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With more stringent effluent discharge standards, many plants are facing a costly upgrading. Such was the situation for the WWTP in Vasteras, Sweden. In 1996 more stringent discharge limits were required, from 15 to 10 mg BOD7/l and from 0,5 to 0,3 mg Ptot/l. New limits were set on nitrogen, 15 mg Ntot/l as annual average.

The original and very costly intention was to upgrade the WWTP to achieve nitrification and introduce wetlands for denitrification. Instead an enhanced contact stabilisation process, based on pre-denitrification with selector functions, was developed, having been in operation since 1998, upgraded at a cost of 25 Mill. SEK (2,5 Mill. USD), exceeding all expectations.

The process, used for upgrading or green field plants, is showing the following major advantages:

20–35 % less volume with the same sludge age, or 20–35 % higher load within the same volume, compared to conventional pre-denitrification, without increased solids load on clarifiers.

the sludge aeration will act as a selector for nitrifying micro-organisms, having a competing edge to other species, using ammonia, not organic matter, as their energy source

a high nitrification rate due to a sludge with more nitrifying micro-organisms

the selector mechanism is not limited to the nitrifiers only. Other microorganisms appear to be responsible for the reduction of surface-active matter

less energy consumption, up to some 35–45 %, due to an increase of the alpha-value

‘automatic’ creation of anaerobic conditions, enabling biological phosphorus removal

BNR plants in cold climates in winter often lose their capacity to nitrify. The Vasteras WWTP has consistently maintained excellent effluent quality even with effluent temperatures as low as 8 C, and at an SRT of some 7–9 days, proving the effectiveness of the process.
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Document Type: Research Article

Publication date: 2002-01-01

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