Xiao-mao Wang

Role of Gelling Soluble and Colloidal Microbial Products in Membrane Fouling

The mechanism underlying gel layer formation on membrane surfaces from soluble and colloidal microbial products (SCMPs) produced under unfavorable operational conditions for membrane bioreactors (MBRs) has been investigated using supernatants from a bench-scale MBR. SCMPs can be grossly classified into gelling and nongelling SCMPs with the gelling fraction associated mostly with the polysaccharide content. The significant role played by multivalent metals in gel formation through metal-ligand complexation has been confirmed. Functional groups of the gelling SCMPs were determined by pH titration and zeta potential measurement as amine/phenolic sites (pK(a) 9.3 and 8.0), carboxylic sites (pK(a) 6.6, 4.9, and ca. 4.0), and phosphoric sites (pK(a) ca. 2.5). The carboxylic sites were more directly involved with multivalent cation complexation; however, the gelling propensity of the SCMP dispersion was minimally affected by pH change in the circum-neutral pH range, suggesting that the strong carboxylic sites were principally responsible for gel formation. The SCMPs demonstrated a high potential for gel formation given the high density of the strong carboxylic acid groups of about 0.44 mmol/g-TOC and a moderate calcium binding stability constant of about 4.9 x 10(3) M(-1).

Effects of ozonation on disinfection byproduct formation and speciation during subsequent chlorination

Ozone has been widely used for drinking water treatment recently. This study was conducted to investigate the effect of dosing ozone on the formation potentials and speciation of disinfection by-products (DBPs, brominated DBPs in particular) during subsequent chlorination. Trihalomethanes (THMs), trihaloacetic acids (THAAs), dihaloacetic acids (DHAAs), dihaloacetonitriles (DHANs), chloral hydrate (CH)and trichloronitromethane (TCNM) were included. The results showed that the yields of THMs, THAAs and DHAAs reached the maxima at 1.83, 0.65 and 0.56 μM, respectively, corresponding to an ozone dose approximately at 2 mg L(-1). The formation potentials of CH and TCNM increased, while that of DHAN decreased, with the increase of ozone dose up to 6 mg L(-1). The bromide incorporation factor values of THMs, THAAs, DHAAs and DHANs increased from 0.62, 0.37, 0.45 and 0.39 at O3=0 mg L(-1) to 0.89, 0.65, 0.62 and 0.89 at O3=6 mg L(-1), respectively. It indicated that the use of ozone as a primary disinfectant may cause a shift to more brominated DBPs during subsequent chlorination, and the shift may be more evident with increased ozone dose. The total percentage of brominated DBPs (as bromide) reached the maximum value of 55% at 2 mg L(-1) ozone dose.

Formation of disinfection by-products: Effect of temperature and kinetic modeling

The temperature of drinking water fluctuates naturally in water distribution systems as well as often deliberately heated for household or public uses. In this study, the temperature effect on the formation of disinfection by-products (DBPs) was investigated by monitoring the temporal variations of twenty-one DBPs during the chlorination of a humic precursors-containing water at different temperatures. It was found that chloroform, DCAA, TCAA, DCAN and CH were detected at the considerable level of tens of μg L(-1). The three regulated DBPs (chloroform, DCAA and TCAA) were found increasing with both contact time and water temperature, while the five typical emerging DBPs (DCAN, CH, TCNM 1,1-DCPN and 1,1,1-TCPN) revealed the significant auto-decomposition in addition to the initial growth in the first few hours. Increasing water temperature could enhance the formation rates of all the eight detected DBPs and the decomposition rates of the five emerging DBPs. Further, a kinetic model was developed for the simulation of DBP formation. The validity and universality of the model were verified by its excellent correlation with the detected values of each DBP species at various temperatures. The formation rates of 1,1-DCPN and 1,1,1-TCPN, and the decomposition rate of 1,1,1-TCPN were faster as compared to the other DBPs. And the formation reaction activation energies of CH, DCAN and 1,1-DCPN were relatively large, indicating that their occurrence levels in the finished water were more susceptible to temperature variations.

