Electrocoagulation of potable water

Coagulation caused by electrolytically produced ions (electrocoagulation) followed by filtration has been studied as a possible alternative to the conventional coagulation process. Electrochemical processes have been used in water and wastewater treatment since 1887. Electrocoagulation is looked upon as an interesting process in use at small water treatment plants. There are several important design aspects of this process which deserve further study.This study presents results showing the correlation between the current density and the aluminium dosing and provides results showing the necessary overpotential. The process has proved efficient with regard to removal of aquatic humus. A comparison of this method with conventional coagulation shows that the aquatic humus is removed equally well with both methods.Conventionally coagulated water (using alum) contains higher concentrations of sulphate and thus has a higher specific conductivity than the electrochemically treated water. The electrochemically treated water contains higher residual aluminium concentrations than the conventionally treated water due to the higher pH values.     

Chemical aspects of coagulation using aluminum salts—II. coagulation of fulvic acid using alum and polyaluminum chloride

This is the second of a two-part series investigating chemical aspects of coagulation using AI salts. Part I of the series examined the hydrolytic reactions of AI. In this paper, the coagulation of fulvic acid (FA) by alum and polyaluminum chloride (PACI) is examined. An Al speciation methodology was used to examine complexation reactions between Al and FA for water treatment conditions. From pH 5 to 7 and at typical coagulant doses, hydrolysis and complexation of Al is described by a simple model based on the reaction stoichiometry between AI, FA and OH⁻. Model results show that when alum is used as a coagulant, Al complexed with FA is hydrolyzed to a ligand number, This is similar to the ligand number for the prehydrolyzed PACI and explains similarities in dose requirements for these coagulants. Effects of temperature on coagulation performance are shown to be largely chemical in nature. Chemical aspects of coagulation are discussed and the importance of complex formation in coagulation is examined.     

Potable water treatment for some asbestiform minerals: optimization and turbidity data

A study has been undertaken to optimize conventional alum coagulation treatment of water for asbestos fibre removal. The results indicate that both the alum and polyelectrolyte concentrations significantly effect the quality of the treated water. Optimum results were obtained with alum concentrations of 30–50 ppm and polyelectrolyte concentrations of 0.3–0.6 ppm. Rapid coagulation/direct filtration has been evaluated and the results found to be comparable to conventional treatment employing flocculation and sedimentation. A survey of turbidities and fibre concentrations for several municipal water supplies indicate that no systematic relationship exists between these two parameters.     

Optimization of alum recovery from water treatment sludge‐case study: Samannoud water treatment plant,Egypt

Alum recovery from water treatment sludge is a promising technique applied for reducing the virgin coagulant demand and the sludge volume and safe disposal of the sludge. The aim of the current study is to optimize alum recovery process from alum sludge in Samannoud water treatment plant, Egypt by acidification technique using sulphuric acid and evaluate the effect of total solids (TS) content in the sludge on alum recovery process. Results showed that the optimal mixing time for alum recovery was 60 min for clariflocculator sludge, whereas 15 min for thickener sludge. Optimum pH value was 1.50 for alum recovery from different sludge types. At optimum conditions, acidification experiments showed that alum recovery percent and sludge volume reduction for clariflocculator sludge were 83 and 91.2%, respectively, versus 35.9 and 45%, respectively, for thickener sludge. Besides, alum recovery and sludge reduction decreased with the increase in TS content in the sludge.     

Influence of fulvic acid on the removal of trace concentrations of cadmium(II), copper(II), and zinc(II) from water by alum coagulation

The effectiveness of Cu²⁺, Cd²⁺ and Zn²⁺ removal from solution by alum coagulation was measured with fulvic acid present and absent. A factorial experimental design and analysis of variance were used to determine the effect on metal ion removal of the individual variables pH, metal ion concentration, alum concentration and fulvic acid concentration and their combinations. The variable levels model water treatment plant conditions. Metal ion losses up to 96% for Cu²⁺. 59% for Cd²⁺ and 82% for Zn^z+ were measured in the presence of fulvic acid. In its absence the maximum metal ion losses observed were 93%, 14% and 53% for Cu²⁺, Cd²⁺ and Zn²⁺ respectively. Fulvic acid enhances metal ion removal under most experimental conditions. The practical implication of the results is that strong complexes between natural water organic matter and metal ions enhance their removal by the alum coagulation process.     

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.     

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⁻.     

