Wastewater treatment plants. Sewage treatment plants: what is wastewater treatment? How we design
Sludge beds of wastewater treatment plants are necessary for sludge dewatering waste water.
The foundation of these sites can be either with a natural foundation (with or without drainage), as well as with surface drainage water Silt pads are planned plots of land (maps) that are surrounded on all sides by earthen ridges.
Sludge from settling tanks or digesters, the humidity of which ranges from 90 to 99.5%, is poured onto areas at certain intervals and dried to 75-80%. A small portion of the sediment seeps into the ground, but a larger percentage evaporates. For sludge beds with a natural base, no drainage is created if they are installed on soil with good filtration capacity. And for dense, low-permeability soil, tubular drainage is created, laid in ditches with crushed stone and gravel.
Treatment plants small sizes The cards have a width of 10 m, and on larger ones it increases to 35-40 m. The cards are separated from each other by protective rollers, with a height of 0.3 m above the working level. The sediment is distributed over the cards using pipes. Sludge beds are promptly cleared of sediment. At large wastewater treatment plants, sludge is removed using bulldozers and scrapers. For treatment facilities with a throughput capacity of more than 10,000 m3/day, sludge platforms are installed with settling and surface drainage of sludge water. As the upper layer of sludge water accumulates, it is pumped into primary settling tanks. Subsequent dehydration of the sludge occurs due to the evaporation of moisture from the surface.
The area of sludge beds depends on the volume of sediment, the type of soil on which they will be organized, climatic conditions and sediment consistency. Natural drying can be accelerated by turning the sludge. During this process, the plant layer is removed and the surface crust is destroyed, which accelerates the drying of the sediment in the warm, dry season and promotes deeper freezing in the winter. Natural cycle sites depend on climatic conditions, which is important when creating a project and subsequent operation of such sites.
Poleka Group of Companies is engaged in the design, installation and subsequent maintenance of treatment facilities, including the maintenance of sludge beds. We offer our clients turnkey services and provide equipment quality guarantees with a service life of up to 50 years. The use of modern and time-tested technologies allows us to create reliable and easy-to-use treatment facilities.
K category: Wastewater treatment
Sludge pads
Digested sludge discharged from digesters, two-tier settling tanks or other structures has high humidity; for example, from two-tier settling tanks the sludge comes out with a humidity of about 90%, from digesters - 96-97%. For further use, the sediment must be dried. There are various techniques for drying sludge; the most common is drying on sludge beds, where the sludge must be dried to an average humidity of 75%, as a result of which its volume is reduced by 3-8 times.
Sludge pads are used on a natural base, a natural base with drainage, on an artificial asphalt concrete base with drainage, with settling and surface removal of sludge water, and compaction pads.
Silt pads consist of graded plots of land (maps) surrounded on all sides by earthen rolls (Fig. 1). The sediment is poured onto the sludge plot maps periodically in layers of 0.2-0.25 m. As the sediment dries, it loses some of its moisture mainly due to evaporation, and some of the moisture is filtered through the soil. The sludge, dried to a moisture content of 75%, is easily loaded onto vehicles and transported to the place of use.
Sludge pads are usually built on a natural foundation with or without drainage if the groundwater level lies at a depth of at least 1.5 m from the surface of the maps and in cases where sanitary conditions allow silt water to penetrate into the ground. At shallower groundwater depths, their level should be lowered. If the danger of contamination
Rice. 1. Sludge pads 1 - ditches of a boundary ditch; g - road; 3 - drain tray; 4 - bars supporting the sludge tray; 6 - drainage well; 7 - prefabricated drainage pipe; 8 - drainage layer; 9 - drainage pipes; 10 - exit to the map; - drainage ditch; 12 - gates; 13 - shield under the drain tray
groundwater is not excluded; the site is built on an artificial foundation that prevents filtered contaminated water from entering the ground stream. In the presence of dense and waterproof soils, as well as insufficient territory, it is recommended to arrange sludge platforms on a natural foundation with tubular drainage enclosed in special drainage ditches filled with crushed stone or gravel with a particle size of 2-6 cm. The distance between drainage ditches should be taken 6-8 m, the initial depth of the ditch is 0.6 m with a slope of 0.003.
