| JP57094387 | TREATMENT OF WASTE WATER |
| JP11138177 | BATHWATER CLEANER |
| WO/1988/006570 | BIOLOGICAL DEPHOSPHATIZATION AND (DE)NITRIFICATION |
| 1. | Coagulant for purifying water and waste water, the coagulant comprising salts of aluminum and/or iron, such as A1C13, Fe2Cl3, characterized in that the coagulant in addition to said salts contains a dissociateable calcium compound. |
| 2. | The coagulant of claim 1, characterized in that the weight ratio between Ca:Al or Ca:Fe, alternatively Ca:(Al+Fe) is 1-10:1. |
| 3. | The coagulant of claim 1 or 2, characterized in that the coagulant is pre-polymerized. |
| 4. | Method for purifying water, in which crude water, after addition of a coagulant containing salts of Al and/or Fe, alternatively during supply of pH/alkalinity controlling compounds, is subjected to direct filtration or transported to a flocculation step, a sedimentation step, a filtration step and optionally a final admixing step to adjust the pH/alkalinity, characterized in that the coagulant added to the crude water, in addition to said salts, contains a dissociateable calcium compound, thus eliminating a separate addition of lime. |
| 5. | The method of claim 4, characterized in that the added coagulant exhibits a weight ratio between Ca:Al or Ca:Fe, alternatively Ca:(Al+Fe) of 1-10:1. |
| 6. | The method of claim 4 or 5, characterized in adjusting the pH/alkalinity by adding sodium carbonate, sodium bicarbonate, lye and/or carbon dioxide. |
| 7. | Method for purififying waste water with respect to suspended material, organic material, phosphorus, heavy metals etc., the water after the addition of a coagulant containing salts of Al and/or Fe being subjected to flocculation, sedimentation, flotation and optionally filtration, optionally in connection with a combined physical/biolocical/chemical purification process, characterized in that the coagulant, in addition to said salts, supplied to the waste water, contains a dissociateable calcium compound, thus decreasing the demand for coagulant dose and pH control. |
| 8. | The method of claim 7, characterized in that the coagulant added exhibits a weight ratio between Ca:Al or Ca:Fe, alternatively Ca:(Al+Fe) of 1-10:1. |
The invention concerns a coagulant for treating water and waste water, and a method for purifying water and waste water, as stated in the introductory of claims 1,4 and 7, respectively.
Technical Ωeld.
Coagulation is a well known process for removing suspended and colloidal material (turbidity, colour, etc.) from water. Known coagulant or coagulants are aluminum chloride, aluminum sulphate, iron chloride, iron sulphate and relatively novel pre-polymerized aluminum salts based on chlorides and sulphates. In this connection, the term "pre-polymerized" means hydrated metal ions mutually connected via oxygen bridges. The coagulants contain basically metal (Al,Fe) chloride, sulphate, hydroxide and water. Norwegian water and water from several countries in the temperate zones is frequently soft, acidic, has low alkalinity and often exhibit a high humus content, resulting in a very corrosive water with respect to the majority of piping equipment. In order to reduce the corrosiveness, lime (CaCOH^) is often supplied to increase the calcium content, pH and alkalinity. Carbon dioxide often has to be added to further increase the alkalinity and to increase the solubility of lime, e.g. from an alkaline filter. Moreover, an increased content of calcium in drinking water is desireablefor health considerations.
Traditional processes for water treatment will thus consist of a coagulating process including a separation step in the form of flotation and precipitation and sedimentation including a filtering step, and in addition a step of water treatment for corrosion control purposes. Such traditional processes are, however, comprehensive and expensive with respect to processing of lime to so as acheive supply of the lime without leading to operation problems. The addition of lime usually takes place by dosage from a raw material container in the form of a lime silo via a worm conveyor to a slurry tank to dissolve the lime in water, and is then subjected to a post- treatment step in a lime water processing unit, prior to supply to the crude water to be treated.
Such traditional methods for adding lime result in large additional investments particularly for small watervorks. Moreover, such dosage systems do exhibit irregularities in operation e.g. because of lime clogging pipes, worm conveyor etc. Traditional lime supply installations require a relatively large area, an aspect that itself can involve increased investment costs.
Object.
The object of the present invention is to provide a simplified method for purifying water and waste water and to provide a coagulant for use in water purification to eliminate the disadvantages of lime dosage and to decrease the amount of coagulant added in such processes.
The invention.
These objects are achieved by a method as stated in the characterizing part of patent claim 4 and 7, and with a coagulant as stated in the characterizing part of patent claim 1. Further beneficial features of the invention appears in the respective independent claims.
