Ask ten operators what "sludge age" means and you may get three different formulas. That is not because anyone is wrong. The industry uses several related numbers, and they have drifted together in everyday speech. This article sorts them out, works an example of each, and explains how to use the result to run the plant.
Three related numbers
Gould sludge age is the oldest. It compares the solids under aeration to the suspended solids entering the aeration basin each day:
Sludge age (days) = aeration basin solids (lb) / influent TSS to aeration (lb/d)
Solids retention time (SRT) compares the solids under aeration to the solids leaving the system each day (wasted plus lost in the effluent):
SRT (days) = aeration basin solids (lb) / (WAS solids + effluent solids) (lb/d)
Mean cell residence time (MCRT) is the same as SRT but includes the solids in the clarifiers (and sometimes RAS lines) in the numerator:
MCRT (days) = (aeration solids + clarifier solids) (lb) / (WAS solids + effluent solids) (lb/d)
SRT and MCRT describe the average time a particle of biomass stays in the system, which is what controls which organisms can grow. Gould sludge age is easier to estimate (it does not need wasting data) but is less directly tied to biology. When most people say "sludge age" today, they mean SRT.
Worked example: SRT
White River WRRF (fictional) design values:
- Aeration volume: 3.6 MG
- MLSS: 3,000 mg/L
- WAS flow: 96,000 gpd (0.096 MGD) at 9,000 mg/L
- Effluent flow: 8.0 MGD at 8 mg/L TSS
Aeration solids = 3.6 x 3,000 x 8.34 = 90,072 lb
WAS solids = 0.096 x 9,000 x 8.34 = 7,206 lb/d
Effluent solids = 8.0 x 8 x 8.34 = 534 lb/d
SRT = 90,072 / (7,206 + 534) = 90,072 / 7,740 = 11.6 days
Notice how much the effluent solids matter. At 534 lb/d they are about 7% of the solids leaving. Leave them out and you overstate SRT. During a storm with washout, effluent solids can be a large part of the total.
Worked example: MCRT
If the four 90 ft clarifiers each hold a 2 ft blanket at an average 6,000 mg/L, the clarifier solids are approximately:
Volume = 4 x 6,362 ft² x 2 ft x 7.48 gal/ft³ = 380,700 gal = 0.381 MG
Clarifier solids = 0.381 x 6,000 x 8.34 = 19,060 lb
MCRT = (90,072 + 19,060) / 7,740 = 14.1 days
MCRT is always longer than SRT. Clarifier inventory changes daily, so SRT based on aeration solids only is steadier and more common for control.
Worked example: Gould sludge age
Using primary effluent TSS to aeration of 96 mg/L at 8.0 MGD:
TSS load = 8.0 x 96 x 8.34 = 6,405 lb/d
Sludge age = 90,072 / 6,405 = 14.1 days
Which data to use
Use averages, not single days. Wasting varies, MLSS varies, and lab results have scatter. A 7 day rolling average SRT is a good control number; some plants use a rolling average over one SRT.
Use the WAS concentration that matches the WAS. If you waste from the RAS line, use RAS TSS. If you waste mixed liquor directly from the basin, use MLSS.
Measure the WAS flow accurately. WAS flow meters on small lines are often the weakest instrument in the plant. Check them with a drawdown test or a timed volume.
Include effluent solids. Especially during storms.
Use the basin volume in service. If one train is out, use the volume of the trains in service.
What SRT controls
SRT decides which organisms can stay in the system. An organism that grows more slowly than the rate you waste it will wash out. That has big consequences:
Nitrification. Nitrifying bacteria grow slowly, and much more slowly in cold water. A rough guide for aerobic SRT needed for reliable nitrification is 3 to 5 days at 20 C and 8 to 12 days or more at 10 C, with a safety factor on top. Losing SRT in a storm or over-wasting in fall is a leading cause of ammonia problems. See Why Is My Effluent Ammonia High?.
Floc quality. Very short SRT (under about 3 days) tends to produce young, dispersed sludge and straggler floc. Very long SRT (over about 20 to 30 days in conventional plants) tends toward pin floc, and some filaments and foam formers (Nocardioforms, M. parvicella) favor long SRT.
Sludge production. Longer SRT means more endogenous decay and less sludge to handle, but more oxygen use.
Oxygen demand. Longer SRT means more oxygen per pound of BOD removed, because the biomass respires itself.
Using SRT to set wasting
Once you have an SRT target, rearrange the formula to calculate wasting:
WAS (lb/d) = aeration solids (lb) / target SRT (days) minus effluent solids (lb/d)
For a target of 11.6 days: 90,072 / 11.6 = 7,765 lb/d, minus 534 = 7,231 lb/d. At 9,000 mg/L, that is 7,231 / (9,000 x 8.34) = 0.0963 MGD, or about 96,300 gpd. The WAS rate calculator does this for you.
Aerobic SRT and total SRT
Plants with anoxic or anaerobic zones need to be clear about which volume goes in the numerator. Total SRT uses all the basin volume that holds mixed liquor. Aerobic SRT uses only the aerated volume, and it is the one that matters for nitrification, because nitrifiers only grow when they have oxygen.
White River has a 1.21 MG anoxic zone ahead of its 3.6 MG of aeration in each MLE train set. The 11.6 day figure above is already on an aeration basis, so it is the aerobic SRT. The total SRT including the anoxic zones is about 11.6 x (3.6 + 1.21) / 3.6 = 15.5 days. The plant's operating target is 10 to 14 days aerobic, with a minimum of 8 days in summer and 12 to 15 days in winter.
If your reports show only one SRT, find out which volume it uses. A plant that thinks it has a 10 day SRT but means total SRT may have only 6 or 7 days of aerobic SRT, which may not be enough in cold water.
Making changes
- Change wasting gradually. A common rule is no more than 10 to 15% change per day.
- Allow time. It takes about one SRT for the process to show the full effect, and two to three SRTs to stabilize.
- Plan for seasons. Start building SRT several weeks before cold weather.
- Watch the clarifier. More SRT means more MLSS, which means more load on the clarifiers. Check with the state point tool.
Common mistakes
Wasting by MLSS target alone. Holding MLSS constant while flows and loads change means SRT drifts. Many experienced operators control by SRT and let MLSS find its own level, within clarifier limits.
Forgetting the effluent solids. Already covered, but it is the most common error.
Using a single grab sample of RAS. RAS concentration changes with RAS rate and time of day. Use a composite or several grabs.
Mixing units. Keep flows in MGD, concentrations in mg/L, and use 8.34 for both numerator and denominator (they cancel, but only if both are there).
Summary
- SRT = aeration solids / solids leaving per day. This is what most people mean by sludge age.
- MCRT adds clarifier solids. Gould sludge age uses influent TSS instead of wasted solids.
- Use rolling averages and accurate WAS data.
- SRT sets nitrification capacity, floc quality, sludge production and oxygen demand.
- Use SRT to calculate WAS, change it slowly, and plan for seasons.