The sizes of the cards are taken depending on local conditions, ensuring ease of use. The width of individual maps is set to 10-40, length - 100-150, working depth of the sediment layer - 0.7-1 m, and the height of the barrier shafts is 0.3 m above the working level. The dimensions of one card are determined in such a way that when releasing sediment at one time, the entire card is filled with a layer of sediment no more than 0.25 m in summer and 0.5 m in winter. The height of the shafts is taken taking into account the freezing of sediment in winter time. The sediment is brought to the sites through pipes or trays laid with a slope of 0.01-0.03. The distance between issues, depending on the size of the cards, is 10-50 m.
The load on sludge beds depends on the type of sludge supplied to them (sludge from digesters, two-tier settling tanks, aerobically stabilized, etc.) and on their adopted design. The actual area should be slightly larger than the useful one, since it is necessary to have a reserve of 20-40% for dividing shafts and roads.
The dried sludge is loaded into vehicles and transported to nearby collective and state farms for use as fertilizer.
Soyuzvodokanalproekt and LISI have developed designs for sludge compaction pads 2 m deep. The pads do not have drainage. The design of the gates allows sludge liquid to be drained at different levels, which ensures better sediment dewatering. The annual load on the sites may be increased. It would be advisable to set up such sites in the southern regions of the country.
At treatment plants with a throughput capacity of more than 10,000 m3/day, sludge platforms can be used, where sludge compaction and surface removal of released sludge water occur. The platforms are made in the form of a cascade with four to seven stages. Each cascade contains four to eight cards. Usable area one card 0.25-1 ha.
The width of the map is 30-80 m, and the length is 80-160 m. The height of the barrier shafts is 2.5 m. The released silt water is collected and pumped to treatment facilities. The amount of sludge water is 30-50% of the volume of dewatered sludge.
Part 2
The sludge discharged from sludge treatment plants has high moisture content. To enable further use of the sludge, it must be dried. As indicated, there are various techniques for drying sludge; The most common of them is drying it on sludge beds.
Silt pads consist of a number of planned plots of land (maps) and are surrounded on all sides by earthen ridges.
Sludge discharged from digesters, two-tier settling tanks or other structures usually has different humidity; for example, from two-tier sedimentation tanks the sludge comes out with an average humidity of 90%, from digesters - with a humidity of 96 - 97%, etc. On sludge sites it must be dried to an average humidity of 75%, as a result of which the volume
Rice. 1. Sludge beds
The sediment is poured onto the sludge pad maps periodically, separate layers(0.2 - 0.25 m). As it dries, it partially loses moisture due to evaporation, while a significant portion of the moisture is filtered through the soil. Sludge, dried to a moisture content of 75%, does not flow, it can easily be taken with a shovel and loaded on- transport means for transportation to the place of use.
Sludge beds are installed on natural or artificial foundations.
If the soil filters water well (sand and sandy loam) and the groundwater level is at a depth at which there is no threat of contamination, sludge pads are built on natural soils. Sometimes, with shallow burial,
groundwater, special drainage will have to be installed to drain filtered water. If, even with irrigated soils, the danger of groundwater contamination is not excluded, the site must be built on an artificial foundation that prevents filtered contaminated water from entering the ground flow.
If the soil on which sludge platforms are constructed is dense and waterproof (loam, clay), then the sites are made on artificial bulk soil, consisting of one or two layers of sand and gravel 0.2 m thick. The construction of sludge platforms with an artificial foundation due to their high cost should be resorted to only in extreme cases. The sizes of maps, depending on local conditions, vary: from 20 - 30 m2 for small stations to 0.2 - 0.3 hectares for large ones. At small treatment plants, for ease of operation, the width of individual cards is set to no more than 10 m with a one-way inlet; for large stations this value can be increased to 35 - 40 m.
The sludge is supplied to the sites and unloading point through pipes or trays laid with a slope of 0.01-0.03. The distance between outlets, depending on the size of the cards, is taken from 10 to 40 - 50 m. Drainage for discharging filtered water is usually made of ceramic unglazed pipes, laid at a distance of 4 - 10 m from each other, with a drainage slope of 0.0025 - 0.003. The depth of drainage at the initial points should be at least 1.0-1.2 m.
If raw, unfermented sludge is subjected to drying, then the water filtered from the drainage should be disinfected before being discharged into the reservoir; in the same case, if the fermented sediment is filtered, then it is not necessary to disinfect it.
The rate of annual load or annual inlet depends on the composition of the sediment, the filtration properties of the soil, the location of the site and is usually assigned in accordance with technical specifications. So, for example, in the middle zone Soviet Union for sludge platforms built on sand, the load rate of sediment from digesters is assumed to be 2 m.
