How can chemicals be added in water treatment to make denitrification and phosphorus removal more effective?

Nitrogen and phosphorus removal is an important function of wastewater treatment systems. To ensure that nitrogen and phosphorus removal treatment meets the standards, it is very important to ensure that sufficient organic matter is provided to the microorganisms. You can't expect a horse to run fast without feeding it.
For example, effective denitrification requires readily biodegradable carbon sources, biological phosphorus removal requires short-chain volatile fatty acids, and in some areas with relatively soft natural water quality, alkalinity needs to be supplemented to maintain the pH conditions required for the nitrification process in the entire aeration tank; in addition, if chemical phosphorus removal is used, whether as a supplement to the biological phosphorus removal process or as the main phosphorus removal method, metal salts and polymers need to be added.

This article discusses the basic principles, dosage calculation, and operational requirements of various reagent addition methods. 01 Carbon source addition for denitrification

1. When do you need to add reagents?

Biological nitrogen removal requires the completion of two processes: nitrification and denitrification. The ammonia nitrogen in wastewater must first be nitrified or converted into nitrite and nitrate, and then, in the denitrification process, nitrate will be reduced to nitrogen gas as an oxygen supplier for the oxidation of simple carbon compounds during cell respiration.

Therefore, the denitrification process aimed at removing nitrate must have a readily biodegradable carbon source. Its sources include dissolved BOD in the influent, decaying matter of cells in the endogenous denitrification process, and various supernatant returns. When the dissolved organic matter in the influent is insufficient and the nitrogen removal requirements are high, chemical substances need to be added to provide the carbon source required for the denitrification process.

2. What are the carbon sources? Where are they added?

Artificial carbon sources used for denitrification include pure chemical reagents such as methanol, ethanol, denatured ethanol, acetic acid, and sodium acetate, or waste sugar, molasses, and waste acetic acid solutions from industrial production processes. Among them, methanol is the most widely used and has been proven to be the most suitable carbon source.

For conventional biological nitrogen removal processes, methanol should be added directly to the anoxic section, and fully mixed with the influent and mixed liquor through the stirrer in the anoxic section. It is necessary to prevent violent turbulence from causing methanol to volatilize from the liquid phase into the air, and also to prevent the consumption of part of the methanol by aerobic respiration of bacteria due to the presence of excess oxygen.

If the wastewater treatment plant adopts a four-stage or five-stage activated sludge process, adding a carbon source to the subsequent anoxic section (second anoxic section) can achieve a higher denitrification rate than endogenous respiration, which can further remove nitrate; for a tertiary denitrification system, such as a denitrification filter, denitrification aerobic biofilter, etc., supplementing the carbon source is very important for the operation of the system.

Because the denitrification process is downstream of the main aeration process, all dissolved BOD in the influent has been removed, so methanol is usually added to the denitrification influent.

3. How to calculate the dosage?

The methanol dosage 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, it is usually considered that 3 mg/L of methanol is added for every 1 mg/L of nitrate removed in denitrification, and then adjusted according to the actual load and operating conditions of the wastewater treatment plant. The correct control of methanol dosage is very important for the operation of the tertiary denitrification system.

Excessive addition not only wastes chemical reagents but also increases the concentration of BOD in the effluent of the denitrification system. For wastewater treatment plants with low effluent BOD concentration requirements, the problem is not very big, but for wastewater treatment plants with a BOD limit of about 5 mg/L or lower, this is an issue that needs to be considered.

4. Safety measures for methanol addition system

The flash point of methanol is 12 ℃, and it is a highly flammable substance. The methanol storage tank, pipelines, and their accessories and electrical systems need to consider corresponding explosion-proof measures. The methanol addition system should generally be installed outdoors and away from other equipment. The methanol storage tank should be equipped with a floating roof and a pressure relief valve and fire extinguisher.

02 Addition of volatile fatty acids in biological phosphorus removal

1. Why add VFA?

The mechanism of biological phosphorus removal is to absorb volatile fatty acids (VFA) in the anaerobic zone while releasing stored phosphorus, and then polyphosphate bacteria absorb excess phosphorus under aerobic conditions. To ensure the reproduction of polyphosphate bacteria and effective biological phosphorus removal, sufficient volatile fatty acids are needed.

There may be VFAs in the influent of wastewater treatment plants, including raw water with a longer residence time in the collection system and multiple lift pump stations, and the decomposition of complex organic compounds in the anaerobic section of the biological nitrogen and phosphorus removal system. If the naturally produced VFA content is insufficient, VFA needs to be added externally to the anaerobic section.

2. What are the commonly used VFAs? How to calculate the dosage?

For biological phosphorus removal systems, a mixture of acetic acid and propionic acid is the best choice for external VFA addition. Acetic acid solution is the most widely used in practice. If VFA needs to be added (for example, to increase dissolved BOD in the influent, part of which will be converted into usable VFA through the anaerobic fermentation process), it usually takes 5-10 mg of external VFA to remove each milligram of phosphorus.

Acetic acid is usually 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 ℃) and a freezing point of 17 ℃, so fire prevention should be considered according to the specifications, and measures must be taken to prevent freezing. Storage tanks, pipelines, and accessories should all use metal materials.

3. Safe storage measures for acetic acid

Acetic acid is corrosive and typically requires 316 stainless steel. When using glacial acetic acid in warm climates, its relatively low flash point necessitates consideration of an inert gas pad or floating roof. In practice, a low-concentration aqueous solution of acetic acid is recommended.

Of course, adding VFAs cannot completely remove the TP concentration in the system effluent; if a very low TP concentration in the effluent is required, chemical phosphorus removal is still necessary, removing phosphorus through precipitation with chemical agents.