Waste to Value: Reusing Paper Sludge in the Fertilizer Industry

This article was authored by:

Carrie Carlson
Technical Writer

Despite growing digitization, the pulp and paper industry remains healthy, with new and alternative applications displacing demand for declining products. 

The processing of pulp and paper products yields a significant amount of waste in the form of sludge, collectively referred to as pulp and paper mill sludge, or PPMS.  

Historically, these pulp and paper residues have been landfilled or incinerated, but as disposal costs rise and sustainability becomes a requirement rather than an option, the industry is looking for a better way to manage them. Land application has emerged as a popular alternative. With more companies turning to land application, processing the material into a premium fertilizer or soil amendment product is being explored. 

Pulp and Paper Mill Sludge Waste

Pulp and paper mill sludge can be broken down into several categories:¹

  1. Waste created from virgin wood fiber production (referred to as primary biosolids)
  2. Waste produced from the de-inking process
  3. Sludge yielded from secondary treatment (referred to as secondary biosolids)
  4. A combination of primary and secondary biosolids (referred to as combined biosolids)

The management of this pulp and paper sludge is widely recognized by the industry as a substantial waste management challenge. 

Landfilling was long the primary method of disposal, but this approach has many downsides and is increasingly being discouraged. When landfilled, sludge poses environmental risks, with potentially significant impact to soil and water. Additionally, landfilling is expensive: the cost to transport sludge is already costly due to its high moisture content, and with growing tipping fees, landfilling continues to become less economically feasible. 

Incineration, an alternative to landfilling, is a viable option, but presents challenges of its own.

Through extensive study, land applying paper mill sludge has been identified as a cost-effective and beneficial approach to managing this substantial industrial waste stream. 

Benefits of Using Pulp and Paper Mill Sludge as a Fertilizer or Soil Amendment

The reuse of pulp and paper mill sludge would not only divert waste from landfills and incinerators, but it would also create a value-added product. This approach would give pulp and paper producers the opportunity to mitigate disposal costs while improving their environmental standing, and even potentially create an additional source of revenue.

Apart from these benefits, pulp and paper mill sludge products boast several characteristics that could improve soil and growing conditions, with a wealth of studies confirming the beneficial effects of land application. 

Though the composition and resulting benefits of pulp and paper sludge vary, PPMS is generally regarded as being able to offer the following advantages in soil applications: 

Organic Matter

Paper sludge is rich in organic matter (OM) – a critical component in promoting healthy, thriving soils. The addition of organic matter improves moisture and nutrient management, helps in resistance to erosion, and enhances soil structure, among many other benefits. 

Acid Treatment

Paper sludge also contains calcium carbonate (CaCO3), a material widely used throughout the agriculture industry for stabilizing or neutralizing acidic soils by increasing soil pH in a practice known as liming. 

Nutrient Content

Although the amounts vary, paper sludge often contains Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), and Magnesium (Mg), making it an attractive nutrient source.¹  

The reuse of sludge as a fertilizer or soil amendment would not only be a benefit to producers, but it would also give farmers an advantage by providing a cost-effective, readily available source of nutrients to supplement their fertilizer costs. 

Processing Paper Sludge for Use in Fertilizers & Soil Amendments

As paper sludge and end products vary significantly, there is no one standard process for utilizing PPMS in a fertilizer or soil amendment product. Sludge may be processed in a variety of ways to suit the needs of a specific application. Depending on the source material and end product goals, processing may involve:

Paper Sludge Drying

Drying may serve as a stand-alone process to simply reduce the moisture content of paper sludge for use, or it may be used as a means of preparing paper sludge for subsequent processing. Drying is also required after granulation. 

Drying creates a stable product by reducing volume and lowering moisture content, thereby increasing solids content. This allows the resulting material to be used as a mulch, carrier, or additive. 

Paper sludge drying is often carried out in a rotary dryer – an industrial dryer favored for its high throughput and tolerance to variation in feedstock. 

Pelletizing (Granulation)

Paper sludge may also be pelletized or granulated to create a premium product. The pelletizing process is incredibly flexible and allows for the inclusion of various additives to create a custom nutrient or product formulation. When producing a granular product, the paper sludge may serve as a small component, or as the chief ingredient. 

Granulation is carried out using a pin mixer, pugmill mixer, granulation drum, disc pelletizer (also known as a pan granulator), or combination thereof (followed by a dryer).


Pelletized paper sludge produced in a pin mixer in the FEECO Innovation Center

Coating

Coating of dried paper sludge or paper sludge-based products is an effective way to improve handling characteristics, enhance performance, and even optimize formulation. 

Coating is best carried out in a coating drum

Testing Critical in Process and Product Development

The lack of standard production practices, combined with the significant variability of pulp and paper sludge often demands thorough process and product development when creating fertilizers and soil amendments. 

As with many fertilizers and soil amendments, testing begins at batch scale to determine the initial feasibility of the concept and moves into pilot-scale testing. The FEECO Innovation Center can test drying, pelletizing, and coating on individual machines, or as part of a continuous process loop integrating all necessary steps. 

Conclusion

Pulp and paper mill sludge presents a significant waste management challenge, but opportunities in land applying this nutrient-rich source of organic matter look promising, with many potential benefits.

FEECO has helped hundreds of companies to transform their waste material into a value-added product – paper sludge included. We use our state-of-the-art testing facility to develop efficient, effective processes and gather data for engineering and manufacturing of custom equipment. For more information on our paper sludge testing capabilities, or our custom rotary dryers, granulation equipment, or coating drums, contact us today! 

SOURCES

  1. Paper Mill Biosolids Application to Agricultural Lands: Benefits and Environmental Concerns with Special Reference to Situation in Canada

About the Author . . .


Carrie Carlson is a technical writer and visual designer.

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