The 70 MGD (264,950 m3/d) system – which employs microfiltration, RO and UV/H2O2 to treat secondary effluent – had experienced numerous silica-scaling incidents, which had forced them to resort to the use of ammonium bifluoride as an RO membrane cleaner.
AWC’s research found a link between phosphate scale and silica formation and rhe findings were outlined in a paper entitled ‘A relationship between phosphate scaling and silica fouling in wastewater RO systems’, which was presented by AWC technical director Mo Malki at the 2011 IDA World Congress in Perth.
They also found that variations in iron coagulant carryover had a direct impact on antiscalant demand. A comparison of OCWD feedwater and operational history confirmed a correlation between surges in ferric ion levels and silica scaling events.
In response to the problem, AWC designed its new A-110 antiscalant to control high phosphate and silica scales in the presence of ferric ion carryover. OCWD began piloting the antiscalant in August 2011 and continued testing at different temperatures and varying pH levels. By early 2014, it was determined to effectively inhibit scale formation in the pilot unit at a dosage of 3.5 mg/L and a pH of 6.9.
The OCWD is currently in the process of expanding the GWRS’s capacity to 100 MGD (378,500 m3/d).
