Complex waste streams, such as sludge reject water from anaerobic digestion, often contain a multitude of cations at varying concentrations, with the ammonium ion (NH₄⁺) typically being the most abundant. Recently, electrodialysis (ED) has been developed as a technology for the removal and recovery of NH₄⁺ from reject water. However, the further development of ED for targeting NH₄⁺ recovery faces lack of reliable performance prediction and standardized operating strategies due to cation competition. It is postulated that selective rejection of divalent cations can be achieved through the use of monovalent-selective cation-exchange membranes (mCEMs). Due to their higher electrical resistance, questions remain regarding the cationic compositions under which mCEMs provide a substantial performance benefit. In the present study, three feed solutions simulating different molar cation compositions of reject water were subjected to ED using both mCEMs and conventional cation-exchange membranes (CEMs). The feed solutions were categorized based on the molar fraction of NH₄⁺ relative to the total cation content. Results showed that the perm-selectivity of NH₄⁺ over Mg²⁺ and Ca²⁺ was enhanced when using mCEMs. Moreover, mCEMs resulted in a higher overall NH₄⁺ removal efficiency compared to standard CEMs. At a relatively low NH₄⁺ molar fraction (0.32), mCEMs outperformed CEMs in terms of current efficiency. Notably, at an intermediate molar fraction (0.58), energy consumption was lower for mCEMs compared to CEMs, but only up to a critical degree of desalination (CDD). At a high molar fraction (0.89), the contribution of mCEMs was insignificant compared to that of CEMs. The CDD was identified as a pragmatic operational parameter beyond which further desalination leads to disproportionate energy penalties. For on-site-process-control of NH₄⁺ removal with ED, the CDD demonstrated that membrane selection and operating thresholds are strongly dependent on reject water composition.