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Sedimentation is the simplest and most frequently used method of separating coarse impurities from wastewater, which, under the influence of gravitational force, settle to the bottom of the settling tank or float to its surface.
Depending on the required degree of wastewater treatment, sedimentation is used either for the purpose of preliminary treatment before treatment in other, more complex facilities, or as a final treatment method, if local conditions require the separation of only undissolved (sedimented or floating) impurities from wastewater.
Depending on the purpose of the settling tanks in the technological scheme of the treatment plant, they are divided into primary and secondary. Primary are called settling tanks in front of biological wastewater treatment facilities; secondary- settling tanks installed to clarify wastewater that has undergone biological treatment.
Sedimentation tanks are classified according to operating mode periodic action, Or contact, into which wastewater flows periodically, and its settling occurs at rest, and settling tanks continuous action, or flow-through, in which settling occurs with slow movement of liquid. In wastewater treatment practice, sedimentation Weighted substances are most often produced in flow-through settling tanks.
Contact settling tanks are used for treating small volumes of wastewater.
Based on the direction of movement of the main water flow in settling tanks, they are divided into two main types: horizontal And vertical; a type of horizontal are radial settling tanks. In horizontal settling tanks, wastewater moves horizontally, in vertical ones - from bottom to top, and in radial ones - from the center to the periphery.
The so-called settling tanks also include clarifiers. Simultaneously with settling in these structures, wastewater is filtered through a layer of suspended substances.
The content of undissolved impurities (suspended solids) released by primary settling tanks depends on the initial content and on the characteristics of these impurities (shape and size of their particles, density, rate of sedimentation), as well as on the duration of settling. The bulk of coarse suspended solids precipitates within 1.5 hours (see Fig. 4.2). Settlement rate and completeness Release of fine particles from water depends on their abilities Towards agglomeration.
The permissible residual content of suspended solids - removal from primary settling tanks - is established depending on the type of biological oxidizers for subsequent wastewater treatment. In accordance with this, the duration of settling is taken.
Suspended substances of more than 150 mg/l should not be removed from settling tanks in front of biofilters and aeration tanks for complete treatment. The duration of settling of urban wastewater in this case should be 1.5 hours.
The choice of type, design and number of sedimentation tanks should be made on the basis of their technical and economic comparison, taking into account local conditions.
Vertical settling tanks are usually used when groundwater levels are low and the throughput of treatment facilities is up to 10,000 m3/day. Horizontal and radial settling tanks are used regardless of the groundwater level when the throughput of treatment facilities is over 15,000-20,000 m3/day. Radial settling tanks with a rotating distribution device are used at stations with a throughput capacity of more than 20,000 m3/day with an initial concentration of suspended solids of no more than 500 mg/l.
The main conditions for the effective operation of settling tanks are: establishing the optimal hydraulic load on one structure or section (for given initial and final concentrations of wastewater and the nature of suspended solids); uniform distribution of wastewater between individual structures (sections); timely removal of sediment and floating substances.
The effect of sedimentation depends on the height of the water layer in which settling occurs.
The settling depth R in full-scale structures is 2-4 m. In laboratory conditions, the kinetics of the wastewater settling process is usually studied at a lower water layer height.
The State Committee on Science and Technology and the Technical Council of the CMEA member countries have accepted that in order to compare the results of studies carried out by different authors, experiments on settling suspended substances at rest should be carried out at a height of the liquid layer H - = 500 mm, taken as the standard.
For aggregation-stable particles, a simple relationship is adopted that makes it possible to recalculate the time T required to obtain a given cleaning effect in settling tanks, based on the results of laboratory studies in cylinders of a reference height for a duration T:
TIH ~tlh At E - Const,
Where I is the height of water in the settling tank, m; h is the height of water in the cylinder, m. For agglomerating suspended substances that predominate in wastewater, the duration of settling remains proportional to the height of the layer, but this relationship is not linear. In this case, the estimated duration of settling of wastewater in a settling tank T with depth R can be determined from the duration of its settling in laboratory conditions T at height H according to the ratio proposed by the Academy of Public Utilities named after. K. D. Pamfilov and the Moscow Civil Engineering Institute named after. V.V. Kuibysheva, in the following form:
T = t(H/h)n, (4.58)
Where n is an exponent reflecting the influence of agglomeration: for well-formed coagulated flocs in wastewater P - 0.5; for gas treatment wastewater ha = 0.45; for municipal wastewater with suspended solids concentration up to 400 mg/l n= 0.25, with increasing initial concentration P increases: for example, at 600 mg/l p=0.3; for mine waters /g=0.35; for wool washing wastewater « = 0.19...0.44 depending on the amount of fat and surfactants in the wastewater. However, not for all types of wastewater there are sufficiently complete experimental data characterizing the sedimentation of suspended particles.
In cases where data is not available and cannot be obtained experimentally for some reason, settling tanks are calculated based on available data for wastewater of similar composition or other calculation methods are used (for example, based on the load of wastewater in m/m2 of the settling tank surface) .
The initial data when calculating settling tanks for any degree of completeness of separation of insoluble impurities from wastewater, regardless of their type, are: 1) the volume of wastewater and the initial concentration of suspended solids in it, Cb; 2) the permissible final concentration Cg of suspended solids in settled water, accepted in in accordance with sanitary standards or due to technological requirements, as, for example, when calculating primary settling tanks in front of aeration tanks for complete purification and biofilters, when C2 should be 100-150 mg/l; 3) conditional hydraulic size u0 of particles that need to be separated from water; water column height H in a laboratory cylinder in which technological analysis (settlement) of wastewater is carried out; 4) exponent P, reflecting the influence of agglomeration of suspended particles during their deposition.
The required working effect of lightening is determined from the expression
Cl~~Cz yuo. (4.59)
According to this effect, the lowest sedimentation rate (hydraulic particle size) is taken u0, mm/s (Table 4.17), or the duration of settling (see Fig. 4.26), by which the main dimensions of the primary settling tanks are determined.
Effect of wastewater settling E and the resulting sludge compaction affect the efficiency and sustainability of wastewater treatment plants, especially in biological wastewater treatment.
An increase in the removal of suspended particles from primary settling tanks leads to an increase in the volume of excess activated sludge in aeration tanks. The humidity of activated sludge (99%) significantly exceeds the humidity of sludge (93-95%) from primary settling tanks. This necessitates increasing the capacity of sludge compactors and all subsequent facilities for processing excess activated sludge.
In order to increase the efficiency of settling tanks, especially when the content of suspended solids in wastewater is more than 300 mg/l, it is necessary to take additional measures: a) add chemical reagents to wastewater - coagulants that help increase the hydraulic size of impurity particles; b) add well-settling suspended substances, in particular, activated sludge, which acts as a sorbent and biocoagulant; c) pre-aerate the wastewater, which promotes flocculation (floc formation and enlargement) of the smallest undissolved impurities in the wastewater.
Chemical reagents are used mainly in the treatment of industrial wastewater, biocoagulation and flocculation - in the treatment of household wastewater and its mixtures with industrial waters.
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1 - pipeline for draining raw sludge and emptying; 2 And 4 - trays with a cross-sectional area of 800X900 and 600X900 mm, respectively; 3 And 14 - siphons for supplying raw waste water, respectively D=700 ANDD=1000 Mm; 5 - inlets; 6 -- scraper trolley; 7 - fat-collecting hot,th = =400 mm; 8 - weir rib; 9 - front trolley; 10 - fat pipe, 5=200 mm 11 - gravity piping to carry away raw sludge and grease for emptying. 12 - emergency siphon with a cross-sectional area of 1200X1200 mm; 13 - gravity pipeline for removal of raw sludge and emptying, D- 200 mm; /5 - gates 400ХБ00 mm; /b - siphon for drainage of clarified water,D=7 00 mm
Stits uQ pbd action FORCES Gravity and speed of horizontal movement of water V Along the sump (Fig. 4.28). The trajectory of the particle is directed here along the resultant of these two velocities.
