Investigation of pollutant removal processes within a combined wetland/aerated pond system for the treatment of airport runoff
Adeola, Samuel Olawale
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An integrated system of aerated ponds together with rafted and horizontal flow sub-surface reedbeds has been Installed to treat the glycol-contaminated runoff from two of Heathrow Airport’s susceptible catchment areas the Southern Catchment and the Eastern Catchment. The original plan was to transfer pre-treated water from the latter through the existing fire main which runs around the airport perimeter to the Mayfield Farm Treatment Facility (MFTF) which receives runoff from the Southern Catchment. However this part of the overall Heathrow treatment scheme for surface runoff has not yet been put into operation. Results from monitoring of the treatment system has shown Significant reductions In Biological Oxygen Demand (BOD) throughout the system with levels decreasing by 76 6±10 5% across the constructed wetland cells at the MFTF following high airport deicing fluids applications. However continued exposure to BOD concentrations exceeding the design target of 110 mg/l has resulted in anaerobic conditions being established in the wetland. Background nitrate and phosphate levels observed in two aerated ponds at the MFTF which are used for storage and initial treatment of airport runoff were low Initial results from laboratory experiments conducted to assess the ideal nutrient levels required to support glycol biodegradation in the aerated ponds showed efficient BOD removal (within 5 days) after repeated glycol additions There were also Increases In bacteria populations In aerated pond water samples from average background levels of 107 to 1010 Colony forming units (CFU) per litre Based on the Interpretation of the laboratory experiments a nutrient dosing protocol commenced on 12 November 2004 at the MFTF to ensure that nutrient levels in the aerated ponds were appropriate to treat the contaminated airport runoff received during the winter months Although the BOO levels recorded In the aerated ponds were still high after the commencement of the nutrient dosing regime in November 2004 the BOD removal efficiency of the aerated reservoir following the Influx of high winter BOD load increased to 47 5% (February 2005) from 25 5% recorded In March 2004. In microbiological analyses conducted on the water samples from the MFTF only 18 strains of bacteria, 3 fungal strains and 2 actinomycetes strains were culturable In the laboratory. A combination of API biochemical tests and PCR (Polymerase chain reaction) analyses showed that at least SIX of these bacteria were Pseudomonas species which have been widely reported to have high potential for biodegradation of organic pollutants. However biodegradation experiments conducted using the consortium of the bacteria fungi and actinomycetes Isolates In sterile distilled water dosed With glycol showed that they lacked the ability to efficiently remove the BOD despite the continuously high BOD reductions recorded in some of the natural systems they were isolated from.