Reducing operational costs in high silica, 18,000m3/day BWRO in Arica, Chile

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Abstract

The desalination industry has significantly evolved in the last decade with 33million m3/day of additional desalination capacity now operating. One third is using membrane technology with 6,458 Reverse osmosis plants coming on-line. More than twice as many brackish water RO plants than sea water plants have been built since the first plants were commissioned in the late 1960’s. The use of groundwater as a source for RO plant will continue to expand as the world’s resources diminish. A limiting factor of operational efficiency of brackish water RO plant is the recovery rate or percentage of product water recovered from the feed stream which is determined largely by the feed water quality at the design stage. Due to a shortage of good quality water in many regions of the world, RO plants are forced to operate with very high levels of silica. Silica chemistry is complex; deposits can be found on the surface of reverse osmosis membranes in a polymeric or crystalline form, and can also reach the membranes as colloidal particles. Traditionally reverse osmosis systems have been operated at low recoveries with frequent cleaning and reduced membrane life as conventional antiscalants are limited in their ability to inhibit silica fouling.

Arica Desalination plant, has a capacity of 750 m3/h, was built by OHL Medio Ambiente INIMA and started its operation in 1998. The plant is fed from 11 wells located at Lluta Valley characterized by high chloride levels (average concentration over 800 mg/L) and dissolved solids (average concentration over 3.000 mg/L). Feed water quality is variable as the wells have differing geology and chemistry and extraction is dictated by availability not quality. Silica levels in the feed water have increased from the design level of 196 mg/L silica in reject to as high as 300 mg/L of silica in reject. Consequently fouling rates were higher than initially predicted which lead to higher pressure, more energy, frequent cleaning and unscheduled plant downtime, making it difficult to comply with contractual water production targets. This paper describes the analytical and on-site research work carried out to better characterize factors negatively affecting plant performance. Silica inhibition and cleaning strategies adopted in this plant to minimize the fouling problems identified are explained. The effectiveness of different strategies adopted is reviewed using real operational data available from 1999 to 2010.

The desalination industry has focussed recently on improving systems energy efficiency and innovations in this area tend to capture the news headlines. New generations of “low energy” membranes and new developments in energy recovery devices, are regular topics at conferences and in the media. In this paper cost savings of operational strategies adopted in Arica Desalination plant are calculated and presented. The importance of operational practices in reducing energy consumption is demonstrated. The use of actual silica inhibition and cleaning strategies shows that membrane desalination in very poor quality waters can be a cost effective affordable solution. Results show a significant reduction in cleaning frequency from more than six per year to once per year and reductions in energy consumption of 0.06kWh/m³.

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