The dimensions of one card are determined so that when releasing sediment at one time, the entire card is filled and that the height of the sediment layer does not exceed 0.25 m. The height of the roller is taken to be no more than 1 m and is determined taking into account the freezing of sludge in winter .
When operating sludge sites, timely cleaning of dried sludge is of great importance. Sludge removal at small stations is still not sufficiently mechanized. At some stations, air-dried sludge is loaded into cars and taken to nearby collective farms for use as fertilizer. Sometimes a narrow-gauge track is laid on dividing ridges along which trolleys move. The sludge, loaded into trolleys, is transported along this route outside the site and there it is reloaded into vehicles. A convenient machine for removing and loading dried sludge into vehicles should be considered the VNIOMS loader, mounted on a caterpillar tractor. The loader has, in addition to a bucket for loading bulk solids onto a vehicle, also a bulldozer bucket, which can be used to rake sediment into dumps.
At large stations, it is advisable to remove sludge also in winter. To do this, the frozen silt is split into separate blocks using a special machine, which are then transported to the collective farm fields.
- Sludge beds
→ Wastewater treatment
Sludge beds and silt ponds
Sludge beds are one of the first sewage sludge treatment facilities. Sludge beds are designed for natural dewatering of sediments generated at biological wastewater treatment plants. However, even in the era of intensive implementation of mechanical sludge dewatering facilities, sludge beds are the most common method of sludge dewatering in Russia. Currently, 90% of all sludge generated in Russia is processed at sludge sites. The attractiveness of these structures is explained by the simplicity of engineering support and ease of operation compared to filter presses, vacuum filters, and drying units.
Sludge beds, to a greater extent than other wastewater treatment and sludge treatment facilities and systems, depend on climatic and natural factors.
Rice. 16.1. Sand playground:
1 – sand pipeline with a diameter of 200 mm from sand traps; 2 – distribution tray with a cross-section of 200×200 mm (i = 0.01); 3 – pipeline with a diameter of 200 mm for drainage water
Depending on the extent to which natural processes are used, sites can be divided into two main categories: natural dewatering and drying and intensive dewatering and drying.
The first category includes sites that use natural processes of evaporation and decantation without significant changes compared to the same processes occurring in the natural environment. As a rule, these are sites on a natural foundation with surface water drainage and compaction sites.
The second category includes sites in which certain factors of the natural cycle are modified and intensified. As a rule, these are sites with artificial drainage, heating, creation of a vacuum in the drainage system, and artificial waterproof coating. The use of one or another type of site depends on local conditions: climate specifics, availability additional sources energy, free space.
Natural dehydration and drying areas. At natural cycle sites, sludge is dewatered through the process of compaction and subsequent removal of sludge water, as well as drying.
Silt beds consist of maps surrounded on all sides by ridges (Fig. 16.2). The size of the cards and the number of issues are determined based on the moisture content of the sediment, the range of its spillage and the method of cleaning after drying.
Rice. 16.2. Silt pads on a natural basis with drainage:
1 – ditch of a protective ditch; 2 – road; 3 – drain tray; 4 – shield under the drain tray; 5- distributing tray; 6 – drainage well; 7 – prefabricated drainage pipe; 8 – drainage layer; 9 – drainage pipes; 10 – exit to the map; 11 – drainage ditch; 12 – gates; K1-K5 – wells
Silt pads on a natural basis are designed on well-filtering soils when groundwater occurs at a depth of at least 1.5 m from the surface of the maps and only when filtration of silt water into the soil is allowed. If the depth of groundwater is less than 1.5 m, then it is necessary to lower its level.
The spill range of sludge with a humidity of about 97% can be 75-100 m. In this case, it is advisable to build areas measuring 100x100 m. The spill range of sludge with a humidity of 93-95% can be 20-25 m, in this case the width of the maps will be limited to 40 -50 m with double-sided inlet. Narrow areas are preferable when planning on an area that has a well-defined slope.
The dried sludge is raked by bulldozers or scrapers and transported by truck. The humidity of the dried sludge is 75%.
Roads with ramps for access to the maps of vehicles and mechanization equipment are constructed on the sludge beds.
In dense and waterproof soils, sludge platforms are constructed on a natural foundation with tubular drainage laid in drainage ditches. The artificial drainage base of sludge beds must be at least 10% of their area.
The following should be taken: working depth of cards - 0.7-1 m; the height of the barrier rollers is 0.3 m above the working sediment level on the map; the slope of distribution pipes or trays is not less than 0.01; the number of cards is at least four.
