United Utilities

Observing the effects of digestion and chemical dosing on the calorific value of sewage sludge

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Courtesy of United Utilities

New legislation being introduced in Europe for the removal of the macronutrients nitrogen and phosphorous is causing the fundamental composition of sludge to change. Nitrogen removal requires longer residence times during activated treatment, and phosphorous removal can involve the supplementation of chemicals such as ferric. In addition, there is increasing pressure to capture energy from sludge due to incentives which encourage renewable energy recovery. Energy from sludge may be recovered as biogas or as fuel which is combusted. However, little is known about the effects of legislative change on the energy value (calorific value) of the sludge. This paper describes the analysis of numerous samples of raw, digested and chemically treated sludge for chemical composition and calorific value and compares the results against theoretically derived results. A simple formula is proposed based on the findings, which can be used as a primary estimate for calorific value.

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