Sillanpää M, Ncibi MC, Matilainen A, Vepsäläinen M. Removal of natural organic matter in drinking water treatment by coagulation: A comprehensive review. Chemosphere. 2017 Sep 25;190:54-71. doi: 10.1016/j.chemosphere.2017.09.113.
Natural organic matter (NOM) is a complex matrix of organic substances produced in (or channeled to) aquatic ecosystems via various biological, geological and hydrological cycles. Such variability is posing a serious challenge to most water treatment technologies, especially the ones designed to treat drinking water supplies. Lately, in addition to the fluctuating composition of NOM, a substantial increase of its concentration in fresh waters, and also municipal wastewater effluents, has been reported worldwide, which justifies the urgent need to develop highly efficient and versatile water treatment processes. Coagulation is among the most applied processes for water and wastewater treatment. The application of coagulation to remove NOM from drinking water supplies has received a great deal of attention from researchers around the world because it was efficient and helped avoiding the formation of disinfection by products (DBPs). Nonetheless, with the increased fluctuation of NOM in water (concentration and composition), the efficiency of conventional coagulation was substantially reduced, hence the need to develop enhanced coagulation processes by optimizing the operating conditions (mainly the amount coagulants and pH), developing more efficient inorganic or organic coagulants, as well as coupling coagulation with other water treatment technologies. In the present review, recent research studies dealing with the application of coagulation for NOM removal from drinking water supplies are presented and compared. In addition, integration schemes combining coagulation and other water treatment processes are presented, including membrane filtration, oxidation, adsorption and others processes.
Barrado-Moreno MM, Beltran-Heredia J, Martín-Gallardo J. Microalgae removal with Moringa oleifera. Toxicon. 2015 Dec 11;110:68-73. doi: 10.1016/j.toxicon.2015.12.001.
Moringa oleifera seed extract was tested for algae (Chlorella, Microcystis, Oocystis and Scenedesmus) removal by Jar-test technique. This coagulant can be used in drinking water treatment. Jar-test has been carried out in order to evaluate the efficiency of this natural coagulant agent inside real surface water matrix. The influence of variables has been studied in this process, including operating parameters such as coagulant dosage, initial algae concentration, pH, agitation time and water matrix. Removal capacity is verified for water with high contamination of algae while the process is not affected by the pH and water matrix. Coagulation process may be modelling through Langmuir and Freundlich adsorption hypothesis, so acceptable r2 coefficients are obtained.
Keeley J, Jarvis P, Smith AD, Judd SJ. Coagulant recovery and reuse for drinking water treatment. Water research 2015 Oct 21;88:502-509. doi:10.1016/j.watres.2015.10.038.
Coagulant recovery and reuse from waterworks sludge has the potential to significantly reduce waste disposal and chemicals usage for water treatment. Drinking water regulations demand purification of recovered coagulant before they can be safely reused, due to the risk of disinfection by-product precursors being recovered from waterworks sludge alongside coagulant metals. While several full-scale separation technologies have proven effective for coagulant purification, none have matched virgin coagulant treatment performance. This study examines the individual and successive separation performance of several novel and existing ferric coagulant recovery purification technologies to attain virgin coagulant purity levels. The new suggested approach of alkali extraction of dissolved organic compounds (DOC) from waterworks sludge prior to acidic solubilisation of ferric coagulants provided the same 14:1 selectivity ratio (874 mg/L Fe vs. 61 mg/L DOC) to the more established size separation using ultrafiltration (1285 mg/L Fe vs. 91 mg/L DOC). Cation exchange Donnan membranes were also examined: while highly selective (2555 mg/L Fe vs. 29 mg/L DOC, 88:1 selectivity), the low pH of the recovered ferric solution impaired subsequent treatment performance. The application of powdered activated carbon (PAC) to ultrafiltration or alkali pre-treated sludge, dosed at 80 mg/mg DOC, reduced recovered ferric DOC contamination to <1 mg/L but in practice, this option would incur significant costs. The treatment performance of the purified recovered coagulants was compared to that of virgin reagent with reference to key water quality parameters. Several PAC-polished recovered coagulants provided the same or improved DOC and turbidity removal as virgin coagulant, as well as demonstrating the potential to reduce disinfection byproducts and regulated metals to levels comparable to that attained from virgin material.
Hu C, Chen Q, Liu H, Qu J. Coagulation of methylated arsenic from drinking water: Influence of methyl substitution. Journal Of Hazardous Materials 2015 Aug 15; Vol. 293, pp. 97-104.
