Instructions for the use of activated alumina for defluorination

High-fluoride water is widely distributed, covering all provinces, municipalities and autonomous regions across the country. Fluorosis seriously damages the health of the general public and is a major endemic disease in my country. In order to protect the health of the people, our company applied for special funds to conduct sampling and field investigation in the high fluoride area of drinking water in my country, and designed a fluoride removal process using activated alumina as the filter material.

illustrate:

1. Before the raw water contacts the filter material, the pH value should be reduced, and the reduced value should be determined by technical and economic comparison, and generally should be adjusted to between 6.0 and 7.0.

2. The raw water can be reduced by adding acid solutions such as sulfuric acid, hydrochloric acid and acetic acid or adding carbon dioxide gas to reduce the pH value.

3. The filtration rate of the filter tank can be adopted in the following two ways:

     (1) When the pH value of the influent water of the filter tank is greater than 7.0, the intermittent operation mode should be adopted. The design filtration speed is 2~3m/h, the continuous operation time is 4~6h, and the intermittent operation is 4~6h.

     (2) When the pH value of the influent water is less than 7.0, the continuous operation mode can be adopted, and the filtration rate is 6-10m/h.

4. The flow direction of raw water through the filter material layer can be bottom-up or top-down. When using a sulfuric acid solution to adjust the pH, it is advisable to use a top-down approach. [When using carbon dioxide to adjust the pH value, the bottom-up method should be adopted.

5. The fluorine content of the effluent at the end of the fluoride removal cycle of a single filter can be slightly higher than 1mg/L, and the end point fluoride content should be determined according to the mixing adjustment capacity, but the fluorine content of the mixed treated water should not be greater than 1.0mg/L .

6. The periodic adsorption capacity of the filter material is mainly selected according to factors such as raw water fluorine content, pH value, filtration rate, filter layer thickness, end-point fluorine content and filter material performance.

(1) When using a sulfuric acid solution to adjust the pH to 6.0 to 6.5, it is generally 4 to 5 g(F)/kg (Al2O3).

 (2) When the pH value is adjusted to 6.5-7.0 by carbon dioxide, it can generally be 3-4 g(F)/kg (Al2O3).

Regeneration of activated alumina:

 1. When the fluorine content of the effluent reaches the terminal fluorine content value, the activated alumina should be regenerated. The regeneration solution is preferably sodium hydroxide solution or aluminum sulfate solution.

2. When using sodium hydroxide for regeneration, the regeneration process can be divided into four stages: first backflushing, regeneration, secondary backflushing (or leaching) and neutralization. When using aluminum sulfate for regeneration, the neutralization stage described above can be omitted.

3. The expansion rate of the filter layer for the first backwash can be 30% to 50%, the backwash time can be 10 to 15min, and the flushing intensity depends on the particle size of the filter material, generally 12 to 16L/m2·s.

4. The regeneration solution should pass through the filter layer from top to bottom; the flow rate, concentration and dosage of the regeneration solution can be adopted according to the following Regulations:

(1) Sodium hydroxide regeneration: NAOh solution with a concentration of 0.75% to 1% can be used, and the consumption of sodium hydroxide can be calculated according to the requirement of 8 to 10 g of solid sodium hydroxide to remove 1 g of fluoride. . The volume of the regeneration solution is 3-6 times the volume of the filter particles, the regeneration time is 1-2h, and the flow rate of the regeneration solution is 3-10m/h.

(2) Regeneration of aluminum sulfate: the concentration is 2% to 3%, and the consumption of aluminum sulfate can be calculated according to the requirement of 60 to 80g of solid aluminum sulfate {AL2(sO4)3·18h2O} for every 1g of fluoride removed. The regeneration time can be selected from 2 to 3h, and the flow rate can be selected from 1 to 2.5m/h. The regeneration solution in the filter must be emptied after regeneration.

