Filtration Methods
Sand Filtration
There are 4 main types of sand filters used in the purification of water:
- Slow sand filters
- Rapid gravity filters
- Pressure sand bed filters
- Upflow sand filters
Slow sand bed filters are very low flow rate filters and tend to be used in rural areas or where the water flow rate required is low. Rapid gravity filters are generally used in municipal water treatment plants, especially for drinking water production and pressure sand bed filters tend to be used in industrial applications. Upflow sand filters are a type of pressure sand bed filter. Whilst it is necessary to chemically coagulate and flocculate the contaminants in the water before it is filtered using rapid gravity, pressure or Upflow sand filters to ensure that these filters work efficiently, this is often not necessary with slow sand filters.
Slow Sand Filters
Apart from not normally needing to flocculate particulate matter in water when using a slow sand filter they can even remove pathogens in addition to producing water with very reasonable taste and no colour. An advantage of slow sand filters is that generally they are not backwashed and require little operator skills being so low tech but a disadvantage is that they cannot accommodate a high flow rate of water. Slow Sand filters are also sometimes used in the treatment of sewage as a final polishing stage for the treated effluent and/or a bed on which to dewater the sludge produced from sewage treatment. When used with treated effluent, these filters traps residual suspended material and bacteria which provides a medium for further bacterial decomposition of nitrogenous material.
Rapid Gravity Filters
Purification of water for drinking purposes typically employs the use of rapid gravity sand filters where the particulates in the water are first coagulated and then flocculated using chemicals to trap as much particulate matter as quickly as possible in the filter. Coagulation of the particulate water is achieved by adding small, highly charged cations, such as with Aluminium sulphate (Alum) and Flocculation is achieved by adding small amounts of charge polymer chains which either form a bridge between the particulate solids (making them bigger) or between the particulate solids and the sand. Both Coagulation and flocculation require a certain amount of contact time in tanks before the water is filtered so that a reasonable size floc can form and the process is very pH dependant so that it is important to adjust the pHof the water to the right level first to ensure the greatest efficiency.
Passing flocculated water through a rapid gravity sand filter strains out the floc and the particles trapped within it which also helps top reduce bacteria that may exist in the water. The medium of the filter is sand of varying grades with the grain size of the sand selected specifically to allow the water to pass through the filter rapidly. These filters are then backwashed with clean water on a regular basis as they become clogged with particulate matter (floc), indicated by a rapid drop-off in flow rate of water through the filter, which is then flushed to waste . This backwash waste water is run into settling tanks so that the floc can settle out and it is then disposed of as waste material. The supernatant water is then run back into the treatment process or disposed off as a waste-water stream.
Pressure Sand Filters
Applying more pressure to the water passing through a sand filter will give a greater flow rate, such as with a simple domestic swimming pool filter. The sand grain size often needs to be reduced compared to a rapid gravity filter however so that a sufficient amount of particulate matter is trapped. Whilst using smaller grains of sand in a filter allows a greater surface area of material on which particulate matter can be removed, the smaller grain size also then requires greater pressure to drive the water through the sand bed. The sand grain size in pressure sand filters is typically 0.6-1.2 mm and if there are large particulates (>100 microns) in the water to be treated they are usually removed with a settling tank first as otherwise the surface of the filter will rapidly be blinded with floc and need backwashing too often to be economically practical as too much water will be wasted in backwashing. Pressure sand filters are typically 0.6-1.8m in depth and operate under a maximum flow rate of about 9m3/m2/hr under a feed pressure of between 2 and 5 bars. The particulate matter is not captured uniformly with depth in a pressure sand filter with more material captured higher up in the bed such as on the top surface and with the concentration gradient decaying exponentially. The build-up of particulate solids causes an increase in the pressure lost across the bed for a given flow rate and the pressure sand filter will need to be backwashed when the pressure drop is around 0.5 bar. The back wash fluid is pumped backwards through the bed until it is fluidised and has expanded by up to about 30% (the sand grains start to mix and as they rub together they drive off the particulate solids). The smaller particulate solids are then washed away with the backwash water and captured where they are diverted to a settling tank to separate the solids for waste. The fluid flow required to fluidise the bed is typically 3-10m3/m2/hr for a few minutes). A small amount of sand can be lost in the backwashing process and the sand bed may need to be topped up from time to time to replace this sand. Most pressure filters in industry employ an automated multi-port valve together with sophisticated pressure and flow sensors so that the filter can be backwashed automatically when required.
Cartridge Filters
This type of filter will typically have a removable housing, into which different types of filtration elements can be placed. A domestic cartridge filter element will often be rated at 30- 50 microns or larger whereas specialist industrial filter elements may be rated at 5 microns or less. The “absolute” rating on a filter is a guarantee that no particle of that size or larger will pass through the filter which is often a requirement for some industrial equipment with very fine tolerances and which is very sensitive to particles in the water. A cartridge filter becomes more effective as it gets used but as the particles get trapped on the filter the water flow rate will also reduce and therefore they will need to be monitored or reductions in flow rate and filter elements replaced when this happens. Elements for these filters can include granulated activated carbon, ceramics or metal alloys for specific filtering requirements. A small amount of silver is also sometimes impregnated into the filter material to help prevent any bacterial growth in them however it is important to ensure that high concentrations of silver aren’t appearing in the water should the filter not have been used for extensive periods of time or the filter material is starting to degrade. Some cartridge filters now also contain selective resins that are designed to remove specific contaminants from the water, such as Nitrates, Fluoride or Lead.
Activated Carbon Fitlers
Granulated Activated Carbon (GAC) is used in filters to remove colour, odour, volatile organics and chlorine from water. The GAC removes these mostly by adsorbtion in that the contaminants literally “stick” to the carbon particle. It is important with these filters however to monitor their efficiency as the carbon eventually loses its efficacy and needs to be replace. The flow rate through the filter must also be adhered to according to the manufacturers’ specificationsa s otherwise they will perform poorly, if at all. It is also important to bear in mind that as much as a carbon filter may trap organic material it is this same material that microorganisms feed on and the filter can become a haven for pathogens of not monitored and the cartridge replaced regularly.