Basic Knowledge of Water Treatment
Release time:
2024-09-10
1What is COD (Chemical Oxygen Demand)?
Chemical Oxygen Demand (COD) refers to the amount of oxygen required to oxidize the substances in wastewater when they are oxidized by chemical oxidants, measured in milligrams per liter of oxygen. It is currently one of the most commonly used methods to determine the organic content in wastewater. Common oxidants used in COD analysis include potassium permanganate (Mn method CODMn) and potassium dichromate (Cr method CODCr). Currently, the potassium dichromate method is commonly used. Under strong acid heating and boiling reflux conditions, wastewater is oxidized to organic matter, and using silver sulfate as a catalyst can increase the oxidation rate of most organic matter to 85-95%. If the wastewater contains a high concentration of chloride ions, mercury sulfate should be used to shield the chloride ions to reduce interference with COD measurement.
- What is BOD5 (Biochemical Oxygen Demand)?
Biochemical Oxygen Demand can also characterize the degree of organic pollution in wastewater, with the most commonly used being the 5-day Biochemical Oxygen Demand, represented as BOD5, which indicates the amount of oxygen required for biochemical degradation of wastewater in the presence of microorganisms over 5 days. In the future, we will frequently use the 5-day Biochemical Oxygen Demand.
3What is the relationship between COD and BOD5?
Some organic substances can be biologically oxidized and degraded (such as glucose and ethanol), some can only be partially biologically oxidized and degraded (such as methanol), while others cannot be biologically oxidized and degraded and are also toxic (such as ginkgolide, ginkgolic acid, and certain surfactants). Therefore, we can divide the organic matter in water into two parts: biodegradable organic matter and non-biodegradable organic matter. It is generally believed that COD can basically represent all organic matter in water, while BOD represents the biodegradable organic matter in water. Therefore, the difference between COD and BOD can indicate the portion of organic matter in wastewater that is biologically non-degradable.
4What is the pretreatment of wastewater? What are the purposes of pretreatment?
The treatment before biochemical treatment is generally referred to as pretreatment. Since biochemical treatment is relatively low-cost and stable in operation, most industrial wastewater is treated using biochemical methods, and wastewater treatment mainly uses biochemical methods as the primary treatment means. However, some organic substances in wastewater can inhibit or are toxic to microorganisms, so necessary pretreatment must be conducted before wastewater enters the biochemical tank, aiming to reduce or remove substances that inhibit or are toxic to microorganisms as much as possible, ensuring that the microorganisms in the biochemical tank can operate normally. The purposes of pretreatment are twofold: first, to reduce and remove or convert substances that inhibit or are toxic to microorganisms into harmless or beneficial substances for microorganisms, ensuring the normal operation of microorganisms in the biochemical tank; second, to reduce the COD load during the pretreatment process to alleviate the operational burden on the biochemical tank.
5What is the function of the wastewater collection tank?
The function of the wastewater collection tank is to collect, store, and balance the quality and quantity of wastewater. The wastewater discharged from various workshops is generally unbalanced in terms of quantity and quality; there is wastewater during production and none during non-production, and there can even be significant variations within a day or between shifts, especially in the fine chemical industry. If the clear and turbid wastewater is not separated, the quality and quantity of concentrated and lightly polluted wastewater can vary greatly, which is very detrimental to the normal operation and treatment effect of wastewater treatment facilities and equipment, and can even be harmful. Therefore, before wastewater enters the main sewage treatment system, a wastewater collection tank with a certain volume must be set up to store the wastewater and homogenize it, ensuring the normal operation of wastewater treatment equipment and facilities.
6Why are colloidal particles in wastewater not easily settled naturally?
Many impurities and suspended solids in wastewater that have a specific gravity greater than 1, large particles, and easily settled suspended solids can be removed by natural sedimentation, centrifugation, and other methods. However, suspended particles with a specific gravity less than 1, tiny particles, and those invisible to the naked eye are very difficult to settle naturally. For example, colloidal particles are micro-particles sized 10-4 to 10-6 mm, which are very stable in water, and their settling speed is extremely slow, taking 200 years to settle 1 meter. The reasons for the slow settling are twofold: first, colloidal particles carry a negative charge, which prevents contact between colloidal micro-particles due to like charges repelling each other, preventing them from adhering to each other and remaining suspended in water. Second, there is a layer of molecules tightly surrounding the surface of colloidal particles, and this hydration layer also hinders and isolates contact between colloidal micro-particles, preventing them from adhering to each other and remaining suspended in water.
7How to make colloidal particles settle?
To make colloidal particles settle, it is necessary to promote contact between colloidal particles, causing them to aggregate into larger particles, i.e., to coagulate, making their specific gravity greater than 1 so that they can settle. There are many methods to achieve this, and commonly used engineering techniques include coagulation, flocculation, and sedimentation.
8What is dissolved oxygen? What is the relationship between dissolved oxygen and microorganisms?
Oxygen dissolved in water is referred to as dissolved oxygen. The organisms in the water and aerobic microorganisms rely on dissolved oxygen for survival. Different microorganisms have different requirements for dissolved oxygen. Aerobic microorganisms require sufficient dissolved oxygen, generally maintaining dissolved oxygen at around 3 mg/L, and it should not be lower than 2 mg/L; facultative microorganisms require dissolved oxygen in the range of 0.2-2.0 mg/L; while anaerobic microorganisms require dissolved oxygen below 0.2 mg/L.
9What is activated sludge?
From a microbiological perspective, the sludge in the biochemical pool is a community composed of a variety of biologically active microorganisms. If the sludge particles are observed under a microscope, various microorganisms can be seen inside—bacteria, fungi, protozoa, and metazoans (such as rotifers, insect larvae, and worms), which form a food chain. Bacteria and fungi can decompose complex organic compounds to obtain the energy necessary for their activities and to construct themselves. Protozoa feed on bacteria and fungi and are consumed by metazoans, which can also rely directly on bacteria for sustenance. This flocculent sludge, rich in microorganisms and capable of degrading organic matter, is called activated sludge. In addition to microorganisms, activated sludge also contains some inorganic substances and organic matter that is adsorbed onto the activated sludge and cannot be biodegraded (i.e., metabolic residues of microorganisms). The moisture content of activated sludge is generally between 98-99%. Activated sludge, like alum flowers, has a large surface area, thus possessing strong adsorption capacity and the ability to oxidize and decompose organic matter.
10What does dissolved oxygen (DO) represent?
Dissolved oxygen (DO) represents the amount of oxygen dissolved in water, measured in mg/L. Different biochemical treatment methods have different requirements for dissolved oxygen. In aerobic biochemical processes, the dissolved oxygen in water is generally between 0.2-2.0 mg/L, while in SBR aerobic biochemical processes, the dissolved oxygen in water is generally between 2.0-8.0 mg/L. Therefore, during the operation of the aerobic pool, the aeration volume should be small, and the aeration time should be short; whereas in the SBR aerobic pool operation, the aeration volume and aeration time should be much larger and longer, and we use contact oxidation, controlling the dissolved oxygen at 2.0-4.0 mg/L.
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