--- title: "Sludge digestion: stages and factors" subject: "Sanitary Engineering" chapter: 8 key: "semester_6/sanitary_engineering:sludge-digestion-stages-factors" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sludge-digestion-stages-factors site: BCT Engineering --- # Sludge digestion: stages and factors Sanitary Engineering · Chapter 8: Sludge Treatment and Disposal - Asked in: 2075 Bhadra, 2+6 marks; 2072 Asoj, 4 marks; 2068 Magh (old course), 3+3 marks (asked 3 times) - Video: [CE402 Environmental Engg.II- Module 6- Sludge digestion tank & Stages of sludge digestion process](https://www.youtube.com/watch?v=TxKFv24RIzM) — Toc H Kochi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/sludge-digestion-stages-factors - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-8 ## Question What is sludge digestion? Explain the stages (periods) during sludge digestion and the factors affecting the digestion process. ## Short answer Sludge digestion is anaerobic biological decomposition of organic matter into stable inert material, water, and biogas (65% CH4, 30% CO2). It occurs in three stages: acid fermentation (pH 5-6, foul odor), acid regression (pH rises to 6.8), and alkaline fermentation (pH 6.8-7.4, high methane production). Key factors: temperature (mesophilic 30-38°C, thermophilic 50-57°C), pH (6.8-7.4), volatile acids/alkalinity ratio (<0.2), solids loading, seeding, mixing, toxic substances, retention time, and C:N ratio (20-30:1). ## Answer #### 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 ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Sludge digestion is A. anaerobic biological decomposition of volatile matter in sludge into stable matter, water and gases B. mixing sludge with chlorine C. drying sludge in sun D. aerobic growth of algae in sludge Answer: A. Bacteria decompose organic matter without air, giving methane and CO2 and a stable sludge. (Key point: Meaning of digestion) 2. What is the correct order of the stages? A. Alkaline fermentation, acid fermentation, acid regression B. Acid regression, acid fermentation, alkaline fermentation C. Acid fermentation, alkaline fermentation, acid regression D. Acid fermentation, acid regression, alkaline fermentation Answer: D. Acids form first (pH 5-6), then regression, then methane formation at pH 6.8-7.4. (Key point: Order of stages) 3. Methane bacteria are inhibited when the pH falls below A. 8.5 B. 6.2 C. 4.0 D. 7.4 Answer: B. pH must be 6.8-7.4; below about 6.2 methane formers are inhibited. (Key point: pH requirement) 4. Which statement about temperature is correct? A. Optimum is about 10°C B. Digestion time halves for every 10°C fall C. Sudden changes of more than 1°C/day upset the process; mesophilic optimum is about 35°C D. Thermophilic range is 5-10°C Answer: C. Mesophilic 30-38°C is best near 35°C; digestion time doubles for every 10°C fall. 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