Effects of humic acid on physical and hydrodynamic properties of kaolin flocs by particle image velocimetry

The physical and hydrodynamic properties of kaolin flocs including floc size, strength, regrowth, fractal structure and settling velocity were investigated by in situ particle image velocimetry technique at different humic acid concentration. Jar-test experimental results showed that the adsorbed humic acid had a significant influence on the coagulation process for alum and ferric chloride. Kaolin flocs formed with the ferric chloride were larger and stronger than those for alum at same humic acid concentration. Floc strength and regrowth were estimated by strength factor and recovery factor at different humic acid concentration. It was found that the increased humic acid concentration had a slight influence on the strength of kaolin flocs and resulted in much worse floc regrowth. In addition, the floc regrowth after breakage depended on the shear history and coagulants under investigation. The changes in fractal structure recorded continuously by in situ particle image velocimetry technique during the growth-breakage-regrowth processes provided a supporting information that the kaolin flocs exhibited a multilevel structure. It was proved that the increased humic acid concentration resulted in decrease in mass fractal dimension of kaolin flocs and consequently worse sedimentation performance through free-settling and microbalance techniques.     

Enhanced coagulation for high alkalinity and micro-polluted water: the third way through coagulant optimization

Conventional coagulation is not an effective treatment option to remove natural organic matter (NOM) in water with high alkalinity/pH. For this type of water, enhanced coagulation is currently proposed as one of the available treatment options and is implemented by acidifying the raw water and applying increased doses of hydrolyzing coagulants. Both of these methods have some disadvantages such as increasing the corrosive tendency of water and increasing cost of treatment. In this paper, an improved version of enhanced coagulation through coagulant optimization to treat this kind of water is demonstrated. A novel coagulant, a composite polyaluminum chloride (HPAC), was developed with both the advantages of polyaluminum chloride (PACl) and the additive coagulant aids: PACl contains significant amounts of highly charged and stable polynuclear aluminum hydrolysis products, which is less affected by the pH of the raw water than traditional coagulants (alum and ferric salts); the additives can enhance both the charge neutralization and bridging abilities of PACl. HPAC exhibited 30% more efficiency than alum and ferric salts in dissolved organic carbon (DOC) removal and was very effective in turbidity removal. This result was confirmed by pilot-scale testing, where particles and organic matter were removed synergistically with HPAC as coagulant by sequential water treatment steps including pre-ozonation, coagulation, flotation and sand filtration.     

Preparation, characterisation and application of novel composite coagulants for surface water treatment

The development of the Inorganic Polymeric Flocculants (IPFs) can be regarded as significant progress in the coagulation-flocculation field. However, the IPFs may be less efficient when compared to the organic polymers (polyelectrolytes) regarding their aggregation abilities. In order to increase further their flocculation efficiency, the combination of a cationic IPF (polyaluminium chloride, PACl) and an anionic polyelectrolyte in one unique reagent is proposed in this study. During this investigation, several composite coagulants were prepared, which differ on the preparation method and polyelectrolyte content. Major typical properties of the prepared coagulants were examined, i.e. pH, turbidity, conductivity, Al species distribution. The composition, structure and morphology of the composite coagulants were studied in detail as well, with the application of FT-IR, XRD and SEM techniques. Their coagulation performance was investigated in the treatment of a model water sample (simulating surface water) and compared to the respective coagulation performance of PACl and the polyelectrolyte applied as separated reagents (common procedure). Finally, the kinetics of coagulation was studied with application of the Photometric Dispersion Analyser (PDA). From the results, it was revealed that interactions take place between the Al species and the polyelectrolyte molecules, which probably lead to the formation of new, “composite” species. The properties of the composite coagulants are significantly affected by these interactions, leading to more effective water treatment. The simplification of the overall treatment process and the cost-effectiveness are considered as the major advantages of the composite coagulants.     