The most widespread are sludge platforms on natural foundations of the cascade type with settling and surface removal of sludge water. After filling the sludge pad cards with sediment and draining the separated sludge water, further dewatering of the sludge is carried out by evaporation of the remaining moisture from the surface.
An improved version of cascade-type platforms are compaction platforms. Sludge compaction platforms are rectangular reinforced concrete tanks (cards) with holes located in the longitudinal wall at different depths and covered with gates. To release silt water released when sediment settles, holes are made along the height of the longitudinal walls of the reservoir tanks, blocked by gates. Sludge water is sent for cleaning to the head of structures in a similar way to sludge beds with settling and surface removal of water. The distance between sludge water outlets is set to no more than 18 m. For mechanized collection of dried sludge, ramps with a slope of up to 12% are installed.
One of possible methods, which accelerates the natural drying of sludge on sludge beds, is the process of turning. At the same time, the vegetation cover is removed and the surface crust is destroyed, which promotes accelerated drying of the sediment in warm, dry times and deeper freezing in winter.
Characteristic feature natural cycle sites is their complete dependence on climatic factors. When designing and operating such sites, it is especially necessary to take these factors into account in order to obtain the desired result - dewatered sludge of a certain moisture content.
Sludge sites for intensive dewatering and drying can be divided into traditional and improved. The first category includes sludge platforms with vertical and horizontal drainage, the second category includes sites with the creation of a vacuum in the drainage system, an artificial waterproof coating with air blowing, and heating.
Cascade type sludge beds with a natural base and surface water drainage through monk wells installed at the ends of the pads are transitional type sludge beds. The walls of the monk wells from the map side represent drainage walls made of double reinforcing mesh with gravel loading with a particle size of 15-20 mm.
Sludge beds with artificial drainage are designed to obtain clean filtrate and increase the rate of dewatering.
Filtration through a horizontal drainage system can be carried out using filter panels with special holes or drainage pipes.
A filter platform with horizontal drainage (Fig. 16.3) is a shallow rectangular tank with waterproof walls and a false bottom made of special panels. These panels have wedge-shaped holes measuring 1-4 mm. The border of the false bottom is made waterproof, and the joints between the panels and the walls are sealed.
Rice. 16.3. Layout of the filter sludge platform:
1 – compaction zone; 2 – partition with wedge-shaped slots; 3 – filtrate level control chamber; 4 – outlet valve that regulates the filtration rate
An outlet valve is provided on one of the walls of the platform, connected to the space under the false bottom. A controlled drainage rate is ensured by introducing a layer of water into the system to a certain level above the false bottom. The sediment is then slowly introduced and, under appropriate conditions, maintained on a layer of water. After supplying the required amount of sludge, the initially introduced water and silt water from the sludge seeps through the false bottom. The filtration rate is maintained constant by a constant pressure in front of the outlet valve. For a successful dewatering process, it is necessary that the sediment and the original aqueous layer do not mix. The sludge dewatering technique at such sites involves the controlled formation of a layer of cake at the interface between the sludge and the filter medium, before any significant amount of the smallest particles fall onto this surface or into the holes of the false bottom and end up in the filtrate. The productivity of the filter platform in terms of dry matter is usually from 2.4 to 4.8 kg/m2 per load.
The drainage system of traditional filter sludge beds with drainage pipes usually includes: – a top layer of sand 15-25 cm high, with an effective diameter of 0.3-1.2 mm and a heterogeneity coefficient of less than 5; – a layer of gravel 20-45 cm high, with a grain size of 0.3-2.3 cm; – drainage pipes, often made of ceramic, with a minimum diameter of 10 cm, with open ends, located at a distance of 2-6 cm from each other.
IN lately began to be used plastic pipes, since ceramic ones are quickly destroyed during mechanized sediment removal.
The sediment is fed onto the filter cards either at one or several points in a layer of 250-450 mm and remains on the cards until it dries. When favorable weather conditions well-fermented sludge dries within 2 weeks, reaching a moisture content of 60-70%
To reconstruct existing sites, a drainage system containing vertical filter elements and pipes for draining sludge water can be used. Such a drainage system is made in the form of sectional and general pipes distributed over the surface of the site, having seats with mesh bottoms into which vertical filter elements are installed.
The common pipe is connected to the sludge water drainage pipe.
