Basic Wastewater-Treatment Terminology: What Is the Activated-Sludge Process?
The activated-sludge process is the main method of wastewater biological treatment centered on activated sludge. It is a sludge-like floc formed by continuously blowing air into wastewater and, after a certain time, the proliferation of aerobic microorganisms. It harbors a microbial community mainly composed of zoogloea, with a strong ability to adsorb and oxidize organic matter.
activated sludge process: a method of sewage biological treatment. In this method, under artificial aeration conditions, sewage and various microbial communities are continuously mixed and cultivated to form activated sludge. Using the biological coagulation, adsorption, and oxidation of activated sludge, organic pollutants in the sewage are decomposed and removed. Then the sludge is separated from the water, most of the sludge 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 selection of the treatment method and the design and operation of the aeration tank and sedimentation tank.
1 Aeration tank: the reaction main body
and polysaccharide viscous substances. At the same time, some macromolecular organic matter is decomposed into small-molecule organic matter under the action of bacterial extracellular enzymes. In the second stage, under sufficient oxygen conditions, microorganisms absorb these organic matters and oxidize and decompose them, forming carbon dioxide and water, part of which supplies
3 Return system: 1) maintain the sludge concentration of the aeration tank; 2) change the return ratio to change the operating conditions of the aeration tank.
4 Excess-sludge discharge system: 1) one of the pathways to remove organic matter; 2) maintain the stable operation of the system.
5 Oxygen-supply system: mainly composed of the oxygen-supply aeration blower and special aerators, providing sufficient dissolved oxygen into the aeration tank.
a. BOD loading rate (F/M), also called organic loading rate, denoted by NS;
Besides the conventional activated-sludge process, there are also multi-point inflow, adsorption-regeneration, extended aeration, and high-rate activated-sludge methods. The first two methods differ from the basic flow in the number and position of the inlet where wastewater flows into the aeration tank. In the multi-point inflow activated-sludge process, only part of the wastewater enters the tank together with the return sludge at the head end. The rest of the wastewater enters the aeration tank in 2-3 batches at 2-3 inlets at a certain distance from the head end (the inlet spacing is generally equal). From the flow perspective, the adsorption-regeneration activated-sludge process (Fig. 2) can be said to be only a variant of the multi-point inflow 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 participating in the process are: organic matter, microorganisms, and dissolved oxygen (air). The former two are primary, and dissolved oxygen only needs to maintain a certain concentration.
Throughout the process, the oxygen demand is different. At the beginning the organic-matter concentration is high and microorganisms proliferate rapidly, so the oxygen demand is large. As the organic matter gradually decreases, the oxygen demand also gradually decreases. In the conventional activated-sludge process, aeration-tank oxygen supply is uniform, which is obviously unreasonable. There are two ways to improve. One is from the aeration method, changing uniform aeration to gradually decreasing aeration. The other is the multi-point inflow method. But multi-point inflow not only reduces the amplitude of oxygen-demand variation but also changes the relative amount of organic matter and microorganisms.
The ratio of organic matter 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 affects the stability of operation and capital cost. A lower sludge loading rate makes the process easier to operate, treatment efficiency more stable, and excess sludge less, but capital construction and operating costs are generally higher. The conventional activated-sludge process loading rate is often between 0.15 and 0.3 kg BOD/kg sludge. The high-rate activated-sludge process uses 1 or above, and the return-sludge volume and air volume can be greatly reduced, saving costs, but the BOD removal rate drops to 60-70%, so it is also called the modified activated-sludge process, used where only medium treatment is needed. The extended-aeration activated-sludge process is the opposite, with a loading rate often below 0.1, aeration time over 24 hours, deep metabolism, little excess sludge, no frequent sludge discharge needed, stable work, and simple management, commonly used in occasions with very small flow.
In practice, people found that the efficiency of transferring pollutants onto the sludge is fast, but the metabolic rate is slow. When treating municipal sewage, often in less than 1 hour the wastewater BOD is reduced by about 90%. But if this sludge is returned to the aeration tank, this ability cannot be reproduced (see aeration), thus creating the adsorption-regeneration method. The regeneration of activated sludge is essentially giving the microorganisms enough time to digest the transferred organic matter. Therefore, some rename it the contact-stabilization method.
