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De-icing glycol from airports is usually considered to be an environmental

problem. However, now at one Norwegian airport, the used glycol is collected

for use as nutrient for bacteria that process wastewater in a nearby treatment

plant.

The Gardermoen plant treats wastewater from the airport, from local industry

and from the 20,000 or so people who live in the surrounding counties of Ullensaker

and Nannestad. The amount of wastewater produced by the airport and its restaurants

and toilets takes up about a third of the plant’s capacity.

‘The plant has worked well from the start and has managed to keep above the

high standards required,’ said Willy Slora, technical manager.

‘Norwegian authorities have placed rigid criteria on the efficiency of waste

treatment here, demanding 95% removal of phosphorus, 90% removal of organic

material and 70% removal of nitrogen. There are only four or five other plants

in Norway which remove nitrogen from wastewater,’ he added.

Re-use of glycol

The plant receives about 5,000 metres³ of wastewater per day from the two

counties and the airport. It also receives glycol-containing wastewater from

the airport, which is the result of the de-icing of aircraft in winter months.

An important part of the denitrification process is the addition of the glycol

to the plant’s bioculture reactors. The glycol is an important source of hydrocarbon

in the denitrification process and replaces the ethanol normally used. Apart

from being a sensible environmental solution, the re-use of glycol also saves

the plant considerable expense.

The plant was originally built to deal with the glycol-containing water as

an adjunct to wastewater treatment. No one expected that the glycol waste would

be sufficient for the plant’s denitrification needs, so a container for storing

ethanol was installed. However, so far the container has not been used. There

has been more than enough glycol waste to feed the bioculture reactors.

The first phase in the treatment of wastewater removes the largest particles,

such as napkins, cotton wool and sanitary articles. These are pressed out mechanically

and sent to containers for storage and disposal at the local landfill. About

15 metres³ of this waste are dumped every fortnight. This is not a satisfactory

solution, Slora says. Future plans for the plant include building an incinerator

for burning the waste.

The remaining wastewater is sent through a grit-and-grease trap to remove sand

and fat, coffee grounds and smaller particles, and then on to two primary clarifier

basins.

A Flygt DS 3057, 2.2 kW submersible pump is installed in the primary clarifier

basin to pump floating sludge. From here the wastewater goes to the biological

part of the process, where it is pumped into two parallel lines, with seven

serially connected reactors or cells. These cells, which are seven metres deep,

are filled to 60% capacity with ‘Kaldnes elements’ – small, hollow plastic wheels,

10 millimetres in diameter, with a convoluted inner and outer surface, which

are kept in constant motion.

The bacterial cultures responsible for the biological degrading of the wastewater

are attached to the surface area of the Kaldnes elements. The addition of these

plastic elements greatly increases the concentration of bacteria in comparison

to a normal activated sludge process.

In the first reactors, the bioculture is aerated to remove organic material.

The aeration system is a coarse bubble system in stainless steel. In final reactors

in the series, nitrogen-containing compounds are converted to nitrogen gas,

which is released into the atmosphere.

The wastewater is treated chemically using aluminium in solution so that phosphates

are bound as particles. Small bubbles in the floatation basins make the particles

float to the surface so that they can be scraped off and sent to the sludge

treatment plant. The water is finally exposed to UV light to remove bacteria,

making it of acceptable quality to be used as drinking water for farm animals.

The sludge from the clarifier basin and flotation is mixed in a tank and carried

to a digester tank, and methane gas is collected from the process. This methane

is used as an energy source for drying the sludge and for heating the plant.

The sludge is finally air-dried in large drums at 600°C, producing granules

that can be used as a soil supplement on farms, in parks and garden areas.

Project details

The Gardermoen wastewater and water treatment plant uses ITT Flygt pumps and

mixers supplied by Purac, a Swedish company. The pumps and mixers have varying

capacities, depending on where in the system they are placed and the medium

in which they work.

At the intake, there are three Flygt CP 3201 pumps of 22kW, each with a capacity

of 10.8 metres³/min, and eight mixers. One of the pumps is run by a frequency

converter. In the grit-and-grease removal basin, there are two Flygt DP 3080

pumps, each with a power of 4 kW.

The water within each anoxic bioculture reactor is agitated by one Flygt SR

4430 mixer with a power of 4 kW, (14 mixers in total). These mixers keep the

culture and the Kaldnes elements in it moving constantly to ensure effective

biological degrading of the wastewater. A Flygt CP 3152 pump of 10.5 kW has

been installed to circulate the wastewater.

The sludge is mixed by one Flygt SR 4640, 3 kW and one SR 4630, 2 kW. Three

Flygt Ready pumps are used for taking samples of wastewater at different stages.

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