Pinkwater represents the largest single category of hazardous waste generated by the Army Industrial Operations Command. It contains TNT, RDX and by-products generated in the production, handling and demilitarization of conventional explosives. The current treatment method is adsorption onto granular activated carbon (GAC), which generates a secondary hazardous waste - the spent activated carbon. This is an expensive process. An alternative to adsorption is biodegradation under anaerobic conditions to convert the nitro-substituent groups to their corresponding amino-substituent groups followed by aerobic degeneration polishing in a conventional wastewater treatment plant. This project investigated the cost-effectiveness of this alternative method by demonstrating a prototype, full-scale Granular Activated Carbon-Fluidized Bed Reactor (GAC-FBR) at McAlester Army Ammunition Plant (McAAP).
The bioreactor contains a fluidized bed of GAC in which the conditions favoring anaerobic degradation are tightly controlled. The conditions include addition of a co-substrate (ethanol), nutrients, and temperature and pH control. The control system can shut down the reactor if the parameters operate outside of acceptable ranges before unacceptably high contaminant concentrations reach the effluent. The GAC in the fluidized bed serves two main purposes. First, it provides the attachment media on which the bacteria grow, allowing very high bacterial densities to be attained in the bed (up to 50,000 milligrams per liter (mg/L), volatile suspended solids are not related to bacterial density). Second, it provides an adsorptive capacity to remove peak concentrations of the contaminant and can subsequently desorb the contaminant as the bacteria transform and degrade the peak concentrations.
The technology continuously removed total nitrobodies from an influent range of 20-80 mg/L to below the McAAP regulatory limit of 1 mg/L. Except during periods of operational upsets, a more stringent level of 100 micrograms per liter was also met. The additional Biological Oxygen Demand (BOD) of the treated effluent was easily handled by the downstream aerobic plant at McAAP. Labor requirements were higher than originally estimated due to equipment malfunctions; however, these would be resolved for future installations.
The two main benefits of the GAC-FBR system are destruction of the hazardous waste, rather than transfer to a spent GAC hazardous waste by-product, and reduced pinkwater treatment costs. This adsorptive buffer is also particularly important in industrial operations where influent concentrations can vary widely. The installed capital cost of the GAC-FBR reactor was $290,000. During the demonstration, the operating cost of GAC-FBR per 1,000 gallons was $23 compared to $27 for conventional GAC treatment, giving a payback period of 7.5 years. It is anticipated that this payback period could be shortened to less than 5 years by bulk purchase of ethanol and labor reductions. This technology has applicability at other Army ammunition plants; however, conventional secondary wastewater treatment systems would need to be in place for the final polishing step. (Project Completed - 2003)