AFM as a pre-treatment prior to membranes
AFM is used for the pre-treatment of water prior to membrane filtration, this may be either seawater, brackish water or freshwater.
The standard approach prior to membranes is usually one of more of the following;
- cartridge filtration down to 1 micron, in two stages
- UF ultrafiltration down to 0.03 microns
- sand filtration
Cartridge filters may be used on their own, however the system is expensive and labour intensive. Coarse cartridge filtration may be used in combination with sand filtration or prior ro UF to help take off some of the loading.
Sand filtration is the most common form of pre-treatment, but all sand filters will become biofilters which will lead to bio-coagulation of the sand grains and transient worm-hole channelling. Slow bed sand filters operating at 0.1m/hr filtration velocity depend upon biofouling. In effect the filters are biological filters and they work really well for the application. However slow bed sand filters take up a huge amount of space, so most sand filtration systems are RGF (rapid gravity) or pressure filtration.
RGF and pressure filters operate at 5 to 10m/hr as a velocity and at 100 times the water flow, the biofilm is more likely to be discharged into the water, and channelling of unfiltered water through the filter bed is inevitable. Back-washing of sand filters will not prevent bio-coagulation, however regular treatment using acid, hypo-chlorous and chlorine dioxide is effective. However is is time consuming, expensive and the use of chlorine prior to membranes is not recommended, Great care has to be taken with marine system and chlorine to prevent the formation of hypo-bromous acid and nitrosamines.
AFM grade 0 may be used without any pre-coagulation or flocculation chemicals, you do not require chlorine. The SDI from an AFM filter will be 50% to 80% better than any sand filter and will usually provide a value under 3. Independent verification under ISO procedures has confirmed AFM at a flow of 20m/hr will remove more than 97% of all particles down to 1 micron, and more than 99% at 5 microns.
AFM grade 1 will give filtration 95% down to 5 microns, however AFM grade 1 can be used with pre-coagulation and flocculation. This improves the filtration performance down to under 1 micron. The coagulation reactions remove dissolved organics including phosphate. This task is impossible for membrane filters, so in effect AFM filtration can provide a simple technology that is sustainable and provides for a higher quality of water than membranes.
For existing systems it is east to retrofit AFM, you just replace the sand with AFM. tere is no requirement to air purge, but it is essential that there is good back-washing procedures in place (click here for back-wash procedure).
More information on AFM in relation to biofouling resistance and membranes can be found at the following link AFM and Biofouling
- AFM can be retrofitted to existing sand filtration systems and will double the performance of the sand filters
- AFM provides for a much higher level of filtration than cartridge filters and is comparable to UF for solids removal
- AFM will remove phosphate and other dissolved chemicals by molecular sieve adsorption, performance is better than UF
- AFM is sustainable, the filter media should never need to be changed