting: rising MLSS, pin floc, brown foam, high air use, rising blankets, old organisms.
- Check WAS meter, WAS concentration, MLSS sampling, effluent TSS and volume before changing anything.
- Waste by SRT, change slowly, and watch the process respond.