Direct reuse of filter backwash wastewater to enhance coagulation effect

The water purification plant produces a large amount of production wastewater every day, including the sludge discharge water from the sedimentation tank and the backwash water from the filter tank, accounting for about 3% to 10% of the water supply of the water plant.

At present, this part of the production wastewater is either directly discharged or reused after treatment.

Studies have shown that these wastewaters are enriched with a large amount of natural organic matter and pathogenic microorganisms and other harmful substances, and direct reuse may have an impact on the water quality of subsequent water treatment processes. However, at the same time, these wastewaters also contain a large number of destabilized particles, polymeric colloids and unreacted coagulants. Reasonable reuse of production wastewater can improve the coagulation effect.

In this paper, through the coagulation and stirring small-scale test, under different reflux ratios and coagulant dosage, the influence of the direct reuse of the sand filter backwash wastewater on the effluent quality was studied, and the enhancement of the direct reuse of the filter backwash wastewater was investigated. The effect of coagulation provides a technical reference for the water supply process transformation and cost management of the water plant.

Test part

1.1 Test conditions

The test was carried out in a water plant in the south. The water plant uses Dongjiang water as raw water. The process flow is shown in Figure 1.

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The water plant uses the filtered water from the sand filter as water for backwashing. The backwashing process includes three stages: gas flushing, gas-water mixed flushing and water flushing. Among them, the filter backwash cycle is 38h, the air flushing time is 4min, and the intensity is 20.5L/(m2·s);

The air-water mixing time is 6min, the air-pulse intensity is 20.5L/(m2·s), and the water-pulse intensity is 5L/(m2·s); In the water flushing stage, surface sweeping is set, and the sweeping intensity is about 1.3-2.6L/(m2·s).

The test was carried out in December. The coagulant used in the water plant was liquid polyaluminum chloride (PAC). The dosage of PAC during the test was 10-15 mg/L.

1.2 Test method

The waste water and raw water in the gas-water mixing stage during the backwashing process of the filter tank were mixed according to a certain proportion, and used as the test raw water for the coagulation and stirring test. During the test period, the water quality of the raw water of the water plant and the waste water during the backwashing gas-water mixing stage of the filter tank is shown in Table 1.

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The coagulation stirring test was carried out in the ZR4-6 six-unit coagulation test mixer, and the actual coagulation and sedimentation parameters of the simulated water plant were carried out in 5 stages. Finally, the supernatant of the test cup was used to detect the relevant water quality indicators. The coagulation test parameters are shown in Table 2. .

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1.3 Detection indicators

CODMn: Determined by acid potassium permanganate titration.

Aluminum: Measured by chrome azure S spectrophotometry.

Iron: Determined by phenanthroline spectrophotometry.

Manganese: Measured by ThermoScientificXseries II ICP-MS.

Results and Discussion

2.1 The removal effect of turbidity

According to the reflux ratios of 0, 2%, 5%, and 10%, the recoil wastewater was mixed with the raw water, and then 9, 15, and 21 mg/LPAC were added to conduct the coagulation stirring test. The turbidity of the supernatant in the cup was measured, and the results are shown in Figure 2.

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The turbidity of the backflushing wastewater is close to 700NTU, which is much higher than the turbidity of the raw water. The reason is that in the gas-water mixing stage of the filter backflushing, the impurities trapped in the filter layer are brought out in large quantities under the action of gas and water, thereby Increased turbidity in water.

It can be seen from Figure 2 that with the increase of the influent reflux ratio, the turbidity of each influent also increases, but after coagulation and precipitation, the turbidity of the effluent decreases significantly, and under the same dosage of PAC, With the increase of the influent and reflux ratio, the turbidity of the effluent decreases continuously, indicating that the direct reuse of the recoil wastewater can achieve the effect of strengthening coagulation.

Studies have shown that the mechanism of reusing production wastewater to achieve enhanced coagulation effect mainly lies in its chemical effect and physical effect. The charge on the colloid surface changes, resulting in electrostatic adsorption and electrical neutralization. The physical effect is to increase the colloid concentration in the raw water. From the coagulation kinetics, it can be seen that this can greatly increase the probability of particle collision and make the colloid easy to coagulate, thereby achieving the effect of strengthening coagulation.

In the test, the turbidity of coagulation sediment effluent with three different PAC dosages was studied. Considering the improvement of water quality, if the turbidity of the coagulation precipitation effluent is considered to be up to the standard when the turbidity is 1 NTU, in the case of not reusing the backflushing wastewater, it is necessary to add 21 mg/LPAC to make the turbidity of the effluent lower than 1 NTU, which greatly improves the the operating costs of the water plant.

