After the organic matter enters the water body, it will be oxidized and decomposed under the action of microorganisms, so that the dissolved oxygen in the water gradually decreases. When the oxidation is too fast, and the water body can not absorb enough oxygen from the atmosphere to supplement the consumed oxygen in time, the dissolved oxygen in the water may drop very low (such as less than 3~4mg/L), which will affect the aquatic organisms Need for normal growth. When the dissolved oxygen in the water is exhausted, the organic matter begins to be digested anaerobicly and odor occurs, which affects the environmental hygiene.

Because the organic matter contained in sewage is often an extremely complex mixture of multiple components, it is difficult to determine the quantitative values ​​of various components one by one. In fact, some comprehensive indicators are commonly used to indirectly characterize the content of organic matter in water. There are two types of comprehensive indicators that represent the content of organic matter in water. One is an indicator expressed in terms of oxygen demand (O2) equivalent to the amount of organic matter in water, such as BOD, COD, and TOD. ; The other is an indicator expressed in carbon (C), such as total organic carbon TOC. For the same kind of sewage, the numerical values ​​of these indicators are generally different, and the order of the numerical values ​​is TOD>CODCr>BOD5>TOC

Excessive biochemical oxygen demand

Biochemical oxygen demand is called biochemical oxygen demand, English is Biochemical Oxygen Demand, abbreviated as BOD, it means that at a temperature of 20 ℃ and aerobic conditions, due to aerobic microorganisms in the process of biochemical oxidation of organic matter consumed in the process The amount of dissolved oxygen, which is the amount of oxygen required to stabilize biodegradable organic matter in water, is in mg/L. BOD includes not only the amount of oxygen consumed by the growth and reproduction of aerobic microorganisms in water or respiration, but also the amount of oxygen consumed by reducing inorganic substances such as sulfides and ferrous iron, but the proportion of this part is usually very small. Therefore, the larger the BOD value, the more the organic content in water.

When soluble organic matter is consumed by bacteria, it is converted into carbon dioxide and biological flocs, and then settles from the effluent. To reduce the organic content of the effluent and improve the BOD level, the mentioned process is a popular method of controlling BOD, which is achieved by promoting the correct balance of "food" and organic matter. This can be achieved by an appropriate aeration method where air is introduced into the effluent to increase the rate of this biological oxidation, which in turn increases the level of settleable solids, which can then be removed from the effluent by the following method. Filter or clarify.

Excessive total suspended and dissolved solids

Depending on the level of TSS and TDS in the wastewater and the level of the discharge standard, the implementation method will be different. Commonly used treatment methods to reduce TSS: coagulation, flocculation, sedimentation, sand or carbon filtration.

The reduction of TDS is a more complicated process. If the contaminants are metal based, such as calcium, magnesium or iron, simple chemical additives in the clarification process can be added to reduce these contaminants. In the case of sodium, chlorine or other highly soluble ions, a desalination process or evaporation process may be required.

More nitrate and phosphate

When a large amount of nitrate or phosphate is not removed from the wastewater, and these nutrients are discharged into the local environment, they will increase BOD and cause extensive weed growth, algae and phytoplankton, deoxidize the water body, and kill the organism And may cause hypoxia. They can enter the wastewater stream in various ways, including humans and food waste, detergents and pesticides.

If the wastewater treatment system does not meet the nitrate and phosphate emission levels, the following methods may be useful:

Nitrate removal: Nitrate can be removed by a variety of methods, including ion exchange, reverse osmosis or conventional biological treatment and denitrification.

Phosphate removal: An effective method for removing phosphate from wastewater streams is usually coagulation/chemical precipitation, depending on the type of phosphate present.

Wastewater contains oil and grease

Oils and greases are "hydrophobic" and they tend to repel from water and adhere to surfaces without water. A large amount of grease in the waste water can block sewers and drains, damage human health and kill aquatic organisms. Oils and fats are usually introduced into wastewater as a by-product of food production. These pollutants are strictly controlled and must not be discharged together with sewage.

Wastewater containing oil needs to be treated with an air floatation machine (DAF). The device dissolves air in water by pressure to remove oil. When air bubbles float to the surface, they adhere to oil and grease, so they can be skimmed from the top of the surface. Another method of successful oil and grease removal includes certain types of filters, such as ultrafiltration or activated carbon.

Source: New Horizons in Water Treatment