Designværktøj til optimering af vandgennemstrømning og tilbageholdelse i infiltrationsbassin og filtermedie ("SUDS" og "LAR")

Abstract

I dette projekt udvikles et designværktøj ud fra simpelt input, der muliggør optimering af vandgennemstrømning og tilbageholdelse af miljøfremmede stoffer i filterjord. Designværktøjet er på pilotskala. Værktøjet er delt op i to modeller, en 1D numerisk vandtransportmodel baseret på MMS-modelen og en analytisk stoftransportmodel. Disse udvikles på baggrund af casen - St. Restrup Fælled og PFAS. Der er udtaget prøver fra St. Restrup Fælled og udført laboratorieforsøg på disse for at kunne sammenligne og vurdere diverse empiriske modeller med virkeligheden. De empiriske modeller undersøges, for at kunne minimere mængden af laboratorieforsøg og dermed tiden det tager, før en beregning kan udføres. Værktøjet kan bruges til at bestemme højden af et infiltrationbassin og designet af en filterjord på baggrund af et ønsket kriterie i forhold til, hvor hurtigt en given mængde af stoffet må passere filterjorden (5%, 50% eller 90%). Den ønsket filterjord designes på baggrund af kornstørrelsefordelingen, ved en iterativ proces. I den iterative proces, kan jordfraktionerne (ler, silt, fint sand, groft sand og organisk materiale), kompakteringen samt overfladearealet af bassinet ændres indtil det ønskede resultat opnås. Ud fra værktøjet vurderes det, at det organiske indhold har størst betydning på stoftransporten, da det forudsæt- tes, at stoffet sætter sig på det organiske materiale. Det er dog ikke realistisk at tilføje en stor mængde af organisk materiale, da dette vil gøre jorden ustabil, og jorden risi- kerer at kollapse. Der anbefales et indhold af organisk på maksimalt 5%. Det vurderes derudover, at et lerindhold på over 35% har en for stor effekt på højden i bassinet og ikke på tilbageholdelsen af stof.

In this project, a design tool is developed based on simple input, enabling the optimization of water flow and retention of environmental contaminants in filter media. The design tool is at pilot scale. The tool is divided into two models: a 1D numerical water transport model based on the MMS model and an analytical chemical transport model. These are developed based on a case study - St. Restrup Fælled for the location and PFAS as the chemical. Samples have been taken from St. Restrup Fælled, and laboratory experiments have been conducted to compare and evaluate various empirical models with the samples taken. The empirical models are being investigated to minimize the amount of laboratory experiments and the time it takes before a calculation can be performed. The tool can be used to determine the depth of an infiltration basin and the distribution of grain size in filter media. These are determined based on a desired criterion regarding how quickly a given amount of substance should pass through the filter media (5%, 50%, or 90%). The grain size distribution is found through an iterative process. In this iterative process, the soil fractions (clay, silt, fine sand, coarse sand, and organic matter), bulk density, and surface area of the basin can be changed until the desired result is achieved. In the designed tool, the content of organic matter is considered to have the greatest impact on chemical transport, as it is assumed that the chemical adsorbs to the organic matter. However, adding a large amount of organic matter is not realistic as it would make the soil unstable and prone to collapsing. A content of organic matter up to 10% is recommended. It is also assessed that a clay content of over 35% has a significant effect on the depth in the basin but not on retention of the chemical. For the St. Restrup Fælled case, a filter media consisting of 20% clay, 2% silt, 35,5% fine sand, 35,5% coarse sand, and 7% clay is designed. With this filter media, it will take 6,7 years, 17,2 years, 7,2 years, and 11,9 years for 50% of PFHxS, PFOS, PFNA, and PFOA, respectively, to travel through 1 meter of the recommended filter media when the basin is 20m x 20m in size. The basin will then have a maximum depth of 0,62m and a volume of 250m2 over the calculated 31 years.

A master thesis from Aalborg University

Designværktøj til optimering af vandgennemstrømning og tilbageholdelse i infiltrationsbassin og filtermedie ("SUDS" og "LAR")

[Design Tool for Optimization of Water Flow and Retention of Chemicals in Infiltration Basins and Filter Media ("SUDS" and "LAR")]