Antiscalant Impacts Water Reclamation Efforts
Water reuse efforts on the West Coast are a priority and have the Groundwater Replenishment System (GWRS) of the Orange County Water District (OCWD) utilizing new technologies for water conservation. The GWRS uses a reverse osmosis (RO) system to purify water beyond drinking water standards. Purified water is used for groundwater aquifer recharge and prevention of seawater intrusion.
The GWRS RO system operates at 85 percent recovery to maximize reuse of the wastewater they are treating. However, at this high recovery, the RO process was experiencing frequent silica scaling that was not controlled despite the antiscalant product and pretreatment strategy. This prompted OCWD staff to use ammonium bifluoride (ABF) as a membrane cleaner. Not only is this chemical dangerous to the operators, but if used over a long period of time it could potentially damage membrane elements and/or related equipment. This led the District to look for an alternative antiscalant.
After two years of intense R&D, American Water Chemicals (AWC) was able to provide AWC A-110. AWC first used a series of analytical methods in a controlled environment in the lab. AWC then used historical feed water data to come up with a plan for a series of research experiments using synthesized water replicating what was to be expected in the field. A link was found between phosphate scale and silica formation. Numerous lab experiments determined that variations in iron coagulant carryover had a direct impact on antiscalant demand and performance. A comparison of OCWD feed water and operational history confirmed a correlation between surges in ferric ion levels and silica scaling events.
AWC A-110 was developed around OCWD’s complex source water, and was designed specifically for control of high phosphate and silica scales in the presence of ferric ion carryover. Piloting of the AWC A-110 product began in August 2011 at the District’s Engineering Research Center, and continued through different temperatures and varying pH levels for a total of three years. By early 2014, trialing successfully determined that the AWC A-110 was effectively inhibiting scale at a dosage of 3.5 ppm and an operating pH of 6.9. OCWD made the decision to begin using the AWC antiscalant in the full-scale, 70-MGD RO system based on significant operational and financial performance improvements. The increase in operating pH alone reduced the District’s annual acid consumption by ~$350,000 per year based on their current production.
Operating at a higher pH level would result in significant operational savings for the District as a result of reduced acid consumption. These savings will substantially increase once the District commissions its GWRS expansion project in 2015 and RO capacity is upgraded to 100 MGD. AWC’s next goal is to increase OCWD’s operating pH to 7.0, which would further increase savings to a whopping $1 million per year, based on 100-MGD capacity.
“We feel a great sense of accomplishment at AWC because our success was entirely based on years of scientific research that can now be applied to all other membrane water reuse facilities” said Mo Malki, the CEO of AWC. He also emphasized that recent R&D work had shown the AWC A-110 product to be extremely effective for controlling aluminum silicate scales. The results of that research were presented more recently in a paper titled “Controlling Aluminum Silicate Scaling in the Membrane Separation Processes” at the 2013 IDA World Congress in Tianjin, China.
AWC A-110 has now been operating at pH 6.9 on the full-scale RO system for a total of eight months without showing any signs of scaling or performance decline. When compared to the prior product, which operated at pH 6.8 but required cleanings for mineral scale control, this is a huge accomplishment.