Floc morphology and cyclic shearing recovery: comparison of alum and polyaluminum chloride coagulants

This study investigated the floc formation and re-aggregation potential for alum, polyaluminum chloride (PACl) and a blend of these coagulants. Bench-scale testing included floc morphology characterization for well-developed floc, post-shear floc, and non-settleable (filter influent) floc. Different applications of coagulants were observed to produce non-settleable floc that was morphologically different. The alum treatment had a decrease in average floc size from coagulated to non-settleable conditions, whereas the PACl and PACl/alum treatments resulted in similar sized floc between these processes. Zeta potential distribution measurements showed that the alum treatment resulted in a negative shift from coagulated to non-settleable conditions whereas the PACl and PACl/alum treatments had no significant shift. A photometric dispersion analyzer (PDA) was employed to compare the differences between coagulant treatments with respect to shear induced aggregate breakup and recovery. The PDA allowed a dynamic monitoring of initial floc aggregation and measured the degree of recovery from cyclic shearing. The degree of recovery from shearing was greatest for the PACl/alum treatment likely as a result of increased collision efficiency due to more effective charge neutralization.     

Physico-chemical treatment of municipal wastewater. Coagulation-flocculation

This article traces the possibilities of physico-chemical treatment of domestic sewage with particular attention to coagulation-flocculation processes. As coagulants the following have been used: ferric chloride, hydrated lime and alum. Different types of coagulant aids have been used too. Besides pilot-plant experiments, a large number of batch experiments has been carried out in order to determine the range of optimal doses of coagulants/coagulant aids/pH and so on. The fate of specific organic and inorganic components (proteins, detergents, low organic acids, phosphorus, nitrogen) as well as TOC, BOD and COD in this chemical treatment have been studied. The suspended and most of the colloidal fractions have efficiently been removed. A considerable fraction, which is defined as soluble, has been removed too. The significance of the findings for wastewater treatment processes have been discussed.     

Use of nanoparticular and soluble anionic celluloses in coagulation-flocculation treatment of kaolin suspension

In this work, the effectiveness of a novel, combined coagulation-flocculation treatment based on alum and soluble or nanoparticular anionic derivatives of dialdehyde cellulose, ADAC, was evaluated by studying the removal of colloidal material in a model suspension containing kaolin. Four different ADACs with varying degrees of substitution, size and water solubility were synthesized by periodate oxidation and sulfonation of cellulose. The effects of ADAC dosage, solution pH and temperature on flocculation were studied by measuring residual turbidity of the settled suspension. Moreover, the charge densities, sizes, ζ-potentials and stability of the ADACs in aqueous solutions were studied. The combined treatment was effective in the removal of colloidal particles, as demonstrated by reduced residual turbidity with remarkably lower total chemical consumption compared with coagulation with alum alone. Of the ADACs, samples with lower solubility that contained cellulose nanoparticles performed better than the fully water-soluble sample. Due to the restricted pH tolerance of alum, the combined treatment was effective only at acidic conditions (pH < 5), although the ADACs were found to be stable in a much broader pH range (pH of 3 to about 9). ADACs also retained strong activity at higher temperatures (30-60 °C) and after several days of storage in aqueous solution.     

Electrocoagulation versus chemical coagulation: coagulation/flocculation mechanisms and resulting floc characteristics

Electrocoagulation (EC) and chemical coagulation (CC) are employed in water treatment for particle removal. Although both are used for similar purposes, they differ in their dosing method - in EC the coagulant is added by electrolytic oxidation of an appropriate anode material, while in CC dissolution of a chemical coagulant is used. These different methods in fact induce different chemical environments, which should impact coagulation/flocculation mechanisms and subsequent floc formation. Hence, the process implications when choosing which to apply should be significant. This study elucidates differences in coagulation/flocculation mechanisms in EC versus CC and their subsequent effect on floc growth kinetics and structural evolution. A buffered kaolin suspension served as a representative solution that underwent EC and CC by applying aluminum via additive dosing regime in batch mode. In EC an aluminum anode generated the active species while in CC, commercial alum was used. Aluminum equivalent doses were applied, at initial pH values of 5, 6.5 and 8, while samples were taken over pre-determined time intervals, and analyzed for pH, particle size distribution, ζ potential, and structural properties. EC generated fragile flocs, compared to CC, over a wider pH range, at a substantially higher growth rate, that were prone to restructuring and compaction. The results suggest that the flocculation mechanism governing EC in sweep floc conditions is of Diffusion Limited Cluster Aggregation (DCLA) nature, versus a Reaction Limited Cluster Aggregation (RLCA) type in CC. The implications of these differences are discussed.     

