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The Development and Practical Application of Slow Sand Filtration and other ░o-Filtrationechniques in Closed Hydroponic SystemsA
New and Practical Way for Removing Plant Pathogens from Irrigation Water
Dr. Walter Wohanka
State Research Institute, Von-Lade-Str. 1, D-65366 Geisenheim, Germany
( [phone], fax [phone], e-mail:
[e-mail]
Irrigation water sources and recycled irrigation water from so-called closed
irrigation systems pose a certain risk of spreading root infecting plant
pathogens. Suitable equipment, good sanitation, antagonism or
disease-suppression may reduce the risk to a certain degree. However, there will
remain a risk requiring some kind of water treatment. Slow Sand Filtration and
similar bio-filtration techniques are now available to prevent dissemination of
plant pathogens with irrigation water.
History
The first report on Slow Sand Filtration is from 1804. Mr. John Gibb from
Scotland developed a purification technique to obtain clean water for his
bleachery. Later, this technique was used for water purification as a crucial
weapon against water-borne diseases of man like cholera, dysentery and typhus.
In the 20th century slow sand filtration has been replaced or at least
supplemented by other water treatment techniques like chlorination,
UV-irradiation, etc..
The idea of using slow sand filtration against water-borne phytopathogens was
introduced in the late eighties to the State Research Station at Geisenheim.
Since that time, slow sand filtration and similar techniques are used world-wide
in closed cultivation systems.
Principles of Slow Filtration
The principle of a slow sand filter is very simple. Raw water percolates very
slowly through a bed of fine filter sand (see figure 1). The flow rate should be
in the range of 10 to 30 cm per hour (2.5 to 7.5 inches/hr) - that means a
filter capacity of 100 to 300 L per m2 (25 û 75 gal/sq. yd.) filter surface and
hour. Soon after the filter process begins, a filter skin forms on the surface
of the filter bed. It consists of organic and inorganic material and a wide
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 scraping off only the top inch. Therefore, the initial
thickness of the filter bed should be 80 û 120 cm (32 û 48 inches).
mechanisms of water purification are not fully understood. However, it seems
very clear that it is not only a mechanical straining effect. There are
physico-chemical and biological mechanisms involved. However, the contribution
of the biological component to the efficacy of slow sand filter is not quite
clear. Recent investigations revealed a very high microbial activity, in
particular in the top layers of the filter bed. Electron microscopic examination
photos 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 100 to 300 L per m2 and hour m2 (25 û 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 [e.g. 15 m2 (18 sq. yd.) surface and as such a capacity of 1.5 to 4.5
m3/h (400 û 1200 gal / hr)]. Often on small farms plastic tanks may be found
with a surface of only 1 m2 (1.2 sq. yd.) and a capacity of 100 to 300 L/m2h (25
û 75 gal/sq. yd. per hr). However, very large filters are also possible. So, for
example, outdoor versions with 70 m2 (84 sq. yd.) surface and a capacity of 7 to
21 m3 per hour (1860 to 5600 gal /hr).

Figure 2: Functional scheme of a slow (sand) filter
The construction of the inlet structure should prevent damaging of the filter
skin on the filter surface by the raw water. Therefore, the raw water should
first flow into a box or a wide pipe and then very gently on the sand surface.
As an alternative the raw water may be sprinkled onto the supernatant surface
(see fig. 1).
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. In
horticultural practice often another outlet structure is recommended. A wide
pipe [20 to 40 cm (8 û16 inches) in diameter] reaching through the filter bed
into the drainage layer (see fig. 1) is most suitable. In this pipe, a plunger
pump is installed. The capacity of this pump should not be higher than the
maximum filtration rate of the slow sand filter. Otherwise, a regulation valve
is necessary. The flow rate is controlled by the outlet structure (valve and/or
pump). To monitor the flow rate, a flow meter is absolutely necessary.
The standard filter medium is "local" sand which have to fulfill certain
requirements (see table 1). The minimum thickness of the filter bed should be 50
to 60 cm (20 û 24 inches), initially 80 û 120 cm (32 û 48 inches) are
recommended. The filter sand is supported by three layers of graded gravel and
some kind of underdrainage (see figure 1). However, the handling of sand causes
some practical problems under typically horticultural conditions. A series of
experiments was conducted to test the suitability of alternative filter media
such as pumice or rock wool. Comparison of the filter materials revealed that
rockwool (Grodan type 012517 or 012519) produced the highest efficiency rates
compared to sand and pumice. The slight differences in efficiency between sand
and rock wool alone would not justify the higher price for rock wool. However,
this filter material also has some other advantages. Rockwool is commercially
available as a standardized material. As a low weight material it is easier to
handle and no drainage layers are necessary. Furthermore, according to our own
practical experience there are fewer problems with clogging.

Table1: quality of filter sand
      effective grain size0.15 - 0.30 mm (100 û 50 mesh)
      uniformity coefficient (UC)< 3, max. 5
      silt content< 1 %
      acid solubility< 5 % after 30 min
      effective grain size (d10): sieve opening through which 10 % (by weight)
      of the grains will pass
      uniformity coefficient (UC): ratio between the sieve opening through which
      60 % (by weight) of the grains will pass and the effective grain size; UC
      = d60/d10


Effectiveness
For evaluation of the effectiveness experimental results and long term
experience from the drinking water industry can not be simply transferred to
horticulture. The main phytopathogens are different from human pathogens and
drainage water from a crop is definitely different from drinking water.
Furthermore, the running conditions for a slow sand filter in horticulture are
quite different from those of a community water supplier. For these reasons
specific experimental work has been necessary to evaluate the effectiveness of
slow sand filtration against phytopathogens under specific horticultural
conditions.
A series of trials revealed a complete elimination of so-called pythiaceous
fungi like Phytophthora or 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 virus diseases can be
considerably inhibited.


Costs of Slow 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 like ultra
filtration or heating. Because of the high investment costs of the other means
of water disinfection the difference becomes particularly relevant on small
farms with low daily water turnover (see fig. 3).

Figure 3:: Costs of water disinfection (acc. Range 1995)

Conclusion
Slow (Sand) Filtration or bio-filtration is a practical way to eliminate various
phytopathogens from contaminated water or nutrient solutions for all
horticultural applications. This technique fulfils the grower's demands very
well: Water disinfecting systems which are simple , cheap 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.