PCDM (POROUS CERAMIC DUAL MEDIA) FILTRATION
The PCDM (Porous Ceramic Dual Media) filtration system developed in Hamilton, New Zealand, as a replacement for conventional sand filters, has attracted strong attention and a loyal customer base for the benefits it offers water users. These include lower operating and capital costs, better performance characteristics and the ability to fit inside existing water treatment plants to increase flow rates and lengthen run times.
The demand for treated water worldwide is increasing both in terms of volumes and water quality requirements. Under new regime’s imposed by various resource management acts around the world and for commercial reasons, many industrial companies are seriously evaluating their water use and treatment needs.
The proven and innovative PCDM technology is being used throughout New Zealand and parts of Australia to either recycle Industrial Water, treat food grade water or produce potable water. It has been evaluated successfully with wastewater and potential future applications include industrial discharges, stormwater and domestic recycling. Uses of the system include beverage manufacturers, food processors, dairy companies as well as metropolitan supplies.
Filtration removes particulate matter from the water. It is the final back-stop prior to disinfection and as such is the last real barrier in a treatment process. To a large degree, filtration determines the real quality of the water treatment. Apart from pressure filters and the PCDM system, sand filtration has been largely unchanged for more than 100 years. The PCDM system is very different to conventional sand filters.It combines a highly porous ceramic tile or nozzle with two separate media, each with unique properties. The combination of these different materials utilizes these different properties to gain significant advantages over conventional filters.
Porous ceramic tiles made from waste glass are fitted to the floor of a PCDM filter to replace the traditional plastic nozzles and the various layers of support media. This allows fine sand to be placed at the bottom of the filter. In this position, the sand is under a greater head of water which increases the efficiency of the filter, often more than doubling the flow compared with similar sized conventional systems.
These tiles have very irregular flow paths and are highly porous. The tiles are physically fitted to the filter floor by either stems or saddles, as well as being epoxied. A significant advantage over the plastic alternative is their ability to handle high temperatures.
Traditional gravity sand filters have a layer of fine sand at the top of the filter. Usually there are several layers of coarser media under the sand to prevent it being washed through the nozzles at the base of the filter. This support media serves no filtration purpose and indeed detracts from the performance of the filter.
The porous tiles and the greater head pressures allow finer sand to be used and therefore the PCDM system is able to filter to finer tolerances. The sand used in traditional filters is usually 0.6 to 1.2 millimetres in diameter, where as the PCDM sand is very evenly graded between 0.3 and 0.6 millimetres.
A layer of silicon sponge, a specially modified pumice on the top of the sand layer, shields the fine sand from the worst of the contaminants. The action of the silicon sponge can be understood in terms of its naturally occurring negative charge, which attracts positively charged particles in the water.
An example of how this works is the virtually complete removal of aluminium from water that has been flocced with alum. Other metals, for example iron and manganese, both common in New Zealand bore supplies, can be captured if they are oxidized into an insoluble state.
Results from PCDM systems verify the filters ability to handle floc carry over. In one case, the level of aluminium carry over was reduced by more than an order of magnitude. It is also noteworthy that the background aluminium residuals are much lower than the traditional system.
In addition to the active surface characteristics, silicon sponge is an effective medium because of its porosity and large irregular surface area. It is normally graded between 1.2 and 2.2 millimetres in diameter but can be manufactured in a number of sizes to suit any application.
The PCDM back washing cycle is very regular and easily automated because of the different specific gravities. Backwashing can be reasonably vigorous and the two media layers will never mix. The silicon sponge has a specific gravity of 1.2 and sand about 2. This means that the two media will always settle into distinct layers according to their specific gravities.
If conventional filters are backwashed to vigorously, the media, because they have similar specific gravity can intermix destroying the filter integrity
The rise rate in the filter is best between 1 and 2 centimetres per second. This is slightly higher than conventional systems because of the amount of contaminants in the water. The filter will often have run for twice as long at twice the rate and therefore has 4 times the level of particulate matter stored in the bed. With this rise rate the bed expands and the two media actually separate. The bottom layer of sand cleans itself first and then the silicon sponge is vigorously agitated. This force is stronger than the electrostatic attraction which is preserved on the media while the agitation removes particulate matter from the media surface.
Another common problem with sand filters is the formation of preferential flow paths through the media. This is a result of inadequate cleaning and can be made worse by the use of some floccing aids. This problem is very difficult if not impossible to replicate with the PCDM system.
Because all the PCDM media are actively filtering, there is a greater storage area for contaminants. This extends the run times between cleaning cycles, often by 100%, significantly reducing operating costs and energy consumption.
It is a very simple system to fit into an existing plant.The existing bed and nozzles are removed and replaced with the ceramic tiles and dual media.
The system is in use throughout New Zealand in a variety of situations including freezing works, dairy companies, food industry as well as metropolitan water plants.A selection of clients includes Coca-Cola, Hamilton City Council, Matamata Piako District Council, Waitomo District Council, Waipa District Council, Banks Peninsula District Council, Clutha District Council, Kapiti Coast District Council, Tauranga District Council, Chelsea Sugar Refinery, Waitoa Dairy Factory, Kiwi Dairy Factory, Horotiu Freezing Works, Huntly Power Station.
In addition there are large numbers of private schemes throughout the North Island. These are primarily being used on farms to remove iron and manganese from bore water. This can increase the growth and productivity of stock by at least 10%.
There are a number of remote, small communities relying on the system without any pretreatment.
PCDM filtration will improve the efficiency of disinfection and reduce harmful effects by / removing particulate matter which shields microbes from UV systems. / Removing organic matter which can form trihalomethanes (THM’s) during chloronation and can also produce a higher chlorine demand. / Reducing the dosage rates for ozone.
In summary the system is recognised as a proven, technologically advanced filter system able to provide rapid paybacks when fitted to existing plants. New plants also benefit through simpler, more reliable operation and less area requirements.