Iron oxide red plant wastewater treatment process

The wet mixed acid process produces iron oxide red, and the raw materials are mainly iron sheet, nitric acid, ferrous sulfate, steam and air, so the production wastewater contains more ferrous ions, NH3, COD, TN and other pollutants.

If the production wastewater is directly discharged into the natural water body without treatment, it will cause pollution to the environment and destroy the ecological balance; if it is directly discharged into the municipal pipe network.

Its water quality is inferior to the takeover standard of the municipal pipe network, which will affect the normal operation of the urban sewage treatment plant and indirectly lead to the problem of environmental pollution.

Although the plant has original sewage treatment equipment and facilities, with the development and progress of the enterprise, its production capacity has gradually increased, resulting in insufficient sewage treatment capacity; at the same time, the environmental protection standards have become increasingly strict, and the total nitrogen requirements are discharged up to standard.

The original treatment process cannot meet the current status of sewage treatment.

As the enterprise attaches great importance to environmental protection, in response to the national environmental protection policy and actively promoting the implementation of the new national environmental protection law, the enterprise’s sewage treatment equipment and facilities have been upgraded and transformed, so that the production wastewater can be discharged stably up to the standard.

Water quality and quantity

According to the data provided by the enterprise, the discharge of wastewater from the production process is about 4000m3/d. At the same time, the discharge of low-concentration wastewater such as floor washing, domestic sewage and initial rainwater in the workshop is about 2000m3/d. The total designed water treatment capacity is 6000m3/d.

According to the production process of red iron oxide, the wastewater mainly comes from:

The supernatant after precipitation of the iron oxide mixed solution discharged from the precipitation process and the filtrate and water washing wastewater discharged from the press filtration and water washing process.

The main pollutants in the wastewater are the ammonium generated by the reaction of iron scale and nitric acid, as well as the incompletely reacted nitric acid and iron ions, which are mainly manifested as NH3-N, TN, COD and other pollution indicators. The designed influent water quality is shown in Table 1.

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After the wastewater treatment of this project, the effluent will be discharged into the municipal sewage pipe network.

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Process flow

According to water quality analysis, COD in wastewater is low, ammonia nitrogen and total nitrogen are high, which is high nitrogen wastewater with C/N imbalance. The wastewater first passes through the original ammonia nitrogen stripping tower to remove most of the ammonia nitrogen and then enters the newly built sewage treatment station. .

Therefore, the main purpose of the newly built sewage treatment station is to remove total nitrogen, so that the total nitrogen, ammonia nitrogen and COD of the effluent are discharged up to the standard.

In addition, due to the use of ferrous sulfate in the production process, the water contains a large amount of sulfate radicals. If it directly enters the biochemical system without treatment, it will affect the effect of COD degradation and denitrification in the biochemical system. Therefore, before entering the biochemical system Pretreatment by chemical precipitation method is required to ensure the normal operation of subsequent treatment.

The newly built sewage treatment station adopts chemical precipitation + secondary A/O process + mechanical filtration.

Because most of the ammonia nitrogen has been removed by the first-stage stripping, the biochemical denitrification adopts the A/O process, which mainly uses the biological denitrification to remove the total nitrogen in the sewage.

Using the current mature and reliable improved secondary A/O method terminal sewage treatment process, the total nitrogen, COD and other pollutants in the sewage can be well removed, and a good pollution control effect has been achieved.

The process flow of the system is shown in Figure 1.

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Process description: The waste water after stripping (the main pollutant is nitrate) enters the intermediate tank, and is lifted into the desulfurization reaction tank by the pump, and the slaked lime solution is added to react with the SO2-4 in the waste water to form calcium sulfate precipitation.

The self-flowing desulfurization sedimentation tank completes the mud-water separation under the action of gravity.

The supernatant liquid flows into the carbon source dosing tank, and the carbon source is added to supplement the carbon source required for denitrification of the biochemical system; then it flows into the pH adjustment tank, and the pH of the wastewater is adjusted to neutral by adding acid and liquid alkali.

