Glass as an active ingredient
Dr Howard Dryden of Dryden Aqua reports on active filtration media, manufactured from processed recycled glass, and the trials undertaken with drinking water and sewageActive Filtration Media (AFM) is a direct replacement for sand in all gravity or pressure sand filters. AFM is a self-sterilising media that physically removes particles from water, but there are also surface adsorption and catalytic properties, which make AFM a superior filtration media.
AFM has been in use for six years for the tertiary treatment of sewage, industrial effluent, river water, lakes, fish farms and swimming pools. Tests have been conducted on the treatment of drinking water by Scottish Water, and further intensive tests are scheduled with several water companies including Scottish Water and Essex & Suffolk Water throughout 2003. The trials are being supported by UK organisations Waste and Resources Action Programme (WRAP), Remade in Scotland, Midlothian Enterprise Trust and by the EEC via Life Environment. Entec UK and Ellison Environmental Services are also active participants in the AFM project.
The current project will conclude trials using AFM for the tertiary treatment of sewage and drinking water. Drinking Water Inspectorate (DWI) certification is in progress and due for completion by the end of 2003. AFM is manufactured from recycled bottle glass and is an environmentally and economically sustainable product. A process developed by Dryden Aqua transforms the glass into active filtration media. A new factory and process plant are presently being installed to increase production to 100t/d.
AFM was developed as a means of resolving the biofouling issues associated with the use of substrates such as sand and zeolites. Indeed most substrates including pea gravel and carbons also have serious biofouling issues. In clean water or chlorinated applications such as swimming pool filtration or drinking water, it may take many months or several years for a stable bio-slime to develop on the surface of sand. As the bacteria biomass develops the performance of the filtration system declines. In dirty water applications, or where there is a high organic loading, it is difficult to use simple sand filters, since the filter media will become biofouled and blocked after a few weeks.
Slow bed sand filters depend upon a biofilm, but in rapid gravity or pressure filters, the bio-slime will cause dead areas, bacteria break-through, bed packing, streaming, increased trihalomethanes (THMs) and a host of other issues.
The surface of sand has an ideal microstructure for colonisation by bacteria. It is impossible to prevent bacteria from becoming established, even in chlorinated systems such as swimming pool water. Vigorous and extended air scrubbing and back-flushing of the sand will help to minimise the filter bed bacterial biomass, but it can not be prevented. Indeed, fluidised beds of sand, carbon and zeolites are used as intensive biofilters.
Any retained organic matter in the sand bed will act as a food source for heterotrophic bacteria. Since the back-flush efficiency can not be 100% effective, it is inevitable there will be bacterial accumulation. Reduced filter performance and bed streaming in turn lead to increased levels of THMs in the product water. The two photographs in Figure 1 serve to illustrate the point. The AFM is essentially clean whereas the sand shows an accumulation of surface debris and bacteria.
AFM is manufactured from processed recycled glass into an activated filtration media with an appropriate particle shape and size distribution. The microstructure of AFM is such that the surface is very smooth in comparison to sand. It is said a filter media needs a rough microstructure in order to trap the solids. This statement is correct, however a rough microstructure makes it much more difficult to remove the retained solids from the sand grains.
Extensive trials of sand and AFM media have been conducted in a variety of water types over a period of five years. In all of the tests, the results have shown that AFM out-performs sand. For example, sand will generally remove particles to 10-15µm, whereas AFM will removes particles to 5-10µm, with quite a high percentage of sub 5µm components. AFM and sand will both physically remove the particles from the water, but AFM also carries a permanent surface negative charge which enhances the performance of the media. The surface negative charge also removes a proportion of dissolved organic molecules by adsorption.
AFM has surface catalytic properties, which generate a high oxidation potential on the surface of the media. At high potentials, bacteria will be killed and organic molecules cracked. AFM therefore has an in-built ability to self clean and sterilise. These results have been confirmed. The photographs in Figures 1 and 2 show AFM has stayed clean in a swimming pool application over a five-year period. We have similar data for AFM after five years in a simple pressure filter used for the tertiary treatment of sewage.Benefits of using AFM
Sand removes particles down to between 15 and 10µm. AFM removes smaller particles due to the surface charge. The charge is very weak and while sufficient to hold on to particles during the run phase, the charge will be broken during a back-flush at flowrates in excess of 25m3/h/m2. If AFM is used at high water flowrates during the run phase, e.g. in excess of 25m3/h, the media will still remove solids from the water, but it will not remove the sub 5 and 10µm particles.
Retained solids do not adhere or bond onto the surface of AFM, therefore during the back-flush phase the solids are easily removed. Indeed AFM only requires about 50% of the back-flush water requirements in comparison to sand. The active catalytic properties of AFM prevent bacterial contamination and oxidise organic matter which tries to adhere to the AFM surface. The filter media therefore stays clean and fully operational for many years. The extremely high attrition strength of AFM means it is unlikely the filter media will ever need to be replaced.
In the tertiary treatment of sewage effluent there is an added advantage. The AFM media is back-flushed back to the start of the works, preventing the loss of the bacterial biomass from the system. The unique properties of AFM mean it can be installed in a simple low-cost pressure or rapid gravity sand filter. There is no need for air scouring, even when it is used for the treatment of sewage effluent. For unattended operation the filters are easily fitted with automatic back-flush valves. The system requires the minimum of attention or maintenance amounting to approximately one day per annum. Figure 5 shows a standard stainless steel filter with a capacity to treat 10-15m3/h of sewage or industrial effluent. For clean water application the water flowrates can be increased. However, for most applications, irrespective of whether if it is potable water or industrial effluent, the flowrates should be between 5 and 10m3/h/m2 of filter bed surface area.
Heavy-duty stainless steel filters are manufactured by Dryden Aqua but any good quality steel, GRP or polyester filters may be used with AFM. Potable water trials
Trials were conducted using potable water by Scottish Water using a GRP filter. Two identical filters were operated in parallel, one with high quality filter sand and the second with AFM. The quality of the water entering the filters was actually very good, with an average turbidity of approximately 1NTU. If you already have a good water quality it is difficult to make it any better. However, in all of the analyses conducted, which included manganese and bacterial levels, the results all gave the same pattern as the iron levels shown in Figure 6. Essentially the performance of the sand and AFM was very similar, but in the majority of the results the AFM out performed the sand. However, on at least two occasions during the period covered, there were very high spikes of poor water quality on the discharge from the sand filter. It is likely the spikes can be attributed to an accumulation of waste in the sand filter, not removed during back-flush and subsequently discharged in the filtered water. The result may be a function of the test bed. However since both filters were treated in exactly the same manner, it serves to illustrate the AFM filter is staying clean and that its long term performance is likely to be more predictable and stable than a sand filter.Additional trails
The observations and results reported for AFM have been obtained over many years. We are now entering a period of more intensive testing in order to fully quantify the behaviour and performance of AFM for the treatment of different water types. The trials are being managed by Dryden Aqua, Ellison Environmental Services and Entec in association with Scottish Water, Essex & Suffolk Water, Northumbrian Water, Thames Water, United Utilities and SITA. Additional test sites are required for drinking water applications. The performance of the AFM will be investigated in great detail, attention will also be focussed on the removal of polychlorinated biphenyls (PCBs), arsenic, oocysts removal and cracking of organic molecules. The reduction in THM levels will be assessed as well as the reduced requirement for coagulants. AFM is not simply a replacement for sand in sand filters. It will certainly perform the same function, but there are many added water quality benefits and savings that can be gained