Introduction Conventional biological nutrient removal (BNR) has long been notorious for being energy and space intensive. Shortcut nitrogen removal technologies skip autotrophic nitrite oxidation, resulting in significant intensification (100-300%) of new and existing facilities. Both shortcut nitrogen technologies rely on meticulously controlled reactor conditions (metabolic selection) and external selector, like screens or hydrocyclones installed on the waste line, (physical selection) to be successful. Their efficacy is not easily quantifiable and applying certain process conditions might not yield the desired result. For example, selective retention of anaerobic ammonia oxidizers (AnAOB) and washout of nitrite oxidizing bacteria (NOB) with external selectors might not yield satisfactory results if process conditions (residual substrate, solids retention time (SRT), ...) does not allow for sufficient growth of the former and washout pressure on the latter [1. 2]. This paper provides easy guidelines on how effectively decouple SRT in shortcut technologies.
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