| Seawater Filtration Dryden Aqua are marine biologists specialising in seawater filtration systems for the aquaculture industry over the last 25 years. It is absolutely essential to have the best possible filtration when trying to cultivate delicate marine fish. It is also essential that the systems are a low cost and reliable, because system failure, even for a few minutes could result in loss of millions of juvenile fish. Dryden Aqua in association with the European Commission under the Life Environment initiative have completed extensive research for the development of AFM ( Activated Filter Media). AFM replaces the sand in your media bed filters. Simply by changing the sand and replacing it with AFM media, you will double the performance of your filters, and present a much better water quality to the membranes. Typically, by changing the sand and anthracite in your filters to just AFM you will reduce the SDI ( Silt Density Index) values by 50% to 80%. For example in a comparable trial with sand and AFM , the discharge from AFM was 1.5 and from sand it was 3.5 on average, the influent SDI was actually too high to measure. Filter velocity was 8 cubm/hr/sqm and pre coagulation with alum was employed. When no coagulation is employed the performance of AFM is not quite as good, however the difference in performance between sand and AFM is much greater. Ultra filtration will give a better performance, however the SDI does not give the full story. When AFM is combined with pre-coagulation and flocculation using flocculants, or even if small quantities of ozone at 0.05mg/l is used to create flocculation reactions, AFM will remove sub-micron particles below 0.01 microns as well as dissolved organics. This would be impossible using membranes, so a relatively low cost procedure using AFM can actually be much more efficient that Ultra Filtration. AFM is more expensive than sand ( by around a factor of 4 times), however huge savings will be made in energy costs, cartridge replacement and in protection of the membranes. AFM has a high negative zeta potential, and water filtered by AFM also appears to reduce the energy requirements on the membrane pumps. | | Why you should use AFM | Replace the sand in your filters with an engineered filtration media specifically designed for the application | - Life cycle cost savings that will give a rapid return in capital which could be as short as a few months
- AFM provides the membranes and cartridge filters with much better protection than would be possible with sand
- AFM filter can cope with shock loads and high organics
- AFM filtration is repeatable and predictable, ideal for automation
- AFM does not support bacterial growth therefore water channelling through the filter bed is eliminated
- AFM filter do not destabilise like sand and dump a high level of solids and colloids back into the product water
- AFM filter media stabilises very quickly after a back-wash
- AFM carries a negative Zeta potential charge so it works synergistically with positive charged low zeta potential flocculants such as PAC, Ferric, polyelectrolytes and NoPhos. AFM will tend to increase the zeta potential of the water and increase membrane permeability by dropping surface tension.
- AFM does not contain free silica
There is no down-side from using AFM except the higher initial capital cost, however there will be a return in capital savings measured in months | 
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| Biofouling and channelling
| | Conventionally, pre-treatment will involve the use of Media Bed Filters, normally using sand filters, followed by cartridge filtration stage down to 1 to 5 microns. Sand filters work well for the application, however the performance of the filters tends to deteriorate over a period of around 6 months due to biofouling of the media. It is virtually impossible to prevent biofouling of sand in the media bed filters. Bacteria will start to colonise the sand immediately the system goes on-line. The growth rate of heterotrophic bacteria is exponential with a doubling in biomass every 20 minutes in water over 25 deg C. Bacteria excrete a monosaccharide alginate as a means of bonding onto the sand. Over a period of a few months the alginate gradually accumulates and becomes more resistant to back-wash abrasion. In effect the alginate starts to glue the sand grains together which leads to worm hole channels through the filter bed. Channels can start to develop in the sand bed after 1 to 6 months and filtration performance deteriorates |  | | Mechanical filtration & back-washing
|  Sand filtration is used for the mechanical filtration of water. As the water passes down through the sand bed, solid particles are trapped in the spaces between the sand grains. The surface of the sand will be coated by a layer of bacteria and the alginates excreted by the bacteria. The alginate acts like a glue and solids present in the water will tend to stick to the alginate coat. This is one of the mechanisms by which bacteria trap their food. The problem is that the bond between the alginate layer on the sand and solids removed by the filter is very strong, so trying to back-wash all of the solids out of a sand filter is very difficult. The bacteria will also react and adapt to any given condition, so irrespective of the degree of agitation or fluidisation during back-wash the bacteria will simply evolve and adapt to their environment. AFM will also remove solids mechanically from the water, however AFM is manufactured to have surface active properties that prevent the media from supporting the growth of bacteria. The electron microscope photographs give a dimension of 60 microns in width. The photo of sand shows the surface to be cluttered with bacteria and debris, however the AFM is perfectly clean. AFM back-washes much more effectively than sand, because there is no alginate glue preventing the release of the solids. The graph below presents a back-wash profile of a sand filter and AFM filter operated under identical conditions. The data was collected by Lyonnaise des Eaux France. 
