Even though the name “drain-to-waste” seems to suggest that using this system creates a lot of waste, this is simply not true. The easiest way to understand a drain-to-waste system is to compare it to a recirculating irrigation system.
In a recirculating system, water that has drained off the growing medium is returned to the water-nutrient tank and then pumped once again over the growing medium. In drain-to-waste, the water that is applied to the growing medium is not returned to the water-nutrient tank; instead, it is fed away from the system into a drain.
The fact that some water is going to be drained off in the drain-to-waste system does not mean that there will be significant waste of water or nutrient, however. When designed properly, this type of system wastes very little water.
Below is a discussion in which we look at these two types of irrigation systems within a soilless hydroponic setting. Instead of soil, growing mediums like expanded clay, rockwool, coir, perlite, sand or gravel, and other organic and inorganic mediums are used. (Soil generally includes fine particles like silt and clay that cause medium loss in a drain-to-waste or clogging in a recirculating system.)
Pros and cons of recirculating water systems
An obvious advantage of recirculating water systems is that the extra water and nutrient applied to the plant’s root systems is saved and then reapplied to the roots again. Since the cost the nutrients are usually the primary concern in regards waste, and recirculation would seem to be the better system.
The significant disadvantages of recirculating systems, however, include an increase in the potential for disease (root rot) and the degradation of the nutrients as they are recycled.
Consider this: When solution is applied and reapplied many times, the opportunity for a weaker plant to develop and produce some type of fungal, viral or bacterial pathogen increases—often substantially.
This increase is proportional to time or the number of recirculations; so, the longer a particular crop is being cared for, the greater their risk for some sort of root-related disease. Unfortunately, this also means the greatest risk is at the end of the growth cycle when the crop is ready for harvest.
Drain-to-waste systems, on the other hand, apply water only once before allowing it to drain off. This helps prevents a disease in one plant from affecting all the others. It also reduces the likelihood of disease in any single plant through the use of fresh water, which minimizes undesirable living organisms that could overwhelm a plant’s natural defenses.
The reliability of nutrient benefits to your crop is another issue where a drain-to-waste system can help. In recirculation systems, the nutrients that are returned to the water-nutrient reservoir have actually been chemically modified through the action of the growing medium and biological activity there.
Precipitates will form from the nutrients and these can drop out of solution and become unavailable to the plant, meaning that your plant might not receive proper nutrition. Periodically flushing out your growing medium—that is, flushing and draining of your water-nutrient solution—can help.
This resets your system reduce or eliminate a buildup of any particular nutrients, allowing you to maintain the right nutrient balance for your crop throughout the growing cycle. If you have a recirculating system and want to perform a flush, this can be accomplished by redirecting the outflow to the “drain-to-waste” position.
If you do not have such a valve, you can simply redirect the hosing away from the solution tank and to a waste drain. When the flush is finished, reset this valve or hosing to its original recirculating position.
Hydroponics and water quality considerations
Another important consideration in any indoor grow set-up, regardless of the type of irrigation system it contains, is the quality of the water being used. Some tap water might be high in total dissolved solids (TDS) and some tap water is high in chorine residual, neither of which is good for the health of the plant.
There are a number of ways to neutralize the amount of chlorine residual in the water. One is simply to expose it to the air for 24 to 48 hours. If you want something faster, you can apply a beneficial bioburden to the water. This bacteriological presence in the water will be killed by the chlorine present, which neutralizes the chlorine.
(As a bonus, any of the beneficial bacteria that survive will only help improve the quality of the water-nutrient solution. Still, be careful which products you use to remove chlorine as you do not want to introduce anything that would be harmful to your crop.)
To protect your plants from high TDS, I would recommend not using the tap water at all; instead, use processed water, such as that created through reverse osmosis.
Keeping waste water to a minimum in a hydroponics garden
So, if we are going to use a drain-to-waste system, how can we keep the waste to a minimum? A well-designed drip system with a timer-controller that can be programmed to run for three to four short cycles on any given day is going to be essential.
Slow drip irrigation is a key here because the faster we apply the water-nutrient solution, the more of it drains off and is wasted. Short run cycles also assure that watering stops when the entire growing medium has been moistened.
Different growing mediums, as well as growing conditions, will greatly affect the time between waterings for your drain-to-waste system. Take rockwool for instance; when watered, it will typically hold a ration of about 80% water to 20% air (remember that plants need oxygen in their roots just like they need moisture).
Depending on plant transpiration and evaporation, this medium will go about 24 hours between waterings. Alternatively, utilizing multiple short waterings throughout the day will allow the growing medium to have sufficient drying time before the next watering cycle while keeping the medium moist enough for optimum plant health.
Either way, alternating the moisture level between wet and barely moist will result in better plant health and greater yields.
As mentioned above, each growing medium has different water retention characteristics. Also, remember that an increase in transpiration is normal as your crop matures, which causes higher water demand. (Higher ambient air temperature will also increase transpiration.)
Once you know you’re plants’ and growing medium’s characteristics, you will to need dial your drain-to-waste system into the ideal watering cycle. This means you’ll need a quality timer with run time and cycle versatility.
There are many different types of hydroponic systems, and what basically divides these into two categories is whether the water is recirculated or drained off to waste. No matter which of these you choose for your crop, be watchful off the common issues specific to each system and deal with them accordingly.
Also take notes and do your research so that it gets easier for you with each new crop. Agriculture and horticulture are sciences; treat your crop this way and you will make success common.