Retention soil filter
Retention soil filters ( RBF ) belong to the group of filter systems or wastewater treatment systems . As components of a drainage system , they serve to further treat the discharge drains of the mixed system or clean heavily polluted drains from separation systems and street drainage as part of rainwater infiltration . In general, the dimensioning of RBF takes place according to the permissible “stack height” (= hydraulic filter load; separation system: max. 40 m × a −1 , mixed system max. 30 m × a −1 ). This results in a net filter area of 0.3–1.3% of the connected, drainage area (A red ).
As a process engineering unit, RBF are mandatory two-stage constructions consisting of an open retention basin for light material separation and particle sedimentation (catch basin) combined with a downstream overgrown and bottom-sealed soil filter. The name given to RBF is a storage space ( retention space ) located above the soil filter, which buffers hydraulic stress / inflow peaks or enables short-term overflows. In engineering terms, a distinction is made between vertical filters (flow from top to bottom) and horizontal filters (lateral flow) as well as the retention time of the water in the base filter (operating mode: permanent backlog or free drainage) for the central floor filter .
RBF clean infiltrating water as it passes through the revitalized and strongly sorbent topsoil into the subsoil. The cleaned water is collected by a drainage system and can be discharged into a body of water via a drainage structure or contribute to the formation of new groundwater through seepage . The eponymous retention space above the floor filter dampens outflow peaks through intermediate storage and throttled discharge and thus contributes to the hydraulic relief of the sewer system.
The development of soil filters for rainwater treatment is based on experience from sewage treatment plants (plants, substrates). Particularly in their mode of operation (only temporary stowage phases), retention floor filters differ significantly from plant-based sewage treatment systems, which are designed for a continuous accumulation of wastewater.
The first retention soil filter was put into operation in 1988 in Sinsheim - Waldangelloch ( Baden-Württemberg ). An operational inspection of this system showed such good cleaning performance that a large number of other systems have been planned and built since then. The dimensioning and construction have been continuously developed.
The main components are:
- Preliminary stage:
- Supply structure
- Retention space
- Planted with reeds
- Filter body
- Drainage structure with throttle device and, if necessary, drainage measurement
- Relief structure with filter basin overflow
Further information on technical requirements and structural design options can be found in various sets of rules (e.g. DWA-Merkblatt 178 , ATV Arbeitsblatt 138 ) as well as the brochures of the State Agency for Environmental Protection Baden-Württemberg and the manual for soil filters of the Environment Ministry of North Rhine-Westphalia .
Since RBFs are briefly exposed to large volumes of water, an essential requirement for the filter material is a permanent flow-technical load capacity and sufficient hydraulic permeability ( permeability coefficient kf 1 · 10 −3 to 1 · 10 −6 m × s −1 ). As a rule, washed sands with a grain size of 0/2 mm with a dominant proportion of medium sand are used for this purpose. Edge-rounded ("fluvial") sands with predominantly rounded grain shape are preferred because of their better hydraulic properties and high physical stability. At the same time, they promote the formation of biofilms and the rooting of the filter vegetation.
Filter substrates are of central importance for the cleaning performance of the RBF against infiltrated water. The soil matrix influences the cleaning capacity through the mechanical filtration of particulate water constituents in the pore system as well as chemical-physical ( adsorption , ion exchange , precipitation , complexation ) and biological processes ( degradation , transformation , absorption). In addition, there is an “indirect binding” of water constituents through the water storage capacity of the soil, which extends the contact time and promotes sorption, absorption and microbial degradation.
The cleaning capacity of sandy filter substrates is essentially based on microbial pollutant conversions; In contrast, sorption processes are limited by the relatively low cation exchange capacity of sands (<5 meq / 100 g); Information for the required filter area is 100 m 2 / ha A red (= 1% of A red ).
The performance of technical filter sands can, however, be specifically adapted by adding reactive materials in order to achieve certain properties ( melioration ). These additives increase the sorption capacity relative to the filter sand or selectively improve the retention of certain groups of substances. In addition, they can specifically influence the hydraulic conditions of the filter material. For reasons of cost, the use of technical ion exchangers or activated carbon is very limited.
Since the filter material serves to protect the groundwater , the highest demands are placed on the selection and preparation of materials, as well as quality controls of all substrate components before and during the filter installation. Detailed selection criteria and requirements for the substrate properties are listed in the retention soil filter manual .
The primary aim of planting the RBF is to obtain a loose, hydraulically permeable substrate through permanent root activity, which counteracts the tendency to silt up and encrustation of the filter surface ( colmation ). An intensive root penetration therefore stabilizes the filter function in the long term. The shadow cast creates a damp and cool microclimate close to the surface, and the insulating vegetation also dampens seasonal temperature fluctuations above the soil filter. Both of these significantly promote the living conditions of microorganisms that settle near the surface. In the rhizosphere , root aerenchyme (ventilation) and exudate stimulate the microbial degradation of pollutants in the soil. Further positive aspects of the filter vegetation are the reduction of the seepage water volume through transpiration as well as the withdrawal of N, P and metals. After dying, the organic substance contributes to the formation of a "space filter" and as an additional sorbent.