For given values of I, L And V it is possible to find such a value of the sedimentation rate u0 at which the resultant will pass through the most distant point of the bottom of the settling tank r. Only suspended particles will be retained in the settling tank, having a sedimentation rate that is the smallest for a given settling tank. It is called the covered speed, i.e. the hydraulic size of those
More fine suspended solids that are retained by a settling tank of the specified length. Smaller particles that fall at a lower speed u0, will be carried out with water.
The efficiency of precipitation of suspended solids from wastewater in primary settling tanks is characterized by the data in Table. 4.17.
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Table 4.17 Efficiency of sedimentation of suspended solids from domestic wastewater in primary settling tanks |
When designing primary horizontal settling tanks for domestic and industrial wastewater similar in composition, it is recommended to take the design depth of the settling (flow) part ~3 m (4 m is allowed), the design horizontal flow velocity V = 5...7 mm/s, settling tank length L - VH/ U0 (Here u0- according to the table 4.17).
Table 4.18 gives the dimensions of typical horizontal primary settling tanks.
Table 4.18
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Main parameters of horizontal primary settling tanks |
The height of the sump side above the water surface usually does not exceed 0.4 m.
A neutral layer 0.4 m high is provided between the flow and sludge parts of the settling tank.
The width of the settling tank is taken depending on the method of removing sediment from it, but in such a way that the number of settling tank compartments is at least two. Usually this width does not exceed 9 m. It is advisable to link the width of the settling tank to the width of the aeration tanks (b and 9 m) in order to be able to combine these structures into sections.
The available standardized prefabricated panels with a height of 3.6 and 4.8 m for rectangular tanks make it possible to select two standard sizes of horizontal settling tanks - 3.2 and 4.4 m - according to the depth of the flow path.
Sludge is removed from settling tanks under hydrostatic pressure and using various mechanisms (scrapers, pumps, elevators, etc.).
The main advantages of horizontal settling tanks are: shallow depth, good cleaning effect, the ability to use one raking device for several compartments. Their disadvantages include the need to use a larger number of settling tanks due to the limited width.
A vertical settling tank (Fig. 4.29) is a round tank with a conical bottom.
Wastewater is supplied to the central pipe and flows down it. When leaving the bottom of the central pipe, it changes direction and slowly rises up to the drain gutter. In this case, coarse impurities fall out of the wastewater, the density of which is greater than the density of the wastewater. To better distribute water over the entire cross-section of the sump and prevent sediment from becoming agitated by descending water, the central pipe is made with a socket, below which a reflective shield is installed.
Each particle of undissolved impurities entering the settling tank tends to move upward along with the layer of water at the same speed V, with which water moves; at the same time, under the influence of gravity, it tends downward at a speed u0, depending on the size and shape of the particles, their density and the viscosity of the liquid.
Waste water contains mechanical impurities of various hydraulic sizes, therefore, when it flows through a settling tank at any constant speed v, the particles of these impurities will occupy a variety of positions. Some of them (at u0>v) quickly settle to the bottom of the settling tank, others (with
U
0
=
V
)
are in a suspended state, the third (with u0 For domestic wastewater the value
V
taken equal to 0.7 mm/s. The duration of settling depends on the required degree of wastewater clarification and is taken to range from 30 minutes (before filtration fields) to 1.5 hours (before aeration tanks and biofilters). The water level in the settling tank is determined by the crest of the overflow (collection) chute into which the settled water flows. From here it is sent for subsequent cleaning. Suspended substances released from wastewater form sediment (approximately 0.8 l/day per inhabitant), which accumulates in the sludge part of the sedimentation tank, the capacity of which is calculated for a two-day volume of sediment. Sludge from vertical settling tanks is removed under the influence of hydrostatic pressure through a sludge pipe with a diameter of 200 mm, the outlet of which is located 1.5-2 m below the water level in the settling tank. Sludge humidity 95%. Vertical settling tanks have advantages over horizontal ones; these include ease of sediment removal and a smaller area occupied by the structure. However, they also have a number of disadvantages, of which the following can be noted: a) great depth, which increases the cost of their construction, especially in the presence of groundwater; b) limited capacity, since their diameter does not exceed 9 m. When designing the vertical speed of movement of wastewater Fig 4 29 Primary vertical settling tank with a diameter of 9 h made of precast concrete 1
-release silt, 2
- release of the crust, 3
-central pipe with reflector, 4
Catchment as ub, 5 - outlet tray. 6 - supply tray 16-11 241
V taken equal to the lowest deposition rate u0 that part of suspended solids for which the settling tank is designed; magnitude u0 stops according to the schedule of sedimentation of suspended particles. The calculated cross-sectional area of the sump is equal to the surface area of the water in it (in plan) minus the area of the central pipe. The working length (height) of the sump is the distance from the bottom of the central pipe to the surface of the water. Square
F the central pipe (or the total area of all pipes, if there are several settling tanks) is determined by the maximum second wastewater flow
Q,
m3/s, and speed in the central pipe Vi, m/s: 1
- reflective shield; 2
- bell; 3 - central Ug, mm ft ZbS6
7
at 9
Go 30 LО 50 60 70 80 30 E,"/. 0,5
Naya pipe Velocity Vu, usually assumed to be 0.03 m/s, should not exceed 0.1 m/s in the presence of a reflective shield. The height of the flow part of the settling tank or the length of its central pipe H =
Vt,
(4.61) But not less than 2.7 m. Total volume of the flow path of all settling tanks (if there are several of them), m3, W =
QKt/
24, (4.62) Where Q is the average daily wastewater flow, m3/day; K is the coefficient of unevenness of wastewater inflow. Total working area of settling tanks, m2, Fx
-
W/
H.
