A “wheel in water” is more effective than a heated treadmill. Primary and secondary settling tanks

K category: Cleaning Wastewater

Secondary settling tanks

Wastewater treated at biological stations contains activated sludge (after aeration tanks) or spent bacteriological film together with destroyed loading material (after biofilters or aerofilters). Secondary settling tanks are used to separate these undissolved impurities from wastewater. They, like primary settling tanks, are horizontal, vertical and radial. The activated sludge settling in the secondary settling tank must be pumped back to the aeration tank. The amount of this circulating sludge is 30-50% of the liquid purified in the aeration tank. It should be borne in mind that more activated sludge settles in the secondary settling tank than is necessary for circulation. This excess should be separated from the total mass of circulating sludge. The amount of excess activated sludge is very large and, with a humidity of 99.2%, amounts to 2.5 liters per day per person. Before it is sent for processing for further use, this excess sludge must be compacted in special structures called sludge compactors.

Until now, vertical settling tanks have usually been used to compact activated sludge. Settled activated sludge after remaining in the conical part of a vertical settling tank for 6 hours. is compacted to a humidity of 96-97%, which leads to a corresponding reduction in its volume to 0.4-0.6 liters per day per person. The possibility of using flotation tanks for these purposes has also been proven.

Experiments currently being carried out have shown that it is possible to use part of the activated sludge as a suspended layer, since adding it to the treated water increases the settling effect in the primary settling tank.

If the treated wastewater enters the secondary sump after biofilters, then the amount of biospit (with a moisture content of 96-97.5%) deposited in the secondary sump is taken on average 0.15-0.2 liters per day per 1 person.

Rice. 1. Secondary settling tanks with suction pumps installed at the Kuryanovskaya station

In practice, it is very difficult to build such sedimentation tanks in which the sludge would be completely retained and there would be no partial removal of it, but in most cases this is not required. It is only necessary that the removal of unretained activated sludge or biofilm does not exceed the allowable value established by

In each case, sanitary rules depend on the capacity of the reservoir.

The designs of vertical secondary clarifiers are almost "r" different from the designs of primary clarifiers. Their calculations and design are carried out in accordance with the standards and technical specifications who recommend different times residence and rate of rise of the liquid depending on the category of the deposited sediment. For example, for vertical settling tanks installed after aeration tanks, the settling time is taken as 1 hour after maximum flow water, the vertical rate of rise of the liquid is taken as 0.4 mm / s based only on waste water (without activated sludge); for settling tanks after biofilters, the settling time is assumed to be 30 minutes, the rate of water rise is 1 mm/sec. As for primary settling tanks, the lower part of the settling tank, designed to store settled sludge in it, is made conical or pyramidal.

Vertical secondary clarifiers are most often designed for small and medium-sized plants. For large stations, radial type sedimentation tanks are used. So, for example, for the deposition of activated sludge after aeration tanks at the Lublin aeration station, radial settling tanks with a diameter of 18.7 m and a height of 3.3 m to the water level are adopted. An essential feature of such settling tanks is the collection of settled sludge with the help of sludge pumps. The positive experience of operation of the sedimentation tank with sludge pumps made it possible to recommend them for other large stations. In particular, secondary settling tanks with sludge pumps of only slightly modified design (Fig. 1) were built at the Kuryanovskaya aeration station. The diameter of the sump is 33 m, the useful height is 3.5 m, the construction height is 4.19 m. Water is supplied to these sumps from below. The design of sludge pumps has also been simplified: instead of four wings of pipes, two wings have been designed, on which 5 suckers are placed, covering the entire surface of the bottom of the sump. The tray for collecting clarified water is made flooded and somewhat moved inward from the walls of the sump. The calculation of these sedimentation tanks is carried out according to the loads, which are taken at 1.0-1.8 m3/hour per 1 m2 of surface according to the average hourly flow rate, the settling time is taken equal to 1 hour.

Secondary settling tanks are installed after biofilters to retain undissolved (suspended) substances (which are particles of dead biological film) and after aeration tanks to separate activated sludge from treated wastewater. Horizontal, vertical and radial settling tanks are used as secondary ones (see section 1.1.2).

The bulk of the activated sludge settled in the secondary clarifier must be pumped back into the aeration tank. However, activated sludge settles more than is necessary for reuse, so its excess should be separated and sent for disposal. Excess sludge at 99.2% moisture content is 4 l/day per inhabitant and has a higher moisture content than raw sludge from the primary clarifier, which increases the total volume of sludge. Sewerage design standards (SNiP 2.04.03-85) provide (depending on the type of silt sediment or biofilm) for different residence times and flow rates in the sump. For example, the duration of settling in secondary vertical settling tanks installed after aeration tanks is taken as 2 hours according to the maximum water flow, and the vertical rate of liquid rise is 0.5 mm/s, for settling tanks after drip biofilters - 0.75 h, and the rate of water rise is 0.5 mm/s.

