Biogas Production from Partially Digested Septic Tank Sludge and its Kinetics

Abstract

In developing countries, like India, there is no systematic approach for septage management. Treatment and possibility of methane recovery from this septage has not been explored previously. Hence, possibility of recovering methane from septic tank sludge in an anaerobic digester was explored with and without ultra-sonication pre-treatment in laboratory scale reactors and a 2 m3 pilot anaerobic digester was operated on raw septic tank sludge over a period of 1 year. Methane production potential of the raw septic tank sludge was 298.88 ± 9.7 L kg VS destroyed, which increased to 409.96 ± 13.25 L kg VS destroyed after sonication pre-treatment. Methane content in biogas increased from 73.15 to 81.83% upon sonication pre-treatment of the sludge. A first-order kinetic model best predicted biogas production from pilot digester fed with raw septic tank sludge, with minimum errors. Laboratory scale studies proved the feasibility and economic benefit of using sonicated sludge in the digester, with a net energy gain of 1.67 W-h/L of sludge digested. The kinetic constants derived from the laboratory experiments can be used for design of digester for field application using sonicated sludge to enhance digestion rate and methane recovery from septic tank sludge for better energy conservation.