A MIXING MODEL FOR A SEPARATED MECHANICAL AERATION SYSTEM1

ABSTRACT: An input-output method, using a network of ideal continuous stirred-tank and plug-flow tublar reactors, is adopted to analyze residence time distribution data for a separated mechanical aeration system. The usefulness of this modeling concept is enhanced by its simplicity, especially in the presence of a first-order reaction. This facilitates use of the model format for wastewater quality prediction. Moreover, first-order rate constants can also be estimated from the model, if conversions due to the reaction rate are available.     

现代厌氧反应器的现状与发展

回顾了厌氧反应器发展的三个阶段，对现代反应器的结构与应用情况进行了详细的介绍，认为目前主要研究与应用的热点是以现代反应器为代表的工艺；以膜技术和分阶段多相多级技术将是未来污水处理技术的主导技术；以复合反应器代表的工艺在处理城市生活污水，处理垃圾渗滤液以及有毒物质废水等将具有更多的优势。在能源资源短缺、环境日益恶化的今天，处理能力强、适用范围广、耗能少等优点的厌氧反应器有着广泛的应用前景与研究价值。     

Integrated management of radioactive strontium contamination in aqueous stream systems

A combination of biomass treatment, fluidized bed/membrane reactor, and a minimum-suspension fluidized bed reactor is proposed to remove strontium cations from aqueous solutions, such as those generated by nuclear reactors. After conducting a series of screening tests, three adsorbents were selected for their suitability and high adsorptive capacity. The proposed combination uses Chlorella vulgaris in a packed column, followed by the fluidized bed/membrane reactor with bentonite powder in suspension. The membrane is primarily used to retain bentonite powder in the reactor. However, the same can be designed to remove additional amount of contaminant from the aqueous stream. The final separation is carried out in a fluidized bed containing resins that are suspended with minimal airflow. In laboratory scale, a flow rate of 600 ml/h was achieved for 30 min during which period the inlet concentration of 100 mg/l was reduced to 2.5 mg/l at the outlet. Bio-encapsulation with thermophilic bacteria and subsequent separation is proposed at this point in order to reduce the concentration to an even lower level. The proposed separation scheme offers an acceptable solution to removing strontium while minimizing the generation of secondary waste.     

Optimizing vermicomposting of animal wastes: effects of rate of manure application on carbon loss and microbial stabilization

Vermicomposting is the process whereby organic residues are broken down by earthworms and microorganisms. Addition of manure has been shown to be of critical importance and determines most of the changes that take place during vermicomposting. Here, we study how the rate of manure applied affects microbial biomass and activity and carbon losses. For this, we designed continuous feeding reactors in which new layers of manure were added sequentially to form an age gradient inside the reactors. We compared two application rates of pig slurry (1.5 and 3kg) and set up six reactors for each one; half of the 12 reactors initially contained a population of 500 earthworms (Eisenia fetida). We found that earthworms increased microbial biomass and were more active in reactors fed with 3kg of slurry. However, the differential rates of respiration were not reflected in C losses. The results thus showed that loss of C was not affected by the rate of pig slurry applied. We conclude that despite the strong effect that the rate of manure has on microbe-earthworm relationships, it did not affect carbon losses. We therefore recommend the use of low application rates of manure when the objective is the microbial stabilization of the residue.     

Dry reforming of methane over oil palm shell activated carbon and ZSM-5 supported cobalt catalysts

