The sedimentation process can be quickened by adding coagulants to the water. Chemical coagulants are commonly used in community drinking water treatment systems though some application in household water treatment occurs. The main chemicals used for coagulation are aluminum sulphate (alum), polyaluminium chloride (also known as PAC or liquid alum), alum potash, and iron salts (ferric sulphate or ferric chloride). Lime (Ca(OH2)), lime soda ash (Na2CO3) and caustic soda (NaOH) are sometimes used to “soften” water, usually ground water, by precipitating calcium, magnesium, iron, manganese and other minerals that contribute to hardness.
How do they remove contamination?
Particles that cause turbidity (e.g. silt, clay) are generally negatively charged, making it difficult for them to clump together because of electrostatic repulsion. But coagulant particles are positively charged, and they chemically attracted to the negative turbidity particles, neutralizing the latter’s negative charge. With mixing the neutralized particles then accumulate (flocculation) to form larger particles (flocs) which settle faster. The flocs can then be settled out or removed by filtration. Some bacteria and viruses can also attach themselves to the suspended particles in water that cause turbidity. Therefore, reducing turbidity levels through coagulation may also improve the microbiological quality of water.
Construction, operations and maintenance
Users follow Tramfloc’s instructions and add the prepared dose of coagulant to the water. The water is then stirred for a few minutes to help create flocs. The flocs can be settled out or removed by filtration. Maximum effectiveness requires careful control of coagulant dose, pH and consideration of the quality of the water being treated, as well as mixing.
Coagulation and flocculation are an essential part of drinking water treatment as well as wastewater treatment. This article provides an overview of the processes and looks at the latest thinking. Material for this article was largely taken from references such as iwawaterwiki.org.
Coagulation and flocculation are essential processes in various disciplines. In potable water treatment, clarification of water using coagulating agents has been practiced from ancient times. As early as 2000 BC the Egyptians used almonds smeared around vessels to clarify river water. The use of alum as a coagulant by the Romans was mentioned in around 77 AD. By 1757, alum was being used for coagulation in municipal water treatment in England. In modern water treatment, coagulation and flocculation are still essential components of the overall suite of treatment processes – understandably, because since 1989 the regulatory limit in the US for treated water turbidity has progressively reduced from 1.0 NTU in 1989 to 0.3 NTU today. Many water utilities are committed to consistently producing treated water turbidities of less than 0.1 NTU to guard against pathogen contamination.
Coagulation is also important in several wastewater treatment operations. A common example is chemical phosphorus removal and another, in overloaded wastewater treatment plants, is the practice of chemically enhancing primary treatment to reduce suspended solids and organic loads from primary clarifiers.
Organic coagulants include polydadmacs, epichlorohydrin/dimethyl amine blends and combinations of both organic and inorganic coagulants. Many applications benefit from the use the Tramfloc® 500, 600, 700 and 800 series of organic coagulants because the resulting sludge produced from precipitating solids is of a much smaller volume and weight than with metallic coagulants of Fe and Al. Organic sludge is usually classified as non-hazardous while metallic sludge can be hazardous and is of much greater weight for disposal. This means that disposal costs are considerably higher for metal containing, inorganic coagulants’ sludges.
Inorganic coagulants are used extensively in municipal potable water treatment plants and organic coagulants are often conjointly applied with the inorganics. There are substantial advantages to the organic coagulants application over the inorganic formulations. The latter do have value in various plants and should not be overlooked in coagulant jar testing.
The commonly used metal coagulants fall into two general categories: inorganic and organic and blends thereof. There are those based on aluminum and those based on iron. The aluminum coagulants include aluminum sulfate, aluminum chloride and sodium aluminate. The iron coagulants include ferric sulfate, ferrous sulfate, ferric chloride and ferric chloride sulfate. Other chemicals used as coagulants include hydrated lime and magnesium carbonate.
The effectiveness of aluminum and iron coagulants arises principally from their ability to form multi-charged polynuclear complexes with enhanced adsorption characteristics. The nature of the complexes formed may be controlled by the pH of the system.
When metal coagulants are added to water the metal ions (Al and Fe) hydrolyze rapidly but in a somewhat uncontrolled manner, forming a series of metal hydrolysis species. The efficiency of rapid mixing, the pH, and the coagulants dosage determine which hydrolysis species is effective for treatment.
