The Treatment of Leachate Using a Hybrid Technique
Despite the advances made in the last few years, the search for a successful and cost-effective method of leachate treatment remains one of the holy grails of the waste management industry. And even though experience of treating the complex build-up of liquid that usually accumulates within landfills is extensive, there remain many projects that fail due to the application of an incorrect selection of the technology required for the treatment of any given landfill leachate.
One clear observation is that each project is different and requires a unique solution. The selection of a 'fit-for-duty' leachate treatment process requires the gathering and analysis of certain basic information fundamental to establishing which processes should applied on any given site. This will include firstly, the anticipated flow rate, based largely upon the extent of water inflow to the site rather than biological activity; water inflow can not only be from rainfall but also from groundwater ingress, as well as biological activity. Secondly, the composition of the leachate to be treated must be considered. This is not only related to the pre-treatment observation, but also a characterisation of how the leachate may evolve with time. Unfortunately, this latter parameter is difficult to estimate and, to ensure that the technology employed remains applicable, a comprehensive strategy of regular sample taking, and analysis must be established. Thirdly, it is important to know the level to which leachate must be treated, a factor that will depend on the discharge consents attached to the site license. Regulatory authorities will determine acceptable levels of chemicals within treated leachate before release to the environment. It is therefore important to determine the objective of any treatment process before chemical engineering analysis and design starts.
Unlike landfill gas, where combustion, either in a flare stack or an engine, is the near universal solution, the range of technologies applicable for leachate treatment is directed specifically to a particular objective and treatment goal. If these change during planning or construction, a different method of treatment is may be needed at some stage in the evolution of the site.
In Hong Kong, the Organics Group has installed leachate treatment equipment on several landfill sites around the city. The scale of these sites is on a level that make them challenging in terms of treatment options. Most of the urban waste in Hong Kong is highly contaminated in ammonia and this, coupled with strict discharge consents mean that a hybrid leachate treatment, consisting of ammonia removal (increasingly, recovery of ammonia) and a pre-discharge polishing within a Sequence Batch Reactor, a combined process that can reduce the incidence of ammonia from more than 6,000mg/litre to less than 10mg/l, as well as significantly reducing the biological oxygen demand.
Hybrid solutions form the industry standard for leachate treatment. But it is recognised that the internal chemistry of a landfill will evolve with time. It is important, therefore, to ensure that flexibility and adequate monitoring is in-built to any leachate treatment solution.
Ammonia Removal and Recovery from Leachate
Until recently, the principal reason for treating ammonia in leachate was to ensure that the discharge flow was compliant with the site licence. The ammonia recovery systems developed in Hong Kong were initially constructed with this as the principal design criterion.
Historically, the most commonly employed methods used to treat the incidence of ammonia in leachate have been to lower the pH or to dilute the accumulated leachate with water in a tank or a lagoon. The addition of lignocellulosic biomass, with a high C:N ratio, has also been employed to increase the C:N ratio of the leachate. Where, for various reasons, these approaches cannot be employed, there are also several technology variants that operators can deploy for ammonia treatment, including biological treatment, membranes, pH-driven processes and thermally driven air stripping.
In 1997 thermal air stripping was chosen as the core ammonia removal process for the WENT landfill site in Hong Kong. In this case, thermal efficiency was not a performance criterion. With a design flow rate of 1,800 m3/day, heat was provided by the burning of as much landfill gas as was necessary to achieve the dual objectives of providing the motive energy for the process as well as the destruction of the ammonia. The initial design duty for this plant was for an influent of 6,700 mg/L, and this was to be reduced to an effluent of 100 mg/L. With later upgrading, the plant now removes 14.5 tonnes of ammonia per day and similar processes have now been installed on sixteen additional sites around Hong Kong. As more plants have been installed, the process for ammonia removal has become more efficient and, rather than using a primary resource such as landfill gas, it is proving increasingly viable to use waste heat, such as heat from landfill gas engines.
Thermal ammonia stripping has formed the core nitrogen removal process for several wastewater or leachate treatment facilities at both landfills and food waste anaerobic digesters in Hong Kong but, as the primary driver has been to meet discharge standards, the recovered ammonia has mostly been directly destroyed by thermal oxidisation.
The future drive towards a circular economy will undoubtedly result in a change of focus as to what to do with ammonia that can be removed and recovered from leachate.Instead of destroying it, recovery of either ammonium hydroxide or anhydrous ammonia has been proven to be not only operationally practicable but also commercially viable.
Ammonia is used in a wide range of applications, from pharmaceuticals and agriculture, to industrial cleaning and explosives. Another application is linked to the energy content of liquid ammonia, which at 11.5 MJ/L, is approximately 30% that of diesel. Ammonia may be used in fuel-cells, which offers the potential for a local, revenue generating means of disposal. Ammonia may also be used in engines and turbines as a fuel. Its high-octane rating of 120 and low flame temperature permits the use of high compression ratios without the penalty of high NOx production.
Ammonia is relatively easy to transport and is a convenient way in which hydrogen can be transported from it point of manufacture to its point of use, a feature that has significant implications as a commercially viable mechanism for integrating both the circular and the hydrogen economies.