Depth filtration - granular medium filters
Depth filtration, also called sand filtration, or just filtration, means removal of particles when water is forced to flow through a layer of a material with particles (granular filter medium) of various sizes and depths, this filtration layer can be of sand or another granular material, depending on the purpose for which the filter is to be used. Because sand is a commonly used the filter is often called a sand filter. The layer is not dense but contains a number of channels and holes created between the particles that constitute the filter medium (Fig. 5.4).
When the water that contains particles goes through the filter medium, particles larger than a certain size will be trapped by several mecha-nisms. They may be too large to go through chan-nels (straining); they may settle both unaltered and due to flocculation and adhesion, and may be adsorbed by chemical and physical forces that attract them to the filter mass.
The first process will occur on the top layer of the filter medium; particles are trapped because they are too large to pass into the channels or pores in the filter media, which will then become blocked. It is, however, of some concern that a few large parti-cles block surface channels of the medium causing a large reduction in filter capacity. The head loss increases and the filter becomes blocked more quickly. Therefore it is more important to utilize settling, adsorption and other effects inside the filter medium; the total depth of the filter medium is then utilized, not just the top layer. However, this requires a low water velocity through the filter medium, which again means that the hydraulic load on the filter surface must be low. When the chan-nels inside the filter are full of particles the head loss will increase.
The maximum particle size that will pass through a depth filter is determined by the grain size of the medium; if the grain size is small, the filter will clog easily. The flow rate through filter medium and the rate of clogging depend not only on the grain size of the filter medium, but also on the characteristics of the particulates in the water to be purified. The performance of a depth filter is dependent both on the type of filter and operating procedures, and also significantly on the characteristics of the filter material.
Depth filters can be classified depending on the direction of the water flow through the filter medium. In an up-flowing filter, the water flows up through the filter medium, while in a down-flowing filter the water flows downwards; the latter type is the most common. Good filtration can be achieved, but the filter medium becomes clogged after a period. In an up-flowing filter the same will occur; here there may also be a break-through of waste-water at one or several places in the filter medium if there is excessive clogging, provided that the water pressure is high enough. In this case almost all the water will go through the filter via these zones and there will be virtually no purification.
A depth filter should be equipped with back-flushing facilities, or regular manual purification of the filter medium will be necessary. In a traditional sand filtration system (on-site system, see later) used for purification of municipal wastewater, this can be the case. Such systems can be used for months or even years without doing anything because the wastewater is only discharged onto the soil surface. Of course, this depends on the hydraulic load on the surface area and the concen-tration of particles in the water.
If a filter is to maintain capacity, back-flushing is necessary. Even when this is carried out, the hydraulic capacity of this filter type is lower than that of, for instance, rotating microsieves. While doing the back-flushing operation, the filter must be stopped; water is then sent the opposite way through the filter media, so removing particles that have settled. This back-flushing water is sent directly to the farm outlet. Most of the running problems with depth filters are the result of improper back-flushing. To achieve proper purifica-tion of the filter medium it is important that the back-flushing water suspends trapped particles. Typical amounts of back-flushing water are in the range 1–5% of the bulk flow.
Depth filters can be separated into those operat-ing at normal atmospheric pressure or those having an over pressure inside the filter, i.e. pressurized filters (Fig. 5.5). In a pressurized filter the medium may be put inside a sealed chamber, with the same overflow arrangements as for an unpressurized filter. When a pressurized filter starts to clog, the pressure will increase and the particles will be pushed further down in the filter medium;
ultimately such a filter will become totally blocked, but it takes longer than with an unpressurized filter. The advantages with pressurized filters are that a larger part of the filter medium height is utilized and the back-flushing interval is increased. To use a pres-surized filter it is necessary to pressurize the water to be purified, normally up to 6–8 bar. Usually the water passes through a pressure increase pump to achieve this, and consideration must be given to avoiding damage to the particles. If such an arrangement is used on the outlet water, the pump will break the particles up and the proportion of small particles which are more difficult to remove will increase. Therefore pressurized filters are not recommended for use on outlet water; however, if there is no alternative the outlet water must be handled as carefully as possible.
