|M.Sc Student||Goldstein Nir|
|Subject||Phosphate Removal as Pretreatment for Wastewater|
Desalination to Improve Recovery
|Department||Department of Civil and Environmental Engineering||Supervisors||Professor Carlos Dosoretz|
|Professor Emeritus Raphael Semiat|
|Full Thesis text|
Water scarcity is becoming a serious problem in the Mediterranean countries. Wastewater desalination is a main option to enhance water supply to agriculture. A major difficulty is scale deposition on the reverse osmosis membranes. Calcium phosphate scaling imposes a severe limitation on water recovery from wastewaters. The chemical precipitation method to remove phosphates as slightly soluble salts is a common treatment method. The goal of this research was to examine phosphate removal methods as pretreatment to wastewater desalination. The study was performed in a 120 m3/d capacity demonstration pilot plant constructed at the Nir Ezion wastewater treatment site. The desalination system incorporates one ultrafiltration stage and two reverse osmosis stages planned to work at a high recovery ratio (≥85%). The treatment methods examined were chemical precipitation with iron based salts. The pilot plant was initially run without a pretreatment to remove phosphate and it was found that calcium-phosphate scaling occurred on the membranes of the RO second stage.
Simulation by the MINEQL software of different stages of the pilot showed that precipitation of calcium-phosphate would occur at the RO second stage. Several iron based salts were tested in laboratory by jar test experiments for their ability for chemical clarification of phosphate. It was found that a Fe:P weight ratio needed with FeCl3 was 6:1, higher then expected from the literature (3:1), because of presence of organic materials which inhibit strengite precipitation. The second salt checked was FeCl3 with adding base to pH>6 to increase the particle sizes by an order of magnitude. The Fe:P weight ratio needed was as high as 20:1. Another option that was checked was FeCl3 with tannic acid as flocculation aid. Adding tannic acid to precipitation with FeCl3 did not affect the phosphate removal as compared to FeCl3, but increased the particles size by two orders of magnitude. Chemical precipitation experiments at the pilot showed that precipitation with FeCl3 did not achieve a sufficient removal of phosphate because of the small size of the particles. Dosage of FeCl3 with added base to pH>6 achieved a sufficient removal of phosphate at high Fe:P weight ratio ~ 20:1.
In conclusion, phosphate removal pretreatment is needed between the stages of reverse osmosis. A possible treatment method is chemical precipitation using FeCl3 with added base to pH>6, but a high Fe:P weight ratio is needed. Another possible coagulant that needs to be further examined is FeCl3 with tannic acid.