臭氧预氧化技术对水厂常规工艺的影响

通过生产性试验考察了臭氧预氧化技术对水厂常规工艺的影响.结果表明,臭氧预氧化技术具有明显的助凝效果,在实际生产中可降低常规工艺的出水浊度,同时可减少矾耗约28％左右;臭氧预氧化技术能提高常规工艺对耗氧量和氨氮的去除率,使二者的去除率分别提高15％和50％左右;若仅从成本方面考虑,采用臭氧预氧化技术降低矾耗是不经济的,但是在特殊原水水质情况下,将臭氧预氧化技术和混凝沉淀常规工艺相结合会明显改善出水水质,减少高矾耗带来的出厂水pH值低及铝离子超标等不利影响.     

Optimisation of combined coagulation and microfiltration for water treatment

The effect of upstream coagulant dosing for full-flow microfiltration of an upland-reservoir water has been investigated. The process, run under conditions of constant flux and pH and based on a ferric salt, is compared with a published study of another full-flow process based on alum dosing and operated at constant pressure and coagulant concentration. The current study includes data for the residual deposit remaining following backflushing by reverse flow. Results are presented in terms of the specific-cake resistance (R'o, m(-2)) as a function of pH or coagulant dose. Reasonable correlation with classical cake filtration theory was obtained, such that R' was assumed to be independent of run time and cake thickness. The following trends have been noted: The optimum pH for the alum-based system appears to be between 7.5 and 8 on the basis of cake resistance. The effect of coagulant dose between 18 and 71 microM Fe3+ is much more significant than a change in pH between 5 and 9 for the alum system: a 53-fold increase in specific flux compared with a 7-fold increase with reference to the limiting R'o values at pH 4.8 and 7.7. A low coagulant dose (0.018mM, 1.0 mg l(-1) Fe3+) appears to have a slightly detrimental effect on downstream microfiltration operation. The low coagulant doses apparently cause incomplete aggregation of colloidal particles such that internal fouling of the membrane takes place. The residual (cleaning cycle) deposit resistance followed roughly the same trend as the backflush cycle-cake resistance with coagulant concentration, but with a much reduced value (about 16 times lower, on average). An optimum coagulant dose of 0.055 mM (3.1 mg/l) Fe3+ can be identified on the basis of operational cost based on coagulant cost and cake resistance, all other aspects of the system being substantially unchanged. It is concluded that coagulation with downstream microfiltration offers a cost-effective means of removing natural organic matter, achieving a THMFP removal of around 80% at the optimum dose.     

Removal of organic precursors by permanganate oxidation and alum coagulation

Much is known about the removal and reaction mechanisms of the naturally occurring precursors of trihalomethanes such as humic and fulvic acids. However, another class of precursors including phenols, anilines and aliphatic diketones has not been thoroughly investigated. This study shows that these truly soluble, monomeric precursors are poorly removed by alum coagulation. The use of permanganate preceding alum coagulation provides a good reduction of chloroform in water samples containing high proportions of monomeric precursors. The process was found to be less effective for water samples containing a high proportion of humic acid. This is possibly due to the degradation products from the permanganate oxidation of humic acid. These degradation products are not readily removed by alum coagulation and become available for subsequent chloroform formation.     

Effect of water treatment processes on Cryptosporidium infectivity

Conventional water treatment processes have the ability to remove Cryptosporidium oocysts through coagulation, flocculation, sedimentation and filtration, provided there is efficient management of plant performance. The potential exists for the breakthrough of oocysts through the treatment train. The effect of the water treatment chemical aluminium sulphate (alum) on Cryptosporidium oocyst infectivity has been assessed using an assay that combines cell culture and real-time polymerase chain reaction techniques. The infectivity of fresh and temperature-aged oocysts (stored up to 6 months at 4 or 15 degrees C) was unaffected by exposure to a range of doses of alum in standard jar test procedures and dissolved air flotation processes and subsequent exposure to chlorine or chloramine. Removal efficiencies and infectivity measures are important in determining risk to public health and will reflect the ability of water treatment plants to act as a barrier to these pathogens.     

