Technical and economic analysis of real anaerobic digester centrate by means of partial nitrification and sustainable heterotrophic denitrification

The reliability of partial nitrification coupled with heterotrophic denitrification for the treatment of real anaerobic digester centrate produced in a wastewater treatment plant was technically and economically assessed in two sequencing batch reactors. Removal efficiencies above 90% were consistently achieved at N-ammonium loads above 1.2 g N L⁻¹ d⁻¹. Ethanol, affluent from a waste water treatment plant (biological treatment inlet) and a zero-cost liquid residue from a chemical industry containing polyethylene glycol and sorbitol were employed as carbon source for denitrification. In this last case, a total organic carbon (TOC) requirement of 4.5 g TOC g⁻¹ NO₂⁻-N was calculated. The denitrification rate was 0.26 g NO₂⁻-N g VSS⁻¹ d⁻¹ (VSS: volatile suspended solids). These results show that a carbon-rich waste can serve as a no-cost feed for denitrifying bioreactors. An in-depth economic analysis considering the main investment and operating costs of the process was developed, showing that it can suppose yearly savings above 50% with respect to the most widely used alternative of returning anaerobic digester centrate untreated to the head of the facility.