Case Study:

Sewage Sludge Destruction

Project Location:

Philadelphia, Pennsylvania

Summary Description

Thermal drying and gasification of 240 wet US Tons per Day of seventy-five percent moisture content sewage sludge.

Background Description

Only within the last several decades have municipal wastewater treatment plants separated the nutrient rich organic materials from the wastewater.  These separation processes generate both discharge water that must meet regulatory limits of water purity, and semisolid sewage sludge.  The discharged sludge, when treated, becomes biosolids.  The initial result of the implementation of sewage treatment plants is that our nation’s rivers, lakes and bays are no longer the depository for the wastewater and are made safer for drinking, recreational use and food harvesting.

The disposal of biosolids may be in landfills, land application as fertilizer or fill material in mine reclamation, distributing and marketing products (such as heat dried, composted or alkaline stabilized biosolids).  Within the guidelines of the United States Environmental Protection Agency, state and local regulatory agencies, each city must decide how best to manage their biosolids disposal.

Recently, both the Centers for Disease Control and Protection and the National Institute of Occupational Safety and Health have issued warnings concerning one of the common methods of biosolids disposal, land application.  The warnings from these two federal agencies are to limit worker exposure through special precautions and restricting public access to areas where biosolids have been land applied.  These reports support the claims of environmentalists and citizens who have asserted that biosolids are harmful to people and the ecosystem.  There is, however, an alternative, innovative method of biosolids disposal that does not involve open distribution of biologically active material.

The Philadelphia Project

Biosolids discharged from the host sewage treatment contain approximately seventy-five percent water and twenty-five percent combustible solids.  To reduce the weight and volume of this material, the water must be removed by evaporation.  The primary energy source for biosolids drying is thermal energy.  In a conventional arrangement, this energy would be provided by the firing of fossil fuel, such as natural gas, propane or fuel oil.  Once the biosolids are dried, they contain an energy potential that is sufficient to satisfy the energy requirement of drying.  Conceptually, all of the thermal energy requirement could be satisfied by the conversion of the dried sludge into thermal energy, thus eliminating the need for and cost of auxiliary fossil fuel.  In spite of this energy potential, dried biosolids have not been used as a fuel source.  When burned in a conventional fashion, biosolids form an agglomerating glass and their burning requires expensive air pollution control equipment.  Through a technology called gasification, this energy potential can be released and recovered.

At the Philadelphia location, gasification, and the thermal energy output, is coupled with a direct contact dryer.  This single coordinated system reduces the sludge feed to approximately one tenth of the weight of the input.  The direct contact drying system will process ten tons an hour of seventy-five percent moisture laden sludge cake to three tons an hour of a nine to ten percent moisture content dried biosolids product.  These three tons are then fed into the gasifier, which converts the biosolids feed into thermal energy and recyclable ash.  The output thermal energy from the gasifier is directed into the biosolids dryer, which is sufficient in energy to dry the input, seventy-five percent moisture laden feed.  In other words, the inherent and residual calorific energy contained in the dried product is sufficient to dry the product without the addition of auxiliary fossil fuel!  The dry ash, or “super heat dried biosolids”, discharged from the gasifier is biologically inert, odor free, disease free and potentially a soil supplement with less than one tenth of the weight of the input wet sludge and returned to the host sludge plant to be blended with a compost product.