In wastewater treatment plants (WWTPs) using the activated sludge process, two methods are widely used to improve aeration efficiency — use of high-efficiency aeration devices and optimizing the aeration control strategy. Aeration efficiency is closely linked to sludge characteristics (such as concentrations of mixed liquor suspended solids (MLSS) and microbial communities) and operating conditions (such as air flow rate and operational dissolved oxygen (DO) concentrations). Moreover, operational DO is closely linked to effluent quality. This study, which is in reference to WWTP discharge class A Chinese standard effluent criteria, determined the growth kinetics parameters of nitrifiers at different DO levels in small-scale tests. Results showed that the activated sludge system could meet effluent criteria when DO was as low as 0.3 mg/L, and that nitrifier communities cultivated under low DO conditions had higher oxygen affinity than those cultivated under high DO conditions, as indicated by the oxygen half-saturation constant and nitrification ability. Based on nitrifier growth kinetics and on the oxygen mass transfer dynamic model (determined using different air flow rate (Q′air) and mixed liquor volatile suspended solids (MLVSS) values), theoretical analysis indicated limited potential for energy saving by improving aeration diffuser performance when the activated sludge system had low oxygen consumption; however, operating at low DO and low MLVSS could significantly reduce energy consumption. Finally, a control strategy coupling sludge retention time and MLVSS to minimize the DO level was discussed, which is critical to appropriate setting of the oxygen point and to the operation of low DO treatment technology.Download high-res image (265KB)Download full-size image

•Oxygen diffusion feature was explored in microenvironment of floc by microelectrodes.•Activated sludge properties in combination with nutrient removal efficiency were investigated under different SRTs at low DO conditions.•Reactors with long SRTs were less sensitive to low DO concentrations.•EPS increased the oxygen mass transport resistance in the floc.•Settling performance of activated sludge improved with longer SRTs.In activated sludge systems, the aeration process consumes the most energy. The energy cost can be dramatically reduced by decreasing the operating dissolved oxygen (DO) concentration. However, low DO may lead to incomplete nitrification and poor settling performance of activated sludge flocs (ASFs). This study investigates oxygen transfer dynamics and settling performances of activated sludge under different sludge retention times (SRTs) and DO conditions using microelectrodes and microscopic techniques. Our experimental results showed that with longer SRTs, treatment capacity and settling performances of activated sludge improved due to smaller floc size and less extracellular polymeric substances (EPS). Long-term low DO conditions produced larger flocs and more EPS per unit sludge, which produced a more extensive anoxic area and led to low oxygen diffusion performance in flocs. Long SRTs mitigated the adverse effects of low DO. According to the microelectrode analysis and fractal dimension determination, smaller floc size and less EPS in the long SRT system led to high oxygen diffusion property and more compact floc structure that caused a drop in the sludge volume index (SVI). In summary, our results suggested that long SRTs of activated sludge can improve the operating performance under low DO conditions.

Hollow nanomaterials have attracted significant attention because of their high chemical and thermal stability, high specific surface area, high porosity, low density, and good biocompatibility. These state-of-the-art nanomaterials have been shown to efficiently adsorb heavy metals, and volatile hazardous substances, photodegrade persistent organic pollutants, and other compounds, and inactivate bacteria. Such properties have enabled the use of these materials for environmental remediation, such as in water/wastewater treatment, soil remediation, air purification, and substance monitoring, etc. Hollow nanomaterials showed higher photocatalytic activity than those without hollow structure owing to their high active surface area, reduced diffusion resistance, and improved accessibility. And, the Doping method could improve the photocatalytic performance of hollow nanomaterials further under visible light. Moreover, the synthetic mechanisms and methods of these materials are important because their size and morphology help to determine their precise properties. This article reviews the environmental applications and potential risks of these materials, in addition to their syntheses. Finally, an outlook into the development of these materials is provided.