Then it goes to the water distribution tank to supplement the phosphorus source; it flows to the pre-anoxic tank, and after the pre-anoxic tank reaction, it enters the two-stage AO treatment process; the total nitrogen in the water is removed by the biochemical denitrification treatment, and the organic matter in the water is degraded at the same time.

After being treated by the two-stage AO biochemical system, the sewage is separated from the mud and water in the secondary sedimentation tank (biochemical sedimentation tank), the supernatant liquid flows into the clear water tank, the sludge is lifted to the sludge tank by the pump suction machine, and then returned to the pre-anoxic tank .

When the biochemical system is in good operation, the clean water pool can exceed the effluent to the metering discharge channel, and discharge it into the municipal pipe network when it meets the standard.

It can also be discharged to the metered discharge channel after being treated by a mechanical filter according to the specific conditions of the operation to meet the standard discharge.

The sludge in the desulfurization sedimentation tank is concentrated in the desulfurization sludge concentration tank, and then pumped to the vacuum belt filter press for sludge dehydration, and the sludge is transported out for processing after dehydration.

The residual sludge in the biochemical system is concentrated in the biochemical sludge thickening tank, and then pumped to the centrifugal dewatering machine for sludge dehydration, and the sludge is transported out for processing after dehydration.

The physicochemical sludge produced by the desulfurization and precipitation is lifted by the pump into the sludge thickening tank, and then enters the vacuum belt dehydrator after the concentration treatment.

Process characteristics

3.1 Chemical desulfurization

According to the production process of red iron oxide, the wastewater contains a large amount of sulfate radicals.

Although there is no indicator of sulfate radicals in the monitoring of pollutants in the effluent, the high concentration of sulfate radicals in the water will inhibit the normal growth of microorganisms, thereby affecting the quality of the effluent.

Therefore, before the wastewater enters the biological denitrification reaction, calcium and sulfate ions are reacted to produce calcium sulfate precipitation by adding lime, so as to achieve the purpose of removing sulfate in the water, so as to ensure the normal operation of the subsequent biochemical system.

3.2 Removal of total nitrogen

Due to the increasingly stringent environmental protection standards, while COD and ammonia nitrogen are discharged up to the standard, total nitrogen is also required to be discharged up to the standard. Biological denitrification is currently recognized as an economical, effective and promising method; the research on biological denitrification has made great progress and is maturing.

Since a part of ammonia nitrogen has been blown off by the ammonia nitrogen stripping tower before the wastewater enters the sewage treatment station, the pre-anoxia + two-stage AO process is used in this design to further remove ammonia nitrogen from the water, and at the same time achieve the purpose of removing total nitrogen.

The physicochemical denitrification method and the biological denitrification method are perfectly combined, and the functions and advantages of the two methods are maximized.

Main structures and design parameters

4.1 Intermediate pool

The intermediate pool plays the role of buffering wastewater and averaging the volume.

1 new intermediate pool, semi-underground steel concrete structure, triangular shape, design size B×L×H=17.0m×29.75m×8.6m, total volume of 2200m3, residence time of about 8.8h. The residence time basically meets the normal operation requirements of the sewage system.

Main supporting equipment: 2 sets of intermediate pool lifting pumps (1 set for use and 1 set for standby), self-control self-priming pump without seal, Q=300m3/h, H=12m, N=22kW; 1 set of ultrasonic level gauge; 1 set of water inlet flowmeter Taiwan; pipe diameter 200mm, range 0 ~ 450m3/h.

4.2 Desulfurization reaction tank

The desulfurization reaction tank fully reacts the wastewater and slaked lime to produce calcium sulfate precipitation, thereby removing sulfate radicals in the water for subsequent biochemical treatment.

Two newly built desulfurization reaction tanks, semi-underground steel concrete structure, monomer design size B×L×H=4.0m×4.0m×8.6m, with a total volume of 275m3 and a residence time of about 1h.

