Custom-made plant tackles ammonia removal from leachate
Organics Group outlines the options available in dealing with nitrogen concentrations on landfill sites, citing experience in Hong Kong and Australia
With an increasing focus on total nitrogen concentrations as a discharge consent item, and not just ammonia, the issue of ammonia removal is rising up the list of priorities for attention.
There are two main options for ammoniacal nitrogen removal: anoxic reactors and ammonia strippers. Anoxic reactors work with bacteria to convert nitrates to nitrogen gas. Ammonia strippers use chemical reactions to convert ammonia ions to ammonia gas, allowing it to be driven off with steam or air.
In the United Kingdom concentrations of ammonia in leachate have historically been low, permitting the use of relatively standard sequencing batch reactors to achieve acceptable levels of denitrification.
With improved capping methods, generally improved water control in landfill sites and an increase in protein matter going into UK landfills, it has been noted by several observers that ammonia levels are rising. Indeed, following the BSE crisis, and the use of landfill sites to dispose of animal carcases, ammonia levels of up to 9,000 mg/l were encountered.
A comparison shows that the choice of an anoxic reactor over an ammonia stripper is not always as clear cut as one would wish. The bacteria involved in anoxic reactions require a carbon source to propagate and function correctly. Where the leachate COD is high, this may provide adequate bacterial food and no additional carbon would be required. Where the COD is not high enough, which is the case for most UK leachates, an additional carbon source is required. Methanol is usually employed to make up the requirement.
The running cost of an anoxic reactor is thus made up of methanol to feed the bacteria and caustic for pH adjustment. The running cost of an ammonia stripper is mostly contained in either chemicals to raise the pH or heat to raise the temperature. Both heat and pH adjustment may be employed to make the conversion from ammonium ions to ammonia gas.
Where the choice is between caustic and methanol the position is not clear cut and would depend upon available sources of both methanol, or another suitable carbon source, and suitable pH adjustment chemicals. This can be very much a site-specific issue.
Where the choice is between methanol and heat, the decision becomes very simple where waste heat is available to drive the necessary reactions. Waste heat can come from landfill gas or an engine exhaust. Recent improvements in efficiency make the use of high grade fuels, such as natural gas or propane, an option to consider.
Organics has developed a number of applications based upon the use of waste heat for ammonia stripping. The first such facility was rated at 1,800 m3/day and is located at the WENT landfill site in Hong Kong. The facility has now been in operation for almost seven years and achieves 98.5% ammonia removal in a single pass. Organics is currently constructing its sixth facility in Hong Kong, this time rated at 2,600 m3/day. In Hong Kong, ammonia removed is combusted in the landfill gas flare used provide heat to the process.
In a separate facility installed for use with an anaerobic digester in Sydney, Australia, waste heat from internal combustion engines running on biogas was used to drive the process. In this facility ammonia gas was removed in an acid scrubber, producing an ammonia salt which has a value as a fertiliser. The technology for thermally driven ammonia stripping is continually under review. Various options for ammonia recovery or destruction are currently being assessed.