Filter fiberglass pipes can be used as filter elements for drainage systems. Such filter pipes are used for well construction. Horizontal design drainage system consists of a fiberglass filter pipe. The vertical filter element is made from a similar pipe, but of a larger diameter, coated with filter material. It is connected to horizontal drainage pipelines using steel tees and flange connections.
Visual observations of the operation of the drainage system during various types loadings showed that a layer with high filtration resistance is formed at the sediment-drainage loading boundary.
It is noted that in initial period the specific filtration rates through the vertical drainage system are higher than through the horizontal one, then they level out. At the final stage of drying, only horizontal drainage works. When re-filling sediment onto an already dried layer, the filtration rate is significantly reduced.
A study of the composition and properties of urban wastewater sludge, carried out by I.S. Turovsky, showed that the load on sludge beds largely depends on the type and water yield of sludge. An analysis of operating data from a number of wastewater treatment plants showed that there is a certain connection between the values of sediment resistivity and the operation of sludge beds. Thus, at the aeration station of Kaliningrad (Moscow region), with a humidity of the fermented mixture of 94.8% and its specific resistance of 25800-1010 cm/g, the load per 1 m2 of sludge beds was 0.35 m3 per year. The drainage quickly became clogged, and the platforms worked only to evaporate the liquid.
The clogging of the base occurs the faster, the worse the sediments are filtered, which is due to the high content of fine and colloidal particles in them. The layer of simultaneous influx of sediment onto sludge beds can be greater than less value specific resistance of sediment. At high values of sediment resistivity, most of the moisture is removed by evaporation.
Improved sites for intensive sludge dewatering and drying. To intensify the sludge drying process, it is proposed to blow it with air directly on the site.
The sludge platform contains a waterproof bottom, side walls, a drainage load, perforated pipes placed on the bottom, an air duct and pipelines for washing and filtered water. Air purging is carried out to the required degree of dehydration.
The use of capillary suction effect accelerates the process of sludge dewatering on sludge beds. A sludge pad using this effect (Fig. 16.4) works as follows. When the cards 1 are filled with sediment, thanks to the forces of capillary suction, water from the sediment is absorbed through the edges of the sheets 4 placed in the corridor 3, evaporating into the environment.
The walls of adjacent cards are installed to form corridors, in which sheets of capillary fiber are also placed - porous material. The sludge beds are equipped with blowers connected to the corridors by air ducts.
Abroad, sludge beds are often protected from precipitation with a glass coating. This coating can significantly improve the performance of sites, especially in cold and humid climates. Experience has shown that in some cases, the coating device can reduce the area required for drying sludge by 33%.
The degree to which the required area is reduced and the load on sludge beds increased as a result of the use of transparent or translucent coatings depends on local conditions, such as the amount of precipitation, temperature, and solar radiation.
Rice. 16.4. Sludge bed using capillary suction effect:
1 – silt maps; 2 – enclosing walls; 3 – corridor; 4 – sheets of capillary-porous material; 5 – blower; 6 – air duct
In our country, closed areas, glazed like greenhouses, are recommended to be used in resort areas to save space and reduce the intensity of odors. The load on digested sludge from digesters is assumed to be 10 m3/(m2year).
Asphalted sludge beds with central drainage and heating are used in Dunedin (USA, Florida). These sites are of interest due to the use of a heating system on them. Thermal energy, obtained by burning biogas from wastewater treatment plants, is used to heat water, which circulates in pipes located in the asphalt part of the sites. The sludge beds are heated, but not closed. Polyelectrolytes are used to condition sediments. The drying time for sediment is on average 5 days and increases to 12 days during the rainy season. The annual load on sludge beds in terms of dry matter ranges from 87.9 to 209.9 kg/(m2.year).
Conditioning the sludge before dewatering sludge on sludge beds significantly reduces the duration of the dewatering process and improves the performance of the dried sludge. The method of conditioning sludge with organic flocculants before feeding it to sludge beds is currently widely used in Germany. The humidity of flocculated and untreated fermented sludge from one of the stations after dehydration on sludge beds was respectively: after 2 days 76 and 87%, after 5 days 73 and 86%, after 10 days 72 and 83%, after 15 days 71 and 80% , after 20-25 days approximately 70-77%). Under normal atmospheric conditions (Germany), the conditioned sludge is dried on sludge beds after 3-4 weeks to a humidity of approximately 75%> and can be removed without difficulty using machinery. Thanks to the coagulation of colloids and tiny particles, the phenomenon of silting of drainage is reduced. Dehydrated sludge has a “permeable hydrophobic structure” and even when it rains it does not absorb water, its humidity does not increase.
