Waste Water Treatment Plants

Aerobic Processes
Aerobic processes have been employed by municipal and industrial wastewater treatment systems for the removal of organics, the biological conversion of ammonia to nitrates, reduction of sludge mass and volume, and reduction of pathogenic organisms. Aerobic digestion consists of two steps; direct oxidation of biodegradable matter, and subsequent oxidation of microbial cellular material.

Organic Matter + O2 + Nutrients CO2 + H2O + NH3 + Cellular Material (1)

If the digestion process is provided with sufficient oxygen and detention time, or a separate nitrification system is utilized, ammonia will nitrify and form nitrates. The nitrification process may result in a decrease of both pH and alkalinity as a result of acid generation during the process:

NH4+ + 2O2 NO3- + 2H+ + H2O(2)

This reaction results in the consumption of about 7.1mg alkalinity per each mg NH4+-N oxidized. Influent treated with THIOGUARD® from the collection system generally arrives at the digester very near pH 7.5, the optimum pH for bacterial growth in both aerobic and anaerobic processes. If the wastewater does not have sufficient alkalinity to compensate for losses in the nitrification reaction, it will result in a pH drop and could, if the pH drops too low, result in bacterial deactivation.

The use of THIOGUARD® offers the following advantages over caustic soda and lime. Technical grade magnesium hydroxide buffers to a controlled pH near 9.0, even when over dosed. This buffering capacity results in better pH control and makes pH excursion less likely to occur. Magnesium hydroxide has higher neutralizing value per dry pound. Therefore, less technical grade magnesium hydroxide is required compared with caustic or lime. Unlike lime, technical grade magnesium hydroxide does not cause large volumes of sludge and scaling in the collection system or treatment plant equipment.

Anaerobic Digestion
Anaerobic digestion is the solubilization and reduction of complex organic substances by microorganisms in the absence of oxygen. The products of digestion are methane, carbon dioxide, trace gases and stabilized biosolids. The microbial population responsible for this conversion can be divided into three groups: solubilization, acid formation and methane formation (methanogens). Proteins, lipids, carbohydrates and complex organics are solubilized by hydrolysis. These products are converted into short-chain organic acids, such as, acetic, propionic and lactic. These acids are then converted into methane and carbon dioxide. The acid forming bacteria are tolerant to environmental changes such as pH and temperature. In contrast, the methane forming bacteria are intolerant to environmental changes.

C6H12O6 þ 3CH3COOH (3)


3CH3COOH + 3NH4HCO3 þ 3CH3COONH4 + 3H2O (4)


3CH3COONH4 + 3H2O þ 3CH4 + 3NH4HCO3 (5)

Equation 3 represents acid formation. The acid is then neutralized, equation 4, by bicarbonate present in the system. The buffer consumed in equation 4 is then regenerated in the methane forming step. There is therefore an equilibrium between buffer formation and consumption. The optimum pH range for methanogens is also 6.5 to 7.5. In a digester upset, net consumption of buffer occurs and the process is in danger of pH failure. When this happens an external source of alkalinity must be added. Technical grade magnesium hydroxide can be added to the digester to neutralize any excess acid not consumed by the methanogens. Technical grade magnesium hydroxide when used in anaerobic digesters will have all the benefits apparent in aerobic processes.

 

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