In this study, cobalt supported oil palm shell activated carbon (Co/OPS-AC) and ZSM-5 zeolite (Co/ZSM-5) catalysts have been prepared for dry reforming of methane. Cobalt ratios of 6.0 and 14.0 wt% were deposited via wet impregnation method to the OPS-AC and ZSM-5 catalysts. The catalysts were characterized by XRD, N₂ adsorption--desorption isotherms, BET surface area, SEM, FESEM-EDX, TPR-H₂, and TPD-NH₃. The dry reforming of methane was performed using a micro reactor system under the condition of 10,000 ml/h.g-cat, 3 atm, CH₄/CO₂ ratio of 1.2:1.0 and temperature range from 923 K to 1023 K. The gaseous products were analyzed by gas chromatography (GC) with thermal conductivity detector (TCD) and further quantified to determine the conversions of CH₄ and CO₂, and the yields of CO and H₂. Experimental results revealed both catalysts exhibited lower conversions of CO₂ and CH₄ with the increase in temperature from 923 K to 1023 K. The reduced conversions may be due to the formation of carboneous substance on the catalyst known as coking. Comparatively, Co/OPS-AC gave higher conversions of CO₂ and CH₄ as well as higher yields of H₂ and CO as it has a higher surface area than Co/ZSM-5 which subsequently rendered higher activity for the reforming of methane. With the increasing cobalt loadings and reaction temperature, OPS-AC(14) catalyst exhibited improved activity and H₂/CO ratio. Based on these results, cobalt supported OPS activated carbon catalyst was suggested to be more effective for CO₂ and CH₄ conversions.     

Modelling anaerobic biofilm reactors--a review

Anaerobic treatment has become a technically as well as economically feasible option for treatment of liquid effluents after the development of reactors such as the upflow anaerobic sludge blanket (UASB) reactor, expanded granular sludge bed (EGSB) reactor, anaerobic biofilter and anaerobic fluidized bed reactor (AFBR). Considerable effort has gone into developing mathematical models for these reactors in order to optimize their design, design the process control systems used in their operation and enhance their operational efficiency. This article presents a critical review of the different mathematical models available for these reactors. The unified anaerobic digestion model (ADM1) and its application to anaerobic biofilm reactors are also outlined.     

Determination of dissolved oxygen limitation in aerobic biofilm reactors

The concentration of dissolved oxygen (DO) strongly influences the performance of aerobic biofilm reactors because organic oxidation is limited by the availability of oxygen. However, it is not necessary to maintain a high DO level in the reactors in order to overcome this limitation. Excessive aeration wastes energy. Therefore, the determination of the onset of DO limitation against organic substrate removal in aerobic biofilm reactors is important for their effective operation. This study is aimed at developing an expression to determine the onset of DO limitation and hence to control the aeration system. The expression developed is as follows: , where S_b and C_b are the bulk concentrations of organic substrate and DO, respectively; D_ws and D_wc are the diffusion coefficients of organic substrate and oxygen in the reactors respectively; and R_b is an overall ratio of oxygen consumption to organic substrate removal in the reactors. The latter is the key parameter in the equation, and is determined by the characteristics of the substrate, biofilm, and reactor. In order to measure the value of R_b, the authors have developed a micro-biofilm reactor. The value of R_b was determined to be 0.13 (mg O₂ mg⁻¹ COD_cr) for glucose removal with this reactor. The equation has, subsequently, been verified with data from batch and continuous experiments.     

Clean production with membrane technology

 Membrane processes, because of unique, specialized, and cost-effective applications, have the potential of playing a significant role in preventing pollution from occurring in manufacturing plants. Opportunities are seen in resource recovery, species purification, and energy savings. Emerging technologies of membrane reactors that combine separation with reaction in one physical unit are important developments also. In this paper we discuss the scope of membrane technologies in industrial applications as well as hurdles that must be overcome. Received: 15 April 1998 / Accepted: 9 August 1998     

Anaerobic digestion of dairy manure with enhanced ammonia removal

Poor ammonia-nitrogen removal in methanogenic anaerobic reactors digesting animal manure has been reported as an important disadvantage of anaerobic digestion (AD) in several studies. Development of anaerobic processes that are capable of producing reduced ammonia-nitrogen levels in their effluent is one of the areas where further research must be pursued if AD technology is to be made more effective and economically advantageous. One approach to removing ammonia from anaerobically digested effluents is the forced precipitation of magnesium ammonium phosphate hexahydrate (MgNH4PO4 x 6H2O), commonly called struvite. Struvite is a valuable plant nutrient source for nitrogen and phosphorus since it releases them slowly and has non-burning features because of its low solubility in water. This study investigated coupling AD and controlled struvite precipitation in the same reactor to minimize the nitrogen removal costs and possibly increase the performance of the AD by reducing the ammonia concentration which has an adverse effect on anaerobic bacteria. The results indicated that up to 19% extra COD and almost 11% extra NH3 removals were achieved relative to a control by adding 1750 mg/L of MgCl2 x 6H2O to the anaerobic reactor.     