Studies of the use of domestic flocculants to intensify the work of sludge beds were carried out on a fermented mixture of sediments and aerobically stabilized activated sludge on laboratory models and in pilot industrial conditions on a sludge bed measuring 600 m2, equipped with vertical and horizontal drainage systems made of fiberglass filters. Best results were obtained using flocculant brands KNF and K-100. At the same time, sludge humidity of 78-81% was achieved approximately twice as quickly as when drying sludge not treated with flocculants. The specific productivity of the site during dewatering of sludge treated with flocculants was 4.5-6 m3/(m2-year). The drainage load consisted of a layer of sand 50-150 mm with fraction sizes of 1-3 mm and 3 layers of crushed stone with fraction sizes from top to bottom of 5-3 mm, 10-5 mm, 15-10 mm. Studies have shown that the load on sludge beds when drying stabilized activated sludge and digested sludge for conditions middle zone In Russia, respectively, 4.5 and 5 m3/(m2-year).
To intensify the work of sludge beds, in addition to treatment with flocculants, it is possible to carry out preliminary washing of difficult-to-filter sediments with purified waste liquid, coagulation of sediments with chemical reagents, as well as freezing and subsequent thawing of sediments. All these types of treatment reduce the resistivity of sediment filtration. Preliminary washing of sludge allows you to increase the load on sludge beds by 70%, and the use of chemical reagents or additive materials when drying sediments increases the load on sludge beds by 2-3 times. The resistivity of aerobically stabilized sludge is significantly lower than that of fermented sludge. In sludge beds on an artificial base with drainage and surface water removal at an average annual air temperature of 3-6°C and an average annual precipitation of up to 500 mm after aerobic stabilizers, according to the Federal State Unitary Enterprise Research Institute VODGEO, the load is 3-5 m3/(m2year) with the humidity of the incoming sediment 96.5-97%. In this case, the drainage area is 8-10% of the site area. The size of the map is based on filling it to a working depth of 1-2 m for no more than 3 days. An additional increase in the productivity of the sludge bed can be achieved by subjecting aerobically stabilized sewage sludge to treatment with ammonium nitrate in an amount of 100-150 mg/l. Ammonium nitrate is introduced into the aerobically stabilized sludge (at the outlet of the aerobic stabilizer) and fed to the sludge bed. In the filled sludge bed, the biological process of denitrification of the nitrate compound occurs, i.e. ammonium nitrate introduced into the sediment. The process is spontaneously carried out by denitrifying bacteria, which are part of the bacterial flora of sediment, and is accompanied by intense gas release of nitrogen, ensuring flotation and thickening of sediment particles. The volume of sediment decreases by 5-6 times, its concentration is approximately 50 g/l. Under the compacted layer of sediment there is silt water containing 6-10 mg/l of suspended solids. After the sediment compaction process is completed (4-7 hours), the drainage is opened and the sludge water is released. The condensed sediment sinks to the bottom and quickly dries, because has a good structure due to the presence large number pores formed by gas bubbles. One cycle of operation of the site from the moment of loading to unloading of dry sludge is no more than 1 month. The load reaches 8-10 m3/m2 per year with a site depth of 1.0-1.5 m.
Principles for calculating sludge beds. The method for calculating sludge beds was developed in the twenties by Imgoff and has existed virtually unchanged to this day. The calculation is based on the load Kf> m3/(m2year), which establishes the permissible volume of precipitation placed on a unit surface of the sludge platform per year.
The total area of sludge beds should be increased by 20-40% for the construction of fencing ridges and access roads.
During negative temperatures the supplied sludge is alive. 80% of the area of sludge beds is allocated for winter freezing, and 20% is intended to accommodate sediment during the period of melting of previously frozen sediment.
Recent studies of the operation of sludge beds have shown that the dewatering process must be considered as complex, consisting of several elementary processes.
The rate of moisture removal as a result of drying, according to research, depends on the wind speed and the lack of humidity in the air above the sites.
The filtration stage is determined by the properties of the sediment and the characteristics of the drainage system, and the decantation rate is determined by the ability of the sediment to compact.
Intensification of the work of sludge beds. Increasing the productivity of sites is possible through the following measures: – compaction of sludge supplied to the sites; – ensuring mechanical tedding and removal of dried sludge from the site; – conditioning of sludge before delivering it to the site; – blowing the sediment with air directly on the site; – installation of a translucent covering or a general greenhouse-type covering over the platform with appropriate ventilation systems; – use vacuum systems to speed up filtration; – installation of sludge heating systems directly on sludge beds.
