Leachate treatment

For domestic waste landfills worldwide Leachate from domestic waste landfills all across the world can be treated to produce a consistently high-quality output, regardless of local climate or geography. Using well established and tested leachate treatment techniques is the key.ÃÂÃÂ The disposal of solid waste to landfill is a waste management strategy common to many countries worldwide. Despite an increasing emphasis on alternative options, solid waste disposal to landfill retains an important role in Europe. In many other areas of the world, the more "sophisticated¿ strategies have little applicability and basic landfilling is more common. Leachate is the liquor that is collected at the base of landfills after rainwater has entered the emplaced waste materials and leached out contaminants. Depending on the nature and age of the wastes, the characteristics of the leachate will change. The leachates from landfilling domestic wastes, which are more likely to be of a biodegradable nature, are discussed in this article. Leachates can be classed as acetogenic or methanogenic depending on the state of degradation of the waste materials in the landfill. Leachate from "young¿ wastes is characterized by high chemical oxygen demand (COD) and biological oxygen demand (BOD) values, and by high ratios of BOD to COD. Methanogenic leachates are derived from older landfills where extensive degradation of organic materials in the wastes has occurred. End products of methanogenesis in the landfill are methane and carbon dioxide. Leachate is categorized by lower COD values, very much lower BOD values, and lower BOD to COD ratios. INTERNATIONAL DIMENSION OF LANDFILLING Regulators of landfilling practice in various countries often say that leachate generated from their landfills is peculiar to their country and different from that in others, and that known and practised techniques for treatment are not appropriate for their situation. Actually, this is not so. Research has shown that landfilling of domestic wastes in well engineered, large, deep landfills produces leachate of similar characteristics, regardless of climate or national income levels. In fact, according to analysis of methanogenic leachates from well engineered landfills in the UK and other parts of Europe, the leachates are very similar to one another and to leachates in New Zealand, Hong Kong, South Africa, Malaysia and other countries in the Pacific Rim. "Leachates from well engineered landfills are very similar to one another¿ The transition to methanogenic conditions occurs more quickly in warmer climates than colder climates, but after full transition from the acetogenic state, concentrations of the major determinands generally show a remarkable similarity. Table 1 compares typical analytical data of the characteristics of landfill leachate from international landfilling sources. CONSTRAINTS ON DISCHARGE There are no technical constraints to producing a treated leachate acceptable for discharge back to the environment. Different countries approach the treatment of leachate in different ways. Often, regulators will use specific national standards on which to base acceptance of treatment. Other countries seek to use techniques that can be optimized under BATNEEC (Best Available Techniques Not Entailing Excessive Cost) principles. Although "appropriate technology¿ would be better adopted, there is still a tendency to opt for the "ideal¿ solution. However, the latter often requires degrees of sophistication of operational skill and expertise. These may not be present and, in the long term, can lead to inefficiencies in treatment. The approach to determining an optimized treatment strategy needs to consider: source and disposal route the possibility of modifying and integrating existing facilities variations from the generally accepted characteristics of the leachate the degree of robustness required to be acceptable at a location often remote from major industrial centres. Treatability trials should be organized where appropriate. Figure 1 depicts such an approach. STATE-OF-THE-ART LEACHATE TREATMENT The first UK aerobic treatment system using the sequencing batch reactor (SBR) process was established on a site in mid- Wales in 1982. It proved a robust and reliable system for achieving specified effluent quality for discharge and was the forerunner of many similar systems based on modified SBR technology. Over 20 years later, advances in the understanding of the technology to match increasingly strict discharge quality requirements have seen corresponding advances in systems designed to meet those requirements. Modifications have taken account of the transition from acetogenesis to methanogenesis in the landfill, variations in ambient temperature, changes in pH, the need for nutrient removal, removal of pesticides, reduction in toxicity, etc. Some of these are discussed below. Acetogenesis to methanogenesis The transition from acetogenesis to methanogenesis changed the emphasis of treatment, relying on nitrification as the primary requirement to convert ammoniacal nitrogen to nitrate nitrogen. This brought on a need for pH