Aeration tank: the heart of all activated-sludge processes, whose function is to stir and mix the liquid to fully contact mud and water and supply oxygen to the microorganisms. There are two mixing methods: one keeps the mud and water that enter the aeration tank simultaneously fully mixed and kept so until flowing out of the tank, without mixing with the mixed liquid already in the tank to avoid short-circuit phenomena. The aeration tank adopts a long strip shape to ensure that the simultaneously entering mud and water all exit the tank simultaneously (Fig. 4), giving the simultaneously entering wastewater the same aeration time. The other mixing method makes the entering mud and water immediately and fully mix with the whole-tank mixed liquid, achieving uniform mixed-liquid water quality, possibly placing microbial growth in the best living environment and running the process under the best conditions. There is also a circular aeration long channel, relatively shallow in depth, with mixed liquid circulating in the channel at high velocity. The aeration time of this aeration channel is close to 24 hours, specially called the oxidation channel or oxidation ditch. It is in fact an aeration tank of the extended-aeration activated-sludge process.
Besides designing the geometric shape as required, the aeration method and equipment are also very important. There are two aeration methods: the bubble aeration method (also called the blast aeration method) and the surface aeration method (also called the mechanical aeration method). The deep-shaft aeration that appeared in the late 1970s is also a bubble aeration, increasing the contact time between bubbles and mixed liquid to improve aeration efficiency.
In the surface aeration method, an aerator set at the liquid surface makes the tank liquid circulate and causes the liquid surface to fluctuate violently and exchange gas in close contact with the air. The aerator is generally various vertical impellers, and horizontal brush or propeller types are also used. Circular aeration channels all use horizontal aerators.
To accelerate oxygen dissolution, 'pure oxygen' aeration appeared in the 1970s, replacing general air with air of extremely high oxygen concentration. Mostly the surface aeration method is used.
Operation: mainly the control of activated-sludge amount and oxygen supply. The activated-sludge concentration of the aeration tank (called mixed-liquid suspended solids) is adjustable, that is, the activated-sludge amount and loading rate are adjustable, and should be adjusted according to specific conditions during operation. Sewage plants using the activated-sludge process are prone to sludge bulking, i.e., the sludge has extremely high water content and is difficult to settle. This will cause sludge to flow out of the sedimentation tank with the water, destroying water quality, and at the same time the loss of sludge reduces the sludge in the aeration tank, gradually invalidating the whole process. When a tendency of sludge bulking is found, the cause should be analyzed immediately and measures taken.
1 The wastewater contains sufficient soluble and readily biodegradable organic matter;
2 The mixed liquid contains sufficient dissolved oxygen;
3 The activated sludge is in a suspended state in the tank;
4 The activated sludge is continuously returned, and excess sludge is promptly discharged to maintain a certain concentration of activated sludge in the mixed liquid;
5 No toxic or harmful substances flow in.
The typical activated-sludge process consists of an aeration tank, a sedimentation tank, a sludge-return system, and an excess-sludge discharge system. Sewage and returned activated sludge enter the aeration tank together to form mixed liquid. Compressed air sent from the air-compressor station enters the sewage in the form of fine bubbles through air-diffusion devices laid at the bottom of the aeration tank, aiming to increase the dissolved-oxygen content in the sewage and keep the mixed liquid in a violent agitation state, i.e., a suspended state. Dissolved oxygen, activated sludge, and sewage mix and fully contact, enabling the activated-sludge reaction to proceed normally.
In the first stage, the organic pollutants in the sewage are adsorbed onto the surface of the activated-sludge particles by virtue of their huge specific surface area
2 Secondary sedimentation tank: 1) perform sludge-water separation to ensure effluent quality; 2) ensure return sludge and maintain the activated-sludge concentration in the aeration tank.
to its own proliferation. As a result of the activated-sludge reaction, the organic pollutants in the sewage are degraded and removed, and the activated sludge itself is
proliferated and grown, and the sewage is purified and treated.
The mixed liquid after the activated-sludge purification enters the secondary sedimentation tank, where the suspended activated sludge and other solid matter in the mixed liquid settle down and separate from the water; the clarified sewage is discharged from the system as treated water. The sludge concentrated by sedimentation is discharged from the bottom of the sedimentation tank, most of which is returned as seed sludge to the aeration tank to maintain the suspended-solids concentration and microbial concentration in the aeration tank; the proliferated microorganisms are discharged from the system and called 'excess sludge.' In fact, the pollutants are largely transferred from the sewage to this excess sludge.
The principle of the activated-sludge process in figurative terms: the microorganisms 'eat' the organic matter in the sewage, so the sewage becomes clean water. It is essentially similar to the natural self-purification process of water bodies, only artificially strengthened, so the sewage-purification effect is better.