The turning process significantly accelerates the natural drying of sludge on sludge beds. The wind speed over the surface of sediment overgrown with vegetation is practically zero, the deficit of water vapor elasticity is characterized by a decrease from the upper tier of leaves to the lower tier to virtually zero, therefore, the rate of water evaporation from sediment densely overgrown with vegetation is zero. The formation of a crust of overdried sediment on the surface of the sediment reduces the drying rate by 4 times.
When turning, the plant cover is removed and the surface crust is destroyed, which contributes to accelerated drying of the sediment in warm, dry times and deeper freezing in winter.
The properties of the processed sludge, especially the ability to compact and specific filtration resistance, influence the choice of sludge platform design: at g 4000 -1010 cm/g - with settling and surface water removal.
Dehydration of fermented sludge, which has a specific filtration resistance of the order of 4000 -1010 cm/g, on maps with horizontal drainage has low efficiency. Filtration rates do not exceed 0.48 kg/(m day), which is 1.5 times less than the evaporation rate with a moisture deficit of 6 mbar. The site drainage quickly becomes clogged and stops passing filtrate. The amount of water released during filtration through drainage is insignificant.
The specific filtration resistance of aerobically stabilized activated sludge is 20-100 times less than the specific filtration resistance of digested sludge, therefore, it is rational to use sites with drainage for dewatering aerobically stabilized activated sludge.
Selecting the optimal sludge dewatering technology can significantly improve the productivity of sludge beds. The filling mode, primarily the height and frequency of filling, depend on the type of sediment, its concentration, the characteristics of preparation and the time of year. When supplying stabilized activated sludge with an initial humidity of up to 98% to the site, the filling height should be 0.8-1 m. In this case, a significant volume of drainage water is discharged through a vertical drainage system.
For fermented sediments the most effective method dewatering on sludge beds is a technology of separate compaction, drying and freezing. As the depth of the compacted sediment layer increases, the compaction speed increases and the likelihood of sediment stratification decreases. It is recommended to compact the sludge at a filling height of at least 2.5 m, and drying and freezing in layers of no more than 0.3 m.
Silt ponds. In developing countries, silt ponds (lagoons), made in the form of ditches or by damming natural depressions or ravines, have become widespread. The cost of constructing sludge ponds is less than that of sludge platforms, primarily due to the use of natural excavations and simplicity of design. A necessary condition in all cases is the occurrence of groundwater below the silt ponds. After filling, the lagoons are covered with a layer of local soil up to 40 cm thick. The sediments rot over several years, after which they are used as fertilizer.
Multi-stage sludge ponds are used, in which liquid sediment and water are transferred to subsequent ponds, and drying and unloading are carried out in the previous ponds. In Daugavpils (Latvia), periodic silt ponds with an area of 12.0 ha were built with filtration of silt water into the ground.
A design has been developed for silt ponds 6 m deep with screening of the bottom and slopes with a polymer film. In such ponds, the furrows (ditches) are filled with sediment layer by layer, and a layer of soil 0.7 m thick is poured on top. After a year or two, trees for forest protection or forest park purposes are planted in this place.
The simplest and most common way of dewatering sludge is to dry it on sludge beds with a natural base (with or without drainage), with surface water drainage and on compaction pads.
The first are planned plots of land (maps), surrounded on all sides by earthen ridges (4.60). Raw sludge from sedimentation tanks or fermented from digesters, two-tier sedimentation tanks or other structures, having a humidity of 90% (from two-tier sedimentation tanks) to 99.5% (unfermented activated sludge), is periodically poured in a small layer onto areas and dried to a humidity of 75-80% .
Moisture from sediment partially seeps into the soil, but most of it is removed through evaporation. The volume of sediment decreases. The dried sediment takes on the structure of wet soil. It can be picked up with a shovel and loaded into trolleys and dump trucks for transportation to the place of use.
Silt pads on a natural foundation without drainage are used in cases where the soil has good filtering capacity (sand, sandy loam, light loam), the groundwater level is at a depth of at least 1.5 m from the surface of the map, and seeping drainage water can be released into soil according to sanitary conditions. At a shallower depth of groundwater, it is necessary to lower its level.
If the soil is dense and poorly permeable, the sites are equipped with tubular drainage laid in ditches filled with crushed stone and gravel. The distance between drainage ditches is recommended to be 6-8 m, the initial depth of the ditch is 0.6 m with a slope of 0.003.
