MRI measurements of dynamic clogging in porous systems using sterilised sludge

Abstract

Constructed wetlands are used in various stages of wastewater treatment around the world. They usually consist of a gravel matrix in which water loving plants are grown and through which wastewater flows. Over time the gravel matrix becomes clogged as a result of particulates accumulation and biofilm growth. The aim of this study is to investigate the spatially resolved changes in the values of MR parameter T2eff (transverse relaxation time) as the clogging is increased. This provides a better understanding of the pattern and location of clogging. In this study two different sizes of glass beads, 8 and 14mm packed in 1m long acrylic tubes, were used to produce laboratory model systems of constructed wetlands through which sterilised sludge is pumped to provide accelerated clogging. The MSME (Multi Slice Multi Echo) sequence was run on a 2.35 T Bruker Biospec scanner to image at six different locations along the length of the tube during the clogging process. The overall hydraulic conductivity of the system was monitored by measuring the flow rate after every three minutes of accelerated clogging and the pressure drop using take off tube manometers. The sensitivity of T2eff to clogging is shown and the evolution of two well defined peaks can be seen on the corresponding T2eff distributions, respectively due to the presence of bound water and free water. Results demonstrate that T2eff decreases as the clogging is increased and T2eff distributions demonstrate that the information about the amount of free water present inside the tube is essentially described by bi-exponential contributions to the overall signal.