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Pulp & Paper: Mechanical pulp sludge

Don Horne   


The adage is the greatest ideas start by planting a seed. That’s precisely what happened when employees at Alberta Newsprint Company (ANC) near Whitecourt noticed that tomato plants were growing in their pulp sludge pile.

Today, mechanical pulp sludge is readily known as “super fertilizer,” and is garnering some much-needed positive publicity for the industry. The science that has resulted in more mechanical pulp sludge being used on cropland as organic fertilizer is also further evidence that — slowly, but steadily — we are evolving back to a world in which the full organic cycle is being taken seriously.

In fact, the wood fibre industry is not the only one recognizing the value of organic waste products. Large dairy, hog, cattle and poultry farms are also investing in anaerobic digestion as a means to dispose their manure in place of land application. For the wood fibre industry, research into mechanical pulp sludge as an alternative to burning or landfill use has been ongoing over the past two decades by the Alberta Innovates Technology Futures (AITF) — with financial support from ANC, Millar Western Forest Products and Slave Lake Pulp, as well as direction by a Mechanical Pulp and Paper Consortium.

In ANC’s case, the company has not only returned pulp sludge to the soil, but its diversion has saved ANC the expense of building the equivalent of two landfills. So what is mechanical pulp sludge? It’s been described as a porridge-like material consisting of 80 per cent water. It is a byproduct of the mechanical pulping process that, in addition to water, consists of short lignin fibres, high in carbon and containing plant nutrients such as nitrogen and phosphorus. Where mechanical pulping has an advantage over kraft pulping is the lack of high concentrations of harmful chemicals in its sludge.


According to AITF researcher Dani Degenhardt, the chemical composition of mechanical pulp is about 45 per cent carbon, 1.5 to three per cent nitrogen and 0.3 per cent to 0.5 per cent phosphorus. What’s also important from a land application standpoint is it has a neutral pH. Researchers have conducted extensive research in three land applications: forest sites, agriculture, and oil and gas sites as possible use in lease and road reclamation.

On the forest research front, Degenhardt says a land-spreading trial was held involving forest cutblock field locations to determine the impact of different applications rates of mechanical pulp sludge on soils and trees. Subsequent trials have involved sludge application on juvenile trees, seedling/suckering trials, spreading frozen and snow-covered soils, thinning trials, operational spreading trials and equipment evaluation. Long-term experiments have indicated that sludge application in the forest has had an overwhelmingly positive effect on tree volume, with the largest gain in stemwood volume of balsam poplar and aspen species at age 25.

Degenhardt adds the greatest benefit comes from applying sludge in marginal soils to stimulate tree growth. Juvenile trees are the best candidates for sludge application as they are able to achieve more volume production than seedlings. In general, trees on the test sites grew about 20 per cent faster, achieving harvestable size in just 60 years versus 70 to 80 years. The sludge application rate is recommended at about 50 tonnes per hectare to reduce vegetative competition.

After initial reluctance from farmers within about a 45-kilometre radius of the ANC plant, the company’s phone now rings off the hook from farmers asking about the availability of the mechanical pulp sludge for land application. They are even stopping the trucking contractor delivering the sludge for field application to find out how they can enrol in the program.

Why? Land application of the mechanical pulp sludge as organic fertilizer is delivering multiple benefits.

First, one project at a site near Mayerthorpe, Alta., showed that a one-time sludge application at 50 tonnes per hectare yielded the same quality and quantity of barley as application of 200 kilograms per hectare of 35-10-0 fertilizer annually for three years. In other words, farmers had access to free organic fertilizer that delivered the same results as they would have achieved by commercial fertilizer for three years. No wonder farmers stopped the delivery trucks.

Second, the sludge acts as a slow-release fertilizer that delivers nutritional benefits for up to five years because the nitrogen exists in an organic form. The Mayerthorpe study confirmed this finding. It also improves soil structure and tilth, thereby increasing soil water-holding capacity. In another study, application of 50 tonnes per hectare increased bromegrass productivity in a hay meadow that cattle owners harvest to feed their herd over the winter, by five-fold compared to control plots. And this increased yield was sustained for six growing seasons.

One obvious concern is the impact of less desirable elements within the mechanical pulp sludge on the soil. Degenhardt says at the time of application, there is a slight increase in electrical conductivity, sodium absorption ratio, nitrate, nitrite and metals such as copper, nickel and zinc, but they are not long lasting. For those interested in more details about this research, the findings and the Mechanical Pulp and Paper Consortium, visit

Tony Kryzanowski writes a bout forestry, alternative energy, and natural sciences for a variety of national and international publications, and is headquartered in St. Albert, Alta.


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