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.