Small-scale experiments: Using mesocosms and microcosms for testing hypotheses in treatment wetland research

Small-scale experiments have been conducted to investigate a wide range of factors suspected to influence treatment wetland (TW) functions and efficiency. One crucial advantage microcosms and mesocosms offer over field-scale systems is that they enable reproduction of experimental treatment units that are as similar as possible, only varying the factor(s) of interest. The goal of this paper is to discuss key methodological issues to consider, as well as the overall potential and limitations of using small-scale experiments as applied specifically to TW research. Planning a successful small-scale experiment to address a scientific question in TW science involves several key decisions to reach the project's goal. Due to financial, space or resources constraints, it is fundamental at the planning stage to carefully determine a suitable compromise between unit size, number of units, number of treatments (both in terms of number of variables and number of levels within each variable) and replication. Producing, properly storing and rigorously distributing the same volume or flow rate of wastewater to each unit is often the most challenging and time-consuming aspect of a small-scale TW experiment. Unavoidable sources of bias inherent in small-scale models must be identified, and when possible, minimized. Small-scale experiments for treatment wetlands can only take on full meaning when the insights obtained through them inform full-scale operations.