Gravel filter
In process engineering, gravel filters are devices that are used to purify water, process fluids and waste water. With a gravel filter, undissolved solids can be separated from water or process fluids, thereby cleaning the fluid . This cleaning process is one of the mechanical separation processes and is called filtration . As can be seen from the name, the cleaning is carried out in containers that contain a bed of gravel as filter material . Important areas of application for these filters are drinking and industrial water treatment as well as wastewater treatment . The filtering is carried out under pressure in closed as well as in atmospheric open containers.
Function description
The use of separation technology with a filter material is very old. Sieve vessels filled with rushes and gorse, a variant of the colatoriums used at the time, were already used in ancient times to separate solids from liquids. In the following, only the medium water is generally described for the liquid to be treated . The information also applies to process liquids and wastewater. The diameter of the particles to be separated is from several millimeters to the nanometer range . A water with such undissolved particles, called raw water , is pushed through the gravel in the filter. The gravel is the filter medium by which the particles are held back and thus filtered off. However, for the active filter layers predominantly grain sizes are used that are smaller than fine gravel and that correspond to the dimensions of the sand . In technology , however, the terms gravel and gravel filters are mostly used instead of sand and sand filters .
The forces active in the filtering process go far beyond a pure sieve effect. In addition to the adhesion with the surface of the gravel grains, catalytic reactions, such as the de-ironing of water, can also enhance the filter effect. The liquid cleaned from the particles is called pure water . The dirt particles are filtered off in normal gravel filters, preferably in the upper gravel layers. This dirt absorption reduces the space between the gravel grains and the flow resistance, called filter resistance or pressure loss , increases in the gravel layer. After reaching a maximum permissible pressure drop or if the quality of the pure water deteriorates due to an increase in the particle content, the operation of a filter must be stopped. With increasing pressure loss, the energy demand increases (necessary water pressure or necessary geodetic height difference) and operation becomes uneconomical. There is also the risk that particles that have already been filtered off will be pressed through the entire layer of gravel and contaminate the pure water. This process is also referred to as the breakthrough of a filter.
The amount of particles that can be filtered off and the quality of the pure water that can be achieved depends on both the type and size of the undissolved particles in the raw water and the type of gravel used. With increasing particle size, the achievable operating values for dirt absorption and the residual content in the pure water become more favorable. Very small particles such as bacteria, some algae and the so-called finely dispersed and colloidal solids are difficult to filter with normal gravel filters. For this purpose, the raw water must prior to entry into a gravel filter by adding, dosage called by flocculants and flocculation agents are treated. The particles are attached to the flakes that form and converted into a form that is easier to filter off. If the dirt particles to be removed are voluminous and / or sticky, a filter skin can also form on the topmost layer of gravel. This quickly leads to high pressure losses and premature backwashing of the filters. Gravel filters are hardly suitable for cleaning raw water contaminated in this way.
A filter is exhausted after reaching the permissible dirt pick-up and must be cleaned. This cleaning process is called backwashing . The gravel filters are cleaned regularly to maintain their functionality. To do this, clean water and compressed air are flushed through the gravel layer from bottom to top, and the filter is backwashed . An optimal backwash consists of three phases. In the first, the gravel bed is loosened with air only and then in the second, most of the dirt is rinsed out of the gravel with air and at the same time a small amount of water. This is followed by the third phase with a large amount of water without air for the final cleaning of the gravel filling and rinsing. During a subsequent start-up, what is known as the first filtrate is usually discarded for safety reasons, as it may initially contain somewhat higher levels of solid particles for a short time.
Filter structure
Two different designs are used for the gravel filters. So-called "open filters" made of concrete are often used in systems with high throughput rates, such as those required for drinking water treatment and the generation of large quantities of process water . These are pressureless filters in which only the difference in height between the raw water and the pure water level in the system generates the pressure required to overcome the filter resistance. “Pressure filters”, on the other hand, are filters with closed containers that are mainly made of steel. The required operating pressure is usually generated with pumps. With these filters, the pure water is not pressureless after filtering and, if necessary, still has the pressure required for further use.
In the simple sketch shown, one of the usual designs for a pressure filter is shown. With the two different filter designs, open or closed, the gravel either lies on a nozzle bottom or drainage pipes are arranged in the lower area of the gravel bed for the drainage of the pure water. Before the development of current plastics, drainage pipes were widely used before the 1960s. After that, filter nozzles made of polypropylene were and are predominantly used. When cleaning process water with over 80 ° C. For reasons of strength, drains or nozzles made of heat-resistant material, for example stainless steel, are required at the raw water temperature.
The filter gravel used must meet certain requirements. The required qualities for the permissible filter sand and filter gravel are listed in Germany in DIN EN 12904 "Products for the treatment of water for human use - quartz sand and quartz gravel". Among other things, it lists specifications for the following:
- Grain sizes to be selected for the support and filter layers
- permissible pore volume with 25 to 35%
- largely free of clay, lime, mica (together <4%) and org. Substances (<0.5%), quartz content around 96%
- The smoothest possible grain surface
- Limitation of the proportion of oversized and undersized grain
Both filter types often have devices for the raw water distribution in the inflow area of the filter, especially with larger dimensions. Furthermore, distribution systems are also more frequently available for the purge air distribution. These distribution systems enable an even distribution of water and air in the filter and prevent local areas with too high a flow that would hinder proper function.
