Abstract The most important objectives to gasify sewage sludge are to produce a clean gas of acceptable composition for synthesis or combustion, and to convert this solid resource into combustible-clean gas at high efficiency. The experiments of the gasification were conducted using a 5 kWe-throated downdraft gasifier. It was concluded that sewage sludge can be gasified to produce low-quality combustible gas, and would be an acceptable alternative source to fossil fuels for the production of the clean energy. It is suggested that the downdraft gasifier should be operated at 3.69–3.71 kg/h±1.43% of the feed rate, at 2.28–2.34 N m 3/kg±1.84% of the air fuel ratio, around 497.74–514 N m 3/m 2 h±1.50% of specific gasification rate and around 93.64–94.15%±1.92% of turndown ratio in order to achieve good quality gas and to avoid clinker formation at the throat of the gasifier because of high ash content of sludge. The thermal efficiency was calculated as between 39% and 40% at the optimum operation levels given above.