What Is the Activated-Sludge Process? A Detailed Explanation of Professional Wastewater-Terminology

The activated-sludge process is a wastewater biological-treatment technology and the main method of biological treatment centered on activated sludge. It mixes and aerates wastewater with activated sludge (microorganisms) so organic pollutants decompose; the biosolids are then separated from the treated wastewater and part can be returned to the aeration tank as needed. [1] The process continuously feeds air into the wastewater,

and after some time aerobic microorganisms proliferate to form sludge-like flocs. On them dwell microbial communities dominated by zoogloea, with strong ability to adsorb and oxidize organics.

The activated-sludge process is a method of sewage biological treatment. Under artificial oxygenation it continuously mixes and cultivates sewage with various microbial communities to form activated sludge. Using the sludge's biological coagulation, adsorption and oxidation, it decomposes and removes organic pollutants. Then sludge is separated from water; most is returned to the aeration tank and the excess discharged from the system.

The main factors affecting activated-sludge efficiency (treatment and economic) are the choice of method and the design and operation of the aeration and settling tanks.

(1) Aeration tank: the main reaction body

(2) Secondary clarifier: 1) performs sludge–water separation to ensure effluent quality; 2) ensures return sludge to maintain concentration in the aeration tank.

(3) Return system: 1) maintains sludge concentration in the aeration tank; 2) changes return ratio to alter its operating condition.

(4) Excess-sludge discharge system: 1) one route to remove organics; 2) maintains stable system operation.

(5) Oxygen-supply system: mainly an aeration blower and dedicated aerators that supply enough dissolved oxygen to the tank.

A typical activated-sludge plant comprises an aeration tank, settling tank, sludge-return system and excess-sludge removal system. Sewage and returned activated sludge enter the aeration tank together to form mixed liquor. Compressed air from the station enters through bottom diffusers as fine bubbles to raise dissolved oxygen and keep the liquor violently agitated and suspended. Dissolved oxygen, sludge and sewage mix and fully contact, letting the reaction proceed normally.

Stage 1: organic pollutants are adsorbed onto the zoogloea surface by the sludge particles, due to their huge specific surface area and polysaccharide slime. Meanwhile some macromolecular organics are broken by bacterial extracellular enzymes into small molecules.

Stage 2: microorganisms under ample oxygen absorb these organics and oxidize them into CO2 and water, part feeding their own growth. The result: pollutants are degraded and removed, the sludge proliferates, and the sewage is purified.

The purified mixed liquor enters the secondary clarifier, where suspended sludge and other solids settle and separate from water; the clarified sewage leaves as treated water. The settled, concentrated sludge is discharged from the bottom, most returned as seed sludge to keep suspended-solids and microbial concentration; the proliferated microbes are discharged as 'excess sludge'. In fact, pollutants are largely transferred from sewage into this excess sludge.

A vivid way to put the principle: microorganisms 'eat' the organics in sewage, turning it into clean water. It is essentially similar to nature's self-purification, only artificially enhanced for better effect.

Besides the conventional process, there are step-feed, contact-stabilization, extended-aeration and high-rate activated-sludge methods. The first two differ from the basic flow in the number and position of influent entries to the aeration tank. 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 2–3 entries some distance from the head (generally evenly spaced). In flow terms, contact-stabilization (Fig. 2) is just a variant of step-feed (Fig. 3) using only the last entry — the latter becomes the former.

Method development is based on process mechanism. The main participants are organics, microorganisms and dissolved oxygen (air); the first two dominate, with DO kept at a certain concentration. Oxygen demand varies through the process: high initial organics mean rapid microbial growth and high demand, decreasing as organics fall. In the conventional process aeration is uniform, clearly unreasonable. Two improvements exist: one changes uniform aeration to tapered aeration; the other is step-feeding. But step-feeding not only reduces the oxygen-demand swing but also changes the organics-to-microorganisms ratio.

The organic-to-microorganism ratio is the sludge loading rate (F:M). It affects metabolic depth, sludge settleability, stability and capital cost. Lower F:M makes operation easier, efficiency more stable and excess sludge less, but capital and operating costs are generally higher. Conventional activated sludge runs F:M 0.15–0.3 kg BOD/kg sludge. High-rate uses above 1, greatly reducing return sludge and air and saving cost, but BOD removal drops to 60–70%, so it is also called modified activated sludge, used where only medium treatment is needed. Extended aeration is the opposite: F:M often below 0.1, aeration over 24 hours, deep metabolism, little excess sludge, no frequent desludging, stable and simple — used for very small flows.

In practice, pollutant transfer to sludge is fast but metabolism slow. Treating municipal sewage often cuts BOD about 90% in under an hour, yet returning this sludge to the aeration tank cannot reproduce that ability (see aeration), giving rise to the contact-stabilization method. Sludge regeneration essentially gives microorganisms enough time to digest the transferred organics, so it is also called the contact-stabilization method.

The aeration tank is the heart of all activated-sludge methods; it mixes liquor for full sludge–water contact and supplies oxygen. There are two mixing modes: one keeps simultaneously-entering sludge and water fully mixed and undisturbed until they leave, avoiding short-circuiting with existing liquor — long narrow tanks ensure simultaneous in/out (Fig. 4) and equal aeration time; the other immediately mixes entering sludge and water with the whole tank's liquor for uniform quality, possibly optimal microbial growth and best operation. There is also a ring-shaped shallow aeration trough with high-velocity recirculation, aeration time near 24 hours, specially called an oxidation trough or ditch — in fact an extended-aeration tank.

Besides geometric design, aeration method and equipment are important. Aeration methods are bubble (blow) aeration and surface (mechanical) aeration. The deep-shaft aeration appearing in the late 1970s is also bubble aeration, raising efficiency by increasing bubble–liquor contact time.

In surface aeration, devices at the liquid surface cause recirculation and vigorous surface agitation for gas exchange with air. Aerators are usually vertical impellers, sometimes horizontal brushes or propellers. Ring troughs use horizontal aerators. To speed oxygen dissolution, 'pure-oxygen' aeration appeared in the 1970s, replacing ordinary air with very high-oxygen air, mostly via surface aeration.

Operation mainly controls activated-sludge amount and oxygen supply; the tank's MLSS is adjustable, i.e., sludge amount and loading are adjustable and should be tuned to conditions. Activated-sludge plants easily get sludge bulking — extremely high water content, poor settling — causing sludge to leave with effluent and spoil water quality while loss of sludge reduces the tank's concentration and gradually fails the process. On signs of bulking, analyze the cause and act promptly.

(1) Wastewater contains enough soluble, readily degradable organics;

(2) Mixed liquor contains enough dissolved oxygen;

e. Nutrient balance: much industrial wastewater lacks nitrogen, phosphorus etc., lowering efficiency. [1]

(4) Continuous sludge return and timely excess-sludge removal keep a certain activated-sludge concentration;

(5) No toxic or harmful substances flow in.

a. BOD loading rate (F/M), also organic loading (NS): oxygen demand is estimated from wastewater BOD5 and daily wasted activated sludge. [1]

b. Water temperature: it affects reaction activity.

c. pH value: sludge activity differs at different pH.

d. Dissolved oxygen: aerobic decomposition requires oxygen's participation. Microorganisms use oxygen to decompose organics, producing high-energy compounds for new cells and respiration. [1]

(3) Activated sludge is suspended in the tank;

f. Toxic substances: although toxic metals and toxic organics in wastewater are not too concentrated to affect plant operation, if not removed in pretreatment they may cause two bad outcomes: volatile organics escape from the aeration tank into air, causing air pollution; toxic metals may precipitate into waste sludge, making it hazardous. [1]