Hydrothermal processes as an alternative to conventional sewage sludge management

Sewage sludge management is one of the biggest concerns to the wastewater industry due to the increasing volumes produced and new stringent environmental regulations. Hydrothermal Treatments (HT) are a good option for converting wet biomass such as sewage sludge into high value products. However, HT are still not well developed when compared with other waste processing treatments. One of the most promising areas for developing hydrothermal processing applications is in sewage sludge treatment facilities. Sewage sludge has been identified as a potential feedstock for hydrothermal processing that could make use of existing facilities currently in place in wastewater treatment works (WWTWs). In order to look for options aimed at reducing the costs of the WWT process and digestate management by delivering a sustainable and novel approach, the aim of this project is to assess alternatives to enhance the way sewage sludge is handled in WWTWs, by focusing on the use of hydrothermal processes and the potential of recovering energy and nutrients. The potential of integrating HT Processes with AD for sewage sludge treatment was evaluated. Hydrochar yields ranged from 38 to 68% at 160°C and from 29 and 40% at 250°C for all thermal treated sewage sludge samples. The soluble fraction of organic carbon increased in primary sludge digestate (525%), secondary sludge digestate (808%) and sewage digestate sludge (675%) after thermal treatments compared with the untreated digestates. Figures from Biomethane Potential (BMP) tests showed that hydrothermal treatment enhanced methane production in all non-AD and AD sludge samples processed. Mass and energy balances were carried out from six proposed process configurations from different sewage sludge feedstocks and their digestates (primary, secondary and 1:1 Mix) in order to evaluate the waste generation, nutrients potential fate, net energy production and potential profit. The results showed the HTC at higher temperatures (250°C) seems to have more economic and environmental benefits. Scenarios that involved primary and mix sludge seemed to be the most suitable options in terms of the organic matter removal, energy harnessing and economic feasibility.