Strategy of Nitrogen and Phosphorus Removal System

(Summary description)The current national and local sewage discharge standards for urban sewage treatment plants and water quality standards for reclaimed water use have strict requirements on the nitrogen and phosphorus content of the effluent from sewage treatment plants. However, at present, due to groundwater infiltration or mixed connection of rain and sewage, the concentration of organic matter in the influent of many sewage plants is low. Therefore, it may be necessary to add chemical substances to ensure the normal operation of the denitrification and phosphorus removal system. For example, effective denitrification requires readily biodegradable carbon sources, biological phosphorus removal requires short-chain volatile fatty acids, and in some areas with softer natural water quality, it is necessary to supplement alkalinity to maintain the pH conditions required for the entire aeration tank nitrification process ;Also, if using chemical except

Strategy of Nitrogen and Phosphorus Removal System

(Summary description)The current national and local sewage discharge standards for urban sewage treatment plants and water quality standards for reclaimed water use have strict requirements on the nitrogen and phosphorus content of the effluent from sewage treatment plants. However, at present, due to groundwater infiltration or mixed connection of rain and sewage, the concentration of organic matter in the influent of many sewage plants is low. Therefore, it may be necessary to add chemical substances to ensure the normal operation of the denitrification and phosphorus removal system. For example, effective denitrification requires readily biodegradable carbon sources, biological phosphorus removal requires short-chain volatile fatty acids, and in some areas with softer natural water quality, it is necessary to supplement alkalinity to maintain the pH conditions required for the entire aeration tank nitrification process ;Also, if using chemical except

Information

The current national and local sewage discharge standards for urban sewage treatment plants and water quality standards for reclaimed water use have strict requirements on the nitrogen and phosphorus content of the effluent from sewage treatment plants. However, at present, due to groundwater infiltration or mixed connection of rain and sewage, the concentration of organic matter in the influent of many sewage plants is low, so chemical substances may be added to ensure the normal operation of the denitrification and phosphorus removal system.

For example, effective denitrification requires easily biodegradable carbon sources, biological phosphorus removal requires short-chain volatile fatty acids, and in some areas with softer natural water quality, it is necessary to supplement alkalinity to maintain the pH conditions required for the entire aeration tank nitrification process ; In addition, if chemical phosphorus removal is used, either as a supplement to the biological phosphorus removal process or as the main phosphorus removal means, the addition of metal salts and polymers is required. This article discusses the basic principles, dosage calculations and operational requirements of various pharmaceutical dosing methods.

01 Addition of carbon source for denitrification

Biological nitrogen removal requires two processes, nitrification and denitrification. Ammonia nitrogen in wastewater must first be nitrified or converted into nitrite and nitrate, and then during denitrification, nitrate will be reduced to nitrogen gas as an oxygen donor for the oxidation of simple carbon compounds during cellular respiration. Therefore, the denitrification process with the goal of removing nitrate must have a readily biodegradable carbon source. Its sources include dissolved BOD in influent water, decayed matter of cells during endogenous denitrification, and backflow of various supernatants. When the influent dissolved organic matter is insufficient and the denitrification requirement is very high, it is necessary to supplement the chemical substances to provide the carbon source required for the denitrification process.

The artificial carbon sources used in denitrification include pure chemicals such as methanol, ethanol, denatured ethanol, acetic acid and sodium acetate, or waste sugar, molasses and waste acetic acid solutions in the industrial production process. Among them, methanol is the most common and proved to be the most suitable carbon source.

For the conventional biological denitrification process, methanol should be directly added in the anoxic section, and fully mixed with the influent water and the mixed liquid through the agitator in the anoxic section. It is necessary to prevent the methanol from volatilizing from the liquid phase to the Air should also prevent part of methanol from being consumed by bacteria aerobic respiration due to the presence of excess oxygen. If the sewage plant adopts the four-stage or five-stage activated sludge process, adding carbon source in the subsequent anoxic stage (second anoxic stage) can obtain a higher denitrification rate than endogenous respiration, which can further remove nitrate; for The three-stage denitrification system, such as denitrification filter, denitrification aerobic biological filter, etc., supplementary carbon source is very important for the operation of the system. Because the denitrification process is downstream of the main aeration process and all the dissolved BOD in the influent has been removed, methanol is usually added to the denitrification influent.

The dosage of methanol is affected by nitrate (NO3-N), nitrite (NO2-N) and dissolved oxygen. The required amount of methanol can be calculated by formula (1).

Methanol requirement:

2.47NO3-N+1.53NO2-N+0.87DO (1)

In actual operation, 3 mg/L of methanol is usually added for every 1 mg/L of nitrate removed by denitrification, and then adjusted according to the actual load and operating conditions of the sewage treatment plant. The correct control of methanol dosage is very important to the operation of the three-stage denitrification system. Excessive dosing not only wastes chemicals but also increases the concentration of BOD in the effluent of the denitrification system. This is not a big problem for sewage treatment plants that do not require high effluent BOD concentration, but it is a major issue for sewage treatment plants with a BOD limit of about 5 mg/L or lower.

