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The deep-bed filter (often called a sand filter, because that has for long been the filter medium used) is a clarification filter and has been the basic means of treating fresh water to render it safe to drink for over 100 years. The simple gravity sand
filter is still its most common form, in terms of volumes of water treated.

The system involves operation of a filter with a deep bed of granular material as its filter medium, usually with the liquid flowing downwards under its own hydrostatic head. When full of dirt, the bed is normally cleaned by flow reversal, which
expands the packed bed, releasing the trapped dirt particles and washing the dirt upwards and out of the vessel containing the bed.

Most commonly filled with sand, deep-bed filters may also use anthracite, coke, garnet and other inert solids as their filter media.

The gravity filter exists in two main types:
1. slow, characterized by a low water flow rate and a finer grade of sand, and
2. rapid, with water flow rates 5–7 times higher, and using a coarser sand.

The main difference between the two types is, however, in their mode of ope ration. The slow sand filter works by a straining action, exercised by a shallow layer of material on the top of the bed, which contains biological matter. This ‘ schmutzdecke ’ has both a filtering and 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 um, from raw water as high as 50 mg/l in solids concentration (or even 500 mg/l).

The slow version runs with water rates of about 0.1–0.2m/h, downwards through a bed of sand in the particle size range of 0.35–0.5mm (uniformity coefficient up to 1.75). It sits, as a layer about 0.6–1.0 m deep, at the bottom of a concrete tank
full of the raw water. For a new bed, time must be allowed for the schmutzdecke to form, which can mean running for a while at about one third of full capacity, and then slowly bringing the rate up to its full value. 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 is around 0.75 m deep and formed of sand in the size range 0.5– 0.6 mm (uniformity coefficient up to 1.7). It accepts a downwards water flow in the region of 5–15 m/h. 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 fluidize the bed completely. The higher water flow rate and the fluidizing action together dislodge trapped solids into the wash water, which is removed to another separator for further treatment   (usually a gravity settler, after flocculation). The backwashing flow is in the region of 30–35 m/h, and 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 (and so has about 50 times the solid concentration of the raw water).

Whilst the backwash flow is a very effective means of cleaning the bed, it has also the feature that when the backflow is stopped, the fully expanded bed sinks back to its compact form, with all its constituent particles settling at velocities dictated by their size and density. The result is a stratified bed, with the coarsest and heaviest particles at the bottom, and the finest at the top.

Unfortunately this is exactly the opposite of what is needed for a downflow depth filter, which should have the raw water meeting the coarsest particles first and the finest last. Nevertheless, the rapid sand filter was used in this manner for many
years, because to have an upflow filter risked the expansion of the bed in the direction of the flow, and the consequent release of trapped solids into the filtrate. It was not until the development of the Immedium filter in the 1940s that upflow became
possible, by virtue of an open grid of parallel bars just below the surface of the bed.

The upflow filter was one of the major changes in the rapid sand filter in the second half of the twentieth century. Another was to operate it under pressure, which is done by building a cylindrical steel pressure vessel to contain the bed. Pressurized operation enables the filter to reach head loss figures 2 or 3 times those of the gravity filters. Pressurized filters offer the chance to operate at increasing pressures to counter the increase in head loss as the medium blocks up with trapped
solids; however, declining rate operation is generally preferred, with the filter being backwashed when the flow falls below some predetermined value.

Another major development was the multimedia deep-bed filter, which uses the stratification action of the return from the fluidized state to create the better size gradations – by using two or three 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.

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