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Биотехнологии The reactors used for aerobic sewage treatment. The scheme of work of homogeneous reactors
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Lectures 6-7

To this point the biological systems discussed include trickling filters and rotating biological contactors. These systems are effective unit processes in treating wastewater. However, trickling filters and RBCs are termperature sensitive, remove less BOD, and trickling filters cost more to build than activated sludge systems. Although they are more expensive to build, the activated sludge systems are much more expensive to operate because of the need for energy to run pumps and blowers.

Activated sludge refers to biological treatment processes that use a suspended growth of organisms to remove BOD and suspended solids. As shown below, the process requires an aeration tank and a settling tank (picture 2).

Picture 2. Activated Sludge Process Description.

In addition, support equipment, including return pumps, waste pumps, flow measurement devices for return and waste, as well as equipment to provide aeration (mixers and/or blowers) is also required.

Note: Activated sludge processes may or may not follow primary treatment. The need for primary treatment is determined by the process modification selected for use. All activated sludge systems include a settling tank following the aeration tank.

Primary effluent (or plant influent) is mixed with return activated sludge to form mixed liquor. The mixed liquor is aerated for a specified length of time. During the aeration the activated sludge organisms use the available organic matter as food producing stable solids and more organisms. The suspended solids produced by the process and the additional organisms become part of the activated sludge. The solids are then separated from the wastewater in the settling tank. The solids are returned to the influent of the aeration tank (return activated sludge). Periodically the excess solids and organisms are removed from the system (waste activated sludge). Failure to remove waste solids will result in poor performance and loss of solids out of the system over the settling tank effluent weir (picture 3).

Picture 3. Active sludge in waste water treatment

Activated sludge is a process in sewage treatment in which air or oxygen is forced into sewage liquor to develop a biological floc which reduces the organic content of the sewage. In all activated sludge plants, once the sewage has received sufficient treatment, excess mixed liquor is discharged into settling tanks and the supernatant is run off to undergo further treatment before discharge. Part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new sewage entering the tank. The remaining sludge is further treated prior to disposal.

Activated sludge: Biomass produced in raw or settled wastewater (primary effluent) by the growth of organisms in aeration tanks in the presence of dissolved oxygen. The term "activated" comes from the fact that the particles are teeming with bacteria, and protozoa. Activated sludge is different from primary sludge in that the sludge contains many living organisms which can feed on the incoming wastewater.

Activated Sludge Process: A biological treatment process in which a mixture of sewage and activated sludge is agitated and aerated. The activated sludge is subsequently separated from the treated sewage by settlement and may be re-used. A common method of disposing of pollutants in wastewaters.

In the process, large quantities of air are bubbled through wastewaters that contain dissolved organic substances in open aeration tanks. Oxygen is required by bacteria and other types of microorganisms present in the system to live, grow, and multiply in order to consume the dissolved organic "food", or pollutants in the waste. After several hours in a large holding tank, the water is separated from the sludge of bacteria and discharged from the system. Most of the activated sludge is returned to the treatment process, while-the remainder is disposed of by one of several accepted methods.

Sewage is domestic, municipal, or industrial liquid waste products. How it is disposed varies by the area, and the local commitment to the environment. In some countries, notably the United States, national law mandates sanitary treatment of sewage, and outfalls are regulated. Surprisingly, many quite wealthy countries have untreated outfalls directly to surface water, often causing disease, pollution and undrinkable tapwater.

Sewage may be carried directly through pipelines to outfalls, or from upstream sources via river systems. Sewage is often from storm water runoff of streets, parking lots, lawns and commercial and industrial areas. In some urban areas, sewage is carried separately in sanitary sewers while runoff from streets is carried in storm drains. Access to either of these is typically through a manhole.

Sewage may drain directly into major watersheds with minimal or no treatment. When untreated, sewage can have serious impacts on the quality of an environment and on the health of people. Pathogens can cause a variety of illnesses. Some chemicals pose risks even at very low concentrations and can remain a threat for long periods of time because of bioaccumulation in animal or human tissue.

The solution, of course, is sewage treatment.

Sewage contains mineral, animal and vegetable matter in suspension, as well as large numbers of bacteria. It may contain paper, food, grease, cigarettes, leaves, faeces, and urine. Other items that occasionally are flushed down toilets include child care-related waste (such as disposable diapers, training pants, baby wipes, bibs, pacifiers, and outgrown clothing), feminine hygiene materials (tampons and pantiliners), medical waste from hospitals, and industrial chemicals. Some items are disposed of in the sewage system for illicit purposes. Drugs are often disposed of this way in a raid, as are legitimate medications at the end of their useful life. In some prisons, inmates flush blankets down the powerful vacuum suction toilets in vain hopes of amusement caused by a clogged line. Sewage odors are unacceptable to most people. In a confined space such as a manhole or lift station housing, gases such as hydrogen sulfide may be concentrated to dangerous levels, requiring special breathing apparatus and rescue apparatus for workers who must enter such spaces. A special hazard of hydrogen sulfide is that it becomes odorless at high concentrations. Another dangerous gas that can form in sewers is methane, which is both toxic and explosive.

Sludge is a solid waste extracted in the process of sewage treatment. When fresh sewage water is added to a settling tank, approximately 50% of the suspended solid matter will settle out in the period of an hour and a half or so. This collection of solids is known as fresh sludge. Such sludge will become actively putrescent in a short time and must be removed from the sedimentation tank before this happens.

This is commonly accomplished by two different ways. In an Imhoff tank, fresh sludge is passed through a slot to the lower story or digestion chamber where decomposition by anaerobic bacteria takes place resulting in liquefaction and a reduction in the volume of the sludge. After digesting for 6 to 9 months, the result is called "digested" sludge and may be disposed of by drying and then landfilling. It has value as fertilizer, being similar to humus. Alternately, the fresh sludge may be continuously extracted from the tank by mechanical means and passed on to separate sludge digestion tanks which operate at higher temperatures than the lower story of the Imhoff tank and as a result digest much more rapidly and efficiently.

Control questions:

True or False Questions:

T F Color can be indicative of activated sludge health.

T F An overabundance of algae in an activated sludge plant may indicate low nutrient levels.

T F Settled bacteria in an aeration basin provide much of the treatment.

T F The activated sludge process is an anaerobic, suspended growth biological treatment method.

T F In the activated sludge process, the wastewater oxygen demand may be separated into two categories: carbonaceous and nitrogenous.

T F A well-designed and operated activated sludge process, treating typical municipal wastewater, should achieve a CBOD effluent quality of 5 to 15 mg/L.

T F The loading on a nitrification system consists of ammonia plus organic nitrogen and is typically referred to as total nitrogen.