correction in the treatment process and alkalinity consumption was found to give good correlation with stoichiometric predictions. The modified technique has been used extensively in treatment internationally, including in Hong Kong, South Africa and Malaysia.Removal of pesticides The second modification was derived from a need to consider the removal of pesticides, or more particularly, herbicides. At the Buckden Leachate Treatment Plant (LTP) in the UK, the presence of isoproturon and mecoprop in leachate demanded additional facilities for treatment to a very high standard. A series of treatability trials showed that mecoprop would be readily removed by controlled aerobic biological treatment, but that isoproturon was more persistent. Ozonation was included as a secondary treatment, followed by polishing in a reed bed to remove byproducts from the ozonation process. Effects of temperature Extensive trials on leachates containing more than 1500 mg/l of ammoniacal nitrogen showed significant reduction in nitrification rates below 10°C. Since winter air temperatures in northern Europe can be significantly lower than this, a means of reducing heat loss from an SBR was needed. SBRs were therefore generally modified from lagoons to tanks, the area of the tank liquor surface was reduced and covered, and the aeration technique changed to submersible venturi jet aerators to take advantage of the heat generated by the aerator motors. In warmer tropical climates such as Hong Kong, Malaysia, South Africa and Mauritius, temperature constraints manifest themselves in the opposite way. Particularly for nitrification, reaction kinetics shows a dramatic reduction in treatment efficiency at temperatures greater than 40°C. With ambient air temperatures often higher than 30°C in such countries, it is important that aeration takes account of the potential for internal SBR "overheating¿. In such circumstances, reactors with a large surface area and surface aerators are more appropriate, and lagoon systems are preferred. Nutrient removal Leachates from domestic landfills are rich in ammoniacal nitrogen but usually contain little phosphorus. The primary biochemical reactions occurring in the treatment of methanogenic leachates thus focus on nitrification. However, the frequent constraints on the discharge of nitrate-nitrogen in the treated leachate may require significant removal of nitrate after initial nitrification processes. In Hong Kong, for instance, 200 mg/l (as nitrogen) is the maximum total of all nitrogen species allowed in the discharge of trade effluents to sewer. The maximum may be much lower for discharge to controlled waters. In such circumstances, it is either necessary to remove the ammoniacal nitrogen as a primary treatment or to denitrify after the nitrification step. In Hong Kong, air stripping of ammonia gas at elevated temperature is now widely practised; the stripped ammonia gas can be incinerated in the landfill gas flare. Where incineration is not possible, the ammonia gas has to be reacted with a suitable acid with disposal of the resulting salt. The latter is not always a viable economic option and biological nitrification/denitrification can be more appropriate. Biological nitrification/denitrification is proposed for landfill leachate treatment plants in Malaysia and Mauritius. Removal of hard COD After aerobic biological treatment, COD values in the effluent are often fairly high in relation to the COD of the raw leachate, particularly for methanogenic leachates. A significant part is soluble COD, which will be carried over into the discharge from the LTP. This hard (recalcitrant) COD has a close correlation with the influent ammoniacal nitrogen, as shown in the results from full-scale treatment plants and detailed pilot-scale studies worldwide (Figure 2). "Effluent¿ hard COD can be managed in a number of ways, depending on the acceptability to the regulating authority. In the UK, specific performance of the treated leachate regarding toxicity is required before it is deemed acceptable for discharge to controlled waters. In Malaysia, the regulatory authority demands reduction of the hard COD irrespective of toxicity, and facilities using activated carbon have been designed into leachate treatment systems. INTRODUCTION OF BEST AVAILABLE TECHNIQUES (BAT) The EU Integrated Pollution Prevention and Control (IPPC) Directive included landfill leachate under its umbrella. As a result, procedures have been developed for a landfill operator to demonstrate that the policies, philosophies and strategies employed at a site ¿ including leachate management ¿ follow the principles of IPPC for Member States. Technical guidance for the treatment of hazardous and non-hazardous landfill leachates is being prepared for the UK Environment Agency. This will result in further advances being required ¿ and made ¿ in the area of leachate treatment. Emission limits will be set as benchmark values for operators and will be used by the regulatory authority in assessing leachate management strategies when it receives applications for permits to run landfill sites. These will generally be based on minimum industry