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2.4 Biofiltration

Technical Description

Technology of biofiltration is based on the controlled use of the ecological characteristics of common mollusc species, such as the freshwater mussel (Dreissena polymorpha). Freshwater mussels are natural filter feeders, which effectively and efficiently filter organic and inorganic matter from the water. Freshwater mussels and other water sedentary organisms (such as bacteria, rotatoria, sponges, coelenterates, infusoria, and crustaceans) which constitute the periphyton can, on a specially prepared, artificial bedding with an open weave, create a biological membrane that can effectively purifying water contaminated with organic matter, bacteria, and other biogenic elements.

Providing the mollusc species with appropriate bedding on which other organisms are also able to grow results in the creation of a "living biofiltration system" that is both self-reproducing and highly efficient in reducing concentrations of pollutants in water. For example, such a biofiltration system, or biological membrane, was installed in Raci Lake during 1994. This system consists of a collector panel with 50 biofiltration units. Each unit is a simple sack made of suitable material with a float inside (typically, a sealed plastic bottle). Collector panel is fixed by ropes anchored to concrete weights placed on the lake bottom. Native lake organisms, naturally occurring in the lake water, settle (as larvae) on the sacking and grow.

Extent of Use

This method has been field tested at a few natural sites in Bory Tucholskie (Raci Lake, Suszek), in the northern part of Poland, and there are two known full-scale applications of this technology; one, used for treatment of waters discharged from a salmon trout hatchery and fish farm, at Laska Bia a, Mytlof, and the other, used to purify contaminated water, at Raci Lake.

Operation and Maintenance

Most of the operation is undertaken during the initial stage of preparing the artificial bedding and positioning the unit within the waterbody. Subsequently, some limited control and repairs must be completed. Because the technique uses biological material that naturally occurs in inland waters, the succession of molluscs on the bedding is fully natural process which does not require any intervention. Materials for the bedding are simple and easy to obtain (some waste materials, such as the plastic bottles used for floats, can be utilized).

Level of Involvement

This technique is usually implemented at the local administration level.

Costs

The technology has very low costs. Most construction, including the preparation of the bedding (using waste materials), can be accomplished with minimal labour and materials costs.

Effectiveness of the Technology

The filtration capacity is a characteristic feature of every mollusc species. Freshwater mussels are the most efficient filter feeders in central European inland waters, with a single freshwater mussel (about 3 cm in diameter) filtering around 100 ml/hour. Because of high density of mollusc population (up to a few thousand per m2), the volume of water filtered is very large. Freshwater mussel populations living on a biological barrier with an area of 100 m2 can filter between 500 and 28 000 m3/day and absorb up to 5.5 g of phosphorus and 11.5 g of nitrogen (depending on biological activity).

Suitability

The method is suitable for use in most natural and artificial waterbodies, and can be applied as a polishing step at existing wastewater treatment facilities. It is especially useful in areas with low to moderate environmental impacts. [Caution: Non-native species should not be introduced into waterways.]

Advantages

This biohydrotechnology does not impact natural movements of water within a water body, and, thus, appears to be a useful means for upgrading water quality and ecological value using native aquatic fauna and flora. Because of the flexibility of the technology, and the possible wide range of modifications, it could be used in most types of waterbodies. The technique should be effective over a range of pollutant types. The technology has a very low cost, can be implemented in a phased manner, and is highly effective and efficient.

Disadvantages

In case of chemically polluted waters, persistent pollutants like heavy metals may interfere with the biological utilization of contaminants.

Cultural Acceptability

Method is based on natural ecological processes and is fully acceptable.

Further Development of the Technology

This method is experimental and some research is needed to optimize the design specifications and verify its efficiency, especially under a range of pollution conditions.

Information Sources

Prof. Dr. hab. Roman Gondko, University of ód , Department of Biology and Earth Sciences General Biophysics Unit, ul. St. Banacha 12/16, 90-237 ód , Poland, Tel. (48-42) 35 44 74, fax: (48-42) 35 44 73.

Janusz Krupanek, Institute for Ecology of Industrial Areas, ul. Kossutha 6, Katowice, Poland, Tel.(48-3) 1546031, fax: (48-3) 1541717, e-mail: jan@amnesia.ietu.us.edu.pl.

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