GrahamTek 16 Inch Spiral Wound RO Technology

The GrahamTek RO system is a new departure for RO technology. The hydraulic conditions are vastly improved, and the sustainable flux is greatly increased. This means lower investment (CAPEX), reduced operating costs (OPEX) and smaller environmental impact due to lower use of chemicals.

These effects are achieved by:

The use of 16 inch diameter elements
Vastly optimized hydraulic conditions
Maximum of 4 elements per pressure vessel
A 2:1 pressure vessel array
A patented Flow Distributor
A patented Electro-Magnetic Field (EMF) Device
A unique equipment design.

The combined effect of these parameters and devices allows operation of the system at extremely high fluxes, most often at least 50% higher and in some cases up to 150% higher than for conventional 8 inch RO technology.

In reverse osmosis of seawater (SWRO), GrahamTek systems operate at sustainable fluxes of 13 to 16 GFD (22 to 27 lmh) at system recoveries of 40% to 50%. If polishing of the permeate is necessary to achieve specific parameters, a low pressure RO (LPRO) system can achieve this. Recirculation of the concentrate from the LPRO system to the feed of the SWRO system will maintain the recovery rate.

In a LPRO system treating secondary municipal wastewater, surface water or brackish water, GrahamTek RO systems typically operate at approximately 24 GFD (40 lmh) and a recovery of 75%.

In comparison to conventional systems, some added advantages of the GrahamTek RO system are:

Higher system salt rejection
Higher system rejection of organic material
Smaller foot print
Lower energy consumption
Reduced usage of chemicals.

The following paragraphs provides some basic explanations and design guidelines for the GrahamTek RO system. It should always be borne in mind that it is always safest to design plants based on the results derived from realistic pilot tests.

Increased Flux in GrahamTek RO Systems

Every membrane filtration system has a critical flux, which is determined by the nature of the feed and the set of operating parameters. The critical flux is defined as the point where the combination of the nature of the feed and the set of operating parameters result in a loss of flux due to fouling, scaling or particle deposit on the surface of the membrane.

Manufacturers supply membrane elements with a set of wet test parameters obtained via tests on fouling and scaling free water. The recommended operating parameters, that will create a sustainable flux, are chosen to fall below the critical flux to ensure stable operation, in most cases these may also be below the wet test parameters.

For instance, an LPRO element, 8 inch or 16 inch diameter, may show 30 GFD (51 lmh) flux in the manufacturer's wet test for a given membrane type. In the case of 8 inch diameter elements, the critical flux may be 12 GFD (20 lmh) under a normal set of operating parameters for secondary wastewater. If the membrane was incorporated in a conventionally designed 16 inch diameter element, the results would be identical, or even a little lower. The operating parameters would typically be chosen at 10 GFD (17 lmh) with the purpose of operating with a safety margin.

In the GrahamTek equipment design, the Flow Distributor, the EMF and the hydraulic conditions radically change the critical flux and thereby the sustainable flux. In the case of secondary wastewater, the critical flux is increased to an estimated 28 GFD (48 lmh), and the sustainable flux can be comfortably chosen at 24 GFD (41 lmh).

Similar conditions are valid for seawater desalination. The wet test data for a seawater element often indicate 20 GFD (34 lmh) flux. Most SWRO plants using 8 conventional inch diameter element operate at a 6 to 10 GFD (10 to 17 lmh) sustainable flux. GrahamTek SWRO equipment consistently operate at 13 to 16 GFD (22 to 27 lmh) sustainable flux.

Fouling, Scaling and Particle Deposits

The difference between critical flux (and thereby sustainable flux) in conventional RO plants using 8 inch diameter elements and GrahamTek RO plants is caused by the much improved control of fouling, scaling and particle deposits on the elements.

The GrahamTek Flow Distributor provides a set of improved hydraulic conditions, which greatly reduce or completely eliminate microbial fouling, which normally happens in the front end of the plant. This is often the limiting parameter for sustainable flux in conventional equipment using 8 inch diameter elements. GrahamTek equipment creates improved hydraulics by an even distribution of the feed flow, generation of micro-bubbles providing a scouring effect, and a swirling movement of the feed providing higher shear forces on the membrane surface.

The GrahamTek EMF works for microbes, particles and scale. Microbes can be considered to be particles, and the EMF will either cause aggregation and thereby reduce the fouling potential and prevent them from depositing on the membrane surface. The EMF also changes the morphology of potentially scaling substances, which can form in the last few elements of the vessel, where the solubility of the substances may be exceeded. For instance, calcium sulfate will precipitate as a 'fluffy' substance, instead of a crystalline substance, which can form a boundary (or cake) layer on the membrane. The 'fluffy' substance is easily swept away by the concentrate flow without depositing on the membrane surface.

The GrahamTek Equipment Design, using a maximum of 4 elements per pressure vessel, ensures optimum concentrate flow, and also provides unique possibilities with respect to maintaining operational flux by the simple means of 'cleaning-on-line' without interrupting the production flow, thus eliminating the adverse effect of particle deposits resulting from inadequate pretreatment of the feed to the equipment.

Cleaning Requirements

It is a generally accepted proposition for conventional RO equipment using 8 inch diameter elements, that a thorough cleaning procedure should be performed, when the plant has lost 10% to 15% flux. This implies that for this type of equipment a flux loss due to fouling, scaling and particle deposits is an accepted fact of life.

GrahamTek RO equipment presents the totally new and unique possibility of maintaining a constant flux over long periods of time without performing tedious, expensive and time consuming cleaning procedures. GrahamTek RO equipment has proved to be able to perform on only scantily pretreated feed water, but the operation will, of course, be better on properly treated feeds.