Background
Sundet waste water treatment plant was built in 1994 to replace an older facility. The environment in the small lakes around Växjö is extremely sensitive and the discharge restriction imposed on the plant is one of the strictest in Sweden.
Since the 1970s, treated water from the plant has been released into the Norra Bergundasjön lake. With high levels of phosphorus having been released into the lake in the past, significant phosphorus reduction measures were needed at the new plant.
Solution
To meet the requirement for less than 0.2 mg of phosphorus per litre of water, Sundet waste water treatment plant uses DynaSand filters both before and after precipitation. The system consists of six rows made up of ten filters in each run-off area. One of the lines also serves as a test line for trying out different operating positions and chemicals.
Process stages:
• Screen with 3 mm slot width
• Aerated grit chamber
• Primary sedimentation tanks with chemical precipitation
• Aerated tanks
• Secondary sedimentation tanks
• DynaSand filters with contact filtration
Result
Water quality
The phosphorus reduction process at the plant is working very well and has now reached 98%, 5-10% of which is removed in the filters. The output phosphorus levels are often less than 0.1 mg/l.
Parameter [mg/l]
After sedimentation
Suspended solids: 5-30
Total phosphorus content: 0.3-0.7
Filtrate from DynaSand filters
Suspended solids: < 5
Total phosphorus content: < 0.2
Chemical dosing before DynaSand filtration
Type of chemicals: Eka mix (8% Al, 1% Fe) [ml/m3]
Dosage: 30-50
Facts about the system
Design flow rate: 1,500 m3/h
Daily flow rate: 28,600 m3/h
Maximum flow rate: 3,000 m3/h
DynaSand filters: 60 x DST50, concrete
