Bäckhammar turns wastewater into high-grade energy
By leading flue gases and wastewater from the plant to algae farms, we are providing a great service for the environment. Algae thrive on substances such as nitrogen and can then be harvested and used to generate high-grade energy. This autumn, the first algae were harvested at the Bäckhammar plant.
This event occurred when the Technical Research Institute of Sweden (SP) carried out a demonstration of the effectiveness of algae. The experiment was conducted on farmed algae that had been collected directly from ponds and dried.
The Bäckhammar plant has now been entrusted with continuing SP’s trials. In an area of a few square meters behind the plant’s biological wastewater treatment facility; a green sludge bubbles away in a few small ponds. Some of the ponds are carpeted in algae; they are chockablock with nutrients. In the autumn, the algae will be harvested by SP and the valuable bio-oil extracted. The oil can then be used for applications including biodiesel, bioplastic and lubrication oil. Naturally, pressing the algae into pellets and burning the dried algae to extract the energy is another possibility.
“In all likelihood, the algae will first be used as fuel. Purifying the algae to bio-oil for food production requires more stringent control of the input materials. We use waste, which does not yet comprise a suitable raw material from a food perspective.”
“On the other hand, plastic from bio-oil is something to be considered given time, depending on the properties of the oil,” says Tarjei Svensen.
To keep the weight as low as possible during transport, the algae will be dried into cakes at the Bäckhammar plant. The extraction of bio-oil will be carried out either at SP or at another member of the consortium. Should the algae project prove successful, rationalization of the work through a central refinery for bio-oil that accepts deliveries of dried algae from a number of interested paper mills in Sweden has been discussed.
A 450-meter conduit has been laid from the Bäckhammar plant’s soda recovery boiler and supplies carbon dioxide to the algae. The original idea was to run a continuous stream of process wastewater through the algae to ensure that levels of nitrogen and phosphorus would not be too low, but this had to be stopped, as was the attempt with municipal wastewater. Apparently, the natural algae strains occurring in the wastewater outcompeted the valuable algae. Now, UV-filtered river water is being used successfully.
The algae are not your run-of-the-mill Baltic Sea algae. They are fat and jelly-like. If you squeeze them, they compress but immediately regain their original shape when the pressure is released. Like all algae, they multiply through photosynthesis. To multiply, they require carbon for the cell membranes, in addition, phosphorus and nitrogen are needed as nutrients and are supplied by the plant’s flue gases as well as by commercial NPK fertilizer added directly to the water.
The project will investigate if it is possible to use alternative sources of phosphorus and nitrogen that are located closer to the plant.
“It’s a hard world out there,” says Tarjei and points at the algae ponds. “The ponds cannot be too deep or the algae will die as photosynthesis is inhibited because sunlight is unable to penetrate. Each pond has its own start colony of algae; we then add fertilizer to speed up the algal bloom and to give them something to chew on. At night, the algae sleep and the process stops.”
The PH value must be kept between 6.5 and 7.5 and the temperature must be 25 to 30 degrees Celsius. The temperature is maintained through the use of heating coils through which the process wastewater from the plant flows.
The facility is environmentally friendly and the costs for operating it are low. Electricity is used to operate a flue-gas fan, a blade wheel to mix the water in the large pond and an ionometer. However, SP plans to replace grid electricity with solar panels to make the facility energy neutral and self-sufficient.
Algae farms such as this one can reduce over fertilization and climate impact. In addition, to provide a perspective of its economic significance, I was provided with a comparison. Research indicates that algae provide 22 times as much oil per hectare than rape seed grown on arable land. Its fuel value is slightly more than half that of fuel oil.
Mikael Hedlund