Characteristic for both filter types, open or closed, is the general grain distribution in the gravel bed. Due to the regular backwashing, gravel grains with small dimensions are mainly arranged in the upper layer and the coarser ones in the lower area. These grain differences lead to the disadvantage that after entering the filter, the water flows through the uppermost layers of gravel with predominantly smaller grain sizes and the solids are deposited there. Fine gravel, however, has less space between the grains. This inevitably leads to disadvantages for the dirt holding capacity and worsens the cleaning effect, since the lower coarse gravel layers have to take over the fine filtering more quickly. Coarse gravel can generally absorb more dirt. Fine gravel enables better and finer solids separation. In order to avoid these disadvantages, room filters and bed filters were developed in which the filter layers are structured differently.
Spatial filter
In addition to normal gravel filters with a flow direction of the liquid from top to bottom when operated, so-called space filters are also used. These filters are flown through from bottom to top during operation. Since the coarser gravel grains are preferably arranged in the lower gravel bed and the finer ones in the upper gravel bed, these room filters can absorb larger amounts of dirt than normal filters before the pressure loss increases too much. Another advantage of the room filters is that raw water can usually be used as rinsing water instead of pure water without any disadvantages.
In order to reduce the risk of a flow breakthrough in the filter bed, according to a Dutch patent from the 1960s, a firmly anchored grid system was arranged in the upper bulk area of the filter medium. Filters with this design are called medium filters. The process sketch on the left shows the usual structure of an immersion filter . Especially with very heavily polluted raw water, these filters enable sufficiently long operating times with good pure water quality between backwashes despite the uptake of higher amounts of dirt.
Stratified bed filter
In all of the aforementioned filters, only gravel is used as the filter medium. A similar effect as with spatial filters can be achieved with multi-layer filters , also known as layered bed filters. With these filters, a layer with a specifically lighter and coarser material is applied to a fine-grained lower layer, which consists mainly of gravel. The absorption capacity for dirt is significantly improved by the upper, coarser filter layer, as this is flown through first and can absorb larger amounts of dirt. Filter materials such as hydroanthracite or activated carbon are suitable for this second, coarser layer .
In addition to these two-layer filters, three-layer filters are also used. With regard to the dirt holding capacity, there are no major differences between these two types. However, there are clear advantages when, in addition to suspended particles (e.g. loam and clay), finely dispersed contaminants such as algae and bacteria have to be filtered off. Such filters are well suited for cleaning surface waters that have higher levels of turbid matter during periods of flooding and, at times, larger amounts of algae in the warmer seasons with a simultaneously low total content of undissolved matter.
The filter structure of such a three-layer filter is, for example, as follows:
- lowest coarse-grained water distribution layers made of gravel
- 1. fine-grained filter layer made of fine sand
- 2. Medium-grain filter layer made of hydroantrazite
- 3. coarser filter layer made of light activated carbon (e.g. based on peat)
The prerequisite for trouble-free continuous operation of the multi-layer filters is the use of the correct grain size and bulk density for the filter materials. A permanent, clean layering can only be achieved with the right selection. Otherwise, the necessary backwashes lead more or less quickly to partial mixing, which greatly deteriorates the filter effect and the dirt holding capacity.
In a comparison of two- to three-layer bed filters, extensions of the running times until clogging or solids breakthrough of 59 to 63% were achieved for the latter filters in a test system.
Applications
As already mentioned, gravel filters are often used in water and wastewater treatment . Typical applications are the cleaning of surface water containing solids and the filtering off of precipitated compounds from chemically treated water, which arise, for example, during decarbonization or the oxidation of dissolved iron compounds . If phosphate precipitation is carried out during wastewater treatment , the resulting water is often finely filtered through gravel filters after such a stage. In all of these examples, undissolved solids are filtered off and pure water is produced with the lowest possible content of undissolved particles.
Individual evidence
- ↑ Harald Anlauf, in: Mechanical solid / liquid separation ; Chemie Ingenieur Technik, 2003, vol. 75, no. 19th
- ↑ European Committee for Standardization: DIN EN 12904 Products for the treatment of water for human consumption - quartz sand and quartz gravel .
- ↑ http://www.urbaner-metabolismus.de/strom.html in Chapter 5, Section 5.7
- ↑ a b Heinz Bernhardt, Helmut Schell in: Experiences with the operation of multi-layer filters, gwf, 129th year, 1988, issue 6, p. 403.
- ↑ Heinz Bernhardt, Helmut Schell in: Experiences with the operation of multilayer filters, gwf, 129th year, 1988, issue 6, p. 404.
- ↑ Heinz Bernhardt, Helmut Schell in: Experiences with the operation of multilayer filters, gwf, 129th year, 1988, issue 6, p. 405.