Reactor Performance

All process streams in an anaerobic digester system should be monitored for the volume of flow per day, and sampled and analyzed for various constituents and physical conditions (WEF, 1998):

• Feed sludge: TS, VS, pH, alkalinity, temperature

• Digester content: TS, VS, volatile acids, alkalinity, temperature

• Digested sludge draw-off: same as digester content

• Digester gas: percentage of methane, carbon dioxide, and hydrogen sulfide

• Supernatant: pH, BOD, COD TS, total nitrogen, ammonia nitrogen, phosphorus

There is no single parameter that can be isolated as the best indicator of digester performance. A decrease in methane production and a correspond ing increase in the CO2 content of the gas produce an increase in volatile acid concentration, and decreases in the alkalinity and pH of the digester content indicate unbalanced anaerobic treatment operation. Anaerobic digestion may become unbalanced temporarily by the sudden change in temperature, organic loading, composition of feed sludge, or toxic loading, or a combination of these. This type of imbalance can be remedied by not feeding the digester temporarily, or by providing sufficient time for the microorganisms to adjust to the new environment.

The relationships among the CO2 content of digester gas and the alkalinity and pH of the digesting sludge has been described in Section 6.2.3 and presented in Figure 5.4. As shown in the figure, at the normal anaerobic digestion condition of pH between 6.6 and 7.4 and CO2 content of 30 to 40%, the bicarbonate alkalinity will be in the range 1000 to 5000 mg/L as CaCO3. The concentration of bicarbonate alkalinity in the digester content should be about 3000 mg/L as CaCO3.

Prolonged and relatively permanent imbalance of the anaerobic system may be the result of the presence of toxic materials or an extreme drop in pH. Imbalance due to an extreme drop in pH is encountered frequently in starting up a new digester. The doubling time of acid-forming bacteria is measured in hours, whereas it is measured in days for methane-forming bacteria. Therefore, the methane-forming bacteria require time to develop in sufficient numbers to convert the volatile acids to methane and CO2. If the pH drops below the minimum value tolerated by the methane-forming bacteria, raising the pH by the addition of chemicals such as sodium bicarbonate is required until the digester returns to normal efficiency.

Figure 5.4 indicates that system pH is controlled by CO2 concentration of the gas phase and bicarbonate alkalinity of the liquid phase. A digester with a given gas-phase CO2 concentration and liquid-phase bicarbonate alkalinity can exist at only one pH. If bicarbonate alkalinity is added to the digester and the portion of the CO2 in the gas phase remains the same, the digester pH must increase. For any fixed gas-phase CO2 concentration, the amount of sodium bicarbonate required to achieve the desired pH change is given by the equation

where D is the sodium bicarbonate dose, (mg/L) and BA is the bicarbonate alkalinity in digester, (mg/L CaCO3).

Inorganic and organic materials that are toxic to anaerobic digestion process are described in Section 5.2.4 and are listed in Tables 5.1 and 5.2. The presence of toxic materials in wastewater treatment plants has been reduced as a result of the implementation of industrial pretreatment programs nationwide. However, if one or more toxic materials caused prolonged imbalance of a digester system, these materials must be removed.

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