What happens to the sludge?
When the sludge truck is full, it goes to a so-called sludge lagoon to be emptied. Sludge lagoons are located in five places - Graninge, Ramsele, Junsele, Näsåker and Gistgårdsön (Kramfors municipality).
The sludge contains a lot of water that needs to be removed in order to handle the sludge. Sludge lagoons are large pits in the ground, dug in places where the ground consists of several metres of sand. They are also very far from groundwater and far from settlements.
As the soil is sandy, the water from the sludge can infiltrate into the ground. At the same time, nitrogen and phosphorus are purified in the sand. The soil acts as a giant infiltration bed - similar to many individual sewers.
The sludge sits in the pit for 1-3 years for the water to drain away. It is then dug up and placed on a surface next to it. At the same time as the sludge is dug out, about 1-2 dm of the sand at the bottom is excavated and new sand is added. The sludge is left for at least a year to dry out further.
In winter, something called freeze-drying takes place. The water in the sludge disappears when it freezes. What remains is a pile of sludge that looks like soil. This sludge is then composted with crushed garden waste.
Once composted, it is used in our final covering of our closed landfills (Rödsta, Ramsele and Edsbacken). The sludge is placed as the top layer, a so-called plant establishment layer.
Can you really do this?
Sollefteå municipality has a permit from the County Administrative Board for the sludge lagoons. Annual sampling and reporting to the environmental authority is done to ensure that the sludge lagoons meet all environmental requirements.
The sampling shows that right next to a sludge lagoon there is a visible impact from nutrients such as nitrogen and phosphorus. But further away, the impact decreases and eventually disappears completely.
The sites are also chosen from the outset because there is sand several metres deep in which treatment takes place, it is far from groundwater and far from settlements.
Isn't it much better to take the sludge to a sewage treatment plant?
The first treatment of wastewater from a house with its own sewerage system must take place in the house's own sewerage system. A typical house consumes about 150 m3 of water in a year. Of this, normally about 2-3 m3 is taken from the sludge well by the sludge truck.
Most of the nutrients are dissolved in the water and not in the sludge and therefore run off on site. So first of all, the water flowing out of the individual sewer should be treated on site.
The difference between a sewage treatment plant and a sludge lagoon
The difference between a municipal sewage treatment plant and a sludge lagoon in terms of nutrient treatment is that in a sewage treatment plant, phosphorus is removed by means of precipitation, for example. The phosphorus then ends up in the sludge instead of in the water.
In a sludge lagoon, some phosphorus ends up in the sludge, but a lot goes into the soil where it is treated.
In the municipality's treatment plant, there is no nitrogen purification, but the nitrogen is discharged into the watercourse that receives the water from the plant.
In a sludge lagoon, it is the sludge or the soil that becomes the recipient of the nitrogen.
Transporting the sludge to sludge lagoons located in several places around the municipality is also an advantage from a transport point of view. The sludge is transported in a shorter time than to a single treatment plant located somewhere in the municipality. This is because it is not possible to transport sludge from one sludge truck to any treatment plant. If the sludge truck is to be able to deliver sludge to a treatment plant, the plant must be rebuilt and adapted for this handling.