Tracer studies, such as the lithium tracer study, are powerful diagnostic tools.
One of the many consulting services Environmental Business Specialists (EBS) specializes in is tracer studies. Tracer studies are a powerful diagnostic tool in evaluating pulp and paper mill wastewater systems. Tracer studies focus on evaluating the mixing regime and hydraulic residence times of reaction vessels, such as aerated stabilization basins (ASB) and settling ponds.
A commonly used tracer is lithium chloride (LiCl) brine. This liquid contains approximately 40% LiCl or 6.5% Li. Lithium chloride is preferred over other tracers because it is not a normal component of pulp and paper mill wastewater, it is inert and does not react with anything in the wastewater system, and it is easily detected by flame atomic absorption spectrophotometry. Recovery rates are also generally greater than those reported for fluorescent dyes or biological spores.
In order to calculate the retention time of a system, samples are collected and lithium concentrations are measured over time. Early in a study, samples are collected more frequently in order to recover the lithium at its peak concentration. As the study progresses, samples are collected less frequency. Sampling duration varies, but generally samples are collected for a period two to three times the theoretical retention time. This is to ensure that the entire amount of tracer has passed through the system and reached the effluent.
In addition to sampling at the effluent, it is highly recommended that profile samples are collected throughout the basin 4 hours and 24 hours after the tracer has been introduced into the system. While long term sampling reveals how long the tracer was in the system, profile sampling would show the route it took. Knowing the flow patterns in the system can help identify short-circuiting or channeling. Both of which could be a result of solids build up or a breach in a baffle.
To get the most benefit from a tracer study, one should be conducted every one or two years and in conjunction with sludge depth study.