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Activated Sludge Basics

The activated sludge process grows a managed population of microorganisms, keeps them in the system with a clarifier and return pumping, and removes the excess by wasting. Everything else is detail.

The big picture

Wastewater carries dissolved and fine organic matter that a clarifier alone cannot remove. In an activated sludge plant, that organic matter becomes food for bacteria. The bacteria grow, stick together into floc, and are separated from the clean water by gravity in a secondary clarifier. Most of the settled biomass is returned to the start of the process to keep working (return activated sludge, RAS). The extra growth is removed (waste activated sludge, WAS).

Aeration basin cross sectionPrimary effluent and return activated sludge enter an aeration basin. A blower feeds fine bubble diffusers on the floor. Mixed liquor flows out to the secondary clarifier.BlowerPrimary effluentRASMixed liquorto clarifierAeration basin (aerobic zone)Fine bubble diffuser grid on the floorAir supplies oxygen and keeps the floc in suspensionFloc + bubblesDO 1.5 to 2.5 mg/L
Aeration basin: primary effluent and RAS mix, air keeps the biomass suspended and supplied with oxygen, and mixed liquor flows to the clarifier.

The main parts

  • Aeration basin. A tank where wastewater and biomass mix with air. The mixture is called mixed liquor, and its suspended solids are mixed liquor suspended solids (MLSS).
  • Secondary clarifier. A quiet tank where floc settles. Clear water leaves over the weirs; settled sludge is drawn from the bottom.
  • RAS. Pumps return settled sludge to the aeration basin, typically at 40 to 75% of influent flow.
  • WAS. A smaller flow of sludge is removed daily to control the amount and age of the biomass.
  • Air supply. Blowers and diffusers, or mechanical aerators, provide oxygen and mixing.

The control numbers

MLSS and MLVSS

MLSS is the concentration of solids in the aeration basin, often 2,000 to 4,000 mg/L in conventional plants. MLVSS is the volatile (mostly biological) part, typically 70 to 85% of MLSS. At the fictional White River WRRF, the design MLSS is 3,000 mg/L and MLVSS is 78%.

SRT (sludge age)

Solids retention time is the average time solids stay in the system. It decides which organisms can grow. Nitrifiers need a longer SRT than BOD removers, especially in cold water.

SRT = solids under aeration (lb) / solids leaving per day (lb/d)

Solids retention time conceptThe solids inventory in the aeration basin is divided by the pounds per day leaving through waste sludge and effluent. The result is the average number of days a particle of sludge stays in the system.Solids inventorylb under aerationGrowth from foodWAS, lb/d (operator controlled)Effluent TSS, lb/d (loss)SRT, days = inventory, lb / (WAS lb/d + effluent lb/d)Example: 90,072 lb / 7,740 lb/d = 11.6 days
SRT compares the inventory under aeration with the solids leaving each day through WAS and the effluent.

F/M

The food to microorganism ratio compares the BOD load with the biomass available to eat it. Typical conventional values are 0.05 to 0.4 lb BOD per lb MLVSS per day. White River's design F/M is 0.142.

F/M = BOD load (lb/d) / MLVSS under aeration (lb)

Food to microorganism ratioA balance compares the pounds of BOD fed each day with the pounds of volatile solids under aeration. Typical F/M ranges for extended aeration, conventional and high rate systems are shown.Food (F)BOD, lb/dMicroorganisms (M)MLVSS, lbF/M = lb BOD per day / lb MLVSS under aerationExtended aeration0.05 to 0.15Conventional0.2 to 0.4High rate0.4 to 1.5Nitrifying plants usually run lower F/M (longer SRT) than BOD-only plants.
F/M compares the daily food supply with the biomass under aeration.

F/M and SRT are related: a plant with a long SRT usually has a low F/M. Most operators control SRT directly and use F/M as a check.

SVI

The sludge volume index tells you how well the sludge settles and compacts. 80 to 150 mL/g is typical. See How Do I Run a Settleometer Test?.

DO

Dissolved oxygen in the aeration basin is usually held between about 1 and 3 mg/L. Too little starves the biomass and grows filaments; too much wastes energy.

How the operator controls the process

Operators have a small number of levers:

  1. Wasting sets SRT and, over time, MLSS.
  2. RAS decides where the sludge sits: basin or clarifier.
  3. Air sets DO.
  4. Flow routing decides which tanks are in service and how flow is split.
  5. Chemicals (alkalinity, coagulants, polymer, chlorine) for specific needs.

Each lever affects more than one thing. Wasting more lowers MLSS and SRT, which lowers oxygen demand but may cost nitrification. Raising RAS lowers the blanket but adds clarifier hydraulic load. Good process control means making small changes, one at a time, and watching the response.

Process variations

  • Conventional plug flow: long, narrow basins; load is highest at the inlet.
  • Complete mix: uniform conditions throughout.
  • Extended aeration: long detention and SRT (often 20 to 30 days), low F/M, common in small plants and oxidation ditches.
  • Contact stabilization and step feed: spread the load or store solids for wet weather.
  • Sequencing batch reactors (SBR): one tank fills, reacts, settles and decants in sequence.
  • BNR configurations: anoxic and anaerobic zones for nitrogen and phosphorus removal. See BNR.
  • Membrane bioreactors (MBR): membranes replace the clarifier.

Where to go next