2.4.7. Structures of deep clearing of sewer water
Structures of total biological clearing ensure the degree of clearing up to 15-20mg/l by БПК повне of cleared water. In the range of cases such degree of clearing doesn’t satisfy the requirements country control authority. In such case it’s foreseen the additional clearing (deep clearing) of sewer water.
For clearing there are used biological ponds, filters, micro filters and the structures of physic-chemical clearing of sewer water.
During the usage of filters for clearing of biologically cleared sewer water it is ensured the decreasing of contamination in the water by hanged up substances by 70-75%, by БПКповне – by 50-60%. Usage of micro filters for the same water allows to obtain a bit smaller depth of additional clearing – correspondently be hanged up substances - 50-60%, by БПКповне – 25-30%. For clearing there are used filters with ordinary grain and floating loading.
At the presence of considerable areas for additional clearing there are can be used biological ponds, that ensure the decreasing БПКповне up to 4-6mg/l. Advantages of usage of biological ponds (especially with natural aeration) are the simplicity of their arrangement and maintenance, minimal exploitation expends. If
bio-ponds are additionally planted by kinds of higher water vegetation (rush, reed mace), this increases the reliability of structure work.
Sufficiently wide practical utilization for additional clearing of biologically cleared sewer water was found by regent methods that allow to solve task of water clearing up, decreasing of БПК and deleting the phosphorus, one of biogenic elements, that causes the eutrofication of reservoirs and intensive biological overgrowing of pipelines and equipment, Technological schemes of reagent additional clearing of sewer water, as a rule, are analogical to those that are used in water-supply for clearing of water from natural sources. As reagents are used sulphuric—acid aluminum, natrium aluminat, chloral iron, sulphats with I- II-valences, silicon acid, lime, synthetic flocculants of different types.
Quality of sewer water that was additionally cleared with the help of flocculants, can be improved during the future sorption of surplus contamination on the active coal and seolits, and also during the usage of oxidation methods with usage of ozone, chlorine and other strong oxidants.
2.4.8. Peculiarities of clearing of small volume of sewer water
During choosing of scheme and structures for clearing of small volume of sewer water (individual building, country house sites, rest base) at the absence of centralized sewerage are used, as the rule, the typical decisions.
For the total biological clearing of small volume of sewer water are used septics, filtrating wells, filtration trenches, compact installations and other structures.
Septic – a horizontal flowing reservoir to which come the uncleared sewer water from the objects of canalization. The septic is exploited without clearing during four-six months. Sediment which festered is septic is periodically one- two times per year is exported to the fields as organic fertilizer. However, septics have the range of considerable disadvantages: they should be built of enough big sizes, as they reckon on two-three day rising of sewer water. Besides this, bubbles of gas that are emitted in the process ob anaerobic disintegration of sediment, come to the surface of sewer water and carry to its surface light particles of mule from which its created compacted thick crust, that make difficult the exploitation of septic. For liquidation of repeated contamination the septic is divided onto the chambers by transversal partitions which have windows for the transferring of sewer water from one chamber into another. Total calculated volume of septic, number and volume of chambers are determined depending on daily expends of sewer water which come onto the purifying structure. Septics are built of the prefabricated
reinforced concrete or brick with manholes with double-covers. Septics are arranged on the distance 5-20m from the buildings depending on daily expends of sewer water.
Filtrating wells (fig.2.42) are used at the calculated inflow of sewer water up to 1m3/day and with the presence of sand and sandy-loam soils. Filtration well is a mine of round or square cross-section in the plan up to 2.5m deep, diameter up to 2m, cross-section up to 2x2m. Well is built from the reinforced concrete, brick and rubble stone. In its lower part are made the filter of the gravel, crushed rock, coke, and other filtration materials up to 1m thick. From the outside filtration well is strewed by the same filtration material as the filter. Thickness of strew layer - 20-25cm, that corresponds to the height of filtration part of the well. At the bottom and on the walls of well in the borders of filter the holes are made. In reinforced concrete wells the holes have the diameter up to 30mm and are located on the filtration surface in the chess order in step of 250mm. In the walls of wells from the brick and rubble stone are remained the vertical joints 2 cm wide, which are filled by cement mortar.
Filtration wells, intended for biological clearing of sewer water, are built behind the septics and are arranged at the distance 8-10m from the dwellings. Calculated area of filtrating surface of the wells depends on the loading of sewer water on the 1m2, and also on the soils in which it is foreseen the construction of filtrating wells. Load on the 1m2 of the filtrating surface of the well for the sand soils is taken to be equal80l/day, for sand-loam ones – 40l/day.
Filtrating trenches (fig.2.44) – are the trenches of rectangular shape filled with filtration material 0.8-1.0m thick and are equipped with the network of irrigation and drainage pipes. Irrigation pipes are laid in the layer of gravel or crushed rock. At the bottom of trench are laid the pipes of drainage network with the inclination 0.005 to the side of filtrate removing. Filtrating trenches are built in the soils with the low filtration or in the fully infiltrated soils (clay, loam).
Sizes of filtrating trench depend on expends of sewer water and the load on the irrigation pipes. Load on the 1m of irrigation pipes is equal to 50-70l/day. Length of filtration trench should be up to 30m, and width – not less than 0.5m.
Instead the filtrating trenches there can be used fields of underground filtration (fig.2.45) – land plots where at the depth 0.5-1.8m and higher above the level of ground water it is laid the dividing and irrigation network from the drainage ceramic, asbestos cement or plastic pipes with the diameter 75-100mm. These fields are located on the territory with the sand and sand-loam soils.
Irrigation network from the ceramic pipes is laid with the cracks between joints of pipes 15-20mm. Joints of pipes at the top are covered with joint bars of
the waterproof sheet material (roofing paper, ruberoid, etc). During laying the irrigation network from the asbestos-cement or plastic pipes, in these pipes at the bottom along the pipe are made the cuttings at the distance up to 0.2m one from another, the length of cuttings equals half the diameter of pipe, width – 15mm. Irrigation pipes essentially are laid on the layer of gravel, crushed rock or slag with the thickness 5cm, and then are strewed. Thickness of strew layer – 17-20cm.
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Fig.2.42. Filtrating well from round reinforced concrete rings: 1-outlet pipe from septic; 2-reiforced concrete rings; 3-orifice; 4-separating trough with cogged spillway; 5-ventilation post; 6-filter; 7-covering; 8-ventilation channel.
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