Disinfection byproducts in drinking water and regulatory compliance: A critical review

Disinfection by-products (DBPs) are regulated in drinking water in a number of countries. This critical review focuses on the issues associated with DBP regulatory compliance, including methods for DBP analysis, occurrence levels, the regulation comparison among various countries, DBP compliance strategies, and emerging DBPs. The regulation comparison between China and the United States (US) indicated that the DBP regulations in China are more stringent based on the number of regulated compounds and maximum levels. The comparison assessment using the Information Collection Rule (ICR) database indicated that the compliance rate of 500 large US water plants under the China regulations is much lower than that under the US regulations (e.g. 62.2% versus 89.6% for total trihalomethanes). Precursor removal and alternative disinfectants are common practices for DBP regulatory compliance. DBP removal after formation, including air stripping for trihalomethane removal and biodegradation for haloacetic acid removal, have gained more acceptance in DBP control. Formation of emerging DBPs, including iodinated DBPs and nitrogenous DBPs, is one of unintended consequences of precursor removal and alternative disinfection. At much lower levels than carbonaceous DBPs, however, emerging DBPs have posed higher health risks.

Change in the fouling propensity of sludge in membrane bioreactors (MBR) in relation to the accumulation of biopolymer clusters

A membrane bioreactor (MBR) and an activated sludge process (ASP) were operated side by side to evaluate the change of sludge supernatant characteristics and the evolution of the sludge fouling propensity. The MBR sludge had a higher organic concentration and more biopolymer clusters (BPC) in the supernatant compared with ASP. BPC increased in both concentration and size in the MBR. The results show that the change in the liquid-phase property had a profound effect on the sludge fouling propensity. MBR operation transformed typical activated sludge to MBR sludge with a higher fouling propensity. Distinct from the ASP, membrane filtration retained soluble microbial products (SMP) within the MBR, and the vast membrane surface provided a unique environment for the transformation of SMP to large size BPC, leading to further sludge deposition on the membrane surface. Thus, membrane filtration is the crucial cause of the inevitable fouling problem in submerged MBRs.

Effect of dissolved oxygen concentration on iron efficiency: Removal of three chloroacetic acids

The monochloroacetic, dichloroacetic and trichloroacetic acid (MCAA, DCAA and TCAA) removed by metallic iron under controlled dissolved oxygen conditions (0, 0.75, 1.52, 2.59, 3.47 or 7.09 mg/L DO) was investigated in well-mixed batch systems. The removal of CAAs increased first and then decreased with increasing DO concentration. Compared with anoxic condition, the reduction of MCAA and DCAA was substantially enhanced in the presence of O2, while TCAA reduction was significantly inhibited above 2.59 mg/L. The 1.52 mg/L DO was optimum for the formation of final product, acetic acid. Chlorine mass balances were 69-102%, and carbon mass balances were 92-105%. With sufficient mass transfer from bulk to the particle surface, the degradation of CAAs was limited by their reduction or migration rate within iron particles, which were dependent on the change of reducing agents and corrosion coatings. Under anoxic conditions, the reduction of CAAs was mainly inhibited by the available reducing agents in the conductive layer. Under low oxic conditions, the increasing reducing agents and thin lepidocrocite layer were favorable for CAA dechlorination. Under high oxic conditions, the redundant oxygen competing for reducing agents and significant lepidocrocite growth became the major restricting factors. Various CAA removal mechanisms could be potentially applied to explaining the effect of DO concentration on iron efficiency for contaminant reduction in water and wastewater treatment.

Iron speciation and iron species transformation in activated sludge membrane bioreactors.

Iron speciation and iron species transformation were investigated in three membrane bioreactors (MBRs) differing in feed iron concentration (and oxidation state) and the presence or absence of an anoxic chamber to simulate various feed stream conditions and operational configurations. The concentration of dissolved Fe(II) was below detection limit (i.e., <0.1microM) in all chambers while the concentration of dissolved Fe(III) was found to be around 0.25microM. H(2)O(2) was detected as a quasi-stable reactive oxygen species with concentrations in the muM range in all MBR chambers. H(2)O(2) acted as the primary potential oxidant of Fe(II) in the anoxic chamber. Batch experiments showed that the rate constant for oxygenation of dissolved Fe(II) in the liquid phase of the activated sludge compartment was as high as 78M(-1)s(-1). The half-life time of dissolved Fe(II) in all chambers was found to be no longer than 1min. The stability constants of the Fe(III)SMP complexes were far from uniform. A large quantity of Fe(II) (over 0.036% of the sludge dry mass) was found to be adsorbed by the bacterial flocs suggesting the active reduction of adsorbed Fe(III). The content of adsorbed Fe(II) was found to increase if the MBR was supplied with iron in the Fe(II) form. Over 60% of iron fed to the reactors was converted to highly insoluble ferric oxyhydroxide in all MBRs. A model has been developed which satisfactorily describes the oxidation of Fe(II) in the activated sludge liquid phase and which provides valuable insight into the relative importance of redox processes occurring which mediate the speciation of iron in the system.