The main differences between primary clarifiers and secondary clarifiers are as follows:

secondary clarifiers do not have devices for collecting and removing fatty and other floating substances;

usually used different system sludge pumping (sludge pumps in secondary clarifiers).

The work of settling tanks is evaluated by the removal of suspended solids, the concentration of return sludge and the moisture content of the sediment. These indicators characterize its main functions:

separation of purified water from activated sludge;

sludge compaction.

Controlling the operation of the secondary settling tank is a very important task of the operating service, since the efficiency of secondary settling directly affects the course of biochemical oxidation in aerotanks and largely determines the content of suspended solids in treated water, i.e. loss of activated sludge biomass and, accordingly, its increase.

If more than the optimal amount of sludge is removed from the secondary settling tank, then an excess volume of water returns to the aerotank, if less, then a lot of settled sludge is collected in the settling tank and the quality of treated water decreases. Therefore, the technological mode of operation of the secondary clarifier is set so that the level of sludge is consistent with that provided for by the project (as a rule, it is 0.5-0.75 m from the bottom of the radial clarifier). The efficiency of the secondary clarifier depends on the compliance of the actual hydraulic load with its design values ​​and the uniformity of its distribution, as well as on the timely continuous and uniform mode of sediment removal. The timeliness of sediment removal can be controlled by the values ​​of the return sludge dose and its level using control airlifts.

The operating experience of Moscow BOS has shown that at a dose of return sludge of 4-6 g / dm 3, the removal of suspended solids from secondary settling tanks is about 15 mg / dm 3, at 6 g / dm 3 - the removal increased from 15 to 20 mg / dm 3 . A significant increase in the removal of suspended solids from secondary settling tanks (up to 40 mg / dm 3) occurs when the concentration of return sludge reaches 8 g / dm 3, which, apparently, is the threshold for typical facilities that purify urban wastewater (A.L. Frolova , personal message).

At each treatment plant, it is necessary to experimentally establish the optimal dose of return sludge at which the maximum possible amount of sludge would be returned to the treatment system while ensuring the minimum removal of suspended solids from the secondary clarifiers.

It is necessary to control the operation of the secondary sedimentation tank for the removal of suspended solids (if it works well, it is less than 10 mg / dm 3), for the moisture content of the removed sediment (the norm is 99.4-99.7%) and for the content of dissolved oxygen. For normal operation of the secondary clarifier, the concentration of dissolved oxygen in it must be at least 2 mg/dm 3 . If this condition is met, the return sludge will enter the aeration tank of good quality and immediately begin the active oxidation of pollutants. If the concentration of dissolved oxygen in the secondary settling tank is less than 0.5 mg/dm 3, sludge rots and floats to the surface of the settling tank, the condition of the return sludge worsens and the operation of the regenerators is disrupted.

Oxygen is involved not only in the respiration of organisms, it removes metabolic products and toxins (in the secondary settling tank, these products accumulate in flakes due to unsatisfactory oxidation of contaminants in aeration tanks). Oxygen consumption in secondary settling tanks is less than in aeration tanks, since the load on the sludge is small. However, in the case of industrial wastewater (with a high concentration of pollutants in the form of suspensions and colloids that are adsorbed by sludge and poorly oxidized in aeration tanks), provided that the sludge is deposited in a secondary settling tank, pollutants continue to oxidize in it, while toxins and products of anaerobic decomposition and metabolism in secondary settling tanks are poorly discharged, and the sludge rots.

Consequently, the degree of sludge recirculation from the secondary settling tank in the case of industrial toxic wastewater should be determined only by the rate of sludge settling in the secondary settling tank, which will ensure a minimum period of sludge remaining in anoxic conditions.

Secondary settling tanks are fundamentally different from primary ones in the properties of the substances settling in them. If in primary sedimentation tanks sediment can lie for some time without rotting, then in secondary sedimentation tanks even a small deposit of sediment causes rotting and deterioration of the aeration regime throughout the entire system. Rotting return sludge disrupts the treatment system and, as a result, its effect is significantly reduced.

Therefore, the system for removing sludge from secondary settling tanks must provide for operation under conditions of daily peak loads, and not average daily loads

and be carried out around the clock, and not periodically, which is sometimes allowed in order to save energy.

It is necessary to control the load of suspended solids on secondary settling tanks based on the dose of activated sludge in the water entering them. It is optimal if the dose of sludge in the water coming from the aeration tank is no more than 1.5-2.0 g/dm 3 . Then the removal of suspended substances from the secondary settling tank will be from 5 to 10 mg/dm 3 under other favorable conditions.

