Frontiers of Environmental Science & Engineering

ISSN 2095-2201

ISSN 2095-221X(Online)

CN 10-1013/X

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EDITORIAL
Urban watershed management under rapid urbanization
Pengfei DU, Haifeng JIA, Shaw L. YU
Front Envir Sci Eng. 2012, 6 (5): 595-595.  
https://doi.org/10.1007/s11783-012-0452-5

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RESEARCH ARTICLE
Water---- and nutrient and energy---- systems in urbanizing watersheds
Rodrigo VILLARROEL WALKER, Michael Bruce BECK, Jim W. HALL
Front Envir Sci Eng. 2012, 6 (5): 596-611.  
https://doi.org/10.1007/s11783-012-0445-4

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Driven by considerations of sustainability, it has become increasingly difficult over the past 15–20 years — at least intellectually — to separate out the water infrastructure and water metabolism of cities from their intimately inter-related nutrient and energy metabolisms. Much of the focus of this difficulty settles on the wastewater component of the city’s water infrastructure and its associated fluxes of nutrients (N, P, C, and so on). Indeed, notwithstanding the massive volumes of these materials flowing into and out of the city, the notion of an urban nutrient infrastructure is conspicuous by its absence. Likewise, we do not tend to discuss, or conduct research into, “soilshed” agencies, or soilshed management, or Integrated Nutrient Resources Management (as opposed to its most familiar companion, Integrated Water Resources Management, or IWRM). The paper summarizes some of the benefits (and challenges) deriving from adopting this broader, multi-sectoral “systems” perspective on addressing water-nutrient-energy systems in city-watershed settings. Such a perspective resonates with the growing interest in broader policy circles in what is called the “water-food-energy security nexus”. The benefits and challenges of our Multi-sectoral Systems Analysis (MSA) are illustrated through computational results from two primary case studies: Atlanta, Georgia, USA; and London, UK. Since our work is part of the International Network on Cities as Forces for Good in the Environment (CFG; see www.cfgnet.org), in which other case studies are currently being initiated — for example, on Kathmandu, Nepal — we close by reflecting upon these issues of water-nutrient-energy systems in three urban settings with quite different styles and speeds of development.

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Flow and mass balance analysis of eco-bio infiltration system
Marla C. MANIQUIZ, Lee-Hyung KIM, Soyoung LEE, Jiyeon CHOI
Front Envir Sci Eng. 2012, 6 (5): 612-619.  
https://doi.org/10.1007/s11783-012-0448-1

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A structured stormwater infiltration system was developed and constructed at a university campus and monitoring of storm events was performed during a one-year operation period. The flow and pollutant mass balances were analyzed and the overall efficiency of the system was assessed. While significant positive correlations were observed among rainfall, runoff and discharge volume (R2= 0.93-0.99; p<0.05), there was no significant correlations existed between rainfall, runoff, discharge volume and pollutant load. The system was more effective in reducing the runoff volume by more than 50% for small storm events but the difference between the runoff and discharge volume was significant even with rainfall greater than 10 mm. Results showed that the pollutant reduction rates were higher compared to the runoff volume reduction. Average pollutant reduction rates were in the range of 72% to 90% with coefficient of variation between 0.10 and 0.46. Comparable with runoff reduction, the system was more effective in reducing the pollutant load for small storm events, in the range of 80% to 100% for rainfall between 0 and 10 mm; while 65% to 80% for rainfall between 10 and 20 mm. Among the pollutant parameters, particulate matters was highly reduced by the system achieving only a maximum of 25% discharge load even after the entire runoff was completely discharged. The findings have proven the capability of the system as a tool in stormwater management achieving both flow reduction and water quality improvement.

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State versus private sector provision of water services in Armenia
Naira HARUTYUNYAN
Front Envir Sci Eng. 2012, 6 (5): 620-630.  
https://doi.org/10.1007/s11783-012-0425-8

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Despite increasing advocacy and adaptation of public-private model of water governance worldwide since the 1990s, today only 5% of the world’s population is served by water utilities with private involvement. The present article examines the experience of the water sector in Armenia with private sector participation. The study describes the process of the introduction of public-private partnerships in the water sector and focuses on analyzing the impact of privatization on water utility performance. The analysis employs the partial indicator method for evaluating the impacts in relation to operational, finance, and environmental performance, done by drawing on the database for the five water companies in Armenia. The empirical evidence shows that private participation in general led to improved overall performance. In particular, private involvement resulted in increased operational efficiency in terms of labor productivity, water metering, continuity of service, and revenue collection efficiency. There were mixed improvements in the operating cost coverage ratio. As for environmental performance, there were gains in the reduction of residential water consumption, accompanied, however, by an increase in non-revenue water.

