Preliminary Investigation of Nitro-hydrolysis for Wastewater Sludge Treatment

Abstract

Activated sludge waste treatment facilities generate a waste stream, in the underflow, that consists of 3 to 5 wt% biosoilds that are not readily biodegradable and must be disposed of as a solid waste. Conventional disposal methods for this biosludge consist of landfills, land farming, wet air oxidation, and incineration, all of which add substantially to the cost of operating a municipal wastewater treatment facility. Bench-scale batch experiments were conducted with a 4.1 wt% biosludge stream obtained from the Knoxville Utility Board (KUB) Kuhawee treatment facility. These experiments demonstrated that treatment with dilute nitric acid at 180°C at 200 psia initiates a hydrolysis reaction that converts 40 to 82% of the biosolids to biodegradable substances suitable for recycle back to the treatment facility. The experimental design consisted of varying the nitric acid concentration from 1.5 to 11.2 wt% and the residence time between 5 and 20 minutes. At these conditions, the initial hydrolysis reaction is rapid, destroying most of the biosolids within 5 minutes. The products of this reaction consisted mainly of carboxylic acids that may be recoverable and sold commercially. The stoichiometry of the reaction was investigated and the kinetics was determined to be first order in nitric acid concentration. Results of these batch experiments performed on municipal sludge partially confirm the patent literature data that used a plug flow reactor and industrial sludge. Future work will expand this study to include a complete factorial design of this process by investigating four possible reaction variables: residence time, reaction temperature, acid concentration, and feed solids concentration in detail, with the objective of developing comprehensive kinetics for the process.

Keywords:

• Excess sludge
• Sludge
• Biosolids
• Wastewater

Acknowledgments

This work was funded by the Oak Ridge National Laboratory, managed by UT-Battle, LLC for the United States Department of Energy under contract DE-AC05-000R22725.