Pipe Static Mixers used in the Desalination Process

The Statiflo Motionless Mixer is a vital component of any inline mixing/control system. Even the most sophisticated control systems will not perform completely effectively if the measuring equipment fails to see a representative sample. Process control systems are usually very expensive, and despite the mixer being only a small fraction of the total cost, its role as an essential component, vital to plant performance, is often overlooked.

Installing a Statiflo Motionless Mixer allows sampling immediately downstream within very few diameters of the injection point. This allows the system to rapidly adjust to monitored process changes with negligible time lag and minimum use of dosing chemicals.

Raw sea water is first course filtered and then treated with coagulant/floculant before secondary fine filtration. The coagulation stage has a vital role to ensure efficient performance of the fine filtration equipment, particularly during periods of algal bloom. Inadequate coagulation can result in:-

Rapid build-up of sand filter pressure drop
Excessive sand filter backwash
Premature blocking of cartridge filters
Algae breakthrough from the fine filters to the membrane section.

Statiflo motionless mixers are frequently used in the coagulation process to avoid these problems. Initially, the mixer distributes injected coagulant into the water stream using a gentle low shear rolling action, promoting floc growth and re-contacting. This mechanism does not rely on relatively high shear rates, commonly associated with agitators, which would increase the risk of floc degradation.

Complete and predictable mixing avoids the need to overdose. Not only are chemical savings possible, but the intact polyelectrolyte molecules have maximum effect on the floc growth and improved filtration rates.

A retrofit Statiflo Motionless Mixer was installed on Jeddah ll SWRO plant were it was reported to have transformed the coagulation/Flocculation process, vastly improving plant performance and capacity.

In areas  where salt concentration are particularly high, membrane efficiency is improved by diluting fresh feed with recycled desalinated water. Desalination can then process at lower osmotic pressure, which has the knock on effect of reducing the design pressure of the complete system.

The use of Statiflo Motionless mixers ensure complete blending to allow high efficiency membrane performance.

It is often necessary to adjust the pH before chlorination to maximise the effectiveness of disinfection. There are several common applications.:-

The first is the continuous dilution of concentrated sulphuric acid or sodium hydroxide to manageable levels. The Statiflo Series 300 Mixer has all wetted parts in PTFE construction to withstand corrosion and heat dilution.

The diluted pH correcting chemicals are then dosed upstream of larger Statiflo Series 500/550 or DSM Motionless Mixers, usually in GRP construction, to uniformly mix throughout the treated water prior to representative sampling of the pH.

Systems can be incorporated with pH probes, controllers and microprocessors for effective automatic control with both feedback and feed forward control loops.

Representative sampling is vital to ensure effective control and to minimize the risk of under and over chlorination, especially at plant start up and low flow conditions. Statiflo Motionless Mixers can be provided with both injectors and sampling points as integral parts of the equipment, for short time lag and rapid response to system changes.

Desalinated water produced in both SWRO and MSF processes has low soluble carbonate salt levels and is recarbonated before distribution. Two methods are available:-

Primary recarbonation involves the injection of large volumes of carbon dioxide gas into water to raise its pH and ensure that adequate levels of carbonate concentrations are maintained. The Industry has traditionally used large diameter contact towers, but more recently Statiflo Motionless Mixers have been adopted to provide a cost effective inline method of mass transfer. This approach ensure rapid dispersion of of gas and very efficient mass transfer the Statiflo Gas Dispersion System offers extremely high efficiencies where water flow rates vary.

Secondary recarbonation, in the final treatment stages, involves dosing saturated lime solution or milk of lime suspension into the desalinated water to simultaneously increase pH and carbonate concentration before consumption. This process is referred to as ‘potabilisation’, particularly on MSF plants. Lime solution/suspension is injected at the inlet of a Statiflo Motionless Mixer for complete mixing for the pH and recarbonation reactions. Under some circumstances, chlorine is also injected at this location.