Evolution of dissolved organic nitrogen (DON) during sludge reject water treatment revealed by FTICR-MS.

This study evaluated the ability to remove dissolved organic matter (DOM), particularly dissolved organic nitrogen (DON), at a molecular level using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) in a full-scale reject water treatment project comprising three steps of short-cut nitrification and denitrification, two-stage AO, and ultrafiltration membrane system. The results indicated that short-cut nitrification and denitrification were effective in reducing the DON concentration from an average of about 180 mg/L to 43 mg/L. The average molecular weight of DOM showed a decreasing trend from 238 Da to 160 Da. The percentage of nitrogen-containing organic compounds (CHON-DOM), which is the primary component of reject water DOM, increased from 65.79 % to 72.35 %, while the percentage of CHO-DOM decreased from 20.67 % to 15.24 %. The percentage of CHOS-DOM remained stable at 12.21 %-13.54 %. The percentage of protein-DOM decreased from 50.32 % to 18.40 %, while lignin-DOM increased from 36.16 % to 55.88 % and carbohydrate-DOM increased from 2.68 % to 9.74 %. The short-cut nitrification and denitrification and ultrafiltration membrane system both generated more unsaturated, highly aromatic DOM. This study provides insights into the effects of different wastewater treatment processes on the evolution of DOM/DON, which can be useful for effective DON control.

Enhanced sludge settlement of two stage PN/Anammox for reject water treatment with respective diatomite addition.

The present study investigated the potential of diatomite addition in enhancing sludge settlement of two-stage PN/Anammox for real reject water treatment, with a focus on sludge settling velocity, nitrogen removal capacity, sludge morphological features, and microbial community changes. The study found that diatomite addition significantly improved the sludge settleability of the two-stage PN/A process, resulting in a decrease in sludge volume index (SVI) from 70 to 80 mL/g to about 20-30 mL/g for both PN and Anammox sludge, although the sludge-diatomite interaction differed between the two types of sludge. In the PN sludge, diatomite acted as a carrier, while in the Anammox sludge, it acted as micro-nuclei. The addition of diatomite also increased the biomass amounts in the PN reactor, with a 5-29 % improvement attributed to its role as a biofilm carrier. The effects of diatomite addition on sludge settleability were more prominent at high mixed liquor suspended solids (MLSS), where sludge characteristics were deteriorated. Furthermore, the settling rate of the experimental group consistently exceeded that of the blank group after diatomite addition, with a significant decrease in SV. The relative abundance of Anammox bacteria was improved, and sludge particle size decreased in the diatomite-added Anammox reactor. Diatomite was effectively retained in both reactors, with less loss observed for Anammox than PN due to its more tightly wrapped structure, resulting in a stronger sludge-diatomite interaction. Overall, the results of this study suggest that diatomite addition has potential in enhancing the settling properties and performance of two-stage PN/Anammox for real reject water treatment.