(4.63)
The total area (in plan) of settling tanks is determined as the sum of their useful area
Fy and areas
F,
occupied by the central pipe (or central pipes): F = F!--f. (4.64) Calculation of vertical settling tanks, according to the method proposed by prof. S. M. Shifrin, is carried out as follows. According to the required clarification effect of wastewater with different initial con In this case, the living cross-sectional area is the lateral surface of a cylinder, the diameter of which is equal to the diameter of the socket of the central pipe, and the height is equal to the size of the gap, i.e. 0.25-0.5 m. Center pipe diameter
D
determined by the speed of downward movement of water in it, equal to 0.03 m/s. The length of the pipe, which must be entirely located in the cylindrical part of the sump, is determined by formula (4.61). The diameter of a vertical settling tank should not exceed its working depth by more than 3 times. The clarification effect of wastewater in vertical settling tanks is practically no more than 40%; theoretically, the calculation is made for a clarification effect of 50%. The number of settling tanks depends on the adopted design type, the diameter of one settling tank and the estimated wastewater flow. The total construction height (depth) of the Yastr settling tank is determined as the sum of the height of the flow part, the neutral layer, the sludge part (or chamber) and the height of the side above the water level, assumed to be 0.3-0.4 m. The height of the sludge chamber depends on its volume and the diameter of the settling tank. The estimated capacity of the sludge chamber is determined by the volume of sediment that falls and the duration of its stay in the chamber. The sludge part of the sedimentation tanks is made conical (for round sedimentation tanks) with an angle of inclination of the bottom walls of 50° to ensure sludge sliding. At the bottom of the cone (or pyramid) a platform with a diameter of 0.4 m is arranged. To prevent floating contaminants from entering the drain, semi-submersible boards (shields) are installed in front of the collection trays (peripheral and radial), located at a distance of 0.3-0.5 m from the tray; they are immersed in water to a depth of 0.25-0.3 m from the surface of the water; the height of the part not immersed in water should be at least 0.2-0.3 m. The main dimensions of typical vertical sedimentation tanks made of precast reinforced concrete are given in Table. 4.19. Table 4.19
The vertical settling tank of a new design with a downward-ascending flow of waste water is a round tank with a peripheral tray for collecting clarified water. The difference between this sump and a standard one is that the central pipe is replaced Fig 4 33 Primary vertical settling tank with downward-upward flow 1 - receiving chamber 2 - feeding tray (or pipeline), 3 - pipeline for removing melting substances 4
- receiving funnel for removing floating substances, 5
-toothedVodosli"iz6
- reflective visor 7 - distribution tray 8
- peripheral tray for clarified water supply, 9-outlet pipeline, 10
- settling tank, //-- ring semi-submersible partition 12
sludge drainage pipeline On a semi-submersible partition that does not reach the bottom, dividing the sump area into two equal parts, and the inlet device is made on the inner surface of the partition along the entire perimeter in the form of an overflow gear distributor with a submerged reflective visor (Fig. 4.33). Wastewater flows through a tray (or pipe) into a receiving chamber, and then into a tray that has a serrated weir, from which water evenly overflows and moves along the perimeter of the inside of the sump. The reflective visor changes the direction of water movement from vertical to horizontal. As it moves from the partition to the center, the water falls down, being distributed evenly over the entire cross-section of the internal downward part of the sump. When wastewater moves downward at low speeds, the flow loses its transporting ability, due to which suspended particles settle. Intensive separation of liquid and solid phases occurs at the flow turn. Then the water moves in an upward flow, overflows over the side of the collection tray and is discharged through the outlet pipe. Floating substances accumulate at the funnel and are periodically removed through the pipe. The sludge is removed under hydrostatic pressure through a sludge pipe. A vertical settling tank of this type increases the retention effect of suspended solids up to 60-70% or, while maintaining the clarification effect of a conventional vertical settling tank, increases the throughput by approximately 1.5 times. The Institute of Urban Economy of the Ministry of Agriculture of the Ukrainian SSR has developed designs for vertical settling tanks with a downward-ascending flow for several standard sizes. A radial settling tank is a tank that is circular in plan (Fig. 4.34). Wastewater is supplied to the center of the settling tank from the bottom up and moves radially from the center to the periphery. A feature of the hydraulic mode of operation of a radial settling tank is that the speed of water movement varies from its maximum value in the center of the settling tank to the minimum at the periphery. Floating substances are removed from the surface of the water in the settling tank by a suspension device placed on a rotating truss and enter a receiving hopper or collection tray. The falling sediment is moved into the settling pit using scrapers mounted on a movable truss. Rotation frequency of the moving truss is 2-3 h-1; rotation is carried out using a peripheral drive with a trolley on a pneumatic machine. The sediment is removed through a pipeline using plunger and centrifugal pumps installed in a nearby pumping station. The floating substances are discharged into the grease collector. Clarified water enters the circular collection tray through one or both of its sides, which are spillways. In order to ensure more reliable equalization of the speed of water movement at the outlet of the settling tank, the spillways of the collection trays are made with gears. The load per 1 m of the spillway does not exceed 10 l/s. In the USSR, radial settling tanks are built with a diameter of 18-54 m (Table 4.20), and at foreign treatment plants - with a diameter of 6-60 m or more. Radial settling tanks are used as both primary and secondary settling tanks. The ratio of the diameter of the sedimentation tank to its depth at the peripheral drainage tray is taken from 6 to 12. Sedimentation tanks retain up to 60% of suspended solids. 1
- silt scraper 2
- distribution bowl 3
- supply pipeline 4 -
wet sludge pipeline, 5 - grease collector, 6-
pumping station, 7-outlet pipeline Table 4 20 Sump diameter, m Depth of settling zone, m Estimated volume from the standing area, m3 Settlement pass Capacity at 7=1.5 h, m3/h The calculation of primary radial settling tanks is carried out for the maximum hourly inflow based on the settling time, which is assumed to be 1.5 hours for domestic wastewater. The capacity of the pit for collecting sediment in the settling tank is determined by the volume of sediment formed within 4 hours. The walls of the pit have a slope of 60°, which facilitates the sliding of the sediment. Depending on the volume of deposited sediment, the scraper mechanism operates continuously or periodically. In the latter case, it turns on 1 hour before the start of sediment removal. The removal process is automated. The sludge moisture content is 95% for gravity removal and 93.5% for pump removal. The diameter of the sludge pipe is determined by calculation, but it must be at least 200 mm. The height of the sides of the sump above the surface of the water in it is usually 0.3. The advantage of radial settling tanks is their shallow depth, which reduces the cost of their construction. The round shape in plan allows the installation of minimal wall thicknesses, which also reduces the cost of structures. Regardless of the productivity of the treatment plant, the minimum number of settling tanks is taken so that at the first stage of construction there are at least two working settling tanks. Often four settling tanks are assembled into a single block. Uniform distribution of wastewater between settling tanks is carried out using a distribution bowl. When choosing standard sizes of sedimentation tanks, it is taken into account that larger sedimentation tanks are more economical compared to small-sized ones. To increase the CLEANING effect, when the BOD of wastewater is more than 130 mg/l, the radial settling tank can have a pre-aerator installed in the central distribution device. Preliminary aeration with excess activated sludge of municipal wastewater makes it possible to remove chromium, copper, and zinc compounds from their composition in a finely dispersed and colloidal state while settling. However, pre-aeration of wastewater increases the wet sludge moisture content to 94.5% compared to the sludge moisture content during conventional settling (93.5%). A type of radial settling tanks are sedimentation tanks with peripheral supply wastewater in them (Fig. 4.35). The main parameters of such primary radial settling tanks are presented in table. 4.21. The water distribution trench surrounds the sump around the circumference and has a constant width and a gradually decreasing depth from the beginning to the end of the gutter. The bottom of the gutter has round inlet holes positioned so that, combined with variable gutter depths and varying hole diameters and spacing, a constant rate of water flow through the gutter is ensured. Constancy of speed prevents sediment from forming in the distribution chute and creates favorable conditions for transporting floating substances to the collector located at the end of the chute. The water coming from the holes is directed by a vertical annular partition into the lower zone of the settling tank. The speed of the downward flow gradually decreases and reaches a minimum at the annular reflector, which directs the flow to the central zone of the settling tank and further to the drainage ring trench. The low flow rate causes the beginning of the precipitation of suspended substances already at the exit from under the annular partition. The movement of water occurs throughout the entire living section of the sump, while local turbulence is practically absent. The entry of clarified water into the settling tank at its bottom provides the shortest path for sedimentation of suspended substances. The noted features of the hydraulic operating mode of such settling tanks determine a higher retention effect / - supply channel; 2- pipeline for removal of floating substances; 3- outlet pipe - water; 4
- a gate with a movable weir for releasing floating substances from the tray; 5
- jet - guide tubes; 6
- distribution tray; 7 - semi-submersible board for detaining floating substances; 8
sludge pipeline Table 4.21
Index Settling tank diameter, m Hydraulic depth, m Depth of settling zone, m The ratio of the diameter to the depth of the settling zone. . . Working volume, m3. . . . Supply system - distribution tray: Depth at the beginning, m. The same, at the end, m. . . Width, m................................... Flow depth at the beginning, m The same, at the end, m. . . Flow speed, m/s. . Diameter of water inlet pipes, mm...... . Distance between pipes, m Discharge system - prefabricated trays with gear weir: Perimeter, m..... . Outlet pipeline diameter, mm................................... Diameter of sludge pit, m Diameter of wet sludge pipeline, mm... . Suspended solids than in conventional radial settling tanks with wastewater supplied from the center. The duration of settling in settling tanks with peripheral water inlet is assumed to be less than in conventional settling tanks, with the same effect of wastewater clarification. 1 - waste water supply; 2 - distribution partition, 3 - direction of water movement to collection trays; 4
- sludge scrapers; 5-sediment removal; 6-release of clarified water A radial settling tank with rotating water distribution and drainage devices, proposed by I.V. Skirdov and developed by Soyuzvodokanalproekt, is shown in Fig. 4.37. The bulk of the water in settling tanks with such devices is at rest, so the sedimentation of suspended substances in them occurs at the same rate as How
and in laboratory conditions. M
Water is supplied to the settling tank and clarified water is discharged using a freely rotating chute, divided into two parts by a longitudinal partition. The tray is limited on the inside by a partition, on the bottom by a slotted bottom, and on the outside by a distribution grid with vertical slots equipped with jet guide vanes. The slotted bottom is made in the form of a louvered grille, through the transverse slits of which heavy particles fall through. The jet guide vanes have a streamlined shape and rotate at any angle; they are placed in such a way that the duration of residence of individual jets in the settling tank is practically the same. A drainage tray with a flooded spillway has waterproof walls and a bottom. From the tray, water is sucked out by a siphon into the outer drainage chute. The siphon is equipped with a flow regulator (a throttle valve connected by a system of levers to a float). There is a guide visor at the bottom of the drainage tray. Required settling time t depends on the depth of the settling zone h0
and deposition rate u0 particles for the retention of which the settling tank is designed, i.e. t=
H0
Ju0
.