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Treatability aspects of urban stormwater stressors
Anthony N. TAFURI, Richard FIELD
Front Envir Sci Eng. 2012, 6 (5): 631-637.  
https://doi.org/10.1007/s11783-012-0420-0

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Pollution from diffuse sources (pollution from contaminants picked up and carried into surface waters by stormwater runoff) has been identified as a significant source of water quality problems in the U.S. scientists and engineers continue to seek solutions that will allow them to optimize existing technologies and develop new ones that will provide the best possible protection to people, wildlife, and the environment. This paper addresses the various pollutants or stressors in urban stormwater, including flow (shear force), pathogens, suspended solids/sediment, toxicants (organic and metals), nutrients, oxygen demanding substances, and coarse solids. A broad overview of the pollutants removed and the removal mechanisms by and of conventional best management practices (BMPs) is also presented. The principal treatment mechanisms of conventional wet ponds, vegetative swales/buffer strips, and wetlands are sedimentation and filtration. These mechanisms have the capability to remove significant amounts of suspended solids or particulate matter and are a vital component of strategies to reduce pollutant loads to receiving waters. In addition, because most of the nation’s receiving water violations are caused by pathogen indicator bacteria, it is of utmost importance that research efforts address this problem. Further research is also needed on the treatment of emerging contaminants in BMPs and on the costs and affects of maintenance and maintenance schedules on the long-term performance of BMPs.

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Stormwater treatment: examples of computational fluid dynamics modeling
Gaoxiang YING, John SANSALONE, Srikanth PATHAPATI, Giuseppina GAROFALO, Marco MAGLIONICO, Andrea BOLOGNESI, Alessandro ARTINA
Front Envir Sci Eng. 2012, 6 (5): 638-648.  
https://doi.org/10.1007/s11783-012-0442-7

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Control of rainfall-runoff particulate matter (PM) and PM-bound chemical loads is challenging; in part due to the wide gradation of PM complex geometries of many unit operations and variable flow rates. Such challenges and the expense associated with resolving such challenges have led to the relatively common examination of a spectrum of unit operations and processes. This study applies the principles of computational fluid dynamics (CFD) to predict the particle and pollutant clarification behavior of these systems subject to dilute multiphase flows, typical of rainfall-runoff, within computationally reasonable limits, to a scientifically acceptable degree of accuracy. The Navier-Stokes (NS) system of nonlinear partial differential equations for multi-phase hydrodynamics and separation of entrained particles are solved numerically over the unit operation control volume with the boundary and initial conditions defined and then solved numerically until the desired convergence criteria are met. Flow rates examined are scaled based on sizing of common unit operations such as hydrodynamic separators (HS), wet basins, or filters, and are examined from 1 to 100 percent of the system maximum hydraulic operating flow rate. A standard turbulence model is used to resolve flow, and a discrete phase model (DPM) is utilized to examine the particle clarification response. CFD results closely follow physical model results across the entire range of flow rates. Post-processing the CFD predictions provides an in-depth insight into the mechanistic behavior of unit operations by means of three dimensional (3-D) hydraulic profiles and particle trajectories. Results demonstrate the role of scour in the rapid degradation of unit operations that are not maintained. Comparisons are provided between measured and CFD modeled results and a mass balance error is identified. CFD is arguably the most powerful tool available for our profession since continuous simulation modeling.

