Sand filters, rapid gravity and pressure, are discussed

Pool Sand Filters for Private Swimming Pools

Rapid Gravity Sand Filters consist essentially of clean bed of fairly coarse graded sand supported on several grades of rock with a distribution and collection system on the top and bottom of the filter.

In order to remove finely suspended or colloidal impurities from the water, a coagulant such as alum is added to the filter. Filtration rates normally operate at 3 gallons/sq.ft./minute.

Pressure Filters

Pressure Filters are similar to an ordinary rapid sand filter except that it is completely enclosed in a steel tank and the entire unit is operated under pressure. Rates of filtration, backwashing, sand and gravel sizes are the same. Filter units of this type require much less head room than the gravity filters but require the same surface area as the gravity filters. Pressure filters must be equipped with pressure gauges on both the inlet pipe and outlet pipe for determining the loss of head or back pressure in the filter medium. The main disadvantage of pressure filters lies in the fact that the operator is unable to observe the filter operations. The sand becomes encrusted or develops "mud" balls or backwashing may become unsatisfactory, all without the operator's knowledge. Air relief valves are necessary on these filters and should be located on the top of the filter shell to release entrained air after backwashing and at other regular intervals.

Backwashing of Sand Filters

This is one of the most important operations and should be carried out when the loss of head equals the distance from the water surface on the filters to the bottom of the sand layer or when the effluent is no longer satisfactory. Backwashing consists of shutting off the inlet line and opening the sewer line, then running clean water from the pool in the reverse direction through the under-drainage system. Most manufacturers recommend that the backwash rate should be 12 gallons per minute per square foot of tilter are.

The rate of flow for backwashing operation is that which will expand the sand bed until the individual grains are not continuously in contact with each other but will "vibrate" back and forth and dislodge any dirty material adhering to the surface of the sand grains. The backwash rate should be great enough to raise the small dirt particles and soil vertically and carry them to waste. Effective sand washing normally occurs when the sand bed is expanded about 40 per cent. Too great an expansion of the sand will make the backwashing less effective as the sand grains may be too far apart as not to infringe on each other to obtain the effective degree of scrubbing action. The size of the sand grains specified by the manufacturer is such as to give the desired expansion at the rate of flow.

To prevent loss of sand in backwashing, a free board of at least 50% of the filter depth is required.

The normal operation of a rapid sand filter is a relatively simple procedure and it is not until some of the component parts fail to function that the operator is faced with any difficult problem. Good operation, however, includes activities on the part of the operator which tend to prevent some of these failures occurring. Many of these impending difficulties may become evident by simple observations during both the filtering and backwashing operations.

For example the presence of "mud balls" in the medium, indicates incomplete washing. The existence of cracks in a sand bed or the pulling way of the sand from the walls indicates that the sand grains are being cemented together by some material in the water and that parts of the filter bed may become ineffective. Uneven distribution of wash water may often be observed directly or it may be indicated by the formation of mounds, craters or sand "boils", particularly if the under-drains or strainers are broken. The possible loss of sand during backwashing may be checked by collecting and making visual observations of water samples taken from the backwash troughs. Observance of any of these conditions should be thoroughly investigated, so that the cause may be eliminated.