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In April 1999, the Charlotte-Mecklenburg Utilities (CMU) began to experience nuisance odor problems with dewatered biosolids produced at the McAlpine Creek Wastewater Management Facility (WWMF). Anaerobically digested primary and waste activated sludge biosolids are dewatered by three horizontal, solid-bowl centrifuges, and conveyed to storage silos by horizontal and vertical shaftless screw conveyors. Periodic testing on digested biosolids for volatile solids reduction and fecal coliform density indicate that the liquid digested biosolids meet Federal and state Class B biosolids requirements prior to dewatering. After dewatering and storage, both objectionable odors and elevated fecal coliform levels render the biosolids unsuitable for land application, and thus the bioslids are disposed of via land filling.

In order to evaluate and develop options to render the dewatered biosolids suitable for land application, a series of bench-scale and full-scale tests were run to investigate the causes and underlying mechanisms that promote fecal re-growth and odor generation.. Based on this and previous studies, protein was found to be a major substrate leading to odorous compound generation, and shear forces imparted on biosolids during dewatering were found to make the protein a better available substrate promoting fecal coliform growth.

In light of this information, a baseline was developed for fecal coliform and odorous compound generation levels, and cake volatile fatty acid composition under normal operation conditions. Following the baseline development, a series of bench-scale tests were performed to investigate the efficacy of chemical addition to slow down or to prevent fecal growth, and at the same time decrease odor generation from the final dewatered product. For this purpose, ferric chloride, sodium hypochlorite and lime were considered as potential agents. The tests revealed that ferric chloride and hypochlorite were potential agents to control fecal re-growth, while lime at the required dosage caused significant odor generation, although it effectively controlled fecal growth. Previous full-scale ferricchloride application has indicated no benefit for fecal control and some odor control benefits. A fullscale test of a combination of ferric chloride and sodium hypochlorite is scheduled for December 2003. The results of the on-going full-scale study will provide better options for fecal coliform re-growth and odor problems experienced at this facility.

Document Type: Research Article


Publication date: 2003-01-01

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