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Characteristics of pollutants behavior in a stormwater constructed wetland during dry days
Jianghua YU, Kisoo PARK, Youngchul KIM
Front Envir Sci Eng. 2012, 6 (5): 649-657.  
https://doi.org/10.1007/s11783-012-0426-7

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A stormwater wetland treating non-point source pollution (NPS) from a 64 ha agricultural watershed was monitored over a period of five months. The results indicated that pH and dissolved oxygen (DO) were increased in the wetland due to the algal growth. The highest total suspended solids (TSS) concentration was observed in the aeration pond due to the resuspension of solids, decreased in the wetland. The respective decreases in total nitrogen (TN) and total kjeldahl nitrogen (TKN) were 15.9% and 28.7% on passing through the wetland. The nitrate and ammonia were increased by 45.4% and decreased by 79.9%, respectively. These variations provided strong evidence for the existence of nitrification. The total phosphorus (TP) and phosphate had respective reductions of 52.3% and 58.2% over the wetland. The total chemical oxygen demand (TCOD) and soluble chemical oxygen demand (SCOD) were also decreased. Generally, the TN, TP and phosphate removal efficiencies were positive. These positive removal efficiencies were mainly due to microbial activities, uptake by plants, and chemical precipitation at high pH. Negative removal efficiencies can be caused by continuous rainfall activities, with short antecedent dry days (ADDs) and unstable hydraulic conditions, some other biogeochemical transformations and algal growth also being important parameters.

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An approach to calculating allowable watershed pollutant loads
Yu GUO, Haifeng JIA
Front Envir Sci Eng. 2012, 6 (5): 658-671.  
https://doi.org/10.1007/s11783-012-0446-3

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To improve the management of discharge pollutants loads in the reservoirs’ watershed, an approach of the allowable pollutants loads calculation and its allocation, based on the water environment model, was proposed. Establishment of the approach framework was described at first. Under the guidance of this framework, two major steps were as follows: modeling and scenario analysis were involved and should be applied to support the decision of discharge loads management; Environmental Fluid Dynamic Code (EFDC) model was selected as the kernel model in this framework. In modeling step, spatial discretization for establishing cell map in model, data preprocessing, parameter calibration and uncertainty analysis (which is considered as the significantly relevant factor of the margin of safety (MOS)), were conducted. As a result of the research, the model-based approach presented as a combination of estimation and precise calculation, which contributed to scenario analysis step. Some integrated modules, such as scenario simulation, result analysis and plan optimization were implemented as cycles in the scenario analysis. Finally, allowable pollutant loads under various conditions were calculated. The Chaihe Reservoir in Liaoning Province, China was used as a case study for an application of the approach described above. Results of the Chaihe reservoir water quality simulation, show good agreement with field data and demonstrated that the approach used in the present study provide an efficient and appropriate methodology for pollutant load allocation.

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Stormwater runoff pollution loads from an urban catchment with rainy climate in China
Liqing LI, Baoqing SHAN, Chenqing YIN
Front Envir Sci Eng. 2012, 6 (5): 672-677.  
https://doi.org/10.1007/s11783-012-0447-2

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An older urban district in Wuhan, China, is transitioning from discharging sewage and stormwater directly into lakes, to directing the sewage to wastewater treatment plants (WWTPs). Dealing with polluted stormwater discharge is a great challenge. Stormwater runoff from an urban catchment with a combined sewer system was sampled and analyzed over a three-year period. Results indicate that wet weather flows account for 66%, 31%, 17%, and 13% of the total load of suspended solids (SS), chemical oxygen demand (COD), total nitrogen, and total phosphorus, respectively. The first flush of COD and SS was significant in all runoff events. More than 50% of the SS and COD loads were transported by the first 30% of runoff volume. Storage and treatment of the first 10 mm from each combined sewer overflow event could reduce more than 70% of the annual COD overflow load. An integrated solution is recommended, consisting of a tank connected to the WWTP and a detention pond, to store and treat the combined sewer overflow (CSO). These results may be helpful in mitigating CSO pollution for many other urban areas in China and other developing countries.

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Water environment security indicator system for urban water management
Tian HAO, Pengfei DU, Yun GAO
Front Envir Sci Eng. 2012, 6 (5): 678-691.  
https://doi.org/10.1007/s11783-012-0450-7

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Water environment security (WES) is defined in terms of three aspects: first, that it meets basic demands due to industrial and domestic usage; second, that it protects public health from acute and chronic threats; and third, that measures are adopted to ensure sustainable ecological functioning of freshwater resources. Limitations associated with current water environment security indicators in China — including inefficiency in terms of environmental monitoring, inappropriate indicators and parameters that do not take sufficient account of local water characteristics, and a lack of management targets — leads to a failure of effective water management. To achieve better water environment management, a systematic approach, encompassing several steps including establish ideal indicator system and narrow down the scope, screen priority pollutants, identify local characteristics and organize discussion workshop, should be followed to establish a comprehensive water environment indicator system. A case study in Suzhou is included to demonstrate the detailed operational procedures used to assess the risks associated with poor management practices relating to water environment security as well as design an appropriate water environment security indicator system.

