In the third in his series "Irrigation or Irritation", technical area sales manager Peter Robinson turns his attention to filtration – including some golden rules to ensure its effectiveness.
The majority of irrigation products originate from Israel, with many still produced there today. It is a long-held belief that when the waters of the Nile were turned to blood the locals dug into the sandy banks and clean water percolated through. This, they say, was the first instance and the ‘invention’ of the sand filter – as well as a good salesman’s yarn!
However, filtration was discovered, the principle remains to separate whatever is undesired from (in the case of irrigation) the clean water we require for plant production. The method generally involves some form of barrier to the unwanted particles. These are most commonly referred to as either media filters or screen filters.
- Media Filters
These consist of a vessel filled most commonly with sand, gravel, carbon and more recently crushed glass granules. They work in two ways; surface filtration where the top of the media interrupts particles in the water flow; and depth filtration where the particles are trapped in the water passages within the media. The filtration achieved is mainly dependent on particle size of the media used.
- Screen Filters
In contrast, these consist of a vessel with a screen with holes, most commonly made from metal, mesh or cloth. The degree of filtration is purely dependent on hole sizes. It is quite common to have a large and smaller screen in the same filter.
There are some golden rules to filtration which are sometimes overlooked or surpassed, for example, in the expansion of systems:
- All filter flow throughputs are manufacturer rated with clean water. Once put to use, their throughput will drop which can be alarming in only moderately dirty water.
- Filters should be specified using a hydraulic calculator with the relevant water quality included
- A rule of thumb is to use a filter rated at double the maximum flow rate of the irrigation system
- Maintenance is key to filter performance. This is particularly true for media (eg sand) filters where regular cleaning and agitation of the media are essential
- Pressure gauges on the inlet and outlet of filters are essential as a reference for cleaning. 0.5 Bars is the trigger point for cleaning
Cleaning of screen filters requires the removal and washing of the filter element. Dirt can become trapped in screens and may require a pressure washer to remove.
Media filters are more challenging to clean and generally consists of a two-stage process. The water flow is reversed through the media and exhausted out of the waste port. During this process, the surface dirt will be removed first. Many growers think this is the filter cleaned once the dirty water stops. However, the reverse flow needs to continue as this ‘lifts’ the media allowing the trapped dirt within to be washed away – the second dirty flush. After this, the filter can be returned to filtration mode.
The cleaning process can be automated, generally using 0.5 Bars differential pressure between the inlet and outlet as the trigger for the cleaning mechanism. Screen filters are cleaned with either a rotating brush or, more commonly, suction scanner nozzles inside the filter element exhausting the dirt through a waste valve. The cleaning mechanism can be driven by an electric motor or water powered turbine.
Media filter automation is simply a process of opening and closing the relevant valves to reverse the water flow and exhaust the dirt. The control system can be electric or hydraulic.
Both cleaning systems rely on the system pressure and flow to remove the dirt to waste. The waste water is often piped to a drain. It is worth noting that the cleaning works as a result of the difference between operating pressure and atmospheric pressure. Any waste pipes — particularly long and/or small diameter — will reduce this differential and adversely affect the cleaning power.