Innovative strategies to maximize carbon and energy recovery from domestic wastewater

Wastewater is not just waste, but a valuable source of (bio)energy, organic carbon and nutrients that can be recovered and reused. Conventional wastewater treatment systems are energy inefficient and focus on the removal of these resources rather than in their recovery. High-rate activated sludge systems allow to recover organic carbon in the form of sludge and increase the (bio)energy produced via biogas formation, allowing to obtain an energy neutral wastewater treatment. Production of sludge in high-rate systems is not without its problems, due to the poor settling capacity of the sludge formed that is thus highly diluted and needs to be further thickened prior to digestion. This increases the land footprint and the operational costs, and requires chemical dosage (polyelectrolytes) to increase sludge concentration. In this work, an alternative strategy to thickening was explored by applying a forward osmosis based concentration, which could reduce the land-foot print and avoid expenses related to polyelectrolyte addition. A second approach investigated was the use of a dissolved air flotation system instead of a settler and a thickener, that could reduce the land footprint by 85 % and generate sludge with a concentration similar to that of a combination of settling and thickening. Moreover, this thesis focused on an alternative route to biogas formation, whereby the sludge organics are fermented to added value carboxylic acids that can be converted into a broad range of chemicals and products, such as plastics, solvents and fuels. Carboxylic acids are currently produced from petroleum resources thus their generation from sustainable carbon sources (waste) could bring further added value to wastewater treatment processes and reduce the use of non-renewable petroleum resources.