Mechanical filtration is not an effective tool to prevent water borne diseases in NFT hydroponic nutrient solutions. It has been found that the total organic carbon content can reach 20 mg.L-1 for 2-3 months without affecting plant growth. There are many other compounds that attribute to the total carbon mass such as chelates and the biomass of microbes. By replenishing all the circulating water on a continuous basis, most of the organic material will be removed in any case. Extensive work has been done on biological filters which show positive results in preventing pythium. However, once the disease reaches the root system there is not much one can do to reduce its effect other than remove all the plants, clean the system and re-plant. It is possible to run a closed hydroponic system for 90 days without filtration and without replacing all the water. The disadvantage of this method is the build-up of certain nutrients in the water causing an imbalance of nutrients (See the post on refilling nutrient solutions and differential uptake of nutrients.
It has been found that after a while, an equilibrium exists between micro-organisms on root surfaces and root exudates. The micro-organisms degrade additional organic material to CO2. It is a well known fact that plants which are under stress release more exudates, however, since plants which grow under hydroponic conditions are under less stress, the concentration of exudates will be less.
The only time when mechanical filtration is required, is when the water source is polluted or has some soil or other inorganic matter. Depending on the amount of inorganic material, the roots of the plants will be suspended in a dense powder at the bottom of the gullies. This suspension has little oxygen for the roots to grow in and will affect plant growth and root development.