The graph shows the back-wash profile of two AFM filters and two sand filters. The data was collected after the filters had been in commission for 8 weeks. The graphs clearly show that AFM back-wash profile was a smooth sine curve, indeed the curve is predictable and exactly the same over months and years. The area under the curve was 25% more than the sand filter, which means that 25% more solids were removed by the AFM filters in comparison to the best filter sand available in the UK. ( Leighton Buzzard deposit). The data was collected before the sand filter started to exhibit worm hole channels, once the sand filters started to channel the difference in performance was in excess of 80%. Operation criteria were, 10 cubm/hr/sqm run phase, back wash was air scour at 90 cubm/hr/sqm followed by counter-current water wash at 45 cubm/hr/sqm for 10 minutes . | | Physio-chemical filtration | | Sand is an aluminosilcate mineral, AFM is an amorphous aluminosilicate, however they both exhibit completely different properties. While the chemical composition is almost identical the difference relates to the shape and structure. For example activated carbon, graphite and diamonds have the same chemical composition, but they are certainly not the same. Zeolites such as clinoptilolite is also an aluminosilcate like sand, however the shape of the crystal lattice structure imparts molecular sieve ion exchange properties. Zeolites need to be regenerated with a caustic brine solution once a week which is a major inconvenience and expense. Activated carbon has to be replaced when it reaches capacity. At Dryden Aqua we wanted to engineer a product that would have surface active properties and adsorption, but not molecular sieve filtration or absorption because both of these properties require a regeneration phase. AFM is an activated alumino silicate amorphous glass in which we have we have manufactured to have a high surface area, high Zeta potential and high catalytic oxidation potential. AFM does not require a regenerator phase, nor does it impart any chemicals to the water, indeed the performance of AFM tends to improve with age. Adsorption and Zeta Potential. 
The negative charge on the surface of AFM attracts +ve charged micron and sub-micron particles as well as dipolar organics molecules. AFM is therefore removing much smaller particles from the water than would be possible for a sand filter. AFM is also acting like activated carbon by removing dissolved organics from the water. The capacity of AFM for organics is much lower than activated carbon, however because the organics and solids are only held on to the surface by a weak charge, every time the AFM media is fluidised during a back-wash, all of the adsorbed materials are eluted in the back-wash water. AFM therefore never becomes exhausted. The surface of AFM also has catalytic properties and will dissociate a small proportion of dissolved oxygen into very short lived free radicals. The the radicals will crack carbon double bonds, and oxidise heavy metals. Also the combination of the catalytic properties and zeta potential stern layer both contribute to prevent the surface of AFM from becoming biofouled. A more detailed explanation is available Click here | Operating parameters
| | AFM will remove between 25% and 80% more solids from the water than sand, in addition AFM will remove a proportion of the micron and sub micron particles as well as dissolved organics and heavy metals such as iron and manganese. With sand the alginate bacterial coat restricts the passage of water through the filter bed. The pressure differential over a sand filter will normally be 15% to 25% greater than an equivalent AFM filter bed, this translates to an equivalent saving in pump energy requirements. The filtered water is also used as a source of back-wash water, with AFM the back wash duration can be as short as 3 minutes as opposed to 10 minutes for sand. Also the back-wash frequency is normally 50% to 75% lower than sand. AFM will always perform better than sand at any given flowrate. AFM has been used in filters up to a flowrate of 80 cubm/hr/squm, however the performance of any media bed filter, be it sand or AFM varies inversely proportionally to the flow of water through the filter, so it is always best to operate at as slow a flowrate as possible, however this increase the size and the cost of the system, a compromise is therefore required, and this depends on the seawater quality and the objectives of the operator. The normal operating flowrate will be between 15 and 25 cubm/hr/sqm, however flowrate below 15 cubm/hr/sqm are recommend to reduce the surface shear forces and increase the adsorption capacity of AFM between back-wash cycles. Therefore in situations where there is likely to be pollution incidents, iron or heavy metal issues, hydrocarbon contamination and heavy loads of solids due to turbulence and storm conditions, we would recommend flows below 15 cubm/hr/sqm Back-wash flowrate is the same as sand, it is important to fluidise the bed by at least 15% at to achieve this task a flow of 45 cubm/hr/sqm is required. Prior to a back-wash it is useful to employ and air scour at 95 cubm/hr//sqm | | |
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