Depth h0
depends on the design of water intake devices; in the case of using trays with a flooded weir, it is usually taken from 0.8 to 1.2 m. The height of the neutral layer is taken from 0.5 to 0.6 m, the depth of the Li sediment layer is from 0.3 to 0.4 m. For a time t The water distribution and collection tray must make one revolution. In this case, it will collect settled water, the volume of which Q = KnR2h0t (4.65) Where K is the experimental utilization coefficient of the settling zone, equal to 0.85; R is the radius of the settling tank. Size
Q
characterizes the throughput of the settling tank. Hydraulic calculation of the water distribution and drainage device comes down to determining the shape (in plan) of the partition between the receiving and distribution parts of the tray, the required immersion depth of the edge of the drainage drain, as well as the height of the difference between the water levels in the sump and the peripheral drainage chute, ensuring uninterrupted operation of the siphon. The shape of the partition in plan does not depend on the estimated wastewater flow. When distributing water using a grid of evenly spaced blades of a curved outline, the width of the water distribution tray b and m is determined depending on the distance I, m, from the center of the settling tank according to the equation Bi = n Vr2 - 12, (4.66) Where n is the ratio of the width of the water distribution tray at its beginning to the radius of the sump R; The value n is recommended to be taken equal to 0.1-0.12. To collect clarified water, it is most advisable to use flooded weirs. With a flood coefficient 6 = 0.8 and a discharge coefficient t = 0.45, the immersion depth is determined by the equation H
0
= 1,24 (QIR
2
)2/
3 12/3
, (4.67) Where Q is the throughput of the settling tank, m3/h; R is the radius of the settling tank, m; / - length (width) of the spillway, m. The difference between the water level in the sump and the drainage peripheral trench Tf>2/is, (4.68) Where hs- pressure loss in the siphon, determined using general hydraulic formulas. The magnitude of the reactive force depends on the mass of waste liquid supplied to the settling tank and the speed of its flow. With practically permissible loads on the settling tanks, it ensures uninterrupted movement of the log without the use of any other (except reactive) forces; in many cases, the reactive force is sufficient to rotate not only the tray itself, but also the scraper truss. The natural aeration clarifier is a vertical settling tank with an internal flocculation chamber (Figure 4.38). Stoch - /
G
The fresh water flows through a tray into a central pipe, at the end of which a reflective shield is attached. Due to the difference in water levels (0.6 m) in the supply tray and the clarifier, air is ejected by the flow of wastewater entering the clarifier. In the flocculation chamber, partial oxidation of organic substances and increased flocculation occur, which contributes to the intensification of the process. From the flocculation chamber, wastewater is directed to the settling zone of the clarifier, in which fine suspended particles are retained when passing through a layer of suspended sediment. The clarified water flows through the edge of the weir into the peripheral tray and then into the outlet tray. The deposited sediment is removed under hydrostatic pressure through a pipe into a sludge well. Floating substances are retained by the inner wall of the collection tray and, as they accumulate, are discharged into the sludge well through a pipe through a ring tray. As a result, the effect of wastewater treatment in the building reaches 75%. The characteristics of the operation of clarifiers are given in table. 4.22. The throughput capacity of a clarifier with a diameter of 9 m with a residence time of waste liquid in it of 1.5 hours is 53.6 l/s, and a clarifier with a diameter of 6 m is 23.6 l/s. Clarifiers are arranged in a block of two or four structures. Thin-layer sedimentation tanks are open and closed tanks. Like conventional sedimentation tanks, they have water distribution, settling and drainage zones, as well as a sediment accumulation zone. The settling zone is divided into a number of shallow layers (up to 15 cm) by shelf sections or tubular elements. Shelf sections are mounted from flat or wavy plates, convenient to use. Tubular sections are characterized by greater structural rigidity, ensuring dimensional consistency along the entire length. They can operate at higher speeds than shelf sections, but they silt up faster, are more difficult to clean, and require increased material consumption. Reducing the settling height ensures a reduction in turbulence, characterized by Re^500, and the vertical component of wastewater flow pulsations, as a result of which the volume utilization factor increases and the duration of settling decreases (up to several minutes). Reconstruction of conventional settling tanks into thin-layer ones allows increasing their productivity by 2-4 times. To sediment suspended substances from water in a thin layer, both in our country and abroad, a large number of thin-layer sedimentation tanks of various designs have been proposed. Schematic diagrams of thin-layer sedimentation tanks are shown in Fig. 4.39. The main patterns of mutual movement of water and separated sediment are as follows: cross pattern - when the separated sediment moves perpendicular to the movement of the working fluid flow; counterflow scheme - the separated sediment is removed in the direction opposite to the movement of the working flow (Fig. 4.40); Direct-flow scheme - the direction of sediment movement coincides with the direction of water flow. The most rational design of a thin-layer sedimentation tank should be considered a sedimentation tank with a countercurrent phase movement pattern, equipped with a proportional distribution device. Fig 4 39 Tubular sections built into the radial (A) and in horizontal (b) thin-layer sedimentation tanks These sedimentation tanks should be used to treat wastewater containing mainly settling impurities. Due to the movement of water in Inclined sections from bottom to top create favorable conditions for sedimentation of suspended substances along a shorter trajectory. The sediment continuously slides against the movement of water and is deposited in the form of large agglomerates into a sludge pit, from which it is periodically removed through a sludge pipe. The floating substances are collected in the cavity between the sections and removed by a submersible tray. To reduce the volume of water removed with them, floating substances are driven to the tray by air jets. Air is supplied by perforated pipes located along the periphery of the sump. Calculation of a thin-layer sedimentation tank is carried out in the following order: 1. The cross-sectional area of the shelf space is calculated by the formula (O -
Q/
V,
(4.69) Where Q is wastewater flow rate, m3/h; V is the flow rate of wastewater in the sections of a thin-layer settling tank, m/h. Speed v,
m/h, determined from the condition of ensuring laminar flow of water in sections according to the equation V- 3600 Re %v/, (4.70) Where Re is the Reynolds number; must be less than 500; %-wetted section perimeter, m; ©x is the cross-sectional area of the section, m2; v- kinematic viscosity, m2/s. In practice, the speed of water movement in the sections is taken equal to 10 u0, i.e. approximately 5-10 mm/s. B=:<й/Н. (4.71) The angle of inclination of the shelves is 45-60°, depending on the angle of sediment sliding in the water. 3. The required settling time, h, is determined from the equation Where u0-hydraulic particle size, mm/s, the sedimentation of which provides the required effect of wastewater clarification. Size u0 determined by the kinetics of wastewater clarification at rest at a settling layer height equal to the section height
Hc in a thin-layer sedimentation tank. The minimum height hc should be taken taking into account the method of removing the deposited sediment and the need to ensure that the section does not become clogged,