The formulas for calculating the main parameters of the operation of secondary clarifiers are as follows:

Wastewater residence time in settling tanks (t h):

W is the volume of the settling zone of one settling tank (or the sum of the volumes of the zones from the melting of all working structures), m3;

q - hourly wastewater consumption for one settling tank (or for all working ones), m 3 / h.

The estimated residence time of wastewater in settling tanks must correspond to the design time, which, as a rule, is 1.5-2.0 hours. It should be remembered that the concentration time of sludge in settling tanks is much shorter (a property of dense settling particles), therefore, with a satisfactory return of activated sludge from secondary settling tanks to aeration tanks, its residence time is no more than 30-40 minutes. As the residence time of activated sludge in secondary settling tanks increases, it cannot withstand the deposits and begins to rot and die from its metabolites.

The hydraulic load on the secondary settling tank N, M3/(m2°h), is determined by the formula:

where P is the area of ​​the working surface of the sump (
), m2.

Example. W (volume of the sedimentation zone in one settling tank) - 4580 m3, a total of two settling tanks are in operation; q (hourly inflow of sewage) - 3965 m3/h; the radius of the sump is 10.6 m. Then the residence time of wastewater in the sump:

If the sludge index is unstable, the hydraulic load on the secondary settling tanks should be correctly calculated taking into account the sludge index, sludge removal, sludge concentration in the water leaving the aeration tanks and the type of settling tanks:

where K is the coefficient of utilization of the volume of the settling zone, accepted for radial settling tanks - 0.4, vertical - 0.35, vertical c. peripheral outlet - 0.5, horizontal - 0.45;

H is the depth of the flow path in the sump, m;

I - silt index in the water leaving the aerotanks, cm 3 /g;

a - the dose of sludge in the water leaving the aerotanks or in the collection channel, g / dm 3;

Example. K - 0.4, H - 6m, a -1.5 g / dm 3, I - 100 cm 3 / g, b - 15 mg / dm 3.

Secondary clarifiers are integral part biological treatment facilities are located in the technological scheme directly after bio-oxidizers and serve to separate dead biofilm from biologically purified water leaving biofilters.

The effectiveness of clarification of biologically purified water in secondary settling tanks determines, as a rule, the final effect of water purification and the operating efficiency of the entire biofiltration station complex.

Classification of secondary sedimentation tanks. Secondary settling tanks are: vertical, horizontal and radial. For treatment plants of small capacity (up to 20,000 m3/day), vertical secondary settling tanks are used, for treatment stations of medium and large capacity (more than 20,000 m3/day) - horizontal and radial ones.

Vertical secondary sedimentation tanks by their design are divided into the following: – round in plan with a conical sludge part, similar in design to the primary ones, but with a lower height of the settling zone; – square in plan (12×12 m, 14×14 m) with a four-bunker pyramidal silt part.

The advantages of vertical secondary sedimentation tanks are the convenience of removing settled biofilm from them under hydrostatic pressure, compact location when they are blocked with biofilters, simplicity of design due to the absence of moving parts, and the possibility of using a suspended layer of sediment. However, they also have a number of disadvantages, the main one being their large depth, which increases the cost of their construction, especially when high level groundwater standing.

When developing projects for biofiltration stations, horizontal and radial secondary settling tanks were practically not used; in very rare cases, radial settling tanks were used.

The optimal number of secondary settling tanks at treatment plants of almost any throughput should be from 2 to 8.

15. Ways to intensify the process of primary clarification of wastewater.

In cases where the concentration of contaminants in suspended solids exceeds 3 300 mg / l, to intensify the process of retaining suspended solids in settling tanks, use various techniques which include:

1 .Aeration of wastewater for 10-20 minutes before it enters the settling tanks, this technique heals the drain. water removes fermentation gases from it. Improves the process of biochemical oxidation of pollution in aerators. Improves suspension sedimentation.

In this way, the suspension clarification effect can be increased by 10%.

2. If excess sludge is added to the waste water in front of the sump, which has previously undergone regeneration, we will get a higher clarification effect. If such sludge is kept together with waste water for 15-20 minutes. With constant aeration, the activated sludge begins to absorb.

Priorator volume is calculated:

W=Q waste (1*R i)*t BC

V bq \u003d N bq

N bk \u003d N ots

Radial:

W=Q one hundred (1+Ri)

H bq =H set –(0.3/0.5).

Most effective method intensification of primary clarifiers colloidal contaminants of fat-like substances in front of biological treatment facilities is flotation bioflocculation. and activated sludge. Activated sludge floats well and is able to remove toxic substances from wastewater, organic pollution. With the flotation method, the effect of the retention of the air. Thing. In a flotation bioflocculator 60%. Reduction of BOD up to 40%, residence time from 20 to 40 minutes. The moisture content of sludge retention is 94-92%; in principle, the moisture content of the sludge can be less than 90%. However, such sludge loses its fluidity and is difficult to transport through pipes. Therefore, aspirators must prevent a decrease in humidity of at least 95%.

