Nitrate adsorption and desorption during biological ion exchange

[Display omitted] •Secondary IEX followed by aqueous denitrification caused resin regeneration.•Inorganic ions/organic acids in sludge displaced more NO3– than NOM in woody mulch.•Higher molecular weight organic acids had higher NO3– displacement.•Hydrophilic NOM with low carbohydrate composition had higher NO3– displacement.•Common ionic buffers have higher NO3– displacement than zwitterionic buffers. Ion exchange (IEX) is an effective NO3– removal technology and research has shown promise for bioregeneration of NO3– laden IEX resins, creating an ideal technology for decentralized systems by removing the need for chemical regeneration that results in brine wastes. This work aimed to advance the applicability of biological ion exchange (BIEX) by evaluating the effect of secondary IEX between organic acids, natural organic matter (NOM), and buffer solutions commonly present in BIEX systems and NO3–, which is often displaced from the resin by these compounds. Organic acids had separation factors ranging from 0 (little to no adsorption affinity) to 2.8 (40–50% of NO3– displaced from the resin), with higher molecular weight molecules yielding higher separation factors. Separation factors for NOM ranged from 0 to 2.1, with higher carbohydrate compositions identified as a major influence for higher separation factors. Batch denitrification experiments were conducted using sludge and woody mulch as sources of bacteria, and charge balances revealed that secondary IEX with inorganic ions and organic compounds facilitated NO3– displacement from the resins followed by aqueous phase denitrification.