Emission characteristics of CO, NOx, SO2 and indications of biomass burning observed at a rural site in eastern China

Atmospheric O3, CO, SO2, and NO* y (NO* y ≈ NO + NO2 + PAN + organic nitrates + HNO3 + N2O5 + ⋯) were measured in 1999-2000 at a rural/agricultural site in the Yangtze Delta of China. In this paper we analyze the measurement results to show the emission characteristics of the measured gases and to infer relevant emission ratios. Positive correlations were found between CO and NO* y with a slope (Δ[CO]/Δ[NO* y]) of 36 (ppbv/ppbv) for the winter and nighttime measurements. The ratio is considerably larger than that (≈10 ppbv/ppbv) observed in the industrialized countries. The highest CO/NO* y ratio (30-40 ppbv/ppbv) occurred in September-December 1999 and June 2000. The good correlation between CO and the biomass burning tracer CH3Cl and the lack of correlation with the industrial tracer C2Cl4 suggests that the burning of biofuels and crop residues is a major source for the elevated CO and possibly for other trace gases as well. The average SO2 to NO* y ratio was 1.37 ppbv/ppbv, resulting from the use of relatively high-sulfur coals in China. The measured SO2/NO* y and ΔCO/ΔNO* y were compared with the respective ratios from the current emission inventories for the study region, which indicated a comparable SO2/NOx emission ratio but a large discrepancy for CO/NOx. The observed CO to NO* y ratio was more than 3 times the emission ratio derived from the inventories, indicating the need for further improvement of emission estimates for the rural/agricultural regions in China. Additional research will be needed to study the implications of rural emissions to atmospheric chemistry and climate on both regional and global scales.Department of Civil and Environmental Engineerin

Membrane fouling in a submerged membrane bioreactor (SMBR): Characterisation of the sludge cake and its high filtration resistance

The attachment of sludge cake to the membrane surface is the main cause of the fouling problem in the submerged membrane bioreactors (SMBR) used in biological wastewater treatment. In this laboratory study, the sludge cake deposited on the membrane was found to have a specific filtration resistance of the order of 1014 m/kg, which is much greater than expected for sludge cake formed during the dewatering of activated sludge. The filterability tests showed that the cake sludge removed from the fouled membrane of the SMBR had an average specific filtration resistance of 4.9 × 1013 m/kg, whereas the sludge cake of the SMBR bulk sludge had an average filtration resistance of only 1.9 × 1011 m/kg. Detailed chemical analysis showed there was a pool of biopolymer clusters (BPC) that was trapped within the sludge cake on the membrane. These clusters could be readily separated from the cake sludge by stirring it into a suspension. The abundance of non-filterable BPC as measured by the total organic carbon (TOC) in the suspended solids (SS) was about 10.3 mg/g SS for the cake sludge, in comparison to 0.4 mg/g SS for the bulk sludge. When the BPC were removed from the cake sludge, the filtration resistance of the cake sludge could be reduced considerably from 4.9 × 1013 to 8.4 × 1012 m/kg. It is argued that the BPC are a special form of organic matter formed by affinity clustering of the free extracellular polymeric substances (EPS) and soluble microbial products (SMP) in the sludge cake deposited on the membrane surface. The accumulation of BPC within the pores of the sludge cake is mostly responsible for the unusually high filtration resistance of the cake sludge during the SMBR operation. © 2006 Elsevier B.V. All rights reserved.postprin

Visualisation and characterisation of biopolymer clusters in a submerged membrane bioreactor

A laboratory wastewater treatment membrane bioreactor (MBR) with a submerged hollow-fibre membrane was used to investigate the major foulants in sludge mixtures. Confocal laser scanning microscopy (CLSM) with a triple fluorescent staining protocol, i.e., SYTO9 for microbial cells, ConA-TRITC lectin for polysaccharides and NanoOrange for proteins, was utilised to visualise the fouling materials. A pool of biopolymer clusters (BPCs) ranging from 2.5 to 60 μm in size was identified in the liquid phase of the MBR sludge and in the cake sludge on the membrane surface. According to the CLSM examination, BPC are free and independent organic solutes that are different from biomass flocs and extracellular polymeric substances (EPS) and much larger than soluble microbial products (SMP). Compared to EPS, BPC contain more polysaccharides and proteins and less humic substances. It is believed that BPC are an important foulant that interacts with biomass flocs to form the sludge fouling layer on the membrane. A filtration test observed with the CLSM shows that BPC are apparently formed by the adsorption and affinity clustering of SMP within the sludge deposited on the membrane surface. The cake sludge on the fouled membrane has a much higher BPC content (16.8 mg TOC/g SS) than the MBR bulk sludge (0.4 mg TOC/g SS). It is argued that BPC behave as a glue to facilitate the growth of an impermeable sludge cake on the membrane surface, thus resulting in serious MBR fouling. These CLSM findings provide the first direct evidence of the presence of BPC in MBR and illustrate their essential role in membrane fouling. © 2008 Elsevier B.V. All rights reserved.postprin

An innovative membrane bioreactor (MBR) system for simultaneous nitrogen and phosphorus removal

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Change in the fouling propensity of sludge in membrane bioreactors (MBR) in relation to the accumulation of biopolymer clusters