standards and be capable of being achieved without incurring excessive costs. Defined treatment systems will therefore depend on discharge requirements: the maxim is that the minimum acceptable standard is complete compliance with a specific discharge consent. CASE STUDIESUK experience of on-site treatment systems for landfill leachate with proven technologies has been successfully transferred to other parts of the world. The following case studies, in which Enviros has been involved, describe typical applications for leachate treatment both in the UK and around the world. Methane stripping at Red Moss Landfill, Lancashire, UK Leachates from methanogenic landfills can contain much higher levels of methane than the minimum that can cause explosions. This is particularly relevant for leachates that would be discharged to foul sewer without any pretreatment. Red Moss is a large, closed landfill in north-west England where leachate is permitted to be discharged to the public sewer without biological treatment. As part of extensive restoration works, a methane-stripping plant was designed and commissioned to remove dissolved methane before the leachate was discharged to sewer. The raw leachate contains up to 10 mg/l dissolved methane, while discharge to sewer requires a concentration of less than 0.14 mg/l to give a safety factor of 10 below the minimum concentration known to provide possible explosive conditions. This stripping plant is typical of many such methane-stripping systems operating in the UK. It consists of two banks of aeration reactors, each bank having four separate reactors, with separate air to each reactor. Table 2 presents typical data from Red Moss. TABLE 2. Typical operating results from the Red Moss methane stripping plant Sampling point Dissolved methane (mg/l) Raw leachate 9.98 Aeration tank1 4.61 Aeration tank 2 0.486 Aeration tank 3 0.133 Aeration tank 4 0.102 Pretreated leachate 0.095 Consent limit 0.14ÃÂÃÂ Leachate treatment at NENT Landfill, Hong Kong During the 1990s, the Environmental Protection Department of the Hong Kong Government developed a solid waste management strategy for Hong Kong. This strategy included the development of three large strategic landfill sites at the west, south-east and north-east New Territories (WENT, SENT and NENT), each with an estimated capacity of 25¿40 million m3. It was recognized early on that leachate management would need to play an integral part. Studies were carried out to predict the characteristics of the leachate from the new landfills, with particular reference to the NENT landfill, the first of the three to be constructed. These studies predicted, correctly, that ammoniacal nitrogen would be a critical parameter. Further studies demonstrated that efficient nitrification could be achieved using on-site treatment processes similar to those operating in the UK and Ireland. The treatment process has flexibility in the number of aerated lagoons provided and the different capacities of those lagoons. Leachate flow rates of up to 800 m3 per day, with 6000 mg/l ammoniacal nitrogen, have been treated successfully. LEFT TO RIGHT The methane-stripping system at the Red Moss Landfill in Lancashire, UK, consists of two banks of aeration reactors l A landfill in north New Territories, Hong Kong, incorporates an on-site leachate treatment plant focusing on efficient nitrification of ammoniacal nitrogen l The Buckden Leachate Treatment Plant in the UK employs extra steps to mitigate the presence of herbicides in the leachate: aerobic biological treatment, ozonation, and final polishing in a reed bedÃÂÃÂ More recently, limits for total nitrogen in treated leachate were made much stricter at NENT. Pretreatment using air stripping of ammonia at elevated temperatures was trialled successfully, with stripped ammonia gas being destroyed using the site¿s landfill gas flare system. A fullscale plant was constructed and commissioned as a preaerobic biological treatment process step. At SENT, a strict limit of 200 mg/l "total nitrogen¿ for the treated leachate was applied and pH-driven air stripping of ammonia was installed and commissioned from the beginning. Aerobic biological treatment at Buckden Landfill, Cambridgeshire, UK During the mid to late 1990s, stricter UK regulation of wastewater discharges into the environment imposed tighter limits on the determinands permitted to be present in leachates treated in on-site treatment plants. It was recognized that on-site treatment needed to advance still further, though on a site-specific basis, in order to meet such discharge limits reliably and cost-effectively. At that time, the landfill site at Buckden in Cambridgeshire was generating leachate containing significant concentrations of herbicides, mecoprop and isoproturon. Treatability trials showed that the presence of these herbicides did not impair the efficiency of biological treatment of the leachate. However, treatment did not remove isoproturon sufficiently and a secondary treatment process, ozonation, was included in the overall management strategy, followed by tertiary polishing in a reed bed. The full-scale plant includes twin