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Temporal variation of heavy metal pollution in urban stormwater runoff
Wen LI, Zhenyao SHEN, Tian TIAN, Ruimin LIU, Jiali QIU
Front Envir Sci Eng. 2012, 6 (5): 692-700.  
https://doi.org/10.1007/s11783-012-0444-5

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Stormwater runoff from three types of urban surfaces, a parking lot, a street, and a building roof, was monitored during four rainfall events that occurred in the one-year period from June 2009 to June 2010. The event mean concentrations (EMC) of dissolved copper (Cu), lead (Pb), zinc (Zn), manganese (Mn), and iron (Fe) exceeded China’s National Water Quality Standards for Surface Water. The degree of heavy metal contamination was related to the type of underlying surfaces. Additionally, the concentration of dissolved heavy metals peaked shortly after the runoff began and then declined sharply as a result of adequate flushing. First flush effects of varying degrees were also observed during all of the monitored rainfall events based on the first flush ratio (FF25). Redundancy analysis revealed that four environmental variables (rainfall depth, intensity, antecedent dry weather period and type of underlying surface) had significant effects on the strengths of the first flush effects, accounting for 72.9% of the variation in the FF25. Dissolved metals presented varying first flush effects on different underlying surfaces that occurred in the following relative order: parking lot>roof>road for low intensity and high runoff volume rainfall events; parking lot>road>roof for high intensity and low runoff volume events. The relative strength of the first flush for dissolved heavy metals was Fe, Mn>Cu, Zn>Pb.

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Biodegradation of 2-methylisoborneol by bacteria enriched from biological activated carbon
Rongfang YUAN, Beihai ZHOU, Chunhong SHI, Liying YU, Chunlei ZHANG, Junnong GU
Front Envir Sci Eng. 2012, 6 (5): 701-710.  
https://doi.org/10.1007/s11783-011-0367-6

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One of the most common taste and odour compounds (TOCs) in drinking water is 2-methylisoborneol (2-MIB) which cannot be readily removed by conventional water treatments. Four bacterial strains for degrading 2-MIB were isolated from the surface of a biological activated carbon filter, and were characterized as Micrococcus spp., Flavobacterium spp., Brevibacterium spp. and Pseudomonas spp. based on 16S rRNA analysis. The removal efficiencies of 2-MIB with initial concentrations of 515 ng·L-1 were 98.4%, 96.3%, 95.0%, and 92.8% for Micrococcus spp., Flavobacterium spp., Brevibacterium spp. and Pseudomonas spp., respectively. These removal efficiencies were slightly higher than those with initial concentration at 4.2 mg·L-1 (86.1%, 84.4%, 86.7% and 86.0%, respectively). The kinetic model showed that biodegradation of 2-MIB at an initial dose of 4.2 mg·L-1 was a pseudo-first-order reaction, with rate constants of 0.287, 0.277, 0.281, and 0.294 d-1, respectively. These degraders decomposed 2-MIB to form 2-methylenebornane and 2-methyl-2-bornane as the products.

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Characterization of humic substances in bio-treated municipal solid waste landfill leachate
Guangxia QI, Dongbei YUE, Yongfeng NIE
Front Envir Sci Eng. 2012, 6 (5): 711-716.  
https://doi.org/10.1007/s11783-012-0421-z

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Considerable organic matter remains in municipal solid waste landfill leachate after biological treatments. Humic substances (HSs) dominate the organic matter in bio-treated landfill leachate. In this study, the HSs from landfill leachate treated by membrane bioreactor (MBR-HSs) were analyzed via elemental analysis, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, and charge polarized magic-angle spinning-13C-nuclear magnetic resonance. The characteristic absorption in the UV wavelength range indicated the presence of high C=C and C=O double bonds within the MBR-HSs. Compared with commercial HSs, MBR-HSs had lower carbon content [48.14% for fulvic acids (FA) and 49.52% for humic acids (HA)], higher nitrogen content (4.31% for FA and 6.16% for HA), lower aromatic structure content, and higher carbohydrate and carboxylic atoms of carbon content. FA predominantly had an aliphatic structure, and HA had less condensed or substituted aromatic ring structures than natural HA. The aromatic carbon content of MBR-HSs was lower than that of humus-derived HSs but higher than that of waste-derived HSs, indicating that MBR-HSs appeared to be more similar to humus-derived HSs than waste-derived HA.