A membrane bioreactor (MBR) and an activated sludge process (ASP) were operated side by side to evaluate the change of sludge supernatant characteristics and the evolution of the sludge fouling propensity. The MBR sludge had a higher organic concentration and more biopolymer clusters (BPC) in the supernatant compared with ASP. BPC increased in both concentration and size in the MBR. The results show that the change in the liquid-phase property had a profound effect on the sludge fouling propensity. MBR operation transformed typical activated sludge to MBR sludge with a higher fouling propensity. Distinct from the ASP, membrane filtration retained soluble microbial products (SMP) within the MBR, and the vast membrane surface provided a unique environment for the transformation of SMP to large size BPC, leading to further sludge deposition on the membrane surface. Thus, membrane filtration is the crucial cause of the inevitable fouling problem in submerged MBRs. © 2011 Elsevier Ltd.postprin

Investigation of the role of biopolymer clusters in MBR membrane fouling using flash freezing and environmental scanning electron microscopy

The technique that employs flash freezing and environmental scanning electron microscopy (ESEM) was utilised for detailed investigation of the fouling materials in a membrane bioreactor (MBR). The method involves the flash freezing of a wet sample in liquid nitrogen for 10. s to preserve its structure for direct ESEM observation with a high image resolution. ESEM images show that the sludge cake formed by simple filtration of the MBR bulk sludge has a highly porous, sponge-like structure with a fairly low resistance. However, the fouling layer attached to the membrane surface contains a thin gel layer under the main body of the sponge-like sludge cake, which is similar to that formed by filtration of a dispersion of biopolymer clusters (BPCs). It is apparent that BPCs tend to accumulate on the membrane surface, and the gel layer is largely responsible for the high filtration resistance of the cake layer on the fouled membranes. © 2011 Elsevier Ltd.postprin

Effect of biopolymer clusters on the fouling property of sludge from a membrane bioreactor (MBR) and its control by ozonation

Organic substances in the liquid phase of the sludge in a membrane bioreactor (MBR) have a profound impact on membrane fouling. In this study, a single-fibre microfiltration apparatus was developed to investigate the fouling propensity of MBR sludge and the effectiveness of ozonation in membrane fouling mitigation. The results show that biopolymer clusters (BPC) in the MBR suspension had a significant influence on the fouling potential of the sludge. An increase in BPC concentration by 20% and 60% from around 3.5 mg/l in the mixed sludge liquor drastically increased the fouling rate by 120% and 300%, respectively. Ozonation of the BPC solution greatly reduced the detrimental role of BPC in membrane fouling. An ozone dose of 0.03 mg/mg TOC of BPC could reduce the mean BPC size from 38 to 27 μm, which was further reduced to 12 μm at 0.3 mg O3/mg TOC of BPC. In addition to BPC destruction, ozonation apparently also modified the surface properties of BPC, resulting in an increase in the filterable fraction and a decrease in the liquid viscosity. Based on the experimental findings, an approach for MBR membrane fouling control is proposed that applies ozonation to the supernatant containing BPC in a side-stream application. © 2010 Elsevier Ltd. All rights reserved.postprin

Source contributions to ambient VOCs and CO at a rural site in eastern China

Ambient data on volatile organic compounds (VOCs) and carbon monoxide (CO) obtained at a rural site in eastern China are analyzed to investigate the nature of emission sources and their relative contributions to ambient concentrations. A principal component analysis (PCA) showed that vehicle emissions and biofuel burning, biomass burning and industrial emissions were the major sources of VOCs and CO at the rural site. The source apportionments were then evaluated using an absolute principal component scores (APCS) technique combined with multiple linear regressions. The results indicated that 71%±5% (average±standard error) of the total VOC emissions were attributed to a combination of vehicle emissions and biofuel burning, and 7%±3% to gasoline evaporation and solvent emissions. Both biomass burning and industrial emissions contributed to 11%±1% and 11%±0.03% of the total VOC emissions, respectively. In addition, vehicle emissions and biomass and biofuel burning accounted for 96%±6% of the total CO emissions at the rural site, of which the biomass burning was responsible for 18%±3%. The results based on PCA/APCS are generally consistent with those from the emission inventory, although a larger relative contribution to CO from biomass burning is indicated from our analysis. © 2004 Elsevier Ltd. All rights reserved

Assessing photochemical ozone formation in the Pearl River Delta with a photochemical trajectory model

A photochemical trajectory model (PTM), coupled with the Master Chemical Mechanism (MCM) describing the degradation of 139 volatile organic compounds (VOCs) in the troposphere, was developed and used for the first time to simulate the formation of photochemical pollutants at Wangqingsha (WQS), Guangzhou during photochemical pollution episodes between 12 and 17 November, 2007. The simulated diurnal variations and mixing ratios of ozone were in good agreement with observed data (R2=0.80, P<0.05), indicating that the photochemical trajectory model - an integration of boundary layer trajectories, precursor emissions and chemical processing - provides a reasonable description of ozone formation in the Pearl River Delta (PRD) region. Calculated photochemical ozone creation potential (POCP) indices for the region indicated that alkanes and oxygenated organic compounds had relatively low reactivity, while alkenes and aromatics presented high reactivity, as seen in other airsheds in Europe. Analysis of the emission inventory found that the sum of 60 of the 139 VOC species accounted for 92% of the total POCP-weighted emission. The 60 VOC species include C2-C6 alkenes, C6-C8 aromatics, biogenic VOCs, and so on. The results indicated that regional scale ozone formation in the PRD region can be mainly attributed to a relatively small number of VOC species, namely isoprene, ethene, m-xylene, and toluene, etc. A further investigation of the relative contribution of the main emission source categories to ozone formation suggested that mobile sources were the largest contributor to regional O3 formation (40%), followed by biogenic sources (29%), VOC product-related sources (23%), industry (6%), biomass burning (1%), and power plants (1%). The findings obtained in this study would advance our knowledge of air quality in the PRD region, and provide useful information to local government on effective control of photochemical smog in the region. © 2010 Elsevier Ltd