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Recirculating vs. Drain to Waste Systems

by Sunny Datko

What is the Difference Between Recirculating and Drain to Waste Systems?

There are two forms of hydroponic systems – there are re-circulating and drain-to-waste systems. In a recirculating system, the runoff from the nutrient solution is collected, replenished and reapplied to the substrate. In a drain to waste system, the runoff is drained into the ground or routed to a holding reservoir. Many gardeners choose to use the drain to waste system because this method offers more control over the composition of the nutrient solution being applied. In a recirculating system the solution will gradually become unbalanced, unless the solution is tested for each nutrient. A rapidly growing vine crop such as tomato can remove a considerable amount of nutrients in a day. In a “drain to waste” system the grower can increase the feeding time and be confident a balanced solution is reaching the crop.

Why Run a Drain to Waste System?

  • Drain to waste requires less maintenance due to the fact that the excess nutrient solution isn’t recycled back into the reservoir, so the pH of the reservoir should not vary.  A recirculating system can have large shifts in the pH levels that require periodic checking and adjusting.  If the pH is not corrected, various problems may occur, including but not limited to poor nutrient absorption.

  • In a recirculating system, pathogens can quickly spread their colonies and infecting other plants in the same reservoir. With drain to waste systems this cannot happen because any water leaving a given plant goes to a drain and not back to the main reservoir.  Therefore the spores cannot infect the reservoir.

  • Drain to waste systems allow you to flush your garden more easily, allowing you to reset the medium and draw out unwanted nutrients from within the plants themselves. In a recirculating system, flushing will pull the salts out of the medium as well as the plant, draw them into the reservoir, and then keep pumping them back into the plants again and again, until the reservoir is drained and the cycle is repeated a few times. Since you’re trying to leach nutrients from the plants, it makes sense that you don’t want to keep recirculating them into the garden.  By using a run to waste configuration, the nutrients are flushed out of the system completely.  All salts and excess minerals are drained from the plants and truly flushed away.

What Are The Disadvantages of Drain to Waste Systems?

  • The main drawback of a drain to waste pass system is that fertilizer is wasted. In general, about 15% to 20% of the nutrient solution applied is runoff. This is not a large amount given the control over nutrition that is possible. As technology becomes available that allows for the testing and replenishment of individual nutrients, a recirculating system that provides a balanced feed, without the waste, will be possible.

Advantages

Disadvantages

Recirculating

-One water supply can feed a larger number of plants

-Conserves water and nutrients, therefore it’s less expensive to operate

-Diseases can spread more easily and rapidly (especially root rot)

-Higher pH fluctuations

-Nutrient levels can become imbalanced

Drain to Waste

-Nutrients are always fresh

-pH levels are more stable

-Nutrient levels stay balanced

-Lowers spread of disease

-Easier to flush out salts

-Higher water and nutrient consumption (15-20% runoff)

-Cannot feed as many plants with the same water supply (will usually have one water supply for 1-6 plants)

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11 thoughts on “Recirculating vs. Drain to Waste Systems

  • It is an enormous problem not being able to test for individual nutrients in recirculated solution. Why hasn’t someone addressed this issue at one of the major nutrient supply companies? They must be able to test for the individual nutrients during production of their products. There are test kits that could detect the presence of the macro and micro nutrients and in some cases determine the density of the various constituent elements. But that’s an expensive proposition if a grower is constantly running tests on the recirculating solution in their system.

    Drain to Waste gives us more control over the quality of the nutrient solution but in terms of the environment dumping the waste solution into the public water supply is a horrible form of unnecessary pollution not to mention the cost factor.

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  • In a “run to waste” system, do you really need to completely saturate your medium each time to the point you have substantial “run-off?” With rockwool, because of its wicking effect, wouldn’t more frequent waterings with smaller amounts of nutrient solution suffice. This would take a little fine tuning to get intervals, flow rates and duration worked out (and flushing with just ph-adjusted water several times/wk), but couldn’t the amount of nutrient that runs directly through the system and out be minimised?

    • Yes, you can try to minimize your run off but you would need to flush way more often because if you are not fully saturating the media, there may be empty pockets where salt can build up and roots can’t grow.

      • Just water the rockwool evenly and you shouldn’t have any issues with dry spots. DTW with rockwool is VERY efficient. I use way less nutes/water overall than any top fed recirculating or ebb and flow I’ve tried.

        I run several different formats of rockwool, from the 3, 6 and 8″ cubes to the chunks/croutons in pots and sometimes their slabs. I mix only the nutes I think I’ll need and rarely if ever feed to runoff. Only feed every other day, give plain ph’d water on the off days. I feed light and only water to runoff about once a week to check the ph. With this method I only need to fully flush about once a month, and then for about a week before taking fruit/harvesting.

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