Process intensification in the textile industry: the role of membrane technology

Process intensification is a concept that was recently introduced in the chemical industry for the purpose of reducing environmental emissions, energy consumption and materials consumption. The principle of process intensification can be used in related industries as well; textile finishing is an exemplary activity where it may have a significant long-term added value. Membrane technology can be a key factor in the recycling and reuse of energy, water and chemicals. In this paper, an integral approach for treatment of aqueous process streams in the textile finishing industry is proposed. The proposed process includes microfiltration pretreatment of used finishing baths, followed by a dual nanofiltration (NF) unit. These can be operated at elevated temperatures so that no further energy is needed for preheating of recycle streams. In the proposed treatment scheme, the first of the NF units uses a loose nanofiltration membrane that retains most of the organic fraction but not the dissolved salts. The second unit uses a tight nanofiltration membrane, which produces a permeate fraction that can be directly reused, and a concentrated brine that is fed to a membrane crystallizer. In this unit, salts are recovered and recycled for use in new dye baths. The concentrate stream from the first NF unit is fed to a membrane distillation unit, where the high temperature is advantageously used for further concentration. The remaining fraction is not reusable, given the fact that most dyes are hydrolyzed after exhaustion of the bath, but has a significant energetic value, which can be utilized for compensation of energy losses and preheating of suppletion water, by using an incineration process with energy recovery. The concept was not tested experimentally, but a simulation for a 500 m3/d production unit shows that it is feasible, although modifications may be necessary depending on the nature of the finishing baths. Furthermore, the membrane choice in the first NF unit is a critical aspect.     

United States uranium resources the 1975–2000 outlook

The status of uranium resources for the USA for the period 1975–2000 can only be estimated within rather broad limits due to a complex interplay of the many factors involved. Minimum estimates for uranium oxide requirements to fuel light water nuclear reactors (LWRs) alone range from 2 × 10⁶ to 2.25 × 10⁶ tonnes U₃ O₈. These limits are deemed reasonable due to the unlikelihood of a firm commitment to the use of fast breeder reactors (FBRs) by the government; tentative decision dates range from 1988 to 2000 or so but, even presuming the former, the impact on uranium needs will not be felt until after 2000. A wider use of converter reactors may lighten the eventual uranium needs close to 2000. These reactors, however, were originally planned only for interim use during the change-over from LWRs to FBRs; hence their increased use will not affect the uranium demand until the next century. Present plans call for an accelerated exploration and development programme involving federal and state agencies with industry and other sources.     

Biological treatment of tannery wastewater in the presence of chromium

Experiments were carried out to determine the feasibility of treating tannery wastewater containing chromium, an inhibiting compound, with sequencing batch reactors (SBR). The maximum chromium concentration tolerated by microorganisms was determined through aerobic and anoxic batch experiments, and the biomass inhibition process was analyzed in a lab scale reactor at increasing chromium concentrations. The results obtained, in batch experiments and in the SBR reactor, have demonstrated that chromium addition had less influence on the denitrification bacteria than on the nitrification bacteria. In addition, it was observed that nitrification and denitrification rates, at the same chromium concentration, were higher in the SBR reactor than in batch experiments with unacclimated biomass. Experimental results confirm that sequencing batch reactors are able to produce a more resistant biomass, which acclimates quickly to inhibiting conditions. A large amount of chromium was found in the sludge from the reactor, while the effluent was devoid of the inhibiting metal.     

