Case study Enkoping use of sewage sludge

B.19 To help meet a Helsinki agreement obligation to reduce nitrogen inputs to the Baltic Sea, the energy company Ena Kraft, which is based in Enkoping in Sweden, is using sewage sludge as a fertiliser for its willow plantations. This is cheaper than usual nitrogen removal processes and has proved so successful that the municipal council has financed additional willow plantations and is processing sewage from private septic plants as well as municipal waste.

B.20 By using sewage to irrigate willow, the Enkoping council diverts 250-300 kg nitrogen/ha/y from surface waters and, ultimately, from the Baltic. Phosphorous and heavy metals are similarly removed. Bottom ash from the Ena Kraft CHP plant is also added into this fertiliser mix.

B.21 By using sewage sludge and recycling the bottom ash, Ena Kraft not only meets its air quality and CO2 targets by using willow as a fuel in its power station and also secures other environmental benefits not normally associated with energy production. Ena Kraft attributes the success of this project to the co-operation of a number of stakeholders including the municipal water and wastewater works, the power plant, the environmental conservation

Figure B-I Metal cycle in Enköping CHP-plant

Total fuel input: 350 GWh

Salix uptake from ground:

Zn: 731

Chips Sawdust Willow Bark

irrigation project 200,000 m3/year

100%

Cd: 0,75 g/ha & year Cu: 194,5 Cr: 26,1 Hg: 0,33 Ni: 12,9 Pb: 15 Zn: 324

Cd: <1,1 g/ha & year Cu: 183 Cr: <13 Hg: <0,4 Ni: 25 Pb: 13 Zn: 341

100%

Bottom ash

Fly ash

Cd

: 10%

Cd

: 90%

Cu

: 50%

Cu

: 50%

Cr:

60%

Cr:

40%

Hg

: 20%

Hg

: 80%

Ni:

30%

Ni:

70%

Pb

: 20%

Pb

: 80%

Zn

20%

Zn

Waste water treatment plant

Clean water & sludge water

Deposit

Flue-gas condenser

Condensed water 30,000 m3/year

Clean water

3,8 milj. m3/year

River board, the municipal council and the farmers. It is a complex system and requires planning, discussion and effort to make it work, but once established it operates well.

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