The Challenges of Identifying, Evaluating and Implementing Antiscalants in RO Reuse Applications: So Many Choices-So Few Winners

摘要

Reverse Osmosis (RO) systems today offer many advantages over their predecessors. New membrane materials, pretreatment chemicals, and improved system designs have all contributed to enhanced efficiencies. RO systems now exhibit improved rejection, enhanced permeability and higher recoveries. Advancements in antiscalant technology have enabled design engineers to push the limits of production - achieving high recoveries void of RO tail-end mineral scaling. Antiscalant selection is based on the type of feed water and concentration of minerals. In treating microfiltered secondary effluent, many of the potential scale-forming constituents are well controlled. Phosphates are often present in elevated concentrations, which can lead to calcium phosphate precipitation. Since phosphate is a multivalent and can be present in different forms (and solubilities), pH suppression is typically employed for control. The past few years has seen an increase in antiscalant products claiming to exhibit calcium phosphate control under increasing alkaline conditions. The Orange County Water District (OCWD) currently operates a 70mgd RO system as part of the Groundwater Replenishment System (GWRS) - an advanced wastewater treatment facility. Each of the 15, 5mgd RO units consists of a 3-stage array, operating at 85% recovery. As part of the operating strategy, both antiscalant and pH suppression are used to limit S^-stage precipitation of sparingly soluble mineral salts. Identifying successful antiscalants is extremely challenging. Unlike other bulk chemicals (e.g., acids and bases), a variety of antiscalants exist with varying chemistries and capabilities. These differences present opportunities for the end-user to optimize their RO process. The combination of product pricing, dosage, and RO pH adjustment can all equate to significant cost-savings if successfully identified. Despite vendor claims, product performance is ultimately dependant on water chemistry and plant operating conditions. Successful performance at one facility in no way guarantees effectiveness at another. Recognizing the potential savings in RO pretreatment chemical cost, OCWD began pilot-scale trials of antiscalants in 2009. Evaluations were conducted as specified in a RFP - with the lowest costing product (antiscalant price, dosage, and acid for pH adjustment) evaluated first. Failure to identify a successful product led to the cancellation of the RFP and a revamping of the evaluation process. Concurrently, the GWRS RO facility began experiencing 3rd-stage permeability loss in all 15 RO units. The antiscalant was ineffective in completely controlling mineral scaling, including calcium phosphate and silicates. This resulted in fast track testing to identify effective antiscalants. This paper will discuss the evaluation process and outline the steps taken when switching antiscalants in the GWRS RO facility. Ensuing RO performance and monitoring efforts will also be discussed.