Mathematical modelling of an intermittent anoxic/aerobic MBBR: Estimation of nitrification rates and energy savings

Assessment of nitrification process in a sequencing batch reactor: Modelling and genomic approach

Nitrification of ammoniacal nitrogen (N-NH4+) to nitrate (N-NO3-) was investigated in a lab-scale sequencing batch reactor (SBR) to evaluate its efficiency. During the nitrification process the removal of N-NH4+ reached 96%, resulting in 73% formation of N-NO3-. A lineal correlation (r2 = 0.9978) was obtained between the concentration of volatile suspended solids (VSS) and the maximal N-NO3- concentration at the end of each batch cycle under stationary state. The bacterial taxons in the initial inoculum were identified, revealing a complex diverse community mainly in the two major bacterial phyla Proteobacteria and Actinobacteria. The FAPROTAX algorithm predicted the presence in the inoculum of taxa involved in relevant processes of the nitrogen metabolism, highlighting the bacterial genera Nitrospira and Nitrosomonas that are both involved in the nitrification process. A kinetic model was formulated for predicting and validating the transformation of N-NH4+, N-NO2- and N-NO3- and the removal of organic and inorganic carbon (TOC and IC, respectively). The results showed how the increase in biomass concentration slowed down the transformation to oxidised forms of nitrogen and increased denitrification in the settling and filling stages under free aeration conditions.

Mechanism of Iris sibirica and aeration combination on promoting the water purification performance of constructed wetland under low temperature

Temperature is an important factor affecting the water purification performance of constructed wetland (CW). In the previous study, the combined measures of Iris sibirica and aeration at the bottom of the first quarter filtration chamber could improve the pollutant removal capacity of CW at low temperature. However, the mechanism between the combined measures of Iris sibirica and aeration on enhancing the performance of domestic sewage treatment is unclear. Our study aims to provide scientific validation for the combined measure through monitoring the concentrations of dissolved oxygen (DO), chemical oxygen demand (CODCr), ammonia nitrogen (NH4+-N), and total nitrogen (TN) along the water flow pathway of the CW and measuring the superoxide dismutase (SOD) activities of the plants and the abundance of nitrogen cycle-related microbial functional genes in the substrates of CW to explore the mechanism of combined measures promoting the removal efficiency of the CW under low-temperature stress. Results showed that aerating at the bottom of the first quarter filtration chamber increased DO concentration in the front part of the CW, which benefited the aerobic removal of pollutants and the activities of microorganisms, and the removal CODCr and NH4+-N occurred mainly in the front part of the CW. SOD activities showed that I. sibirica had better resistance to low temperature than Canna indica did. The combined measures of I. sibirica and aeration activated the activities of microorganisms, increased the abundance of the denitrification process genes along the water flow pathway and formed a clear nitrification-denitrification zone in the CW, thus promoted the nitrogen removal efficiency at low temperature. Therefore, this study confirmed the feasibility of the combined measures from a mechanistic perspective.

Operational strategies enhancing sewage sludge minimization in a combined integrated fixed-film activated sludge - oxic settling anaerobic system

A lab-scale integrated fixed-film activated sludge (IFAS) reactor was mplemented with the oxic-settling anaerobic (OSA) cycle for reducing sewage sludge production through the addition of an anoxic/anaerobic sludge holding tank (SHT) along the sludge recycle line. The IFAS-OSA system was operated under the different hydraulic retention time (HRT) in the SHT (HRTSHT) of 12 h and 6 h, at an oxidation-reduction potential (ORP) < -91 mV and solid retention time (SRT) between 39 and 126 d. Furthermore, the effect of temperature increase in the SHT (TSHT) from ambient (19.8-25.6 °C) to mesophilic (35 °C) conditions was investigated. The system performances were monitored in terms of sludge minimization and dewaterability efficiencies as well as carbon and nutrients reduction. The observed sludge yield (Yobs) for the IFAS system was 0.37(±0.06) mg VSS/mg COD. After OSA implementation Yobs decreased by 32% and 46-65% at HRTSHT of 12 h and 6 h, respectively, indicating that prolonged exposure to anoxic/anaerobic conditions was not beneficial for sludge reduction. The lowest Yobs of 0.09(±0.05) mg VSS/mg COD (76% lower than that in the IFAS system) was obtained at an HRTSHT of 6 h and when TSHT was set at 35 °C. OSA implementation did not affect COD and NH4+ oxidation of the IFAS system (90-96% and 99%, respectively) and improved total nitrogen (TN) reduction (31-53%) due to improved denitrification in the SHT. On the contrary, sludge dewaterability worsened following OSA implementation, which was linked to the increased levels of exopolymeric substances in the suspended biomass.