“…Due to the complexities of multi-period mitigation guidelines, the mitigation rates may vary with the preference of local authorities, which would lead to various possibilities in nutrient load emission permits. For better reflecting the flexibility of the decisions, TN and TP emission permits can be treated as fuzzy sets (He et al 2008 (Gao et al 2014;Liu et al 2014). Accordingly, we set the TN emission permit for urban drainage as (7720, 7810, 7900), (6940,7030,7110), and (6250, 6325, 6400) tonnes/year during periods 1, 2, and 3, respectively.…”