Confirm Cancel. From the journal Chemical Product and Process Modeling. Cite this. You currently have no access to view or download this content. Please log in with your institutional or personal account if you should have access to this content through either of these. Showing a limited preview of this publication:. Abstract Motivated by the oxidative power of hydrogen peroxide and its environmentally attractive properties, the present study aimed to determine the optimum conditions for the production of ferric sulfate coagulant from ferrous sulfate The independent variables studied were the temperature 7.
Keywords: environmental process ; optimization technology for process ; process modeling ; chemical product. Received: Revised: Accepted: Published Online: Barbosa Mazza, V. Chemical Product and Process Modeling , 15 3 , Chemical Product and Process Modeling, Vol. Chemical Product and Process Modeling. Copy to clipboard. As every water treater knows, coagulants are used in the first step of water treatment; they destabilize the water so micro flocks can begin to form. This is followed up by a polymer, which turns it into a larger flock by creating a bigger charge.
While not an exhaustive list, here are some important positives and negatives to consider about these chemicals:. Substitutes include other aluminum and iron salts, like sodium aluminate and ferric sulfate, but these may or may not work. Proprietary products, instead of commodities, may offer a better solution. ATS is a great replacement for alum because it does an amazing job, but with a much, much smaller dose. For example, a water treater who feeds 25 parts of alum or a related product might get away with feeding just parts of ATS And because ATS is completely soluble, it will hardly increase solids due to the coagulant.
Alternatively, if a water treatment plant is using ferric chloride, a recommended replacement product is ATS This product is not only a successful replacement but also offers a much better quality of water for the same dollars spent.
Arsenic removals have ranged from approximately 59 to percent at dosages of 0. Best results are obtained at a pH of 5.
As V removals ranged from approximately 70 to The best available technologies for fluoride removal from water are generally considered to be activated alumina adsorption and reverse osmosis.
However, in some cases, fluoride removal by aluminum coagulation has been shown to be cost effective. It appears that several aluminum based coagulants are equally effective, based on the aluminum content added for treatment.
Fluoride removal using aluminum based coagulants is strongly affected by pH and aluminum dosage. Optimum pH varies from 6. A further factor is the residual aluminum remaining after treatment. Higher aluminum dosages often produce lower residual aluminum due to adsorption of fluoride to aluminum hydroxide flocs, rather than producing aluminum-fluoride complexes that remain in solution. Aluminum dosages are generally high for appreciable fluoride removals. For example, to reduce fluoride from 3.
To further reduce the fluoride to 1. In many sensitive catchment areas, chemical phosphorus removal is also required for wastewater treatment. Figure 4 shows the 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. 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 s 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 sludgeproduced, 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 activatedsludge systems. Such urban developments also may not have the areas available for appropriate technology options such as stabilization pond processes.
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: percent TSS; percent COD; percent BOD; percent nitrogen; and 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. The total reaction time from the point of ferric chloride addition to entering the primary clarifiers was approximately 8 minutes at peak flow.
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:.
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. 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. 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. Figure 5 shows the relationship between residual aluminum and treatment 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.
Figure 5. Compilation of residual aluminum determined from jar tests using membrane filters ranging from 0. 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.
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 beforefiltration.
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. 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 s 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. The effects of agitation speed, settling time, pH, coagulant dosages and temperature were examined. FeCl3 was found to be superior to other coagulants tested. At pH 4 and 12, fair removal of suspended solids was observed at a reasonably low coagulant dose, i.
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