Chemical aspects of coagulation using aluminum salts—I. Hydrolytic reactions of alum and polyaluminum chloride

This is the first of a two-part series of papers investigating the chemistry of Al coagulants. This paper examines hydrolysis reactions of alum and polyaluminum chloride (PAC1). Part II of the series addresses the coagulation of fulvic acid by these coagulants.

Monomeric, polymeric and precipitated Al were identified based on a timed spectrophotometric analysis. At typical A1 doses used in water treatment, alum showed no evidence of polymer formation. PAC1 consists of preformed polymers which are stable upon dilution below pH 6 and over the time frames encountered in water treatment. Solubility studies showed that alum and PAC1 precipitate to form different solid phases. Alum precipitates are adequately described by amorphous A1(OH)₃(s) solubility. The polymeric structure of PAC1 is retained upon precipitation yielding a solid phase with different light scattering characteristics, electrophoretic mobility and solubility than alum floc. A1 hydrolysis is interpreted as a coordination reaction between A1 and OH⁻. Effects of low temperature on alum are shown to be a result of changes in OH⁻ caused, in part, by the temperature dependence of the ion product of water. Hydrolysis products in PAC1 are preformed and therefore less sensitive to in situ hydrolysis than alum. Results suggest that when using alum, some of the adverse effects of low temperatures may be mitigated by an increase in pH, thereby maintaining a constant concentration of the complexing ligand, OH⁻.     

Breakage and re-growth of flocs formed by charge neutralization using alum and polyDADMAC

The formation, breakage and re-growth of flocs were investigated using alum and polyDADMAC to explore the reversibility of floc breakage. There is a significant reversibility of the breakage process, i.e. the broken flocs can re-grow to the size before breakage, when charge neutralization dominates the coagulation mechanism. However, for higher alum dosage, the break-up process displayed a distinct irreversibility. When coagulated in charge neutralization, the re-growth process of alum was nearly the same as that of polyDADMAC. The average size, coagulation rate and fractal dimension of flocs before and after breakage were nearly the same, including alum and polyDADMAC. While at higher alum dosage, the average size, coagulation rate and fractal dimension of flocs after breakage were much lower than that before breakage. Most important is that the number of small flocs after breakage and re-growth was much less than before breakage when charge neutralization dominated the coagulation mechanism. On the contrary, at higher alum dosage, the small flocs, after breakage and re-growth, increased. The fractal dimension of flocs with alum increased as coagulation time increased until a limiting floc size was reached, while for higher alum dosage, it decreased, whether before or after breakage. The determining parameter for floc re-growth is probably not the fractal dimension, but rather the chemical characteristics of the flocs surface.     

Interaction between Cryptosporidium oocysts and water treatment coagulants

The electrokinetic properties of gamma-irradiated Cryptosporidium oocysts in the presence of coagulants (ferric chloride and alum) and coagulant aids (DADMAC based cationic polyelectrolytes) have been studied. The zeta potential of the oocysts was unaffected by the addition of ferric chloride at all pH values (3-10) studied. Addition of alum resulted in reversal of the oocysts charge, which suggests that the initial stage in the coagulation process leading to floc formation proceeds via the adsorption of hydrolysed aluminium species. The cationic polyelectrolyte Magnafloc LT35 was adsorbed onto iron flocs at doses of 0.1 mg/L even against an electrostatic barrier. The cationic polyelectrolyte only adsorbed and caused charge reversal at the oocyst surface at around 0.4 mg/L, suggesting a lower affinity for this surface. These results indicate that the oocysts, unlike inorganic colloidal materials such as metal oxides, appear to possess a lower surface density of active or charged sites. The lower density of sites, combined with the rapid precipitation of iron salts, may be responsible for the lack of specific adsorption of either hydroxylated ferric species or primary iron hydroxide particles on the oocysts. Further, this suggests that a process of sweep flocculation, where oocysts are engulfed in flocs during coagulation and floc formation, is the more likely mechanism involved. By comparison, it is likely that the specific interaction of hydrolysed aluminium species with the oocysts surface would result in a stronger link at the oocyst-floc interface and that the flocculation process may initially proceed via charge neutralisation.     

