AFM, sand or Zeolites

in rapid gravity and pressure filters?

AFM biological index

Introduction

Dryden Aqua comprise of a team of biologists specialising on water quality and water treatment.  Key personnel have conducted Ph.D. research on biological filtration and water filtration using Zeolites.  Our expertise has been applied over many years to the development of water filtration system in a diverse range of  industries,   from drinking water and swimming pools, to fish farms, sewerage effluent and even toxic industrial discharges.

Gravity and pressure sand filtration technology is simple,  inexpensive and very effective,  however it is difficult to use the process in biologically active water such as sewerage effluent or fish farms. Sand acts as an excellent support for bacteria, and in a matter of a few days to several weeks, the sand filter has become completely biofouled. Even in chlorinated systems such as drinking water and swimming pool applications  we know that the sand becomes biofouled,  it will take longer to have a impact on the performance of  the filter,  but there will be a slow, gradual deterioration.  During this period,  the filter will be consuming more chlorine, and generating high concentrations  THM`s  and bacteria. We also know that it is essentially impossible to disinfect sand or indeed any surface, especially if it has an irregular micro structure.   .

Photograph of the surface of a Zeolite, showing the zeolite crystals which measure approx. 5 microns in width. Holes between the crystals provide the perfect environment for bacterial colonization.

A great deal or research has been centred on the surface bacterial decontamination of food, and we know that you can only achieve around 98% decontamination of leafy salads by free chlorine level in excess of 1000mg/l.   The UK institute of Food Research has some additional information that makes interesting reading,  http://www.ifr.bbsrc.ac.uk

One last point, when a bacteria is in suspension in the water and not attached to a substrate particle, the bacteria can normally be destroyed by very low levels of chlorine, e.g. 0.1mg/l,  however as soon as the bacteria come in contact with a substrate such as sand, or food item, it just takes around 20 seconds for a firm bond to become established, and for the resistance of the bacteria to increase dramatically. 

It  is impossible to prevent bacterial colonisation of sand,  and it is impossible the remove the bacteria, even by super chlorination.  The rate of contamination can be slowed down,  but it can not be prevented,  it is therefore an inevitable situation that the performance of a sand filter will deteriorate slowly from the day of commissioning. In the case of drinking water and swimming pool applications or where the filtered water is in contact with food products,  there is a significant risk of filter failure and bacterial contamination of the product water.

Zeolite

Zeolitic minerals such as clinoptilolite are often used for water filtration. The zeolites have unusual ion selectivity properties, and are excellent as a means of removing ammonium ions (+ve) and heavy metals such as lead from freshwater with a low cationic content.  We have conducted many years of intensive R & D on the use and application of zeolites, and we presently use zeolites such as clinoptilolite as biological ammonium buffers for the stabilisation of activated sludge effluent treatment systems. We also use zeolites for the selective removal of ammonium ions in freshwater applications where the water temperature are below 8 degrees centigrade. When zeolites are used for ion exchange filtration, a regeneration cycle is normally implemented on 48 hour cycles. If zeolites are used as a replacement for sand in pressure sand filters, we find that they perform extremely well during the first few days and possibly up to several weeks in swimming pool applications. However zeolites will be absorbing and adsorb biological nutrients,   and they have an even rougher macro porous structure than sand,  these properties make zeolites a wonderful support for bacteria, but a dreadful mechanical filtration media in sand filters.

Zeolites are cationic ion exchange minerals, this means they will only absorb ions with a positive charge. Uric acid is a very large ion that will not fit into the ion exchange pores,  in an event it also has a negative charge which means uric acid ion exchange will not occur with natural cationic zeolites or any type.

AFM filtration

AFM was developed as an alternative to sand and zeolites to over come the bio fouling issues.  The surface of AFM is extremely smooth and non porous, this limits the places where bacteria can hide.  The surface of AFM also has a high localised oxidation potential, which cracks organics and prevents the firm bonding of bacteria. Indeed, we have found that the weak surface charge attracts the bacteria to the surface of  AFM where the bacterial cells walls are oxidised to prevent a biological glued bond forming.  The bacteria are however held in place,   but because there is no biological bond,  the bacteria are all removed during the back-wash phase.  In waste water treatment applications, AFM  is used to capture bacteria from aerated tanks and activated sludge systems,  the bacteria are then recycled back into the systems. We have had systems running in sewerage effluent for over 5 years,  the fact that the media continues to work, and the water treatment works has a 100% discharge compliance record is testimony to the product. 

The unique properties of AFM make it a far better filtration media than sand or zeolites in all applications that we have tested. Research into the activation and customisation of AFM properties continues at Dryden Aqua.