טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentMoti Kartashevsky
SubjectApplication of Granular Ferric Hydroxide for Phosphate
Removal as a Complementary Process in Secondary
Effluent Desalination
DepartmentDepartment of Civil and Environmental Engineering
Supervisors Full Professor Dosoretz Carlos
Professor Emeritus Semiat Raphael
Full Thesis textFull thesis text - English Version


Abstract

This research deals with phosphate scaling in reverse osmosis (RO) membranes during secondary effluent desalination at high recovery rates.

Calcium phosphate precipitation, and in particular calcium hydroxyapatite (HPA)-Ca5(PO4)3OH scaling, is a major factor limiting high recovery rates in reverse osmosis (RO) desalination of treated effluents. The main objective of this study was to evaluate the operational feasibility of phosphate removal in continuous regime by adsorption on granular ferric hydroxide (GFH), from a 1st-stage brine (2-fold concentrated) of a 2-stage desalination train of secondary effluents. The desalination scheme was based on the 10 m3/h Technion-pilot plant for secondary effluents desalination located at the wastewater treatment plant Nir Ezion near Atlit.

Bench scale-fixed bed adsorption on GFH was tested in order to ensure low turbidity, cope with fluctuation of phosphate concentration and preserve the pressure of the brines stream for the 2nd-RO stage. The optimal hydraulic conditions for the adsorption process and a methodology for GFH reuse and regeneration were formulated. Simultaneous analysis of bed height/mass transfer zone (z/MTZ) and adsorbent  phase concentration (q) as function of empty bed contact time (EBCT) and hydraulic load (HL), yielded optimal hydraulic conditions at EBCT >3.5 min and 10<HL<15 m/h (Langmuir adsorption parameters at 25 were qmax=29.52 mg P/g GFH and KL=0.75 L/mg P). Breakthrough P concentration was set 0.2 mg/L to maintain an HPA-solubility index ≤ 7. Findings indicated that GFH can be reused to treat 3300 bed volumes at a target breakthrough concentration of 0.2 mg P/L h (feed phosphate concentration of 9.3±0.9 mg/L as P). Nine adsorption/regeneration cycles were attained with a recycled NaOH solution.

Based on the empirical data obtained from the fixed columns, the 2-stage-Technion desalination scheme was analyzed for addition of a GFH adsorption stage on the RO1 concentrate stream. The results showed 30% reduction in energy requirement for desalination (0.553 to 0.389 kWhr/m3) due to GFH use. Total water recovery rose to 90% and HPA scaling was limited to below the SI value of 7. The SIHPA value of 7 is the target value which one should not exceed in order to avoid HPA scaling with high recoveries.

In conclusion, achieving high water recovery (>90%) and good water quality of the permeate stream (TOC<1 ppm, SOC< limit of detection, total dissolved solids (TDS)<50ppm), marks a step forward for direct or indirect reuse of wastewater effluent.