Rice fertilised with urban sewage sludge and possible mitigation strategies: an environmental assessment

Abstract

Among the different cereals, rice plays a relevant role in terms of production and dietary intake. Although the highest rice producers are Asian countries, this crop is also cultivated in Europe and, within Europe, Italy is the most relevant country. There are not studies evaluating in detail the environmental load arising from the application of urban sewage sludge in rice fields that, being flooded, are characterized by anaerobic conditions. The goal of this study was to evaluate the environmental performance of rice cultivation fertilised with urban sewage sludge in the Pavia district. Moreover, three mitigation strategies have been proposed and evaluated: the substitution of urban sewage sludge with compost, the introduction of an additional aeration and the collection of straw.The results suggest that the main contributors to the environmental impact of rice are methane emissions associated with the degradation of the organic matter during the flooding period, nitrogen emissions associated with the application of fertilisers, and diesel used for the field operations. The differences among the proposed alternative scenarios are considerable, suggesting that strategies to improve the environmental performance of rice are possible. In more details, in order to reduce the environmental burdens of rice fertilised with urban sewage sludge, the two most effective possibilities are: the substitution of urban sewage sludge with compost and the implementation of an additional aeration period during the cultivation. The first option determines an improvement in all the categories analysed, and particularly in toxicity-related impact categories, with reductions of 99.6%, 78.8% and 68.7% for human toxicity-cancer effects, human toxicity-non cancer effects and freshwater ecotoxicity, respectively, followed by Climate Change (−25.4%) and Mineral Fossil Resource Depletion (−19.2%). The introduction of an additional aeration is beneficial for the Climate Change and Photochemical Oxidant Formation, with a reduction of 9.7% and 1.6%, respectively.Concerning the results of the Climate Change impact category, the use of standard emission factors for the estimation of methane emission involves uncertainty; the development of country-specific emission factors would be instrumental in overcoming this point of weakness. Finally, despite the focus on the Italian context, this study proposes different mitigation strategies, which can be applied to other rice systems in order to reduce their environmental impact.