The planting is often done with reeds ( Phragmitis communis ), although this is not a typical vegetation on sandy substrates. Since it does not settle spontaneously, it has to be artificially established. This can be done with seedlings, rhizome planting, the insertion of reed balls or the laying of reed mats. Seedlings are only fully developed after three vegetation seasons, during which the filter surface is endangered by silting up ( colmation ) or the emergence of spontaneous vegetation. In contrast, prefabricated reed mats achieve covering vegetation more quickly, which means that the RBF is operational earlier.
Since non-congested RBFs are not part of the ideal settlement zones for phragmitis due to longer periods of dryness , their physiological requirements are often not or only insufficiently met and the reed vegetation is lost. For this reason, other suitable plants with a broader ecological moisture amplitude ( pointer values according to Ellenberg ) such as grasses (sowing or turf), irises , cattails , plumes of water , rushes, etc. are used in a targeted manner.
The costs of an RBF are made up of the construction and material costs as well as the subsequent operating costs. When calculating the construction costs, it should be noted that in addition to the pure material costs and labor costs, other factors such as B. Transport costs of substrates and plants u. must be taken into account. Literature data from NRW therefore show for the various building trades z. T. wide spans. On average, 51% of the construction costs in NRW are made up of earthworks and drainage works, followed by inflow and drainage structures (21%) and the filter seal (15%). The planting contributes an average of 3% to the construction costs.
The literature (Sieker, 2001) shows the following average construction and operating costs:
- Construction costs: approx. € 2.40 / m 2 A red
- Operating costs (maintenance, green maintenance): 0.01 € / m 2 A red × a
- Annual costs (estimated): 0.13 € / m 2 A red
In the planning phase, the costs can be reduced through early and fundamental integration of rainwater management in the catchment area, through consideration of natural spatial boundary conditions (geology, topography, hydrogeology) and the settlement structures (spatial planning, building structure) as well as the nature of the precipitation water and the treatment goals. Other cost-reducing factors are the installation of suitable sand near the site, the shared use of existing facilities and a compact construction.
The mean useful life of RBF is estimated at 25 years of operation.
- Abwassertechnische Vereinigung eV: ATV - A 138 Planning, construction and operation of systems for the infiltration of rainwater. GFA, Hennef 2002.
- Abwassertechnische Vereinigung eV: ATV - A 166 Structures for central rainwater treatment and retention. Constructive design and equipment. GFA, Hennef 1998.
- German Association for Water Management, Wastewater and Waste eV (DWA): Leaflet DWA-M 178: Recommendations for the planning, construction and operation of retention floor filters for further rainwater treatment in mixed and separated systems. Hennef 2005.
- Hessian Ministry for the Environment, Rural Areas and Consumer Protection (HMULV) (Ed.): Rainwater treatment through retention soil filter systems . 2007.
- Ministry for the Environment, Forests Rhineland-Palatinate (Ed.): Natural handling of rainwater. Concept and executed examples. Mainz 2000.
- State Institute for Environmental Protection Baden-Wuerttemberg (Ed.): Soil filter for rainwater treatment in the mixed and separated system. Karlsruhe 2002.
- Ministry for the Environment and Nature Conservation, Agriculture and Consumer Protection (MUNLV) of the State of North Rhine-Westphalia (Hrsg.): Retentionsbodenfilter. Handbook for planning, construction and operation. 1st edition. Düsseldorf 2003.
- F. Remmler, U. Schöttler: Qualitative requirements for near-natural rainwater management from the point of view of soil and groundwater protection. In: F. Sieker (Hrsg.): Natural rainwater management. Analytica, Berlin 1998, ISBN 3-929342-31-6 , pp. 104-125.
- K. Seidel: Purification of waters by higher plants. In: Natural Sciences. 53/12, 1966, pp. 289-297.
- H. Sieker: Evaluation of measures for rainwater management with regard to costs and effects. In: S. Heiden, R. Erb, F. Sieker (Eds.): Flood Protection Today - Sustainable Water Management. Erich Schmidt Verlag, Berlin 2001, pp. 83–110.
- M. Uhl, R. Adams, RW Harms, F. Schneider, D. Grotehusmann, U. Kasting, G. Lange: ESOG Introduction of surface water runoff from roads into bodies of water - final report - file number IV - 9 - 042 252 on behalf of the Ministry for the environment and nature conservation, agriculture and consumer protection of the state of North Rhine-Westphalia. Munster 2006.
- F. vd Kammer, PH Jacobs: Decentralized rainwater purification on the B 75: Problem solving with reactive filter systems. In: Rainwater infiltration - a possibility of decentralized rainwater management. (= Reports from water quality and waste management Technical University of Munich. No. 175). 2003, pp. 187-203.