Pilot scale anion selectrodialysis for water reclamation and nutrient recovery from UASB effluent after nitrification, ultrafiltration and UV C treatment

Abstract

In the frame of the increasing need of nutrient recovery and recycling the objective was to fractionate and concentrate nutrients from nitrified, ultra-filtered and ultraviolet C treated UASB effluent with anion selectrodialysis (aSED). Under these conditions it was possible to keep the system in operation for 14 days or even for 3 weeks with intermittent cleaning in place every week. When membranes developed in the frame of sulphuric acid production (PC-Acid 100 OT) were used as anion exchange membranes (AE membranes) instead of standard AE membranes (PC-SA) a 4.5 times higher current efficiency for phosphate transport was noted. The diluate of the aSED is reusable as e.g. cooling water for condensers. The product stream which is enriched in phosphate (5–6 mmol.L −1 ) and the brine stream which accumulated potassium (150 mmol.L −1 ) and nitrate (90 mmol.L −1 ) are suitable for nutrient recovery. During the optimization process the current efficiency for phosphate transport increased tenfold, from 0.7% to 7%. The product stream was fed to a lamella separator (with a molar Ca/P ratio of ± 1.93) to recover 98% of the phosphate. The recovered precipitate was free of magnesium and contained the equivalent of 30 to 38 m% P 2 O 5 ; XRD analysis showed the presence of hydroxyapatite. The Ca/P ratio of this precipitate decreased proportionally with the degree of aeration to expel dissolved inorganic carbon from the feed to the aSED, indicating less coprecipitation of calcium carbonate within the recovered calcium phosphate. • Anion selectrodialysis combines water reclamation with N, K and P fractionation. • Ca 3 (PO 4 ) 2 precipitated from the selectrodialysis product in a lamella separator. • The calcium phosphate (max. 82 m% ≈ 38% P 2 O 5 ) is only contaminated with CaCO 3 . • Up to 80% of K and nitrate-N is recovered and is concentrated 3.2 times. • By optimization the specific current efficiency for phosphate increased tenfold.