Reverse osmosis membranes act as very effective filters. Therefore any suspended solids, colloids or heavy metals entrained within the water run the risk of fouling the membrane surface. An effective pre-treament system to remove these impurities before the water gets to the membranes is essential. The pre-treatment system will be designed to take into account the feed water and water quality needs.
Multi-media filtration (MMF)
This level of filtration removes course particles in the 20-25 micron range. It is suitable for a variety of intake waters. These filters use multi-layers of different media including various coarseness of sands, pumice, anthrice or garnet as filters.
Microfiltration is designed to remove particles in the 0.1 - 1 micron range. The microfiltration process removes contaminants through a microporous membrane. These membranes affectively remove pathogens and large bacteria.
Ultrafiltration is another membrane filtration process not fundamentally different from microfiltration except in terms of the size of molecules it retains. The pores of ultrafiltration membranes can remove particles of 0.001 - 0.01microns from fluids. This removes most viruses, bacteria, colloids and silts (SDI) from the intake water.
Desalination is the removal of dissolved salts from a solution. This can be achieved in a number of ways, with the most common forms falling into three main categories - membrane, chemical or thermal processes. Reverse Osmosis (RO) is a process that uses a pump and a semi-permeable membrane to cause the separation of dissolved salts from a liquid. The pump provides the driving force to the membrane that is greater than the osmotic pressure of the liquid. The semi-permeable membrane allows water and some ions to pass, but retains most of the dissolved salts. The reverse osmosis membrane also removes more than 98% of residual biological and colloidal matter from the feed water, resulting in a highly purified product stream. Osmosis is the natural process which occurs when water spontaneously flows from a purer solution, through a semi-permeable membrane into a more concentrated solution. Osmotic pressure is the pressure associated with osmosis. Reverse Osmosis is used for desalting sea water (SWRO) and brackish water (BWRO).
Membrane Bioreactor (MBR)
The term MBR describes the two stages of treatment that includes an activated sludge bioreactor to process BOD and nutrients and then a membrane barrier to separate solids from water. Membranes can be provided in the MF and UF pore sizes depending on the reuse application. MBR is suitable when a high level of treatment is required for reuse water, for example where high risk of human contact exists and therefore high levels of downstream disinfection is required.
Moving Bed Bioreactor (MBBR)
The term MBBR describes treatment that includes a bioreactor that contains loose plastic media to which bacteria attach and grow. This bacteria uses BOD and nutrients and reduces their concentration in water. The MBBR is followed by a clarifier to separate solids from water. MBR is suitable when a medium level of treatment is required for reuse water, for example where moderate risk of human contact exists. The MBBR process is more compact than activated sludge processes of the same capacity.