Water quality monitor is based on efficient filter technology

Cross water filters utilise a patented ‘zero gravity’ filter coil with raised nodes on the surface which produce a precise filtration gap, selectable between 12 and 400 microns. Automatic backwashing is usually triggered by selecting a set point for the increased pressure drop across the coil that occurs during normal operation. Backwashing causes the coil to open evenly along its entire length and completely flushes any debris out of the system.

Because backwashing is swift and restores the filter coil to a completely clean condition, it provides a reliable set point from which the frequency of backwashing can be seen to accurately reflect any increase or decrease in the amount of debris or sediment carried by the water passing through the coil.

If backwashing data is logged, the filter becomes a water quality monitor, providing a permanent record of the condition of the water passing through it. Furthermore, if there is a sudden change or deterioration in water quality, signified by an immediate increase in the frequency of backwashing, this can be utilised to trip an alarm or automatically take action such as closing off the water supply.

Water quality monitors based on this principle have been tested at water treatment works, fitted to bypasses on the outlet side of slow gravity sand filters. Under normal circumstances, sand filters are very effective methods of filtering water supplies, but problems can arise with conditions such as ‘mudballing’ and ‘canalisation’. In extreme circumstances such faults can allow unfiltered water to get straight through and into the mains supply. Turbidity monitors are often used to counter this threat, but they do not always succeed in identifying a problem in time, making it difficult to establish in which filter bed the problem originally arose.

The Cross water quality monitor is designed with a number of features to enable such a fault to be instantly recognised and, if necessary, enable the supply from the filter bed to be shut down before any unfiltered water reaches the mains. The filter control system logs the date and time of each backwash and draws a graph from which it is easy to establish how the water quality fluctuates.

A serious deterioration – denoted by a rapid increase in backwash frequency – triggers an alarm in the control room and at the same time the filter backwash water is diverted into a collection vessel rather than being run to drain. The operator therefore has immediate access to a sample of the problem water which can be analysed to identify the cause.

Results from a six month trial have convinced Cross that the filter has the proven ability to immediately recognise a problem developing in the slow gravity sand filter. The company now believes that there must be many other applications and processes which could benefit from this technology.

Examples quoted include anyone who wants to keep control of the quality of water incoming to their plant. Furthermore, those who may need tight control of any effluent or waste water leaving their plant in order to conform with strict environmental standards could also benefit from the reassurance provided by the Cross system.

Contact Details and Archive...

• Cross Manufacturing 1938 Ltd

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