A Safe Alternative

While land application is an approved disposition of biosolids, many private and public organizations have raised serious doubts about the associated environmental impacts and public health risks.  In response to these concerns, some regions have halted or restricted land application of biosolids.  Sewage sludge is generated every hour of every day and must be disposed.  Today, land application is the primary outlet because it is the lowest cost method of disposal available for large quantities of biosolids.  If this currently inexpensive disposition conduit were eliminated, or severely restricted, is there an environmentally friendly alternative method of disposal that:

• Can dispose of the sewage sludge right off the de-watering operation at 25% solids and 75% moisture,
• Will not dramatically increase the cost of disposal,
• Will reduce biologically active wet sludge to a biologically inert, carbon rich ash,
• Will dispose of over eighty-five percent of the weight of the sludge,
• Uses the biosolids as the fuel source requiring NO other fuel for the drying / disposal process?

After several years of research and technology breakthroughs, Primenergy can demonstrate that the answer to all of these questions is yes.

Our solution for sewage sludge is to take the sludge at 25% solids into a sludge dryer, either indirect and direct dryers will work, and further reduce the moisture content of the sludge while producing a biosolids renewable energy fuel for a gasifier.  This biosolids fuel will provide 100% of the thermal energy necessary to operate the sludge dryer.  The only additional energy used in this whole process is the electricity needed to run the electric motors and controls.

The process of gasification is the conversion of a combustible solid into a synthesis gas.  The heat energy released from the biosolids fuel provides the dryer with the necessary heat to evaporate the water from the sludge.

The beneficial use of dried biosolids as a renewable energy source destroys biosolids and is not just a method of disposal.  Primenergy has demonstrated that this process is sustainable without the addition of supplemental fuel.  This elimination of a conventional fuel source reduces the cost of operations and will compare favorably with current operational costs. 

An added benefit of our process is the ability to take all classes of sewage sludge, “A” or “B” biosolids, and produce the same final product at the same cost to the clients.  Using Primenergy’s drying and gasification process, the cities won’t have to spend millions of dollars to up-grade their wastewater treatment plant to produce biosolids for land application.  Our integrated process will produce the same end product - a biologically inert, carbon rich ash that is about 12% to 14% of the weight of the beginning sludge.

The following block flow diagram is provided for visualization of the overall design. 

The steps in this integrated process design are:

1. High moisture content sewage sludge, either digested or undigested, is introduced into a thermal dryer.  The two common types of dryers are direct contact, which uses heated air as the drying medium, or indirect contact, which uses steam or heated oil as the drying medium.
2. Dried biosolids are discharged from the dryer.  Moisture content is reduced to approximately ten percent, creating a quality biosolids fuel.
3. The dried biosolids are fed into the gasification and energy conversion process.
4. The remnant ash is discharged from the gasification process.  At this point over eighty-five percent of the weight of the wet sludge has been removed.
5. The synthesis gas is oxidized in a series of stages and the released thermal energy is captured either in heated air or in the production of steam.
6. The thermal energy is directed to the dryer, providing the energy necessary for the evaporation of the moisture contained in the wet sludge.
7. Environmentally compliant dryer exhaust, spent productions of combustion and moisture, are vented to the atmosphere.

At Primenergy, we are dedicated to the use of domestic, renewable biomass energy.  At our Tulsa, Oklahoma location, we have a commercially sized demonstration gasification system.  This research and development facility is available to demonstrate the sustainable performance of the conversion of biomass into energy.  Environmental compliance can be assured not only through the superior performance of gasification, but proven by on-site third party emissions testing.

back