Treatment performance assessments of different wetland mesocosms
, PhD thesis, University of Salford.
Despite their global acceptance as a sustainable and cost-efficient technology for water pollution control including urban wastewater contaminated with hydrocarbons, treatment performance of vertical-flow constructed wetlands can be hampered by clogging of the substrate media pores of the wetland filters. This clogging usually leads to blockage of filter substrate, progressive diminution of porosity and reduction of active pore volume, permeability and substrate hydraulic conductivity subsequently leading to poor water quality production by the wetland filters. This operational problem hinders the wider application and acceptability of these systems worldwide. In this study, different laboratory-scale vertical-flow constructed wetlands filled with gravel and planted with common reed were constructed and operated between June 2011 and March 2014 to assess treatment performances and their relationship with clogging, and diesel spill treatment as a function of hydraulic and organic loading rates, media size, and contact and rest time. Furthermore, to evaluate the hydrocarbon spill, approximately 130 grams of diesel fuel was poured into each of four wetland filters. This is the equivalent of a one-off inflow concentration of 20 g/l. A range of hydraulic loading rates was applied across the systems using real urban wastewater. Analysis of total petroleum hydrocarbon concentrations of outflow waters along with other water quality parameters was carried out to monitor both clogging and treatment performance variations.
Overall, all constructed wetland systems have shown relatively high removal efficiencies for the key water quality parameters regardless of filter set-up before the hydrocarbon spill and no clogging observed. The removal efficiencies dropped for those filters impacted by the diesel spill. The filter with the highest COD loading but no diesel contamination performed the best in terms of COD and BOD removal. Furthermore, filters contaminated by diesel performed worse in terms of COD and BOD, but considerably better regarding nitrate-nitrogen removal without any apparent negative impact of within bed clogging
Pertaining to seasonal variability, findings show that COD, nitrate-nitrogen and ammonia-nitrogen have shown a seasonal trend with high removal in summer compared to other seasons, while BOD removal was efficient in winter compared to summer and turbidity was greatly removed in autumn compared to other seasons. However, no clear seasonal pattern of ortho-phosphate-phosphorus and SS removal were noted. Furthermore, in the hydrocarbon contamination period, all filters regardless of the pollution, design or operation had higher removal in winter than autumn for COD, BOD, ammonia-nitrogen and ortho-phosphate-phosphorus while no seasonal trend was observed for other water quality parameters.
Investigation regarding treatment performance and clogging evolution revealed that none of the systems has shown any signs of clogging after about three years of operation even with high rate Filters 7 and 8. The simulation model confirms the empirical findings that considerable filter clogging restricting the operation has not occurred. However, a small aggregate diameter, a short contact time, a long resting time and a low COD inflow concentration were most beneficial in reducing SS accumulation within the wetland filters.
Treatment of the hydrocarbon pollutants was also assessed, and the investigation revealed that all the hydrocarbon components treated in the wetland filters were highly degraded (>80% removal efficiency) in all contaminated filters with some even attenuated below the detection limit.
The overall outcome of this research may give useful information to wetland engineers and scientists to redesign and model configuration and operation of vertical-flow systems to increase performance and sustainability by maximizing contaminants removal efficiency for pollutants found in urban wastewater and preventing clogging occurrence in the systems. Consequently, this will help in saving cost for stakeholders in terms of operation and maintenance, and allow for progressive management of the wetland systems. Moreover, it will provide beneficial judgement for petroleum and related water industries to have confidence to incorporate wetland systems in their wastewater treatment technologies with no fear of clogging, particularly for control of hydrocarbon spills that may be released in sewage discharged to the municipal treatment plants and can also be discharged with industrial wastewater.
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