ROTEC Technology

Increasing the recovery rate of a RO system is fairly simple, requiring no structural changes to the system aside from increasing system pressure and minor operational adaptations. However, there are significant obstacles to increasing the recovery beyond the typical rates. Mineral scaling, the process in which mineral deposit forms on the membrane surface of a RO element, causing membrane plugging and loss of water permeability, is the main obstacle in BWRO desalination. It sometimes occurs even with standard recovery operations. Increasing recovery rates leads

to numerous operational issues, such as more frequent membrane cleaning, and consequently, a shorter membrane life span. Therefore, the addition of expensive anti-scalant chemicals is required to inhibit and moderate the effects of this phenomenon. In SWRO systems, osmotic pressure issues limit recovery rates more than mineral scaling as the feed water contains relatively low concentrations of sparingly soluble salts, leading to less scaling. SWRO suffer mostly from another type of fouling (mainly biofouling).

ROTEC made further improvements to Flow Reversal technology to adapt it to industrial desalination settings, which are dominated by multi-stage RO systems that use a tapered flow arrangement of pressure vessels (PVs) to maintain correct hydraulic flows in the PVs of sequential stages.

The tapered flow arrangement of PVs involves placing fewer pressure vessels in parallel in downstream stages than in upstream stages. With the lower amount of feed flowing in the downstream stage divided between fewer PVs, a minimum crossflow rate is preserved in the downstream pressure vessels. This helps prevent fouling by colloids, organics and biomaterials, while reducing the concentration of salts at the membrane surface. Simultaneously, it prevents too large a stream from flowing into the individual pressure vessels of the first stage.

A simplistic flow reversal method, in which the feed is fed to the original last stage and concentrate is removed from the entrance of the original first stage, would cause the flow to be too high into the last-stage pressure vessels and too low out of the first-stage pressure vessels.

To prevent this, ROTEC developed a patented approach in which individual blocks can be repositioned between the stages, so that the block of pressure vessels from the last stage is switched with a block of equal number of pressure vessels from the first stage by using a sophisticated array of valves and valve sequencing.