N2O accumulation from denitrification under different temperatures.

The effects of temperature on nitrous oxide (N2O) accumulation during denitrification and denitritation were investigated. Batch experiments were performed to measure N2O accumulation at 25 and 35 °C. More N2O accumulation was observed during denitritation at the higher temperature as compared with full denitrification and low temperature tests. The highest nitrite concentration tested in this study (25 mg/L NO2 (-)N and pH 8.0) did not show inhibitory effect on N2O reduction. It was found that the major cause of more N2O accumulation during denitrification at higher temperature was due to higher N2O production rate and lower N2O solubility. Specific nitrate, nitrite, and N2O reduction rates increased 62, 61, and 41 %, respectively, when temperature rose from 25 to 35 °C. The decrease of N2O solubility in mixed liquor at 35 °C (when compared to 25 °C) resulted in faster diffusing rate of N2O from liquid to gas phase. It was also more difficult for gas phase N2O to be re-dissolved. The diffused N2O was then accumulated in the headspace, which was not available for denitrification by denitrifiers. The results of this study suggest higher temperature may worsen N2O emission from wastewater treatment plants (WWTPs).

Effect of free nitrous acid on extracellular polymeric substances production and membrane fouling in a nitritation membrane bioreactor.

The membrane bioreactor (MBR) with nitritation based nitrogen removal processes has attracted growing interest in recent years, although membrane fouling in the nitritation MBR is a challenging issue. In this study, the inhibitory effect of free nitrous acid (FNA) on microbial extracellular polymeric substances (EPS) production and membrane fouling in a nitritation MBR was investigated. Results showed that EPS played a critical role in the biofouling process, and EPS production was affected by FNA concentration. As FNA concentration increased from 5.10 × 10-3 mg N/L to 1.34 × 10-2 mg N/L, protein (PN) and polysaccharide (PS) contents increased from 8.20 to 60.28 mg/g VSS and 4.74-30.46 mg/g VSS, respectively. However, when FNA concentration was 1.48 × 10-2 mg N/L, PN and PS reduced by 20.0% and 10.9%, respectively, indicating that the higher FNA concentration could reduce EPS production. The EPS reduction could be attributed to reduction in the loosely bound (LB) and tightly bound (TB) EPS but not the soluble microbial products (SMP). It was further revealed that higher FNA concentrations up to 1.48 × 10-2 mg N/L consequently mitigate trans-membrane pressure (TMP) rate in terms of dTMP/dt by 25.5% in the nitritation MBR. High throughput sequencing analysis revealed that the increase in FNA led to enrichment of Nitrosomonas but reduction in heterotrophic bacteria. This study showed that the appropriate FNA concentration affected EPS production and hence membrane fouling, leading to the possibility of membrane fouling mitigation by in-situ generated FNA in the nitritation MBR.

Statistical optimization of glyphosate adsorption by biochar and activated carbon with response surface methodology.

The introduction of glyphosate, found in herbicides, to waterbodies is of concern due to its toxicity and hence potential threat to public health and ecological systems. The present study has compared glyphosate removal from aqueous solution with activated carbon and biochar. Box-Behnken design, and percent contribution with Pareto analysis techniques were used in surface response and efficiency calculations modelled the process conditions and their effects. The adsorption data better fitted the Freundlich isotherm model than the Langmuir model. The rate of glyphosate adsorption was found to follow a pseudo-second-order model. pH of the solutions was regulated by buffering during the adsorption process. Higher efficacy of glyphosate removal was obtained by optimising parameters such as operating pH, initial glyphosate concentration, temperature, adsorbent dose, and contact time. The conditions yielding the best removals were pH 8.0, 0.2 mg/L, 50.0 °C, 11.4 g/L, 1.7 h for activated carbon and pH 5.0, 0.7 mg/L, 50.0 °C, 12.3 g/L, 1.9 h for biochar, for the aforementioned parameters respectively. The maximum removal capacity and efficiency were 0.0173 mg/g and 98.45% for activated carbon, and 0.0569 mg/g and 100.00% for biochar. The test results indicated biochar could be important from the perspective of performance and affordability.