At small treatment plants, for ease of operation, the width of individual cards is taken to be no more than 10 m. At medium and large stations, the width of cards can be increased to 35-40 m. The sizes of cards should be assigned taking into account the placement of sediment released at a time with a layer thickness in summer 0.25-0.3 m and in winter 0.5 m.
The cards are separated from each other by protective rollers, the height of which is 0.3 m above the working level.
Additional information on sewerage:
INSPECTION WELLS AND CONNECTING CHAMBERS
DIFFERENCE WELLS
LLC DESIGN PRESTIGE
The sediment is distributed over the cards using pipes or wooden trays, laid mostly in the body of the separating roller with a slope of 0.01-0.03 and equipped with outlets.
Sludge areas must be promptly cleared of dried sediment. At small treatment plants, sludge is manually loaded into machines and transported to nearby collective and state farms for use as fertilizer.
Sometimes narrow-gauge tracks for trolleys are laid on the dividing rollers, on which the sediment is transported outside the area and unloaded into cars there.
In winter, frozen sediment is split into separate blocks by special machines, which are then transported to collective farm fields.
At medium and large stations, scrapers and bulldozers are used to remove sediment. The waste collected in dumps is loaded into vehicles using a peat or manure loader mounted on the base of a DT-54 tractor, or a multi-bucket loader. The latter is the most economical of the mechanisms used - its productivity is up to 40 m3/h.
In areas with an average annual air temperature of 3-6° C and an average annual precipitation of up to 500 mm, for treatment facilities with a throughput capacity of more than 10,000 m3/day, it is recommended to construct sludge platforms with settling and surface drainage of sludge water. Figure 4.61 shows sludge platforms of this type built at the Kuryanovskaya aeration station. Sludge platforms with surface drainage of sludge water are designed in the form of several (4-7) independently operating cascades. Each cascade consists of four to eight cards arranged in stages. The discharge of sediment from the supply pipelines is provided onto the upper cards. As it accumulates, the upper layer of silt water (or sediment) is discharged to the underlying map through reinforced concrete bypass-wells. The settled sludge water from the lower map of the cascade is pumped into the primary settling tanks of the treatment plant, since the content of suspended solids in it can reach 1.5-2 g/l. The volume of settled sludge water is 30-50% of the volume of dewatered sludge, the humidity of which is reduced from 97 to 94-95%. Further dehydration of the sludge occurs due to the evaporation of moisture from the surface of the sludge.
The useful area of one map is taken to be 0.25-1 hectares with a width to length ratio of 1: 2-1: 2.5.
Sludge compaction pads developed by the Soyuzvodoka-nalproekt Institute together with the Department of Sewerage LISI. The sites consist of rectangular tanks with waterproof bottoms and walls. The walls are constructed from prefabricated reinforced concrete unified panels 2.4 m high, the bottom is monolithic. Working depth of the site is 2 m.
If there is not enough space to set up open sludge areas, then sometimes covered sludge areas are built like greenhouses, covering them with glazed frames. Such sites were built in Kislovodsk. According to experimental data, the annual load on them is 9-10 m3/m2 when drying sediments from digesters.
The area of sludge sites depends on the volume of sediment, the nature of the soil on which the site is located, climatic conditions, as well as the structure of the sediment.
When designing sludge beds with surface drainage of sludge water, the load is assumed to be 1 m3/m2 per year.
Sludge compaction sites are calculated according to the load q, which depends on the working depth of the site and the number of unloadings per year, taken depending on the properties of the sediment and climatic conditions from 1 to 5.
It is necessary to check the sufficiency of the area obtained in the calculation, taking into account the freezing of sediment in winter. Long lasting! The period during which freezing occurs is determined by the number of days with an average daily temperature below -10° C. Part of the moisture (25%) is filtered and evaporates in winter. 80% of the surface of the sludge pads is allocated for freezing, and 20% is left for use during the spring thaw.
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Selecting a section Sewerage Selecting a section Water supply Additional information......
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Small sewerage includes networks and structures designed for drainage and......
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For the technical and economic characteristics of the operation of treatment facilities, technological accounting of the results of the work of individual......
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At treatment plants with a throughput capacity of up to 1000 m3/day, disinfection is allowed......
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As already mentioned, wastewater from cities and towns, if they......
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The volume of excess activated sludge formed at stations is usually 1.5-2.5......
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