3. Use of thin-layer blocks

Thin-layer settling tanks differ from the usual ones by the presence of seedlings in them. Elements placed in the settling zone in the aisles of which sedimentation of contaminants occurs in thin layers liquid. This process proceeds quickly, since the path of movement of precipitating particles is 10 times less than in conventional settling tanks. They require less area than conventional settling tanks.

According to the way the fluid moves:

Horizontal

Vertical

Radial

By design, the thin-layer elements of the sump can be divided into tubular and stick (lamellar) The working element of the sump is a pipe of various sections. They are made of polyvenyl plastic. I usually use blocks about 3 m wide, 0.75 m high, 0.5 m high.

Plate ones consist of a series of parallel installed plates between which the movement of liquid, depending on the direction of movement of water and unloading of sediment, settlers are divided into direct-flow in which the direction of movement of water and sediment coincides, and counter-flow movement towards each other. Cross-flow water moves perpendicularly in the direction of movement of sediment More lamellar countercurrent have become widespread.

According to the material of manufacture. thin-layer elements are divided into 2 types.

1 In some thin-layer blocks are made of flexible materials

(thin films).

2 From materials with sufficient rigidity.

Their purposefulness is based on the fact that reducing the height of the flow while maintaining the same speed of its movement proportionally reduces the settling time. Also, dividing the flow height into smaller segments, at the same time increasing the settling area and reducing the load on suspension.

The deposition process is influenced by such factors as the angle of inclination of the module and the distance between the plates.

Usually, settling is carried out in a periodic mode. Water clarification, washing of the settling tank.

If the angle of inclination is 45 0 -60 0, flushing is not required. The sediment slides off on its own. Tubular ones have better hydraulic characteristics. Greater structural rigidity, operate at higher speeds.

However, they have an important design drawback - the difference in hydraulic characteristics between the tubular and tubular space due to their different geometric shapes.

Because of this, in the intertubular space there is an accumulation of solid forms of load. Leads to clogging between the tubular just. And there are anaerobic processes.

Modules having the form of honeycombs are deprived of such a disadvantage, the geometric dimensions in each cell are the same.

Modules can be of various heights, slopes and shapes to provide the best solution for each specific application. the calculation is reduced to the determination of the overall dimensions of thin-layer elements, the load and the determination of the flow velocity.

The durability of the structure is one of the factors determining the reliability of the investment. Our company offers to purchase metal tanks - various sizes. These tanks are part of a complex for wastewater treatment. The liquid, as it passes, is freed from most of the mechanical contaminants. The operating principle of such receivers is based on the action of gravitational force. This makes the design one of the most reliable and economically profitable options problem solving.

In industry there is a distinction primary and secondary settling tanks, horizontal and vertical. The designs differ from each other in configuration, dimensions and operating technology. Here you can order the necessary containers and reservoirs By affordable price. Products are available in a wide range of sizes. If necessary, we will produce tanks according to individual requests.

Basic characteristics of primary and secondary settling tanks of treatment systems

In a wastewater treatment complex, the primary settling tank is the initial link. The liquid to be cleaned enters this container. It separates the mechanical components. Sand and others particulate matter sink to the bottom. Subsequently, the resulting sludge is removed from the tank through a special tray. The task of the biological separation is assigned to the secondary settling tanks of the treatment complex. In them, the liquid is treated with reagents that cause impurities to form silty sediment. There are the following types of such collections:

• In the direction of flow. Large stations predominantly use secondary horizontal settling tanks, as they are the most productive. Water flows into them through a water overflow by gravity, while the secondary radial settling tank requires the organization of a liquid supply.
• By the number of tiers. There are one- and two-tier structures. The choice of scheme is influenced by the required performance rate. Multi-level complexes use a secondary vertical clarifier oriented to light loads. Such reservoirs are of low height and can be used by small stations.

Advantages of our company's tanks

You should give preference to our products for the following reasons:

• High performance characteristics. The most important advantages of the presented systems are dimensional accuracy and tightness. The smooth metal walls of the container do not allow algae to grow on them. The use of special coatings significantly increases the service life of the tank.
• Professionalism in development and execution. We have our own design department. Its engineers specialize in the selection of materials, designing the geometry of such vats. We will provide you with a solution tailored to your specific operational requirements.
• Production control. The secondary and primary clarifiers of the sewage treatment plant you have purchased comply with the standard. To provide High Quality Product samples are tested before sending to the client.