Drying and Treating Bio-Solids


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1. What is the difference between microwave and conventional biosolids processing?

In conventional heating, the process time is limited by the rate of heat absorption by the material from the surface inward. As biosolids are a poor conductor of heat, the process time for heating is slow, with non-uniform heat distribution. The biosolids tend to be heated excessively on the surface and edges, but very randomly within the material. If a vessel must be heated first, prior to the introduction of the biosolids, even more process time, energy and labor is expended.

However, with the microwave system, the entire volume of biosolids is heated at a constant rate after application of power. The microwave energy is immediately transferred into and absorbed throughout the material. The uniformity of heat distribution within the biosolids is considerably improved, with a significant reduction in process time. With the microwave system, process time is typically three per cent or less of that required with conventional heating.

2. How does processing biosolids with microwaves meet EPA requirements?

The biosolids processed by the microwave drying/sterilization system meet the EPA-defined time-temperature regime for biosolids with 7% solids or greater, Class A requirements and passed EPA-specified tests for Total Solids Content, Helminth Ova, Fecal Coliform and Salmonella. The microwave achieves in minutes, with a continuous process, what may take hours with conventional equipment in a batch process.

3. What precautions have been taken to prevent microwave radiation leaks?

First, it should be made clear that microwave "radiation" is a misnomer. No ionizing radiation is propagated by the microwave system. The microwave
operating frequency is actually an ultra high frequency (UHF) radio frequency, as specified by the U.S. Federal Communications Commission (FCC).

Secondly, no measurable amount of microwave energy has been detected near the biosolids entry or exit tables, around any access doors, or near any micro- wave entry port flanges. Choke pin trips and choke flange doors effectively ground all microwave energy in the entry/exit and access door areas, respectively. Microwave entry port flanges are sealed with a microwave-absorbing gasket.

4. What steps have been taken to guarantee safe operation?

The microwave system is designed to operate via program under PLC control, with all safety devices installed in a fail-safe mode. Critical safety devices are hard-wired external to the PLC, in the event of a PLC malfunction, insuring a safe, controlled shutdown. All access doors have dual safety switches interlocked to the high voltage and safety shutdown circuits.

In addition, fire detection and extinguishing, as well as microwave power density monitoring, prevents excess power from being applied to the biosolids.

All equipment included with the microwave system meets the applicable standards set forth by NEMA, NEC, NFPA, ACGIH, OSHA, U.S. EPA and state EPA/DEQ.

5. Are there any odor issues, which would prevent location in residential neighborhoods?

No odor issues exist since microwaves are preferentially absorbed by ammonia, amines and some mercaptans (sulfur-based compounds). These odor-causing compounds are destroyed, leaving only water vapor and steam to be exhausted to the atmosphere. Due to the high-efficiency burners provided for the source of hot convection air, the levels of Hydrocarbons, Particulates, CO, NOx and SOx are below the minimum standards set by the Ohio EPA.

6. Since our community is looking for a cost-effective solution to replace our existing wastewater treatment method, how can the microwave system lower costs?

First, the microwave system continuously dries and sterilizes the biosolids to Class A requirements without any further treatment, thus saving energy
and chemical costs.

Second, through volumetric reduction, less dry tons are required to be transported to a landfill or be land-applied, thus saving hauling and tipping fees.

Finally, through efficiently drying the biosolids from the inside-out - instead of the outside-in - starting at the application of power through discharge from the applicator, reduced energy and process times are realized.

7. How many people are required to operate and maintain the microwave system?

As the unit is designed to operate unattended, only one factory-trained person is necessary per shift.

8. What is the approximate size of the microwave system?

Depending on the model required for the application, a typical microwave system including main control panel, microwave generator panels, and applicator will require a footprint of approximately 42' L x 30' W x 12' H.

9. How much space does the microwave system need for operation and maintenance?

For operation, the microwave system needs no additional space. For maintenance, a building 50' L x 50' W x 12' High (at the bottom of the roof truss) is recommended to provide adequate access to all of the equipment.

10. What is the approximate operating cost per hour?

Based upon utility costs in central Ohio in 2006, biosolids can be dried and sterilized for less than ten cents per pound.

11. What is the approximate maintenance cost per year?

With the high reliability components selected for the microwave system, the only consumable part is the filament within the magnetron, which is guaranteed for a period of one year after startup. This results in a maintenance cost of one dollar per operating hour. However, the magnetron may be returned to the magnetron manufacturer, rebuilt , recertified, and returned to service - several times throughout its life. The cost to rebuild the magnetron versus replacement cost of the magnetron is 50% of the new price.

12. How much does the microwave system cost?

Based upon the amount of throughput, number of hours of operation per year, and type of peripheral equipment and accessories required by the customer, the capital cost of the microwave system typically ranges from $U.S. 1,600 - 2,200 per kilowatt amortized over five years - excluding interest.

13. How does the microwave system reduce the amount of biosolids dumped in landfills?

Through volumetric heating of a continuously moving bed of biosolids, the volumetric reduction is about 75%. The additional benefit of Class A biosolids allows the use of microwave-processed biosolids to be used as a soil amendment, which can be a revenue source, instead of a landfill expense.

14. How safe is using microwave-processed biosolids for land applications or soil amendment?

Since the microwave-processed biosolids exceed the requirements specified in U.S. EPA, Part 503 for Class A, Exceptional Quality biosolids, these biosolids may be applied to all agricultural, forest, reclamation sites lawns and home gardens without restriction.