Soil filters are particularly suited for the treatment of sewage from small settlements (e.g. population equivalents of up to 50) and individual buildings, but also prove a sound choice in areas where sewer construction, due to the high investment needs, would be too expensive.
Soil filtration is a relatively simple yet efficient process (see diagram): Wastewater is preclarified in a tank where suspended solids settle out and is then pumped in doses to a sand and/or gravel bed. Here microorganisms do most of the cleaning by breaking down organic matter. Soil filters are usually equipped with side and bottom seals. After trickling through the system, the purified wastewater is collected at the bottom by drainage pipes and, depending on its cleanliness, either discharged into a nearby river, ground- infiltrated or used for irrigation.
The European CEN/TR 12566 standard (Small Wastewater Treatment Systems for up to 50 PT) distinguishes three types of filters:
- buried filters (covered with a layer of native soil; the surface can subsequently be used for gardening etc.)
- (gravel-)covered filters
- open/plant-covered filters (identical with the Austrian ÖNORM B 2505 standard; these are often reed beds [Phragmites australis]).
With a view to observing the stringent national outflow limits for small wastewater treatment plants in Austria, especially the latter type has undergone continuous testing and development in the last decade. Such research often leads to unexpected findings, as is demonstrated by the change of meaning which reed plants have experienced in the course of time.
While experts formerly believed that the reef roots themselves play a vital role in wastewater cleaning, they now know that reed (Phragmites) rather supports the efficacy of microorganisms dwelling in the soil by providing the filter bed with shade, isolating it and loosening up the ground with its roots. Reed also prevents the growth of undesirable ruderal vegetation, such as stinging nettle.
In Austria, the sewage treatment plant of Ernsthofen, Lower Austria, has become established as a leading centre of scientific research and practical work on soil filters over the last three years. With local authority approval and in collaboration with the Institute for Sanitary Engineering and Water Pollution Control of the Vienna University of Natural Resources and Applied Life Sciences (BOKU), the Vienna-based private research institute Ökoreal is currently undertaking research activities in the area examining whether soil filters can be downsized.
The results may be of great practical relevance: Karl Rohrhofer from Ökoreal estimates that a significant reduction of the current minimum space requirements (4 m2 per capita according to ÖNORM), while observing mandatory outflow limits, would cut down construction costs by 30 percent! Operating costs, being extremely low already, are not expected to change.
Says the expert: “In two government-funded (BMLFUW) research periods we have already managed to prove that filters in Austria can be downsized to at least 3 m2 per capita (in gravelcovered two-stage systems). It also became clear that the use of a gravel cover assures more consistent outlet values during the cold season.”
Open filters: will they remain the optimal solution?
In the wake of expert disputes and an official decree already leading to a ban on open soil filters in France, research activities in Ernsthofen are now likely to continue beyond 2007 – with possible consequences also at EU level. Scandinavian countries, due to their local climate, use only buried soil filters.
Experts in these countries often claim that such filters, while not requiring more space than the open systems traditionally used in Central Europe, have equally positive outlet values. Such claims are hard to verify: most systems in Northern Europe are unsealed and this makes the monitoring of outlet values difficult. Karl Rohrhofer says evidence thereof could be established in Ernsthofen under scientifically proven conditions in an additional test area set out next to the existing one. He suggests a twostage system to allow matching the outcome with previous research results.
Asked about the likely outcome of these experiments, Rohrhofer believes that the cleaning efficiency of buried soil filters can only match that of open/plant-covered systems if forced aeration is introduced. As most Scandinavian soil filters are bottomless, however, there is no legal proof. According to the Ernsthofen test results, open (with or without vegetation) and covered soil filtration systems still prove the best option in terms of cost-effectiveness and cleaning, provided they have a two-stage design.
Despite Rohrhofer’s original intention to launch an EU-wide research programme, scientific tests with outcomes likely to favour open soil filters are in fact not sought after by all European countries. The French, for example, have banned open soil filters for some time due to sanitary reasons and meanwhile require the installation of buried systems.
Time will show whether in Austria promotion of soil filter research is continued. Without doubt this would be of great importance as some wastewater problems in rural areas are still waiting to be solved. Moreover, not every existing (conventional) small wastewater treatment plant can boast good outlet values – much depends on the quality of maintenance.
In many cases, soil filters could prove a sound alternative given their low maintenance requirements. And once a soil filter does become defective, it won’t be out of order for long as the foul smell will stir any nearby resident or farmer into action!
Contact & Information:
DI Karl Rohrhofer
Carl-Reichert-Gasse 27, A-1170 Wien
Tel.: +43/1/480 50 10-0
(Source: aqua press Int. 1/2007, Mag. Christof Hahn)