Gryaab´s Ryaverket plant opened in 1972 and has continuously increased its capacity as more municipalities have been connected to the wastewater network and environmental requirements have been tightened. But since Gryaab cannot expand on adjacent land, some innovative thinking has been needed to cope with the load on the plant. Gryaab has some ingenious solutions for constructing two-level basins and for vertical water treatment. This has attracted great interest all round the world. The fact that everything is made use of is also something that visitors find extremely interesting; the biological waste products, i.e. the fecal matter, is turned into biogas and organic fertilizer, and rubbish, such as sanitary napkins and wet wipes, is burnt and the resulting heat fed to the district heating network.

• The biogas plant at Gryaab digests biological waste products, principally fecal matter. Every year the plant produces biogas equivalent to 60 gigawatt hours. This is the same amount of energy that 5,000 gasoline powered cars consume over a year.
• When the cars are run on the biogas that is produced instead of on gasoline, emissions of fossil carbon dioxide are reduced by 15,000 tons.
• Every year 50,000 tons of organic bio-mulch are produced and used as soil in agricultural and civil engineering applications.
• 1,200 tons of rubbish flushed down into the sewage system is dewatered and then burnt at Sävenäs combined heat and power plant.
• The disc filter plant will filter out very small particles through a strainer. When the plant comes into operation, a maximum of 0.3 mg of phosphorus per liter will be released into the environment together with the treated water.

Wastewater flows into Gryaab at an average rate of 4,000 liters a second, which is equivalent to 25 full bathtubs every second. After twelve hours the treated water is released back into the Göta Älv river.

When the water arrives at Gryaab, it first passes through a coarse bar screen where things that should have been placed in the garbage can, for example tampons and rags, are filtered out. The water is then pumped up 20 meters to the treatment plant itself. There it passes through a sand trap and the grit suspended in the water falls to the bottom. The sand is then used as material, for example in road construction.

Before the water passes to the next stage of the process, a primary settling basin, it is filtered through a fine bar screen. The fine bar screen traps such items as cotton buds and toilet paper that has not yet dissolved.

In the primary settling basin, particles that are heavier than water fall to the bottom where they form a sludge that can be scraped off. The sludge is transported to the facility´s biogas plant. To begin with, the sludge is very liquid. So it is transported on a straining belt that allows water to drain off.
The atmosphere in the biogas plant is anaerobic and kept at a constant 37°C, which are the ideal conditions for the bacteria. It takes about three weeks for the biological waste products to be degraded by the bacteria in the biogas plant. The sludge is stirred constantly during degradation. It is in this process that the biogas is formed.
Every year the plant produces biogas equivalent to 60 gigawatt hours. This is the same amount of energy that 5,000 gasoline powered cars consume over a year.

The sludge that remains is centrifuged and then composted for two months to turn it into nutrient-rich organic fertilizer.

However, the water still contains organic material and nitrogen and phosphorus, that can cause eutrophication and oxygen deficiency in the sea. The organic material is broken down by bacteria in so-called activated sludge basins. Bacteria help to convert the nitrogen into nitrogen gas, which is released into the atmosphere. This process takes place in nitrifying trickling filters and the unoxygenated zones of the activated sludge basins.

Phosphorus is removed from the water by means of a chemical process. In 2010 the world´s largest disc filter plant will come into operation at Gryaab. The disc filter plant will filter out very small particles through a strainer. When the plant comes into operation, a maximum of 0.3 mg of phosphorus per liter will be released into the environment together with the treated water.

Before the water is released into the Göta Älv river, it passes through a heat pump that extracts the heat from the out-flowing water. 7% of Göteborg´s district heating comes from Gryaab´s wastewater treatment plant.

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Design: Gryaab together with k-konsult, subsequently acquired by SWECO Environment, www.sweco.se Machinery and equipment: Läckeby Water AB, www.lackebywater.se Construction: NCC www.ncc.se