There has been considerable development of pre-hydrolyzed inorganic coagulants, based on both aluminum and iron to produce the correct hydrolysis species regardless of the process conditions during treatment. These include aluminum chlorhydrate, polyaluminum chloride, polyaluminum sulfate chloride, polyaluminum silicate chloride and forms of polyaluminum chloride with organic polymers. Iron forms include polyferric sulfate and ferric salts with polymers. There are also polymerized aluminum-iron blends.
The principal advantages of pre-polymerized inorganic coagulants are that they are able to function efficiently over wide ranges of pH and raw water temperatures. They are less sensitive to low water temperatures; lower dosages are required to achieve water treatment goals; less chemical residuals are produced; and lower chloride or sulfate residuals are produced, resulting in lower final water TDS. They also produce lower metal residuals.
Pre-polymerized inorganic coagulants are prepared with varying basicity ratios, base concentrations, base addition rates, initial metal concentrations, aging time, and aging temperature. Because of the highly specific nature of these products, the best formulation for a particular water is case specific, and needs to be determined by jar testing. For example, in some applications alum may outperform some of the polyaluminum chloride formulations.
PoIymers are a large range of natural or synthetic, water soluble, macromolecular compounds that have the ability to destabilize or enhance flocculation of the constituents of a body of water. Natural polymers have long been used as flocculants. For example, Sanskrit literature from around 2000 BC mentions the use of crushed nuts from the Nirmali tree (Strychnos potatorum) for clarifying water – a practice still alive today in parts of Tamil Nadu, where the plant is known as Therran and cultivated also for its medicinal properties. In general, the advantages of natural polymers are that they are virtually free of toxins, biodegradable in the environment and the raw products are often locally available. However, the use of synthetic polymers is more widespread. They are, in general, more effective as flocculants because of the level of control made possible during manufacture.
Important mechanisms relating to polymers during treatment include electrostatic and bridging effects. Polymers are available in various forms including solutions, powders or beads, oil or water-based emulsions, and the Mannich types. The polymer charge density influences the configuration in solution: for a given molecular weight, increasing charge density stretches the polymer chains through increasing electrostatic repulsion between charged units, thereby increasing the viscosity of the polymer solution.
One concern with synthetic, organic polymers and coagulants relates to potential toxicity issues, generally arising from residual unreacted monomers. However, the proportion of unreacted monomers can be controlled during manufacture, and the quantities present in treated waters are generally low.
Removal of Natural Organic Matter
Natural organic material (NOM) is usually associated with humic substances arising from the aqueous extraction of living woody substances, the solution of degradation products in decaying wood and the solution of soil organic matter. These substances are objectionable for a number of reasons: they tend to impart color to waters; they act as a vehicle for transporting toxic substances and micro-pollutants, including heavy metals and organic pollutants; and they react with chlorine to form potentially carcinogenic by-products.
The degree to which coagulation can remove organic material depends on the type of material present. The specific ultraviolet absorption (SUVA) is related to the concentration and type of dissolved organic carbon (DOC) present, as follows:
SUVA = UV254/DOC (l/mg m) Where: UV254 is the ultraviolet absorbance measure at a wavelength of 253.7 nm, after filtration through 0.45-µm filters (m-1); DOC is the dissolved organic carbon measured after filtration through 0.45-µm filters (mg/l).
In general, lower molecular weight species such as the fulvic acids are more difficult to remove by coagulation. Higher molecular weight humic acids tend to be easier to remove.
The United States Environmental Protection Agency (US EPA) introduced enhanced coagulation for the removal of NOM. Enhanced coagulation is an elaboration of long-practiced techniques for removing organic color by coagulation. It requires the removal of NOM material, while still achieving good turbidity removal. These dual objectives can be met by selecting the best coagulant type, applying the best coagulant dosage and adjusting the pH to a value where best (or adequate) overall coagulation conditions are achieved. The enhanced coagulation approach recognizes that the constituents of any given water govern the practical degree of treatment achievable. Therefore, a water-specific point of diminishing returns (PODR) is identified, at which a coagulant increment (10 mg/l for alum) results in a TOC removal increment of less than 0.3 mg/l. Organic coagulants’ removal and enhanced coagulation are effective with traditional coagulants like aluminum sulfate, ferric chloride and ferric sulfate, as well as formulations like polyaluminum chloride (PACl) and acid alum. Acid alum formulations are aluminum sulfate with 1 to 15-percent free sulfuric acid. Their effectiveness with TOC removal applications is due to the enhanced depression of pH.
