Jill Ruhsing Pan

Optimal condition for modification of chitosan: a biopolymer for coagulation of colloidal particles

Chitosan, an acetylated derivative of chitin, is a biodegradable cationic polymer. Chitosan can be a promising substitute for alum in the coagulation process, because of its potential feasibility in coagulation without posing any health threat as the residual aluminium and other synthetic polymers do. In this study, various pretreatment conditions were tested in search of the optimum chitosan modification. Batch tests with synthetic source water suggest that the optimal pretreatment condition to prepare modified chitosan coagulant is deacetylation by 45% alkali solution for 60 min, followed by dissolution in 0.1% hydrochloric acid.

Coagulation dynamics of fractal flocs induced by enmeshment and electrostatic patch mechanisms

The size and structure of flocs during floc formation were monitored for various coagulation mechanisms. Two distinctive mechanisms, namely, enmeshment and electrostatic patch, govern the dynamics of kaolin particles coagulation by polyaluminum chloride (PACl). They were investigated by small angle static light scattering (SASLS) and solid-state (27)Al NMR. In addition, a novel wet SEM (WSEM) was used in-situ to image the morphology of the aggregate in aqueous solution. Synthetic suspended particles were coagulated by two PACl products, a commercial product (PACl) and one laboratory product (PACl-E). The PACl-E contained more than 60% Al(13) while the PACl contained only 7% Al(13), with large percentage of colloidal Al. For coagulation by PACl at neutral pH and high dosage where the strong repulsion between particles occurs, the enmeshment ruled by reaction-limited aggregation (RLA) results in larger sweep flocs as well as higher fractal dimensional structure. For coagulation by PACl-E at alkaline pH and low dosage, the flocs were coagulated predominately by electrostatic patch with Al(13) aggregates. At such condition, it is likely that diffusion-limited aggregation (DLA) predominately rule PACl-E coagulation. The fractal dimension (D(s)) values of PACl and PACl-E flocs formed at enmeshment and electrostatic patch increased with dosage, respectively. When breakage of flocs occurs, the breakage rate of PACl-E flocs is slower than that of sweep flocs. By WSEM imaging, the adsorption of spherical Al precipitates onto the particles was observed to form sweep flocs with a rough and ragged contour, while the PACl-E flocs were formed with a smooth and glossy structure.

Coagulation behavior of Al13 aggregates

The coagulation behavior of Al(13) aggregates formed in coagulation of kaolin was investigated by small angle static light scattering (SASLS), solid-state (27)Al NMR and tapping mode atomic force microscope (TM-AFM). A kaolin suspension was coagulated by PACl containing high content of Al(13) polycation (PACl-Al(13)). The results indicated that Al(13) was predominant in destabilizing kaolin particles for PACl-Al(13) coagulation even though at alkaline pH (pH 10). At such high pH, Al(13) aggregates were observed when the dosage of PACl-Al(13) was increased. In addition, the mechanism of coagulation by PACl-Al(13) at alkaline pH was affected by dosage. When the dosage was insufficient, coagulation was caused by electrostatic patch, which led to compact flocs with high fractal dimension (D(f)). Interparticle bridging dominated the coagulation when the coagulant dosage approached the plateau of adsorption, which caused the looser flocs with low D(f). The in-situ AFM scanning in liquid system proved that the existence of linear Al(13) aggregates composed of a chain of coiled Al(13) in coagulation by PACl-Al(13) at a high dosage and alkaline pH. Meanwhile, several coiled Al(13) aggregates with various dimensions were observed at such condition.

Effect of Al(III) speciation on coagulation of highly turbid water

In Taiwan, the turbidity of raw water for fresh water treatments can sometimes reach as high as 40000 NTU due to intensive rainfall, especially in typhoon seasons. In response, water works often apply large quantities of coagulants such as polyaluminium chloride (PACl). In this study, simulated and natural highly turbid water was coagulated with two PACls, a commercial product (PACl-1) and a laboratory product (PACl-E). The Al species distributions of PACl-1 and PACl-E under various pH conditions were determined, and the corresponding coagulation efficiency was evaluated. The PACl-E has a wider range of operational pH, while the efficiency of PACl-1 peaks at around neutral pH. For simulated water up to 5000 NTU, the PACl-E was superior to PACl-1 at low dosage and in the pH range studied. Similar results were discovered with natural water, except that when the turbidity was extremely high, the coagulation efficiency of PACl-E decreased significantly due to the presence of large amounts of organic matter. The coagulation of PACl-E was closely related to the content of polycationic aluminium (Al(13)) while that of PACl-1 was dictated by the amount of Al(c). The sludge from PACl-E coagulation had better dewaterability when the optimum dosage was applied. The experimental results suggest that for natural water up to 5000 NTU, PACl containing high Al(13) species is recommended for coagulation. In cases when the water contains high organic matter, efficient coagulation depends upon enmeshment by amorphous aluminium hydroxide.

Time requirement for rapid-mixing in coagulation

Abstract Mechanisms of coagulation have been widely investigated by many researchers. Little is understood about the critical role of rapid-mixing in water treatment. In this study, a photometric dispersion analyzer (PDA) was employed to monitor clay coagulation by alum and polymeric aluminum chloride (PACl). The effect of rapid-mixing time on particle removal was studied. A standard jar test was performed to determine the degree of destabilization of particles. Maximum ratio outputs of PDA measurements were inversely related to residual turbidities. Adequate rapid-mixing times evaluated from both PDA and mixing tests were close to each other. Studies of coagulation mechanisms at various rapid-mixing times indicated that mixing time had significant impact on charge neutralization and sweep coagulation. The difference in residual turbidity can be attributed to the characteristics of microflocs. The rise in residual turbidity in sweep coagulation suggested the breakage of microflocs.

