Basic Wastewater Treatment Terms: What Is the Activated Sludge Process?
The activated sludge process is the main biological wastewater treatment method centered on activated sludge. It is a sludge-like floc formed by continuously blowing air into wastewater; after a certain time, aerobic microorganisms multiply and form it. It hosts a microbial community dominated by zoogloeal masses and has a strong ability to adsorb and oxidize organics.
The activated sludge process is a method of sewage biological treatment. Under artificial oxygenation, sewage and various microbial communities are continuously mixed and cultivated to form activated sludge. The biological coagulation, adsorption, and oxidation of the activated sludge decompose and remove the organic pollutants in the sewage. The sludge is then separated from the water; most of it is returned to the aeration tank and the excess is discharged from the activated sludge system.
The main factors affecting the working efficiency (treatment efficiency and economic benefit) of the activated sludge process are the choice of treatment method and the design and operation of the aeration tank and settling tank.
1) Aeration tank: the main reaction unit
2) Secondary settling tank: 1) performs sludge-water separation to ensure effluent quality; 2) ensures return sludge to maintain the sludge concentration in the aeration tank.
3) Return system: 1) maintains the sludge concentration in the aeration tank; 2) changes the return ratio to change the operating conditions of the aeration tank.
4) Excess sludge discharge system: 1) is one route for removing organics; 2) maintains stable system operation.
5) Oxygen supply system: mainly composed of an oxygen-supply blower and special aerators to provide sufficient dissolved oxygen to the aeration tank.
a. BOD loading rate (F/M), also called the organic loading rate, denoted NS;
Besides the conventional activated sludge process, there are also step feed, adsorption-regeneration, 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 the step-feed activated sludge process, only part of the wastewater enters the tank at the head together with the return sludge. The rest of the wastewater enters the aeration tank at 2-3 points a certain distance from the head (entries generally equally spaced), in 2-3 doses. In terms of flow, the adsorption-regeneration activated sludge process (Fig. 2) is just a variant of the step-feed process (Fig. 3) with only the last of several wastewater inlets used; the latter then becomes the former.
The development of method types is based on the mechanism of the process. The main substances involved are organics, microorganisms, and dissolved oxygen (air). The first two are primary; dissolved oxygen only needs to be maintained at a certain concentration.
The oxygen demand differs throughout the process. At the start the organic concentration is high, microorganisms multiply rapidly, and oxygen demand is large. As organics gradually decrease, oxygen demand also decreases. In the conventional activated sludge process, oxygen supply to the aeration tank is uniform, which is obviously unreasonable. There are two improvement methods. One changes uniform aeration to tapered aeration, looking at the aeration method. The other is the step-feed method. But step feeding not only reduces the variation in oxygen demand but also changes the relative amounts of organics and microorganisms.
The ratio of organics to microorganisms is called the sludge loading rate (F/M). It affects the metabolic depth of the process and the settleability of the sludge, and also the stability of operation and capital cost. A lower sludge loading rate makes the process easier to run, with more stable treatment efficiency and less excess sludge, but generally higher capital and operating costs. The conventional activated sludge process usually has a loading rate of 0.15-0.3 kg BOD/kg sludge. The high-rate activated sludge process uses above 1, greatly reducing return sludge and air volume and saving cost, but the BOD removal rate drops to 60-70%, so it is also called the modified activated sludge process, used where only moderate treatment is needed. The extended-aeration activated sludge process is the opposite: the loading rate is often below 0.1, aeration time exceeds 24 hours, metabolism is deep, excess sludge is small, frequent sludge discharge is unnecessary, operation is stable and easy to manage, and it is often used for very small flows.
In practice it was found that the transfer of pollutants to the sludge is fast, while the metabolic rate is slow. Treating municipal sewage, the wastewater BOD is often reduced by about 90% in less than an hour. But if this sludge is returned to the aeration tank, that ability cannot be reproduced (see aeration), leading to the creation of the adsorption-regeneration method. Regeneration of activated sludge is essentially giving the microorganisms enough time to digest the transferred organics. Thus some renamed it the contact-stabilization method.
