Toward long solids retention time of activated sludge processes: benefits in energy saving, effluent quality, and stability

The activated sludge process is the most common method of secondary municipal wastewater treatment; solids retention time (SRT) is the key control parameter for this process. Typically, operating at long SRT is considered only for nitrification, but there are additional benefits of high SRT operation. This paper presents experimental and literature evidence to demonstrate three major additional benefits of long SRT operation: increased oxygen transfer efficiency; improved biomass particle size distribution, which results in more efficient clarification with fewer effluent particles and suspended solids; and enhanced removal of many emerging contaminants such as pharmaceuticals, personal care products, and endocrine disrupting compounds. This paper presents experimental results from several treatment plants that showed increasing oxygen transfer efficiency and particle size with increasing SRT, and evidence documenting improved removal of emerging contaminants and biodegradable organic carbon. A long-term survey of three treatment plants concludes that operating at higher SRT is not as energy intensive as typically assumed.     

Green procurement in the Asian public sector and the Hong Kong private sector

Since the 1980s, green purchasing/green procurement has increasingly been recognised as an effective means of addressing and reducing negative environmental impacts related to product production and consumption around the world. This paper reviews the procurement practices and policies of five Asian countries and Hong Kong, in the context of an international overview of green purchasing and practices. The extent of government involvement in green procurement is found to directly determine its overall effectiveness, and up-to-date information resources clearly enhance delivery and performance. A case study in Hong Kong demonstrates how public leadership on green procurement has advanced green procurement in the private sector.     

Transport and fate of nitrate at the ground-water/surface-water interface

Although numerous studies of hyporheic exchange and denitrification have been conducted in pristine, high-gradient streams, few studies of this type have been conducted in nutrient-rich, low-gradient streams. This is a particularly important subject given the interest in nitrogen (N) inputs to the Gulf of Mexico and other eutrophic aquatic systems. A combination of hydrologic, mineralogical, chemical, dissolved gas, and isotopic data were used to determine the processes controlling transport and fate of NO(3)(-) in streambeds at five sites across the USA. Water samples were collected from streambeds at depths ranging from 0.3 to 3 m at three to five points across the stream and in two to five separate transects. Residence times of water ranging from 0.28 to 34.7 d m(-1) in the streambeds of N-rich watersheds played an important role in allowing denitrification to decrease NO(3)(-) concentrations. Where potential electron donors were limited and residence times were short, denitrification was limited. Consequently, in spite of reducing conditions at some sites, NO(3)(-) was transported into the stream. At two of the five study sites, NO(3)(-) in surface water infiltrated the streambeds and concentrations decreased, supporting current models that NO(3)(-) would be retained in N-rich streams. At the other three study sites, hydrogeologic controls limited or prevented infiltration of surface water into the streambed, and ground-water discharge contributed to NO(3)(-) loads. Our results also show that in these low hydrologic-gradient systems, storm and other high-flow events can be important factors for increasing surface-water movement into streambeds.     

Acute and chronic toxicities of tributyltin to various life stages of the marine polychaete Hydroides elegans

Experiments were performed to determine the acute and chronic toxicities of tributyltin (TBT) (nominal concentrations) to polychaete Hydroides elegans. Firstly, static tests were performed to examine the lethal effect of TBT on five life stages of H. elegans. The 48-h LC(50) (95% CI) for the eggs, 2-cells, trochophores, juveniles and adults was 0.18 (0.15-0.20), 0.97 (0.77-1.23), 2.36 (2.11-2.65), 2.86 (2.63-3.12) and 4.36 (4.04-4.71) microg TBT l(-1), respectively. Secondly, a 17-d static renewal chronic test was conducted to study sensitivity of the early development (egg to juvenile) to TBT. The survivorship, percent settlement and time to reach settlement were significantly reduced at 0.01 microg TBT l(-1). Effects of TBT on the juvenile growth and maturation were also studied through a 44-d chronic test. Lower survivorship and reduced tube growth were observed only at >or= 1 microg TBT l(-1), however, only H. elegans exposed to or= 0.1 microg TBT l(-1). Our results indicate that the early development of H. elegans is highly sensitive to TBT and this polychaete can be routinely employed as a test organism for both acute and chronic ecotoxicity bioassays in tropical and subtropical regions such as southern China.     

