Common Wastewater Treatment Methods

Wastewater treatment has never been so crucial since it more and more comes from many sources today. So many factories, especially those in chemical, printing and dyeing, electroplating industries, produce a great deal of wastewater. In agriculture, harmful ingredients of pesticides flow from farms into rivers that used to supply fresh water for people’s life. Even in everyday life, domestic wastewater is produced every day. The harmful ingredients in various cosmetics products, washing products can easily enter the sewer, and may be further discharged into rivers or groundwater. Some of the harmful components of industrial and domestic waste also enter the groundwater with the leachate. This wastewater will pollute both surface water and groundwater.

Tertiary Wastewater Treatment

The whole process of wastewater treatment can be divided into three stages, called primary, secondary and tertiary stages. The following is a brief introduction of this tertiary treatment of wastewater.

Primary Treatment

The primary treatment of wastewater is mainly carried out with physical methods. Stone, sand, fat, grease and other components contained in wastewater can be removed by means of grating, precipitation or air filtration. The primary treatment is mainly intended to reduce the load of subsequent biological treatment and, when possible, to achieve a separation of solid and liquid contaminants.

Secondary Treatment

The secondary treatment of wastewater mainly refers to the use of various biological methods. Through biological action, especially microbial action, suspended solids and harmful biodegradable pollutants in wastewater can be effectively removed. Most organic matter will be broken down during the process. Organic pollutants will be converted into harmless gaseous products such as carbon dioxide, water, and some solid products called microbial communities or biological sludge.

Tertiary treatment

The tertiary treatment of wastewater belongs to advanced treatment. It includes the removal of nutrients from wastewater and the denitrification, dephosphorization and disinfection of wastewater by the methods of chlorine, ozone or ultraviolet light. Activated carbon adsorption or reverse osmosis can also be used to remove the remaining pollutants in the wastewater. And later, ozone or chlorine can be used to achieve the elimination of bacteria and viruses.

Common Wastewater Treatment Methods

There are multiple methods for wastewater treatment. Some of them are physical methods, while some others are chemical or biological. Different treatment methods exist since the quality of wastewater varies greatly between one type and another. And no single solution can solve all the existed wastewater problems. The following is a brief introduction of various wastewater treatment methods:

1. Sediment Filtration

Sediment filtration is a physical wastewater treatment method, which can be effectively used to filter sand, dirt, silt, scale and other components that cause the turbidity in well water and natural water sources. The tools used are mainly cylindrical filters made of paper, rope, polypropylene and cellulose. This method can effectively reduce the chroma, turbidity and odor of water, and remove heavy metals, pesticide residues and other organic matters. It can be used to filter all kinds of wastewater in both industries and daily life. The drawback of this method is that some bacteria and contaminants can sometimes pass through the filter and not be filtered. And since the filters that have been used contain a great deal of harmful microorganisms, it needs to be cleaned at a regular basis. If discarded, they need to be disposed of properly to avoid causing further contamination to the environment.

2. Microbial Treatment

Just as its name implies, microbial treatment is a biological treatment method. It can be used to treat harmful pollutant components that are difficult to treat by physical and chemical methods. Microbial life activities can be used to degrade dissolved or colloidal organic matters in wastewater, so as to achieve the purification of wastewater. The advantage of microbial treatment is that it is economic. It has a great removing effect for ammonia nitrogen and other components in water. The drawback of this method is that the effect of microbial treatment will be greatly affected by temperature conditions. Therefore, it will be less effective in cold winter.

3. REDOX

REDOX refers to the treatment of pollutants in wastewater by oxidation or reduction. Pollutants are transformed into harmless substances by oxidation or reduction, thus achieving the purpose of wastewater treatment. Commonly used oxidants are ozone, chlorine, sodium hypochlorite, etc. And commonly used reductants are iron filings, ferrous sulfate, sodium bisulfite, sodium borohydride, etc. These chemical methods will be more effective in treating stubborn contaminants that are difficult to remove by using conventional methods. It also has a great effect on the treatment of wastewater containing phenol and sulfur. The advantages of REDOX method also lie in its fast reaction speed and relatively simple process. At the same time, it is also capable of increasing the content of dissolved oxygen and reduce the COD and BOD of the wastewater. However, due to the need to use a large number of oxidation agents, the processing cost of this method is high. And the technical requirements for operation are also high.

