W.J. Ng

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.

A new method for characterizing denitrifying phosphorus removal bacteria by using three different types of electron acceptors.

This study investigated the characteristics of denitrifying phosphorus removal bacteria by using three different types of electron acceptors as well as the positive role of nitrite in phosphorus removal process. Denitrifying phosphorous removal bacteria was enriched under anaerobic-anoxic (A/A) condition. To understand A/A sludge better, sludge from two other sources were also studied. These include sludges obtained from a lab-scale anaerobic-anoxic-aerobic (A/A/O) system and a local sewage treatment plant. Three types of possible electron acceptors (oxygen, nitrate and nitrite) were examined for their roles in phosphorus uptake. The results obtained indicated that oxygen, nitrate and nitrite were able to act as electron acceptors successfully. This observation suggested that in addition to the two well-accepted groups of phosphorus removal bacteria (one can only utilize oxygen to take up phosphorus, P(O), while the other can use both oxygen and nitrate, P(ON)), a new group of phosphorus removal bacteria, P(ON(n)), which could use oxygen, nitrate or nitrite to take up phosphorus was identified. The relative population of these three types of bacteria could be calculated from results obtainable from phosphorus uptake batch experiments with either oxygen or nitrate or nitrite as electron acceptor. The results obtained in this study showed that A/A sludge had similar phosphorus removal performance as the A/A/O sludge. However, it has better denitrifying phosphorus removal capability, which was demonstrated by the relative population of the three groups of bacteria. The results also suggested that nitrite was not an inhibitor to phosphorus removal process. Instead, it is an alternative electron acceptor to oxygen or nitrate.

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.

Chemical regeneration of exhausted activated carbon—I

Abstract This paper represents the first of a series of reports on the chemical regeneration of exhausted activated carbon following experimental research studies carried out at the University of Birmingham, U.K. A wide range of regenerants, inorganic and organic, was evaluated in the treatment of carbon samples exhausted with mono-substituted benzene compounds. Organic chemical regenerants with solubilising powers were found to be generally much more effective than inorganic chemical regenerants with oxidising powers. Results showed that the efficacy of organic solubilising regenerants decreased as their molecular weights and sizes increased; it is likely that the smaller the regenerant, the further it could penetrate into the micropores of the carbon and displace the adsorbate. The relationship between the molecular weight of the adsorbate and that of the organic regenerant was observed to be of significant importance; the regenerant chosen should be smaller than the smallest adsorbate to be removed.

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.

The repeated exhaustion and chemical regeneration of activated carbon

Abstract Following experimental research studies carried out at the University of Birmingham, U.K., this paper presents data on the repeated application of the cycle of exhaustion, chemical regeneration and re-exhaustion of activated carbon. The adsorbates studied were nitrobenzene (a small organic molecule of mol. wt approx. 123), Rhodamine B (a much larger adsorbate of mol. wt approx. 480) and humic acid (an ill-defined substance consisting of macromolecules of molecular weight predominantly in the range 20,000–50,000). The regenerants used were all organic with solubilising powers to encourage the physical displacement of the adsorbate molecule by the regenerant molecule. As a general conclusion, it may be stated that chemical regenerants could be used to regenerate granular activated carbon repeatedly with little loss of adsorption capacity. Earlier studies which considered the effect of water on the powers of the carboxylic acid regenerants were confirmed in the results reported here. Bonding between the acids and water hindered the regeneration process.

Two-phase anaerobic treatment kinetics of palm oil wastes

A laboratory-scale two-phase anaerobic digestion system was used to treat a palm oil mill effluent (POME) containing around 63,000 mg l−1 COD. Phase separation was accomplished through control of the hydraulic retention times of two reactors operated in series. Acid and methane phase biokinetic coefficients were evaluated. Steady state parameters indicate good process stability with high gas yields.

A Study on Arsenic Removal from Household Drinking Water

Abstract Arsenic removal from household drinking water has been receiving considerable attention in the field of water supply engineering. To develop the optimal coagulation protocol, the effectiveness of several operation options such as coagulants, coagulant aids and additives, as well as flocs separation systems were investigated in this study through the use of orthogonal array experiment based on Taguchi method. Arsenic removal mechanism during household coagulation (via manual mixing) was also discussed. The results showed that the addition of kaoline and powder activated carbon (PAC) did not enhance arsenic removal efficiency of ferric sulfate or aluminum sulfate. Similarly, mixture of ferric sulfate and aluminum sulfate (MFA) as well as polymeric ferric silicate sulfate (PFSiS) was also unable to improve the overall arsenic removal efficiency. The mechanism of arsenic removal during coagulation was somewhat different from those experienced in conventional processes. Coprecipitation appeared to be the crucial mechanism for arsenic removal. It is noted from this study that arsenic adsorption isotherm under household operation condition could be described by Langmuir equation. An efficient flocs separation system subsequent to coagulation was essential to achieve the effectiveness of overall arsenic removal. The results obtained from field experiment demonstrated that the method of ferric sulfate coagulation/sand filtration for arsenic removal from household drinking water was acceptable and affordable.

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.