Modelling stochastic behaviour of horizontal flow constructed wetlands subjected to variable loads

Constructed wetlands (CWs) are a wastewater treatment technology that exploits the physical, chemical, and biological processes occurring in soils to improve water quality. The low operation and maintenance costs allow for a widespread use of CWs, especially for treating wastewater from small communities (up to 2000 people). Despite the wide application of CWs, many aspects of CW behaviour in real environments still need to be investigated. For instance, the stochastic behaviour of CW outflows is generally neglected during the typical CW design procedure, and CW size should highly overestimated to face real outflow variability [Kadlec and Wallace, 2009]. Hence, further efforts are still need to improve the ability to properly design CWs. Here a process-based model approach is used to better understand the response of a horizontal flow (HF) CW subjected to stochastic loads. Data from laboratory experiments of HF-CWs subjected to variable loads [Galvão and Matos, 2012] are used to calibrate a model of the HF-CW set-up using the HYDRUS wetland module [Langergraber and Šimůnek, 2012]. The calibrated model fits the data well and is then used to investigate the HF-CWs response to stochastic loads. Firstly, we force the HF-CW model with a sudden step, i.e. from a lower load to a higher value. This is aimed to understand the typical response time of the system and compare it with the intrinsic timescales of HF-CW. Different step heights are considered, combined with different HF-CW parameters. Secondly, we force the CW with a periodic signal with typical periods, T, chosen in relation to the intrinsic timescales of the HF-CW and its response time (evaluated from the previous point); different amplitudes are tested. Thirdly, we force the HF-CW with a noisy input signal synthetically generated with stochastic methods [Ridolfi et al., 2011], which allow to control the statistical properties of the signal. Even in this case, the typical scales of the noise component are chosen in order to match the timescales of CW. The stochastic analysis proposed here can be a new tool to better understand these highly dynamic systems. Particularly, the main goal is to improve the HF-CW design procedure, limiting the safety overestimation of HF-CW size needed to face the HF-CW stochastic behaviour [Kadlec and Wallace, 2009]. REFERENCES Galvão, A. and Matos, J. (2012) Response of horizontal sub-surface flow constructed wetlands to sudden organic load changes. Ecol. Eng.49, 123-129. Kadlec, R.H. and Knight, R.L. (1996) Treatment Wetlands. 2nd edition CRC Press Boca Raton, FL, USA. Langergraber, G. and Šimůnek, J. (2012): Reactive Transport Modeling of Subsurface Flow Constructed Wetlands Using the HYDRUS Wetland Module. Vadoze Zone J 11(2) Special Issue "Reactive Transport Modeling", doi:10.2136/vzj2011.0104. Ridolfi, L., D'Odorico, P. and Laio, F. (2011) Noise-Induced Phenomena in the Environmental Sciences. Cambridge University Press.