4. Ultraviolet Sterilization

Ultraviolet sterilization is a water treatment method that uses short-wave ultraviolet light to kill bacteria, viruses and various microorganisms in wastewater. Ultraviolet light can damage the DNA of the harmful microbes, preventing them from reproducing properly. Wastewater is usually purified by going through a 360-degree stainless steel chamber or by placing ultraviolet lamps into open channels. Ultraviolet light can kill up to 99.99% of harmful microorganisms in wastewater. In addition to common bacteria and viruses, cryptosporidium and giardia, which are difficult or even impossible to be removed by chlorination, can also be effectively killed and removed. The advantage of ultraviolet water treatment is that it is a non-chemical process that it will not add any chemicals to the water. It also produces no by-products, and thus will not change the taste and smell of the water as chlorination does. Compared with other treatment methods, ultraviolet water treatment stands out for its low energy consumption and low maintenance cost. The disadvantage of ultraviolet water treatment is that it cannot completely eliminate chlorine residue or heavy metals. So, it cannot purify wastewater to be safely drinkable. To realize the sterilization requirements of drinking water, it needs to be used with a reverse osmosis system.

5. Adsorption by Activated Carbon

Activated carbon is a commonly used adsorbent used in wastewater treatment. Activated carbon has a good removing effect on phenolic compounds, benzene compounds, petroleum and other dissolved organic matter in wastewater. Since the 1960s, activated carbon has been largely adopted in the treatment of oil refining wastewater, explosive wastewater, printing and dyeing wastewater, chemical wastewater and electroplating wastewater. In a regular wastewater treatment processes, activated carbon adsorption is usually placed after biochemical treatment to treat pollutants that are not easily decomposed. The presence of organic matter in the water can be further reduced with this treatment. The wastewater treated by activated carbon can meet the requirements of discharge standards. And the treated water can be reused in the production process again. Activated carbon is well received for wastewater treatment. But in terms of its material cost, activated carbon has its disadvantages of high price, small adsorption capacity, and the difficulty in the recycling and reuse of activated carbon. The stacking of waste activated carbon after use will bring environmental problems. It will bring an increase in operating costs if incineration is used for its disposal.

6. Ion Exchange

Water treatment adopting ion exchange method is a reversible chemical reaction between ions in the liquid and solid phases. Common ion exchange methods include hard water softening and deionization. When it is applied, ion exchange beads are used to exchange charged ions with minerals and metals in the water. By doing this, the contaminated ions in wastewater can be purified, separated, and decontaminated. Ion exchange method can remove calcium and magnesium ions, which cause water to harden, and replace them with sodium or potassium ions. The advantages of ion exchange water treatment are its simple structure, high removal efficiency and easy operation and control. However, it has a high requirement for wastewater pretreatment. The regeneration of ion exchanger and the treatment of regeneration solution are also difficult problems to handle.

Conclusion

Through the above introduction and comparison, we can see that, based on different treatment principles and methods, different types of water treatment methods have their own advantages and disadvantages. Sediment filtration has the problems with its thoroughness. The implementation of microbial treatment will sometimes be influenced by environmental conditions. The economic cost and operational requirements for REDOX and the use of activated carbon will be high due to the use of large amounts of chemicals and adsorption materials. The problem of ion exchange method is reflected in its high requirement for the pretreatment of the wastewater and the difficulty in the treatment of its by-products.

Through the comprehensive comparison of the above methods, although ultraviolet sterilization has certain drawbacks, but relatively speaking, it has the highest feasibility. Ultraviolet sterilization is a non-chemical treatment method. It neither uses nor produces any harmful chemicals during the treatment process. So, it will not change the taste and smell of the water, which is highly workable in beverage and drinking water production industries. Ultraviolet sterilization has a super high killing rate (99.99%) against harmful microorganisms and a second-level elimination speed. Its benefits also lie in its low power consumption, easy operation and maintenance of the equipment. For more introduction of ultraviolet water treatment, please go to out Blog: Advantages and Disadvantages of UV Water Treatment. Ultraviolet Water treatment can also be affected by some influencing factors such as UV dose and water quality, but such influences can easily be overcome by pretreatment of the wastewater or adjustment of the equipment.