Enhanced Nitrification of High-Ammonium Reject Water in Lab-Scale Sequencing Batch Reactors (SBRs)

Dewatering anaerobic digested sludge leaves a liquid fraction known as reject water, a liquid organic fertilizer containing high amounts of ammonium nitrogen (NH4-N). However, its concentration should be enhanced to produce commercial fertilizer. Thus, reject water nitrification for stabilization as well as for nitrate capture in biochar to be used as a slow-release fertilizer is proposed. This study attempted to accomplish enhanced nitrification by tuning the operating parameters in two lab-scale sequential-batch reactors (SBRs), which were fed reject water (containing 520 ± 55 mg NH4-N/L). Sufficient alkalinity as per stoichiometric value was needed to maintain the pH and free nitrous acid (FNA) within the optimum range. A nitrogen loading rate (NLR) of 0.14 ± 0.01 kg/m3·d and 3.34 days hydraulic retention time (HRT) helped to achieved complete 100% nitrification in reactor 1 (R1) on day 61 and in reactor 2 (R2) on day 82. After a well-developed bacterial biomass, increasing the NH4-N concentration up to 750 ± 85 mg/L and NLR to 0.23 ± 0.03 kg/m3·d did not affect the nitrification process. Moreover, a feeding sequence once a day provided adequate contact time between nitrifying sludge and reject water, resulting in complete nitrification. It can be concluded that enhanced stable nitrification of reject water can be achieved with quick adjustment of loading, alkalinity, and HRT in SBRs.