Optimization of hybridized seawater desalination process

Abstract

Saline Water Conversion Corporation (SWCC), Research and Development Center (RDC) came up in recent years with a breakthrough in feed treatment to all desalination processes. Selective ions, biomass and suspended solids rejection characteristic of nanofiltration (NF) membranes were utilized to retard the ingress of hardness forming ions and other foulants into desalination downstream processing steps, e.g. SWRO and MSF. NF pretreatment using brackish water hardness removal membranes were applied to seawater pilot, demonstration and eventually commercial desalination plants. A good number of such membranes were tested in order to select the best. The screening scheme revealed couple of interesting facts related to characteristics of various brands and grades of NF membranes. In an earlier stage of this multi phase project these membranes were categorized as groups A, B and C. These groups in the same order were found to give from high hardness and salinity rejection with low flow and product recovery down to low rejection with high flow and recovery. Based on the above findings, it was decided that the most suitable membranes would be those of group B which could give good flow and recovery with reasonable rejections. Membranes of group B were tested at various plant sizes, production schemes and configurations. This paper will concentrate on 4, 8 and 9 inch membrane elements used in NF or NF/SWRO combination to provide make-up to pilot plant MSF of 20 kl/d distiller. Such hybridized production configurations were optimized by testing overall performance. Few prime parameters were closely analyzed to come up with the best combination of NF membrane brand and their operating parameters along with MSF and/or SWRO—MSF hybrid conditions. In the latter case SWRO reject was used as make-up to the MSF pilot plant distiller. Moreover, the testing program locked into further hybridization, whereby MSF heated reject water was used as feed to NF pretreatment section. This was done to establish the effect of feed temperature rise benefits on NF and/or NF—SWRO energy input, yield(s) and product(s) flow and quality. The MSF operation was monitored and its operating results were analyzed. Main components of such analyses were gain output ratio (GOR), fouling factor (FF), top brine temperature (TBT), chemical consumption and product to make-up yield ratio and its quality.