Phosphate adsorption on granular ferric hydroxide to increase product water recovery in reverse osmosis-desalination of secondary effluents

Calcium phosphate precipitation, and in particular calcium hydroxyapatite (HAP)-Ca5(PO4)3OH scaling, is a major factor limiting high recovery in reverse osmosis (RO) of treated effluents. This study evaluated the feasibility of phosphate adsorption in continuous regime on granular ferric hydroxide (GFH) from 2-fold concentrated RO-brines (1st stage of a 2-stage RO desalination train) of secondary effluents. Fixed bed adsorption on granular media was chosen to ensure low turbidity and preserve brine pressure for the 2nd RO stage. Optimal hydraulic conditions for adsorption and a method for GFH regeneration were formulated. Simultaneous analysis of bed mass transfer zone and adsorbent concentration yielded optimum at empty bed contact time (EBCT) N 3.5 min and 10 b hydraulic load (HL) b 15 m/h. Breakthrough P-concentration was set at 0.2 mg/L to maintain HAPsolubility index ≤ 7. A constant adsorption rate of 1.6 mg P/g GFH was maintained during nine adsorption/ regeneration cycles at feed phosphate concentration of 9.3 ± 0.9 mg/L (as P), approx. 3300 bed volumes (BV) per batch. A NaOH regeneration solution was effectively recycled over 2300 BV until reactivation was required. Considering that phosphate concentration in effluents of well performing wastewater treatment plants is far below 10 mg/L, in practice column capacity will be considerable higher than 3300 BV. © 2015 Elsevier B.V. All rights reserved.

Because of the relatively high calcium carbonate alkalinity and phosphate concentrations in secondary effluents, calcium phosphate scaling becomes a major factor limiting high RO recovery in effluent desalination. Phosphate adsorption on GFH applied on a 2-fold concentrated RO brines resulted in effective removal (N 95%), maintaining a stringent breakthrough concentration ≤ 0.2 mg/L regardless of fluctuations in feed concentration. An almost constant adsorption rate of 1.6 mg P/g GFH per BV for approx. 3300 BV per batch was maintained during nine adsorption/regeneration cycles with a hydraulic pressure drop of 0.2 bar/m of media height. The alkaline regeneration solution was effectively recycled over 2300 BV until reactivation was required. Downflow-fixed bed adsorption on granular media, in addition, released low turbidity (0.6 ± 0.4 NTU) and preserved the pressure of the brines for further RO stages. From graphical analysis of z/MTZ and q as function of EBCT and HL, the optimal operational conditions drawn were EBCT at least 3 min and HL in the range of 10 to 15 m/h. Iron leaching from the GFH column was negligible, depicting a high chemical and physical stability of the media. Running RO simulations for a 2-stage RO plant with overall recovery of 90% and GFH adsorption, depict a 3.2 b SIHPA b 6.2 can be ensured within a pH range of 6.5–7.5 during the RO2 stage, below the maximum value allowed to prevent apatite scale formation (SIHPA ≤ 7). To conclude, phosphate adsorption on granular ferric hydroxide appears as a technically feasible and compatible technology for preventing phosphate scaling during effluent desalination achieving high water recovery and good water quality.