A conceptual approach based on suspended solids to estimate clogging time in constructed wetlands

Clogging index: A tool to quantify filter bed clogging in horizontal subsurface flow macrophyte-assisted vermifilter

This study aimed to develop a mathematical index for quantifying filter bed clogging in horizontal subsurface flow macrophyte-assisted vermifilter (HSSF-MAVF). The developed clogging index (CI) simulates the porosity reduction in the HSSF-MAVF bed due to the combined actions of deposition of suspended solids, biofilm generation, and plant growth. A series of experiments based on HSSF-MAVF were conducted to examine the key parameters related to clogging of the vermifilter such as hydraulic loading rates (HLR), organic strength (COD), and total suspended solid (TSS) at different operating conditions for the treatment of synthetic dairy wastewaters. The index was then validated using the data collected from the experiments. The predicted CI was observed to be highly capable of replicating the clogging phenomenon, as observed in this experimental study within the error range of 2-8%. Based on the visual observation and value of CI, filter beds can be grouped as unclogged (CI < 25), partially clogged (25 < CI < 40), and clogged (40 < CI). Clogged filter beds with higher CI resulted in a 5-15% reduction in the COD removal performance of HSSF-MAVFs. Moreover, the CI also envisages one to understand the individual contributions of biofilm growth, suspended solid deposition, and plant roots growth on the filter bed clogging during the operation of vermifilter and thus helps in deciding the proper setting of operational conditions to prolong the time of HSSF-MAVF operations within the acceptable range of bed materials clogging. PRACTITIONER POINTS: A mathematical index was developed to quantify clogging of the HSSF-MAVF. Flow is the most sensitive parameter based upon the sensitivity analysis. Dairy wastewater was used for the validation of CI.

The State of the Art of Clogging in Vertical Flow Wetlands

Clogging in vertical flow (VF) wetlands is an important process influencing water purification processes. The main contributing factors are the growth of microorganisms within the filter media, the accumulation of suspended solids on top of the wetland, as well as within the filter media. Both processes lead to a decrease of the available pore space, hence changing the soil’s hydraulic properties. This will alter the water flow and cause malfunctioning of the system. This paper summarizes the state of the art of the prevailing physical, biological and chemical processes influencing clogging in VF wetlands. Different design and operational parameters are discussed to give a better understanding on their influence to prevent malfunctioning. Based on a literature review, a detailed overview on experimental as well as modelling studies carried out is presented. The main conclusions are that on the one hand, important insights on clogging processes in VF wetlands have been gained but, on the other hand, design parameters such as intermittent loading operation and the grain size of the filter media are not well represented in those studies. Clogging models use different conceptual approaches ranging from black box models to process based models.

Influence of clogging and resting processes on flow patterns in vertical flow constructed wetlands

Vertical flow constructed wetlands are widely used for removing pollutants from wastewater. Substrate clogging is an operational challenge of constructed wetlands, which can result in impeded water flow and finally a significant decline in the ability of the system to treat the wastewater. The entire clogging process in a vertical flow constructed wetland (VFCW) was quantitatively analyzed by measurements of hydraulic conductivity. Tracer tests and model simulations were carried out to investigate internal flow patterns during the clogging and resting processes. This analysis revealed that hydraulic conductivity gradually decreased with operation time. Further, the distribution time of the flow field was different under different degrees of clogging. Non-uniformity in water flow was primarily observed in the first 400min after adding the tracer (NaCl) in the early clogging stage, as opposed to the last 400min in the late clogging stage. Variation in water flow divergence was closely correlated with piston flow; the reaction efficiency was highest in the early stages of clogging. In the later stages, stronger flow mixing was observed. Resting operations can reduce the dispersion of internal flow and improve reaction efficiency. After resting for approximately 15days, tracer concentration fluctuations decreased and internal flow back-mixing was alleviated. A simulation further described the internal flow pattern and elaborated and validated the tracer experiment. The outcomes of this study will assist in understanding how internal flow behavior varies in response to the clogging process and reveal details of the internal clogging mechanism in VFCWs.

An integrated model of substrate clogging in vertical flow constructed wetlands

This paper presents an integrated model of substrate clogging in a vertical flow constructed wetland (VFCW). The model simulates the reduction of pore space in the wetland substrate due to combined influences of various physical, biogeochemical and plant-related processes. A series of experiments based on laboratory-scale VFCWs were conducted to examine and measure key parameters related to clogging of the wetland substrate during operation under different conditions. The model was then validated using data collected from the experiments. The results showed that the model was able to replicate the clogging phenomenon as observed in the experiments, in particular, the characteristic clogging time. The model also predicted well individual contributions to clogging by accumulated inert suspended solids, microbial biomass and plant root materials during the wetland operation. Although the validation was based on the laboratory data, the results indicated that the model describes well the processes underlying the clogging and has the potential to become a tool for assessing the performance of prototype CWs in relation to clogging at both the design and operation stages.

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.