Issue 24: Slow Sand Filtration

Issue 24
September/October – 1995
Story Title: Slow Sand Filtration
Author: Roger Fox

The slow sand filtration method for disinfesting recyled water, is currently being researched by Gail Barth from the South Australian Plant Research Centre. Her recent presentation to the Australian Hydroponic Conference drew much interest, as ROGER FOX reports.

Slow sand filtration has been used for purifying drinking water for over a century, but only recently has its potential for use in horticulture come to be studied. At the recent Australian Hydroponic Conference in Sydney, Gail Barth from the South Australian Plant Research Centre outlined the technique and its potential role in assisting horticultural producers to meet current and future environmental guidelines.

Ms Barth first became interested in the technique after visiting trials at the Geseinheim Research Centre in Germany, in 1991. There, Dr Walter Wohanka has, over a number of years, been exploring the efficiency of sand filters in removing plant pathogens from water. Gail subsequently applied to the Australian Hydroponic Association and the HRDC for funding to investigate sand filters further, and to encourage their trialling by Australian producers. Approved last year, these Australian trials are soon to begin in earnest.

Slow sand filtration is just that – slow. Flow rates are in the order of 100 to 300 L/hour per square metre of surface area, compared to rates of 10-15 L/sec/sqm (36,000-54,000 L/hr/sqm) used in media filters for screening water used in micro or drip irrigation systems. But their reliability, and simplicity, is a major recommendation.

Early investigations in Germany demonstrated that the filters were reliable in eliminating Phytophthora and Pythium from recirculating nutrient solutions or drainage water. High efficiency was observed against Cylindrocladium, Verticillium dahliae,Thielaviopsis and Xanthomonas bacteria. There is also a report of high efficacy against a virus, pelargonium flower break (Berkelmann et al, 1993).

Ms Barth also said that pathology work with Fusarium spp has demonstrated a 99.9% reduction rate of microconidia (small resting spores) which were poorly filtered by early designs of sand filters. It is assumed by researchers that this level of efficacy is sufficient to prevent serious problems with distribution of Fusarium through recirculating filtered water. Fusarium microconidia are more resistant to heat and UV treatment than other pathogens and are most likely to be most poorly controlled by any disinfestation method.

Around the world, a range of disinfestation methods are being used to recycle water in nurseries and greenhouses. Heat treatment, ozonisation and ultraviolet (UV) radiation are all used to disinfect water against fungi, bacteria and viruses. However, these methods require high capital investment in equipment, need to be used in conjunction with traditional sand filters to remove particulate matter, and are not always effective at removing persistent pathogens. These techniques are often only economical in large production units of more than 1ha of greenhouses (Nienhuis, 1988).

In Australia, container nurseries with water recycling programs are most commonly using chlorination or bromination to treat their run-off. Combined with media filters to screen out particulate matter and solids, and storage tanks for treatment, such systems can be quite effective in disinfecting water of serious nursery crop pathogens such as Phytophthora and Pythium. Collection ponds or dams are often integrated into the system if they can be incorporated into the site; this alleviates some of the filter requirements and reduces the number of holding tanks.

The Slow Sand System
Ms Barth drew on the work of Dr Wohanka in Germany, to describe the mechanics of how slow sand filtration could work in practice. The diagram based on the work of Wohanka 1992 shows a slow sand filter, with a reservoir above the sand and a water inlet below the sand head. Having the inlet below the sand head avoids disturbing a skin that forms on the surface of the filter, soon after it begins operating. This skin consists of organic and inorganic material and a range of biologically active micro-organisms which break down organic matter.

The filter appears to have biological activity in the top 40cm of sand, thus it is recommended that the filter thickness should be a minimum of 50-60cm. Over time, some cleaning of the filter bed may be necessary and it is recommended that an initial thickness of 80-120cm is more appropriate, to allow for scraping off a few centimetres during the cleaning process. Beneath the sand filter are three layers of gravel, which prevent sand from getting into the growing system.

Slow sand filters can be housed in such containers as concrete or corrugated iron tanks, or large plastic bins. A pump may be required at the outlet point if the gravity feed rate of the water is too slow. The pH and conductivity of the water are not affected by the process.

Ms Barth pointed out that an important point to arise from Dr Wohanka’s work, is the fact that the efficiency of sand filtration is dependent on the particle size distribution of the sand (see Table 1). Areas such as this will be further explored in the Australian research trials.

Ms Barth’s research project will be conducted at the recently completed Plant Research Centre in Adelaide. To date, her team have established an ebb and flow recirculating system inside a greenhouse, where crops can be isolated and inoculated with pathogens. They also have an outdoor capillary bed nursery, where they are assessing the management of beds with saline irrigation water. There is potential to also incorporate sand filters into this system to evaluate their possible use in container nurseries. Interestingly, granulated rockwool has shown potential as an alternative medium to sand in this system, and according to Ms Barth, has been very effective also.

Table 1
Grain Size (mm) * * * * Proportion (%)
0.063 – 0.150 = 0.35%
0.150 – 0.160 = 1.10%
0.160 – 0.315 = 33.95%
0.315 – 0.500 = 20.55%
0.500 – 0.630 = 10.90%
0.630 – 0.800 = 8.15%
0.800 – 1.000 = 6.00%
1.000 – 1.250 = 4.30%
1.250 – 1.600 = 3.80%
1.600 – 2.000 = 4.30%
+ 2.000 = 5.75 %