Return activated sludge (RAS) is the conveyor belt of the activated sludge process. It takes the biomass that settled in the secondary clarifier and sends it back to the aeration basin, where it meets the incoming wastewater. Get the RAS rate right and the clarifiers hold a shallow, healthy blanket while the aeration basin keeps its inventory. Get it wrong and you either fill the clarifier with sludge or pump so much water around the loop that the clarifier cannot thicken.
There is no single "correct" RAS rate. There is a correct rate for today's flow, today's MLSS and today's settleability. This article covers four practical ways to estimate it, how state point analysis ties them together, and the habits that keep RAS control simple.
What RAS actually does
RAS has two jobs:
- Keep biomass in the aeration basin. Without RAS, the MLSS would wash out within hours.
- Remove solids from the clarifier fast enough. The clarifier underflow must carry away the solids that arrive with the mixed liquor, or the blanket grows.
RAS is not the main control for MLSS concentration over the long run. That is the job of wasting (SRT). RAS mostly decides where the sludge sits: in the aeration basin or in the clarifier. A plant with too little RAS has a lower MLSS in the basin and a deep blanket. A plant with too much RAS has a thin RAS, a shallow blanket and a hydraulically loaded clarifier.
Method 1: Mass balance
At steady state, the solids going into the clarifier equal the solids coming out in the RAS (ignoring the small amount in the effluent and WAS). That gives:
RAS flow = Q x MLSS / (RAS concentration minus MLSS)
Worked example (White River WRRF design values): Q = 8.0 MGD, MLSS = 3,000 mg/L, RAS concentration = 9,000 mg/L.
RAS flow = 8.0 x 3,000 / (9,000 minus 3,000) = 24,000 / 6,000 = 4.0 MGD, a RAS ratio of 50%.
This is the most reliable method when you have good, current RAS and MLSS data. Its weakness is that RAS concentration depends on the RAS rate itself, so it describes where you are, not where you could be. If you double the RAS rate, the RAS gets thinner and the calculation changes.
Tip: if the mass balance says you need 4.0 MGD but you are returning 3.0 MGD, expect the blanket to grow.
Method 2: Settleometer
The 30 minute settled volume (SSV30) gives a quick estimate of the ratio between clear water and settled sludge:
RAS ratio = SSV30 / (1,000 minus SSV30)
With SSV30 = 330 mL/L: ratio = 330 / 670 = 0.49, so at 8.0 MGD the RAS flow is about 3.94 MGD.
The logic: if the sludge settles to 330 mL in a liter, the clarifier underflow needs to remove about 330 parts of sludge for every 670 parts of clear water leaving over the weir. The method assumes the clarifier thickens about as well as the cylinder does in 30 minutes, which is roughly true for many plants but not all. It is easy to do every shift and gives a good first estimate.
Method 3: SVI
SVI gives an estimate of how thick the sludge can get. The settled sludge concentration in the cylinder is about 1,000,000 / SVI mg/L. Using that as the expected RAS concentration:
RAS conc (estimated) = 1,000,000 / SVI, then use the mass balance.
With SVI = 118 mL/g: RAS conc = 1,000,000 / 118 = 8,475 mg/L. RAS flow = 8.0 x 3,000 / (8,475 minus 3,000) = 4.38 MGD.
This method is a cross check. Many clarifiers thicken better than the cylinder (because of deeper sludge), so the actual RAS may be thicker than predicted. Use your own records to calibrate: compare predicted RAS concentration with measured RAS TSS over several weeks.
Method 4: Blanket depth
Every operator ends up here. Pick a target blanket (often 1 to 3 ft; White River uses 1 to 3 ft) and adjust RAS to hold it:
- Blanket rising day over day: increase RAS (and check wasting and SVI).
- Blanket very shallow and RAS thin: decrease RAS.
- Blanket deep at peak flow but fine at night: consider flow paced RAS.
Measure the blanket in every clarifier at the same time each day with a core sampler or sludge judge. Blanket depth is the best feedback you have, because it reflects real clarifier behavior.
Tying it together with state point analysis
State point analysis shows the clarifier's thickening capacity as a curve and your operation as two lines:
- The overflow line has a slope equal to the surface overflow rate (Q / area). It shows clarification.
- The underflow line starts at the state point (where the overflow line meets the MLSS) and slopes down at the underflow rate (RAS / area). It shows thickening.
If the underflow line stays under the flux curve, the clarifier can thicken what you send it. If it cuts through the curve, solids accumulate and the blanket rises. Increasing RAS makes the underflow line steeper, which moves it away from the curve, up to a point. Too much RAS increases the total solids flux applied to the clarifier and the state point rises.
Our state point tool does all of this from either settling test data or an SVI estimate and reports the minimum RAS flow that keeps the clarifier underloaded. For White River's design case (8.0 MGD, MLSS 3,000 mg/L, four 90 ft clarifiers, SVI 118), the clarifiers run at about 55% of their limiting flux. During the September storm (21 MGD peak hour, SVI 172), the same clarifiers were overloaded at about 1.2 times their limiting flux, and the tool suggests RAS near 8 MGD to clear the curve. That equals the full firm capacity of the RAS pumps (four duty pumps at 2 MGD each), is well above the 5.2 MGD operators actually reached, and helps explain why blankets rose to 6 to 7 ft even after RAS was raised to 5.2 MGD.
Common mistakes
Changing RAS to control MLSS. RAS shifts mass between the basin and clarifier. To change MLSS for good, change wasting.
Big step changes. Change RAS in steps of 10 to 20% and give the blanket a few hours to respond.
Ignoring individual clarifiers. The plant total may look right while one clarifier holds a 5 ft blanket because its pump is weak. Balance them.
Running maximum RAS all the time. High RAS thins the RAS, raises the hydraulic load on the clarifier, sends more flow through the aeration basin (shorter contact time), and costs pumping energy. It can also carry nitrate and DO into anoxic or anaerobic zones.
Running minimum RAS in summer. Deep blankets in warm water lead to rising sludge from denitrification. See What Causes Rising Sludge?.
Constant RAS or flow paced RAS?
Two common strategies:
Constant RAS flow. Simple. The RAS ratio drops during the day as flow rises, so solids move into the clarifier during peaks and back to the basin at night. Clarifiers act as storage. Works well when clarifiers have plenty of capacity.
Flow paced (constant ratio) RAS. RAS follows influent flow. Keeps the blanket steadier but increases clarifier loading at peak. Common at plants with automatic controls.
Many plants use a mix: constant RAS with a step increase above a set influent flow, or flow paced RAS with a minimum and maximum.
A daily RAS routine
- Measure blanket depth in every clarifier.
- Run the settleometer and calculate SVI.
- Check RAS TSS and compare with MLSS (mass balance).
- Compare the RAS rate with the settleometer and SVI estimates.
- Run state point when SVI changes or a storm is forecast.
- Adjust in small steps and record what you did and why.
RAS control is not complicated, but it needs attention every day. The four methods each tell you something different. When they agree, you can be confident. When they do not, the difference itself is information: maybe the RAS meter is off, maybe one clarifier is struggling, or maybe the sludge is changing.