Breakage and re-growth of flocs: effect of additional doses of coagulant species

Several polyaluminum chloride (PACl) coagulants were prepared, with different OH/Al ratios (B values), and characterized by Ferron assay. These were used in studies of floc formation, breakage and re-growth with kaolin suspensions under controlled shear conditions, using a continuous optical monitoring method. Particular attention was paid to the effect of small additional coagulant dosages, added during the floc breakage period, on the re-growth of broken flocs. The results showed that the re-growth ability was greatly dependent on the nature of the PACl species added as second coagulant. The re-growth ability of broken flocs was greatest when the second coagulant was PACl₀ (i.e. AlCl₃, with B = 0) and least with PACl₂₅ (B = 2.5). In the latter case there was no effect on floc re-growth, irrespective of the initial coagulant used. PACls with intermediate B values gave some improvement in floc re-growth, but less than that with PACl₀. Additional dosage of PACl₀ gave re-grown flocs about the same size or even larger than those before breakage. The re-growth of broken flocs is significantly correlated with the species Al_a (monomeric) and Al_b (polymeric), as determined by Ferron assay. The amorphous hydroxide precipitate formed from PACl₀, (mainly Al_a) can greatly improve the adhesion between broken flocs and give complete re-growth. However, for PACl₂₅, mostly composed of Al_b, the nature of the precipitate is different and there is no effect on floc re-growth.     

Effects of chemical amendments on aquatic floc structure, settling and strength

Using a shear-cell/flow-cell combination integrated with an inverted microscope, the behaviour of Hamilton Harbour sediments was studied mixed with three different amendments: alum, chitosan (both coagulants) and a polyacrylamide (a flocculant). Samples from the shear cell were drawn into the flow cell, where floc structure and size were assessed throughout the floc formation and breakage stages using computer image analysis. Settling velocity, density and porosity were also assessed, with results suggesting that amendment addition may be an effective method for the management of high-turbidity environments, provided there are no toxicological effects. In an assessment of performance, it was found that the polyacrylamide flocculant showed the greatest promise in reducing turbidity levels as it produced the largest flocs with the highest settling velocity. Although more prone to break-up, these flocs still remained larger than those formed with alum or chitosan at the same shear. All flocs, regardless of amendment, broke up due to a fracture mechanism rather than by microscale erosion. By improving our understanding of how these amendments may influence floc properties and behaviours, more effective management tools may be developed for the remediation and control of high-turbidity aquatic environments.     

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 ²⁷Al 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% Al₁₃ while the PACl contained only 7% Al₁₃, 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 Al₁₃ 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.     

Inactivation of particle-associated viral surrogates by ultraviolet light

This study investigated whether colloid-sized particles can enmesh and protect viruses from 254-nm ultraviolet (UV) light and sought to determine the particle characteristics (e.g. size, chemical composition) that are most relevant in causing a protective effect. Two viral surrogates (MS2 coliphage and bacteriophage T4), three types of particles (kaolin clay, humic acid powder, and activated sludge), two coagulants (alum and ferric chloride), two filtration conditions (none and 0.45 microm), and two UV doses (40 and 80 mJ/cm2 for MS2 coliphage; 2 and 7 mJ/cm2 for bacteriophage T4) were considered in a series of bench-scale UV collimated beam experiments. Transmission electron microscopy was used to qualitatively confirm the phage particle-association after coagulation. Humic acid and activated sludge floc particles shielded both viral surrogates to a statistically significant degree (with >99% confidence) relative to particle-free control conditions, while the kaolin clay particles provided no significant protection. The results of the study suggest that particles <2 microm in diameter are large enough to protect viruses from UV light and that particulate chemical composition (e.g. UV-absorbing organic content) may be a critical factor in the survival of particle-associated viruses during UV disinfection.     

