Phosphorus reduction by chemical precipitation and tertiary filtration in sand filters at Växjö WWTP, Sweden
The recipient lake is very sensitive. High amount of phosphorus discharge into lakes or sea cause algae blooms, which led to eutrophic oxygen depletion and failure of the system to support aquatic life. In order to reduce suspended solids and phosphorus to a minimum this technical solution was chosen.
Components installed in the solution
In case of Sundet WWTP:
• Screen with 3 mm slot width;
• Aerated sand traps;
• Primary sedimentation tanks with chemical precipitation;
• Aerated tanks;
• Secondary sedimentation tanks;
• DynaSand filters with contact filtration.
DynaSand filters operate continuously. Uninterrupted operation provides several benefits over conventional backwashing sand filters. In conventional sand filters particles are collected in the sand bed with increasing pressure drop as a result, until the sand must be cleaned by backwashing before treatment can proceed. The time between backwashes can be very short, which reduces the effective operating time.
The continuous sand filter technology of DynaSand are an optimal solution to the need for continuous water and waste water treatment process while delivering a constant quality of the filtrate even at a high surface load.
Sundet WWTP is situated outside Växjö, in southern Sweden, and it was built in 1994 to replace an older facility. The plant has a capacity of 95 000 PE (real value 67 670 PE) and the average daily inflow rate is 20 000 m3/day.
The environment in the small lakes around Växjö is extremely sensitive. With high level of phosphorus having been released into the lakes in the past, significant phosphorus reduction measures were taken at the wastewater treatment plant built in 1994. The plant was faced with the challenge of ensuring that the phosphorus content in each litre of water discharged did not exceed 0.2 mg. To fulfil this requirement, DynaSand filters, from Nordic Water Products, were installed as a tertiary filtration. The system consist of six rows made up of ten filters in each run-off area.
The Sundet WWTP has a two-stage phosphorus removal process. The first stage is located between the initial mechanical treatment and biological treatment processes. Chemicals are added to precipitate phosphorus flocs, which are then removed by sedimentation. The second stage occurs in the continuous sand filters, which are the last element in the treatment process chain. Before the sand filtration, an additional chemical addition is made.
The plant was the first DynaSand system in Sweden to use concrete basins. Total filter area is 300 m2 and the filtration rate is five to ten meters per hour. The phosphorus reduction process is now 99%, with 4-7% removed in the filters. The output phosphorus levels are 0.05-0.1 mg/l.