Sources and treatment processes of common pollutants in wastewater treatment!

Wastewater contains numerous pollutants from diverse sources. This article introduces the sources and treatment methods of several common pollutants. What are the sources of readily biodegradable oxygen-demanding organic matter? What are the treatment methods?

　　Wastewater contains numerous pollutants from various sources. This article will introduce several common pollutant sources and treatment methods.

　　1. What are the sources of readily biodegradable oxygen-demanding organic matter? What are the treatment methods?

　　Oxygen-demanding organic matter (readily biodegradable) in wastewater mainly includes humic acid, proteins, esters, sugars, amino acids, etc. These organic compounds exist in wastewater in suspended or dissolved states and can be decomposed into simple inorganic substances such as CO2 under the action of microorganisms. The decomposition of these organic substances in natural water bodies consumes dissolved oxygen in the water, hence the name oxygen-demanding organic matter.

　　Wastewater containing these substances, once entering water bodies, will cause a decrease in dissolved oxygen content, leading to blackening and foul odor. Domestic sewage and industrial wastewater discharged from food, papermaking, petrochemical, chemical fiber, pharmaceutical, dyeing, and other industries all contain a large amount of oxygen-demanding organic matter. Statistics show that the oxygen-demanding organic matter discharged by China's papermaking industry accounts for about 1/4 of the total oxygen-demanding organic matter discharged from industrial wastewater. Although the concentration of organic matter in urban wastewater is not high, the total amount of oxygen-demanding organic matter discharged from urban wastewater is also large due to the large volume of water. The key issue to be addressed in secondary biological treatment of wastewater is to remove most of these substances from the wastewater. The composition of oxygen-demanding organic matter is complex, and it is quite difficult to separately determine the concentration of various organic matters. In practice, indicators such as CODCr, BOD5, TOC, and TOD are commonly used to represent them. Generally speaking, the higher the values of the above indicators, the more dissolved oxygen is consumed in the water, and the worse the water quality. When BOD5 in natural water bodies is below 3 mg/L, the water quality is good; when it reaches 7.5 mg/L, the water quality is already poor; when it exceeds 10 mg/L, it indicates that the water quality is very poor, and the dissolved oxygen is close to zero.

　　Readily degradable organic matter can be removed using biochemical methods, such as plug-flow activated sludge process (e.g., aeration tank), sequencing batch reactor (SBR, CASS process), biofilm, or MBR.

　　2. What are the recalcitrant organic pollutants? What are the treatment methods?

　　Biodegradable organic matter refers to organic compounds that cannot be degraded by untrained activated sludge but can be degraded to some extent after a period of acclimatization. Some toxic macromolecular organic matter in wastewater, such as organochlorides, organophosphorus pesticides, organometallic compounds, and polycyclic and other long-chain organic compounds represented by aromatics, all belong to organic matter that is difficult to be degraded by microorganisms. Some organic compounds cannot be degraded by microorganisms at all and can be called inert organic matter.

　　Therefore, wastewater containing this type of organic matter should be treated separately by cultivating special microorganisms or using anaerobic processes to convert part of CODCr into BOD5, improve biodegradability, and then mix it with other wastewater for secondary biological treatment.

　　3. What are the sources of organic nitrogen and ammonia nitrogen in wastewater? What are the treatment methods?

　　Organic nitrogen mainly exists in the form of protein, as well as urea, muramic acid, fatty amines, uric acid, and other compounds containing and not containing amino groups. Some organic nitrogen, such as glue, chitin, and quaternary ammonium compounds, are difficult to biodegrade. Industries that produce these organic nitrogen or use them as raw materials will discharge wastewater containing these organic nitrogen. Industries such as steel, petroleum refining, chemical fertilizers, inorganic chemicals, ferroalloys, glass manufacturing, meat processing, and feed production discharge industrial wastewater containing ammonia nitrogen. The initial ammonia nitrogen content in fresh wastewater such as leather and animal excreta is not high, but due to the deamination reaction of nitrogen in the wastewater, the concentration of ammonia nitrogen will increase rapidly after a period of storage or residence in drainage pipes.

　　Biological methods can be used to treat organic nitrogen industrial wastewater. While microorganisms remove organic carbon, advanced oxidation converts nitrogen in wastewater into ammonia nitrogen through biological assimilation and biomineralization. Treatment methods for ammonia nitrogen wastewater include stripping, air stripping, ion exchange, activated carbon adsorption, biological nitrification, and denitrification.