Methanol has a flash point of 12°C and is a highly flammable substance. Methanol storage pools, pipelines and their accessories and electrical systems need to consider appropriate explosion-proof measures. The methanol dosing system should usually be installed outdoors and away from other equipment. Methanol storage tanks should be fitted with floating roofs and pressure relief valves and fire extinguishers.

02 Dosing of volatile fatty acids in biological phosphorus removal

The mechanism of biological phosphorus removal is through the absorption of volatile fatty acids (VFA) in the anaerobic zone and the release of stored phosphorus, while the phosphorus-accumulating bacteria absorb excess phosphorus under aerobic conditions. In order to ensure the reproduction of phosphorus-accumulating bacteria and effective biological phosphorus removal, sufficient volatile fatty acids are required. There may be VFA in the influent of the sewage treatment plant, including the long residence time of the collection system, the raw water with multi-stage lifting pump station and the decomposition of complex organic compounds in the anaerobic section of the biological nitrogen and phosphorus removal system. If the content of naturally occurring VFA is insufficient, it is necessary to add VFA to the anaerobic stage.

For biological phosphorus removal systems, the mixture of acetic acid and propionic acid is the best choice for adding VFA. The most commonly used in practice is an acetic acid solution. Productivity tests have shown that if additional VFA is required (e.g. to increase soluble BOD in the influent, some of which will be converted into usable VFA through the fermentation process in the anaerobic stage), the additional VFA requirement is typically 1 mg of phosphorus removed. 5 to 10 mg of VFA is required. Usually acetic acid is in the form of glacial acetic acid (approximately 100% solution) and 84% and 56% solutions. Although glacial acetic acid is not as volatile as ethanol, it has a relatively low flash point (40 °C) and a freezing point of 17 °C, so it should be considered to prevent combustion according to the specification requirements, and measures must be taken to prevent solidification. Metal materials are required for storage tanks, pipes and accessories. Acetic acid is corrosive and 316-gauge stainless steel is commonly used. If glacial acetic acid is used in warm climates, due to its relatively low flash point, an inert gas blanket or floating roof should be considered. In practical applications, it is recommended to use a low-concentration aqueous acetic acid solution. Of course, the addition of VFA cannot completely remove the TP concentration of the system effluent; if the effluent TP is required to be very low, chemical phosphorus removal is still required, and phosphorus precipitation is removed by adding chemicals.

03 Addition of alkalinity

Alkalinity is a measure of the ability of sewage to neutralize acids. Alkalinity is closely related to pH and is critical for wastewater treatment plants with biological nitrogen and phosphorus removal processes. The consumption of alkalinity in the nitrification process leads to a decrease in the pH of the sewage, and the use of iron or aluminum salts for chemical precipitation to remove phosphorus will also cause a decrease in alkalinity. The decrease in pH leads to a decrease in the nitrification reaction rate, and nitrification stops when the pH is about 6; when the pH is lower than 7, the polysaccharide bacteria will compete with the phosphorus accumulating bacteria, affecting the ability of the phosphorus accumulating bacteria to utilize VFA, thereby affecting the biological phosphorus removal effect. In addition, alkalinity also reflects the buffer capacity of sewage, that is, the ability to cope with pH changes of different influent water quality.

Therefore, in order to ensure the nitrification reaction, some sewage treatment plants need to add alkalinity. There are many chemicals that can be used to replenish alkalinity. The choice of chemicals is influenced by local natural conditions, local chemical prices, and operator preferences.

Chemicals that can be used to replenish alkalinity are sodium hydroxide (NaOH), calcium hydroxide (slaked lime) [Ca(OH)2], and calcium oxide (quick lime) (CaO). The price of sodium hydroxide is higher, but compared with calcium hydroxide, it is more convenient to use and operate, and the annual operating cost of the storage and dosing system is lower; calcium hydroxide is usually sold in the form of solid substances, which must be slurried before use. The lime slurry pool is prone to scaling; calcium oxide needs to be matured, the labor environment during the aging operation is harsh and labor-intensive, and it takes a lot of manpower to maintain the operation of the equipment. When designing the supplementary alkalinity dosing system, 50-100 mg/L (CaCO3 meter) is generally used as the target alkalinity of the effluent. In actual operation, each plant must be evaluated individually to determine what level of effluent alkalinity can ensure a stable effluent pH value.

When determining the dosage, it is necessary to consider the effect of the subsequent process on the pH and alkalinity of the effluent. Usually chlorine will increase the acidity and further reduce the pH value of the effluent; sodium hypochlorite will increase the alkalinity; use iron salt or aluminum salt precipitation to remove phosphorus. Increase alkalinity consumption. Typically for aluminum salts, 5.56 mg of CaCO3 is consumed per mg of aluminum hydroxide produced. For iron salts, 2.69 mg of CaCO3 are consumed per mg of ferric hydroxide produced. Sodium hydroxide is a strong base, and if it is added in excess, the pH will rise significantly. The diluted sodium hydroxide solution must be stored frozen below 0 °C. The freezing point of 50% sodium hydroxide solution is about 12.8 ℃, so its storage tank and pipeline must be heated and insulated. Once the liquid temperature falls below 12.8°C, sodium hydroxide will crystallize and come out of solution. Crystallized sodium hydroxide is difficult to dissolve again. Sodium hydroxide is diluted on site with in-plant water or drinking water, and scaling is prone to occur at the mixing point. Therefore, the pipe interface at the mixing point of the dilution system should be designed to be easy to clean; the dosing point of sodium hydroxide is also prone to scaling. It is recommended to add sodium hydroxide to the return sludge pipe, because the flow rate in the return sludge pipe is relatively high Large, it can protect the pipeline from scaling.

