AFM, Glass Media and Sand Filtration Systems, a Comparison of Technologies and Optimization of Performance.
Media Bed mechanical sand filtration systems comprise of, gravity flow, pressure, and moving bed continuous backwash filtration systems. In all cases, the most common mechanical filtration media is quartz silica sand. The quality of quartz sand is a variable depending upon the country and the location of the deposit.
There is a requirement for a consistent quality of filter media for all industries using media bed filtration in order to standardize and optimize the filtration process. This aspect becomes more important for filters, that have a pressure gradient across the bed such as horizontal filters, or filters that have not been installed on a perfectly level base. The performances of seven different types of filtration media were physically evaluated by IFTS (1) one of the leading independent accredited laboratories in Europe for the evaluation of products used by the water industry.
Sand has been used for over 200 years in Europe as a means of filtering Drinking water. A company in Scotland in 1804 was the first documented report of a company using sand in a slow bed sand filter (2). Slow bed sand filters typically operate at water flow velocity of 0.1m/hr and use a coarse grade of sand and gravel. The filters depend on the maturation of the sand as a biological filter before they provide adequate mechanical water filtration.
Slow bed sand filters provide excellent water quality and are still used for the treatment of drinking water. Approximately 15 percent of all water supplies in the UK currently use slow bed filters, but they are being phased out in favor of RGF (Rapid Gravity Filters) and pressure sand filters in order to save space. RGF filters for drinking water operate at water flow velocity of 6m/hr whereas pressure filters typically operate at 12m/hr.
The water flow velocities of RGF and pressure filters are therefore 60 to 120 times faster than slow bed filters. The higher water velocities change the bio-dynamics of the filtration process which impacts on filter performance leading to bio-instability and transient wormhole channeling of unfiltered water through the filter bed.
The performance of any media bed will be inversely proportional to the flow velocity, which is a function of the filter diameter, its surface area and bed depth. The bar graph compares the performance of AFM and Leighton Buzzard sand from England. The slower the filter flow velocity the higher the performance, the relationship is exponential but the coefficient depends on the media characteristics and particle size used for performance evaluation. One of the key issues in the drinking water industry is the ability to remove a parasite called Cryptosporidium, which is almost completely resistant to chlorine and only measures 4 micron in size. If sand filters are operated at water flows in excess of 12m/hr it becomes increasingly difficult to ensure adequate water quality and the removal of parasite. Water treatment system tend to operate at the highest possible water flow rates in order to save space and reduce capital cost. AFM has been shown to provide performance advantages over sand, which permits higher water flow rates and reduced capital cost of illustrations. Typically 50% higher water flow rates can be used with AFM over sand while still maintaining good performance.
Filtration performance also depends upon filter configuration; horizontal filters save space and maximize surface area. Bed depth is shallower which reduces absorption capacity for small particles. Also, a differential pressure gradient across the bed reduces performance when compared to vertical filters that have a consistent pressure gradient and a deep bed. The differential pressure promotes biofouling of sand, biodynamic instability and transient wormhole channeling; the problems are largely resolved by using AFM which does not suffer from biofouling.
During the run-phase large solids will accumulate on the top of the filter bed and small solids will penetrate the bed. Small particles attracted by electrical (Van Der Waals) forces may become trapped on the surface of the media. Sand and most media carry a negative charge on Zeta Potential. In water treatment, coagulants and flocculants such as; Lanthanum chloride, aluminium chloride, ferric chloride, PAC (polyaluminium chloride) or polyelectrolytes may be applied to drop the zeta potential, increase coagulation and flocculation as well as increasing electrical attraction. In some industries including pre-treatment prior to membranes or the aquarium industry, the use of chemicals would not be advisable. Reduction of the zeta potential and coagulation can nevertheless be achieved by the rapid movement of water, cavitating static mixers.
In addition to mechanical and electrical attraction, there will also be some degree of molecular sieve filtration. This will be the case with activated carbon, and to a lesser extent with new sand. The ability of sand to adsorb is a function of the silicon to aluminium ratio and how the molecules are configured. An example of natural ion exchange molecular sieve sand is the zeolitic sand clinoptilolite.
Zeolites are used in water treatment as a mechanical filtration media and also as an ion exchange mineral for the selective removal of ammonium and radioactive nuleotides from freshwater. Zeolites cannot be used for marine systems or water with a high TDS because the competing cation will prevent ion exchange. In freshwater systems zeolites provide a good substrate for the growth of autotrophic nitrifying bacteria, a characteristic that is likely due to the adsorption of ammonium into the mineral and its availability to be metabolized by autotrophic species such as Nitrosomonas spp.