Hc =
50...150 mm. 4. The length of the shelf space is determined from the expression L= Ktpv,
Where K is the safety factor equal to 1.1 -1.5. The total construction length of a thin-layer sedimentation tank consists of the length required for installing water distribution and drainage devices, and the length of the shelf space. 5. The volume of the sludge part of the settling tank is determined by the equation (C0 - Ct)
Q-100 (ioo-pL (4-73)
Where W is the volume of sediment; C0 is the initial concentration of suspended solids in wastewater; Ct
- concentration of suspended substances in clarified water; Q - estimated wastewater flow; R
- sediment humidity, %"" dew - sediment density. To calculate sedimentation tanks, their dimensions are first determined, and then the values of the calculated values are specified. One of the main values is the average design speed in the flow part of the settling tank, taken as a first approximation for radial (in cross-section at half the radius) and horizontal settling tanks and = 5...7 mm/s, for settling tanks with a rotating distribution device and vertical y = 0. The length of horizontal settling tanks is determined by the formula VH
AG"O The radius of vertical, radial settling tanks, with a rotating distribution device and with a peripheral inlet - according to the formula Where v is the average design speed in the flow part of the settling tank, H is the depth of the flow part of the settling tank (from the boundary of the neutral layer to the water level), m; K - coefficient depending on the type of sump and the design of water distribution and catchment devices; is assumed to be equal to 0.5 for horizontal settling tanks, 0.45 for radial settling tanks, 0.35 for vertical settling tanks, and 0.85 for settling tanks with a rotating distribution device; u0—sedimentation rate of suspended particles in the settling tank (hydraulic size), mm/s; Q-calculated wastewater flow, m3/h. Hydraulic size is determined by the formula I --------------- i----- - o, (4.76) 0
at(KH/h)n
"
}
Where a is a coefficient that takes into account the effect of water temperature on its viscosity; accepted according to the table. 4.23; t-duration of settling in a cylinder with a layer of water L, corresponding to the specified clarification effect, s; determined experimentally or accepted approximately for the main types of suspended substances according to table. 4.24; n-empirical coefficient, depending on the properties of the suspension, is determined experimentally; w is the vertical component of the speed of water movement in the settling tank, taken from the table. 4.25. Table 423 Table 4.24 Duration of wastewater settling at rest depending on the clarification effect Duration of sedimentation of suspended substances, s, in a cylinder 500 mm deep Lightening, % Notes: 1. The settling time is given for a water temperature of 20 °C. For intermediate concentrations of suspended solids and the clarification effect, the duration of settling is determined by interpolation 2. The kinetics of sedimentation of suspended substances from wastewater and the exponents n must be determined by standing at rest in vessels with a diameter of at least 120 mm. Meaning ( KHjh)
N in the calculations of primary settling tanks for urban wastewater can be taken according to table. 4.26. After defining L And R for horizontal and radial settling tanks the value of and is specified: Where IN
- settling tank width, m; accepted within 2-5 R; for radial settling tanks (in cross section at half the radius) Table4.26 Values(KH(h)n
Pfor settling tanks Height from Verti With rotating Stoinika For the sake of Horizon distribute N,
m Kal Alnykh Talnykh Telny mouth Swarm If the updated value differs significantly from the previously accepted one (when calculating w),
quantities L And R should be re-determined taking into account the obtained value V. For settling tanks with a rotating distribution device, the rotation period, h, of the distribution device is determined by the formula T
= nR
*HK
/Q
.
(4.77)
The volume of sludge removed from primary settling tanks is determined in accordance with the effect of wastewater settling. The volume of the sludge chamber is assumed to be equal to the volume of sediment that falls over a period of no more than two days. In some cases, in the absence of sufficient data characterizing the kinetics of sedimentation of suspended substances, settling tanks can be calculated based on the load of wastewater on the surface area of the settling tank Q or by speed v
AND settling time t,
accepted based on operating data from sedimentation tanks that clarify water of similar composition. For domestic wastewater q-
2...3.5 m3/(m2-h), V
-
S...7
mm/s and f=l...l.5 h. The hydraulic operating mode of settling tanks greatly influences the effect of their operation. The more perfect the design of the settling tank, the higher the efficiency of retention of suspended substances. The perfection of designs is associated with the conditions of water entry into the sump, i.e., with the speed of water entry and the depth of the casing in the radius - 17-11 Alnom or distribution baffle in a horizontal settling tank. The hydraulic operating mode is assessed by the coefficients of volumetric use and efficiency of settling tanks. The volumetric utilization factor of the settling tank is determined by measuring water flow rates throughout the entire depth of the settling zone (in several sections) and establishing the active zone, and the efficiency coefficient is the ratio of the clarification effect in a full-scale settling tank to the clarification effect in the model (at rest) with equal settling time. These coefficients are taken into account in the calculations to one degree or another. Thus, when calculating horizontal settling tanks [formula (4.74)], the coefficient /(=0.5 is introduced when determining their length, in calculating a radial settling tank [formula (4.75)] TO==0.45, and when calculating the settling tank designed by I.V. Skirdov, the volumetric utilization coefficient is assumed to be 0.85. However, these coefficient values are not described in the form of a mathematical relationship. For this purpose, the Department of Sewerage MISS named after. V.V. Kuibyshev conducted studies on models and in a full-scale settling tank. After mathematical processing of the experimental results, the following dependencies were obtained: 0,76 - 0,05
/і
2
4-
0
,11
H
1 + 0.00275,in; (4"78) /Co. u = 1 - 0.000825 (L/I)3 -J - 0.02335 (L/I)2 - 0.1755 (UN), (4.79) Where K"oi is the coefficient of volumetric use, depending on the immersion depth of the distribution device L = 0.25# and the water entry speed uvh (under the distribution device vBX accepted within 20-25 mm/s); K"op - volumetric utilization coefficient, depending on the geometric ratio of the length of the settling zone
L or
R to the depth of Self. Meaning K"ol in equation (4.78) is valid only for
L/
H-
10. Otherwise, the volumetric utilization coefficient is determined by the formula AGo. And=*o. H*S. e/*S. H. (4.80) Where K°i is the coefficient of volumetric use of the settling tank, determined by formula (4.79) at
L
(
H
-[
Q
KnQVl is the same, but for any value of L/R other than 10. Values of efficiency coefficients t| found depending on the duration of settling
T,
h, which is determined during technological analysis (Fig. 4.41), /(oi and actual viscosity of wastewater p according to the formulas: for domestic wastewater C=e "*k<>-*"
. (4.81)
For industrial wastewater Т=е VWp2 t<4.82) Gder. n, [xm - dynamic viscosity of wastewater, respectively, in a full-scale settling tank and during a technological analysis of the clarification of the same wastewater on a model; Pi> Рз - sludge density of domestic and industrial wastewater, respectively. 3H = 3MG1, (4.83) Where ^m is the effect of water clarification on the model (at rest); taken from Fig. 4.42 at the same time values at which the efficiency was determined, i.e. at t=
0...
