To many wastewater engineers, ammonia removal from wastewater represents a problem that directly affects the operational bottom-line profit as, if a carbon source that acts as food for anoxic bacteria is required to remove ammonia from wastewater, operational costs can be prohibitively expensive. Ammonia released to the environment with concentrations ranging from 2.5 to 25 mg/l is recognised as being lethal to aquatic life, because when it is biologically oxidised to nitrate it exerts an oxygen demand on the receiving water that reduces oxygen in the water to levels where life cannot survive. Ammonia also acts as a fertiliser, causing the profuse growth of stringy bacteria or fungi, and consequent harmful disruption of the natural environment.
The obverse to the coin of the challenges related with ammonia is that there is a growing recognition that ammonia in wastewater need not be a problem, but it can also be a resource. If ammonia is recovered from wastewater it can not only reduce the incidence of global warming greenhouse gas emissions, it can also contribute to a significant reduction of operational costs.
Ammonia removal from wastewater is therefore not only beneficial to the environment it is also a potential resource that, if concentrations are high enough, can represent a significant commercial opportunity.
The ammonia molecule is comprised of four atoms, one nitrogen and three hydrogen with the hydrogen atoms carrying the thermal energy, and ammonia as a fuel burns to give water and nitrogen. Consequently, its use as a viable fuel is currently being investigated for several applications including ammonia powered fuel cells and, as ammonia can be transported more readily that hydrogen, as a manner in which the distribution network for a hydrogen driven economy can be successfully implemented.
There are several techniques for removing ammonia from wastewater. One that has been successfully implemented on several sites is that of treating highly ammoniated effluent using waste heat to thermally separate ammonia from wastewater. This technology has been employed to treat and remove ammonia from both landfill leachate and wastewater from food waste anaerobic digestion plants. Waste heat from, for example, landfill gas engines, raises the temperatures of the wastewater to a point where, if it is run counter-current to a stream of air, the ammonium ion is preferentially stripped off and fixed within the air stream as a gas; a feature that ensures it can either be destroyed under controlled conditions in a flare stack or, if appropriate, it can be concentrated and recovered in a useable, commercially viable form.
Ammonia is increasingly recognised as a compound that, whilst being highly polluting if released to the environment untreated, can also be recovered and thus represent a viable commercial resource. Using waste heat as the basis for ammonia removal and recovery meets the combined objectives of ensuring a long-term sustainable two-pronged solution to the challenges of ammonia pollution and the recycling of a waste material; this latter, one of the defining tenets of the circular economy.