•Innovative optofluidics-based DNA structure-competitive aptasensor was developed for rapid detection of Pb2+ in real samples.•This novel portable approach for Pb2+ detection is high sensitivity, adequate selectivity, reusability, low reagent volumes, and rapidity.•This aptasensor demonstrates sufficient specificity, good recovery, precision, and accuracy in actual water samples.•This DNA structure-competitive mechanism provides new opportunities for the analysis of trace analytes.Lead ions (Pb2+), ubiquitous and one of the most toxic metallic pollutants, have attracted increasing attentions because of their various neurotoxic effects. Pb2+ has been proven to induce a conformational change in G-quadruplex (G4) aptamers to form a stabilizing G4/Pb2+ complex. Based on this principle, an innovative optofluidics-based DNA structure-competitive aptasensor was developed for Pb2+ detection in an actual aquatic environment. The proposed sensing system has good characteristics, such as high sensitivity and selectivity, reusability, easy operation, rapidity, robustness, portability, use of a small sample volume, and cost effectiveness. A fluorescence-labeled G4 aptamer was utilized as a molecular probe. A DNA probe, a complementary strand of G4 aptamer, was immobilized onto the sensor surface. When the mixture of Pb2+ solution and G4 aptamer was introduced into the optofluidic cell, Pb2+ and the DNA probe bound competitively with the G4 aptamer. A high Pb2+ concentration reduced the binding of the aptamer and the DNA probe; thus, a low-fluorescence signal was detected. A sensitive sensing response to Pb2+ in the range of 1.0–300.0 nM with a low detection limit of 0.22 nM was exhibited under optimal conditions. The potential interference of the environmental sample matrix was assessed with spiked samples, and the recovery of Pb2+ ranged from 80 to 105% with a relative standard deviation value of <8.5%. These observations clearly illustrate that with the use of different DNA or aptamer probes, the sensing strategy presented can be easily extended to the rapid on-site monitoring of other trace analytes.

The effects of anionic, cationic, and non-ionic surfactants (SDS, SDBS, CTAB and Tween20) on oxygen mass transfer (OMT) in fine bubble aeration systems were investigated. The overall gas-liquid volumetric mass transfer coefficient (KLa), specific interfacial area (a), and liquid-side mass transfer coefficient (KL) parameters were used to assess the influence of the surfactants. At the same concentration, the different surfactants were observed to influence the KLa value as follows: KLa (SDBS)>KLa (SDS)>KLa (tween20)>KLa (CTAB). For all used surfactants, the overall trends showed a significant decrease in the KLa value at low concentrations (0–5mg/L), while the KLa value recovered somewhat at high concentrations (10–20mg/L). The decrease to the KL value was found to be much larger than increase in the a value in the presence of surfactants. Furthermore, a simple model was established that provides an OMT prediction for different surfactants.

•Carotenoid and bacteriochlorophyll in PSB were enhanced by high salinity.•The highest productions of two pigments were observed with 50 g L−1 NaCl.•Variation of pigments and dehydrogenase activity might be related to resistance.•Organic pollutants in high salinity wastewater were also removed.The variation of carotenoid and bacteriochlorophyll in photosynthetic bacteria (PSB) by high salinity stress (20–100 g L−1) was investigated. Results showed that PSB could grow in high salinity wastewater and the productions of two pigments were affected by the salinity significantly. Suitable salinity stress could enhance their productions while excessive salinity would impede the productions. When the NaCl concentration was 50 g L−1, the PSB were stimulated continuously during the whole culture period, and the productions of carotenoid and bacteriochlorophyll were 1.17 and 1.45-fold of the control group. The variation of dehydrogenase activity indicated that high salinity changed the metabolic activity greatly, and the highest level of dehydrogenase activity appeared at 50 g L−1 NaCl condition. Mechanisms analysis showed that the variations of pigments and dehydrogenase were related to self-protection. At the same time, pollutants in high salinity wastewater were removed by PSB. The COD and NH3-N removals under 20–100 g L−1 NaCl concentration were around 33.5–66.1%. The biomass yields were over 0.2 mg-biomass (mg-COD-reduction)−1 under all situations and salinity benefitted the biomass yield.Download high-res image (176KB)Download full-size image