What is the size you are considering ?

We have been operating bench scale and pilot scale wetlands in both batch and continuous flow. If you want to examine removal efficiencies at relatively small scale, you could use 'cubitainers' or other types of containers and fill it with desired planting substrate and plants. Subsequently you add a known amount of wastewater and follow-up the target-parameters (will depend on your specific research topic) by periodic and systematic sampling of the the wastewater in your system. If you want to know water quality at various depths in your system, you need to foresee simple tubes from which to sample when filling the container with substrate. Alternatively, you could foresee a pump (or tap in smaller systems) and extract a desired amount of water to be sampled. In semi-continuous flow field systems, you can also build "channels", fill it and plant it and add a known volume of wastewater each day and also extract a known amount. 

Do mind that in both types of systems (batch and semi-continuous) you will have plant evapotranspiration. By monitoring and controlling your water level, you will be able to quantify this.