Steel Plant Wastewater Treatment and Reuse Technology

Only by improving the utilization rate of wastewater in iron and steel production can realize the recycling of wastewater and reduce the cost of water resources, and the recycling mode of purification and reuse of wastewater in iron and steel enterprises is a specific mode to improve the utilization rate of wastewater.

Project overview

A new steel plant needs to build a central water treatment plant. According to the water consumption of steel production, the design scale of the production wastewater treatment system is 12,500 m3/d (including rainwater treatment), and two water treatment lines are set up. Its main design inlet and outlet water quality indicators (see Table 1).

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Process flow

In waste water treatment, according to the characteristics of steel waste water quality and the amount of influent water, the vortex concave air flotation process is adopted, which can effectively remove grease, colloids and suspended solids in waste water, and remove petroleum and suspended solids (SS). The removal rate exceeds 80%, and the removal rate of BOD and COD can reach more than 60%.

In the wastewater reuse treatment, the traditional flocculation-sedimentation process is used, which has great advantages in cost and operation safety. Mixing rainwater and production wastewater for treatment and reuse greatly reduces the central water treatment plant’s own water consumption, and achieves the goal of sustainable development, energy conservation and emission reduction in the iron and steel industry.

The production wastewater treatment and recycling system adopts the following process flow:

The production wastewater first enters the lift pump room, and the water inlet of the lift pump pit is equipped with a filter screen to intercept coarse particles.

It is lifted from the pump to the fine grid to remove floating debris and large particles of impurities, and then enters the wastewater conditioning tank for homogenization and uniformity. Then it enters the flocculation reaction tank and adds coagulant for flocculation reaction. The water from the reaction tank enters the inclined tube sedimentation tank to remove the heavier flocs. The effluent from the sedimentation tank flows into the recycling water tank, and is supplied by the recycling water supply pump to the recycling water users of the whole plant. .

The reused rainwater transported by the rainwater grit chamber directly enters the wastewater conditioning tank and is mixed with the production wastewater to be treated and treated together. The sludge discharged from the sedimentation tank and the scum discharged from the air flotation tank are discharged into the sludge tank, and then sent to the sludge thickening tank after being pressurized by the pump.

Design parameters of main buildings, structures and equipment

(1) Lift the pump room.

1 underground reinforced concrete production wastewater lifting pump house, size: 12m×6m×9m. It is used to lift wastewater to ensure the smooth flow of wastewater in subsequent treatment structures. Set up wastewater first-level lift pump group P301: 3 sets (2 for 1 standby), Q=275m3/h, H=12m, N=22kW.

(2) Between the grilles.

A total of 1 underground grille room is built in the design. Size: 8.5m×4m×2.5m, divided into two grids. The effective grid width is 900mm, the tooth rake spacing is 8mm, and the trench width is 1000mm.

At the same time, it is equipped with 2 rotary solid-liquid separators, which have a high degree of automation, high separation efficiency, low power consumption, no noise, and good corrosion resistance. It can ensure continuous and stable work without supervision. The self-purification ability will not occur, so as to reduce the daily maintenance workload.

(3) Conditioning tank for production wastewater.

One underground reinforced concrete waste water conditioning tank, size: 25m×12m×5.7m, divided into two grids. Hydraulic retention time 2h.

The water volume of the adjustment pool fluctuates greatly, and the combination of fixed frequency and variable frequency water pumps can be used when selecting the water pump.

Equipped with waste water adjustment tank skimmer: 2 sets, its oil collection capacity is 10kg/h, the recovery rate of floating oil is 98%, and the water content of the oil is less than 3%; wastewater secondary lift pump group P302: 3 sets, 2 for use and 1 for backup ( One of them is variable frequency speed regulation), pump performance parameters: Q=550m3/h, H=15m, N=37kW.

The setting of the skimmer can effectively remove the oil insoluble in the wastewater, reduce the wastewater treatment load for the subsequent vortex concave air flotation process, and ensure the removal rate of the oil in the wastewater.

(4) Vortex concave air flotation tank.

