METHOD OF COAGULATION SEDIMENTATION PROCESS

Abstract

A method of coagulation sedimentation process for water to be treated, including a process of selecting the G.sub.R value and T.sub.R value that allow energy consumption for rapid agitation to be reduced and turbidity of sedimentation-treated water to be lowered. The method including: a step of injecting an inorganic coagulant into the water to be treated and a rapid agitation step in which injection is carried out, by first setting the same value of G.sub.R.Math.T.sub.R with respect to the G.sub.R value in a range of 150 s.sup.1 to 2000 s.sup.1 and the T.sub.R value in a range of 1 minute to 5 minutes, which are ranges commonly employed in the prior art, and then selecting the G.sub.R value with a smaller numerical value than the G.sub.R values in this range, and selecting the T.sub.R value with a large numerical value that is longer than 10 minutes.

Claims

1. A method of coagulation sedimentation process for water to be treated, the process having an inorganic coagulant injection step of injecting an inorganic coagulant into water to be treated, a micro flocculation step of fine suspended particles in the water to be treated with mixing and agitating the water to be treated in a rapid agitation tank into which the inorganic coagulant has been injected to micro-flocculate in advance, a flocculation step including a step in which micro flocs are further flocculated according to contact with already existing flocs in a sedimentation basin, and a sedimentation separation step of effecting sedimentation and separation of the flocs in the sedimentation basin, wherein a G.sub.R value as a rapid agitation intensity and a T.sub.R value as a rapid agitation time, represented by following formulas, are selected by following processes: $\begin{matrix} G_{R} = \sqrt{\frac{(C .Math. A .Math. v^{3})}{2 .Math. .Math. V}} & [Formula .Math. .Math. 1] \end{matrix}$ Where C: agitation constant, A: agitation blade area (m.sup.2), v: agitation blade peripheral speed (m/s), : kinematic viscosity coefficient (m.sup.2/s), V: agitation tank volume (m.sup.3) 1. In order to satisfy a necessary prescribed level for a concentration of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m at a stage where a rapid agitation step has been completed, initial values for the G.sub.R value and the T.sub.R value are respectively set to a G.sub.R0 value in a range of 150 s.sup.1 to 450 s.sup.1 and 5 minutes, and a minimum G.sub.R1 value is set with 75 s.sup.1 as a lower limit and a maximum T.sub.R1 value is set with 10 minutes as a lower limit with G.sub.R1.Math.T.sub.R1=G.sub.G0.Math.5 minutes being satisfied, 2. Within 5 minutes after starting a rapid agitation operation, after having detected a G.sub.R2 value which is an upper limit for the G.sub.R value where there is no decrease in the concentration of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m at the stage where the rapid agitation step has been completed, a maximum G.sub.R2 value and a minimum T.sub.R2 value are set by a relational expression of G.sub.R25 minutes=R.sub.R2.Math.T.sub.R2, and, 3. As operating conditions at the rapid agitation tank, a G.sub.R3 value and a T.sub.R3 value are selected to satisfy G.sub.R1G.sub.R2G.sub.R2, T.sub.R2T.sub.R2T.sub.R1, and to satisfy G.sub.R3.Math.T.sub.R3600,000.

2. The method of coagulation sedimentation process for water to be treated according to claim 1, wherein as a final stage of the flocculation step, a floc-forming inclined plate whose pitch width is from 5 mm or more to 50 mm or less is provided and the inorganic coagulant at a stage after the micro flocculation step is limited for used amount so that turbidity of the water to be treated after passage through the inclined plate is at a ratio of 4/5 or less than a ratio before the passage.

3. The method of coagulation sedimentation process for water to be treated according to claim 1, wherein the minimum G.sub.R1 value is selected as the G.sub.R3 value and the maximum T.sub.R1 value is selected as the T.sub.R3 value.

4. The method of coagulation sedimentation process for water to be treated according to claim 1, wherein, in a final stage of the rapid agitation tank, the G.sub.R3 value and the T.sub.R3 value are selected so that the turbidity is to be a minimum value.

5. The method of coagulation sedimentation process for water to be treated according to claim 1 wherein, after setting 450 s.sup.1 as the G.sub.R0 value, and 150 s.sup.1 is set as the G.sub.R2 value, and moreover 150 s.sup.1 is selected as the G.sub.R3 value, while 15 minutes is set as the T.sub.R2 value and 15 minutes is selected as the T.sub.R3 value.

