DEVELOPMENT OF BIOLOGICAL SEWAGE TREATMENT PLANTS
Sewage recycling and neutralization is among the most important problems of ecology. There are so many various technological means based upon physicochemical or biological degradation of the harmful components contained in sewage.
Sewerage systems and treatment plants started to be intensively constructed in Europe in the 19th century. The earliest designs aimed at sewage detoxication, the so-called land filtration methods, were based on the self-purification ability of soil. Such kind of treatment is carried out in sewage farms or filter beds. However, in order to treat sewage under natural conditions, it is required to alienate significant areas; besides, the level of sewage purification in farms or filter beds drops in winter due to retardation of biological processes at low temperatures. Household sewage contains large amounts of pathogenic bacteria and heminth eggs, more than 50% of which, while getting on soil and vegetables, remain viable for long periods of time. Besides that, the complexity of technological control over the processes of purification at natural biological treatment plants resulted in a poor use of the above method in both our country and a number of industrially developed foreign states.
As compared to natural conditions, biological treatment becomes more intensive under artificial conditions. This process may be controlled and adjusted, hence intensified. It is the possibility to regulate the purification level that brought forth the creation of varied technological methods whose efficiency criterion is the achieved level of purification (ecological factor) and the cost of purification (business factor).
This or that technology application may be limited by its cost which, at the period of the treatment plant operation, depends on the power inputs and maintenance staff number. Industrially developed states have all the required conditions for normal operation of treatment plants. Besides, most countries of this type are located in favorable climatic zones, which promotes optimal biochemical processing at treatment plants.
Russia is in a more difficult situation, since the winter atmospheric temperature at some regions may drop down to -50°C. Frequent are interruptions and damages in the power supply systems. A paramount issue of the day is, above all, the reliability of the aeration equipment at its long operation (as it is frequently needed to be repaired and replaced). Under such conditions, development of such new technologies which would provide high and sustained sewage purification is still a relevant and claimed issue.
Before the 1970-s, sewage purification plants normally used biological filters characterized by reliable operating practices and low power-consumption. However the efficiency of sewage biofilter purification does not exceed 80%, while the optimal range of application of biofilters is incomplete biological purification of a 50-70% efficiency of organic matters’ oxidation.
In the 1980-s Yugoslavian and Bulgarian engineers designed stabilizing plants for purification of the sewage produced by small settlements. These plants included biofilters with plastic sheets placed above the reservoirs where partial oxidation of impurities and mineralization of waste biofilm took place. The water was driven through the biofilter and the stabilizer content was stirred with the help of a circulation pump. Yet due to the low quality of purification and swollen biomass ejection these installations never became wide spread.
Another variety of biofilters is submersible biofilters consisting of reservoirs with horizontal rollers having various devices to form mircoflora attached to them. These plants also fail to provide high-quality purification which would satisfy the demands of the day, and this happens due to the insufficient time of contacts between the sewage and biocenosis, as well as the limited sorptive capacity of active biomass. At an abrupt rise of hydraulic loads or organic matters loads there occurs an admission of non-purified sewage.
In aerotanks, colonies of microorganisms (active sludge) performing sorption and biological degradation of those organic matters, stay in a suspended state in the water under purification. To form an active sludge suspension state, the content of the aerotank is stirred with the help of various devices, most frequently with air because in this latter case the processes of creating aerobic conditions in the aerotank and active sludge suspended state occur simultaneously. The principal aerotank modifications include displacing aerotanks and mixing aerotanks.
Displacing aerotanks are operated in a flowing mode: the clarified sewage which has to be biologically treated arrives to the aerotank inlet where active sludge is also driven. While moving, microorganisms use, in their metabolism, organic matters, i.e. a substrate extracted from the sewage. This way the water is purified. Meanwhile the load on the active sludge in its organic impurities changes from maximum at the plant entry to minimum at its outlet. In the event the sewage contains toxic matters in concentrations inhibiting biological processes, the active sludge is cyclically exposed to these matters shocking impact as it over again arrives to the aerotank.
Relatively constant conditions for active sludge are created in mixing aerotanks.
A modification which occupies an intermediate position between displacing aerotanks and mixing aerotanks is an aerotank with a dispersed water supply. Here to a certain degree combined are the advantages of both above-mentioned aerotanks: for example, like in mixing aerotanks, the load on the active sludge may be averaged within the plant, sudden active sludge overload, organic and toxic matters inhibition may be avoided; at the same time one may achieve high quality of purification typical for displacing aerotanks.
The operating experience shows that with the coming sewage BOD up to 300 mg/l, it is expedient to apply displacing aerotanks, while where BOD equals 150 mg/l, active sludge regenerating sections should be introduced in them. At the same concentration range, it would also be efficient to apply aerotanks with a dispersed water supply. In case of the impurities concentration BOD > 300 mg/l, expedient becomes usage of displacing aerotanks.
Operation of aerotanks may be intensified by way of increasing the sludge dose. However, as the operating experience shows, if the dose exceeds 6 g/l, the sludge is driven out of the system. To avoid this, another modification was designed. It is a settling aerotank incorporating an aeration pond and a secondary settler within one plant.
