Battling the dreaded pythium and other root rot pathogens in a hydroponic system can be a demoralizing and long-term issue for some growers, with little in the way of effective chemical or other controls.

Even when roots look pristine, white and healthy, we all worry from time to time if nasties are starting to invade our hydroponic nutrient or causing issues without us even knowing what might be occurring at the microscopic level.

One of the best defenses against nutrient-borne pathogens is other beneficial microbes. In fact, if present in high populations and combined with effective microfiltration, control over problematic phytopathogens becomes an extremely effective practice.

Slow sand filtration

Slow sand filtration, sometimes termed biofiltration, is a method of drinking water purification that has been around for well over a century. Since the 1980s, slow sand filtration has been under investigation for its use in cleaning up irrigation or waste water from horticultural operations.

Even more recent and precise studies have evaluated biofiltration as a means of eliminating nutrient-borne plant pathogens like pythium, phytophthora, verticillium, fusarium and others.

So, large-scale commercial hydroponic operations around the world are now incorporating huge slow sand filters as a way of not only dealing with waste water, but to also purify and control nutrient-borne pathogens in recirculating hydroponic systems.

What is particularly exciting about slow sand filtration methods for smaller growers and indoor gardens is that the system can be scaled down to fit any size system and is relatively easy to construct and run with little cost to the grower.

How does slow sand filtration work?

A slow sand filter works on a number of different levels. Firstly, the filter material (traditionally sand, though other materials can also be used) screens out any organic or suspended matter—for example, algae small pieces of root or plant material, sediment, etc.—from the nutrient solution.

Secondly, and more importantly, the filter material provides a home with a large surface area for a wide range of beneficial microbial inhabitants. These beneficial microbes are what provide the biological filtration that has been shown to remove pathogens at up to 99% efficiency.

The principle behind slow sand filtration is that the nutrient solution applied must flow very slowly through the bed of fine material. If rates of nutrient solution flow are too rapid, the removal of plant pathogens is compromised and the filter might not be effective for disease control.

Nutrient solution flow rates for biofiltration must be within the range 2.4 to 6 gal. per square foot of filter surface area per hour. So, for a small hydroponic indoor garden with a recirculating system where the nutrient solution needs to be treated, a slow sand filter with 1 sq. ft. of surface area and set at a depth of at least 23 in. will be able to filter 2.4 to 6 gal. of nutrient solution per hour.

Once the nutrient solution has slowly flowed through the filter and collected from the base, it can be returned to the hydroponic system. Using this recommended flow rate it is possible to calculate the size of slow sand filter required based on the amount of nutrient solution that needs to be treated every day.

Some growers treat their incoming water supply with slow sand filtration before it is used to make up a nutrient solution or is added as top-up water to a reservoir. This is a good idea whenever rain, pond, stream, river or shallow well water is used as the hydroponic supply because these sources can all contain plant pathogen spores.

Those with clean and good-quality water supplies can simply use a slow sand filter to keep the recirculating nutrient clean and free of disease pathogens and to help inoculate the nutrient solution with beneficial microbes.

How to construct a slow sand filter

There are small slow sand filters that can be purchased as a complete, ready-made unit. Typically, these are designed for the aquarium industry and might not have the capacity or type of filter material that is optimal for hydroponics.

Most growers who utilize slow sand filtration construct their own filters based on the size required for their hydroponic system and nutrient requirements. For a small indoor garden, a slow sand filter can be made from a plastic bin, deep bucket or large diameter plastic pipe—all that must be provided is a depth of at least 23 in. or more.

The top of the filter needs to be open to the air as oxygen is a vital component of biological filtration. The base of the filter is filled with coarse, clean drainage sand or gravel (about 5/16 to 7/16 in.); the middle levels of the filter with finer sand (about 1/16 to 5/16 in.) and the top layer with the finest grade of sand (0 to about 1/16 in.).

The finest top layer of sand should be at least 15-in. deep, as this is where the majority of the biological filtration will occur. Some research has been carried out into using granulated rockwool as the filter body material as an alternative to sand and this has proven to be highly effective particularly in smaller filters. Granulated rockwool can also have the advantage of being cleaner and less likely to leak fine sand into the lower layers of the filter.

Filter operation

Nutrient solution or water to be treated must be slowly dripped or sprayed onto the top of the slow sand filter so as not to dislodge or disrupt the filter surface. Spraying the water/nutrient solution to be treated onto the filter surface helps oxygenation, which is important as the bacteria in the filter bed require oxygen to function.

A shallow layer of water (supernatant water) must remain over the surface of the sand to keep it moist, while the slow flow rate is controlled by the outlet in the base of the filter system with an in-line tap.

Nutrient solution flowing through a slow sand filter will undergo biological filtration; however, this process will not change the physical or chemical nature of the solution. As such, pH, EC and levels of individual ions won’t change during filtration.

What can occur is that dissolved oxygen levels in the nutrient solution can drop as the solution flows through the filter material, which becomes highly populated with microbial life, thus increasing the biological oxygen demand (BOD).

Aeration of the solution in the nutrient reservoir or before it is added back into the hydroponic system can be achieved with air stones and pumps or by the cascade/fountain method.

Slow sand filters, once set up, should be relatively trouble-free; however, depending on the organic loading of the water supply or nutrient solution, they could need some maintenance. Over time, the top of the filter bed can become clogged and need the top 1 to 2 in. removed and replaced to allow nutrient to flow freely through the sand.

These processes should only be carried out when absolutely necessary, as this top layer is rich in biologically active micro-organisms that help break down organic matter.

The effectiveness of a slow sand filter is dependent on a number of factors, the main one being flow rate. While the flow rate of nutrient solution through the filter is recommended to be within the range 2.4 to 6 gal. per square foot of filter surface area per hour, there is a negative correlation between flow rate and removal of plant pathogens.

For that reason, flow rates of 2.5 to 3 gal. per square foot per hour would be recommended when problems with pythium and other pathogens exist. The efficiency of the filter is also dependent on the health and species diversity of the microbial populations that develop inside the filter.

While it’s not a requirement to inoculate a new filter with microbial mixtures, as these will naturally develop over time, it is advisable to age a new slow sand filter for a few weeks before it is required to treat nutrient or water.

Microbial populations also work most effectively in warm temperatures, with sufficient oxygen and with moisture; so, the filter material should not be permitted to dry out, even when not in use.

Slow sand filtration, if constructed and run at the correct flow rates, is highly effective against a wide range of root disease pathogens that can be carried in the water supply or nutrient solution. As a passive disinfection technique, slow sand filtration is easy to set up, cost effective, environmentally friendly and low maintenance.

Its only limitations are that some space is required to house the filter and that it is largely ineffective against viruses and nematodes. For indoor growers experiencing ongoing root rot pathogen problems, slow sand filtration could be one of the best options for safe, long-term disease control.