A method for comparative evaluation of whole-lake and inflow alum treatment

A method is described to evaluate two methods of phosphorus (P) management in lakes using aluminum sulfate (alum)--in-lake and tributary (inflow) treatment--and compare the resulting in-lake P levels. For in-lake treatment, a technique is described to calculate the optimum alum dose based on measurement of "mobile P" in lake sediments. Mobile P is defined as loosely sorbed and Fe-P, the fraction of sediment P subject to release under anoxic conditions. A linear relationship (r2 = 0.90) was found between P-release rate and the mobile-P content in sediment cores. Addition of alum to aliquots of sediment showed predictable relationships between (i) alum dose and aluminum-bound P (Al-P) formed and (ii) mobile-P loss and Al-P formation. The decrease in sediment P release that would result from in-lake alum treatment was estimated from the residual mobile P after treatment. A method also is presented to estimate the amount of alum needed to bind potentially mineralizable sediment organic P. For inflow treatment, jar tests with urban runoff in metropolitan St. Paul and Minneapolis, Minnesota (USA) were used to study effects of alum dose on P removal from water. With sufficient mixing, a dose of 8 mg AlL(-1) reduced total P (TP) and soluble reactive P to low levels regardless of pH, TSS, and TOC, but doses 相似文献   

Suspended particle destabilization in retained urban stormwater as a function of coagulant dosage and redox conditions

Source area runoff entrains a hetero-disperse particle size distribution (PSD). When retained for clarification, larger sediment and settleable particles are mainly influenced by gravitational forces, while the suspended particles, in particular the clay-size particles, are subject to coagulation phenomena. Such phenomena occur in untreated runoff as well as runoff treated with a coagulant, albeit to differing rates and extents. Runoff PSDs and water chemistry indices including zeta potential (xi) are potentially modified during inter-event stormwater retention in best management practices (BMPs). This study examined xi of clay-size particles (<2 microm) in retained runoff, captured from an instrumented watershed, subject to batch coagulation and variable redox conditions. Separate parallel tests were also conducted with wastewater. Significant turbidity, particle mass (measured as total suspended solids (TSS)) and volume concentration (as total volume concentration (TVC)) reduction generated by alum and ferric chloride consistently occurred at a xi in the range of -15 to about -10 mV. Alum addition produced a charge reversal at dosing above 60 mg/L (18 x 10(-5)M) while ferric chloride did not reverse charge. With respect to turbidity and TSS reductions, alum outperformed ferric chloride, without the need for pH control. While xi illustrated no clear trend during aerobic retention, anoxic retention resulted in a trend for xi approaching the isoelectric point. The decrease in negative xi towards the isoelectric point appears to be a result of the coupled pH depression under reductive conditions and an increase in conductivity. Results have significant implications for BMPs that retain runoff between events.     

Enhanced coagulation using a magnetic ion exchange resin

The objective of this investigation was to examine the effectiveness of a magnetic ion exchange resin (MIEX) to enhance the coagulation of disinfection by-product precursors in nine surface waters, each representing a different element of the USEPA's 3 x 3 enhanced coagulation matrix. The effect of MIEX-pretreatment on the requisite alum dose needed for subsequent coagulation of turbidity was also evaluated. Enhanced coagulation with MIEX was found to be very effective for removing trihalomethane (THM) and haloacetic acid (HAA) precursors from the nine waters examined. THM and HAA formation potential was reduced by more than 60% in all of the waters studied; reductions approaching 90% were seen in the waters with the highest specific ultraviolet absorbance values. The residual total organic carbon concentration, ultraviolet absorbance, and THM and HAA formation potential were all substantially lower as a result of MIEX and alum treatment compared to alum coagulation alone. MIEX pre-treatment also lowered the coagulant demand of each of the waters substantially.     

Colour and turbidity removal with reusable magnetite particles—VI: Pilot plant operation

A novel process which utilizes fine magnetite particles for the removal of colour and turbidity from water has been tested in a 60 1 min⁻¹ pilot plant on water from an unconfined aquifer in Perth, Western Australia. The performance of the pilot plant closely matched jar test results. The process design was optimized to achieve a high quality product water on a raw feed which was difficult to treat in the conventional alum coagulation and filtration plant. A comparison of the conventional and magnetite treatment systems is made, which shows that the magnetite process is viable as an alternative water treatment technique.     

常规工艺对浊度的去除效率及浊度预警水平

分析了原水泵站及水厂各工艺单元出水一整年的浊度数据，发现1月-4月的原水浊度较低，如果碱铝投加量低于一定限值（如8．88mg／L）则易造成出水浊度偏高，需要控制混凝剂投加量以保证混凝效果。清水池内浊度容易升高，这可能与滤池初滤水的水质较差有关。根据各工艺单元对浊度的一般去除效率和出厂水浊度标准，提出了原水和工艺单元出水的浊度预警水平，这对于水厂常规工艺的运行具有指导性意义。