Growth of Floc Structure and Subsequence Compaction into Smaller Granules through Breakup and Rearrangement of Aluminum Flocs in a Constant Laminar Shear Flow.

We have constructed an outer-cylinder-rotating Couette device for high-speed shear flow in laminar flow conditions and visualized the structure formation and subsequent rearrangement of PACl (flocculant made of aluminum hydroxide gel) and kaolinite flocs by visible light imaging. In a previous report, we analyzed the case of relatively low shear rate (G-value = 29 1/s) and confirmed that the flocculation process could be separated into two stages: a floc growth stage and a breakup/rearrangement stage. Once the large bulky flocs that reached the maximum size appeared, they rearranged and densified through structural fracture and rearrangement. In this report, this process was further investigated by conducting experiments under two different high shear rates (58 and 78 1/s) at which breakup and rearrangement became more pronounced, and three different aluminum kaolinite ratios (ALT ratios) that were over and under the optimum dosage (neutralization point by Zeta potential). Visualization results confirmed that, during the growth stage, the flocculation rate could be approximated by a scaling relationship between floc size and elapsed time, which depended on the ALT ratio. After reaching the maximum size, the floc rapidly became compact and dense following adsorption of the gel, incorporating fine fragments from erosion breakup. The over and under dosages created a lot of fragments of erosion breakup, but less so in the optimum dosage. In the optimum ALT ratio, fragments did not remain because they were incorporated into the flocs and densified, and the floc size was thought to be maintained. The floc circularity distribution peaked at around 0.6 and 1, suggesting that the flocs were spherical in shape due to erosion breakup.

Overlooked effect of ordinary inorganic ions on polyaluminum-chloride coagulation treatment.

Application of poly-aluminum chloride (PACl) coagulant is a popular mode of water treatment worldwide because of the high capacity of PACl to neutralize charge. The manufacture and use of PACls with various basicities in different regions around the world suggest that the characteristics of the raw water are important determinants of the efficacy of PACl application. However, attention has not been fully paid to the effects of water quality other than the substances to be removed. In this study, two typical PACls with different basicities were used to investigate why the performance of PACls depends on the characteristics of the raw water. We focused on the concentrations of inorganic ions in the raw water. Use of high-basicity PACl (HB-PACl) with a high content of polymeric-colloidal species (Alb+Alc) resulted in very slow floc formation and little turbidity removal in raw water with low concentrations of sulfate ions. The performance of the HB-PACl was inferior to that of normal-basicity PACl (NB-PACl), although the charge-neutralization capacity of the HB-PACl was higher. Rates of floc formation were strongly correlated with the rate of aluminum precipitation by hydrolysis reaction, which was identified as an indicator for evaluating the compatibility of raw water with PACl treatment. Among the common ions in natural water, the sulfate ion had the greatest ability to hydrolyze and precipitate PACl because of its divalency and tetrahedral structure. This conclusion followed from experimental results showing similar effects for selenate and chromate ions as for sulfate ions and somewhat smaller effects for thiosulfate ions. Bicarbonate ions and natural organic matter affected PACl hydrolysis-precipitation, but chloride ions, nitrate ions, and cations had little effect on PACl hydrolysis-precipitation. Interestingly, the abilities of sulfate ions to hydrolyze HB-PACl and NB-PACl were very similar, but bicarbonate ions were less effective in hydrolyzing HB-PACl than NB-PACl, and bicarbonate ions contributed little to the hydrolysis-precipitation of HB-PACl in raw water with normal alkalinity. Therefore, sufficient coagulation with HB-PACl therefore usually requires a certain concentration of sulfate ions in water to be treated. The implication is that which anions are most influential to the hydrolysis-precipitation of PACl, and thus to PACl's coagulation ability depends on the constituents of the PACl.

Hypersaline produced water clarification by dissolved air flotation and sedimentation with ultrashort residence times.

