A pilot study was carried out in a waste water treatment plant (WWTP) to recover phosphorous by Struvite precipitation from anaerobic digester centrate by means of Ostara's Pearl technology with the objective to assess technically and economically the performance and reliability of this process treating sidestreams with different concentrations of phosphorous and also to assess the quality of the fertilizer obtained. Different scenarios were studied: phosphorous inlet ranged from 31 to 150 mg PO 4 -P/L. The phosphorous removal varied from 60 to 81% depending on the influent, and the quality of the Struvite obtained was suitable to be used as a fertilizer. The economic assessment of implementing Struvite Pearl recovery process at full scale in WWTPs with anaerobic digestion (AD) and P-physicochemical or bio-P removal was carried out, taking into account investment (CAPEX) and operating costs (OPEX) of the Struvite recovery process and the economic savings due to phosphorous and ammonium removal in centrates. The results showed that with the current reagent strategies and investment costs, the Struvite recovery process was not economically feasible for WWTPs with AD and P-physicochemical removal. However, in the case of bio-P removal WWTPs with AD, the process can be an economic solution (pay back between 5 and 10 years, OPEX recovered due to the selling Struvite) which creates a high value re-cycled fertilizer and saves the environment due to the higher efficiency of the fertilizer usage.
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.
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