Main supporting equipment: 2 vertical mixers, single N=7.5kW; 1 online pH meter.

4.3 Desulfurization sedimentation tank

The desulfurization sedimentation tank separates the waste water from the slaked lime production sediment sludge, the supernatant flows to the citric acid dosing tank, and the physicochemical sludge is discharged to the physicochemical sludge thickening tank through the lift pump.

  新建脱硫沉淀池1座,半地下式钢砼结构,单体设计尺寸B×L×H=28.0m×8.0m×8.6m,表面负荷1.12m3/m2×h。

Main supporting equipment: 1 reciprocating mud scraper, B=8m, L=28m, single N=7.5kW; 2 desulfurization sludge lift pumps, vertical sewage pump, Q=50m3/h, H=25m, N=5.5kW.

4.4 Carbon source dosing pool

The addition of carbon source to the carbon source dosing pool not only supplements the carbon source required for denitrification of the biochemical system, but also has a certain adjustment effect on the pH value.

A new carbon source dosing tank is built, with a semi-underground steel concrete structure, the design size of the monomer is B×L×H=5.0m×6.0m×8.6m, the total volume is 258m3, and the residence time is about 1h.

Main supporting equipment: 1 vertical mixer, single N=7.5kW; 1 online pH meter.

4.5 pH distribution tank

The pH adjustment tank adjusts the pH of the wastewater to neutral by adding acid and liquid alkali.

One newly built pH adjustment tank, semi-underground steel concrete structure, the design size of the monomer is B×L×H=5.0m×6.0m×8.6m, the total volume is 258m3, and the residence time is about 1h.

Main supporting equipment: 1 vertical mixer, single N=7.5kW; 1 online pH meter.

4.6 Distribution pool

Supplement the phosphorus source through the distribution tank.

One new water distribution tank, semi-underground steel concrete structure, single design size B×L×H=5.0m×6.0m×8.6m, total volume of 258m3, and residence time of about 1h.

Main supporting equipment: 1 vertical mixer, single N=7.5kW.

4.7 Pre-anoxic pool

The pre-anoxic tank improves the biodegradability of wastewater, degrades the organic matter in the water and removes COD, and denitrification occurs at the same time, and the total nitrogen in the water is removed.

A newly built pre-anoxic tank, semi-underground steel concrete structure and trapezoidal structure, with a total volume of 4500m3 and a residence time of about 18h.

Main supporting equipment: 6 submersible mixers, single N=7.5kW; 1 online pH meter.

4.8 Primary Hypoxia Pool

The first-level anoxic tank degrades the organic matter in the water to remove COD, and further denitrification occurs to remove the total nitrogen in the water.

Two new first-level anoxic tanks, semi-underground steel concrete structure.

Main supporting equipment: 16 submersible mixers, single N=7.5kW; 2 online pH meters.

4.9 First-class aerobic pool

The first-level aerobic tank degrades the organic matter in the water to remove COD, and at the same time, the nitrification reaction occurs to prepare for the denitrification reaction. Two new first-level anoxic tanks, semi-underground steel concrete structure.

Main supporting equipment: 2 sets of microporous aerators; 4 sets of submersible mixers, single N=5.5kW; 2 sets of online pH meters; 2 sets of Roots blowers, 42m3/min, 0.85kgf/cm2, N=90kW and two Grade aerobic shared.

4.10 Secondary Hypoxia Pool

The secondary anoxic tank degrades the organic matter in the water to remove COD, and further denitrification occurs to remove the total nitrogen in the water. Two new first-level anoxic tanks, semi-underground steel concrete structure.

Main supporting equipment: 8 submersible mixers, single N=7.5kW; 2 online pH meters.

4.11 Secondary aerobic pool

The secondary aerobic tank degrades the organic matter in the water to remove COD to ensure that the effluent COD meets the standard; at the same time, the nitrification reaction occurs to prepare for the denitrification reaction.