...1,5toc(determined in a vessel by a depth equal to the depth of the designed settling tank). The obtained data is plotted on a graph (Fig. 4.43) and a curve is drawn depending on the effect of wastewater clarification E depending on the duration of settling t for the sump. 1 - at rest (in division); 2
- moving (in kind) According to the required wastewater treatment effect and schedule
9=f
(t
)
for the settling tank, the required duration of settling of wastewater in the settling tank is determined tn.
30
60
20
For horizontal settling tanks U?!
= VN(L- /0), (4,85)
Where R-sump radius equal to L,
m; H - depth of the settling zone, m; should be taken 1.5-3 m; IN- width of the horizontal settling tank; 10 - distance from the distribution tray to the semi-submersible board in the horizontal sump. We determine the wastewater flow rate, m3/h, which should be supplied to one settling tank: Qi =
WtJtBt
(4.86)
Where їн is the duration of wastewater settling, adopted according to Fig. 4.43. Finally, the required number of working settling tanks is determined: N
= Qo
6ui
/Q
1
,
(4.87)
Where Fobsh is the flow rate of wastewater entering the treatment plant, m3/h. The primary settling tank is the construction of a mechanical treatment unit designed for gravitational settling of finely dispersed contaminants, mainly organic, and as a result, a reduction in BOD and COD. The plan shape is round or rectangular. The number of settling tanks is determined by calculation and must be at least two. Primary settling tanks can be: These settling tanks differ in the movement of the flow of purified water. Primary horizontal settling tank is a rectangular tank consisting of several corridors. Vertically, the structure can be divided into a working part (where sedimentation occurs) and a sludge part (where sediment is collected). There should be a distance of at least 0.4 m between these conditional zones. At the beginning of the horizontal settling tank, a pit is arranged where the sediment is raked (with scrapers) or washed off. It is removed from the pit by hydraulic elevators or pumps. The disadvantage of this type of structure is its large area. Plus - high efficiency. Vertical primary settling tank- This is a cylindrical structure with a conical bottom. The purified water is supplied from above into a pipe, which is located in the center of the structure. There is a reflective shield under the pipe. Hitting it, the water changes direction and moves from bottom to top. For better flow distribution, the central pipe is made with a widening at the lower end. Clarified water is collected in collection trays, which are located on the edge of the tank. The sediment accumulates in the cone (sludge) part of the settling tank and is removed from there under the influence of pressure (hydrostatic) through the sludge pipe. The disadvantages of the design are the large depth and the impossibility of application with a treatment plant capacity of up to 10,000 m3/day. Radial primary settling tank– a special case of a vertical settling tank. The difference is that in this type of structure the water moves from the center to the periphery, rather than from the bottom up. Therefore the design is different. The radial settling tank, like the vertical one, is round in plan. But the wastewater supply pipe is located at the bottom. The drainage is also supplied to the center; the highest speeds are observed here, which decrease as they approach the collection trays (periphery). The sediment that accumulates at the bottom is moved by scrapers into the sludge pit (in the center), from where it is removed by a centrifugal or plunger pump. The disadvantage of the design is low efficiency. Plus - not high cost. Varieties of this type of sedimentation tank are a sedimentation tank with a peripheral inlet and with rotating water distribution and collection devices. All types of sedimentation tanks are equipped with devices for collecting floating substances. Based on the mode The work of primary settling tanks is divided into: When choosing the type of sedimentation tank, the economic factor, the composition of the wastewater, geological and hydrogeological conditions, terrain conditions, estimated costs, etc. are taken into account. If the cleaning efficiency is not sufficient, then you can add another cleaning stage or intensify the design of the structure. In this area, much attention is paid to the wastewater inlet system into buildings, since the flow distribution has a great influence on treatment. In horizontal settling tanks, for example, perforated panels are used for this purpose, located at the beginning of the tank (1/3 of the length from the inlet); in vertical ones - a reflective shield. It is possible to use aeration in radial settling tanks to remove mechanical particles from organic matter. Mikhail Ivanov Water purification using the sedimentation method is used in hydraulic structures, centralized water supply and sewerage systems. There are several types of sedimentation tanks: horizontal, vertical, static, dynamic and plate. Settling tanks are reservoirs or open containers in which mechanical impurities are removed from water by settling. During this process, particles of the dispersion phase, depending on the density of the substance, either float to the surface of the water or settle to the bottom of the reservoir. Particles that settle to the bottom form sediment. In some cases, sedimentation is accompanied by particle enlargement. Horizontal settling tanks high installation cost; Vertical settling tanks Biological treatment Static and dynamic settling tanks Tubular and plate settling tanks Settling tanks are artificial reservoirs or natural reservoirs in which suspended impurities contained in them are separated, floated or deposited from industrial and domestic wastewater under the influence of gravity or using reagents. A sewer settling tank can be used as a filter for preliminary and final wastewater treatment in a drainage system. The primary stage is the separation of mechanical impurities or suspended particles and the beginning of biological wastewater treatment. In biocoagulator settling tanks, activated sludge is mixed with source water, colloidal and finely dispersed impurities are separated. Secondary settling tanks of treatment facilities, final treatment devices, are called environmental protection ones, since after them the clarified water is discharged into natural reservoirs. Radial settling tank for water utility They are used in water supply and drainage systems in industry and private households, for the treatment of sewage, waste water discharge during the hydromechanization process in quarries and mines, in mining and processing factories, for domestic and drinking water supply. Purpose: During the construction of structures and roads, settling ponds are created to accumulate and clarify surface runoff. Industrial wastewater contains fatty substances that negatively affect the operation of treatment systems: To prevent consequences, grease traps are built into sewer channels, which are made in the form of a box with a tire. Its functional purpose is to retain and remove oil and grease. During the spring thaw and rainy periods, pollutants of a wide variety of nature are washed into sewer drains. Stormwater wells and chambers made of concrete can reduce pollution. They collect storm flows from a certain area, so the calculation necessarily includes a volume that can accommodate the discharge of water from a given area. Gabions are used to capture pollutants in runoff from roads. To remove some suspended compounds, flocculators with a built-in chamber are installed. In devices, using laminar flows or turbulent flows, particles collide with each other and interact with reagents, which promotes rapid flocculation and removal of the resulting sediment. Rolling mills use settling tanks to collect scale. In ceramic workshops, the drains contain a large amount of clay, which leads to sewer blockages if it is not removed in time using sedimentation tanks. After aeration during biochemical wastewater treatment, the water enters the sludge settling tank, from where the activated sludge is returned to the aeration tanks for further continuous water purification. Wastewater must be separated from liquid household waste - solid waste or feces pumped out of cesspools. For this purpose, sedimentation tanks are used. Polygons or filter fields are used to place them. Homeowners make septic tanks for their garden plots with their own hands. The partition that creates the overflow edge prevents the entry of contaminants thrown away from the reflective shield. Types of settling tanks depending on operating mode: Batch thickeners look like shallow pools. After filling the tank, time is allowed for suspended solids and colloidal particles to settle to the bottom of the structure. Clarified water is drained through taps located above the sediment level. The sludge in the form of a viscous liquid mass is scooped out manually or poured into a pipe hole at the bottom of the device. Settling tanks