A framework for P-cycle assessment in wastewater treatment plants

Highlights: • Full-scale wastewater treatment plants (WWTP) are ideal places for the recovery of phosphorus (P) to prevent P losses • An universal and comparable framework for the assessment of the P-cycle was established, based on WWTP wide P balances • Several process streams in a WWTP reach the economically feasible threshold for the (biological) recovery of P • chemical P removal affects the microbial P removal capacity Phosphorus (P) in wastewater has a variety of negative effects and is usually permanently lost as a non-renewable resource. To mitigate future P shortage, P must be recovered from wastewater, preferably by bio-based technologies to avoid toxic side streams. A standardized procedure for the determination of P types and P concentrations in all liquid and solid process stages was established, which is applicable to all full-scale wastewater treatment plants (WWTPs). Based on this, an equally universal calculation framework for P-cycle assessment based on volume flow and mass load rates was designed to identify the most promising process streams for biological P recovery. As an example, in 16 process streams of a typical WWTP, concentrations of free, bound and total P were calculated and microbial communities were analyzed by flow cytometry over 748 days. The most promising process streams for the recovery of free P were anaerobic digester sludge, centrate and the water-extracts of the biosolids with 0.510 kg P m⁻³, 0.075 kg P m⁻³ and 1.023 kg P m⁻³, while the best process streams for the recovery of bound P were return sludge, excess sludge, anaerobic digester sludge, and the solids of the biosolids with 0.300 kg P m⁻³, 0.268 kg P m⁻³, 0.213 kg P m⁻³ and 1.336 kg P m⁻³, respectively. Microorganisms capable of P accumulation were active in all process stages and it was observed that chemical P precipitation antagonizes biological P removal. The framework for P-cycle assessment was able to identify process streams that are economically viable to make future in-stream technologies for biological P removal feasible.