04 Dosing of chemicals in chemical phosphorus removal

The basic principle of chemical phosphorus removal is to convert soluble phosphorus into chemical precipitates, which are removed during the sludge sedimentation process. The chemical substances used for chemical precipitation and phosphorus removal in wastewater include iron salts, aluminum salts and calcium salts, among which iron salts are more commonly used.

The dosage of chemical phosphorus removal agents should be considered in combination with the entire treatment system. The absorption of phosphorus by biological phosphorus removal should be fully utilized to make effective use of chemical agents and minimize sludge production. According to different targets of phosphorus concentration in effluent, chemicals can be added at different dosing points, as shown in Figure 1. If chemical phosphorus removal is carried out in the primary sedimentation tank, the demand for phosphorus by downstream microorganisms also needs to be considered. If the addition of chemicals removes excess phosphorus, the biological system will face nutrient deficiencies.

Iron or ferrous compounds can be added before the primary settling tank and precipitated in the primary settling tank. The phosphorus removal effect of iron salts depends on the length of the reaction time. The complete reaction takes 5 to 10 minutes, so a mixed reaction zone of iron salts and sewage is required to form insoluble precipitates. If there is no condition to set up a mixing reaction zone, the agent needs to be added to the more upstream zone to ensure sufficient residence time. Iron salts can also be added before the secondary sedimentation tank, and iron salt precipitates are formed upstream of the sedimentation tank and separated from the system in the sedimentation tank. Ferrous salt is added before the aeration tank, because the oxidation of ferrous ions to iron ions needs to consume additional oxygen; excessive addition will increase the ion concentration in the effluent, so ferrous ions cannot be added in the secondary sedimentation tank. Once the excess or unreacted ferrous ions are brought into the disinfection system, the chlorine gas will be consumed and a precipitate will be formed (increasing the TSS concentration of the effluent).

In addition, if an ultraviolet disinfection system is used, iron will interfere with the absorption of ultraviolet rays, form deposits on the lamps, and speed up the cleaning frequency of the lamps. It is recommended that each WWTP conduct a small test to determine the actual molar dosing required to achieve the target value of dissolved phosphorus in the effluent. Usually the molar dosage of iron salt required for phosphorus precipitation is based on the desired dissolved phosphorus concentration in the effluent rather than the influent phosphorus concentration. If the concentration of phosphorus in the primary sedimentation tank is reduced to 1 mg/L, the molar ratio of Fe3+:P to be added is 1.67:1 or the mass ratio is 3:1; 0.5 mg/L dissolved in the secondary treatment system The iron salt Fe3+: P needs to be added in a molar ratio of 2.27: 1 or a mass ratio of 4.1: 1.

In addition, the addition of iron ions could not make the soluble phosphorus concentration in the effluent below 0.10 mg/L. To achieve this concentration, the molar ratio of iron salt to phosphorus needs to be added to be 12:1. Iron or ferrous salts are acidic, so storage and handling issues need to be considered. Ferric chloride, ferrous chloride, ferric sulfate, or ferrous sulfate can be stored in fiberglass reinforced plastic (FRP) or polyethylene storage tanks. The metering pump can be a peristaltic pump, a screw pump or a diaphragm pump. It should be added as close to the dosing point as possible to reduce the influence of electroplating. The pump body needs to be made of polyvinyl chloride (PVC). Pipes, valves and fittings should be made of PVC or perchlorinated vinyl (CPVC). Operators should wear personal protective equipment such as gloves and face shields when working or handling any solution of iron salts.

05 Chemical dosing control

In order to monitor and control the dosing of chemical agents, it is necessary to measure the nitrogen and phosphorus concentration and related alkalinity indicators in the effluent of the sewage plant, as well as the concentration of the influent and the relevant stages of the treatment process of the sewage plant. Analysis can usually be performed in a laboratory or using an on-line monitoring system. The dosage of chemical agents can be controlled by manual control, automatic flow control, automatic feedforward control through flow rate and influent concentration, and automatic feedforward and feedback control through flow rate, influent concentration and effluent concentration.

06 Conclusion

Dosing chemicals will inevitably increase the cost of facility construction and daily operation. Whether it is necessary to add chemicals should be determined according to the standards of discharge or utilization; the effective use of chemicals depends on accurate dosage and appropriate mixing measures. Finally, it must be emphasized that reliable protective measures should be taken to ensure the safety and health of operation and maintenance personnel.

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