for wastewater treatment in the direction of movement of the initial mixture are divided into two types: The design of a horizontal settling tank is a tank with several corridors. A conditional division by height distinguishes the working area and the sludge part. In the first, the sedimentation process takes place, in the second, settled sludge is collected. According to the standards, a distance of 25 cm is provided between the zones. The sediment is collected in a pit at the point where the wastewater enters, from where it is pumped out or raked. The shape and size of settling tanks are selected depending on the suspension concentration and size. With increasing concentration and particle size, the diameter of the thickener decreases. Increasing the temperature reduces the viscosity of the liquid, which increases the speed of purification. The principle of operation is the continuous supply of the original liquid and the drainage of clarified water. Fallen sludge is removed periodically. To increase the deposition area, structures with inclined partitions are used. They repeatedly change the direction of flow from bottom to top and back. At the same time, the residence time of the source liquid in the tank increases, and better water purification is achieved. Used in industrial production. Based on their design, continuous settling tanks are divided into single-, double- and multi-tiered. The use of two-tier and multi-tier devices is determined by the degree of wastewater pollution. The countercurrent principle is used, in which the movement of sediment goes in the direction of series-connected tanks, and the flow of clarified liquid in the opposite direction, to replenish losses in previous devices. Advantages of rowing devices: The main disadvantage is the bulkiness of the device. The diameter varies between 1.8-30 m. In industrial installations, the diameter of the sediment reservoir can be 100 m. For compactness, in order to save space in production facilities, multi-tier settling tanks are used. They are a structure consisting of several modules stacked on top of each other. In turn, multi-tier devices are divided into two types: closed and balanced. In precipitating devices of the first type, the mixers are installed on one shaft with one drive. To eliminate leaks, sealing seals are used. The process of unloading sediment, as well as draining clarified liquid, is carried out separately for each settling tank. Sequential sludge removal is used in balanced type settling tanks. The sediment from the previous one is poured into the glass of each subsequent tier. As it moves down, the sludge thickens and is removed from the lower tier. In this case, sealing glands are not required in the previous tiers, since in them the sediment pressure on the bottom is much lower than in the lower settling tank. Primary settling tanks are typically used to remove suspended solids and clarify wastewater before the biological treatment stage. Secondary settling tanks are used for 2 purposes: to clarify wastewater after biological treatment and to compact activated sludge and recycle it to biological treatment (into an aeration tank, into an anaerobic reactor) in order to increase the concentration of sludge and the oxidative (fermentation) capacity of the bioreactor. Settling tanks are also divided into vertical, horizontal and radial. Vertical settling tanks are used when wastewater flow is no more than 10 thousand m3/day. Horizontal settling tanks are used for medium and large water treatment plants (with a wastewater flow rate of 10–100 thousand m3/day). Radial - with wastewater flow rates over 100 thousand m3/day. Radial settling tanks can have a diameter of up to 100 m, usually 24–50 m. In 2-tier settling tanks (emscher), the upper tier is used for settling, the lower – for digestion and compaction of sludge Typically, sedimentation and biological treatment of wastewater do not provide satisfactory removal of bacterial contaminants: the degree of removal of pathogenic and other macroorganisms is only 90–95%. Many pathogenic microorganisms survive in wastewater for up to 2 weeks, and some for up to 10 weeks. Helminth eggs end up in water bodies with waste water in an amount of 500–1000 eggs/m3, even if the water is well purified from bacteria. Therefore, the sanitary and epidemiological safety of water is ensured only if it is disinfected. At the same time, the degree of reduction in bacterial contamination of wastewater at complete biological treatment stations with disinfection increases to 99.5–99.99%.
The concentrations of suspended particles in it are found using Fig. 4.30 hydraulic particle size that should be separated in the designed settling tank. Then, according to the found value u0y using fig. 4.31, determine the radius of the settling tank. S. M. Shifrin recommends taking the average speed of entry of wastewater into the settling zone of the willows (the speed in the section between the socket of the central pipe and the reflective shield) equal to 1.2 cm/s.
Water settling is a fairly common way to remove coarse mechanical impurities. This method is used in waterworks systems, centralized water supply and sewerage, at hydroelectric power stations, irrigation structures, as well as in the treatment of municipal wastewater and after biological wastewater treatment.
At pumping stations and hydroelectric power plants, incoming water from open sources is subjected to sedimentation in order to prevent abrasion of hydraulic turbine blades and pump parts by solid impurities larger than 0.25 mm. It is advisable to use sedimentation tanks in irrigation systems to prevent clogging of irrigation canals with silt.
In centralized water supply systems, settling tanks are used at water treatment plants for preliminary clarification of water with a turbidity of more than 2 g/l. Only those impurity particles whose sizes exceed 10-5 cm are subject to settling. Particles with sizes from 10-7 to 10-5 cm, forming a colloidal microheterogeneous system, do not settle during settling due to the balancing of gravity forces and the energy of Brownian motion for particles with small by the masses.
To remove colloidal impurities, it is necessary to cause their enlargement by sticking together or cause them to lose stability as a result of coagulation. Traditionally, coagulants are used to remove colloidal impurities from water, and the resulting flocs are removed in settling tanks or other equipment. To ensure complete settling, the water flow speed is reduced as much as possible to 0.25–0.5 m/sec. The second factor influencing the completeness of precipitation is the duration of settling, which is usually 1.5–2.0 hours.
Sedimentation tanks are classified according to various criteria, in particular, depending on the direction of the main flow of purified water. Based on this feature, settling tanks are divided into horizontal, vertical and radial.
The most common are horizontal settling tanks, which are used at water treatment plants with a capacity of 15–50 thousand m3 per day. As a rule, up to 60% of suspended impurities are removed from them.
A horizontal settling tank is a rectangular reinforced concrete tank consisting of several compartments. Its length can reach 48 m, and its width is usually 3–5 times less. The depth of this structure should not exceed 4 m. The thickness of the water layer, as a rule, is 2.0–2.5 m. Water enters the horizontal sump through a series of holes in the end wall, then is distributed throughout the tank and flows along the entire length of the sump. The purified water is discharged through a spillway on the opposite side of the structure.
To collect sediment, several pits are located at the bottom of the settling tank. The sediment that does not fall into the pit is cleaned from the bottom with a special scraper device. The movement of scrapers along the sump is carried out using gear and chain transmission. When moving along the bottom, the scrapers collect sediment, and when moving along the surface of the water, they collect impurities that have floated to its surface, directing them to a special chute. Removal of sediment from pits can be carried out by draining through pipes at the bottom, lifting through sludge pipes under water pressure and using a plunger pump.
The disadvantages of horizontal settling tanks are:
low reliability of the scraper mechanism;
the presence of stagnant zones where sediment is not removed.
Another common type is vertical settling tanks, which are cylindrical tanks with conical bottoms. Most often they are used for primary sedimentation of water at water treatment plants or for removing suspended matter after coagulation.
In vertical settling tanks, water enters through a pipe from above to the lower part of the unit and is distributed over the entire cross-sectional plane. The sediment is collected in the lower conical part, and the purified water rises upward in an ascending flow and overflows through a circular weir into a collection tray. Before the drain there is a special partition that removes floating impurities.