SBRs, each designed for heat efficiency and capable of treating up to 100 m3/day. These are followed by 2000 m2 of reed bed and then an ozonation plant. This is capable of dosing up to 500 g/hour of ozone, typically dosing at a rate of 150 mg/l at maximum treatment rates. After final treatment in a 500 m2 tertiary reed bed, the treated leachate is discharged into the River Great Ouse. Since the plant was commissioned in 1994, it has provided over 10 years of successful leachate treatment. Between 1995 and 2000, the plant regularly achieved COD values of between 250 and 350 mg/l in the treated leachate. Most impressive of all, the plant has never breached a toxicity-based consent demanding that rainbow trout should not be harmed by 96 hours of exposure to 50% treated leachate (by volume). Treatment of leachate in South Africa Enviros has successfully completed treatability trials on leachates from three landfills in South Africa, designing and commissioning two large full-scale leachate treatment plants since 1999. Full-scale treatment plant in South Africa: Vissershok Landfill Vissershok LandfillVissershok Landfill receives up to 2000 tonnes of municipal solid waste (MSW) each day from the city of Cape Town, including some low to medium hazardous waste. Up toÃÂÃÂ 80 m3/day of leachate are generated, which have to be treated to very high standards. Pilot trials using SBR treatment technology were provided by Enviros and operated for a significant period to demonstrate nitrification and denitrification processes. The strategy used for treatment incorporated buffering of flows from different phases of the landfill, aerobic biological treatment in an SBR, with final polishing through a reed bed planted with Phragmites. Ammoniacal nitrogen concentrations are routinely reduced from over 1200 mg/l to less than 1.0 mg/l, and COD values are reduced by 50%¿60% from raw leachate values of over 2000 mg/l. Treated leachate shows no detectable toxicity when subjected to the Microtox test. The quality of the treated leachate is sufficiently good that the final "effluent¿ is now used instead of potable water for dust damping across the site. Full-scale treatment plant in South Africa: Mariannhill Landfill Mariannhill LandfillThe Mariannhill Landfill in Durban receives up to 200,000 tonnes of MSW annually. The landfill is next to a developed, sensitive horticultural area, and there is a recognized commitment by Durban City Council to retain and restore the natural flora and fauna of the overall Mariannhill area. Operation of the landfill needs to consider these matters, including the provision of a successful strategy for leachate management. In this case, the treated leachate is discharged into a recent man-made aquatic system, developed specifically as a rescue area for wetland species. Leachate generated from the landfill is about 50 m3/day and is treated using biological nitrification techniques. Ammoniacal nitrogen is reduced from about 500 mg/l to less than 1.0 mg/l. Treated leachate replaces potable water for dust suppression and plant irrigation. As with the Vissershok landfill, treatment of the leachate to high standards contributes to the critical protection of water resources in South Africa. Treatment of leachate in Malaysia Strategies for the treatment of leachate in Malaysia are constrained by environmental policies regulated by the Department of the Environment (DoE). Limits set by the DoE for discharges to places such as watercourse and sewerage include removal of residual hard COD after biological treatment. Treatability trials have demonstrated no detectable toxicity in treated leachate after SBR treatment to remove ammoniacal nitrogen, but to ensure full compliance, trials with granular activated carbon (GAC) have also been carried out. These have substantially reduced residual COD from typical values of up to 1200 mg/l. The results of these trials are being incorporated into the full-scale design of leachate treatment plants for two landfills in Malaysia. CONCLUSIONSThe characteristics of leachate from large, deep, domestic waste landfills are remarkably similar throughout the world. The technologies developed for treating leachate have evolved to achieve compliance with increasingly strict discharge consent limits. A philosophy of initial aerobic biological treatment using SBR technology has proved a reliable and robust strategy. Using this as a foundation for treatment, polishing in reed beds or advanced treatment using dissolved air flotation (DAF), GAC or ozonation can be added to provide treated leachate quality matched to site-specific discharge requirements. The minimum acceptable standard must always be complete compliance with a site-specific discharge consent. Well established and tested techniques are available to provide reliable and consistent treatment of landfill leachates. There are no technical obstacles to the treatment of landfill leachate to extremely high standards. Martin Carville is Technical Manager and Howard Robinson is Technical Director at Enviros Consulting, Shrewsbury, UK.Fax: +44 01743 245558e-mail: martin.carville@enviros.com howard.robinson@enviros.com \\n\n