Preparation and characterisation of new-polyaluminum chloride-chitosan composite coagulant

In this study, the formulation of a novel polyaluminum chloride-chitosan composite coagulant that improves the coagulation process for natural organic matter (NOM) removal was investigated. The performance of the composite coagulant was tested using two water sources (synthetic and natural water) to develop a better understanding on the behaviour of the composite coagulant. Fourier Transform-Infra red (FT-IR) spectroscopy, ferron analysis and zeta potential studies were performed to characterise the composite coagulant. FT-IR analysis showed that there is an intermolecular interaction between Al species and chitosan molecules, while ferron analysis indicated that the distributions of Al_a, Al_b, and Al_c in PACl-chitosan are different from those in PACl. At a low Al dosage (2.16 mg L⁻¹), a much higher removal of NOM from synthetic water, as evidenced from UV₂₅₄ and Dissolved Organic Carbon (DOC) measurements, was achieved by the composite coagulants in comparison to that removed by PACl or PACl and chitosan added separately. For natural water from the Myponga Reservoir, both polyaluminum chloride (PACl) and PACl-chitosan composite coagulants demonstrated similar dissolved organic carbon (DOC) percentage removal, whereas PACl-chitosan gave a slight improvement in removing the UV₂₅₄ absorbing components of NOM.     

Effects of coagulation processes on aldehydes formation in groundwater treated with common oxidative agents

This work is concerned with the effects of coagulation processes with two different coagulants (polyaluminum chloride (PACl) and Al2(SO4)3) on aldehydes formation during oxidation with common oxidants (ozone, chlorine and chlorine dioxide) in a particular groundwater source in Northern Banat region, Yugoslavia. Aldehydes concentrations in coagulated water were lower than in raw water. In contrast, obtained results showed that specific contents of these disinfection byproducts (microg mg(-1) TOC) showed an increase after coagulation processes in a number of samples. Results indicate that the choice of the coagulant-oxidant combination may be important as well as the type of filtration bed, retention time, and filter washing regime in the removal of aldehydes from water.     

Removal of particle-associated bacteriophages by dual-media filtration at different filter cycle stages and impacts on subsequent UV disinfection

This bench-scale study investigated the passage of particle-associated bacteriophage through a dual-media (anthracite-sand) filter over a complete filter cycle and the effect on subsequent ultraviolet (UV) disinfection. Two model viruses, bacteriophages MS2 and T4, were considered. The water matrix was de-chlorinated tap water with either kaolin or Aldrich humic acid (AHA) added and coagulated with alum to form floc before filtration. The turbidity of the influent flocculated water was 6.4+/-1.5 NTU. Influent and filter effluent turbidity and particle counts were measured as well as headloss across the filter media. Filter effluent samples were collected for phage enumeration during three filter cycle stages: (i) filter ripening; (ii) stable operation; and (iii) end of filter cycle. Stable filter operation was defined according to a filter effluent turbidity goal of <0.3 NTU. Influent and filter effluent samples were subsequently exposed to UV light (254 nm) at 40 mJ/cm(2) using a low pressure UV collimated beam. The study found statistically significant differences (alpha=0.05) in the quantity of particle-associated phage present in the filter effluent during the three stages of filtration. There was reduced UV disinfection efficiency due to the presence of particle-associated phage in the filter effluent in trials with bacteriophage MS2 and humic acid floc. Unfiltered influent water samples also resulted in reduced UV inactivation of phage relative to particle-free control conditions for both phages. Trends in filter effluent turbidity corresponded with breakthrough of particle-associated phage in the filter effluent. The results therefore suggest that maintenance of optimum filtration conditions upstream of UV disinfection is a critical barrier to particle-associated viruses.     

Dewatering of alumino-humic sludge: impacts of hydroxide

The paper draws together information on factors which influence the conditioning and dewatering behaviour of an alum sludge gained from the coagulation of a low-turbidity coloured water. A principal focus is the potential impact of aluminium hydroxide on the sludge character. Background information is provided on the composition of the source floc for the domain pH 6.0-6.5 and Al>2.0mg/l. From this, there were many pointers to the presence of Al(OH)(3)(s) within the floc. A series of comparisons were made between an alum sludge and a hydroxide suspension at a concentration equivalent to the coagulant fraction within the sludge. The parameters studied included floc size, floc density, polymer adsorption and dewatering behaviour at different time-scales. In all cases, there were strong similarities in the behaviour of the two suspensions-indicating the potential impact of the hydroxide. There was also evidence of common features being displayed by both the organic fractions and the hydroxide. It was suggested that some of the behavioural features might emanate from a common fractal structure within the source floc, the fractal dimension (approximately 1) being insensitive to composition.     

