The use of ammonia recovered from wastewater as a zero-carbon energy vector to decarbonise heat, power and transport – a review

Ammonia (NH3) is an energy vector with an emerging role in decarbonising heat, power and transport through its direct use as a fuel, or indirectly as a hydrogen carrier. Global ammonia production is having to grow to enable the exploitation of NH3 for energy decarbonisation, which it is projected will consume >50 % of manufacturing capacity by 2050. Ammonia recovered from wastewater can be directly exploited as a sustainable source of ammonia, to reduce the demand for ammonia produced through the energy intensive Haber-Bosch process, while fostering a triple carbon benefit to the water sector, by: (i) avoiding the energy required for aeration of biological processes; (ii) reducing nitrous oxide emissions associated with ammonia oxidation, which is a potent greenhouse gas; and (iii) producing a zero-carbon energy source that can decarbonise energy use. While previous reviews have described technologies relevant for ammonia recovery, to produce ammonia as a zero-carbon fuel or hydrogen carrier, wastewater ammonia must be transformed into the relevant concentration, phase and achieve the product quality demanded for zero carbon heat, power and transport applications, which are distinct from those demanded for more conventional exploitation routes (e.g. agricultural). This review therefore presents a synthesis of established and emerging technologies for the extraction and concentration of ammonia from wastewater, with specific emphasis on enabling the production of ammonia in a form that can be directly exploited for zero carbon energy generation. A précis of technologies for the valorisation of ammonia as a clean energy or hydrogen resource is also introduced, together with discussion of their relevancy and applicability to the water sector including implications to energy, carbon emissions and financial return. The exploitation of ammonia recovered from wastewater as a zero carbon energy source is shown to offer a critical contemporary response for the water sector that seeks to rapidly decarbonise existing infrastructure, while responding to ever stricter nitrogen discharge limits.