AFM prove effective alternative to sand

Research shows active filter media (AFM) provide a more effective barrier to the spread of disease than sand. Howard Dryden of Dryden Aqua explains

Sand filtration will be the principle means of treating drinking water for many years. The process has served us well but, with the increasing demands on our water resources and pressure to continue to improve water quality, we have perhaps reached the limit of sand filter performance.

As a consequence UV, ozone, carbon filters and membrane systems are now becoming more commonplace. But there is a technique that can greatly improve the performance of pressure and rapid gravity filters without resorting to high capital cost systems.

The World Health Organisation estimates that 6% of the disease in Europe is waterborne, and the highest percentage of this is due to gastroenteritis caused by the 4 micron diameter oocysts of cryptosporidium and giardia. Research conducted by Dryden Aqua, with support from the European Commission under the Life Environment initiative, has confirmed that active filter media (AFM) provide a much more effective barrier to the passage of oocysts than is possible for sand.

Considering that the oocycts are resistant to chlorine, AFM filter media provide an opportunity to easily improve the performance of most drinking water treatment systems and reduce the incidence of disease. The public and environmental health implications are substantial. Key areas of sand filter performance are:
  • Solids removal
  • Removal of oocysts
  • Reduce chlorine demand and reaction products; trichloramines and trihalomethanes
  • Reduce backwash water volume
  • Generate life-cycle cost savings and employ sustainable procurement
Sand filters will remove particles down to around 10 microns. But, when used in conjunction with good flocculation, it is possible to consistently remove solids down to micron and sub-micron levels. This performance can only be maintained if there is zero channelling of water through the filter bed.

It is impossible to prevent bacterial growth on sand, indeed sand is an excellent substrate for the growth of bacteria. Even when a sand bed is continually fluidised at 50% expansion, the sand will become a very effective biofilter.

No amount of air scouring or backwashing will remove the biofilm. Alginates excreted by bacteria glue the bacteria on to the sand.

The problem is the alginates also glue the sand grains together to form channels, and over a period of a few months to several years (depending on water quality and temperature), the alginates will become harder and more stable. With water temperatures above 15¢ªC, the growth rate of heterotrophic bacteria can be as short as 15 minutes.

The exponential growth and production of alginate will either form stable channels through the sand, or there will be a rapid increase in pressure differential across the filter. The consequences are either deterioration in water quality, or an increase in back wash frequency, or more likely a combination of both problems.

Bacteria are required in slow bed filters, but in rapid gravity and pressure sand filters, they are the main reason for poor water quality and filter failure. AFM solve this problem.

AFM are manufactured by Dryden Aqua and designed to replace sand in all types of sand filter. AFM have surface catalytic properties and a high negative zeta potential.
The surface active properties prevent colonisation by bacteria so bacteria induced bed channeling is essentially eliminated. Coagulants and flocculent such as PAC have a positive charge, when added to water they reduce the zeta potential and impart a positive charge to the suspended solids.

The high negative zeta potential on AFM attracts the particles and holds them within the filter bed. AFM filter media will remove at least 30% more particles from the water than a sand filter. This statement is supported by independent trials conducted by two UK water companies.

Back-washing
What goes into a filter must come back out. In the trials, more waste was removed from an AFM filter during a backwash in comparison with an identical filter bed containing sand.

The backwash profile of AFM was also reproducible, which indicates stable steady-state conditions. In comparison, the backwash performance of sand was not stable, and up to 30% less waste was eluted from the sand.

In sand filters, a proportion of the solids will be glued to the sand by alginates. Aggressive air scouring and extended backwash times are therefore required to try and keep the media clean.

With AFM, the solids are only held by a weak electrical charge that is broken during backwash to release all of the solids. A great deal less water is therefore required to backwash AFM.

The surface of AFM exhibits catalytic activity in the presence of oxygen. A proportion of the dissolved oxygen molecules are dissociated on the surface of AFM, which increases the oxidation potential of the media. The surface of AFM is therefore self sterilising.

The zeta potential generates a high charge density, which attracts positively charged particles. But, at the same time, the slip zone prevents the particles from reaching the surface of the media. AFM therefore attract solids, and hold on to them, but they are prevented from bonding to the surface.

Their self-cleaning properties mean that the media are just as effective in treating wastewater such as sewage effluent as in treating clean water, because the media does not become biofouled.

Chlorine is usually added after the sand filters, but in double filtration systems, chlorine may be used before the second sand filter, and under these conditions trichloramines will be produced. Trichloramines are formed by reaction between ammonium and chlorine at a pH less than 5.

The biofilm on the sand grains is acidic and is the principle location for trichloramine production. AFM have no biofilm, and the surface is not acidic, so trichloramines cannot be produced.

NH3 + HOCl > NH2Cl + H2O pH = 6-8 = Mono-chloramine
NH2Cl + HOCl > NHCl2 + H2O pH = 5-6 = Di-chloramine
NHCl2 + HOCl > NCl3 + H2O pH = <5 = Tri-chloramine

At the chlorine levels normally used before sand filters, the chlorine will not kill the biofilm, indeed it can actually promote the growth of bacteria species that produce copious quantities of alginates such as pseudomonas spp Bacteria excreted alginates as a defence mechanism, so the presence of a low chlorine concentration can actually increase alginate production and make the situation worse.

Rapid gravity and pressure sand filters behave as biofilters, but the high water flow rates and backwash regime results in an unstable system. Bacteria cells are constantly being scoured off the media.

If insufficient bacteria are eroded, the biofilm will become unstable, anaerobic zones develop, and methane and hydrogen sulphide are produced. The filters can then dump a slug of bacteria and waste AFM particles, with permanent surface negative charge trapped in the biofilm, back into the product water.

Sand filters will remove organic matter and solids from the water, but by acting as biofilters they also convert soluble nutrients back into organic matter in the form of bacteria cell biomass, which is then discharged back into the water supply. The bacterial cell biomass, alginates and waste products are now available to react with chlorine to form THM.

Alternative media
The mechanical filtration performance of rapid gravity and pressure sand filters are inherently unstable because the bacteria cell biomass is in a state of flux. The biofilm is resistant to chlorine, indeed if chlorine were used before a filter, the levels of THMs and trichloramine would be substantially increased.

The bacteria also coagulate the sand grains, which increases the backwash frequency and volume of water required to keep the sand clean.

Eventually, the bacteria will cause channeling of water through the filer bed providing a conduit for the passage of oocysts and at the same time the filter will discharge bacteria and organics back into the product water, which increases the THM level. In around 25% of the water supplies in the UK, ammonium is added to the water after the sand filters to form mono-chloramine in an effort to prevent chlorine forming THMs.

All of the inherent problems of sand filters could be eliminated if biofouling of the sand is prevented. Unfortunately, this is neither easy nor practical for municipal drinking water systems. One possible solution is to use an alternative filter media that actively rejects biofouling. AFM are the first active filter media certified for treating drinking water that meet this criteria.

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