Treatment and reuse of some produced waters is made difficult due to their hypersalinity, high concentrations of myriad other dissolved and suspended components, specialized technology requirements (modularity, portability, and short residence times), and lack of existing information on their processing. In this work, produced water containing ∼100,000 mg/L total dissolved solids from the Permian Basin was coagulated with aluminum chlorohydrate (ACH) and flocculated with an anionic high molecular weight organic polymer prior to dissolved air flotation (DAF) and sedimentation to reduce turbidity to < 4 NTU and iron < 0.8 mg/L (>95% removal in both cases) with a total coagulation-flocculation-sedimentation/flotation residence time of only 5 min. Two advantages of DAF over sedimentation were noted: (i) DAF required only half the dosage of the pre-hydrolyzed ACH coagulant to remove ∼90% of turbidity and iron even without the organic polymeric flocculant and (ii) DAF even operated successfully without ACH coagulation (i.e., using only the organic polymeric flocculant) evidencing its lower chemical dosing needs. Further, DAF attained all water quality and operational goals at a recycle ratio of only 12% demonstrating that it outperformed sedimentation to generate clean brine at relatively reduced excess energies necessary for air saturation. Higher DAF recycle ratios reduced turbidity and iron removal possibly due to floc breakage. Colloids were effectively destabilized by double layer compression (due to high water salinity), charge neutralization (via adsorption of Al13 polycations), and enmeshment (precipitation of amorphous aluminum). They were flocculated via interparticle bridging (by the anionic organic polymeric flocculant) to create large, compact flocs facilitating ultrashort flotation/sedimentation times. Direct evidence for these individual coagulation and flocculation mechanisms were obtained using electrophoretic mobility measurements, thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, optical microscopy, computational image and video analysis, and scanning electron microscopy - energy dispersive X-ray spectroscopy.

Co-coagulation of micro-nano bubbles (MNBs) for enhanced drinking water treatment: A study on the efficiency and mechanism of a novel cleaning process.

MNBs (Micro-nano bubbles) are widely used in cleaning processes for environmental treatments, but few studies have examined the interaction of MNBs with coagulation. In this study, a novel process, i.e., MNBs-coagulation, was developed for enhanced drinking water treatment. The humic acid (HA) removal efficiency was used to evaluate the effectiveness of MNBs-coagulation for drinking water treatment. The hydrolysis component ratio of polymeric aluminum chloride (PACl) with and without MNBs, the complexation strength of HA and PACl, and flocculent functional group characterization were used to analyze the mechanism of the MNBs-coagulation process to enhance drinking water treatment. The results of a Jar test showed that the MNBs-coagulation process could improve the removal efficiency of HA (up to a 27.9% increase in DOC removal). In continuous-flow experiments to remove HA, MNBs-coagulation can increase the removal efficiency of UV254 by about 26.5% and with no significant change in turbidity. These results are attributed to the inherent hydroxyl radical generating properties of MNBs, the forced hydrolysis of PACl by MNBs to increase the Alc percentage, and the ability of MNBs to increase the complexation strength of HA with PACl. At the same time, the MNBs-coagulation process has a strong anti-interference ability, almost no interference from anions and cations such as Cl-, SO42- and Ca2+, and has a good performance in natural surface water. In summary, MNBs-coagulation has strong potential for practical applications to enhance the efficiency of drinking water treatment.

Effect of type of coagulants on removal efficiency and removal mechanisms of antibiotic resistance genes in anaerobic digestion of primary sludge produced via a chemically enhanced primary treatment process.

The potential impact of the trivalent coagulant cations on the removal mechanisms, removal efficiencies and removal patterns of antibiotic resistance genes (ARGs) during anaerobic digestion (AD) of chemically enhanced primary treatment sludge (CEPTS) was investigated using polyaluminium chloride (PACl), ferric chloride (FeCl3) and mixed FeCl3-PACl. The removal efficiency of 23 ARGs and intI1 improved to 72.1% in AD of primary sludge with 100 mg/L FeCl3 and was lowest (only 54.4 %) in AD of primary sludge with 25 mg/L PACl. The removal of ARGs in AD of CEPTS with addition of single or mixed types of Al-based coagulant began to increase rapidly at the onset of batch operation. On the other hand, both the rapid increase in the removal efficiency of ARGs in AD with FeCl3 and the maximum removal efficiency were attained later than in the other ADs.

Stray particles as the source of residuals in sand filtrate: Behavior of superfine powdered activated carbon particles in water treatment processes.