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Impact of gasoline engine deposits on light duty vehicle emissions: in-use case study in Beijing, China
Xin YUE, Ye WU, Xianjiang HUANG, Yao MA, Yuan PANG, Xiaofeng BAO, Jiming HAO
Front Envir Sci Eng. 2012, 6 (5): 717-724.  
https://doi.org/10.1007/s11783-012-0438-3

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Tailpipe emissions from light-duty gasoline vehicles usually deteriorate over time. The accumulation of engine deposits due to inadequate gasoline detergency is considered to be one of the major causes of such emission deterioration. Six in-use light-duty gasoline vehicles in Beijing were tested to investigate the impact of engine deposits on emissions of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx). Emissions under cold start and hot running test conditions from the six light duty vehicles were measured before and after engine deposits were removed. Results show that although individual vehicles reacted differently for each of the pollutants, elimination of engine deposits on average reduced HC emissions under hot running conditions by 29.4%, CO emissions under cold start conditions by 23.0% and CO emissions under hot running conditions by 35.5% (t<0.05 in all cases). No pollutant emissions increased with statistical significance (t<0.05) after the removal of engine deposits. Variations of emission changes upon removal of engine deposits were observed. Such variations are in line with previous studies, implying that the impact patterns of engine deposits on vehicle emissions may be subject to many influencing factors that are not fully understood and difficult to control under all conditions. A statistical view of the impact of engine deposits on vehicle emissions may be appropriate for evaluation of emissions reductions across a city or a country. It is necessary to maintain sufficient and effective gasoline fuel detergency in practice to keep the engines clean and in turn reduce vehicle emissions.

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Characterization of submicron aerosols in the urban outflow of the central Pearl River Delta region of China
Zhaoheng GONG, Zijuan LAN, Lian XUE, Liwu ZENG, Lingyan HE, Xiaofeng HUANG
Front Envir Sci Eng. 2012, 6 (5): 725-733.  
https://doi.org/10.1007/s11783-012-0441-8

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Submicron aerosol particles (with aerodynamic diameters less than 1 μm, PM1) were sampled and measured in Heshan, an urban outflow site of Guangzhou megacity in Pearl River Delta in South China, using an Aerodyne High-Resolution Time-of-Flight Aerosol Mass Spectrometer (HR-ToF-AMS) in November 2010 during 2010 Guangzhou Asian Games. The mean PM1 mass concentration measured was 47.9±17.0 μg·m-3 during the campaign, with organic aerosol (OA) and sulfate being the two dominant species, accounting for 36.3% and 20.9% of the total mass, respectively, followed by black carbon (17.1%, measured by an aethalometer), nitrate (12.9%), ammonium (9.6%) and chloride (3.1%). The average size distributions of the species (except black carbon) were dominated by an accumulation mode peaking at ~550 nm. Calculations based on high-resolution organic mass spectrum showed that, C, H, O and N on average contributed 58.1%, 7.3%, 30.7%, and 3.9% to the total organic mass, respectively. The average ratio of organic mass over organic carbon mass (OM/OC) was 1.73±0.08. Four components of OA were identified by the Positive Matrix Factorization (PMF) analysis, including a hydrocarbon-like (HOA), a biomass burning (BBOA) and two oxygenated (SV-OOA and LV-OOA) organic aerosol components, which on average accounted for 18.0%, 14.3%, 28.8% and 38.9% of the total organic mass, respectively.

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Forecasting industrial emissions: a monetary approach vs. a physical approach
Yang DONG, Yi LIU, Jining CHEN, Yebin DONG, Benliang QU
Front Envir Sci Eng. 2012, 6 (5): 734-742.  
https://doi.org/10.1007/s11783-012-0451-6

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Forecasts of industrial emissions provide a basis for impact assessment and development planning. To date, most studies have assumed that industrial emissions are simply coupled to production value at a given stage of technical progress. It has been argued that the monetary method tends to overestimate pollution loads because it is highly influenced by market prices and fails to address spatial development schemes. This article develops a land use-based environmental performance index (L-EPI) that treats the industrial land areas as a dependent variable for pollution emissions. The basic assumption of the method is that at a planning level, industrial land use change can represent the change in industrial structure and production yield. This physical metric provides a connection between the state-of-the-art and potential impacts of future development and thus avoids the intrinsic pitfalls of the industrial Gross Domestic Product-based approach. Both methods were applied to examine future industrial emissions at the planning area of Dalian Municipality, North-west China, under a development scheme provided by the urban master plan. The results suggested that the L-EPI method is highly reliable and applicable for the estimation and explanation of the spatial variation associated with industrial emissions.

