Waste Heat or Digester Gas

The use of digester gas or the recovered heat from cogeneration at municipal WWT sites represents the largest opportunity for drying in the United States and abroad. It has several advantages:

  • It uses existing, yet unused, energy source available at the site
  • It reduces Green House Gas Emissions (GHG’s) – opening up grant and tax credit opportunities
  • It reduces landfill costs
  • It reduces pollution due to seepage (sludge greater than 80% in moisture leads to water contamination)
  • It greatly reduces operating costs by using available energy
digester gas sludge drying

digester gas sludge drying

Digesters are large holding areas (tanks) of wet sludge (over 95% water) that have active microbes that “eat” the suspended solids. The result of the digestion process is a greatly reduced volume of solids and the release of gasses, which is mostly constituted of methane. There are both anaerobic and aerobic digesters. Most municipal WWT sites use anaerobic digesters, this is due to the large amount of released methane and the fact that in an anaerobic digester, the facility encloses both the sludge/microbe blend and the odorous gasses that are produced.

In some municipal WWT facilities, digester gas (methane) is used for cogeneration (CHP), which produces waste heat as a byproduct, or it is flared into the atmosphere. Flared digester gas is an environmental challenge as it produces Green House Gas emissions. However, Cogeneration is an expensive proposition, and has a very slow rate of return. Thus, much of digester gas is flared and unfortunately becomes an unused, yet recoverable fuel source. Gryphon holds a distinct advantage over the competitors in that digester gas or exhausted heat from cogeneration may be used as the thermal energy source for drying. And most importantly, we can remove 30-40% more water from the sludge with the same amount of available energy..