H. Chua

Assessment of heavy metal cations in sediments of Shing Mun River, Hong Kong.

The extent of heavy metal cation contamination in the Shing Mun River has been assessed. Sediment samples were taken at eight strategic locations along the river system. The highest concentrations of copper (Cu, 1.66 mg/g), lead (Pb, 0.354 mg/g), zinc (Zn, 2.2 mg/g) and chromium (Cr, 0.047 mg/g) were found in the Fo Tan Nullah, a major tributary of the Shing Mun River. The highest concentrations of aluminum (114 mg/g) and cadmium (Cd, 0.047 mg/g) were found in the Shing Mun Main River Channel. These contaminated sediments, accumulated over the years on the river bed, could act as secondary sources of pollution to the overlying water column in the river.

Nutrient release and sediment oxygen demand in a eutrophic land-locked embayment in Hong Kong

Tolo Harbour is a large eutrophic land-locked estuarine embayment in Hong Kong. The rapid urbanization, commercio-industrial activities and lack of legislative control around the Tolo catchment produced large quantities of untreated or partially treated municipal sewage, agricultural wastes and cottage industrial effluents which were discharged into Tolo Harbour via rivers and watercourses. Control measures were implemented to reduce the external nutrient loading into the harbour since the early 1980s. Nutrient data for the period 1982 to 1997 were analyzed for temporal trends. Over the period of observation, the total inorganic nitrogen and total phosphorus both show an increasing trend, despite a decade of efforts in reducing nutrient loading. The release rates of potentially mobile nitrogen (N) and phosphorus (P) from the sediments collected from Tolo Harbour were determined by N and P release experiments under oxic conditions. The experimental results showed that the sediment released significant amount of nutrients, especially orthophosphates and ammonia nitrogen. The maximum release rates were 15.0 and 206.0 mg/m2/day, respectively. Although the external nutrient loading has been reduced, nutrients could gradually be released back into the water column from the contaminated sediments and delay improvement of the water quality.

Using rice straw fermentation liquor to produce bioflocculants during an anaerobic dry fermentation process

In this study, fermentation liquor from rice straw was used to produce bioflocculants during a dry fermentation process. Acetic acid and butyric acid were the predominant VFAs during the process. A compound bioflocculant producing inoculum, F(+) was inoculated into compound media in which fermentation liquor and conventional bioflocculants medium at different ratios. The maximum flocculation activity, 92.45% was achieved when 100-day fermentation liquor and conventional bioflocculants medium were mixed at an equal ratio. Furthermore, 454-pyrosequencing technology was used to measure bacterial diversity on the 25th day of operation, which was a period of rapid VFA accumulating. A total of 2110 sequences were obtained, and were found to be affiliated with 8 phylogenetic groups, including Actinobacteria, Acidobacteria, Bacteroidetes, Firmicutes, Proteobacteria, Chloroflexi, Spirochaetes and Synergistetes. Notably, Firmicutes (92.3%) was the dominant microbial population, followed by Actinobacteria (2.37%) and Proteobacteria (1.04%).

Sub-lethal effects of heavy metals on activated sludge microorganisms.

The effects of heavy metals, at sub-lethal concentrations, on activated sludge microbial ecosystem were investigated. Adsorption capacity and rate of copper, chromium, lead and zinc on microbial flocs were much faster than that of organic matters. Metals affected not only the adsorption rate of organic matters but also the COD adsorption capacity of the activated sludge. Effects of heavy metals, on wastewater treatment performance of a sequencing batch reactor were also studied. Metal-laden wastewater at sub-lethal levels affected the performance to different extents, depending on the hydraulic retention time. Metal ions acted as a strong competitor against the organic compounds for active sites on the bioflocs instead of acting as a toxic microbial inhibitor, thus hampering organic adsorption and affecting the COD removal efficiency under shorter HRTs.

Responses of an anaerobic fixed-film reactor to hydraulic shock loadings

The responses of an anaerobic fixed-film reactor (AFFR) to hydraulic shock loadings were studied. The AFFR was started up with a synthetic wastewater of 3000 mg COD/l at 5.00 d hydraulic retention time (HRT), achieving 98.1% COD removal efficiency. When stable operation was attained, the HRT was sporadically adjusted to 2.50, 1.25, 1.00 and 0.50 d to simulate two, four, five and 10 times hydraulic shock loadings, respectively, while the COD loading was maintained constant. Under two, four and five times hydraulic shock loadings, the COD removal efficiency was temporarily reduced to between 84.4 and 88.3%, and the pH of the treated effluent and the biogas production were also affected. The AFFR could recover from the temporary inhibition due to these shock loadings and resumed normal operation within eight days. On the other hand, under 10 times hydraulic shock loading, the treatment performance deteriorated drastically. Volatile fatty acids (VFAs) accumulated in the AFFR liquor, resulting in reactor souring and failure. When the HRT was returned to 5.00 d, the biofilter could be recovered within a few days. The ability of the AFFR to recover from critical hydraulic shock loadings and system failure was attributed to the immobilized-biofilm design, which enabled the temporarily inhibited biofilms to be retained in the AFFR and regain activity when favourable conditions were restored.