6. The method of coagulation sedimentation process for water to be treated according to claim 1 wherein, as the amount of the water to be treated increases, the G.sub.R3 value increases and the T.sub.R3 value decreases, and as the amount of the water to be treated decreases, the G.sub.R3 value decreases and the T.sub.R3 value increases.

7. The method of coagulation sedimentation process for water to be treated according to claim 1, wherein the amount of the inorganic coagulant used increases as the amount of the water to be treated increases, and the amount of the inorganic coagulant used decreases as the amount of the water to be treated decreases.

8. A method of coagulation sedimentation process for water to be treated, the process having an inorganic coagulant injection step of injecting an inorganic coagulant into water to be treated, a micro flocculation step of fine suspended particles in the water to be treated with mixing and agitating the water to be treated in a rapid agitation tank into which the inorganic coagulant has been injected to micro-flocculate in advance, a flocculation step including a step in which micro flocs are further flocculated according to contact with already existing flocs in a sedimentation basin, and a sedimentation separation step of effecting sedimentation and separation of the flocs in the sedimentation basin, wherein a G.sub.R value as a rapid agitation intensity and a T.sub.R value as a rapid agitation time, represented by following formulas, are selected by following processes: $\begin{matrix} G_{R} = \sqrt{\frac{(C .Math. A .Math. v^{3})}{2 .Math. .Math. V}} & [Formula .Math. .Math. 2] \end{matrix}$ Where C: agitation constant, A: agitation blade area (m.sup.2), v: agitation blade peripheral speed (m/s), : kinematic viscosity coefficient (m.sup.2/s), V: agitation tank volume (m.sup.3) 1. In order to satisfy a necessary prescribed level for turbidity at a stage where a rapid agitation step has been completed, initial values for the G.sub.R value and the T.sub.R value are respectively set to a G.sub.R0 value in a range of 150 s.sup.1 to 450 s.sup.1 and 5 minutes, and a minimum G.sub.R1 value is set with 75 s.sup.1 as a lower limit and a maximum T.sub.R1 value is set with 10 minutes as a lower limit with G.sub.R1.Math.T.sub.R1=G.sub.R0.Math.5 minutes being satisfied, 2. Within 5 minutes after starting a rapid agitation operation, after having detected a G.sub.R2 value which is an upper limit for the G.sub.R value where there is no increase in the turbidity at a stage where the rapid agitation step has been completed, a maximum G.sub.R2 value and a minimum T.sub.R2 value are set by a relational expression of G.sub.R25 minutes=G.sub.R2.Math.T.sub.R2, and 3. As operating conditions at the rapid agitation tank, a G.sub.R3 value and a T.sub.R3 value are selected to satisfy G.sub.R1G.sub.R3G.sub.R2, T.sub.R2T.sub.R3T.sub.R1, and to satisfy G.sub.R3.Math.T.sub.R3600,000.

9. The method of coagulation sedimentation process for water to be treated according to claim 8, wherein as a final stage of the flocculation step, a floc-forming inclined plate whose pitch width is from 5 mm or more to 50 mm or less is provided and the inorganic coagulant at a stage after the micro flocculation step is limited for the used amount so that the turbidity of the water to be treated after passage through the inclined plate is at a ratio of 4/5 or less than a ratio before the passage.

10. The method of coagulation sedimentation process for water to be treated according to claim 8, wherein the minimum G.sub.R1 value is selected as the G.sub.R3 value and the maximum T.sub.R1 value is selected as the T.sub.R3 value.

11. The method of coagulation sedimentation process for water to be treated according to claim 8, wherein, in a final stage of the rapid agitation tank, the G.sub.R3 value and T.sub.R3 value are selected so that the turbidity is a minimum value.

12. The method of coagulation sedimentation process for water to be treated according to claim 8 wherein, after setting 450 s.sup.1 as the G.sub.R0 value, 150 s.sup.1 is set as the G.sub.R2 value and 150 s.sup.1 is selected as the G.sub.R3 value, while 15 minutes is set as the T.sub.R2 value and 15 minutes is selected as the T.sub.R3 value.

13. The method of coagulation sedimentation process for water to be treated according to claim 8 wherein, as the amount of the water to be treated increases, the G.sub.R3 value increases and the T.sub.R3 value decreases, and as the amount of the water to be treated decreases, the G.sub.R3 value decreases and the T.sub.R3 value increases.