The sediment during the settling process is collected in the lower part of the apparatus and removed through a special hopper. Scraper mechanisms are installed only in settling tanks with significant amounts of sediment. Typically, vertical settling tanks can remove up to 40% of suspended impurities.
Vertical settling tanks are simpler in design and operating conditions than horizontal ones. Their important advantage is the significant size of the annular weir in the upper part. This will greatly reduce the flow speed, resulting in a reduced likelihood of sediment being carried out. The disadvantages of this type of apparatus include the difficulty of removing sediment from the unloading hatch in the absence of a scraper mechanism.
One of the varieties of vertical devices are radial settling tanks. Their height is much smaller - 0.1–0.15 m, and their diameter is usually large - 16–60 m, and in some cases can reach 100 m. Radial settling tanks are used for clarification of very turbid waters and water purification in industrial water supply systems water supply Water is supplied to such devices through pipes in the central part, and drained through a circular weir at the top of the device. The sediment settled to the bottom is collected with rotating scrapers.
Radial settling tanks are used at treatment plants with a capacity of more than 20 thousand m3 per day and ensure the removal of 50% of suspended solids.
Sedimentation tanks play a particularly important role in wastewater treatment using biological treatment methods. The specificity of this type of cleaning requires that settling be applied twice.
Primary settling tanks are placed before wastewater enters the bioreactor to remove excess amounts of suspended solids and mechanical impurities, mainly sand. Biotreatment units should receive wastewater with an optimal concentration of 100–120 mg/l, since with a higher degree of clarification, the bioreactor is underloaded, which causes “starvation” of the activated sludge.
At the same time, insufficient clarification of wastewater after primary sedimentation can lead to an increased content of nutrients in the wastewater, resulting in a significant increase in activated sludge, which will cause secondary pollution.
To ensure the most complete removal of biomass residues, after biological treatment, wastewater is secondarily settled in settling tanks. Typically, for these purposes, radial settling tanks are used, equipped with suction pumps - devices for removing sediment. The wastewater remains here, as a rule, for 1.5–2 hours. The collected biomass sediments are removed from the settling tanks and sent for further processing.
In some cases, during secondary settling, two-tier settling tanks are used, in which the processes of collecting sediments and their fermentation occur in separate tanks combined in one apparatus.
Another method of classifying settling tanks is based on the method of pumping water into them. In batch settling tanks, settling occurs after filling. They belong to the type of static settling tanks. If the pumping of wastewater into a settling tank and the removal of clarified water from it occur constantly, then such a settling tank belongs to continuous-operating devices - the so-called dynamic settling tanks.
Static settling tanks are usually used to purify wastewater from impurities of oil and petroleum products. They are steel or reinforced concrete tanks that operate not only in settling mode, but also as storage tanks or buffer tanks, which are necessary for the uniform supply of wastewater for further treatment. Through inlet pipelines, these structures are filled with wastewater, which settles here. Then the floating impurities are removed and the clarified water is drained. To remove sediment, drainage from perforated pipes is placed at the bottom of the sump.
Unlike static sedimentation tanks, dynamic sedimentation tanks have a continuous flow of water. This group includes the above-mentioned horizontal and vertical settling tanks.
In addition, devices have been developed in which water flows at different angles of inclination. Such devices are necessary in order to reduce the settling time, which, based on the laws of sedimentation, increases with increasing height of the water layer. Moreover, the bulk of the sediment falls in the initial period. When the water layer is reduced, the settling time is reduced, and when the procedure is repeated many times, the cleaning efficiency increases.
This led to the creation of thin-layer sedimentation tanks, which, according to their design, are divided into tubular and plate.
The working element of a tubular settling tank is a pipe with a diameter of 2.5–5 cm and a length of about 1 m. The length of the pipe depends on the degree of water contamination and flow speed. These pipes have either a slight slope of up to 10°, or a more significant slope of more than 60°. Low angle sedimentation tanks usually operate in batch mode. First, the water is clarified by passing it through tubes, and then the sump is washed.
Such settling tanks are used to purify water with a small amount of mechanical impurities. The efficiency of their removal sometimes reaches 85%.
When using tubular sedimentation tanks with large angles of inclination of the tubes, not only the clarified water flows down, but also the settled sediment slides off, which makes it possible not to wash the tubes.
The performance of tubular sedimentation tanks does not depend on the diameter of the tubes, but largely depends on their length. Tubes for such cleaning are usually made of plastic, and for high-capacity units special blocks are used. These blocks consist of many tubes about 3 m long, 0.75 m wide and 0.5 m high. The use of such blocks allows the construction of settling tanks of any capacity.
A similar method of settling is implemented in plate settling tanks. They consist of a series of parallel plates, between which water flows. The movement of clarified water and the resulting sediment in these devices can occur in one direction (such devices are called direct-flow devices) or in opposite directions (counter-flow settling tanks).
The disadvantages of all types of thin-layer sedimentation tanks are the need to first remove impurities with large particle sizes from the water, as well as particles floating on the surface. When using thin-layer sedimentation tanks to purify wastewater from contamination with oil and petroleum products, it is first necessary to remove clots of impurities from the wastewater, which can quickly render the equipment unusable.
The advantages include increased cleaning speed, efficiency due to the small volume of the apparatus and low construction costs.Scope of use
The purpose of the ball settler is to remove water and salts from oil before using the natural fossil in the production of gasoline. Hydrophobic settling tanks are used in oil production to separate formation waters for their further use, cascade settling tanks are used to separate washing solutions from oil when washing ships.Principle of operation
The classic type of settling device is a vertical settling tank with a cylindrical shape. The contaminated water flows down through the central pipe, hitting the reflective shield. After this, the flow changes direction, and the dispersed suspension precipitates. The clarified water rises up to the overflow edge, after which it is poured into a peripheral tray to collect clean water. Sludge is removed through a sludge line from the settling section.Classification and design of water settling tanks
The advantage of settling over other methods of separating suspensions is that the process is cheap. Subsequent filtration is accelerated if the filtered material is pre-condensed. For this reason, settling is used in the primary phase of treatment to remove suspended solids.Horizontal settling tank
Vertical settling tank
Used for small volumes of source liquid. The vertical settling tank is made in the form of a cylinder with a conical base. It is equipped with several drain taps located at different height levels. Sediment is removed through a special hatch at the bottom.Semi-continuous
They are used in systems for treating large volumes of wastewater. Execution in the form of large concrete tanks or a system of series-connected settling tanks.Continuous action
With paddle mixers
A radial settling tank is one of the special cases of vertical devices. The movement of source water occurs under the influence of centrifugal force from the center to the periphery. Devices with paddle mixers are cylindrical in shape. The initial suspension is fed into the tank continuously. The mixers have inclined blades. When they rotate, the sediment is partially agitated on one side, and dehydrated on the other.With conical shelves
The suspension is distributed using conical shelves directed downwards. The source water reaches their surface through holes located across one shelf. The clarified water flows in an ascending flow to the exit in the upper part of the structure. The gravitational force drags the particles to the bottom; due to inertia, they do not have time to change their direction. Advantage:For separating emulsions
Horizontal devices designed for separating emulsions are equipped with baffles with holes that prevent disturbance from the incoming emulsion. The cross section ensures a calm laminar (movement of water mass in layers) fluid flow. The flow rate is several mm/sec. The emulsion is divided into component parts: light and heavy. Lighter liquid is drained through a hole in the upper part, heavier liquid is drained through a drain in the lower part of the device.Efficiency of biological wastewater treatment