Influence of aging on phosphorus sorption to alum floc in lake water

Alum is often added to eutrophic lakes to limit the release of phosphorus from sediments. This study quantified the effect of age and extent of crystallization on the phosphate (PO4-P) sorption capacity of alum floc. Aluminum hydroxide flocs were formed from alum addition at a dose of 25 mg/L of Al3+ to Big Bear Lake waters returned to the laboratory; flocs were then aged for 4, 20, 50, 120, and 180 days in the treated lake waters. The physical and mineralogical properties of the alum floc were characterized using surface area and thermal analyses. Phosphate sorption to the floc was evaluated using filtered lake water and NaCl/NaHCO3 solutions spiked with PO4-P concentrations ranging from 0 to 100 mg/L. The Langmuir model provided reasonable fits to data (r2 = 0.97-1.00), from which sorption constants and sorption maxima were determined. Phosphate sorption decreased with increased floc age and crystallinity and decreased surface area. Phosphate sorption maximum of the alum floc aged for 6 months was about 50% lower than freshly precipitated floc, while the binding constant, Kads, decreased approximately 65% over this same time period.     

Re-use of water treatment works sludge to enhance particulate pollutant removal from sewage

This paper attempted to study the feasibility of reusing water treatment works sludge ("alum sludge") to improve particulate pollutant removal from sewage. The main issues focused upon were: (1) the appropriate dosage of the alum sludge, (2) the appropriate operating conditions, and (3) the possible mechanisms for enhancement by adding alum sludge. Actual alum sludge and sewage were applied to a series of jar tests conducted under various conditions. It has been found that both the SS and COD removal efficiencies could be improved by the addition of the alum sludge, which was mainly attributed to the removal of relatively fine particles with a size of 48-200 microm. The appropriate dosage of the alum sludge was determined to be 18-20 mg of Al/L. Increasing the mixing speed or reducing the floc size of the alum sludge enhanced the SS and COD removal and the dispersed alum sludge could remove particulate contaminants with smaller size than the raw sewage. ToF-SIMS evidence revealed that the aluminum species at the surface of the alum sludge were effectively utilized for improving the SS and COD removal. It was postulated that the sweep flocculation and/or the physical adsorption might play key roles in the enhancement of particulate pollutant removal from sewage.     

The perils of foreign intervention: the Nile perch disaster for Lake Victoria

The present study involved the use of a pilot scale water treatment plant to treat turbid surface water from a stream using processed Moringa oleifera seed and alum as primary coagulants. At low initial turbidity of 21.5 to 49.3 NTU, residual turbidities of 2.7, 1.8 and 1.4 NTU were achieved after treatment using Moringa oleifera, alum, and alum with Moringa oleifera as coagulant‐aid, respectively. For medium turbidities varying from 51.8 to 114 NTU, lowest residual turbidities of 2.9, 1.2 and 1.4 were achieved after treatment using Moringa oleifera, alum, and alum with Moringa oleifera as coagulant‐aid, respectively. For high turbidity varying from 163 to 494 NTU, minimum residuals of 1.4, 1.9 and 0.9 NTU were achieved after treatment using Moringa oleifera, alum, and alum with Moringa oleifera as coagulant‐aid, respectively. The proposed mechanism for turbidity removal by Moringa oleifera seed in this study is a combination of partial‐charge neutralization and micro‐bridging or an electrostatic patch mechanism based on the results of zeta potential measurements.     

