By Sara Arabi and Andrew Lugowski
Landfill leachate treatment is a major engineering challenge due to the high and variable concentrations of dissolved solids, dissolved and colloidal organics, heavy metals and xenobiotic organics.
Specific leachate management practices, such as recirculation (bioreactor landfill) and blending landfill gas with leachate, impact quality, resulting in characteristics that vary greatly from site to site. Cold temperature in winter is also a challenge to designing leachate treatment facilities (LTF) in Canada.
Traditionally, landfill leachate has been hauled or pumped to off-site wastewater treatment facilities for disposal. Disposal to off-site facilities has generated opposition from plant owners due to more stringent effluent discharge criteria. When discharged to a wastewater treatment facility, leachates can interfere with ultraviolet disinfection by strongly quenching UV light. Leachate may also contain heavy metals and high ammonia concentration that may be inhibitory to the biological processes.
On-site leachate treatment is an alternative to the increasing costs associated with hauling leachate to a local wastewater treatment plant. These treatment facilities are designed to fulfill the specific needs of individual landfill sites and allow discharge to a sanitary sewer or water body without any hauling or disposal costs.
Technologies for landfill leachate treatment include biological treatment, physical/chemical treatment and “emerging” technologies such as reverse osmosis (RO) and evaporation.
Biological leachate treatment is a proven technology for organics and ammonia removal in young and mature leachate. The anoxic/aerobic processes achieve nitrification and denitrification and reduce the oxygen demand for landfill leachate treatment.
Biological treatment methods include the activated sludge process (ASP), sequencing batch reactors (SBR), membrane bioreactors (MBR), aerobic lagoons and constructed wetlands. Physical-chemical treatment methods include oxidation, coagulation/flocculation, activated carbon, stripping, evaporation, filtration and RO. The choice of technology depends largely upon characteristics of the leachate, discharge limitations (e.g., direct or indirect discharge), and site constraints.
Green Lane LTF
The Green Lane Landfill is located in St. Thomas, Ontario, and was originally opened in 1978 for the disposal of local residential wastes. The site currently accepts domestic, commercial and solid non-hazardous industrial wastes from a province-wide service area. 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 remaining site life is estimated to be 20 years.
Extensive treatability studies were conducted prior to the design of the LTF, including respirometry tests to determine bio-kinetics and bench-scale coagulation/precipitation testing for colour removal. Coagulation and flocculation testing was conducted to compare alum and ferric chloride with respect to the initial and final pH, dosage, sludge production and colour reduction.
A bench-scale treatability study proved to be useful in determining the feasibility of potential treatment processes for removing contaminants from the leachate. It also resulted in the collection of significant data for the full-scale on-site LTF design. The leachate was characterized by high COD (1,000 – 5,000 mg/l), ammonia (150 – 600 mg/l), total dissolved solid (TDS) (2,000 – 6,000 mg/l) and intense colour (up to 1,200 Pt-Co).
A primary design criterion for the treatment plant was to incorporate significant flexibility in the treatment processes to allow for expected variations in leachate quality and quantity throughout the landfill site life and beyond. In order to reliably achieve the treatment required, the biological system was designed as Modified Ludzack-Ettinger (MLE) process for an extended retention time. Hydraulic retention time (HRT) was found to be required for hydrolysis of slowly biodegradable compounds in the leachate and to achieve low enough organics, prior to aerobic nitrification.
The full-scale LTF, designed at a capacity of 300 m3/day, includes pre-treatment comprising chemical addition and primary clarification to remove metals, hardness and TDS. The MLE process is followed by ozonation to remove colour, and filtration to treat leachate to acceptable regulatory levels for a surface water receiver. Phosphoric acid and methanol are added as supplemental phosphorus and carbon sources, respectively (see Figure 1).
The pre-treatment and biological tanks are housed in a building and greenhouse cover, to eliminate cold winter temperature concerns.
The 2008-2013 average effluent concentrations are BOD < 5 mg/l, ammonia < 3 mg/l , and total phosphorus (TP) < 0.5 mg/l, which are all below the compliance criteria of 5 mg/l, 3 – 5 mg/l, and 0.5 mg/l, respectively. The average effluent total suspended solids (TSS) concentration ranged from 4 – 11 mg/l which is below the regulatory requirement of 15 mg/l.
Long-term consistent ammonia removal efficiency of 99% and COD removal efficiency of 60% – 80% were achieved. On average 70% – 90% colour removal was achieved in the LTF. While there were no regulatory requirements for total nitrogen (TN) or nitrate, the LTF achieved on average 60% – 70% TN removal efficiency.