Green Lane’s new system
The management of leachate produced by a landfill is a major engineering challenge. To design and operate an efficient leachate treatment system, which is essential to minimize potential impact on the environment, several key factors must be considered. The characteristics of the landfill waste, the local climate, the hydrogeological conditions, the operational procedures and the landfill’s age all impact the quantity and quality of the leachate.
The proper characterization of leachate, backed by treatability studies, is essential to select a reliable system. Leachates are typically characterized by extreme values of parameters such as pH, ammonia nitrogen (NH3-N), alkalinity, dissolved metals, and oxygen demand.
An example of an effective landfill leachate collection and treatment process can be found at the 44-hectare Green Lane Landfill in St. Thomas, Ontario (near London).
Conestoga-Rovers & Associates (CRA) undertook the evaluation of the existing and future leachate as part of efforts to expand the landfill. The landfill is owned and operated by Green Lane, a division of St. Thomas Sanitary Collection Service Limited.
Originally opened in 1978 for the disposal of local residential wastes, the 130-hectare site currently accepts domestic, commercial and solid non-hazardous industrial wastes from a province-wide service area. The site capacity is approximately 5.7 million tonnes, and approximately one-quarter of this capacity has been utilized to date. With current landfilling rates and air-space limitations, the site life is estimated to be about 17 years. The amount of waste disposed of at the site in 2001 was approximately 230,000 tonnes.
Once a bench-scale leachate treatability study was complete and the site expansion was approved, CRA completed detailed design, approvals, and construction oversight of a leachate treatment facility.
The new system consists of a combination of toe drain and continuous base collection. Leachate is channeled by various means to a manhole located adjacent to a leachate holding tank. Leachate is pumped to the holding tank and is then pumped to the recently commissioned leachate treatment facility.
Following complete treatment the final effluent from the facility is discharged to Dodd Creek via a polishing basin. The polishing basin consists of a pond area and a stream corridor. Stormwater collected in a stormwater management pond is also pumped periodically to the polishing basin to augment the volume of treated effluent flowing from the plant. These flows augment discharge from the polishing basin during dry periods and contribute to otherwise low or no base flow conditions in Dodd Creek in the summer months.
In order to allow the treated leachate to be discharged into a small creek, the Ontario Ministry of the Environment established very stringent final effluent criteria (see table).
In light of the results obtained from the treatability study and expected raw leachate characteristics, the following treatment system was implemented for the site (see diagram):
pretreatment of the leachate with air and oxidation/pH adjustment to reduce total dissolved solids concentration;
secondary treatment by a suspended biological growth (BNR) system, designed for ~ 90 per cent total nitrogen removal;
tertiary treatment in the form of ozonation followed by sand filtration; and,
post-tertiary enhancement by a polishing basin.
A primary design criterion for the treatment plant was to incorporate significant flexibility in the treatment processes to allow for the expected variations in leachate quality and quantity throughout the landfill site life and beyond.
The objectives of the primary system are to reduce colour, remove iron, hardness, and colloidal matter as well as to reduce organic and TKN loading to the secondary biological treatment system. The estimated hydraulic retention time (HRT) for this portion of the plant is approximately eight hours at the full plant flow.
The secondary treatment system is comprised of an activated sludge process also known as the BNR system. The rational for using this process rather than a conventional activated sludge process (such as extended aeration) is to utilize the high nitrates from nitrification of ammonia and TKN as electron acceptors for organic matter stabilization. This reduces oxygen demand and organic loading into the aerobic portion of the overall treatment system.
Other advantages include further reduction in nitrates, improved buffering capacity of the leachates, excellent filament control, improved sludge settleability and better effluent quality.
The tertiary treatment of ozonation and filtering is to reduce colour, remove colloidal particles and suspend solids. The estimated HRT for the secondary and tertiary treatment processes is about 90 hours at the full plant flow.
Prior to completion of the facility in the fall of 2001, leachate was trucked to a municipal sewage treatment plant. The average volume of leachate transported off-site for treatment was approximately 32 m3/day.
Since December 2001 the plant has treated an average leachate volume of 45 m3/day. The maximum estimated volume of leachate the landfill is expected to produce (and that the treatment plant is sized to handle) is 131 m3/day.
The capital cost of this facility is approximately $3.6-million with annual operating and maintenance costs expected to be less than $150,000.
Andrew Lugowski, P. Eng. is a senior consultant with Conestoga-Rovers Associates in Waterloo, Ontario.