Methylated arsenic can be found in virtually all earth surface environments. So far, however, little information has been collected regarding their removal by coagulation. In this study, the removal of monomethylarsenate (MMA) and dimethylarsenate (DMA) from drinking water by coagulation was investigated from the viewpoint of methyl substitution. Results indicated that FeCl3 was more efficient than AlCl3 and polyaluminum chloride (PACl) in methylated As removal. For the initial arsenic concentration of 200 μg/L, an FeCl3 dosage of 0.2 mmol Fe/L was sufficient to attain about 95% removal of MMA, while a dosage of 0.6 mmol Fe/L achieved about 57% removal of DMA. Arsenic removal efficiency was negatively correlated with the degree of methyl substitution. With the increase in methyl group number, the quantity of negatively charged arsenic species decreased and molecular size increased, leading to the decrease of methylated As removal by coagulation. Adsorption on preformed hydroxide flocs was the major mechanism during coagulation. Both FTIR and XPS results indicated that the As−O group of As might substitute the O−H group of Fe/Al hydroxide to form a Fe/Al−O−As complex. Furthermore, the use of traditional oxidants and coagulation aids exhibited limited help for improving coagulation removal of DMA.
Cui J, Jing C, Che D, Zhang J, Duan S. Groundwater arsenic removal by coagulation using ferric(III) sulfate and polyferric sulfate: A comparative and mechanistic study. Journal of Environmental Sciences (China). 2015 Jun 1;32:42-53. doi: 10.1016/j.jes.2014.10.020.
Elevated arsenic (As) in groundwater poses a great threat to human health. Coagulation using mono- and poly-Fe salts is becoming one of the most cost-effective processes for groundwater As removal. However, a limitation comes from insufficient understanding of the As removal mechanism from groundwater matrices in the coagulation process, which is critical for groundwater treatment and residual solid disposal. Here, we overcame this hurdle by utilizing microscopic techniques to explore molecular As surface complexes on the freshly formed Fe flocs and compared ferric(III) sulfate (FS) and polyferric sulfate (PFS) performance, and finally provided a practical solution in As-geogenic areas. FS and PFS exhibited a similar As removal efficiency in coagulation and coagulation/filtration in a two-bucket system using 5mg/L Ca(ClO)2. By using the two-bucket system combining coagulation and sand filtration, 500L of As-safe water (<10μg/L) was achieved during five treatment cycles by washing the sand layer after each cycle. Fe k-edge X-ray absorption near-edge structure (XANES) and As k-edge extended X-ray absorption fine structure (EXAFS) analysis of the solid residue indicated that As formed a bidentate binuclear complex on ferrihydrite, with no observation of scorodite or poorly-crystalline ferric arsenate. Such a stable surface complex is beneficial for As immobilization in the solid residue, as confirmed by the achievement of much lower leachate As (0.9μg/L-0.487mg/L) than the US EPA regulatory limit (5mg/L). Finally, PFS is superior to FS because of its lower dose, much lower solid residue, and lower cost for As-safe drinking water.
Liu R, Liu B, Zhu L, He Z, Ju J, Lan H, Liu H. Effects of fluoride on the removal of cadmium and phosphate by aluminum coagulation. J Environ Sci (China). 2015 Jun 1;32:118-25. doi: 10.1016/j.jes.2014.10.024. Epub 2015 Apr 22.
This study focuses on the effects of pH and fluoride at different molar ratios of fluoride to Al (RF:Al) on the removal of cadmium (Cd(2+)) and phosphate by Al coagulation. Fluoride at RF:Al≥3:1 inhibits the removal of Cd over wide Al dose ranges from 5 to 10mg/L as Al. The removal of phosphate decreases significantly at high RF:Al of 10:1 whereas at lowered RF:Al (i.e., ≤6:1), an adverse effect is observed only at insufficient Al doses below 2mg/L. Fluoride shows inhibitive effects towards the removal of Cd at pH7 and 8 and that of phosphate at pH6. Fluoride decreases the ζ-potential in both systems, and the decreasing extent is positively correlated to the elevated RF:Al. The Al fluoride interactions include the formation of Al-F complexes and the adsorption of fluoride onto Al(OH)3 precipitates, i.e., the formation of Al(OH)nFm. Al-F complex formation inhibits Al hydrolysis and increases residual Al levels, and a more significant increase was observed at lower pH. Al-F complexes at high RF:Al complicate the coagulation behavior of Al towards both negative and positive ionic species. Moreover, fluoride at low RF:Al shows little effect on Al coagulation behavior towards Cd(2+) and phosphate, and the spent defluoridation adsorbent, i.e., aluminum (Al) hydro(oxide) with adsorbed fluoride at RF:Al of below 0.1:1, may be reclaimed as a coagulant after being dissolved.