The PSI crossflow microfiltration process occurs in an array of permeable textile tubes. Manifolds are cast onto each end of the cloth filter to form modules which are connected to a pump for liquid inlet and to a back pressure valve at outlet. Upon introducing liquid flow into the tubes and regulating outlet pressure, suspended and colloidal matter in the liquid to be treated forms a thin film cake layer on the internal surface of each tube. This layer is called a "dynamic membrane" for its membrane-like characteristics. Other terms used are "filter layer" and "pre-coat layer". Should the quantity of suspended matter in the feed liquid be insufficient to form a filter layer, a small amount of filter aid compound is added to the initial feed. Filter layers or membranes of widely different characteristics can be produced by using different treatment chemicals.
To become treated product liquid, or permeate, the feed water filters radially through the membrane layer and out of the textile tube walls for collection at the base of each filter module. The debris removed from the liquid becomes concentrated and is swept out of the tubes with the remaining liquid which is called reject concentrate.
It is from the longitudinal or "crossflow" passage of the feed liquid along the filter cloth tubes that the process derives its name. PSI crossflow micro-filtration plants are of modular construction employing a number of manifolded filter modules. Modules are connected together either in parallel or in series with each other.
Ease of cleaning is an important feature of the PSI crossflow microfiltration technology, distinguishing it from standard membrane microfiltration. In most cases, cleaning is simply a matter of momentarily stopping the feed resulting in tube collapse which causes the thin cake or membrane material to be dislodged and flushed out with the reject flow. In other applications, chemical cleaning in place is used. The core technology is based upon the highly specialized woven textile tubular array and its post weaving treatment as well as on the formation and maintenance of dynamic layers, or membranes, and cleaning techniques. A uniformly high quality permeate is achieved with the PSI crossflow microfiltration process. Removal of virtually all suspended solids down to about 0.1 micron has been demonstrated in countless laboratory and field trials. Other experimental work indicates that the system can be developed to produce a low pressure process to reject high molecular weight dissolved solids. The process has many advantages over conventional treatment process: Most liquids are treatable without the addition of filter aid chemicals Specialized membranes can be formed to produce the desired quality of treated liquid by using standard chemistry. PSI crossflow microfiltration is a low pressure process (20 - 35 PSI) with a low crossflow velocity (3 - 6 fps) The single stage treatment produces a treated liquid having no suspended solids greater than approximately 0.10 micron. Cleaning of the filter cloth is easily accomplished Water recovery is very high: Usually greater than 97% Coagulation, settlement, clarification and filtration occur in the same process - usually without the need expensive of polymers or coagulant additives Liquids of wide pH range, from strongly acidic to strongly alkaline, can be processed at temperatures up to 90¡C Extensive civil works and structures are not required Operation is simple either manually or in the automatic mode