Conservation of water resources and the minimization of discharges are paramount in today's industries. The reuse of process water effluents plays an important role in conserving water. This paper investigates the recycle of effluent process water from a metal finishing operation using reverse osmosis to provide make-up water for reuse. A case history demonstrates how membrane separation is used to produce water suitable for use and to minimize or eliminate aqueous discharges. This paper also examines the use of pretreatment chemicals to minimize membrane fouling
The use of reverse osmosis (RO) to remove salts and impurities from water has been a recognized technology to improve water quality. Design consideration of reverse osmosis systems is dependent on dissolved solids, organic, and suspended solids content. Naturally occurring ions comprise the total dissolved solids (TDS) of natural waters. RO design consideration must take into account the amount of TDS, or ionic strengtE and desired water product quality. RO is used to produce a variety of high purity needs including industrial boiler feed, pharmaceutical waters, electronic industry supply, and other process industries.
RO is also used in food processing of milk, beverages, vegetable and fruit juices and meat by-products. ~1~ The RO process is used in water consolidation and waste minimization either by itself or in combination with microfiltmtion (MF), ultrafiltration (UF), evaporation or other water processes. The use of RO in wastewater is a valuable application because permeate water and can be reused and wastewater stream becomes a resource in the process. The benefits include reduced discharge, reduced purchases, and conservation of water resources.
Reverse Osmosis Process
The performance of an RO system is largely controlled by the composition of the feed water. Feed water quality will determine the amount and the type of pretreatment necessary to make an RO an economical process. The balance is the primary limiting factor of most RO systems in operation today.
The major cause of RO system failure is membrane fouling, which results from the accumulation of foulants on the surface of the membranes. The fouling of an RO membrane is a complex phenomenon involving the deposition of several different but related types of foulants on the membrane surface. RO system fouling problems are becoming more prevalent as the use of low quality feed waters increases. In addition, surface water treated with cationic organic flocculants poses very different and challenging fouling problems. Operating costs increase when performance problems arise. These costs are those associated with membrane cleanup, replacement, and system downtime. The success of an RO system depends largely on three factors: system design, petreatment (including chemical conditioning), and operation and maintenance of the system.
The types of foulants most commonly encountered in an RO system include: inorganic fouling (scaling), colloidal fouling, biological fouling, and organic fouling. The high levels of dissolved scale forming ions in most feed water necessitate some form of pretreatment to control scale formation on the membrane surface. Colloidal fouling is typically controlled by mechanical filtration and/or the use of dispersants.
In RO, total solids are reduced through membrane separation. The concentrate, or reject, contains more highly concentrated ions than are found in the feedwater. The membranes typically remove 95 to 99 percent of the dissolved salts? The concentration of TDS, total suspended solids (TSS), and other water born contaminants can lead to fouling and decreased efficiency