Although superfine powdered activated carbon has excellent adsorption properties, it is not used in conventional water treatment processes comprising coagulation-flocculation, sedimentation, and sand filtration (CSF) due to concerns about its residual in treated water. Here, we examined the production and fate of very fine carbon particles with lacking in charge neutralization as a source of the residual in sand filtrate after CSF treatment. Almost all of the carbon particles in the water were charge-neutralized by coagulation treatment with rapid mixing, but a very small amount (≤0.4% of the initial concentration) of very fine carbon particles with a lesser degree of charge neutralization were left behind in coagulation process. Such carbon particles, defined as stray carbon particles, were hardly removed by subsequent flocculation and sedimentation processes, and some of them remained in the sand filtrate. The concentration of residual carbon particles in the sand filtrate varied similarly with that of the stray carbon particles. The stray and residual carbon particles were similarly smaller than the particles before coagulation treatment, but the residual carbon particles had less charge neutralization than the stray carbon particles. The turbidity of water samples collected after sedimentation was not correlated with the residual carbon concentration in the sand filtrate, even though it is often used as an indicator of treatment performance with respect to the removal of suspended matter. Based on these findings, we suggest that reduction of the amount of stray particles should be a performance goal of the CSF treatment. Examining this concept further, we confirmed that the residence time distributions in the coagulation and flocculation reactors influenced the concentration of stray carbon particles and then the residual carbon particle concentration in sand filtrate, but found that the effect was dependent on coagulant type. A multi-chambered-reactor configuration lowered both the stray carbon particle concentration after coagulation treatment and the residual carbon particle concentration in sand filtrate compared with a single-chambered reactor configuration. When a normal basicity PACl that consisted mainly of monomeric Al species was used, the stray carbon particle concentration was decreased during coagulation process and then gradually decreased during subsequent flocculation process because the monomeric Al species were transformed to colloidal Al species via polymeric Al species. In contrast, when a high-basicity PACl that consisted mostly of colloidal Al species was used, coagulation treatment largely decreased the stray carbon particle concentration, which did not decrease further during subsequent flocculation process. These findings will be valuable for controlling residual carbon particles after the CSF treatment.

Experimental investigation and modelling the effect of humic acid on coagulation efficiency for sludge blanket clarifier.

The physicochemical process of coagulation has largely been used for turbidity removal for water treatment. However, lately, the intrusion of NOM (Natural organic matter) in the surface water sources due to climate change has impeded the dosing approaches and has presented a requirement to evaluate the effect of NOM on turbidity removal efficiency and the performance of coagulation reactors in general. In this work, a previously developed performance model for hydraulic flocculators was modified and tested for a sludge blanket clarifier (SBC) which is a type of hydraulic flocculator. The experimental runs were conducted by preparing synthetic sample waters by using humic acid (for NOM) and kaolin clay (for turbidity). PACl (Poly aluminium chloride) was used as a coagulant. The expression of attachment efficiency has been modified to include the interactions of humic acid (HA) and kaolin, which were not previously accounted for in the model. The coverage functions were used to calculate the attachment efficiency of HA-PACl and PACl-Clay. The standalone coverage function ГPACl-HA efficiently predicted the doses where the removal efficiency was maximum. However, the coverage function ГClay-PACl was impacted by the hydrodynamic conditions in SBC and over-speculated the Clay-PACl interactions. The RMSE value was low for the modified equation indicating that in SBC the interactions between the organic and inorganic impurities are significant. The HA-Kaolin interactions were found to be significant in the modified model in case of a low HA range of 4 and 8 mg/L of HA.

Coagulation-flocculation: a potential application for mosquito Larval Source Management (LSM).

Larval mosquitoes have a more limited home range and lower resistance to adverse environment than adults, thus can be ideal targets for vector control in some cases. Coagulation-flocculation technology, which could be used for water treatment in breeding sites of several vector mosquito species, can significantly change both the distribution of organic particles and surface sediment characteristics in water environment. The aim of this study was to explore the effect, principle and possibility of using coagulation-flocculation technology in immature mosquitoes killing. In this study, dechlorinated water was treated with Poly Aluminum Chloride (PACl, sewage treatment using), and we observed the impacts of PACl treatment on the development and survival of immature Culex pipiens pallens mosquitoes. When exposed to PACl treatment, physical effect is believed to be a main reason of coagulation-flocculation caused high larvae mortality: Ⅰ) alum floc layer increases the difficulty of larvae foraging, leads larvae starving to death; (Ⅱ) the little floc particles could attach to the lateral hair of larvae, which impede floatation process and then surface respiration by larval mosquitoes. The alum floc layer had a good killing effect on the mosquito larvae, presented the half lethal time (LT50) of 2d, the 90% lethal time (LT90) of 8.7±7.3 ∼ 14±4.5 d, and the pupation rate of 0 ∼ (6.5±0.5)%, respectively. Our results indicates alum floc, produced by PACl coagulation-flocculation, was shown to be highly active against 1st∼2nd instar larvae, the high mortality rate of immature mosquitoes as a result of physical effect. The observations suggest that coagulation-flocculation technology offers a novel potential approach to a sustainable and low-impact mosquito control method.