TOC or NOM reductions depend on the type and dosage of coagulant, the pH, temperature, raw water quality and NOM characteristics. In general, the optimum pH for ferric salts is in the range 3.7 to 4.2, and for aluminum sulfate in the range 5.0 to 5.5.
In some cases, the removal of lower weight organics has been improved by supplementing treatment with metal coagulants with powdered activated carbon (PAC). In one case with raw water TOC of 2.4 mg/l, a combination of an alum-polymer blend coagulant at 25 mg/l with PAC at 10 mg/l was optimal to achieve a 39-percent TOC reduction. In another case, a water with a low humic content and low SUVA (1.43 l/mg.m) was treated with 65 mg/l FeCl3 and 23 mg/l PAC. Fifty-six percent of the TOC was non-humic and 46-percent of the TOC had molecular weights less than 1,000.
The U.S. EPA surface water treatment rule requires 99.9-percent (3-log) Giardia removal or inactivation, and at least 99-percent (2-log) removal of Cryptosporidium. Adequately designed and operated water treatment plants, with coagulation, flocculation, sedimentation and filtration are assigned a 2.5-log removal credit for Giardia, leaving only 0.5-log inactivation to be achieved by disinfection.
Coagulation and flocculation, with dissolved air flotation (DAF) for clarification, has achieved average log removals of Giardia and Cryptosporidium of 2.4 and 2.1, respectively. Optimum coagulation conditions were governed by turbidity and NOM removal requirements, rather than by pathogen removals. Overall Giardia and Cryptosporidium removals, including the filtration step were approximately 5-log.
Cryptosporidium oocyst surfaces are believed to consist of polysaccharide layers. The negative charge carried by the oocysts is believed to arise from carboxylic acid groups in surface proteins. Removal of Cryptosporidium using alum coagulation appears to be by a sweep floc mechanism. Zeta potential measurements suggest that removal does not appear to be by a charge neutralization mechanism at lower DOC concentrations. At higher DOC, it appears that the mechanism is mediated by a NOM-assisted bridging between aluminum hydroxide and oocyst particles.
Significant virus removals have been reported using metal coagulants and organic coagulants. Removals of up to 99.9% have been reported for both aluminum and ferric salts. Various polyelectrolytes (cationic) have effected removals of greater than 99% but have the disadvantage that if other material is present in the form of color, turbidity, and COD, removal of such material is poor. Using metal coagulants and poyelectrolytes conjointly has the advantage that better floc characteristics are produced. If a variety of substances are present in water, it is possible that the use of both metal coagulants and polyelectrolytes will effect a higher overall removal. However, this very much depends on the conditions pertaining for each case. When using organic coagulants as flocculant aids, floc formation improves but does not appear to improve virus removals beyond those achieved using metal coagulants alone.
Viruses are essentially DNA (deoxyribonucleic acid) or RNA (ribonucleic acid) units contained within a protein coat. The destabilization mechanism involves coordination reactions between metal coagulant species and carboxyl groups of the virus coat protein. Because of the similarity of the destabilization mechanisms for organic color and viruses, optimum removals tend to occur at similar pH values. The optimum pH for virus removal with aluminum sulfate has been found to be in the region of 5.0 with percentage virus removals in the range 97.7 to 99.8%. Using a cationic polyelectrolyte as flocculant aid, virus and turbidity removals were increased to 99.9 and 98.5% respectively.
Coagulants do not fully inactivate viruses. Therefore, a potential health hazard exists with the ultimate disposal of water treatment plant sludges. Furthermore, complete virus removal by destabilization with metal coagulants has not been reported. For a safe drinking water, disinfection of the water before distribution is required. However, there is some inactivation that accompanies virus removal by coagulation. Some reports have shown that the infectious virus concentration only recovers partially after re-dissolution of aluminum hydroxide precipitates. This phenomenon has been interpreted as virucidal activity of the aluminum. PACl coagulants appeared to have a higher virucidal activity compared with alum. The presence of NOM in waters appears to inhibit the virucidal activity of the aluminum.
Chemical Phosphorus Removal
In many sensitive catchment areas, chemical phosphorus removal is also required for wastewater treatment. There is a general relationship between effluent residual phosphorus concentration and the ratio of metal added to P removed. Inordinate dosages, beyond stoichiometric, are required to achieve very low effluent concentrations.
