Comparison of thermal destruction technology for complete biosolid processing

Sewage sludge production is rising around the world. This is due to population growth, stricter legislation, and new investment in wastewater infrastructure. Whilst, typically considered a nuisance, sludge has numerous benefits which can be exploited, such as the reuse of nutrients or extraction of energy. Previously in Europe, reluctance for land recycling of sludge coupled with cheap energy led to the development of sludge treatment strategies which were heavily reliant on energy intensive processing. However, increasing energy costs, mounting importance of nutrient recovery and growing influences of sustainability have led to the development of new sludge strategies. This paper highlights a number of case study options available for sludge and includes results of a study looking at optimal energy recovery strategies. The study suggests that use of anaerobic digestion is preferable (to no use) when dewatered cake is processed further for energy recovery (for example co-firing at power station) with regards to overall energy balance. When drying options are also considered, use of anaerobic digestion with thermal hydrolysis gives the greatest overall energy recovery potential. This is due to a large decrease in drying requirements as a result of greater solids destruction (less to dry) coupled with better dewaterability (less water to evaporate). Drying and energy recovery of raw sludge gives a negative energy balance.