DynaSand® filters have been successfully protecting the lakes of Växjö in southern Sweden from excess phosphorus and algae blooms for over 20 years.
Two decades ago, the community Växjö in southern Sweden faced considerable environmental problems in the form of excessive levels of phosphorous in its local lake system. Växjö is located in central southern Sweden, about 400 km south of Stockholm. Their issue with phosphorous sedimentation in the lakes had been going on since around the turn of the century and originated from the historic discharge of raw sewage. Historically there was also a linen factory, a butchery and other industries located in the area that had an impact on the phosphorus sedimentation in the lake system.
It was estimated that the local lake Norra Bergsundasjön could absorb a maximum of 1 ton of phosphorus per year without damaging the eco system in the lake. During the 10 years before this estimate, an average of approximately 1.9 tons had been emitted from Växjös old treatment plant and discharged to the lake. Beyond the emissions from the old treatment plant, an additional 2.8 tons of phosphorus was leaking into the lake water from the phosphorus rich sediment in Norra Bergsundasjön and other surrounding lakes. The phosphorus emissions to the lake were consequently almost five times larger than what the lake could handle, resulting in a widespread algae blooming.
This issue was approached in two steps, where the first step was to dredge the phosphorous leaking sediment out of the lakes. The second step was building a new sewage treatment plant which incorporated a two-stage phosphorous removal process, where the second stage involved contact filtration ie. flocculation. This was provided by the DynaSand® continuous sand filter, where the flocculation was performed within the filter.
The new sewage treatment plant was built in 1994 and was named Växjö Sundet. Phosphorus removal at Växjö Sundet is achieved in two stages to meet the plants outlet requirement of 0.2 mg/ l of total phosphorus. The first stage is located after the initial mechanical treatment and before the biological stage. Chemicals, generally iron sulfate, are added to precipitate phosphorus into flocs which are then removed by sedimentation.
The second phosphorus removal stage is filtration through continuous sand filters. The filter stage is the last element in the treatment process chain and consists of 60 DynaSand® filters. The 60 DynaSand® filters are divided into six separate treatment lines, each equipped with 10 DynaSand® units housed in concrete structures; a compact solution, about the size of a tennis court, with a total filter area of 300 m2. The filtration rate in Växjö sundet is 5 to 10 m per hour. The procedure in the second phosphorous removal stage is very similar to the first in the aspect of adding ferric salts to precipitate the phosphates which are later removed. The same precipitant is added just before the flow enters the sand filters where the solid phosphates are removed. The solid phosphates are removed from the sand in the integral sand washer. The separated phosphates are channelled back to the plant inlet for re-treatment.
The sewage treatment plant Växjö Sundet has now been in operation for over two decades and maintains an excellent performance record. The plant has a population equivalent of 85,000 of which 25,000 is from industrial sources.
The phosphorus content in the outlet water corresponds directly to the amount of chemical dosing (iron sulfate). Annual averages of 0.05 mg/l have been achieved. The incentive to get as low as 0.05 mg/l is limited, but increased chemical dosage during some years show how ambitious the phosphorus reduction could be.