SAND SLOW FLOW BIO-FILTERS
Excerpted from the hand-out of a talk given by Dr. Walter Wohanka.
State Research Institute, Geisenheim, Germany.
Excerpted from the hand-out of a talk given by Dr. Walter Wohanka.
State Research Institute, Geisenheim, Germany.
The idea of using slow sand filtration against water-borne plant pathogens was introduced in the late eighties to the State Research Station at Geisenheim, Germany. Since that time, slow sand filtration and similar techniques have become adopted world-wide in closed cultivation systems.
Principles of slow sand filtration. The principle is simple. Raw water percolates very slowly (through a bed of fine filter sand. The flow rate should be in the range of 2.5 - 7.5 inches/hr (or 25 - 75 gal./sq. yd.) filter surface per hour. Soon after the filter process begins, a filter skin forms on the surface of the filter bed consisting of organic and inorganic material and a variety of active microorganisms. Depending on the raw water quality, cleaning of the filter bed will be necessary after a few weeks or months to prevent clogging. This is done by scrapping off only the top inch. Therefore, the initial thickness of the filter bed should be 32 - 48 inches.
It seems clear that the mechanism is not only a mechanical straining effect but there are also physico-chemical and biological mechanisms. Electron microscopic examination of sand grain surfaces have shown micro-colonization and the development of biofilms.
Construction of Slow- or Bio-filters. The capacity of a slow sand filter depends mainly on the filter surface. The recommendations are in the range of 25 - 75 gal./sq. yd. per hour. By varying the filter surface, bio-filters are adaptable to any size of cultivation area or cultivation systems.
In European greenhouses, mostly standard water tanks are used as filter containers of 18 sq. yd. surface and a capacity of 25 - 75 gal/sq. yd. per hr. However, very large filters are also possible. For example, outdoor versions with 84 sq. yd. surface and a capacity of 1860 - 5600 gal./hr.
The construction of the inlet structure should prevent damaging the filter skin on the filter surface by the raw water. Therefore, the raw should first flow into a box or a wide pipe and then very gently on the sand surface. The raw water may be sprinkled onto the supernatant surface. The flow rate is controlled by special outlet structures. In the simplest way with a gravity driven filter only a valve at the outlet is necessary to reduce the flow rate. The outlet opening should be on a higher level than the filter surface to prevent total drainage and thus drying up of the filter bed.
The standard filter medium is "local" sand which has to fulfill certain requirements such as: effective grain size of 0.15 - 0.30 mm; uniformity coefficient: <3; silt content: <1%; acid solubility <5% after 30 min. the minimum thickness of the filter bed should be 20 - 24 inches; initially, 32 - 48 inches are recommended. the filter sand is supported by three layers of graded gravel and some kind of underdrainage. an alternative material to sand is rockwool. rockwool is slightly more efficient than sand, it is standarized material easily available and of low weight. it does not need drainage layers.
Effectiveness. A series of trials revealed complete elimination of so-called Pythiaceous fungi such as Phytophtora and Pythium. The efficiency rates against bacteria and fungi with small spores have been also very high but some propagules of such organisms may pass the filter bed. In practice, the efficacy seems to be sufficient against these pathogens. Viruses and nematodes are not satisfactorily eliminated by slow filtration. However, the development of viral diseases can be considerably inhibited.
Costs of slow sand filtration. From a grower's point of view, the best point about bio- or slow-filtration is the very low price compared to other water disinfecting systems, such as ultra filtration or heating. Because of the high investment costs of the other mean means of water disinfection, the difference becomes particularly relevant on small farms with low daily water turnover.
Conclusion. Slow (sand) Filtration or bio-filtration is a practical way to eliminate various plant pathogens from contaminated water or nutrient solutions for all horticultural applications. The technique fulfills grower's demands very well: water desinfecting systems which are simple, inexpensive and, of course, effective. Slow (sand) filters are very easy to construct (even by the grower himself) and adaptable to any size of cultivation area and to any recirculating irrigation system.
