Paul Knowles

Clogging in subsurface-flow treatment wetlands: measurement, modeling and management.

This paper reviews the state of the art in measuring, modeling, and managing clogging in subsurface-flow treatment wetlands. Methods for measuring in situ hydraulic conductivity in treatment wetlands are now available, which provide valuable insight into assessing and evaluating the extent of clogging. These results, paired with the information from more traditional approaches (e.g., tracer testing and composition of the clog matter) are being incorporated into the latest treatment wetland models. Recent finite element analysis models can now simulate clogging development in subsurface-flow treatment wetlands with reasonable accuracy. Various management strategies have been developed to extend the life of clogged treatment wetlands, including gravel excavation and/or washing, chemical treatment, and application of earthworms. These strategies are compared and available cost information is reported.

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

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.

A Finite Element Approach to Modelling the Hydrological Regime in Horizontal Subsurface Flow Constructed Wetlands for Wastewater Treatment

A Finite Element Analysis (FEA) model is used to explore the relationship between clogging and hydraulics that occurs in Horizontal Subsurface Flow Treatment Wetlands (HSSF TWs) in the United Kingdom (UK). Clogging is assumed to be caused by particle transport and an existing single collector efficiency model is implemented to describe this behaviour. The flow model was validated against HSSF TW survey results obtained from the literature. The model successfully simulated the influence of overland flow on hydrodynamics, and the interaction between vertical flow through the low permeability surface layer and the horizontal flow of the saturated water table. The clogging model described the development of clogging within the system but under-predicted the extent of clogging which occurred over 15 years. This is because important clogging mechanisms were not considered by the model, such as biomass growth and vegetation establishment. The model showed the usefulness of FEA for linking hydraulic and clogging phenomenon in HSSF TWs and could be extended to include treatment processes.

Long term monitoring of constructed wetlands using an NMR sensor

Subsurface flow wetlands have become a popular technology for the treatment of waste water all over the world. These systems become clogged over time, and must be renovated at great expense. We present a nuclear magnetic resonance sensor which is sufficiently small and inexpensive that several of them could be embedded in a constructed wetland to allow spatially resolved long term continual monitoring of the clogging process. We demonstrate the suitability of this sensor by first measuring NMR of sludge from an operational wetland, and secondly by monitoring the evolution of the fluids NMR spin lattice relaxation time (T1) during clogging in a model wetland. Measurement of clogging rates in two locations are made and found to be 10.7×10−2min−1 and 4.9×10−3min−1 for regions near the inlet and the centre respectively.

Measurement Techniques for Wastewater Filtration Systems

Filter-based microbiological wastewater treatment systems (such as subsurface flow constructed wetlands, trickling filters and recirculating sand filters) require a thorough understanding of system hydraulics for their correct design and efficient operation. As part of the treatment process, the filter media will gradually become clogged through a combination of solids filtration and retention, biomass production and chemical precipitation. Eventually the media may become so clogged that hydraulic malfunctions ensue, such as untreated wastewater bypassing the system. To achieve good asset lifetime a balance must be struck between these essential treatment mechanisms and the hydraulic deterioration that they cause. For many wastewater filtration systems the exact mechanism of clogging is not obvious, and few specialised techniques have been developed which allow the cause and extent of clogging to be measured in typical systems. The resultant lack of understanding regarding clogging hinders the ability of operators to maintain good hydraulic performance. In this chapter, for the first time, we compare three different families of standard hydraulic measurement techniques and discuss the information that they can provide: hydraulic conductivity measurements; clog matter characterisation and hydrodynamic visualisation. Each method is assessed on its applicability to typical wastewater filtration systems using horizontal subsurface flow constructed wetlands as a case study. Furthermore, several new techniques will be considered which have been specifically developed to allow in situ determination of hydraulic health for subsurface flow constructed wetland wastewater filtration systems. These include in situ constant and falling head permeameter techniques and embeddable magnetic resonance probes. Discussion is given to the ways in which different methods can be combined to gather detailed information about the hydraulics of wastewater filtration systems before exploring methods for condensing heterogeneous hydraulic conductivity survey results (that vary by several orders of magnitude) into a single representative value to describe the overall hydraulic health of the system.