Biosolids have residual caloric content (energy) that can be tapped for many uses. The energy in biosolids can be used to produce power and heat, as well as to reduce the carbon footprint of the water resource recovery facility (WRRF).  Since the 1930’s anaerobic digestion has been used to produce biogas from biosoliods. Initially the biogas was used in boilers for heating but is now used in a variety of ways including the following:

Uses for Biogas:

  • Fuel for boilers to heat used in the process and for heating building
  • Combined heat and power

The biogas is burned in engines that power generators creating electricity. The heat from the engines is used for process heating.

Biogas is also used to fuel to create heat for biosolids drying. This displaces fossil fuel usage reducing the carbon footprint of the facility. The dried biosolids are most often used for soil amendment displacing chemical fertilizer use but can also be used for fuel in industrial processes such as cement manufacturing or in traditional soli fuel boilers for producing electricity.

Renewable Natural Gas (Vehicle fuel and pipeline fuel)

While standard natural gas is a fossil fuel that requires tremendous energy to extract, renewable natural gas (RNG) can be produced from the anaerobic digestion of organic wastes such as sewage; fats, oils, and greases; the organic fraction of municipal solid waste; agricultural waste; and industrial organic waste. RNG is pipeline quality gas derived from biogas that is fully interchangeable with natural gas.

RNG can be used for on-site fleet vehicle fueling as well as export to local gas utilities by means of pipeline injection. The Federal Renewable Fuel Standard (RFS) provides significant incentives for RNG used as transportation fuel. Some states have their own renewable fuels programs that are allowed to be added to the federal program. The value can be several multiples more than natural gas itself.

Like all technologies anaerobic digestion has advanced over time so that biogas yields increase and the character of the solids produced is improved. Such advanced include thermophilic digestion, multistage digestion, thermal hydrolysis and preconditioning. Thermophilic digestion occurs at higher temperatures and results not only in higher gas production but also a pasteurized Class A biosolids product. Thermal hydrolysis combines heat and preconditioning of the biosolids upstream of digestion. The precondition involves pressurization and rapid depressurization. This process makes the organic matter more readily digested thus increasing the biogas production. The heating pasteurizes the biosolids producing a Class A product. There are other variations of preconditioning that accomplish only the increase in biogas production by increasing the bioavailability of the organic matter.

Thermal oxidation is another long used technology that has improved to include energy production. Thermal oxidation is defined as the combustion of the organic solids in wastewater sludge or biosolids to form carbon dioxide and water. The remaining solids are an inert material commonly called ash.  Significant heat is generated in the process and this heat is often captured to produce steam to run turbines generating electricity.  The heat is also used for process heating and can be used for building heat.

In a world with ever increasing energy demands the search for new ways to tap into the energy of biosolids is always ongoing. There are emerging technologies in development to accomplish this.  These include Pyrolysis, and gasification.  Both Pyrolysis and gasification are long used technologies and are used in other industries.  But it was not generally applied to biosolids in the past due to the high temperatures involved and the high moisture content of the biosolids.  However technology has overcome the moisture issues.

Pyrolysis is a chemical/physical process that breakdown organic compounds their elements.  This mainly produces a carbon char and some syngas.  The process takes place in with very little oxygen at temperatures around 400oC to 700oC depending on the material being processed.  The lack of oxygen separates it from thermal oxidation and thus produces a different product.  The product is a carbon char that, unlike ash, can be burned as a fuel to generate heat needed for the process and to create steam to be used for electricity generation.

Pyrolysis is the first stage of gasification. In gasification the elements released in Pyrolysis are recombined into syngas compounds in including methane, carbon monoxide and hydrogen.  Gasification takes place with very little oxygen at temperatures of 850oC to 1200oC.  There have been several pilot scale operations of these technologies but only a few large scale operations.

The technology has not reached Sustained long term large scale use but is improving. There are new refinements in reactor types, catalysts and process heat recovery being piloted and it holds promise for the future.

See, resourcerecoverydata.org; and