Comparison of coagulation behavior and floc structure characteristic of different polyferric-cationic polymer dual-coagulants in humic acid solution

Three polyferric-cationic polymer dual-coagulants were comparatively evaluated in terms of coagulation behavior and floc structure characteristic in the coagulation of humic acid (HA) solution. The first dual-coagulant, PFC-PDADMAC, was prepared by premixing of polyferric chloride (PFC) and polydiallyldimethylammonium (PDADMAC) before dosing. The other two were achieved by dosing PFC and PDADMAC in different order. For the given neutral condition, all three dual-coagulants gave similar HA removal before reaching optimal dosage. The strongest charge neutralization and narrowest effective coagulation dosage range were obtained when PFC was dosed firstly. While the weakest charge neutralization and the broadest effective coagulation dosage were obtained when PDADMAC was used as the primary coagulant. The HA removal of all three dual-coagulants was slightly pH dependent for optimum coagulant doses. Fe(III) hydrolysis species distributions of the dual-coagulants in coagulation process were measured by ferron method. PFC-PDADMAC gave the highest content of active Fe(III) coagulating species which is responsible for the coagulation performance of ferric coagulant. The evolution of floc size and floc fractal dimension (D_f) in coagulation process was measured under optimum dose and neutral condition by laser diffraction instrument and small-angle laser light scattering (SALLS), respectively. All three dual-coagulants gave similar final floc size but different floc growth rate and floc structure. Both the growth rate and D_f were in the same order: PFC dosed firstly > PDADMAC dosed firstly > PFC-PDADMAC.     

Consecutive chemical cleaning of fouled PVC membrane using NaOH and ethanol during ultrafiltration of river water

Chemical cleaning of fouled hollow-fiber polyvinyl chloride (PVC) membrane with the consecutive use of NaOH and ethanol during ultrafiltration of river water was investigated in the study. Results showed that through the chemical cleaning with 1% NaOH for 30 min, a negative cleaning efficiency of −14.6% was observed for the PVC membrane. This might be due to the increase of membrane hydrophobicity, which was reflected by the increase of contact angle from 69.7° to 87.6°. On the other hand, the cleaning efficiency of 85.1% was obtained by the consecutive cleaning with 30 min of 1% NaOH and 30 min of ethanol. Individual ethanol cleaning could remove 48.5% of the irreversible resistance, indicating that NaOH cleaning also made its contribution (36.6%) to the removal of membrane foulants. Scanning electronic microscopy (SEM) and atomic force microscopy (AFM) analyses demonstrated that both NaOH and ethanol were not only able to eliminate the foulants on membrane surface, but also able to remove the in-pore fouling of the PVC membrane. The synergetic effects for removing membrane foulants were observed between the NaOH and ethanol. Furthermore, ethanol could also restore the hydrophilicity of the membrane by decreasing the contact angle from 87.6° to 71.4°. Considering that ethanol is easy to be used and reclaimed, the consecutive chemical cleaning by alkali and ethanol is recommended for PVC membrane in filtration of surface water.     

Critical flux and chemical cleaning-in-place during the long-term operation of a pilot-scale submerged membrane bioreactor for municipal wastewater treatment

The critical flux and chemical cleaning-in-place (CIP) in a long-term operation of a pilot-scale submerged membrane bioreactor for municipal wastewater treatment were investigated. Steady filtration under high flux (30 L/(m² h)) was successfully achieved due to effective membrane fouling control by sub-critical flux operation and chemical CIP with sodium hypochlorite (NaClO) in both trans-membrane pressure (TMP) controlling mode (cleaning with high concentration NaClO of 2000-3000 mg/L in terms of effective chorine was performed when TMP rose to 15 kPa) and time controlling mode (cleanings were performed weekly and monthly respectively with low concentration NaClO (500-1000 mg/L) and high concentration NaClO (3000 mg/L)). Microscopic analysis on membrane fibers before and after high concentration NaClO was also conducted. Images of scanning electron microscopy (SEM) and atomic force microscopy (AFM) showed that NaClO CIP could effectively remove gel layer, the dominant fouling under sub-critical flux operation. Porosity measurements indicated that NaClO CIP could partially remove pore blockage fouling. The analyses from fourier transform infrared spectrometry (FTIR) with attenuated total reflectance accessory (ATR) and energy dispersive spectrometer (EDS) demonstrated that protein-like macromolecular organics and inorganics were the important components of the fouling layer. The analysis of effluent quality before and after NaClO CIP showed no obvious effect on effluent quality.     

Characteristic transformation of humic acid during photoelectrocatalysis process and its subsequent disinfection byproduct formation potential

In this study, degradation of humic acid (HA) via photoelectrocatalysis (PEC) process and corresponding disinfection byproduct formation potential (DBPFP) were investigated. Particularly, structure variation and subsequent DBPFP of HA during PEC treatment were correlated. The PEC process was found to be effective in reducing dissolved organic carbon concentration by 75.0% and UV absorbance at 254 nm by 92.0%. Furthermore, 90.3% of haloacetic acids formation potential and 89.8% of trihalomethanes formation potential were reduced within 180 min. Based on molecular weight and resin fraction results, it was demonstrated that HA with large aromatic, hydrophobic and long aliphatic chain organics were transformed into small and hydrophilic organics during PEC process. Combined with the fourier transform infrared spectra and ¹³C nuclear magnetic resonance spectra analysis of HA fractions, it was concluded that phenolic hydroxyl and conjugated double bonds tended to be attacked by hydroxyl radicals during PEC process; these groups were reactive with chlorine to produce disinfection byproducts (DBP), especially trihalomethane and trichloroacetic acid. By contrast, amino, carboxyl and alcoholic hydroxyl groups were relatively difficult to be oxidized during PEC process; these groups had the potential to form dichloroacetic acid during chlorination. Results of these studies confirmed that PEC process was a safe and effective technique to decrease DBP formation significantly in water treatment plant.     