Desorption of copper and cadmium from soils enhanced by organic acids

The adsorption/desorption behavior of copper and cadmium on soils was investigated in this study. The adsorption isotherm of copper and cadmium conformed to Langmuir equation better than Freundlich equation. The effect of ionic strength, pH, and organic acid, including ethylenediamine tetraacetic disodium acid salt (EDTA), citric acid, oxalic acid and tartaric acid, on the desorption of copper and cadmium was studied. The desorption of copper and cadmium increased with the increase of ionic strength, while the desorption decreased with the rise of pH. The desorption of copper and cadmium enhanced by organic acids was influenced by pH. EDTA showed excellent enhancement on the desorption of both copper and cadmium; citric acid demonstrated great enhancement on the desorption of copper but negligible enhancement on the desorption of cadmium; oxalic acid enhanced the desorption of copper only at pH around 6.4 and enhanced the desorption of cadmium in the pH range from 6.4 to 10.7; tartaric acid slightly enhanced the desorption of copper but negligibly enhanced the desorption of cadmium. The desorption mechanism in the presence of organic acids were explained as the competition of complexation, adsorption and precipitation. The net effect determined the desorption efficiency. This study provided guidance for the selection of organic acids to enhance the electrokinetic (EK) remediation of copper and cadmium from contaminated soils.     

引入路径权重蚁群算法在应急救援中的应用

规划最优路径是应急救援时一项很重要的工作。在基本蚁群系统模型的基础上,基于层次分析法解算出路径权重矩阵,并将其引入状态转移规则,这样求出的最优路径不只是考虑路径距离,而且考虑了道路实际状况。仿真实验结果表明,引入路径权重后的自适应蚁群算法能根据实际路况权重矩阵规划出更合理的救援路径,缩短了救援时间。     

An empirical survey of the benefits of implementing pay for safety scheme (PFSS) in the Hong Kong construction industry

Introduction

The Government of the Hong Kong Special Administrative Region (SAR) has implemented different safety initiatives to improve the safety performance of the construction industry over the past decades. The Pay for Safety Scheme (PFSS), which is one of the effective safety measures launched by the government in 1996, has been widely adopted in the public works contracts. Both the accident rate and fatality rate of public sector projects have decreased noticeably over this period.

Method

This paper aims to review the current state of application of PFSS in Hong Kong, and attempts to identify and analyze the perceived benefits of PFSS in construction via an industry-wide empirical questionnaire survey. A total of 145 project participants who have gained abundant handson experience with the PFSS construction projects were requested to complete a survey questionnaire to indicate the relative importance of those benefits identified in relation to PFSS. The perceived benefits were measured and ranked from the perspectives of the client and contractor for crosscomparison.

Results

The survey findings suggested the most significant benefits derived from adopting PFSS were: (a) Increased safety training; (b) Enhanced safety awareness; (c) Encouragement of developing safety management system; and (d) Improved safety commitment. A wider application of PFSS should be advocated so as to achieve better safety performance within the construction industry.

Impact on Industry

It is recommended that a similar scheme to the PFSS currently adopted in Hong Kong may be developed for implementation in other regions or countries for international comparisons.     

Whole-mount in situ TUNEL method revealed ectopic pattern of apoptosis in cadmium treated naupliar larvae of barnacle (Balanus amphitrite Darwin)

The effect of cadmium on stage II naupliar larvae of barnacle (Balanus amphitrite Darwin) was investigated. Barnacle larvae were exposed to 0-15 microM CdCl(2) for 24 h. Apoptotic cells were stained by the terminal deoxyribonucleotidyl transferase mediated dUTP nick end labelling (TUNEL) method. Incidence of apoptosis, as measured by numbers of animals with ectopic pattern of apoptosis as well as numbers of apoptotic cells per animal, was assessed using confocal microscopy. An increase in incidence of apoptosis was observed in the experimental animals with an increase in cadmium concentration. Mortality increased, and motility decreased, when barnacle larvae were exposed to an increasing concentration of cadmium. The relationship between the occurrence of apoptosis and swimming behaviour was investigated in larvae exposed to 10 microM CdCl(2). Significant increases in apoptosis were detected in the non-motile and dead nauplii. This study suggested that whole-mount in situ TUNEL method may be used to study increased occurrence of apoptotic cells in crustacean larvae.