Plant-wide modelling of phosphorus transformations in wastewater treatment systems: Impacts of control and operational strategies.

The objective of this paper is to report the effects that control/operational strategies may have on plant-wide phosphorus (P) transformations in wastewater treatment plants (WWTP). The development of a new set of biological (activated sludge, anaerobic digestion), physico-chemical (aqueous phase, precipitation, mass transfer) process models and model interfaces (between water and sludge line) were required to describe the required tri-phasic (gas, liquid, solid) compound transformations and the close interlinks between the P and the sulfur (S) and iron (Fe) cycles. A modified version of the Benchmark Simulation Model No. 2 (BSM2) (open loop) is used as test platform upon which three different operational alternatives (A1 , A2 , A3 ) are evaluated. Rigorous sensor and actuator models are also included in order to reproduce realistic control actions. Model-based analysis shows that the combination of an ammonium ( [Formula: see text] ) and total suspended solids (XTSS ) control strategy (A1 ) better adapts the system to influent dynamics, improves phosphate [Formula: see text] accumulation by phosphorus accumulating organisms (XPAO ) (41%), increases nitrification/denitrification efficiency (18%) and reduces aeration energy (Eaeration ) (21%). The addition of iron ( [Formula: see text] ) for chemical P removal (A2 ) promotes the formation of ferric oxides (XHFO-H , XHFO-L ), phosphate adsorption (XHFO-H,P , XHFO-L,P ), co-precipitation (XHFO-H,P,old , XHFO-L,P,old ) and consequently reduces the P levels in the effluent (from 2.8 to 0.9 g P.m-3 ). This also has an impact on the sludge line, with hydrogen sulfide production ( [Formula: see text] ) reduced (36%) due to iron sulfide (XFeS ) precipitation. As a consequence, there is also a slightly higher energy production (Eproduction ) from biogas. Lastly, the inclusion of a stripping and crystallization unit (A3 ) for P recovery reduces the quantity of P in the anaerobic digester supernatant returning to the water line and allows potential struvite ( [Formula: see text] ) recovery ranging from 69 to 227 kg.day-1 depending on: (1) airflow (Qstripping ); and, (2) magnesium ( [Formula: see text] ) addition. All the proposed alternatives are evaluated from an environmental and economical point of view using appropriate performance indices. Finally, some deficiencies and opportunities of the proposed approach when performing (plant-wide) wastewater treatment modelling/engineering projects are discussed.