Hydrothermal treatment and recycling of organic by-products from sludge – confirmation of by-products availability for biological nutrient removal

The main objective of this study was to confirm the technical feasibility of using hydrothermal processing of sludge to combine partial treatment and generation of useful organic by-products for internal recycling to support biological nutrient removal (BNR). The experimental program involved assessing hydrothermal sludge treatment and production of organic by-products containing volatile fatty acids, especially acetic acid, under a variety of reaction temperatures, durations, and uses of oxidant. Three sludge types were treated; primary, secondary, and a mixture of the primary and secondary. The organic by-products were then used to provide the main carbon source to support BNR in two short sets of sequencing batch reactor experiments. The results confirmed the bio-availability of the by-products to support simultaneous biological phosphorus removal and denitrification. The BNR microorganisms readily used the dissolved by-products, especially acetic acid, for active biological phosphorus release and uptake and denitrification. Mass balance analysis confirmed that hydrothermal generation of by-products from sludge can be adequate for internal recycling in BNR wastewater treatment plants. Key words: wastewater sludge, hydrothermal treatment, recycling of organic by-products, organic carbon source, volatile fatty acids, acetic acid, biological nutrient removal, phosphorus removal, denitrification.