The normal filter medium is sand or gravel. The size of the sand or gravel particles in the filter medium depends on the characteristics of the par-ticles in the water to be purified. By using small par-ticles in the medium the head losses and clogging velocity of the filter are increased whilst the capacity is decreased. In a one size sand filter most of the filtration occurs in the first few centimetres. Instead of using one type of medium over the whole filter depth, i.e. a single size medium, media of several sizes (multimedia) may be used to increase utilization of the filter. In such filters, provided the water enters from the top of the filter and flows down, the largest media are on the top and the smallest on the bottom. The largest particles are then removed in the top layer and increasingly small particles are removed through the lower layers of the filter. This will ensure a more even col-lection of particles distributed over a larger part of the filter and not just in the top layer. To avoid mixing of the media during back-flushing, media with different relative density can be used. The largest media must have the lowest density and the smallest the highest. When back-flushing from below this will ensure that the largest particles stay on the top. In aquaria, a two-media filter compris-ing a layer of crushed anthracite or ilmenite above a layer of fine sand has been used. A third bottom layer with even finer particles, such as crushed ilmenite or garnet, could also be included. Regard-less of the filter media, it is of great importance that the back-flushing is done correctly so that all the media are back-flushed, otherwise there will be zones that are still clogged and that will not be uti-lized when the filter is returned to operation.
Each medium can be characterized by its grain size, uniformity of grain size, grain shape and rela-tive density, all factors that are important for filter-ing performance. Appropriate grain parameters can considerably reduce the resultant head losses.
Several classifications can be used on granular media filters, and these can also be classified with respect to their hydraulic capacity giving (1) slow sand filters, (2) rapid sand filters, and (3) continu-ously back-flushed filters.21 Type 1 is unpressurized and type 2 is pressurized; type 3 is constructed to enable continuous back-flushing without interrupt-ing the water flow as is necessary for back-flushing types 1 and 2. Typical reported loading rates are 0.68 l/s/m2 or less for slow sand filters, up to 1.4 l/s/m2 for rapid sand filters and up to 5.4 l/s/m2 for contin-uously back-flushing sand filters.
In slow sand filters 2–5 mm grains are utilized;10 pressurized rapid single sand filters have grain sizes ranging from 0.3 to 4 mm.22 Reported removal rates are for 0.3 mm sand about 95% of particles down to 6 μm, and for 0.5 mm sand about 95% of parti-cles down to 15 μm. A typical pressurized rapid sand filter has a capacity of 136 kg sand, a surface area of 0.29 m2, a maximum pressure of 3.5 kg/cm2 and a design flow rate of 238 l/min.
Depth filters can also be of the cartridge type. The cartridge can be made of different materials, such as plastic, ceramic, spun fibre or resinbound fibre, often specific to the supplier. The cartridge has a defined depth, and the waste is collected either inside or outside the cartridge, or has to pass through the cartridge. The typical cartridge filter is used once and then replaced. Cartridge filters can be used for removal of small particles and are avail-able for sizes below 1 μm. They are used on small water flows with low particle loads; otherwise the cost of operation becomes too high.
Diatomite (DE) or pre-coated filters may also be classified as depth filters.23 Diatomaceous earth filters comprise a filter with a screen coated with DE (fuller’s earth), a granular material of fossil diatoms. A cake of DE is formed and put on the filter screen that functions as a skeleton. The water to be purified has to pass through this filter cake and the particles are trapped. When the filter isdirty it can be back-flushed, or a new filter cake can be installed. DE filters can tolerate higher water flows than cartridge filters, but are best suited to small water flows.
Filter bags are another simple type of filter for use with small water flows. The filter bag is woven and so forms a surface, the mesh size of which depends on the closeness of the weave. Particles are trapped on the surface of the bag. When no more water will pass through, the filter bag has to be removed and replaced with a new or clean bag. The bags can be cleaned in a washing machine, for instance.