The aeration tank is the heart of all activated sludge processes; its role is to agitate the mixed liquor so that sludge and water fully contact and oxygen is supplied to the microorganisms. There are two mixing methods. One keeps the sludge and water entering the tank at the same time fully mixed and held until they leave the tank, without mixing with the mixed liquor already in the tank to avoid short-circuiting. A long, narrow aeration tank ensures that the sludge and water entering together also leave together (Fig. 4), giving the simultaneously-entering wastewater the same aeration time. The other mixing method makes the entering sludge and water immediately mix thoroughly with the whole tank's mixed liquor, achieving uniform mixed-liquor quality and possibly keeping the microorganisms in an optimal living environment so the process runs under the best conditions. There is also a circular aeration long channel, relatively shallow, with the mixed liquor returning at a higher velocity. Its aeration time is close to 24 hours and it is specially called an oxidation channel or oxidation ditch, actually an aeration tank of the extended-aeration activated sludge process.
Besides designing the geometric shape as required, the aeration method and equipment are also important. There are two aeration methods: bubble aeration (also called diffused-air aeration) and surface aeration (also called mechanical aeration). The deep-shaft aeration that appeared in the late 1970s is also a bubble aeration that increases the contact time of bubbles with the mixed liquor to raise aeration efficiency.
In surface aeration, an aerator set at the liquid surface makes the tank liquid recirculate and causes the surface to fluctuate violently for close gas exchange with air. The aerators are generally various vertical impellers, but horizontal brush or propeller aerators are also used. Circular aeration channels all use horizontal aerators.
To speed up oxygen dissolution, 'pure oxygen' aeration appeared in the 1970s, replacing ordinary air with air of very high oxygen concentration. Mostly surface aeration is used.
Operation is mainly the control of activated sludge amount and oxygen supply. The activated sludge concentration in the aeration tank (called mixed-liquor suspended solids) is adjustable, meaning the activated sludge amount and loading rate can be adjusted; operation should be regulated according to the specific situation. Activated sludge plants are prone to sludge bulking, i.e., very high sludge water content and poor settleability. This causes sludge to flow out with the water from the settling tank, spoiling the water quality, and at the same time the loss of sludge reduces the sludge in the aeration tank and the whole process gradually fails. When a bulking tendency is found, the cause should be analyzed and measures taken immediately.
1) The wastewater contains sufficient soluble, easily degradable organics;
2) The mixed liquor contains sufficient dissolved oxygen;
3) The activated sludge is in a suspended state in the tank;
A typical activated sludge process consists of an aeration tank, a settling tank, a sludge return system, and an excess sludge removal system. Sewage and returned activated sludge enter the aeration tank together to form a mixed liquor. Compressed air from the compressor station enters the sewage as fine bubbles through an air diffusion device laid at the bottom of the aeration tank, to increase the dissolved oxygen content of the sewage and keep the mixed liquor in violent agitation and suspended. Dissolved oxygen, activated sludge, and sewage mix and fully contact so the activated sludge reaction proceeds normally.
5) No toxic or harmful substances flow in.
4) The activated sludge is continuously returned and excess sludge is removed in time to keep the mixed liquor at a certain activated sludge concentration;
In the first stage, the organic pollutants in the sewage are adsorbed onto the surface of the zoogloeal masses by the activated sludge particles, because of their huge specific surface area
and polysaccharide sticky substances. At the same time some macromolecular organics are decomposed into small-molecule organics by bacterial extracellular enzymes. In the second stage, under sufficient oxygen, the microorganisms absorb these organics and oxidize and decompose them into carbon dioxide and water, part of which is used
for their own growth and reproduction. As a result of the activated sludge reaction, the organic pollutants in the sewage are degraded and removed, and the activated sludge itself
grows and multiplies, and the sewage is purified.
The mixed liquor after activated sludge purification enters the secondary settling tank, where the suspended activated sludge and other solids in the mixed liquor settle and separate from the water, and the clarified sewage is discharged from the system as treated water. The settled and concentrated sludge is discharged from the bottom of the settling tank, most of it returned to the aeration tank as seed sludge to maintain the suspended solids and microbial concentration in the aeration tank; the grown microorganisms discharged from the system are called 'excess sludge'. In fact, the pollutants are largely transferred from the sewage to this excess sludge.
An intuitive description of the activated sludge principle: the microorganisms 'eat' the organics in the sewage, so the sewage becomes clean water. It is essentially similar to the natural self-purification of water bodies, only artificially enhanced for better sewage purification.
