Complementary methods to investigate the development of clogging within a horizontal sub-surface flow tertiary treatment wetland

Abstract

A combination of experimental methods was applied at a clogged, horizontal subsurface flow (HSSF) municipal wastewater tertiary treatment wetland (TW) in the UK, to quantify the extent of surface and subsurface clogging which had resulted in undesirable surface flow. The three dimensional hydraulic conductivity profile was determined, using a purpose made device which recreates the constant head permeameter test in-situ. The hydrodynamic pathways were investigated by performing dye tracing tests with Rhodamine WT and a novel multi-channel, data-logging, flow through Fluorimeter which allows synchronous measurements to be taken from a matrix of sampling points. Hydraulic conductivity varied in all planes, with the lowest measurement of 0.1md(-1) corresponding to the surface layer at the inlet, and the maximum measurement of 1550md(-1) located at a 0.4m depth at the outlet. According to dye tracing results, the region where the overland flow ceased received five times the average flow, which then vertically short-circuited below the rhizosphere. The tracer break-through curve obtained from the outlet showed that this preferential flow-path accounted for approximately 80% of the flow overall and arrived 8h before a distinctly separate secondary flow-path. The overall volumetric efficiency of the clogged system was 71% and the hydrology was simulated using a dual-path, dead-zone storage model. It is concluded that uneven inlet distribution, continuous surface loading and high rhizosphere resistance is responsible for the clog formation observed in this system. The average inlet hydraulic conductivity was 2md(-1), suggesting that current European design guidelines, which predict that the system will reach an equilibrium hydraulic conductivity of 86md(-1), do not adequately describe the hydrology of mature systems.

Measurement Techniques for Wastewater Filtration Systems

“…There are a variety of measurement techniques available to determine the hydraulic conditions within the filter in situ (Knowles et al, 2009a;Lin et al, 2003), whilst determination of the composition and quantity of clog matter usually requires samples of the gravel matrix to be extracted prior to laboratory analysis. Each of these measurement techniques is discussed in this section along with the weaknesses and strengths of each strategy, which are summarised in Table 1.…”

Characterization of Clogging Material From Horizontal Subsurface Flow Constructed Wetland Systems

“…Accumulation of different solids in interstitial space associated with the formation of a biofilm adhered to the support medium decreases the useful system volume, leading to a reduction in bed infiltration/percolation capacity (CASELLES-OSORIO et al, 2007;HUA et al, 2014;KNOWLES et al, 2010). According to PEDESCOLL et al (2009), clogging occurs due to material accumulation associated with treatment and operational factors.…”

Multilayer Substrate Configuration Enhances Removal Efficiency of Pollutants in Constructed Wetlands

“…Clogging may cause a large proportion of the water to pass over the bed as surface runoff, thus significantly reducing the effective volume of the system for removing waste [4]. The resulting ineffective wetland bed leads to incomplete biodegradation and reduces the performance of CWs [5,6].…”