Aerobic and anaerobic digestion

Aerobic digestion

Sludge (usually waste activated sludge or mixed primary and secondary sludge) is aerated for 10-20 days (at 15-20°C) in an open tank, in the same way as in the activated sludge process. Without fresh food the aerobic bacteria oxidise their own cell mass (endogenous respiration):

$$C_5H_7NO_2+5O_2\rightarrow5CO_2+2H_2O+NH_3+\text{energy}$$

It reduces volatile solids by 35-50%.

• Advantages: simple operation, low capital cost, no odour problems, a supernatant of lower BOD, good for small plants.
• Disadvantages: high power for aeration, no methane gas is produced, and the sludge dewaters less easily.

Anaerobic digestion

Sludge is held in a closed, heated tank without air, with decomposition in three stages by two groups of bacteria. Acid-forming bacteria convert complex organics to volatile acids, and then methane-forming bacteria convert the acids to CH₄ and CO₂:

$$\text{organics}\rightarrow\text{volatile acids}\rightarrow CH_4+CO_2+H_2S$$

It is the common method for primary and mixed sludge in medium and large plants: 40-60% volatile solids are destroyed, and the biogas (65% CH₄, calorific value about 22 MJ/m³) is used for heating and power. The digested sludge is stable, inoffensive and can be dried on beds and used as manure.

Effect of temperature and pH

• Temperature: the rate of digestion increases with temperature. Anaerobic digestion works at the mesophilic range 30-38°C (optimum about 35°C, digestion time about 20-30 days) and the thermophilic range 50-57°C (faster, 10-15 days, but less stable). Below 20°C digestion becomes very slow (60 days or more); the digestion time about doubles for each 10°C fall. Sudden changes of more than 1°C per day upset the methane bacteria, so digesters are heated and insulated.
• pH: acid formers tolerate pH 5-6 but methane formers need 6.8-7.4. If volatile acids accumulate, pH falls below 6.2 and digestion stops ("sour" digester, foul odour, little gas). The pH is corrected by reducing the feed rate and adding lime or bicarbonate to maintain alkalinity of 1500-5000 mg/l.