The present invention concerns a simplified purification process by using a novel coagulant of the type comprising Al or Fe salts, such as A1C1 3 , Al 2 (SO 4 ) 3 , FeCI- 2 , FeCl 3 , optionally pre-polymerized, the coagulant being characterized by an additional content of calcium. Thus, the step of lime dosage can be eliminated in a water purification process. The calcium content will also decrease the required dosage of coagulant, and results in the production of less mud in a purification process. Moreover, the wreck water volume after filter backflushing is reduced, when compared with the use of known coagulants, as described in further detail below. Moreover, the content of calcium will result in an exteded pH-range for optimum coagulation, an effect that was discovered in connection with purification of hard versus soft water. The calcium compound in the present coagulant exists in such form that it dissosiates into calcium ions by the addition of water. One example of such calcium compound is calcium chloride. Another example of such calcium compounds is calcium carbonate, but because of its low water solubility, the coagulant should be
accompanied by the addition of acid or CO 2 gas, either in the production of a liquid coagulant solution or during the admixture to the water to be treated.
The mass ratio between calcium and aluminum, or optionally iron, with respect to the elements, can be varied according to the field of use and to the desired calcium content in the treated final water, and lies typically in a range from 1:1 to 10:1, respectively, (Ca:Al or Ca:Fe; alternatively Ca:(Al+Fe)). For treating drinking water one would normally choose a weight ratio in the upper region, whereas a weight ratio in the lower region is used for treating waste water. A normal coagulant dose for treating drinking water with Fe or Al salts is from 2 to 5 mg/litre, based upon the elements. Thus one can obtain a calcium content in treated final water of from 15 to 25 mg Ca pr. litre and thus fulfil the authorities\' requirements with respect to corrosion control. With respect to waste water, the coagulant dose is normally about 10 times higher. The benefit in this connection arises from the fact that the increased calcium content (the increased hardness) results in significant simplification of the chemical purification process, characterized by decreased coagulant doses and relaxed demand for pH control during the coagulation.
For the benefit of customers, the coagulant is preferably provided in liquid form based on pre-polymerized metal salts, normally sulphates or chlorides, of iron or aluminum, in which calcium is present as Ca + , e.g. as dissociated calcium chloride. Accordingly the addition of the coagulant becomes relatively convenient. Calcium compounds other than calcium chloride and calcium carbonate can also be used, provided that they form calcium ions and do not represent any negative effect with respect to the environment or health. Accordingly, in a process for treating drinking water, the step of lime addition can preferably be omitted, as described above, and a coagulant according to the present invention used instead, the coagulant per se containing a dissociated or dissociateable calcium compound. Such process can be accomplished as a direct filtering process, as described in a following example, or as a multiple step process by an initial addition of coagulant, flocculation, sedimentation, filtration and optionally a final admixing of pH controlling/alkality controlling compounds, such as carbon dioxide, sodium bicarbonate, sodium carbonate or sodium hydroxide. Controlling pH and alkalinity is prefereably and usually accomplished by the addition of coagulant to
alkalinity is prefereably and usually accomplished by the addition of coagulant to establish an optimal pH range in connection with the coagulating process.
The coagulating agent according to the present invention can be supplied in the form of a powder and dissolved in the water to be treated under stirring or similar action, or injected directly into the water as a water based coagulant, optionally based on pre-polymerized Al/Fe compounds.
Example.
The object of the present example is to illustrate the effect of the use of a coagulant according to the present invention, in comparison with commercial coagulants in treating drinking water in a direct filtering process.
The following coagulants were used: A: Pre-polymerized Al ions, with a concentration of Al of 102.62 gram pr. litre coagulant solution. B: Powdery Al based coagulant dissolved in water to a concentration of Al of
23.71 gram Al per litre coagulant solution. C: A coagulant according to the present invention consisting of Ca ions and pre-polymerized Al ions, having a concentration of Al of 17.49 grams per litre coagulant solution, and a concentration of Ca of 150.64 grams per litre coagulant solution.
Several filtering cycles were run during the tests, by using the respective coagulants to determine optimum coagulant dose and coagulation pH. The values for coagulant doses and coagulation pH determined thereby are set forth in Table 1 below. The purification process was performed as a common direct filtration, i.e. purification without flotation, sedimentation and so on. The filtering rate was 7-14 m/h, and the pH was adjusted by adding NaHCO 3 (aq) in a concentration of 80 mg/litre.
Table 1.
Optimal coagulant dose and coagulant pH
During the tests samples were taken from filtrated water once per filter cycle. The samples were analyzed with respect to colour, turbidity, pH, alkalinity and remaining content of aluminum. The results of these analyses are set forth in Table 2 below.
Table 2.
Characerization of purfied water by using different coagulants.
The ripening water, i.e. water produced immeditely after the finish of a backflush of the filter, was also examined in connection with the different coagulants. The results are set forth in Table 3 below.
Table 3.
Duration of the ripening period and maximum turbidity of the ripening water.
Filter mud resulting from the cycles using different coagulants was examined with respect to solid content and mud volume after thickening of flushing water for at least four hours. The results from these examinations are set forth in Table 4 below.
Table 4.
Solid content and mud volume in sedimented flushing water.
As appears fom the results above, coagulant C according to the present invention produced ripening water with best quality (lowest turbidity) and least ripening water of the coagulants applied. The required coagulant dose was clearly lowest by using the coagulant according to the present invention (a magnitude of 25% lower), and provided least production of mud. The content of calcium in the present coagulant also secures a purified water that satisfies the requirements concerning corrosion control.
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