Those countries where the areas destined for treatment plants are limited, have undertaken attempts to construct tower aerotanks with pneumatic airing. However these did not become wide spread since the needed amount of air was cut twice, while the specific consumption of energy raised 3 to 4 times.
In 1982 Rostov Scientific Research Institute started developing new types of sewage purification plants. This system is now based on the combination of the following devices: biofilters and settling aerotanks. Aeration is carried out in a combined way: through water saturation with air oxygen during biofilters irrigation and through air water-jet ejection during water drainage to the pipes. A detailed description and calculation of the plant can be found among other materials of the 1st International Congress in Paris (http://www.WAhq.arq.uk) and the 2nd Congress in Berlin (http://www.world-water-congress.de).
At present we have completed designing combined works (CW) which permit to purify (97-99%) the sewage whose impurities concentration BOD reaches 1,000 mg/l and suspended matters BOD equals 500 mg/l. The CW design philosophy is defined by their productivity: CW20 - 100 m3/day; CW200 - 500 m3/day; CW700 – 1,500 m3/day; CW2,000 – 100,000 m3/day. The CW effectiveness won a gold medal of the 45th International Inventions Salon in Brussels in 1996.
To purify concentrated and strong sewage and, if needed, remove biogenic elements, the flow chart may be complemented by biocoagulators, aerobic and anaerobic bioreactors of original designs, installations for physicochemical sewage treatment. The originality of the designs and efficiency of the plants have been conformed by more than 20 Russian patents, international applications PCT/RU096/00202, PCT/RU098/00126.
Since 1985 CW are operated at 30 new and reconstructed sites in different regions of Russia. The CW operating experience permitted to give a work-out to the designs on the reconstruction of the previously inefficient plants and showed that the applied technology provides a permanently high quality of sewage purification, as well as 2 to 4 times reduction of power discharge intensity and staff reduction by not less than 30%.
Those CW constructed as enclosed spaces may be located directly in dwelling quarters since the discharge and processing of the waste air used for biochemical processes provide ecologically clean operation of the plant.
The growth of large cities leads to the requirement to construct new manifolds and, consequently, raise the consumption of the power supplied to sewage treatment plants. One of the most up-to-date ways to solve this problem is partial or complete decentralization of drainage systems. However in a number of cases its solution is hampered by difficulties in alienation of significant areas for the construction of treatment plants and observance of the required size of the appropriate sanitary zone.
The future treatment plants should be characterized by minimum size, ecological safety as they will be placed within the city boundaries, while the quality of the sewage purified should permit its use for municipal technical needs.
Working in the above direction, Rostov Research Institute is further improving the engineering solutions used in combined installations and works.
If those CWs operated today use as their biofilter filling corrugated asbestos-cement slate sheets, it is further planned to use a flat filling made of fiberglass plastic of a high specific surface area. To exclude silting, the sheets have roughness which changes depending on the height, wavy partitions and slits.
One of the novelties is the use, in biofilters, of ceramic spherical filling elements with evenly placed recesses whose axes meet in the centre of the sphere. Independent of the way the elements are placed, some part of the recesses are filled with the sludge mixture, which provides a longer contact between the effluent and the immobilized microflora. The availability of active sludge in the filling permits to reach the purification level of 80 to 85% in the biofilter. What concerns the settling tank, here it is raised up to 98% (by the active sludge).
Fine cleaning of municipal sewage demanded the development of a new technology of biochemical purification with a simultaneous removal of carbon-nitrogen compounds.
To accelerate the rates of reproduction of the nitrifying microorganisms and control this process, the irrigation system chutes have helium-neon lasers (HNLs) fixed over them. As the sludge mixture in the nitrifier is exposured to radiation, the speed of growth of the ammonificating bacteria grows 1.5 times. Plastic sheets with holes and bristles are used as the feeding and filtering element.
The requirement to completely automatize the engineering process of sewage purification arises from a probability that the people working at the treatment plant may get contaminated. The minimal number of the equipment units comprising the installations and works designed by the Rostov Research Institute, the reliability and simplicity of control, permit to solve this problem too. On the other hand, the new treatment process controlling equipment should meet the following additional demands:
· all nitri-denitrification controlling sensors and instruments have to be included in the computer software;
· the CW room should contain gas-analyzers and ventilation regulating equipment;
· the CW circulation pumps should be provided with equipment adjusting the number of revolutions in case of fluctuations in the coming sewage flow and composition. This will permit to optimize the control procedure, cut the heat loss in winter, reduce the power consumption in the event of temporary falls in the treated sewage quantity and concentration.
According to our assessments, more than 80% of the sewage treatment plants operated in Russia fail to provide a sufficient level of purification. The percentage of old and ineffective sewage treatment systems operated in America and Western Europe is making about 20%.
The engineering solutions and structural concepts proposed by Rostov Institute will permit to provide high and stable quality of sewage purification along with small capital investments needed for the reconstruction of the existing works. They will also make possible to 1.5 – 3 times cut the power consumption for the plants of a 100 –100,000 m3/day capacity and by 25-50% reduce the number of the maintenance staff.
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