Cleaning Sand Filters

Aster Sand-filter

Before I explain the procedure to clean a sand-filter - also known as Back-washing - I'll explain how they work and how to make the filter contents last a lot longer.

Unsurprisingly, sand-filters contain SAND, but it's a very special type of sand called Silex. Most sand is more or less round, as a result of being knocked about in water (visualize waves on a beach), which chips the edges off.

Silex is different - it's produced by crushing silica rock. The resulting sand has flat faces and sharp edges; not round at all.

Now imagine one flat-faced grain of sand with another one pressed closely against it, leaving a tiny gap. It is the size of these gaps that determines what size of particle the Silex can trap between the grains. When the Silex is new it will trap particles of about 15 microns or larger but, over time, it wears out and can only trap particles of 25-30 microns or larger.

Why does it wear out? As the water circulates through the filter, from the top to the bottom, suspended particles are trapped between the grains of Silex: and the water is cleaned. Eventually, the filter has trapped so much debris that the pump cannot push much water through it - and it's time to back-wash.

During a back-wash (full instructions below but please keep reading) the flow of water through the filter is reversed. The pump no longer has to push water through a full filter. Now water flows strongly from the bottom of the filter to the top and out to the drains - and in doing so it 'Fuidizes' the filter media, stirring the whole filter contents and loosening the tiny trapped particles. The smaller, light trapped particles are flushed away, leaving the heavier Silex grains in the filter ready for the next filtration cycle.

During Fluidization the grains of Silex are violently bashed together just like on that beach you were kind enough to visualize above - resulting in tiny bits being broken off. These microscopic particles are much too small to be caught in any sand-filer, and too much small to sink to the bottom of the pool, so they circulate endlessly through the entire pool, including the filter.

As the Silex particles pass through the filter they erode the sharp edges of the Silex and produce even more micro-particles, accelerating wear-and-tear and bringing forward the time when the Silex must be replaced. 

However, there is one, almost miraculous, product that will catch the tiniest particles of Silex - Jolly GelJolly Gel. It is placed in the pump-basket (the pump pre-filter) and dissolves over a couple of days. It has an affinity for filter media so it stays in the filter and does not get out into the pool.

Jolly Gel places a porous, glutinous membrane over the filter media down to about 6 inches (150 mm), filling-in the tiny gaps between the grains. The membrane is 'sticky' at the microscopic level and catches the tiniest particles, down to 2-3 microns, on contact.  It is so efficient at doing this that a cloudy pool can be brought to the point of 'invisibility' in 24 hours.

The membrane is flushed away to the drains on the next back-wash, leaving a perfectly clean pool and a perfectly clean filter. As there is now nothing left in the pool for the filter to trap it takes a long time before the filter needs to be back-washed again; typically every 4 - 6 weeks instead of weekly. This saves an average approx. 40m³ (10,500 US gallons) of water each year in a 50m³ (13,200 US gallon) pool, and also saves the chemicals and HEAT that are lost by back-washing, but the biggest saving is in the extra life, at least DOUBLE, of the filter media and anything else that is eroded - like the Tantalum Oxide coating on the expensive plates of a salt-water chlorinator or the tips of a pH probe.

TIP: -  The micro-particles of Silex are far too small to be seen with the naked eye but try looking at night with the pool lights on. If you see a fan of light in the water you have the problem outlined above. But now you know the cure!

See the panel on the right to buy Jolly Gel - you'll be glad you did!



- to clean (back-wash) the sand filter

  • Turn the pump switch to ‘off’
  • Open the ‘Bottom Drain’ and ‘Waste’ valves; close the ‘Skimmer’ and ‘Vacuum’ valves
  • Turn the pump on and observe the wastewater colour in the tell-tale on the Rotary valve
  • When the water runs clear, stop the pump at the switch
  • Set the Rotary valve to ‘Rinse’
  • Start the pump and run for 20-30 seconds
  • Set everything to ‘Filtration’ - detailed below

Pump room - Filtering - 'Normal Running' Positions

  • Rotary Valve to ‘Filter’
  • ‘Skimmer’ and ‘Jets’ Valves fully open
  • Bottom Drain Valve half open
  • ‘Vacuum’ and ‘Waste’ Valves closed
  • Timer running and set for 4 hours in every 12 hours
  • Always have the pump/filter running when the pool is in use

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Ken Walker - MyPoolGuru©