Electricity generation from cattle dung using microbial fuel cell technology during anaerobic acidogenesis and the development of microbial populations

A microbial fuel cell (MFC) was constructed to investigate the possible generation of electricity using cattle dung as a substrate. After 30 days of operation, stable electricity was generated, and the maximum volumetric power density was 0.220 W/m(3). The total chemical oxygen demand (TCOD) removal and coulombic efficiency (CE) of the MFC reached 73.9±1.8% and 2.79±0.6%, respectively, after 120 days of operation. Acetate was the main metabolite in the anolyte, and other volatile fatty acids (VFAs) (propionate and butyrate) were present in minor amounts. The PCR-DGGE analysis indicated that the following five groups of microbes were present: Proteobacteria, Bacteroides, Chloroflexi, Actinobacteria and Firmicutes. Proteobacteria and Firmicutes were the dominant phyla in the sample; specifically, 36.3% and 24.2% of the sequences obtained were Proteobacteria and Firmicutes, respectively. Clostridium sp., Pseudomonas luteola and Ochrobactrum pseudogrignonense were the most dominant groups during the electricity generation process. The diversity of archaea dramatically decreased after 20 days of operation. The detected archaea were hydrogenotrophic methanogens, and the Methanobacterium genus disappeared during the periods of stable electricity generation via acidogenesis.

Role of cell surface components on Cu2+ adsorption by Pseudomonas putida 5-x isolated from electroplating effluent

A gram-negative bacterium Pseudomonas putida 5-x with high Cu2+ accumulating capability was isolated from electroplating effluent in Kwun Tong, Hong Kong. The pretreated cells without superficial layer-capsule, isolated cell envelopes and the separated peptidoglycan layer materials were obtained from fresh P. putida 5-x cells, their Cu2+ adsorption capacities and properties were compared with that of the fresh cells. Pretreatment by 0.1 mol L(-1) HCl enhanced Cu2+ adsorption capacity due to the degradation of cell superficial layer-capsule of P. putida 5-x cells. Isolated cell envelopes possessed five times more Cu2+ adsorption capacity than that of fresh intact cell. The Cu2+ adsorption of separated peptidoglycan layer materials indicated that the peptidoglycan layer only played 10-15% part of the Cu2+ adsorption capacity, and implied other cell surface components such as outer membrane or inner membrane might play an important role in such high Cu2+ binding of the cell envelopes. The adsorption process of fresh cells, pretreated cells and cell envelopes of P. putida 5-x could be described with Freundlich isotherm, while the adsorption of Cu2+ by separated peptidoglycan layer materials was better described with Langmuir isotherm.

Optimal Production of Polyhydroxyalkanoates in Activated Sludge Biomass

Polyhydroxyalkanoates (PHAs) have been recognized as good candidates for biodegradable plastics, but their high price compared with conventional plastics has limited their use. In this study, actiated sludge microorgan isms from a conventional wastewater treatment process were induced, bycontrol-lingthe carbon: nitrogen (C:N) ratioin the reacorliquor, toaccumulate PHAs. In addition, an intermittent nitrogen feeding program was established to optimize the volumetric PHA productivity in a wastewater treatment process. The optimal overall polymer production yield of 0.111 g of polymer/g of carbonaceous substrate consumed was achieved under a C:N ratio of 96:1 by feeding nitrogen in the reactor liquor onceevery four cycles. At the same time, the amount of excess sludge generated from the wastewater treatment process was reduced by22.9%.

Synthesis of Polyhydroxyalkanoate (PHA) from Excess Activated Sludge Under Various Oxidation-Reduction Potentials (ORP) by Using Acetate and Propionate as Carbon Sources

Accumulation of poly hydroxyalkanoate (PHA) from excess activated sludge (EAS) was monitored and controlled via the oxidation-reduction potential (ORP) adjusting process. The ORP was adjusted and controlled by only regulating the gas-flow rate pumped into the cultural broth in which sodium acetate (C2) and propionate (C3) were used as carbon sources. Productivity of PHA and the PHA compositions at various C2 to C3 ratios were also investigated. When ORP was maintained at +30 mV, 35% (w/w) of PHA of cell dry weight obtained when C2 was used as sole carbon source. The PHA copolymer, poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), accumulated by EAS with different 3-hydroxyvalarate (3HV) molar fractions ranged from 8% to 78.0% when C2 and C3 was used as sole carbon source, By using ORP to monitor and control the fermentation process instead DO meter, the ORP system provided more precise control to the PHA accumulation process from EAS under low dissolved oxygen (DO) concentrations. Adjusting the C2 to C3 ratios in the media could control the composition such as the 3HV/3HB ratios of the PHBV. Furthermore, it might be an effective way to adjust the 3HV molar fractions in PHBV by controlling the DO concentration via the ORP monitoring system. The 3HV molar fractions in the PHBV declined with increasing ORP from -30 mV to +100 mV by adjusting the gas-flow rate (i.e. the DO concentration). It is concluded that the DO plays a very important role in the synthesis of 3HV subunits in PHBV co-polymer from the EAS. Therefore, a hypothetic metabolic model for PHA synthesis from EAS was proposed to try to explain the results in this study.

Synthesis of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) from activated sludge

Poly-3-hydroxybutyrate (PHB) and poly(3-hydroxybutyrate- co-3-hydroxyvalerate) (PHBV) was produced using a co-culture of activated sludge. When butyric acid was used as sole carbon source, PHB was produced. When valeric acid was added to the medium, PHBV was produced. The 3-hydroxyvalerate (3HV) mole fraction in the PHBV reached a maximum of 54% when valeric acid was used as sole carbon source. When the 3HV units in the co-polymer increased from 0.0 to 54.0 mol%, the melting temperature ( T m ) decreased from 178 to 99°C. The composition, and hence the mechanical properties, of the co-polymer produced by activated sludge can be controlled by adjusting the medium composition.