Mn oxide coated catalytic membranes for a hybrid ozonation-membrane filtration: Comparison of Ti, Fe and Mn oxide coated membranes for water quality

In this study the performance of catalytic membranes in a hybrid ozonation-ceramic membrane filtration system was investigated. The catalytic membranes were produced by coating commercial ceramic ultrafiltration membranes with manganese or iron oxide nanoparticles using a layer-by-layer self-assembly technique. A commercial membrane with a titanium oxide filtration layer was also evaluated. The performance of the coated and uncoated membranes was evaluated using water from a borderline eutrophic lake. The permeate flux and removal of the organic matter was found to depend on the type of the metal oxide present on the membrane surface. The performance of the manganese oxide coated membrane was superior to that of the other membranes tested, showing the fastest recovery in permeate flux when ozone was applied and the greatest reduction in the total organic carbon (TOC) in the permeate. The removal of trihalomethanes (THMs) and haloacetic acids (HAAs) precursors using the membrane coated 20 times with manganese oxide nanoparticles was significantly better than that for the membranes coated with 30 or 40 times with manganese oxide nanoparticles or 40 times with iron oxide nanoparticles.     

Performing a microfiltration integrated with photocatalysis using an Ag-TiO2/HAP/Al2O3 composite membrane for water treatment: Evaluating effectiveness for humic acid removal and anti-fouling properties

Membrane filtration has been increasingly used for water treatment and wastewater reclamation in recent years. To further improve the effectiveness of membrane process and reduce membrane fouling, a highly reactive photocatalytic membrane, Ag-TiO₂/hydroxiapiate (HAP, Ca₁₀(PO₄)₆(OH)₂)/Al₂O₃, was employed to realize microfiltration (MF) coupling photocatalysis for surface water treatment. The effectiveness on the potential of membrane was investigated by removing humic acid (HA) test under different feed total organic carbon (TOC), light intensity and transmembrane pressure (TMP). The HA removal and anti-fouling property of as-prepared membrane was improved under UV irradiation, likely due to photocatalytic degradation of foulants along with filtration simultaneously. Under given feed water composition, increasing the light intensity resulted in increased removal of HA from aqueous solution. However, a limiting TMP seems to exist beyond which the increased HA removal cannot be sustained. Fouling behavior analysis indicated that the transition in fouling mode from initial pore blocking to cake filtration occurred much slower as UV irradiated. Furthermore, a superior efficiency on removal of trace organic contaminants, as well as milder flux reduction, was presented from surface water treatment, which demonstrated that the integrated system with enhanced performance is foreseen as an emerging technique for water treatment.     

Removal of co-present chromate and arsenate by zero-valent iron in groundwater with humic acid and bicarbonate

The interactions of co-present Cr(VI) and As(V), and the influences of humic acid and bicarbonate in the process of Cr(VI) and As(V) removal by Fe⁰ were investigated in a batch setting using simulated groundwater with 5 mM NaCl, 1 mM Na₂SO₄, and 0.8 mM CaCl₂ as background electrolytes at an initial pH value of 7. Cr(VI) and As(V) were observed to be subject to different impacts induced by co-existing As(V) or Cr(VI), humic acid and bicarbonate, originating from their distinct removal mechanisms by Fe⁰. Cr(VI) removal is a reduction-dominated process, whereas As(V) removal principally involves adsorption onto iron corrosion products. Experimental results showed that Cr(VI) removal was not affected by the presence of As(V) and humic acid. However, As(V) removal appeared to be inhibited by co-present Cr(VI). When the Cr(VI) concentration was 2, 5, and 10 mg/L, in the absence of humic acid and bicarbonate, As(V) removal rate constants were decreased by 27.9%, 49.0%, and 61.2%, respectively, which probably resulted from competition between Cr(VI) and As(V) for adsorption sites of the iron corrosion products. Furthermore, the presence of humic acid significantly varied As(V) removal kinetics by delaying the formation and aggregation of iron hydroxides due to the formation of soluble Fe-humate complexes and stably dispersed fine iron hydroxides colloids. In the presence of bicarbonate, both Cr(VI) and As(V) removal was increased and the inhibitory effect of Cr(VI) on As(V) removal was suppressed, resulting from the buffering effects and the promoted iron corrosion induced by bicarbonate, and the formation of CaCO₃ in solution, which enhanced As(V) adsorption.     

Synergistic effect of coupling zero-valent iron with iron oxide-coated sand in columns for chromate and arsenate removal from groundwater: Influences of humic acid and the reactive media configuration

A column study was conducted using a combination of zero-valent iron (Fe⁰) and iron oxide-coated sand (IOCS) for removing Cr(VI) and As(V) from groundwater. The removal efficiency and mechanism of Cr(VI) and As(V), the effects of humic acid (HA), and the various configurations of Fe⁰ and IOCS were investigated. The results showed that the use of an Fe⁰ and IOCS mixture in a completely mixed configuration can achieve the highest removal of both Cr(VI) and As(V), whilst the effects of HA were marginal in using these reactive materials. The solid phase analysis revealed the occurrence of the synergistic effect in these reactive materials as Fe²⁺ can be adsorbed onto the IOCS and transform the iron oxides to magnetite, providing more reactive surface area for Cr(VI) reduction and reducing the passivation on the Fe⁰. As(V) can then be removed by adsorption onto these iron corrosion products. HA can be adsorbed onto the IOCS so that the impacts of the deposition of HA aggregates on the Fe⁰ surface can be reduced, thus enhancing the Fe⁰ corrosion.