Sludge digestion: stages and factors

Sludge digestion

Sludge digestion is the biological decomposition of the organic (volatile) matter of sludge by bacteria in the absence of air (anaerobic), which converts it into stable inert matter, water and gases (methane and CO₂, about 65% and 30%). It reduces the volume and putrescibility of the sludge and kills most pathogens. It is done in closed digestion tanks.

Stages (periods) of digestion

 Gas / pH
  CH4 ^                         ______  alkaline fermentation
      |                  ______/
  pH  |      _______----/  acid regression
  5   |_____/  acid fermentation
      +-----------------------------------> time
       (about 15 days)   (about 3 months)  (about 1 month)

(Periods shown are typical at about 20°C; they are much shorter in heated digesters.)

1. Acid fermentation (acid formation): acid-forming bacteria hydrolyse carbohydrates, fats and proteins into volatile fatty acids (acetic, propionic, butyric), alcohols, CO₂, H₂S and ammonia. The pH falls to about 5-6, the sludge is acidic with a foul odour, and little gas is produced.
2. Acid regression: the volatile acids and nitrogen compounds formed in the first stage are attacked and converted to acid carbonates and ammonia compounds, with small amounts of H₂S, CO₂ and N₂. The pH rises slowly to about 6.8. The sludge has a high BOD, tends to foam and still has an offensive (H₂S) odour.
3. Alkaline fermentation (methane formation): slow-growing methane bacteria break down the acids and remaining proteins/amino acids to CH₄ and CO₂. The pH rises to 6.8-7.4 (slightly alkaline), gas production is highest (65-70% CH₄), and the digested sludge is black, stable, has a tarry odour and a low BOD, and settles and dries easily.

Factors affecting digestion

1. Temperature: mesophilic range 30-38°C (best at 35°C, digestion in 20-30 days) and thermophilic range 50-57°C. Digestion time roughly doubles for every 10°C fall; sudden temperature changes of more than 1°C/day upset the process.
2. pH: must be kept at 6.8-7.4; below 6.2 the methane bacteria are inhibited. An alkalinity of 1500-5000 mg/l (as CaCO₃) provides buffering.
3. Volatile acids / alkalinity ratio: should be below 0.1-0.2.
4. Solids content and loading: feed of 4-8% solids, with volatile solids loading of about 0.5-1.6 kg VS/m³/day for conventional-rate and 1.6-3.2 kg VS/m³/day for high-rate digesters; overloading causes acid conditions.
5. Seeding: mixing fresh sludge with 10-25% ripe digested sludge keeps the active methane bacteria present.
6. Mixing: gas or mechanical mixing improves contact between sludge and bacteria.
7. Toxic substances: heavy metals, detergents, chlorine, disinfectants and very high ammonia stop digestion.
8. Retention time: 30-60 days for conventional-rate and 10-20 days for high-rate digestion.
9. Nutrients (C : N ratio): should be about 20-30 : 1.