How is a blend valve used in recirculation?

I attended one of the recirculation workshops in which you were a co-presenter. I am finally thinking of taking the plunge with my cocopeat system, but I am confused about the use of a blend valve in controlling the recirculation volumes.


Media-based hydroponic systems
From years ago, similar to today, most commercial hydroponic systems have been media based. Crops are planted into bags or boxes of a suitable growing medium and fed with nutrient solution through drippers. To keep the nutrient solution managed, a proportion of the feed (typically about 30%) is drained off.

In Holland in the 1980s, the progressive banning of methyl bromide for sterilising the soil in glasshouses led to a move out of the soil into soilless culture (hydroponics). These were the media-based systems described earlier, mainly using rockwool. Some of these systems were ‘closed’, that is, recirculated, but most were ‘open’, that is, ‘free drainage’.

Unfortunately, in the early 1980s a rose grower using recirculation had a well publicised total crop loss because of a diseased plant infecting the entire crop.

By the 1990s, rising levels of nutrients in the canals led to the banning of run-off from greenhouses from hydroponic systems and especially from soil growing. This led to all Dutch greenhouses having to convert to recirculation by the year 2000.

Because of earlier crop losses, the starting point was to prevent the possible spread of disease in the recirculating solution. To achieve this, the Dutch industry decided that it was essential to sterilise with a ‘total kill’ of potential recycled pathogens. This meant that the first potential systems to be disregarded were continuous flow systems such as NFT (nutrient film technique), because the volume of solution to be sterilised was unworkably huge.

The end result was that the media-based systems were retained, but with the run-off solution collected, sterilised and recycled.
In summer conditions, with a mature crop such as tomatoes, the uptake of water (for cooling the plants by transpiration) will be relatively higher than the uptake of nutrients. This results in a rise in EC (electrical conductivity) from the dripper to the drain for each container.

Typical figures for tomatoes would be a dripper feed EC of 2.6 mS/cm rising to 3.7 mS/cm in the drain, when running off 30% of the feed as drain for recycling.

The run-off is collected, sterilised, and stored in a holding tank, typically sized to hold two-days volume at maximum throughput. Keeping the feed and drain flows in balance is where the blend valve is used.

Blend valve
Blend valves are also known as three-way valves. Most people have met a blend valve in normal life. A single tap combining hot and cold water to send warm water to a sink, shower, or bath, is a blend valve. It changes the proportion of hot and cold water to give the temperature you want. In a greenhouse with pipe (hydronic) heating, the industrial version of this measures the temperature of the boiler water and mixes it with cold water via a blend valve to give the temperature needed for the water to go through the heating pipes.

Blend valve used with bore water. The hydroponic one is identical with recycle replacing bore water. (Image courtesy Global Water)

Blend valve used with bore water. The hydroponic one is identical with recycle replacing bore water. (Image courtesy Global Water)

In the case of nutrient solutions, the measurement used for blending is the EC of the raw water, recycle and mixed streams. The EC of the mixed stream is known as the ‘Pre-EC’, because it is before the addition of the fertiliser input and any acid/alkali added for pH control.

The Pre-EC is a calculated figure, which keeps the relative flows in balance spread over a day or two.

Pre-EC = % run-off x run-off EC (both for the previous full day)

For the figures mentioned above, of 30% run-off and run-off EC 3.7, the Pre-EC calculates to:

Pre-EC = 30/100 x 3.7 = 1.1 mS/cm

Presuming a raw water EC of 0, the setting of the blend valve at 1.1 mS/cm would be made up of 70% from raw water at EC = 0, and 30% from the recycle stream at EC = 3.7 mS/cm. Before going on to the drippers, a further 1.5 EC would be added by ‘A’ and ‘B’ fertiliser addition to bring the EC up to the feed value of 2.6 mS/cm.

So your management procedure is to measure and record daily the % run-off and drain EC and use these to calculate the pre-EC setting for the next day. Check your recycled volume to ensure that it is keeping in balance. Ω

June 2015 / Issue 156