超滤技术在含铅废水处理中的实际运用

面对日趋严格的重金属排放标准,许多传统处理技术,如化学沉淀法、生化法、吸附法等,在某种程度上都难以单独胜任。与此同时,膜技术的快速发展为含重金属废水的深度处理提供了一个崭新的方向。某钢丝绳企业采用超滤技术对含铅废水进行深度处理,由于其过滤精度达0.002~0.1μm,可过滤铅离子组成的胶体(0.05~0.1μm范围),使总铅排放浓度稳定达到0.1 mg/L以下,此实际运用结果对进一步研究超滤技术在含铅废水处理中的运用具有积极意义,相关设计参数可供参考借鉴。     

生态环境损害赔偿诉讼：定义与定位矫正

面对生态环境损害赔偿诉讼这一新型诉讼形式,准确把握其定义和定位至关重要。现有生态环境损害赔偿诉讼按国家自然资源所有权私权化路径进行制度构建,其现行定义为特殊私益诉讼,现行定位为环境行政替代工具。这种定义与定位导致其与环境公益诉讼割裂,运行序位上优先于环境民事公益诉讼,引起国家机关角色错位。生态环境损害赔偿诉讼与环境民事公益诉讼的诉讼标的相同,救济和保护的利益均为环境公益,其定义应回归公益诉讼本位;政府在环境公益保护上有着广泛的职权和手段,是环境公共治理的优先主体,仅在少数行政不能的情形下才有借助司法的必要性和合理性,其在定位上应作为环境行政执法的补充机制在有限范围内发挥作用。据此,生态环境损害赔偿诉讼在制度体系上应与既有环境民事公益诉讼置于同一诉讼系属统筹立法,在诉讼序位上应让位于社会组织提起的环境民事公益诉讼。     

Solid Waste Treatment as a High-Priority and Low- Cost Alternative for Greenhouse Gas Mitigation

The increased concern about environmental problems caused by inadequate waste management, as well as the concern about global warming, promotes actions toward a sustainable management of the organic fraction of the waste. Landfills, the most common means to dispose of municipal solid waste (MSW), lead to the conversion of the organic waste to biogas, containing about 50% methane, a very active greenhouse gas (GHG). One unit of methane has a global warming potential of 21 computed for a 100-year horizon or 56 computed for 20 years. The waste sector in Israel contributes 13% of total greenhouse gases (GHG) emissions for a time horizon of 100 years (for a time horizon of 20 years, the waste sector contribution equals to more than 25% of total GHG emissions). The ultimate goal is to minimize the amount of methane (CH₄) by converting it to CO₂. This can be achieved by physicochemical means (e.g., landfill gas flare, incineration) or by biological processes (e.g., composting, anaerobic digestion). Since the waste in Israel has a high organic material content, it was found that the most cost-effective means to treat the degradable organic components is by aerobic composting (investment of less than US$ 10 to reduce emission of one ton CO₂ equivalent per year). Another benefit of this technology is the ability to implement it within a short period. The suggested approach, which should be implemented especially in developing countries, could reduce a significant amount of GHG at relatively low cost and short time. The development of a national policy for proper waste treatment can be a significant means to abate GHG emissions in the short term, enabling a gain in time to develop other means for the long run. In addition, the use of CO₂ quotas will credit the waste sector and will promote profitable proper waste management.     

Application of cavitational reactors for water disinfection: current status and path forward

Cavitational reactors are a novel and promising form of multiphase reactors, based on the principle of release of large magnitude of energy due to the violent collapse of the cavities. An overview of cavitational reactors in the specific area of water disinfection, in terms of the basic mechanism, different reactor designs including recommendations for optimum operating parameters and applicability of the cavitation phenomena for disinfection of different micro-organisms have been presented. A design of a pilot scale sonochemical reactor has been presented, which forms the basis for development of industrial scale reactors. Economic analysis for comparison of cavitation phenomena with other conventional techniques of disinfection has been discussed. It appears that though cavitation is quite successful in treatment of water at laboratory scale operations, comparatively higher cost of treatment as compared to the conventional chemical methods is a hindrance in its industrial scale application. Intensification of cavitational activity and efficient design of industrial scale hydrodynamic cavitation reactors is required for ensuring successful application of cavitational reactors at industrial scale operation.     

