Life Cycle Assessment of Different Strategies to Overcome Ammonia Inhibition in Anaerobic Digestion Process

Anaerobic digestion (AD) is a widely used technology to simultaneously dispose biowaste and recover energy (in the form of CH₄). However, the hydrolysis of urea (e.g. manure-based feedstock) and the degradation of protein and amino acid releases ammonia, which could inhibit the AD process when it is at high levels (> 0.15 g NH₃-N L^−1). Thus poor substrate utilization and unstable reactor performance are usually observed when protein-rich feedstock is used. In recent years, different strategies are proposed to overcome the ammonia inhibition problem, aiming to improve the methane yield of the feedstock. For example, bioaugmentation with ammonia tolerant methanogenic consortia, dilution of the substrate with water, addition of absorbing material and air stripping are extensively investigated by a lot of researchers. These methods increase the methane production to a certain level, but they also need extra operations and costs, which cause additional environmental impacts, such as energy and resource consumption. Up to now, few studies can be found discussing about the overall environmental impacts of these proposed methods. Therefore, in order to determine which method to use from the sustainable perspective, it is of great importance to evaluate and compare the environmental impacts of all the methods. In this study, life cycle sustainability assessment (LCSA) methodology is employed to assess these methods with respect to both the life cycle costing and the environmental impacts on different damage categories, such as climate change, human heath, resource, etc. The results derived from this study will be used to identify the weak point of each method and support the strategy choosing to overcome ammonia inhibition in the future.