There are 2 above-ground vortex concave air flotation pools. The main technical parameters of the single-seat vortex concave air flotation tank:

The working principle is as follows: the pretreated wastewater enters the aeration area equipped with a vortex concave aerator, and the aerator transfers the air on the water surface to the water through the exhaust pipe through the rapid rotation of the hollow impeller at the bottom and pulverizes the air. form micro-bubbles, which combine with solid pollutants in wastewater.

Due to the density imbalance between the gas-water mixture and the liquid, a vertical upward buoyancy is generated, and the micro-bubbles attached to the suspended solids (ss) rise to the liquid surface, and are intermittently scraped into the sludge by the chain scraper through the slag scraper. The collection tank is discharged from the system, and the purified water overflows from the overflow tank into the subsequent treatment facility.

The vortex concave air flotation process saves investment because the equipment occupies a small area, and the device does not have pressure vessels, air compressors, circulating pumps and other equipment. It also has the advantages of low operating costs, remarkable treatment effects, and simple operation.

(5) flocculation reaction tank.

2 above-ground reinforced concrete flocculation reaction tanks. The size of a single flocculation reaction tank: 6m×6m×3.5m, divided into 4 grids connected in series, the first grid is a mixing tank, and the last 3 grids are a flocculation tank. Main supporting equipment of single tank: 1 fast mixer (for mixing tank), speed 20r/m; 3 slow mixers (for flocculation tank), rotation speed 10r/m;

1 set of liquid coagulant dosing equipment; 1 set of polyacrylamide (PAM) dosing equipment. Using PAM as a flocculant means that when PAM is used for sewage treatment, it has high removal efficiency of organic matter in water, less dosage, fast settling speed and low water production cost.

  (6)斜管沉淀池。

2 seats. The sedimentation size of each inclined tube: 8m×6m×4.5m; type: semi-underground type; pool structure: reinforced concrete.

Set up double wire rope traction mud scraper: 1; mud scraper running speed 1m/min, pool width 6m. The inclined tube sedimentation tank shortens the particle settling distance, increases the sedimentation area, and improves the treatment efficiency. It also avoids the disadvantages of the large floor space of the horizontal flow sedimentation tank and the construction difficulty caused by the large depth of the vertical flow sedimentation tank.

(7) Reuse pool and recycle water supply pump field.

A total of 1 reclaimed water pool and reclaimed water supply pump field are built in the design.

The reuse pool is semi-underground, with dimensions: 25m×18m×3.9m. Pump field size: 17m×4.5m, 0.3m above the ground. Pump field equipment parameters: P303 water supply pump group: 3 units, 2 for use and 1 for backup (one of which is variable frequency speed regulation), Q=500m3/h, H=62m, N=132kW.

4 Conclusion

The rationality of wastewater reuse engineering design is an effective way to solve the shortage of water resources in the iron and steel industry.

Designers should pay attention to the following points when designing steel industry wastewater treatment and reuse projects:

(1) The choice of treatment process is extremely important, which is the key to ensuring that the water quality of wastewater reuse can meet the standard.

In order to avoid wasting investment and ensure the safety of water supply, the water quality and quantity data of production wastewater need to be scientifically and reasonably determined. Different production processes of iron and steel enterprises adopt different ways of recycling industrial sewage.

The main pollutants in the effluent wastewater are suspended solids, oil, etc., with high hardness, high chroma and high turbidity; generally, the BOD5/COD value is low, and the biodegradability is poor, so the biochemical treatment process can be ignored. .

(2) Minimize self-water consumption as much as possible and utilize rainwater resources, which not only achieves energy conservation and emission reduction, but also greatly reduces operating costs. The reuse rate of various backwash wastewater should be improved to achieve energy saving and environmental protection.

(3) The engineering design ensures smooth process flow, reasonable general drawing and process layout to ensure smooth production, reliable operation and convenient maintenance.

(4) The level of automation control follows the principles of advanced, mature, reliable, practical, and beneficial to water quality control, in order to improve labor productivity and ensure water quality. This is the key to the stable operation of the system.

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