Hydrauliken i Ångersjöns fosforfällor
bestämning av flödesvägar och åtgärdsförslag för optimal fosforbindning vid avloppsrening med reaktiva filter
Discharge of untreated wastewater leads to unnaturally high levels of nitrogen and
phosphorus in rivers, lakes and the sea, which results in unbalanced ecosystems with
eutrophication and lack of oxygen. Part of the phosphorus in surface waters is coming from
on-site wastewater treatment from scattered households with unsatisfactory purification.
Blast furnace slag is a by-product from iron production, and has in laboratory trials showed good phosphorus binding capacity. To test blast furnace slag as a reactive filter media for phosphorus separation in a full scale experiment, a wastewater treatment plant has been built behind the picnic area at Ångersjön alongside the road E4 in central-eastern Sweden. The plant contains two parallel lines, one with blast furnaces slag and the other with Filtralite-P as phosphorus sorbent. Both filters have showed decreasing phosphorus purification effect with time in spite of a low wastewater load. Inhomogenous hydraulic in the phosphorus filters was suspected to result in poorer absorption of phosphorus than expected. This hypothesis was investigated in the present study.
There is no gradient in phosphorus contents between filter material located at the entrance as compared to the outlet in none of the filters. Phosphorus contents decline with height in both filters, indicating that most probably the larger part of the wastewater runs in the lower section of the filter as a result of that both the inlet and the outlet were at the bottom. The phosphorus level of the filter with blast furnace slag decreases more as a function of the distance from the bottom than the filter with Filtralite-P. This may indicate that the waterflow is more uniformly distributed in the filter with Filtralite-P.
The absence of any gradient in phosphorus level between the entrance and the outlet and
continuously increasing phosphorus concentrations in the outlet water indicates that the filter was saturated. Analyses of total phosphorus content in material from the filter with Filtralite-P, done at an external laboratory show higher amounts of phosphorus than my own analyses with another method show. The total phosphorus content indicates that the filter is saturated according to the specifications of the Filtralite-P supplier. Regarding the blast furnace slag, a higher capacity to adsorb phosphorus has been documented in other investigations than the amount adsorbed in the filter at Ångersjön.
To optimize the hydraulics in the filters and thereby the phosphorus adsorbing capacity, the inlet or outlet should be placed in the upper horizon of the filter and the water level should be kept as high as possible.