Sulfate ion in raw water affects performance of high-basicity PACl coagulants produced by Al(OH)3 dissolution and base-titration: Removal of SPAC particles by coagulation-flocculation, sedimentation, and sand filtration.

Many PACl (poly-aluminum chloride) coagulants with different characteristics have been trial-produced in laboratories and commercially produced, but the selection of a proper PACl still requires empirical information and field testing. Even PACls with the same property sometimes show different coagulation performances. In this study, we compared PACls produced by AlCl3-titration and Al(OH)3-dissolution on their performance during coagulation-flocculation, sedimentation, and sand filtration (CSF) processes. The removal targets were particles of superfine powdered activated carbon (SPAC), which are used for efficient adsorptive removal of micropollutants, but strict removal of SPAC is required because of the high risk of their leakage after CSF. PACls of high-basicity produced by AlCl3-titration and Al(OH)3-dissolution were the same in terms of the ferron assay and colloid charge, but their performance in CSF were completely different. High-basicity Al(OH)3-dissolution PACls formed large floc particles and yielded very few remaining SPAC particles in the filtrate, whereas high-basicity AlCl3-titration PACls did not form large floc particles. High-basicity PACls produced by Al(OH)3-dissolution were superior to low-basicity PACl in lowering remaining SPAC particles by the same method because of their high charge neutralization capacity, although their floc formation ability was similar or slightly inferior. However, high-basicity Al(OH)3-dissolution PACl was inferior when the sulfate ion concentration in the raw water was low. Sulfate ions were required in the raw water for high-basicity PACls to be effective in floc formation. In particular, very high sulfate concentrations were required for high-basicity AlCl3-titration PACls. The rate of hydrolysis, which is related to the polymerization of aluminum species, is a key property, besides charge neutralization capacity, for proper coagulation, including formation of large floc particles. The aluminum species in the high-basicity PACls, in particular that produced by AlCl3-titration, was resistant to hydrolysis, but sulfate ions in raw water accelerated the rate of hydrolysis and thereby facilitated floc formation. Normal-basicity Al(OH)3-dissolution PACl was hydrolysis-prone, even without sulfate ions. Aluminum species in the high-basicity AlCl3-titration PACl were mostly those with a molecular weight (MW) of 1-10 kDa, whereas those of high-basicity Al(OH)3-dissolution PACls were mostly characterized by a MW > 10 kDa. Normal-basicity Al(OH)3-dissolution PACl was the least polymerized and contained monomeric species.

Effect of in-situ formed Al hydrates through long-term aging on enhanced particle destabilization by PACl coagulation.

Aging of polyaluminum chloride (PACl) coagulants could significantly influence hydrolyzed Al speciation ruling coagulation performance. The goal of this study was to investigate Al species transformation through long-term aging and its impact on coagulation performance. Two kinds of commercial coagulants (PACl-1 and PACl-2) were stored for in-situ aging tests in six months to evaluate Al species variation with time and the performance of coagulation with natural turbid waters. The results showed that CaSO4 precipitation easily occur in a commercial PACl coagulant with time as it contains SO42- and Ca2+. It also activates the precipitation of gibbsite Al(OH)3 in PACl with aging. Through 180 days aging, both monomeric Al (Ala) and polymeric Al (Alb) substantially transform into colloidal Al (Alc) and precipitated for both PACl coagulants. At low turbidity (10 NTU), PACl-1 has a superior turbidity reduction rate than PACl-2, while PACl-2 performs a little better turbidity reduction at high turbidity (1000 NTU) regardless of aging time. With aging time, an obvious decrease in turbidity reduction for PACl-1 coagulation is observed at low turbidity of 10 NTU, while the improvement in turbidity reduction for PACl-2 coagulation by enhanced sweep flocculation can be achieved as Alb mostly transform into Alc after 150 days aging. It is concluded that dominant in-situ formed Alc after a long time hydrolysis can improve PACl coagulation efficiency in turbidity reduction by enhanced sweep flocculation, especially for low turbidity water, but the increase in preformed Alc in PACl would worsen particle destabilization after aging.