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Ecosystem service tradeoff between traditional and modern agriculture: a case study in Congjiang County, Guizhou Province, China
Dan ZHANG, Qingwen MIN, Moucheng LIU, Shengkui CHENG
Front Envir Sci Eng. 2012, 6 (5): 743-752.  
https://doi.org/10.1007/s11783-011-0385-4

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Besides crops, agriculture supplies all three major categories of ecosystem services (ES). However, agriculture also supplies an array of ecosystem dis-services (EDS) that may harm other ecosystems. The flows of ES and EDS are directly dependent on the management of agricultural ecosystems. The traditional method of Chinese agriculture, which supports sustainable agriculture, has been proven to increase ES and reduce EDS. However, there is a lack of a detailed understanding of the ES and EDS associated with traditional agriculture, and also of differences between traditional and modern agriculture.

In this study, an investigation was conducted on the ecosystem services (ES) and ecosystem dis-services (EDS) of traditional and modern agriculture in Congjiang County, Guizhou Province, China. Afterwards, the economic values of ES and EDS were quantified experimentally and calculated based on the market price. The results show that: the net economic value of traditional rice-fish agriculture was 3.31×104 CNY·ha-1 (6.83 CNY= 1 USD as of July, 2009) and that of rice monoculture was 1.99×104 CNY·ha-1. Significant differences existed between traditional rice-fish and rice monoculture fields for their economic values of some ES or EDS.

A benefit and cost analysis (BCA) model was used to adjust the conflict between the economic income and environmental loss from traditional and modern agriculture. The BCA model not only calculates the net income but also monetizes the EDS of the agricultural systems. The results showed that the net income of rice-fish agriculture was 1.94×104 CNY·ha-1 higher than that of rice monoculture. However, the benefit to cost ratio (BCR) of rice-fish agriculture was lower than that of rice monoculture, indicating that the traditional agricultural model was not the most optimized choice for farmers. The value of the rice-fish agriculture was much higher than that of the rice monoculture. Thus, when considering the benefits that rice-fish agriculture contributes to the large-scale society, these agricultural methods needs to be utilized. Furthermore, the labor opportunity costs were calculated and the comprehensive value of rice monoculture was negative. Finally, the compensation standard was calculated based on the comprehensive benefit analysis. The lowest level was 1.09×103 CNY·ha-1, and the highest level was 1.21×104 CNY·ha-1.

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Enhancement of sludge gravitational thickening with weak ultrasound
Panyue ZHANG, Tian WAN, Guangming ZHANG
Front Envir Sci Eng. 2012, 6 (5): 753-760.  
https://doi.org/10.1007/s11783-011-0368-5

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Gravitational thickening is the prevailing method to reduce sludge volume but the process is slow and usually requires addition of polyelectrolyte(s). This paper investigated the potential benefits of sonication on enhancing the sludge gravitational thickening with very low energy dose, so called “weak ultrasound”. Results showed that weak sonication significantly changed the sludge settlability and the main mechanism was release of the loosely bounded extracellular polymeric substances. The changes in sludge behaviors by sonication were strongly influenced by power density and sonication duration. Lower sound frequency was slightly better than higher frequency. Weak sonication (<680 kJ·kg-1 DS) improved the sludge gravitational thickening while high ultrasonic energy deteriorated the process. Considering both the sludge thickening efficiency and energy consumption, the optimum conditions were 0.15 W·mL-1, 7 s, and 25 kHz. Under such conditions, the energy dose was only 155 kJ·kg-1 DS, much lower than literature reports, and the sludge settling time was shortened from 24 h to 12?h. Weak sonication could substitute expensive polyelectrolyte coagulant for sludge thickening. Combination of weak sonication and polyelectrolyte could further reduce the settling time to 6 h. The final water content of the thickened sludge was not changed after sonication or polyelectrolyte addition.

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18 articles