Different types of sludge are produced in the waste water purifying process at mill sites. Sludges are wet and fibre-rich waste fractions that can be divided into two groups: primary sludge and bio sludge. Bio sludge mainly consists of water-purifying bacterial mass, whereas primary sludge is fibre mass composed of process rejects. Primary sludge typically contains about 20 - 40% inorganic matter. The UPM sites in Finland incinerate about 400,000 tons of forest industry side stream sludge per year.
After collecting the sludge that is formed during the process, all liquid that can be easily separated from the sludge is pressed out mechanically. After this, the sludge is incinerated in power plants UPM owns fully or in part. Since the sludge may contain even 60-75 percent water and is therefore heavy, transporting them elsewhere is not cost-effective. Typically, the fuel inside UPM's boilers contains about 3-26% sludge.
"The composition of sludge, just like other waste fractions, varies by site. This brings about challenges in their utilization and application. In some cases, the sludge may contain more alkali substances that are not wanted in boilers," says Katja Viitikko, head of the side stream research program in UPM Research & Development.
The fuel value of sludge can be enhanced by drying. The Zero Solid Waste to Landfill project is mapping out dryers that are currently on the market and also those currently under development. Their costs and suitability for UPM's processes are being surveyed.
The new recycling methods require well-functioning cooperation networks
Although sludge is incinerated in power plants, it is not a high-quality fuel. Sludge has poor heat value and it could well be replaced with an alternative material that produces more energy. Researchers are trying to find other practical applications for sludge.
Mixed sludge can be utilized as a soil improvement material. The sludge is transported to fields as it is. Whether or not it may be utilized as a proper fertilizer with ash, for example, depends on the sludge batch's nutrient and heavy metal content.
"One idea is to productise sludge and develop a fertilizer that is more refined than the soil improvement materials. In the field of fertilizer development we have to cooperate with partners, make the value chain profitable and evaluate the position of UPM in the value chain carefully," Viitikko says.
A good example of cooperation like this is the fertilizer project, made public today by UPM, with a Finnish fertilizer company Yara Finland (the press release can be found here).
The different ways to utilise sludge have been researched in the forest industry for decades and because of this the Zero Solid Waste to Landfill goal has already been achieved. New cooperation networks can still be created and good, cost-effective and ecologically sustainable operational models found and developed further.
"The rise of the circular economy concept brings about many new opportunities for utilising sludge, particularly in cooperation with external partners. Biogas and organic fertilizers are good examples of the new developments in the field," says Katja Viitikko.