S.L. Ong

Investigation of assimilable organic carbon (AOC) and bacterial regrowth in drinking water distribution system

This paper investigated the variation of assimilable organic carbon (AOC) concentrations in water from several typical water treatment plants and distribution systems in a northern city of China. It is concluded from this study that: (1) The AOC in most of the product water of the studied water treatment plants and the water from the associated distribution systems could not meet the biostability criteria of 50-100 microg/L. (2) Only 4% of the measured AOC concentrations were less than 100 microg/L. However, about half of the measured AOC values were less than 200 microg/L. (3) Better source water quality resulted in lower AOC concentrations. (4) The variation of AOC concentrations in distribution systems was affected by chlorine oxidation and bacterial activity: the former resulted in an increase of AOC value while the latter led to a reduction in AOC. (5) The variation of AOC concentration followed different patterns in different distribution systems or different seasons due to their respective operational characteristics. (6) Less than 30% of AOC could be removed by a conventional treatment process, whereas 30-60% with a maximum of 50-60% could be removed by granular activated carbon (GAC). (7) The observation via scanning electron microscope (SEM) on distribution pipe tubercle samples demonstrated that the pipe inner wall was not smooth and bacteria multiplied in the crevice as well as in the interior wall of distribution pipes.

Engineering design approaches for minimum fouling in submerged MBR

Abstract A contributor to success in managing membrane operations is to be able to identify suitable engineering design approaches to slow down the membrane fouling rate. Two such approaches were discussed in this study, namely: (1) offsite chemical and mechanical cleaning, and (2) air scouring. A longer subsequent operation time was observed after a membrane was cleaned chemically compared with mechanically. For air scouring during membrane operation, the investigation aimed at determining the crossflow velocity induced by coarse and fine bubble diffusers. Higher crossflow velocities were obtained with the latter. Uniformly distributed fine air bubbling might possibly have caused less uplift resistance and induced higher crossflow velocities. Such air scouring, at critical aeration intensity, was able to prolong the membrane operation for up to 8 months without needing chemical or mechanical cleaning.

Effects of pH and temperature on the survival of coliphages MS2 and Qβ

The RNA F-specific coliphages, MS2 and Qβ, have been used as virus indicators in water and wastewater studies. It is therefore useful to have a good understanding concerning the effects of environmental factors on their survival in order to choose an appropriate candidate for assessing microbial safety in relation to water quality management. The effects of pH and temperature on the survival of these two coliphages were investigated. MS2 survived better in acidic conditions than in an alkaline environment. In contrast, Qβ had a better survival rate in alkaline conditions than in an acidic environment. The inactivation rates of both coliphages were lowest within the pH range 6–8 and the temperature range 5–35°C. The inactivation rates of both coliphages increased when the pH was decreased to below 6 or increased to above 8. The inactivation rates of both coliphages increased with increasing temperature. Qβ behaved peculiarly in extreme pH buffers, i.e. it was inactivated very rapidly initially when subjected to an extreme pH environment, although the inactivation rate subsequently decreased. In general, MS2 was a better indicator than Qβ. However, within the pH range 6–9 and at temperatures not above 25°C, either MS2 or Qβ could be used as a viral indicator.

The effect of water treatment processes on the biological stability of potable water

Abstract The AOC (assimilable organic carbon) method was used in this study to evaluate the biological stability of potable water. The potable water was drawn from a waterworks located in a petrochemical industrial area in China. Infrared spectra analysis was adopted in this study to examine the relationship between treatment process and organic compounds in water. By monitoring the AOC concentration in groundwater, potable water and the effluent of each treatment process (biological pre-treatment, ozonation, GAC adsorption), the following were noted: (1) a considerable AOC concentration could still be detected in potable water after advanced treatment processes, such as ozonation and activated carbon adsorption. Relatively poor biological stability was also noted in the potable water; (2) GAC was the most effective process with a total AOC removal efficiency of more than 80%. The corresponding biotreatment removal efficiency was only 45%. However, ozonation yielded a negative overall removal efficiency of −119%; (3) while biological pre-treatment and GAC adsorption contributed towards a biologically stable water, ozonation yielded oxidation products which were biologically unstable. Ozonation, if used, should therefore be combined with the GAC or biological processes. Treatment processes (like ozonation) which increased the amount of organics in carbonyl group would likely lead to poor product water biological stability.

Feasibility of submerged anaerobic membrane bioreactor (SAMBR) for treatment of low-strength wastewater

Two 6-L submerged anaerobic membrane bioreactors (SAMBR) with SRT of 30 and 60 d (denoted as R30 and R60, respectively) were set up and operated for five months, with a mixture of glucose as substrate. Feasibility of SAMBR was studied for treatment of low-strength wastewater. First two months were identified as acclimation stage. A COD removal efficiency was achieved stably at around 99% and biogas productions were maintained at 0.023 and 0.028 L CH4/gMLVSSd for R30 and R60, respectively. Even though R60 contained higher MLVSS concentration, no significant difference of treatment performances between both reactors was found due to the low organic loading rate and high purification function of membrane. In the investigation of membrane fouling, less irreversible fouling was observed for R30 compared to R60. High non-flocculent concentration of R60 would be responsible for membrane internal pore blocking and deteriorated effluent quality.