FILTRATION BEHAVIORS OF GIARDIA AND CRYPTOSPORIDIUM}IONIC STRENGTH AND PH EFFECTS

The laboratory-scale filtration tests of Giardia cysts and Cryptosporidium oocysts in both 2 mm-phi glass beads and 2 mm-phi polystyrene beads filters were conducted to investigate their filtration behaviors. The protozoan parasites were used as target particles, while the chemical system altered by changing the electrolyte concentration and pH. The results significantly indicate that ionic strength have a positive effect on the removal efficiencies for Giardia cysts and Cryptosporidium oocysts. The removal efficiency of two filters for Giardia cysts slightly decreased from pH 2.4 to 8.7 and decreased significantly in pH as pH up to 8.7, while that for Cryptosporidium slightly rippled beyond pH 8.7, and with the decrease in pH up to pH 8.7. The experimental collision efficiencies from the interactions between colloids and the filter media were calculated with a semi-empirical approach of the single sphere model and clean-bed filtration theory. The results also indicated that experimental collision efficiencies for (oo)cysts corresponded to the (oo)cysts removal efficiencies in all trials, and oocysts exhibits higher collision efficiencies than cysts.

Behavior of Membrane Scaling During Crossflow Filtration in the Anaerobic MBR System

Abstract Fouling of membrane bioreactor (MBR) has been studied intensively. Because of the high concentration of carbonates, scaling can be a serious problem in anaerobic bioreactor, which attracts little attention. In this study, the wastewater was treated with an anaerobic process followed by either a submerged or a side‐stream aerobic membrane reactor. The wastewater was spiked with calcium to investigate the effect of scaling on membrane filtration. Very little scaling was detected in the external membrane system (the side‐stream MBR). Results from chemical cleaning of internal membrane system indicated that the flux decline caused by membrane scaling was far more severe than that by membrane fouling. However, the flux decline from membrane scaling can be effectively recovered by the chemical cleaning of EDTA and NaOCl.

Enhance the photocatalytic activity for the degradation of organic contaminants in water by incorporating TiO2 with zero-valent iron.

Titanium dioxide (TiO(2)) has become the most popular photocatalyst in treating persistent organic pollutants. The main disadvantage of TiO(2) is the diminishing photocatalytic activity over time due to the electron-hole pair recombination. Many studies have aimed to prolong the photocatalytic life of TiO(2). Among them, incorporation of zero-valent iron (ZVI) is one of the approaches. In this study, a novel nano TiO(2)/Fe(0) composite (NTFC) was synthesized from a nano neutral TiO(2) sol and a nano zero-valent iron (nZVI), both prepared in our laboratory. The structure, composition and physical property of the NTFC are characterized. The photocatalytic activity of the NTFC was evaluated by the reductive decolourization of an azo dye, Acid Black-24 (AB-24), and was found superior to those of nZVI and nano neutral TiO(2) sol. Evidence suggests that the enhanced activity of NTFC is highly correlated to the ratio of ferrous to ferric ion in the system. The quantities of ferrous and ferric ions in the nZVI and NTFC systems were monitored separately. In the nZVI system, the concentration of ferric ions decreased significantly with time while a high level of ferrous ions was maintained in the NTFC suspension. The ferrous/ferric ratio of the NTFC suspension was substantially increased after irradiation by UV. Evidence from EPR analysis suggests that the excited electrons in the conduction band of the TiO(2) can be trapped by the half reaction of Fe(3+)/Fe(2+), reducing the probability of electron-electron hole pair recombination and sustaining the catalytic life of TiO(2). Corrosion tests further proved that by incorporating TiO(2) with zero-valent iron the surface oxidation of nZVI can be effectively prevented.

Correlation between dewatering index and dewatering performance of three mechanical dewatering devices

Abstract Efficient sludge conditioning can improve sludge dewatering characteristics and promote the separation of flocs from the liquid phase to achieve a high solid content. To optimize this process, a reliable dewatering index must be established. Although numerous researches investigated sludge conditioning, few are devoted to the correlation between the conditioning index and the dewatering efficiency. In this study, sludges were conditioned with both synthetic and natural polymers, and their dewatering characteristics after three different mechanical dewatering devices, the filter press, belt press, and the centrifuge, were compared. The result shows that the traditionally used dewatering index may not reflect sludge conditioning properly. Correlation between sludge conditioning indices and dewaterability depends upon both the type of sludge and the means of sludge conditioning and dewatering. It is therefore concluded that dewatering index must be properly chosen for optimum sludge conditioning.

Fouling Mitigation by TiO2 Composite Membrane in Membrane Bioreactors

Membrane bioreactors (MBRs) have attracted widespread attention in advanced water treatment because of their production of consistent and high-quality effluent. However, membrane fouling during operation has greatly hindered their application. Studies have sug- gested that a coating of TiO2 nanoparticles on membranes may reduce membrane fouling by enhancing the hydrophilicity of the membrane. In this study, two membranes were coated with TiO2 nanoparticles for membrane fouling mitigation. To evaluate the filtration performance of the TiO2 composite membranes, a synthetic wastewater was prepared to model the municipal wastewater for the MBR operation. The mixed liquor from the MBR was used in the filtration test to evaluate the performance of the TiO2 composite membrane. Filtration tests showed that membrane fouling was reduced substantially, which was attributable to the increased hydrophilicity of the membrane. Results also shows that optimal amount of coating is important in fouling mitigation. An ultrasonic washing test suggests that most of the TiO2 particles were firmly coated on the surface of the composite membrane. DOI: 10.1061/(ASCE)EE.1943-7870.0000419.