 5. The secondary recoil strength can be 3~5L/m2s, the flow direction is from bottom to top through the filter layer, and the recoil time can be 1~3h. For rinsing, use raw water with 1/2 normal filtration flow, and rinse the filter particles from the upper part, the rinsing time is 0.5h, and the rinsing time is 0.5h.

6. Using aluminum sulfate as the regenerant, the pH value of the effluent at the end of the secondary recoil should be greater than 6.5, and the fluorine content should be less than 1mg/L.

7. Sodium hydroxide is used as regenerant, and neutralization should be carried out after secondary backflushing (or leaching). For neutralization, 1% sulfuric acid solution can be used to adjust the pH value of the influent water to about 3. The flow rate of the influent water is the same as the normal defluorination process. The neutralization time is 1 to 2 hours until the pH value of the effluent water drops to 8 to 9 hours.

 8. Raw water can be used for the first backflushing, the second backflushing, the rinsing and the preparation of the regeneration solution.

9. The effluent from the first backflush, the second backflush, the rinsing and the neutralization is strictly prohibited for drinking and must be discarded.

 Filter:

1. The filter tank can be open type or pressure type, generally a round tank.

2. Concentrated acid should be added after dilution, and should be injected into the center of the original water pipe. The addition of carbon dioxide gas should be done through a microporous diffuser.

3. The structural material of the filter should meet the following conditions:

(1) Meet the sanitary requirements for drinking water quality;

(2) Adapt to the ambient temperature;

(3) Adapt to pH2～13;

(4) Easy maintenance and replacement of accessories.

4. When using the filter head water distribution method, a layer of quartz sand with a thickness of 50-150mm and a particle size of 2-4mm should be laid under the adsorption layer as a supporting layer.

5. When calculating the height of the filter tank, the height from the surface of the filter layer to the top of the tank should be 1.5-2.0m.

6. The diameter of the backwash inlet and outlet pipes must be selected according to the strength of the first backwash, and the open filter backwash outlet pipes may not be installed with valves.

7. The filter should be equipped with the following accessories:

(1) Inlet and outlet water sampling pipes;

 (2) Influent water flow indicator;

 (3) Sight glass for observing the filter layer.

Defluorination station:

1. Pretreatment can be carried out if necessary before defluorination treatment, and the disinfection process should be placed after the defluorination treatment process. The defluorination station should be equipped with a waste liquid treatment device. Regenerated activated alumina [waste liquid, secondary] backflushing wastewater, rinsing wastewater and neutralizing wastewater must be treated before discharge.

 2. The defluorination process can be measured by continuous operation. When there are regulating structures in the station, it can be measured by the highest daily average hourly water supply; when there are no regulating structures, it shall be designed according to the highest daily water supply.

3. The filter should be built indoors, and its layout should leave enough space to ensure the convenient operation of valves and instruments.

4. The operation of multiple filters can be determined in series or in parallel according to the actual situation.

5. The operation cycles of multiple filters should be staggered from each other, and the treated water can be mixed in pipelines or in storage tanks.

6. When setting the storage tank, its minimum volume can be calculated according to 50% of the maximum daily water consumption.

7. Emergency shower and eyewash equipment must be provided for operators near areas exposed to acid, and operators must wear protective clothing when working. Chemicals that neutralize acids and bases (such as sodium bicarbonate and boric acid solutions) must be prepared to handle spills, and containment containers must be available in areas where spills may occur.

8. A laboratory bench should be set up in the defluorination station to mainly detect fluoride and pH value.

9. The pipes of the defluorination station can generally be composed as follows:

(1) Raw water inlet pipe;

(2) Treated water outlet pipe;

 (3) Waste water discharge pipe;

(4) Sampling tube. The material of acid and alkali solution pipes should be plastic (such as polyvinyl chloride) or stainless steel.

The wastewater can be treated by chemical precipitation or evaporation, and the concentrated wastewater or sediment can be landfilled or fluoride recovered.