Within the stoichiometric range of phosphorus removal, there is a tightening of the optimal pH range as the metal coagulant dosage increases. However, beyond the stoichiometric range, when final phosphorus concentrations are progressively lower, the pH range widens again, towards the side of higher pH. For example, with alum the optimum pH range for effluent P concentrations down to approximately 0.2 is 5.5 to 6.0. However, as the Al ratio is increased for lower P concentrations, the required pH range widens to 6.0 to 7.0. Within the stoichiometric P removal range, a precipitation model describes the interactions between metal and phosphorus. However, at very low P concentrations, more complex models that include precipitation, adsorption and floc specific surface are required.
The benefits of sequential chemical addition for coagulation operations have been shown on many occasions. This is also the case with phosphorus precipitation. For very low final concentrations, overall coagulant dosages can be significantly reduced.
The degree of phosphorus removal depends not only on the coagulant added, but also on the mode of solid-liquid separation employed. This is particularly important for those cases where very low final phosphorus concentrations are achieved. Effluent suspended solids contribute significantly to effluent total phosphorus concentrations. For very low phosphorus residuals, and high metal coagulant dosages, the phosphorus content of effluent suspended solids is significantly reduced. The reason is that at very high metal dosages, a larger proportion of the precipitates formed are metal hydroxides.
Physical-chemical treatment of wastewater was widely practiced until the late 19th century, until the advent of the trickling filter for biological treatment. The early 1970s saw a partial revival of interest that has continued to the present day, particularly for treatment plants that are overloaded during peak flow events. The addition of coagulant chemicals to primary clarifiers, or to other dedicated physical separation processes, is an effective way of reducing the load to downstream biological processes, or in some cases for direct discharge. This practice is generally referred to as chemically enhanced primary treatment, or CEPT.
Principal disadvantages that might preclude a wholly physical-chemical solution to wastewater treatment are the problems associated with the highly putrescible sludge produced, and the high operating costs of chemical addition. However, much of the current interests in physical-chemical treatment stem from its suitability for treatment under emergency measures; for seasonal applications, to avoid excess wastewater discharges during storm events; and for primary treatment before biological treatment, where the above disadvantages become of lesser impact. CEPT can also be an effective first step for pollution control in developing countries – particularly in large urban areas that have evolved with sewerage systems but without centralized wastewater treatment, and have limited financial resources for more complete, but capital intensive biological treatment options such as activated sludge systems. Such urban developments also may not have the areas available for appropriate technology options such as stabilization pond processes.
The efficiency of CEPT, in terms of BOD or COD removal, depends on wastewater characteristics. With CEPT, one can expect to remove particulate components, together with some portion of the colloidal components. Therefore, with such a wastewater, it is feasible to achieve removals of more than: 95-percent TSS; 65-percent COD; 50-percent BOD; 20-percent nitrogen; and 95-percent phosphorus. In practice, removals may be lower or higher: for example, in warmer climates, with larger collection systems, and relatively flat sewers, one would expect a higher degree of hydrolysis of particulate matter resulting in higher soluble fractions, and lower overall removals with CEPT. On the other hand, if the collection system is relatively small, the climate is cold, and wastewater is relatively fresh, there may be a higher proportion of particulate material, and CEPT removals could be higher.
Staged coagulation-flocculation can enhance CEPT performance. For example, at primary clarifier overflow rates of over 6 m/h (3,600 gpd/ft2) during peak flow treatment, TSS and BOD removals of 80 to 95%, and 58 to 68% were achieved, respectively, using 60 mg/l ferric chloride, followed by 15 mg/l polyaluminum chloride, followed by 0.5 mg/l anionic polymer. The total reaction time from the point of ferric chloride addition to entering the primary clarifiers was approximately 8 minutes at peak flow.
Factors affecting coagulation operations
Temperature significantly affects coagulation operations, particularly for low turbidity waters, by shifting the optimum pH. This can be mitigated by operating at an optimum pOH as given by:
pH + pOH = pKW; where pKW = 0.01706xT + 4470.99/T – 6.0875
and T = temperature in °K = 273.15 + °C.
One advantage of the pre-polymerized coagulants such as PACl and polyferric sulfate is that they potentially can be tailored for particular raw water conditions such as temperature and other parameters, and can be less sensitive to changes in temperature.
Sequence of chemical addition
Traditionally, the sequence of chemical addition for coagulation operations is to first add chemicals for pH correction, then add the metal coagulant, then add the flocculant aid. Not all these chemicals are necessarily added, but the sequence logic is often as described. However, there are instances when other sequences are more effective, including inverting the sequence of metal coagulant and polymer addition, and the sequence of metal coagulant addition and pH adjustment. The best sequence for a particular application can be determined by jar test experiments.