Impact of free nitrous acid shock and dissolved oxygen limitation on nitritation maintenance and nitrous oxide emission in a membrane bioreactor.

This study investigated the initiation and maintenance of nitritation in a membrane bioreactor (MBR) with long solids retention time (SRT) of 43.8 days. Nitritation was initiated within 65 days in the MBR via dissolved oxygen (DO) limitation (<0.5 mg/L). However, nitrite oxidizing bacteria (NOB) (Nitrospira and Nitrobacter) acclimated to the low DO environment and proliferated from day 81, leading to nitrate accumulation. Thereafter, the combined strategy of DO limitation and in-situ generated free nitrous acid (FNA) shock successfully restored and maintained stable nitritation for >70 days. Quantitative polymerase chain reaction (qPCR) results showed that cell abundances of Nitrospira and Nitrobacter decreased by between 50.0 to 68.9% and 60.6 to 96.4%, respectively following the FNA shocks. The maximum ammonium loading rate achieved was 1.81 kg N/(m3 day) with ammonium removal ratio and nitrite accumulation ratio of over 0.97 and 0.96, respectively. Average emission rate of N2O from the MBR was 2.1 ±â€¯0.72% of ammonium removed. FNA shock on day 195 reduced the N2O emission by 13.6%. The strategy developed in this study verified that spiked FNA shock together with DO limitation can be used for maintaining nitritation in MBRs with long SRTs. This method can potentially allow for maintaining nitritation at relatively low capital and operating expenditure when treating high concentration ammonium wastewater.

Understanding and optimization of the flocculation process in biological wastewater treatment processes: A review.

In the operation of biological wastewater treatment processes, fast sludge settling during liquid-solids disengagement is preferred as it affects effluent quality, treatment efficiency and plant operation economy. An important property of fast settling biological sludge is the ability to spontaneously form big and dense flocs (flocculation) that readily separates from water. Therefore, there had been much research to study the conditions that promote biological sludge flocculation. However, reported findings have often been inconsistent and this has possibly been due to the complex nature of the biological flocculation process. Thus, it has been challenging for wastewater treatment plant operators to extract practical information from the literature. The aim of this review is to summarize the current state of understanding of the factors that affect sludge flocculation so that evaluation of such information can be facilitated and strategize for intervention in the sludge flocculation and deflocculation process.

Soluble microbial products (SMPs) in a sequencing batch reactor with novel cake filtration system.

The formation, composition and characteristics of soluble microbial products (SMPs) were investigated in a novel system which coupled a sequencing batch reactor with a cake filtration system. Both suspended solids (SS) and turbidity were significantly removed, resulting in effluent SS of 0.12 mg L-1 and turbidity of 0.72 NTU after cake filtration. The average concentrations of proteins and carbohydrates decreased respectively from 4.0 ± 0.4 and 7.1 ± 0.6 mg/L in the sequencing batch reactor (SBR) mixed liquor, to 0.85 ± 0.21 and 1.39 ± 0.29 mg/L in the cake filtration effluent. Analysis of the molecular weight (MW) distribution of SMPs revealed a substantial reduction in the intensity of high-MW peaks (503 and 22.71 kDa) after cake filtration, which implied the sludge cake layer and the underlying gel layer may play a role in the effectiveness of cake filtration beyond the physical phenomenon. Three-dimensional excitation emission matrix fluorescence spectroscopy indicated that polycarboxylate- and polyaromatic humic acids were the dominant compounds and a noticeable decrease in the fraction of these compounds was observed in the cake filtration effluent. Analysis with GC-MS set for detecting low-MW SMPs identified aromatics, alcohols, alkanes and esters as the dominant compounds. SMPs exhibited both biodegradable and recalcitrant characteristics. More SMPs (total number of 91) were accumulated during the SBR start-up stage. A noticeable increase in the aromatic fractions was seen in the SBR effluent accoutring for 39% of total compounds, compared to the SBR mixed liquor (28%). Fewer compounds (total number of 66) were identified in cake filtration effluent compared to the SBR effluent (total number of 75).