Inside the Aeration Tank: What Actually Governs an Activated Sludge Plant

The activated-sludge process is the main method of biological wastewater treatment built around activated sludge. Wastewater is mixed and aerated with activated sludge so that organic pollutants decompose; the biosolids are then separated from the treated wastewater, and part of them can be returned to the aeration tank as needed. Air is fed continuously, and after a period aerobic microorganisms proliferate into sludge-like flocs. Those flocs carry microbial communities dominated by zoogloea, with a strong capacity to adsorb and oxidise organics.

Five Subsystems, One Process

A typical plant comprises an aeration tank, a settling tank, a sludge-return system and an excess-sludge removal system, plus an oxygen-supply system of blowers and aerators. Efficiency depends on how those are chosen and how they are designed and operated.

• The aeration tank is the main reaction body.
• The secondary clarifier performs sludge-water separation to protect effluent quality, and returns sludge to hold concentration in the aeration tank.
• The return system maintains sludge concentration and, by changing the return ratio, alters the operating condition of the tank.
• The excess-sludge discharge system is one route for removing organics and keeps the system stable.
• The oxygen-supply system delivers enough dissolved oxygen to the tank.

Sewage and returned activated sludge enter the aeration tank together to form mixed liquor. Compressed air enters through bottom diffusers as fine bubbles, raising dissolved oxygen and keeping the liquor violently agitated and suspended so that oxygen, sludge and sewage fully contact.

How the Organics Actually Leave

Stage one is physical. Organic pollutants adsorb onto the zoogloea surface, helped by the huge specific surface area of the flocs and their polysaccharide slime. At the same time, bacterial extracellular enzymes break some macromolecular organics into smaller molecules.

Stage two is biological. With ample oxygen, microorganisms take up those organics and oxidise them into CO2 and water, with part of the carbon going into their own growth. Pollutants are degraded and removed, sludge proliferates, and the sewage is purified.

The purified mixed liquor then enters the secondary clarifier, where suspended sludge and other solids settle out. Clarified sewage leaves as treated water. Settled, concentrated sludge is withdrawn from the bottom, most of it returned as seed sludge to maintain suspended-solids and microbial concentration, and the proliferated microbes are discharged as excess sludge. In effect, pollutants are largely transferred from sewage into that excess sludge.

The Ratio That Drives Everything

The organic-to-microorganism ratio is the sludge loading rate, F:M. It affects metabolic depth, sludge settleability, stability and capital cost. A lower F:M makes operation easier, efficiency more stable and excess sludge smaller, but capital and operating costs are generally higher.

Conventional activated sludge runs at 0.15-0.3 kg BOD/kg sludge. High-rate operation uses a figure above 1, which greatly reduces return sludge and air requirement and saves cost, but BOD removal drops to 60-70 percent - hence the name modified activated sludge, used where only medium treatment is needed. Extended aeration sits at the other end: F:M often below 0.1, aeration over 24 hours, deep metabolism, very little excess sludge, no frequent desludging, stable and simple, suited to very small flows.

Why the Aeration Method Matters

Oxygen demand is not constant through the process. High initial organics mean rapid microbial growth and high oxygen demand, which falls as organics are consumed. Uniform aeration in a conventional layout is therefore hard to defend. Two improvements exist: tapered aeration, and step-feeding, which both flattens the oxygen-demand swing and changes the organics-to-microorganisms ratio.

In step-feed activated sludge, only part of the wastewater and return sludge enter at the head; the rest enters in 2-3 batches at evenly spaced points downstream. Contact-stabilization is essentially the variant using only the last entry point. Its logic comes from an asymmetry in the process: pollutant transfer to sludge is fast but metabolism is slow. Treating municipal sewage often cuts BOD about 90 percent in under an hour, yet returning that sludge to the aeration tank does not reproduce the ability immediately. Sludge regeneration gives microorganisms time to digest what they have taken up.

On equipment, aeration is either bubble aeration or surface mechanical aeration. Deep-shaft aeration, appearing in the late 1970s, is bubble aeration that raises efficiency by lengthening bubble-liquid contact time. Pure-oxygen aeration also appeared in the 1970s, replacing ordinary air mostly in surface-aeration installations. A ring-shaped shallow aeration trough with high-velocity recirculation and aeration time near 24 hours is, in effect, an extended-aeration tank.

Keeping It Running

Operation comes down to controlling activated-sludge amount and oxygen supply. The tank's MLSS is adjustable, so sludge amount and loading can be tuned to conditions. Plants fail most often through sludge bulking - extremely high water content and poor settling - which carries sludge out with the effluent, spoils water quality, and drains tank concentration until the process fails. On the first signs, the cause has to be found and acted on quickly.

Among the design and control parameters, BOD loading rate (F/M), also called organic loading (NS), matters most for sizing: oxygen demand is estimated from wastewater BOD5 and the daily wasted activated sludge.

The underlying conditions are not negotiable: enough soluble, readily degradable organics; enough dissolved oxygen in the mixed liquor; activated sludge held in suspension; continuous return with timely excess-sludge removal; and no toxic or harmful substances arriving. Water temperature affects reaction activity, pH changes sludge activity, and nutrient balance matters because much industrial wastewater lacks nitrogen and phosphorus. Toxic metals and toxic organics, if not removed in pretreatment, cause two bad outcomes - volatile organics escape from the aeration tank into the air, and toxic metals precipitate into waste sludge and make it hazardous.