Residual aluminum in treated water is undesirable for aesthetic reasons, but also because of a possible link between aluminum and adverse neurological effects such as Alzheimer’s disease. Although ingestion from drinking water constitutes a relatively small proportion of daily intake, residual aluminum in treated waters can be minimized by proper adjustment of pH. However, the optimum pH to minimize residual aluminum also depends on other substances in solution. For example, the presence of fluoride in the raw water shifts the pH of minimum Al residual upwards towards 7, depending on the fluoride concentration. For these reasons it is prudent for plant managers to consider the application of organic coagulants rather than the metal containing coagulants.
The presence of NOM also complicates the issue. Because of complexation of aluminum species with humic substances, the residual aluminum is linked to the removal of NOM. For example, at low alum dosages applied to humic waters, residual aluminum concentrations after treatment can be relatively high. At higher applied alum dosages, where a larger proportion of the humic substances are removed, residual Al concentrations after treatment are often significantly lower. This reduction in residual aluminum with higher aluminum dosages has also been found during fluoride removal.
The contribution of colloidal material to the aluminum residual emphasizes the importance of achieving low final treated water turbidities, at least less than 0.1 NTU, to minimize final aluminum residuals. When addressing high aluminum residuals, it is also important to determine whether the aluminum is in the particulate form, which would indicate improvements to filter retention, or whether it is soluble, which would require improving the chemistry of coagulation – particularly the pH before filtration.
The rapid mixing stage is possibly the most important component of coagulation-flocculation processes, since it is here that destabilization reactions occur and where primary floc particles are formed, whose characteristics markedly influence subsequent flocculation kinetics. In general it is likely that the metal coagulant hydrolysis products that are formed within the time range 0.01 to 1.0 seconds are the most important for effective destabilization. In many instances, traditional 30 to 60 second retention times during rapid mixing are unnecessary and flocculation efficiency may not improve beyond rapid mix times of approximately 5 seconds or less. Indeed, beyond a certain optimum rapid mix time, a detrimental effect on flocculation efficiency may result.
The type of rapid mixer often installed in practice is given the general name back-mix reactors. These are often designed to provide a 10 to 60 second retention time with a root mean square velocity gradient, G, of the order 300 s-1. Back-mix reactors normally comprise square tanks with vertical impellers. In many instances these back-mix reactors have been abandoned or not used extensively due to the poor results often attained.
Other modes of rapid mixing include in-line mixers either with or without controlled velocity gradients. In general, in-line mixers provide the best rapid mixing conditions. In-line mixers without velocity gradient control include static mixers, orifice plates, diffuser grids in an open channel, and hydraulic jumps in open channels. In each of these cases, the velocity gradient and degree of mixing is dependent on the flow rate.
In-line mixers with velocity gradient control include in-line mechanical mixers with variable speed impellers, and in-line jet mixers. In the latter case, the velocity gradient is varied by the flow rate though the jet nozzle. Typically these are mounted concentrically within an enclosed pipe, with the jet discharging against the flow.
Chitosan as a food grade coagulant
Chitosan coagulant can also be used in water processing engineering as a part of a filtration process. Chitosan coagulant causes the fine sediment particles to bind together, and is subsequently removed with the sediment during sand filtration. Chitosan coagulant also removes phosphorus, heavy minerals, and oils from the water. Chitosan coagulant can be an important coagulant in the filtration process. Sand filtration apparently can remove up to 50% of the turbidity alone, while the chitosan coagulant with sand filtration removes up to 99% of the turbidity. Chitosan coagulant has been used to precipitate caseins from bovine milk and in cheese making.
Chitosan is also useful in other filtration situations, where one may need to remove suspended particles from a liquid. In combination with bentonite clays, gelatin, silica gel, isinglass, and other fining agents, it is used to clarify wine, mead, and beer. Added late in the brewing process, chitosan improves flocculation, and removes yeast cells, fruit particles, and other detritus that cause hazy wine. Chitosan combined with colloidal silica is becoming a popular fining agent for white wines, because chitosan does not require acidic tannins (found primarily in red wines) with which to flocculate.
Coagulant Economics/Cost Effectiveness
Our coagulant formulations are available in a wide variety of physical forms, packaging options, delivery methods to meet the smallest to the largest user’s volume requirements. Please contact our sales department at email@example.com for further information about our coagulant formulations’ samples, product selection, budgetary estimates, quotations and technical recommendations.