Haloacetic acid removal by sequential zero-valent iron reduction and biologically active carbon degradation

An innovative haloacetic acid (HAA) removal process was developed. The process consisted of a zero-valent iron (Fe(0)) column followed by a biologically active carbon (BAC) column that were efficient in degrading tri- and di-HAAs, and mono- and di-HAAs, respectively. The merit of the process was demonstrated by its performance in removing trichloroacetic acid (TCAA). An empty bed contact time of 10 min achieved nearly complete removal of 1.2 μM TCAA and its subsequent products, dichloroacetic acid (DCAA) and monochloroacetic acid (MCAA). HAA removal was a result of chemical dehalogenation and biodegradation rather than physical adsorption. Preliminary kinetic analyses were conducted and the pseudo-first-order rate constants were estimated at ambient conditions for Fe(0) reduction of TCAA and biodegradation of DCAA and MCAA by BAC. This innovative process is highly promising in removing HAAs from drinking water, swimming pool water, and domestic or industrial wastewater.

Membrane fouling in ultrafiltration of natural water after pretreatment to different extents.

The combined fouling during ultrafiltration (UF) of surface water pretreated to different extents was investigated to disclose the roles of polysaccharides, proteins, and inorganic particles in UF membrane fouling. Both reversible and irreversible fouling decreased with enhanced pretreatment (biologically active carbon (BAC) treatment and sand filtration). The sand filter effluent fouled the membrane very slowly. The UF membrane removed turbidity to less than 0.1 nephelometric turbidity unit (NTU), reduced polysaccharides by 25.4%-29.9%, but rejected few proteins. Both polysaccharides and inorganic particles were detected on the fouled membranes, but inorganic particles could be effectively removed by backwashing. The increase of turbidity in the sand filter effluent to 3.05 NTU did not significantly increase the fouling rate, but an increase in the turbidity in the BAC effluent to 6.11 NTU increased the fouling rate by more than 100%. The results demonstrated that the polysaccharide, not the protein, constituents of biopolymers were responsible for membrane fouling. Membrane fouling was closely associated with a small fraction of polysaccharides in the feed water. Inorganic particles exacerbated membrane fouling only when the concentration of fouling-inducing polysaccharides in the feed water was relatively high. The combined fouling was largely reversible, and polysaccharides were the predominant substances responsible for irreversible fouling.

Haloacetic acids in swimming pool and spa water in the United States and China

The objective of this study is to investigate the occurrence of haloacetic acids (HAAs), a group of disinfection byproducts, in swimming pool and spa water. The samples were collected from six indoor pools, six outdoor pools and three spas in Pennsylvania, the United States, and from five outdoor pools and nine indoor pools in Beijing, China. Five HAAs (HAA5), including monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, and dibromoacetic acid were analyzed. Total chlorine, pH and total organic carbon concentration were analyzed as well. Results indicated that the levels of HAA5 in swimming pools and spas in the United States ranged from 70 to 3980 μg·L−1, with an arithmetic average at 1440 μg·L−1 and a median level at 1150 μg·L−1. These levels are much higher than the levels reported in chlorinated drinking water and are likely due to organic matters released from swimmers’ bodies. The levels of HAA5 in swimming pools in China ranged from 13 to 332 μg·L−1, with an arithmetic average at 117 μg·L−1 and a median level at 114 μg·L−1. The lower HAA levels in swimming pools in China were due to the lower chlorine residuals. Results from this study can help water professionals to better understand the formation and stability of HAAs in chlorinated water and assess risks associated with exposures to HAAs in swimming pools and spas.