Efficient dewatering of sediment from rainwater basins
Authors: Agnieszka Behrend, Simon Østergaard Jensen, Amal Tarek Hassan, Alexandra Roed Roin
Rainwater basins sediment, the byproduct of rainwater collection systems, contains everything from runof gathering to improper waste disposal. Rainwater basins need to be emptied from precipitated materials when required, which is dependent on sediment accumulation rate. Usually extraction of the sediment happens every 10 – 30 years, where large amounts of the sediment with an average water content of 70 – 80% need to be handled. The high water content and known heterogeneity of the sediment due to varying organic and inorganic matter content, requires a handling method with efficient time range and space to store big volumes of the extracted sediment.
The commonly used dewatering method for rainwater basins sediment, geotextile bags, have a performance time of 6 – 12 months. The space required for the geotextile bag storage is defined based on the rainwater basin size and therefore, volume of the sediment extracted. Geotextile bags are well-know and efective tool for removing sediment-trapped water under one condition – that the dewatering time is efficient. Since it is highly improbable that this condition can be met, there is a need of improving the sediment treatment efficiency by developing pre-treatment method or improving the existing dewatering method.
The aim of this project is to find a method that requires less space, takes less time and gives better dewatering result under newly assigned parameters. To do so, three rainwater basins where chosen in diferent runof areas, respectively a residential area, a trafficked area and an industrial area. The composition of the rainwater sediment in those three basins was characterized in regards of heavy metals, water, organic and inorganic matter content. Main variations were defined and addressed to the design of newly developed methods.
Three dewatering techniques were chosen for lab-scale testing: a pressurized geotextile bag with addition of polymers, a pressurized geotextile bag without addition of polymers and a hydraulic piston. None of those methods had any pretreatment unit applied except the sediment extraction method, which working principle is similar to a vacuum cleaner. Those dewatering techniques showed satisfactory results within efficiency in removing trapped water, with lowering water content to 45%. Time needed to fulfill the procedure was indicated as few hours. Besides the positive efect on dewatering efficiency within applied methods, the negative efect of polymers addition on dewatering method has been proven.
To unify the composition of the sediment, a possible pre-treatment method for organic and inorganic matter segregation was tested. Flotation tank, which worked as separation unit, gave satisfying results in relation to separation of two fractions: one considered as organic, when floating on the top of water layer; and the second one considered as inorganic when settling in the bottom of the tank. Methods for dewatering and utilization of those two fractions are considered as separate units.
The project benefits supply companies, which need to handle sediment that accumulates in rainwater basins.
Environmental impact and transportation costs can be reduced due to lowered volume and unified composition of the sediment.