Two new secondary aerobic pools, semi-underground steel concrete structure.

Main supporting equipment: 2 sets of microporous aerators; 2 sets of online pH meters.

4.12 Secondary Sedimentation Tank

The secondary sedimentation tank realizes the separation of mud and water, the supernatant is discharged to the clear water tank, and the sludge in the sedimentation tank is returned to the pre-anoxic through the pump suction machine.

Two newly built secondary sedimentation tanks, semi-underground steel concrete structure, single design size B×L×H=32.0m×8.0m×8.1m, surface load is about 0.6m3/m2h.

Main supporting equipment: traveling pump dredger, SB-16.45, gauge 16.45m; electromagnetic flowmeter

4.13 Shimizu Pond

A new clear water tank is built, with a semi-underground steel concrete structure, the design size of the monomer is B×L×H=16.0m×1.5m×8.1m, the total volume is 190m3, and the residence time is about 05h.

Main supporting equipment: 1 ultrasonic level gauge.

4.14 Mechanical filters

6 new mechanical filters, carbon steel anti-corrosion structure, single size Φ3600mm, single processing capacity 50m3/h. Main supporting equipment: 6 sets of rotor flowmeters; 6 sets of pneumatic valves for each mechanical filter, a total of 36 sets of pneumatic valves.

4.15 Biochemical sludge thickening tank

A new biochemical sludge thickening tank is built, with above-ground carbon steel anti-corrosion, the design size is Φ6×5m, and the total volume is 140m3.

Main supporting equipment: 1 sludge thickener, Φ6m; 1 ultrasonic mud level meter; 2 biochemical sludge lift pumps, Q=5m3/h, H=12m, N=1.1kW; centrifugal sludge dewatering machine, treatment The volume is 5m3/h.

4.16 Physicochemical sludge thickening tank

A new physical and chemical sludge thickening tank is built, with above-ground carbon steel anti-corrosion, the design size is Φ10×5.5m, and the total capacity is about 400m3.

Main supporting equipment: 1 sludge thickener, Φ10m; 1 ultrasonic mud level meter; 2 physicochemical sludge lift pumps, Q=100m3/h, H=15m, N=5.5kW; vacuum belt filter press, DU30/2500, 1 set.

Expected treatment effect and reduction of pollutants

5.1 Expected treatment effect According to previous engineering cases and theoretical calculations, the expected treatment effect of each treatment unit is shown in Table 3.

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5.2 Reduce the amount of pollutants

According to the analysis of water quality characteristics, this sewage treatment process mainly removes the total nitrogen in the water, avoids excessive nitrogen discharge into the natural water body and causes the occurrence of eutrophication in the water body, and reduces its adverse impact on the natural environment.

The treatment scale in this sewage treatment is 6000m3/d, the total nitrogen in the influent is 1800mg/L, and the total nitrogen in the effluent is 70mg/L, so the daily reduction of total nitrogen is 6000×(1800-70)=10380kg/d=10.38t /d, the annual reduction of total nitrogen is 10.38×365=3788.7t/a.

According to the calculation, it can reduce 3788.7t of total nitrogen pollutants directly into the natural environment every year, making a huge contribution to the protection of the natural environment.

Summary

The production wastewater from the iron oxide red plant is treated by the ammonia nitrogen stripping tower.

All indicators of wastewater after chemical precipitation + pre-anoxia + secondary AO + advanced treatment can be discharged up to the standard, and the total nitrogen in the effluent is ≤ 70mg/L. Chemical desulfurization is an important pretreatment process, which is the basis for the normal operation of the subsequent biochemical system.

The ammonia nitrogen stripping method of physicochemical denitrification is combined with the pre-anoxia + two-stage AO method of biological denitrification, so that both ammonia nitrogen and total nitrogen in the effluent can be discharged up to the standard.

It has certain reference significance for wastewater treatment projects with high ammonia nitrogen and C/N imbalance.