Disinfection By-Products in Water Produced by Ozonation and Chlorination

Water produced by advanced treatment of a groundwater was evaluated to determine the amount of DBPs (Disinfection By-Products) including trihalomethanes (THMs). Both Gas Chromatography (GC) and Gas Chromatography/Mass Spectrometry (GS/MS) were adopted for detection and identification of DBPs such as trihalomethanes (THMs), halo-acetic acids (HAAs) and aldehydes. Two disinfection modes (ozonation followed by chlorination and chlorination alone) were compared to determine the DBPs generation. The mutagenitic acivity of ozonated water, chlorinated water after ozonation and potable water was assessed using the Ames test. Chloroform, dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA) were the main constituents of THMs and HAAs, respectively. THMs accounted for more than 85% of all DBPs measured, whereas haloacetic acids accounted for around 14%. Ozonation followed by chlorination proved to be better in terms of THMs and HAAs control. The combined system produced 28.3% less DBPs compared to chlorination alone. Ozonation was found capable of reducing mutagenic matter in the groundwater by 54.7%. The combined system also resulted in water with no mutagenicity.

Denaturing gradient gel electrophoresis polymorphism for rapid 16S rDNA clone screening and microbial diversity study

A method based on denaturing gradient gel electrophoresis (DGGE) banding polymorphism was developed for rapid small-subunit (SSU) rDNA clone screening and microbial diversity estimation. Clones with correct rDNA insertion were initially identified using whole-cell PCR with vector-specific primers. Subsequently, the correct PCR products were re-amplified with nested primers in a DGGE analysis. An average of five to 10 individual DGGE-PCR products could be mixed together and loaded into a DGGE well, and at least 125 to 250 clones could be screened and compared within one DGGE gel. This approach eliminated the reproducibility problem associated with DGGE, increased the ability to screen a large set of clones at a relatively affordable cost, and was shown to be more sensitive than the restriction fragment length polymorphism method in identifying 10 different but phylogenetically closely related 16S rDNA fragments from 126 clones obtained from an anaerobic sludge sample.

Treatment of RO brine-towards sustainable water reclamation practice.

Treatment and disposal of RO brine is an important part in sustaining the water reclamation practice. RO brine generated from water reclamation contains high concentration of organic and inorganic compounds. Cost-effective technologies for treatment of RO brine are still relatively unexplored. Thus, this study aim to determine a feasible treatment process for removal of both organic and inorganic compounds in RO brine generated from NEWater production. The proposed treatment consists of biological activated carbon (BAC) column followed by capacitive deionization (CDI) process for organic and inorganic removals, respectively. Preliminary bench-scale study demonstrated about 20% TOC removal efficiency was achieved using BAC at 40 mins empty bed contact time (EBCT) while the CDI process was able to remove more than 90% conductivity reducing it from 2.19 mS/cm to only about 164 microS/cm. More than 90% cations and anions in the BAC effluent were removed using CDI process. In addition, TOC and TN removals of 78% and 91%, respectively were also attained through this process. About 90% water recovery was achieved. This process shows the potential of increased water recovery in the reclamation process while volume for disposal can be further minimized. Further studies on the sustainable operation and process optimization are ongoing.

Investigation into biofilms in a local drinking water distribution system

Biofilm growth within a water distribution system could lead to operational problems such as pipe corrosion, water quality deterioration and other undesirable impacts in water distribution systems. With the high ambient temperatures experienced in Singapore, the operating environment in water distribution systems is expected to be more conducive to biofilm development. We have recently conducted a survey on biofilms potentially present in a local water distribution system. The survey results indicated that residual chlorine (+− standard deviation) decreased from 1.49 +− 0.61 mg/l (water plant outlets) to 0.82 +− 0.21 mg/l (block pipes) or 0.18 +− 0.06 mg/l (unit pipes), respectively. Consumed chlorine, instead of residual chlorine, was found to be correlated with biofilm bacterial population. Assimilable organic carbon (AOC) level was 160 +− 66 μg acetate C/l, and AOC : PO4-P : NO3-N was about 8 : 13 : 1. Carbon source seemed to be the limiting nutrient for bacterial growth. The concentration of iron increased from <0.04 mg/l (water plant outlets) to 0.22 +− 0.10 mg/l (all sites). All samples showed negative results in a coliform test. The average heterotrophic plate count (HPC) for the suspended bacteria was 20 colony-forming units (c.f.u.)/ml (2 days, 35 ◦C) or 290 c.f.u./ml (8 days, 35 ◦C). The average HPC for the biofilm bacteria was 6500 c.f.u./cm (2 days, 35 ◦C) or 29 000 c.f.u./cm (8 days, 35 ◦C). High HPC values in samples B2a, B2b and B3a (representing biofilm samples at site 2 from block/unit pipes and biofilm samples at site 3 from block pipes, respectively) illustrated that the relevant sample sites had a higher probaboility of biofilm growth. * Corresponding author: Dr J. Y. Hu Center for Water Research Department of Civil Engineering Faculty of Engineering National University of Singapore 10 Kent Ridge Crescent Singapore 119260 T 65 6874 4540 F 65 6779 1635 E cvehujy@nus.edu.sg 1 Center for Water Research, Department of Civil Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 2 Water Department, Public Utility Board, 111 Somerset Road, #15-01, Singapore 238164

Kinetics of β-mannanase fermentation by Bacillus licheniformis

Bacterial growth, konjac powder utilization and β-mannanase production by Bacillus licheniformis NK-27 in batch fermentation were used to develop a model of the process. The optimal set of parameters was estimated by fitting the model to experimental data. The results predicted by the model were in good agreement with the experimental data.