With a title like that, some definition of the scope of this article is essential to allow an understanding of its purpose and content. Obviously, as the title proclaims, the article is about the technologies, and associated equipment, involved in the filtration of bulk flows of water — so it will not cover treatment by sedimentation, nor will it cover separations involving organic liquids, nor the filtration of aqueous solutions such as occur throughout industry (like sugar syrups or brines).
The filtration techniques to be reviewed here include not only the familiar solid/liquid separation processes that form the basis of filtration, but also the closely related diffusion processes of reverse osmosis, nanofiltration and ultrafiltration with membranes. Also covered are the liquid/liquid separation processes used to remove or recover oil from an aqueous suspension.
The water filtration applications listed in an earlier paragraph all have some characteristics in common. As already mentioned, they all have reasonably large flow rates, necessitating high capacity filters. They all deal with a liquid feed that is relatively low in suspended solid content or the presence of other contaminants, so that they will need treatment by clarification processes rather than the often more complex solids recovery operations. This low influent contamination level means that the system parameters involved in the filtration process will all be relatively mild: a liquid no more corrosive than aerated water; liquid viscosities and densities close to that of pure water; and temperatures mainly close to ambient.
These applications, especially, are all fairly demanding in terms of the quality of the filtered water – the concentration of residual contaminant solids and their maximum particle size, or concentration of dissolved material. Not too surprisingly, therefore, the filtration techniques used in them are reasonably similar, although some final product specification requirements can be much more stringent than others – for example, the on-site treatment of industrial waste waters need only produce an effluent flow of similar composition to that of the liquid feed to municipal waste treatment, because the treated industrial wastes are most likely to end up in the municipal sewer. At the other extreme, the production of water for semi-conductor washing usually takes, as its feed material, water already purified to drinking water standards.
Filtration processes involved
The objective of the filtration processes used in water treatment is mainly to achieve the separation of solid particles that are suspended in the water to the extent, perhaps, of 0.5% by weight or as little as 10 mg/l (10 ppm), particles that are as large as grit or sand (say 1 mm in diameter or more) or as small as colloidal organic materials, or pathogens such as bacteria or viruses (say 0.1 μm or less).
In addition to this primary purpose, the filtration system may address the removal of oil droplets from suspension in water, or the removal of the colloidal or large dissolved organic molecules that are usually responsible for unpleasant tastes or odours or colour in the water, or the removal of the large dissolved ions from inorganic salts responsible for causing hardness in the water, or, finally, the removal of the smaller ions causing salinity (or, more correctly, in this case the removal of pure water from residual brine).
It is, of course, true that other processes besides filtration may also occur during the basic separation process, or as part of the overall purification system, such as disinfection (as with the legionella bacillus in cooling water treatment, or the long term prevention of the growth of bacteria in drinking water). It may also be important to remove dissolved gases from the product water, as with oxygen removal from boiler feed water.
The types of equipment used to achieve these separations are relatively few in broad classes, although extremely varied in physical embodiment. These broad classes are:
- • for suspended solids separation:
- – screens and strainers
- – deep bed filters
– cartridge filters
– microfiltration membranes
• for oil droplet separation:
- – coalescing filters
• for dissolved material removal:
- – RO, NF and UF membranes
The deep bed filter is probably most important in terms of volumes of water filtered, whereas the membranes are the vital component for final product purity.
Screens and strainers
The coarsest water filtration is undertaken by strainers, either the in-line or larger units. The in-line units can often be blown clear of collected solids (otherwise having to be dismantled for cleaning), while the larger ones run continuously, with collected solids washed or scraped off the collecting surface. The filtration medium is usually a perforated metal plate or piece of wire mesh, in the form of a cylinder for the in-line units, or a large plate or wire screen for the continuous strainers.
The straining function, which may be required to remove large objects accumulated in a sewer flow, is usually performed in screens. The screens used for large water flows may have arrays of vertical bars, with moving scrapers, or circular arrays of wire mesh, rotating through screening and washing zones, and often designed specifically for the channels in a water works.
The normal way of filtering influent streams – of fresh or waste water –— is to use a screen as a roughing filter, often followed by a microstrainer, which takes the form of a rotating drum, closed at one end, rotating about a horizontal or nearly horizontal axis. The open end fits into a wall that partitions the contaminated liquid from the filtrate. The feed suspension enters the inside of the drum, at the bottom, and the filtrate flows by gravity to the outside of the drum. Sprays at the top of the rotating drum wash through the drum and discharge the solids into a trough running parallel to the axis of the drum, containing a screw conveyor. The drum is usually covered with a woven monofilament fabric or wire mesh with an aperture size of 25 μm, although particles as fine as 3 μm can be collected because a cake is allowed to build up. This is a very effective prefilter for high flow conditions.
