Comparison of Microbial Spore Tracers to Lithium Salts in Performing Hydraulic Residence Studies, 2003 Environmental Conference Proceedings

The operating performance of aerated stabilization basins, settling basins, and hold-and-release basins is dependent on knowledge of the Hydraulic Residence Time (HRT). The theoretical HRT basin volume/flow is often compromised by inlet-outlet conditions, vertical stratification, mixing devices and sludge accumulation. A conservative tracer is often applied to determine HRT and mixing regime. The traditional methods used are dyes and metal salts, but the background color of pulp and paper wastes makes use of dyes difficult. Metal salts can be bulky and expensive to apply.

In the 1980’s experiments were conducted using microbial spores to serve as a tracer. Microbial spores can be selected which do not grow in the wastewater. Their small size means that a kilo of 1011 colony-forming-unit (cfu) spores can substitute for >10,000 lbs of the metal salts solution. However spores behave like a particulate or colloidal solid rather than a dissolved salt; there was a chance the spore would settle leading to poor recovery of the tracer. Advances in fermentation and preparation of very small diameter spores improved the technology in the 1990’s.

A conservative bacterial spore, Bacillus globigii, has been applied to several aerated stabilization basins and settling ponds in tracer studies. This organism requires a sugar to grow that is not present in wastewaters. Samples collected are plated onto an agar that allows the development of bright red spore colonies that are easily enumerated. In one of the cases presented, the spore tracer was used side by side with lithium chloride salt in an investigation of an ASB with a large settling area. The tracers had HRT’s within 5% of each other. The recovery of the lithium salt was 82% while the spore tracer recovery was 69%. The tracer data were used to create mathematical models of the ASB system and to design potential improvements. The relative cost of the microbial tracer was less than 40% of the cost of the metal salts tracer.