Nordic Water devises solution for lake clean-up

The university city of Vaxjo, in southern Sweden, is surrounded by beautiful lakes that enhance the city environment by supporting bathing, fishing and watersports, as well as allowing outstanding views of the city from across the waters, and from the city to the forests beyond. Visitors note the inviting and attractive environment, which has helped Vaxjo to earn its title of “The Greenest City in Europe”.

But longer-term residents of the city will tell of an unhappier past, when this major industrial centre was dirty, noisy, and polluted from a wide range of businesses from heavy engineering to linen manufacture and abattoirs, serving the region that looks to Vaxjo as its hub.

These, along with the rapid growth of the city, caused significant problems.

For generations, the lakes had been treated as rubbish dumps, with serious industrial outflows and the discharge of increasing levels of raw sewage close to the populated areas. It was calculated in the early 1970s that just one of the interconnecting system of lakes, Norra Bergundasjon, was receiving some five times the amount of phosphorous that it could naturally absorb, and this had led to the Vaxjo lakes developing damaging and unsightly algae blooms that in turn led to eutropic oxygen depletion and effective death of the lake as a host environment for water life.

The first step in the city’s path to becoming the greenest city was to restore the lakes, and this work started in the 1970s with dredging to remove the phosphorous that was leaking from the lakebed sediments. Lake Trummen – the largest lake in the system – was the first in the world to undergo dredging for these environmental reasons, and along with the other lakes, this has been a repeated operation since that time.

Next, the phosphorous content of waters discharged into the lake system had to be reduced and at the same time, a nitrogen removal stage was decided upon. This two-stage process needed to start in the treatment plant with the conversion of ammonia to nitrates, a process that if allowed to proceed naturally in the lake would actually have consumed more of the all-too-rare oxygen in the water at that time.

Oxygen-rich
But then by introducing nitrates to the oxygen-depleted lake waters, the bound oxygen contained in the nitrates would be consumed by micro-organisms. This unusual solution is appropriate in these circumstances, but would tend to cause further problems if attempted in, for example, oxygen-rich coastal waters where the nitrates would be unlikely to break down.

This additional stage created a challenge. The existing treatment plant was too close to other properties to allow space for extension to accommodate a nitrification plant. But with every challenge comes an opportunity, and here this gave the opportunity to design and build a new plant from scratch, incorporating the best of the available technology at the time.

The Sundet plant has a two-stage phosphorous removal process. The first stage follows the initial mechanical treatment and before the biological stage. Chemicals are added to precipitate phosphorous flocs, which are then removed by sedimentation. Locating this first stage of phosphorous removal before the biological treatment allows the removal of the phosphorous sludge to benefit the nitrification process.

The second stage occurs in the continuous sand filters, which are the last element in the treatment process chain.

The filter stage comprises 60 DynaSand filters, from Nordic Water Products. Each of the six separate treatment lines in the plant is equipped with ten DynaSand units, housed in concrete structures / basins.

The total filter area is 300m2 and the filtration rate is 5 to 10m/h. The procedure in this stage is very similar to the first, the main difference being that the flocs are removed by filtration, to a greater extent. The same floc-building agents are added just before the flow enters the filter plant, without retention basins. It means that both flocculation/coagulation and floc removal occur in the continuous moving filter bed.

The flocs removed by the filters are then channelled back to the inflow to the plant for re-treatment.

Design flow
The Växjö plant was constructed for a population equivalent of 80,000, of which 20,000 was designated from industrial sources. The average daily inflow at the time of commissioning was 28,700m3, with a design flow of 1,500m3/h, with a maximum of twice that volume. The total cost of the plant, including associated pump stations and pipework, was SEK180M (£16M).

But 12 years on, how is the plant performing? And have the lakes recovered?

First, the phosphorous. Total phosphorous levels have been consistently reduced by around 80% over the filters, achieving average outlet levels around 0.1mg/l – design criteria allowed for a maximum level of twice that, at 0.2mg/l. Next, suspended solids have been reduced at the outlet by between 40 and 80%, but typically within the 60-80% range: this represents around 4mg/l in normal operation, against design parameters allowing up to 8mg/l. Finally, Chemical Oxygen Demand (COD) has also fallen, allowing more of this vital gas to remain in the water – typically the DynaSand filters reduce the figures by between 25-45%.

By these criteria, the Sundet Municipal wastewater treatment plant must be considered a success story. But by less scientific measures, accessible to a wider audience, the long-term goal of cleaning up the lake system must also be seen as a success. The city’s residents and visitors can now swim, enjoy watersports, even go fishing for crayfish in the heart of Vaxjo.

The initiative has truly encouraged the city to embrace environmentalism – and this is demonstrated in a number of ways. By using timber more in construction; by using timber waste and sawdust to power and heat the city, as it moves towards a carbon-free goal; and by using power station residual heat to further benefit the residents, rather than go to waste.

And motorists feel the effects too – cyclists are advantaged, and petrol-powered vehicles pay a high tariff to use city streets.

www.nordicwater.se