Deep bed filters
Among the most widely used filters for clarifying high flows of water entering treatment works is the deep bed (or sand) filter. This has a relatively deep bed (of the order of 1 m deep) of granular material as its filter medium, with the feed flowing through the full depth of the material. The deep bed filter has been the basic means of treating fresh water to render it safe to drink for over 100 years.
The gravity filter exists in two main types: the slow filter, characterised by a low water flow rate and a finer grade of granular material, and the rapid filter, with water flow rates 5-7 times higher, and using a coarser material. The main difference between the two types is, however, in their mode of operation. The slow sand filter works by a straining action, achieved by a shallow layer of organic material on the top of the bed, which contains biological matter. This layer has both a filtering and a biological destruction part to play in the water cleaning process. By contrast, the rapid sand filter aims for a truly deep bed action, with contaminant solids adsorbed onto the bed material for most of its depth. Both are capable of giving treated water that is free of solid particles above 0.5 μm, from raw water as high as 50 mg/l in solids concentration (or even 500 mg/l).
The slow version has its water flow downwards through the bed of sand. For a new bed, time must be allowed for the biological layer to form. Once established, however, the slow sand bed can operate satisfactorily for considerable periods of time (weeks or even months) before the flow rate drops too far. Then the top layer must be scraped off the bed, and removed to another container for cleaning.
The rapid filter was originally developed with liquid flow downwards through the bed of granules. It must be cleaned much more frequently than the slow filter, perhaps as often as daily. It is cleaned by reversing the water flow (using clean filtrate), at a much faster rate than the processing flow, so as to expand and fluidise the bed completely, so that trapped solids are dislodged into the wash water. The backwashing flow is usually augmented with air scouring at the base of the bed, or hydraulic jets on the surface. Backwash lasts only a few minutes and uses 1%-5% of the throughput.
After the backwash, the fully expanded bed sinks back to its compact form, with all its particles settling at velocities dictated by their size and density. The result is a stratified bed, with the coarsest particles at the bottom, and the finest at the top. This is the opposite of what is needed for a downflow filter, which should have the raw water meeting the coarsest particles first and the finest last. The obvious change to upwards flow risked the expansion of the bed in the direction of the flow, and the consequent release of trapped solids into the filtrate, and it was not until the Immedium filter of the 1940s that upflow became possible, with an open grid of parallel bars just below the surface of the bed to restrain unwanted expansion.
A major development was the multimedia filter, which uses two or more different materials of markedly different density as well as different sizes. Materials such as anthracite, sand and garnet are graded such that the lightest (anthracite) has the coarsest grains and the densest (garnet) the finest. Then in the resettling, the density factor is greater than the size factor, and the finer particles sink to the bottom. A downwards flow of raw water then reaches the coarsest layer first, as it should. A modern version of this design is the Spruce filter, which has four layers of different solids, the bottom one, magnetite, being positively charged. An effluent quality of 3 mg/l and less than 0.2 μm is claimed.
Other modern versions are the pressurised bed, and the moving bed, which allows continuous operation, by having the bed of sand or other materials move downwards through the filter. The dirty solids are carried from the base of the bed by a jet of air to a wash zone above the filter, to be washed clean of trapped solids and then returned to the top of the bed in the filter.
The cartridge filter uses a replaceable filter element, generally cylindrical in shape and long with respect to its diameter, which operates by filtering a fluid from the outside of the cartridge to its inside. It normally comprises a central open-structured core, on which is placed the filter medium, and is contained in a cylindrical housing. The medium can be a thin flat sheet, or, much more likely, a pleated sheet, to maximise the filtration area, or a thicker layer of bonded granules or fibres for depth filtration applications. Cartridges are made to a set of generally accepted standard dimensions, so as to be interchangeable as to source.
Individually, the cartridge filter has a relatively low liquid capacity, and it would need to be used in multiple-unit batteries to reach the kinds of flow rates being covered by this article. However they can be fitted with a wide range of filter media, and so can be selected to match almost any filtration task.
An important modern development in terms of filter media has been that of the microfiltering membrane, with an open structure enabling operation at relatively low transmembrane pressures. These are available with cut points down to 0.1 μm or below, thus enabling use for removing pathogenic species from drinking water.
As with the cartridge filter, unit capacities may be low, requiring their use in arrays of modules, but the MF membrane is becoming a very valuable tool in most kinds of water filtration.
In a coalescing filter, liquid droplets of one liquid phase suspended in another liquid, with which they are completely immiscible, are caused to combine to form larger drops, and so to become separated from the other liquid. Most coalescers take the form of cylindrical vessels whose interiors are full of some kind of plastic or wire mesh, with which the dispersed droplets collide, and are captured, to merge with other neighbouring droplets, and so grow in size. The mesh should preferentially be wetted by the dispersed phase. Eventually the captured drops become so large that they fall (or float) from the mesh and form a separate layer. A coalescing filter may also effect some separation of solid particles as well.