Sludge disposal — challenges and solutions

A sand/sludge drying bed at a wastewater treatment plant in Elburn, Illinois, helps dewater sludge by letting the liquid drain and evaporate.
Sludge is a solid byproduct produced during wastewater treatment. It contains a mixture of water, nontoxic organic compounds, toxic pollutants (heavy metals and chemicals), inorganic compounds, pathogens and other microbial pollutants. It is typically classified into two types:
  • Primary sludge. This is a byproduct of the preliminary treatment of wastewater containing a high concentration of inorganic matter and decomposing microbes.   
  • Secondary sludge. This is produced during the biological treatment of wastewater and primarily consists of organic matter and aerobic microbes.

Additionally, tertiary sludge is produced during further wastewater treatment, such as adding a flocculation agent. It is worth noting the sludge composition may significantly vary depending on the type of wastewater and the treatment processes applied.

Sewage sludge poses risks to environmental and human health, and sludge disposal is always a challenge for wastewater treatment plants because of the high treatment costs. While technological developments in wastewater treatment have enabled safe and sustainable sludge disposal, municipalities require the expertise of wastewater engineers to determine the most suitable and cost-effective sludge disposal method for publicly owned treatment works (POTWs).

Common challenges associated with sludge disposal

The chart below depicts common methods of biosolid reuse and disposal, including land applications (in agriculture, home gardens, etc.), landfilling, incineration, deep-well injection and more.

Land application accounts for a majority of biosolids use and disposal.
Wastewater treatment plants have conventionally disposed of sludge on land through landfilling or composting. Burning the sludge (incineration) is another common method of sludge disposal to reduce volume and eliminate pathogens. However, these methods pose environmental challenges.

Challenges of common sludge disposal methods

Sludge disposal 



The most common and relatively inexpensive sludge disposal method. Landfilling involves containing and isolating the sludge in designated landfill sites. However, landfilling sludge results in the production of greenhouse gases and potential leachate contamination. 


Sewage sludge combustion is an effective method to convert sludge into a predominantly inorganic solid product. However, incineration is more expensive than landfilling and may cause air pollution if not properly controlled.   

Land application 

Reusing sludge as a fertilizer, soil amendment or feedstock for anaerobic digestion makes it a reusable resource instead of a waste byproduct. However, sludge should be properly treated and disinfected before being used on land.  

Sustainable sludge management addresses the challenges of conventional sludge disposal methods and involves minimizing the potential adverse effects of sewage sludge. The following are six common approaches to sustainable sludge management:
  • Improve the quality of sludge through disinfection.
  • Reduce the total amount or volume of sludge.
  • Recover phosphates from sludge for reuse.
  • Use organic carbon compounds and inorganic compounds.
  • Incorporate changes to the municipal wastewater treatment process.
The effective disinfection and stabilization of sewage sludge is critical to producing a stable biosolid product for land application and protecting the environment and human health.

 Different types of sludge disinfection methods

A biosolids storage building in South Beloit, Illinois is field site for temporary or seasonal storage of biosolids.
The 40 CFR Part 503, Standards for the Use or Disposal of Sewage Sludge under the Clean Water Act establishes the requirements and management practices for the use and disposal of sewage sludge (biosolids). The U.S. Environment Protection Agency (EPA) has divided biosolids into Class A and Class B based on their pathogen reduction requirements.
  • Class A. Produced through anaerobic digestion and additional treatment such as heating, pasteurization and other complementary technologies to eliminate pathogens, including viruses. Class A biosolids are rated "exceptional quality" or EQ, and represent the cleanest and safest form of biosolid for land application.
  • Class B. In Class B biosolids, pathogens may exist even after meeting the 40 CFR Part 503 requirement. The EPA site restrictions require enough time for pathogen degradation in Class B biosolids before using them for land applications.
The following are some key sludge disinfection technologies used to produce clean biosolids.


Types of sludge disinfection methods




Physical stressors (temperature, pressure, irradiation, desiccation and others) solubilize organics, destroy cell membranes and break other critical cellular structures. Using physical stressors helps eliminate pathogens from biosolids. 


Some standard physical sludge disinfection methods include:

  • Thermal treatment.

  • Heat treatment.

  • Pasteurization.

  • Heat drying.

  • Air drying.


Chemicals stressors, including oxidants, pH and noncharged disinfectants, are directly added to the sludge or produced through chemical reactions during treatment to stabilize and disinfect sludge. 

Commonly used chemical sludge disinfection methods include:

  • Alkaline treatment.

  • Lime stabilization.

  • Ferrate oxidation.

  • Ozone.

  • Chlorine dioxide.

  • Hydrogen peroxide.


Biological sludge treatment methods include:

These methods make use of noncharged disinfectants, such as ammonia and organic acids, to encourage microbial degradation in sludge. Biological methods are effective in stabilizing sludge and limiting chances of future putrification and odor production. 

Sludge disinfection methods implemented across a majority of wastewater treatment plants involve:  

  • Adjusting the pH by adding lime as a chemical stressor.
  • Composting for longer periods.
  • Digesting in thermophilic sludge digesters at 140 degrees F.
  • Drying at 160 degrees F, which also removes moisture from the sludge and reduces its weight.
Under 40 CFR Part 503, all POTWs should implement sludge disinfection methods to comply with the pollutant limits, pathogen and vector attraction reduction requirements and more. Failure to adhere to the requirements may result in enforcement action against the concerned wastewater treatment plant.

Partnering with water engineers for effective sludge disposal

At Fehr Graham, we are an experienced team of wastewater engineers committed to improving the efficiency of wastewater treatment plants and minimizing negative environmental effects. From developing innovative wastewater designs and planning upgrades to testing and sampling biosolids for routine land application and landfill disposal, we are your one-stop resource for wastewater treatment.

To learn more about how Fehr Graham can help your local POTW with effective sludge disposal, contact us or call 815.235.7643.