ANTISCALANT CAPABILITIES FOR MINERAL SCALE CONTROL IN RO REUSE APPLICATIONS

摘要

An important component in any reverse osmosis (RO) plant design is identifying an appropriate antiscalant for the treated feed water quality and RO design recovery. Antiscalant selection is largely based on identifying products with the ability to prevent precipitation of a selected few problematic mineral scales. Calcium phosphate often has a high scale potential in advanced water treatment plants using RO to treat microfiltered secondary effluent. Historically, RO plant designs have relied upon pH suppression to control calcium phosphate scale. In recent years, many proprietary chemical vendors have developed antiscalants that claim effectiveness at controlling calcium phosphate deposition in the tail end of the RO system,where concentrations typically exceed the solubility limits. Orange County Water District (OCWD) began considering antiscalant capabilities for calcium phosphate scale control in 2006, while operating the 5mgd demonstration plant - a predecessor to the 70mgd Groundwater Replenishment System (GWRS). At that time,a substantial increase in sulfuric acid cost made reliance on antiscalants to control calcium phosphate an attractive process optimization effort. OCWD employed a competitive bid process for antiscalants that required vendors to provide the antiscalant product, dose, and RO feed pH setpoint. These parameters would allow optimal performance of the antiscalant in preventing scale formation, especially in the tail end of the RO system. Employing the same procurement method in 2009, after the full-scale GWRS came online, OCWD found that several products claiming calcium phosphate and silica scale control were not effective. Additionally, through operation of the full-scale facility during the summer months (when feedwater temperatures reached 30C), the incumbent antiscalant was not completely effective at controlling calcium phosphate because of the temperature dependent nature of the precipitant. It became clear that many vendors' claims of product effectiveness were inaccurate - as several products were unable to completely control calcium phosphate and silica scale. As a consequence, OCWD began independent evaluations of antiscalant products using its own set of prescribed conditions for dose, RO feed pH and recovery. This effort will determine which products are capable of allowing operation at elevated RO feed pH setpoints; simultaneously identifying acceptable antiscalant products and optimizing sulfuric acid consumption. Initially conducting these independent trials outside of a competitive bid process alleviates the inherent influence of price and requirement to test the lowest bidder's product. This paper will discuss and compare results from the various trials performed by OCWD in an effort to identify antiscalant products that are capable of offering stable performance at the GWRS conditions for RO feed pH and recovery.