However, after directly reusing the filter for backflushing the wastewater, the removal effect of turbidity is enhanced with the increase of the reflux ratio. At a reflux ratio of 10%, adding 9mg/L of PAC can already make the effluent turbidity. At about 1NTU, the dosage of coagulant is greatly reduced, which can save water resources and reduce costs while improving the removal rate of effluent turbidity after coagulation and sedimentation.

2.2 Removal effect of organic matter

According to the reflux ratio of 0, 2%, 5%, and 10%, the recoil wastewater was mixed with the raw water, and then 9, 15, and 21 mg/LPAC were added to conduct the coagulation stirring test. The supernatant in the cup was measured for TOC and CODMn, and the results are shown in Figure 3 and Figure 4.

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It can be seen from Figures 3 and 4 that, similar to the turbidity, the TOC and CODMn of each influent gradually increased with the increase of the influent reflux ratio.

After coagulation and precipitation, the TOC and CODMn of each water sample were removed to a certain extent. Under the same dosage of PAC, the TOC and CODMn of the effluent decreased gradually with the increase of the influent reflux ratio. However, the concentration remained basically unchanged. After increasing the dosage of PAC, the removal effect of TOC and CODMn was strengthened to a certain extent.

The possible reason is that there is a large amount of aluminum salt precipitates in the recycled backflushing wastewater, and its enhanced coagulation can effectively remove insoluble organic matter in the water, but the removal effect of dissolved organic matter is limited. The added PAC can be used to remove more dissolved organic matter, so it has a certain strengthening effect on the removal of TOC and CODMn.

2.3 Removal effect of metal

According to the reflux ratios of 0, 2%, 5%, and 10%, the recoil wastewater was mixed with the raw water, and then 9, 15, and 21 mg/LPAC were added to conduct the coagulation stirring test. The supernatant in the cup was measured for its aluminum and iron, and the results are shown in Figure 5 and Figure 6 .

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It can be seen from Figure 5 that under the same dosage of PAC, increasing the reflux ratio of backflushing wastewater in the influent can improve the removal effect of aluminum in water, but after increasing the dosage of PAC, the concentration of aluminum in the effluent is on the contrary. There is an increase, indicating that the use of polyaluminum chloride as a coagulant increases the aluminum content in the water to a certain extent. It can be seen from Figure 6 that the reuse of backflushing wastewater can reduce the iron concentration in the influent, but the iron concentration in the effluent is basically the same under different reflux ratios and different dosages of PAC.

2.4 Control effect on microorganisms

According to the reflux ratio of 0, 2%, 5%, and 10%, the recoil wastewater and raw water were mixed with 9, 15, and 21 mg/LPAC, respectively, for coagulation and stirring tests. , adding 1.5mg/L (calculated by available chlorine) sodium hypochlorite to react for 1h, and measuring the total number of bacteria and total coliform bacteria before and after disinfection. The results are shown in Table 3.

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It can be seen from Table 3 that the reuse of recoil wastewater will lead to an increase in the number of microorganisms in the coagulation and sedimentation effluent, and with the increase of the reflux ratio, the increase in the number of microorganisms gradually increases. When the reflux ratio is 10%, the number of bacteria The total number increased by a maximum of 2 times, and the total coliform group increased by a maximum of 8 times, which was much higher than the normal raw water quality.

Sodium hypochlorite was added to the coagulation and sedimentation effluent by simulating the disinfection process of the clear water tank of the water plant. The results showed that the total number of bacteria and total coliform bacteria were not detected in each test water sample. However, after the conventional disinfection process of the water plant, the biological safety of the effluent can be guaranteed.

At the same time, it was found that although reducing the dosage of PAC will affect the removal effect of coagulation and sedimentation on microorganisms, after disinfection, the microorganisms in the water can still be completely killed, so reducing the dosage of PAC will not affect the risk of effluent organisms.

Conclusion

①The direct reuse of filter backwash wastewater can achieve the effect of strengthening coagulation. Under the reflux ratio of 10%, adding 9mg/LPAC can reduce the turbidity of the precipitated effluent to 1NTU, which can greatly reduce the coagulation while improving the effluent quality. drug consumption.

② After the recoil wastewater is reused, its enhanced coagulation can improve the removal ability of insoluble organic matter in water, and at the same time, the added coagulant can act more on insoluble organic matter, thereby improving the removal rate of organic matter.

③ Reusing backflushing wastewater can reduce the dosage of coagulant, reduce the risk of excessive aluminum in the effluent, and at the same time will not affect the removal of metallic iron, and can ensure the biological safety of the effluent after conventional disinfection treatment. Effectively improve the effluent quality of water plants and reduce operating costs.

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