Pre-treatment / Pre-filtration for Water Purification equipment.
Proper pre-treatment for water treatment is a very critical phase in water purification and water treatment. Pre-filtration when using Reverse Osmosis will prevent scaling, premature RO membrane failure and unplanned maintenance. Below is common problems identified due to lack of proper pre-treatment.
Fouling in reverse osmosis occurs when contaminants accumulate on the membrane surface effectively plugging the RO membrane. There are many contaminants in municipal water that aren’t visible to the human eye and harmless for human consumption, but large enough to quickly foul (or plug) an RO system. Fouling can result in a higher pressure drop across the RO system and a lower permeate flow. This translates into higher operating costs and eventually the need to clean or replace the RO membranes. Fouling will take place eventually to some extent given the extremely fine pore size of an RO membrane no matter how effective your pre-treatment and cleaning schedule is. However, by having proper pre-treatment in place, you will minimize the need to address fouling related problems on a regular basis.
The following can cause fouling:
1. Particulate or colloidal matter (dirt, silt, clay, etc.)
2. Organics (humic/fulvic acids, etc)
3. Microorganisms (bacteria, etc). Bacteria present one of the most common fouling causes, reason being, a disinfectant such as chlorine cannot be used on the Reverse Osmosis membranes, this enables the microorganism to thrive and multiply on the membrane surface. This may produce biofilms that cover the membrane surface and result in heavy fouling.
4. The upstream movement of the filter media into the Reverse Osmosis unit. The Reverse Osmosis system can foul without adequate post filtration caused by an underdrain leak developed by the GAC (Granular Activated Carbon) beds and softener beds may develop an underdrain leak and if there is not adequate post filtration in place the media can foul the RO system.
By performing analytical tests, you can determine if the feed water to your RO has a high potential for fouling. To prevent fouling of an RO system, mechanical filtration methods are used. The most popular methods to prevent fouling are the use of multi-media filters (MMF) or microfiltration (MF). In some cases, cartridge filtration will suffice.
Certain dissolved (inorganic) compounds become more concentrated then scaling can occur if these compounds exceed their solubility limits and precipitate on the membrane surface as scale. The results of scaling are a higher pressure drop across the system, higher salt passage (less salt rejection), low permeate flow and lower permeate water quality. One example of a common scale that tends to form on an RO membrane is (CaCO3) calcium carbonate.
Modern thin film composite membranes are not tolerant of chlorine or chloramines. Oxidizers such as chlorine will ‘burn’ holes in the membrane pores and can cause irreparable damage. The result of a chemical attack on an RO membrane is a higher permeate flow and a higher salt passage (poorer quality permeate water). Therefore microorganism growth on RO membranes tends to foul RO membranes so easily since there is no biocide to prevent its growth.
Part of the prefiltration scheme should be pre and post RO system plumbing and controls. If ‘hard starts’ occur mechanical damage to the membranes can occur. Likewise, if there is too much backpressure on the RO system then mechanical damage to the RO membranes can also occur. These can be addressed by using variable frequency drive motors to start high-pressure pumps for RO systems and by installing check valve(s) and/or pressure relief valves to prevent excessive back pressure on the RO unit that can cause permanent membrane damage.