McDougall LTF, Parry Sound
The McDougall Landfill in Parry Sound, Ontario, has served as a solid non-hazardous waste disposal facility since 1976. Groundwater studies at the site indicated that residual leachate impacts resulting from the unlined landfill were present in on-site groundwater. The contaminants of concern in groundwater were iron and manganese. The landfill owner decided to move forward with the design of a combined facility to treat the impacted groundwater, as well as leachate from ongoing landfilling activities.
Extensive bench-scale treatability tests were performed on the leachate to identify processes suitable for reducing iron. This included iron oxidation by aeration and by strong oxidants, followed by precipitation. Biological process modelling was conducted prior to the conceptual design of the LTF. Treatability tests proved that iron oxidation by aeration is sufficient for the reduction of iron. Based on what had to be removed from the waste, it was determined that a chemical pre-treatment step was not required before biological treatment.
Impacted groundwater and leachates are treated by a biological process designed to achieve nitrification and denitrification at a design capacity of 120 m3/day. The major challenge in the design of this LTF was the high variability of influent total Kjeldahl nitrogen concentrations (60-250 mg/l).
In order to achieve stable operation in spite of highly variable carbon-to-nitrogen ratio, the overall LTF was designed for extended retention time of 2.3 days (anoxic and aerobic HRTs of 15 and 40 hours, respectively). Methanol and phosphoric acid are added as carbon and phosphorus sources.
Effluent from the clarifier is discharged to the polishing pond and a stormwater/infiltration pond.
A major concern in operating biological systems in cold climates is the reduced nitrification performance or freezing, during periods of low ambient temperatures.
Long-term effluent quality averages are: ammonia of 0.1 – 2 mg/l, and BOD of < 7 mg/l. These are lower than the respective effluent limits of 5 and 15 mg/l. The LTF achieved 99% ammonia removal for this period. Effluent nitrate concentrations were in the range of 22 – 47 mg/l, reflecting excellent denitrification. Iron and manganese in the impacted groundwater were reduced to Provincial Water Quality Objectives (0.3 mg/l iron) in the aeration system without pre-treatment.
On-site leachate treatment systems are an attractive alternative to reduce the cost and environmental risk of hauling leachate to off-site treatment facilities.
Proper characterization of leachate, backed by treatability studies (bench tests or pilot studies) is helpful to select a reliable leachate treatment facility that can effectively accommodate variable influent characteristics. Applicable technologies for on-site leachate treatment include a variety of physical/chemical processes as well as biological processes.
The two case studies describe the suitability of biological processes for treatment of young and intermediate leachate. Nitrification and denitrification processes for biological treatment of leachate have been successfully implemented for removal of organics and nitrogen.
The advantages of such processes include stable nitrification, higher degree of total nitrogen removal, reduction of oxygen demand and lower sludge production rate.
Provisions for addition of a supplemental carbon source, such as MicroC™ premium carbon sources or methanol and phosphorus, may need to be included for treatment optimization.
Sara Arabi and Andrew Lugowski are with Conestoga-Rovers & Associates. This article appeared in ES&E’s January/February 2015 issue.