Prospects for a self-sustainable sewage treatment system: a case study on full-scale UASB system in India's Yamuna River Basin

The government of India decided to launch a project to implement 16 full-scale Upflow Anaerobic Sludge Blanket (UASB) reactors (with a total capacity of 598,000 m(3)/d) in the Yamuna River basin under its Yamuna Action Plan (YAP). A polishing pond called the Final Polishing Unit (FPU) was utilized for post-treatment. This paper evaluates the sewage treatment efficiency of the combined system of full-scale UASB reactors and polishing ponds under Indian climatic conditions. Results have shown that the effluent from the sewage treatment plants (STPs) investigated failed to comply with applicable discharge standards in terms of BOD, SS, and fecal coliform removal. Therefore, it is proposed that such proper operation and maintenance as removing excess sludge and scum be conducted in order to increase treatment efficiency. Moreover, trained and experienced workers are also required to operate and maintain the systems, along with a scientific approach.     

Simulation Study of a PEM Fuel Cell System with Steam Reforming

Abstract

This article summarizes the results of a study for a 100 kWe DC electrical power PEM fuel cell system. The system consists of a pre-steam reformer, a steam reformer, high and low temperature shift reactors, a preferential oxidation reactor, a PEM fuel cell, a combustor, and an expander. Acceptable net electrical efficiency levels can be achieved via intensive heat integration within the PEM fuel cell system. The calculations take into account the auxiliary equipment such as pumps, com pressors, heaters, coolers, heat exchangers and pipes. The process simulation package “Aspen-HYSYS 3.1’’ has been used. The operation parameters of the reactors have been determined considering all the technical limitations involved. A gasoline type hydrocarbon fuel has been studied as hydrogen rich gas source. Thermal efficiencies have been calculated for all of the major system components for selected operation conditions. The fuel cell stack efficiency has been calculated as a function of cell numbers (500, 750, 1000, and 1250 cells). Efficiencies of all of the major system components along with auxiliary unit efficiencies determine the net electrical efficiency of the PEM fuel cell system. The obtained net electrical efficiency levels are between 34 (500 cells) to 41% (1250 cells).     

Steam demand reduction of water–gas shift reaction in IGCC power plants with pre-combustion CO2 capture

This paper presents the results of system assessments that were conducted to compare conventional and advanced water–gas shift reactor sections. The latter are specifically tailored for Integrated Gasification Combined Cycles (IGCC) power plants with pre-combustion CO₂ capture. The advanced shift reactor section comprises four staged reactors with distributed feeds of synthesis gas and quench water in between the reactors. Conventional shift reactor sections consist of two sequential reactors with intermediate cooling, where the entire synthesis gas stream passes both reactors.The advanced shift reactor section reduces the steam requirement of the water–gas shift reaction up to 70% in comparison with conventional configurations, at carbon dioxide capture ratios of approximately 85%. This reduction allows for lower electric efficiency penalties, thus higher net electric outputs for IGCC power plants with CO₂ capture. For each case, the CO₂ capture ratio was optimised for the lowest specific lost work per amount of captured CO₂. Both the number of reactors and the total catalyst volume are higher for the advanced shift reactor sections, resulting in increased capital expenses. In case of four staged reactors, the additional expenses are expected to be outperformed by the increased revenues associated with the higher net electric output.     

Comparison of development scenarios of a black water source-separation sanitation system using life cycle assessment and environmental life cycle costing

The objective of the study is to compare different development scenarios of a black water source-separation sanitation system (BWS) that could be environmentally and economically more viable than a conventional system (CONV). Scenarios performance is evaluated using life cycle assessment and environmental life cycle costing. System boundaries include the processes related to the collection and treatment of wastewater and organic kitchen refuse collection and the recycling of by-product (digestate/sludge and biogas) produced in the treatment step. The BWS scenario that entails a vacuum toilet flow-volume reduction to 0.5 L/flush results in significantly higher performances than the ones of CONV for the climate change and resources indicators, while involving a significantly lower performance with regards to human health and a comparable cost. The BWS scenario based on digestate mass reduction with reverse osmosis and acidification prior to its transport to farmland achieves comparable performances to the ones of CONV for all indicators. The BWS scenario with digestate treatment by means of phosphorus precipitation (struvite) and nitritation–anammox reactors gives performances that are comparable to the ones of CONV for all indicators, with the exception of climate change, for which this scenario has a significantly lower performance if the electricity is produced by hydropower. When single-pathway scenarios are combined, the multi-pathway scenarios thus created can produce results that are significantly superior to the CONV result for the climate change, resources and human health indicators although the cost remains comparable.