Membrane Technology

For the preservation of the affectivity and life span of a Reverse Osmosis (RO) installation, a sufficient pre-treatment is required. A proper selection of pre-treatment methods for feed water will improve affectivity and extend the life span of the system by preventing or minimizing biofouling, scaling and membrane plugging.

To perform an uninterrupted and reliable pre-treatment of the feed water a special approach is used. A pre-treatment that is not geared to the installation may cause a system overload. When this occurs the system parts need cleaning much more often to restore productivity and salt retention. Cleaning costs, system performance and standstill time are very significant in that situation.
The kind of pre-treatment system that is used greatly depends on feed water quality. Consequentially, sufficient feed water pre-treatment is dependent on:

· The source of the feed water
· The composition of the feed water
· The function of the feed water

When the source of the feed water that needs treatment is specified, a complete and exact water analysis is performed. This action is an important step for the design of a pre-treatment system and the entire Reverse Osmosis system, because this often determines the type and size of the pre-treatment.

Feed water analysis

Most water types that are treated in a Reverse Osmosis system are either:
· Brackish water with a low salt content and a total dissolved solids content of up to 5,000 ppm, or
· Brackish water with a high salt content and a total dissolved solids content of between 5,000 and 15,000 ppm, or
· Seawater with a total dissolved solids content of around 35,000 ppm

Seawater with a total dissolved solids content of around 35,000 ppm is regarded standard seawater, because of its global abundance. Globally, the differences in total dissolved solids content of seawater can be very sufficient. The Baltic Sea for example, has a total dissolved solids content of about 7,000 mg/L, whereas the total dissolved solids content of the Red Sea is 45,000 mg/L.
Standard values for the concentrations of dissolved solids in seawater have been determined. During seawater analysis, one should always keep in mind that land influences are of great significance for the composition of seawater. A sample that is taken on open sea can have a very different composition than a sample that is taken near the shore.
The limiting factor for seawater treatment with a Reverse Osmosis system is the osmotic pressure, caused by a high total dissolved solids content. The limiting factor for the treatment of brackish water with a Reverse Osmosis system is mainly its chemical nature. This means precipitation and scaling (caused by calcium carbonate or sulphates). The chemical composition of brackish waters varies greatly and is very location specific.
To create a satisfactory process design, a very accurate water analysis should be carried out. On a water analysis form the concentrations of inorganic salts and other data of the feed water are collected. The water analysis should be fully balanced. When this is not the case sodium or chlorine ions should be added, in order to neutralize the water balance.

Scaling prevention and control

Scaling is the accumulation of (partially) insoluble salts in a membrane. When a Reverse Osmosis installation has a recovery of 50%, the concentration of salts in the concentrate flow is double the concentration of salts in the feed water flow. When recovery increases, the chances of scaling increase, as well. Because of this it is of great importance that the saturation limits of the (partially) insoluble salts are not exceeded.
Partially insoluble salts that cause problems in a Reverse Osmosis system are mainly calcium carbonate (CaCO₃), calcium sulphate (CaSO₄) and silica, but also calcium fluoride (CaF₂), barium sulphate (BaSO₄) and strontium sulphate (SrSO₄).

Calcium carbonate scaling can be prevented by the addition of acids. Adding anti-scalents can prevent the precipitation of barium and strontium salts, silicates and iron. One must always keep in mind that anti-scalents can add to biofouling (microrganisms fouling). This can be prevented by tightening the demands of applied chemicals and by using chemicals with a different composition.
The use of anti-scalents cannot always be avoided. Decreasing recovery by 25% will reduce the chances of over-saturation of precipitating salts (scaling).
The membrane elements will need replacement within one or two years, because that causes the use of anti-scalents to be limited. This method is applied for relatively small installations, like installations that produce drinking water from seawater for domestic use.
Another cleaning method is opening the concentrate lid to a forward flush under low pressure.

Prevention of fouling by colloids

In Reverse Osmosis elements colloidal pollution can seriously diminish the performance by decreasing productivity. An early sign of this pollutant is usually an increasing pressure gradient. The sources of this pollution in feed water can vary greatly. They are usually bacteria, clay, and iron corrosion products.
Chemical products that are used during pre-treatment may also cause fouling of membranes. The best available technique for the determination of the fouling potential of feed water by colloids is the MFI (Modified Fouling Index) measurement. This is an important type of measurement that takes place prior to the design of a pre-treatment system. This measurement must be done regularly when the Reverse Osmosis system is put to use.

In order to maintain a MFI of (less than) five, there are a number of pre-treatment methods to decrease the MFI:
· Sand filtration and candle filtration
· Ultra filtration and micro filtration
· Coagulation and flocculation
Biofouling prevention

All surface water contains microrganisms, algae, fungi, viruses and higher organisms. Microrganisms are colloidal pollutants, because they are never larger than 1 to 3 nm. Because of this, bacteria can be removed with techniques that remove colloids.
The consequences of biological growth within a membrane system negatively influence the system, just like other contaminants. The symptoms of fouling can vary by using a higher pressure at the input side of the membrane. With a higher pressure the membrane will telescoping (shoving apart inside the membrane) and will make membrane damage. Sometimes bio film can develop in the permeate; causing it to become polluted again.

The number of microrganisms in the surface water, in the feed water and in the concentrate can provide us with valuable information on the degree of contamination of the water. The types and amounts of nutrients present in the feed water are factors that determine bio film growth. Despite the fact that there are several investigators that determine the growth of bio films, it has not been fully comprehended yet.
The best way to discover biofouling during its development phase is to check a testing surface in the feed water flow. The ‘Robin Sampler’ is a simple device, which exposes small testing surfaces to water. These surface check-ups can be performed regularly and can be tested for the growth and attachment of bio film.
A careful periodic inspection of candle filters and pipelines can also be useful. The presence of mucous or strange smells can be an indication of the occurrence of bio film.