Cleaning water the smart and economical way

There are numerous ways of using filters to provide for clean water in greenhouses. It is important to think carefully in what to invest in beforehand. “Many companies have a 130 micron main filter and 100 micron filters at the watering sections. This is not very convenient – it is better the other way around.”

Filtration is the process during which solid parts are taken from a gas or liquid by forcing them through a porous medium.

Water is becoming scarcer, which forces companies to deal with it more efficiently.”

Efficiency and quality intersect when it comes to filters. The quality is warranted by using the right filters for the company and by correctly deploying these. At the same time, filters will have to work efficiently. “Rinsing filters requires water and time. Choosing the right filters, however, means unnecessary rinsing is in the past.”
Some insight into the workings and possibilities of filters is very useful for a company to obtain high quality water and to deal with it efficiently.

What to look for

People mistakenly look for capacity when buying a filter. “The first thing to look for, however, is the pollution load of the water. That is the amount of  dirt that a filter has to take out of the water” Sometimes a switch is made from rainwater to surface water in dry periods in horticulture –  both types of water are highly diverse in dirtiness.

This should be taken into account when choosing a filter. The next step is to determine what the filter rating should be. This filter rating is determined by the product that needs protecting, for example the dripper or the nozzle.

Subsequently, it should be clear what the capacity is, it determines the size of the filter. “In addition, questions such as: do you want a filter that cleans automatically cleans or by hand, are there special circumstances surrounding pressure, temperature or chemical substances and where the filters can be placed best in the company or the pipeline network are addressed.”

Depth filters

In principle there are two ways in which water can be filtered water in horticulture: via depth filtration or via surface filtration.

Depth filtration is applied in sand filters. The level of filtration in sand filters is determined by the diameter of the sand grains and the flow rate.

The diameter of the sand grains is decisive for the fineness filtered for.  “The density of a sand filter is a sixth of the diameter of the sand grain. In the case of sand with a 600 micron diameter (a micron is one thousandth of a millimetre), that gives you a filter with a density of 100 micron.”

In reality, however, sand filters also hold particles that are many times smaller. And the level on which that takes place depends on the speed with which the water goes through the filter. The slower the water flows, the finer the particles filtered.

This has to do with Van der Waals force, magnetism and other links between dirt particles and water. “A normal sand filter will filter particles of 50 to 80 micron from the water if you send 55 cubic metres of water per hour through a one square meter sand filter. But if you only filter 10 cubic metres of water per hour per square meter, then even particles of 25 micron are filtered out.”
This also shows that the capacity of a filter in sand filtering is not very meaningful.

The capacity depends on the desired filtering and not only on the size of the filter.

Backwashing sand filters is a meticulous job. Backwashing is done by washing water through the filter in the opposite direction and leading it to the drain. “The speed with which this is done should be high enough to lift the sand so it can scrape loose the dirt on the particles, but no so fast that the sand is flushed from the filter. If the backwashing speed is too low then the sand will not be lifted but channels will arise in the sand bed, causing the sand filter to not properly filter anymore. The problem in sand filters is that after backwashing a residue of unfiltered water stays behind in and under the filter. This can be drained but backwashing with filtered water is better yet.

Surface filters

The second much-employed method for filtering water is surface filtering.

The advantage of these filters is that they work continuously and that they can ‘wash’ and filter simultaneously. So-called SAF’s (Suction Automatic Filter) are often used in horticulture. SAF-filters have a multiple layer screen with small holes though which the water is being pressed.
The filter size varies from 10 to 800 micron. The filter water is entered on the inside of the cylinder and presses itself outward through the holes, leaving dirt residue behind on the inside. A number of nozzles – that are connected to a drain - move along the inside of the cylinder during washing. Because there is hardly any pressure in the nozzle, water from outside the filter is pressed in the nozzle where the nozzle touches the filter – causing the dirt to be washed to the drain. The water flow will thus be turned around where the nozzle touches the filter. Moving the nozzle in a twisting motion along the filter will ensure the cleaning of the entire filter during filtering.

The big advantage of this system is that filtration can go on continuously and that you can filter enormous amounts of water using a relatively small filter. Also, these filters use relatively little wash water and are very easy to automate.”

An extra fine filter screen is opted for in the case of malleable materials such as algae and other organic material in the water..

Right choice

The tip I want to give to growers is to know beforehand what type of water is available and what they want to deploy the water in. “An automatic filter is much more expensive, but regularly washing the filters by hand isn’t cheap either.  Also keep in mind space-saving and the amount of wash water that has to be drained – possible in the sewer. So do the math to find out what’s most efficient in the end .” and finally.

What about that claim that it’s better to have a main filter of 80 and watering section filters of 130 instead of the other way around?

At first glance, it seems logical to go from coarser to finer. But that’s not true. You’ll have guaranteed good water in your net if you use a 80 micron as a main filter - all you have to do is regularly wash the main filter.

With the 130 micron you will subsequently collect any dirt developed in the pipes by, for example, fertilizer sediment etc. and because that’s usually not a lot you’ll only have to wash the watering section filters just once in a while. If it is done the other way around, all filters have to be washed very often. Quite some water and time is lost doing that.”

Choosing filters

- Keep the amount of dirt in the water in mind when choosing a filter

-The level of filtration in sand filters is determined by the diameter of the sand grains and the flow rate. This means that the given capacity of a filter is not very meaningful.

- The advantage of these surface filters is that they work continuously and that they can simultaneously wash and filter.

- An automatic filter is expensive, but having to manually wash the filter is not cheap either.

- It is better to have a main filter of 80 micron and watering section filters of 130 instead of the other way around.