What kinds of raw water pretreatment processes do you know?

Algae removal process route

1. Air flotation

The air flotation method for algae removal is to supply dissolved air water by the dissolved air system, adhere the algae in the water through the bubbles in the water, make them float on the pool surface, and scrape them with a slag scraper. Air flotation is a physical method to float and separate algae from water. It is very suitable for algae removal. The effect of algae removal is good without adverse effects. The sensory indicators have also been greatly improved. The air flotation process needs to increase the return pressure water and dissolved air system, which increases the investment and operation costs, and the difficulty of operation and management. The air flotation process has good algae removal effect, but it is not complete. If the algae reaches more than 100million, it must be combined with other processes to ensure the effluent quality.

2. Chemical algae killing

Chemical algae killing is to use disinfectant to kill algae in water, and remove them by subsequent sedimentation and filtration process. This method is simple and easy to operate. However, if disinfectants such as liquid chlorine are added in large quantities, they will also produce three toxic substances. At the same time, there is also a risk that algal toxins will exceed the standard. Ozone pre oxidation has a good auxiliary effect on algae removal. In Waterworks with ozone activated carbon advanced treatment process, pre ozone is often used to replace the original liquid chlorine for algae removal. Ozone kills algae by dissolving and splitting algae cells. The algae toxins released after the destruction of algae bodies can also be partially decomposed and removed by ozone. At the same time, ozone has a certain effect on removing the odor generated by algae secretions.

Organic matter removal process route

1. Pretreatment

The pretreatment process is generally used as an auxiliary measure for other processes. In the early stage, the substances that exceed the standard more and have higher indicators are reduced or their properties are changed to facilitate the removal of subsequent processes.

Pretreatment technology mainly includes biological pretreatment and strong oxidation treatment technology.

Application of biological pretreatment technology

Biological pretreatment is to remove ammonia nitrogen and some organic matter through biological action. The biological pretreatment technology of micro polluted water source has been studied and applied for more than 30 years at home and abroad, and has been widely recognized by people. As the pretreatment of slightly polluted water source, the main advantages of biological treatment are: significant removal of NH3-N, NO2-N and AOC, and certain removal of organic matter, color, smell, TOC and turbidity. The disadvantage is that it occupies a large area, and the treatment effect is greatly affected by the water quality and temperature of the water source.

Application of pre oxidation technology

Pre chlorination, pre ozone technology, permanganate pre oxidation technology and chlorine dioxide pre oxidation technology are mainly adopted.

Prechlorination

Prechlorination has been widely used in China to remove algae and degrade organic matter at a low cost. However, the disinfection by-products generated by chlorine and organic matter in water are very harmful to human body, and the use of prechlorination in slightly polluted water sources should be gradually eliminated.

Preozone

Pre ozone technology is mainly used to eliminate iron and manganese in groundwater, remove color and smell, degrade macromolecular organic matter in water, and improve flocculation and clarification. In the application of pre ozone project, its main purpose is to aid coagulation. If necessary, it is considered to strengthen the removal of algae, color and organic pollutants. The ozone dosage is generally 0.2 ~ 2.0mg/l.

Some studies have shown that the effect of pre ozone to control disinfection by-products is also relatively stable. When the pre ozone dosage is about 1.0mg/l (0.23mgo3/mgdoc), the removal rate of trihalomethane precursor is about 23%, and the algae removal rate is as high as 47% in the high algae period. In the process of ozone pre oxidation, ozone does not control the precursor of disinfection by-product by reducing the content of organic matter in water, but mainly oxidizes hydrophobic organic matter with large molecular weight. Most of these organic compounds have aromatic structures or unsaturated double bonds, which are easy to be attacked and become smaller, and turn into hydrophilic substances. Ozone pretreatment can control the amount of disinfection by-products by changing the physical and chemical properties of organics in water and reducing the chlorination activity of organics in water. However, it should be noted that when the raw water contains a high concentration of bromine ions or ozone is added in excess, ozone pre oxidation will convert bromine ions into bromate ions and increase the concentrations of bromotrihalomethane and bromoacetic acid in the water.

The pre ozone process occupies less land, and the process effect is not affected by season, temperature and other factors, and the effect is stable. However, ozone needs to be prepared on site, and the operation cost is high.

Permanganate preoxidation

Potassium permanganate is a strong oxidant, which can selectively interact with organic pollution in water and destroy unsaturated functional groups of organic matter. It has been used to remove odor and color in water since the 1960s, with good results. In recent years, the permanganate composite agent has been developed, which has significant effects on surface water, such as oxidation and coagulation aid, algae removal, odor removal, removal of trace organic pollutants, and can also reduce the formation potential of trihalomethanes. The intermediate and new ecological components produced by permanganate composite agent in the oxidation process can enhance the removal of trace organic pollutants in water. In addition, the new ecological manganese dioxide can adsorb a variety of trace organic and inorganic pollutants in water, which can improve the removal effect of a variety of organic pollutants and heavy metals in water.

Chlorine dioxide preoxidation

The application of chlorine dioxide pre oxidation is still relatively few, but chlorine dioxide pre oxidation has a good removal effect on aromatic hydrocarbons, which can control the formation of trihalomethanes (THMs), reduce the generation of total organic halogens, and have a good decolorization effect on colored substances in water. Using chlorine dioxide pre oxidation, the organic by-products formed are less and the toxicity is less. The inorganic by-products mainly include chlorite and chlorate. It has been reported that the adverse effects of chlorite and chlorite mainly lie in its strong oxidation and toxic effect on the human nervous system, and long-term drinking can lead to anemia. At present, the research in this field needs to be further deepened.

Chlorine dioxide also needs to be prepared on site, and according to different preparation methods, the reaction conditions need to be strictly controlled to prevent explosion. When chlorine dioxide is used for pre oxidation to remove organic matter, iron and manganese, its dosage is 1 ~ 1.5mg/l, and the specific dosage needs to be determined according to the water quality. The dosing concentration must be controlled below the explosion-proof concentration, and safety explosion-proof measures must be set. Inert materials shall be used where chlorine dioxide contacts; A separate room shall be set for each kind of medicament, and measures shall be taken to remove and contain the leftover or leaking medicament.

Through the above comparison, it can be seen that the selection purposes of pre ozone process and biological pretreatment process are different: pre ozone mainly degrades organic matter and removes color and smell through strong oxidation; Biological pretreatment is to remove ammonia nitrogen and some organic matter through biological action. In addition, the project is located in the Yellow River Estuary, where the temperature is low in winter. The biological pretreatment method is adopted, which has poor effect on the treatment of organic matter, covers a large area, needs to keep the tank warm, and the cost is high. Without thermal insulation, it is difficult to discharge sludge from the biological reaction tank, which will affect the operation. Considering that the raw water quality will be improved after strengthening pollution control, biological pretreatment will not be considered for the time being in this scheme, and the pre chlorination method will undoubtedly generate a large number of chlorination disinfection by-products, which will endanger the safety of drinking water. Permanganate and ozone pre oxidation are relatively economical and have good removal effect on heavy metals, which can be considered together with the later advanced treatment process