Return sludge employed in enhancement of color removal in the integrally industrial wastewater treatment plant

In this study, the recirculation of chemical sludge and integrated sludge were employed for enhancement of color removal in an integrally industrial wastewater treatment plant. The jar test was conducted for simulating chemical coagulation process with different coagulants, aluminum sulfate and polyaluminum chloride (PACl), after different activated sludge systems (with air or high-purity oxygen). The results showed that with sludge recirculation the process of coagulation for the color removal has up to 35% enhancement in comparison with no sludge recirculation. Meanwhile, the color removal enhancement of coagulant aluminum sulfate was approximately 2 to 3-times the color removal enhancement of coagulant PACl at the optimum ratio of return sludge. In coagulation process with influent color of 1220 true color unit (TCU), the optimum dosage of return sludge was 3010mg/L, when coagulant PACl or aluminum sulfate was employed. In coagulation process with influent color of 536 TCU, the optimum dosages of return sludge were 2340 and 4680mg/L using PACl and aluminum sulfate, respectively.     

Novel ferromagnetic nanoparticle composited PACls and their coagulation characteristics

Effects of magnetic nanoparticles on inorganic coagulants and their coagulation performances were studied in the present work. The Fe₃O₄-SiO₂ core-shell particle (FSCSP) and superfine iron (SI), were compounded with polyaluminium chloride of basicity 2.0 (PACl2.0), providing magnetic PACl2.0s (MPACl2.0s). The physiochemical properties of ferromagnetic nanoparticles were investigated using transmission electron microscopy (TEM), the BET method and a zeta potentiometric analyzer. The Al species distributions of the MPACl2.0s and PACl2.0 were examined by liquid ²⁷Al NMR. Jar tests were employed to evaluate the coagulation performances. Floc properties were assessed by use of the electromotive microscope (EM) and small angle laser light scattering (SALLS). The results showed that modified layers of nanoparticles mitigated agglomeration. FSCSP had a larger specific area and pore volume than SI. The addition of ferromagnetic nanoparticles obviously increased the content of Al_un. MPACl2.0s performed better than PACl2.0 in turbidity removal and DOC removal when dosed less than 0.06 mmol/L as Al. Generally, PACl2.0 + FSCSP (50 mg/L) performed best. Large, loose and weak flocs were produced by MPACl2.0s, which were preferred for the magnetic powder recycling. A plausible structure, Al species-nanoparticles cluster, contributing to the unique properties of MPACl2.0 flocs, was proposed.     

Removal of THM precursors by coagulation or ion exchange

The removal of natural organic matter (NOM) from drinking water supplies can be achieved by different processes, among them coagulation and adsorption. Synthetic waters made from concentrates of humic substances from reservoir and river waters were tested in the laboratory for ease of removal of NOM by coagulation with cationic organic polymers and with alum, and by adsorption on anion exchangers. For polymers such as high molecular weight polydiallyldimethylammonium chloride (polyDADMAC) and cationic polyacrylamides of high charge, performance was nearly as effective as alum, with colour removals 86–100% of those obtained for alum. Ion exchange using the best commercially available resins designed for this purpose, a gel polystyrene and a macroporous acrylic resin, was more effective than alum treatment for two of the natural waters studied, but inferior for a third. The resins were overall superior to cationic polymers.

The NOM was separated into four fractions based on hydrophobic and hydrophilic properties. Alum was not as effective as ion exchange for the elimination of individual ionic NOM fractions. It was better than cationic polymers for removal of humic and fulvic acids, although polyDADMAC was as good for one water. For the removal of charged compounds alum then polyDADMAC were the best performers for that water. Unequivocal evidence was obtained that coagulants remove material that is not adsorbed by resins, and vice versa. A combination of coagulation with a cationic polymer and adsorption by an anion exchanger removed essentially all of the NOM. The preference of the coagulants was for the larger, more hydrophobic molecules, and of resins for smaller highly charged hydrophilic molecules. Each fraction had trihalomethane formation potentials in the range 11–24 μg/mg, except for one water that was more reactive. Hence, the actual amount of each fraction in the original water becomes a crucial factor.