14. The method of coagulation sedimentation process for water to be treated according to claim 8, wherein the amount of the inorganic coagulant used increases as the amount of the water to be treated increases, and the amount of the inorganic coagulant used decreases as the amount of the water to be treated decreases.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0046] FIG. 1 is a block diagram of a treatment flow process on which the present invention is based.

[0047] FIG. 2 is a pair of graphs representing change in concentration of micro flocs and fine suspended particles with particle sizes of 0.5 to 1.0 m at different G.sub.R values, using raw water obtained from the Yodo river as water to be treated, where (a) shows horizontal axis of the T.sub.R value and (b) shows horizontal axis of G.sub.R.Math.T.sub.R.

[0048] FIG. 3 is a graph showing a comparison of the remaining states of the micro flocs with different particle sizes where the value of G.sub.R.Math.T.sub.R was set to 450 s.sup.15 minutes or 150 s.sup.115 minutes (however, with particle sizes of 0.5 to 1.0 m, the micro flocs partially remain in a state of the fine suspended particles instead of total flocculation), using raw water obtained from the Yodo river as the water to be treated.

[0049] FIG. 4 is a graph showing the state of change in concentration and the state of change in turbidity of the micro flocs with different particle sizes corresponding to the G.sub.R values (however, with particle sizes of 0.5 to 1.0 m, the micro flocs partially remain in the state of the fine suspended particles instead of the total flocculation), after injecting 20 mg/L of kaolin and 13.0 mg/L of PAC into a testing rapid agitation tank, following this with a slow agitation tank, and setting the T.sub.R value to 5 minutes.

[0050] FIG. 5 is a graph showing change in the remaining amount of the micro flocs with different particle sizes (however, with particle sizes of 0.5 to 1.0 m, the micro flocs partially remain in the state of the fine suspended particles instead of the total flocculation), corresponding to variation in the T.sub.R value with the G.sub.R value set to 1500 s.sup.1, in the same testing rapid agitation tank as in FIG. 4.

[0051] FIG. 6 is a graph showing the state of change in the number of the larger micro floc particles exceeding 30 m, corresponding to variation in the T.sub.R value with the G.sub.R value set to 1500 s.sup.1, and STR value in the same testing rapid agitation tank as in FIG. 4.

DESCRIPTION OF EMBODIMENTS

[0052] The present invention achieves successive purification of water to be treated by employing the treatment flow process as shown in FIG. 1, and specifically a rapid agitation tank 1, a high-speed coagulation sedimentation basin 2 with a floc-forming inclined plate 20, a coarse grain filtration tank 3 and a sand filtration tank 4.

[0053] Technical significance of the basic constructions (1) and (2) will be explained at first.

[0054] Particle sizes of suspended particles in the water to be treated are generally distributed across a range of 0.5 m to 60 m, but the proportion consisting of micro flocs and fine suspended particles with particle sizes of 0.5 to 1.0 m exceeds 90% in almost all the water to be treated.

[0055] This proportion is especially notable in countries with developed water supplies under advanced water management, such as in Japan and Europe and the Unites States.

[0056] In coagulation sedimentation treatment of the water to be treated, therefore, a function of allowing decrease in turbidity depends on degree to which remaining amount of the micro flocs and the fine suspended particles can be decreased by aggregation of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m.

[0057] Explaining this point in detail, the concentration N of particles due to flocculation and micro flocculation, according to the general solution of the above-mentioned Smoluchowski equation, is in a proportional relationship with the turbidity, but in the vast majority of cases 4G.sub.RT.sub.R/ is much smaller than 1, i.e. 4G.sub.RT.sub.R/<<1 holds, and such that [Formula 3] can be approximately represented as:

NA(14G.sub.RT.sub.R/).[Formula 8]

[0058] The value of G.sub.R.Math.T.sub.R and a degree of decrease in N, i.e. the degree of decrease in the turbidity, are proportionally related by this approximation.

[0059] On the other hand, FIG. 2(a) shows cases where the G.sub.R value is respectively set to 150 s.sup.1, 450 s.sup.1, 650 s.sup.1, or 1500 s.sup.1 and the T.sub.R value is successively set to 5 minutes, when using raw water obtained from the Yodo river as the water to be treated, and it is seen that when the G.sub.R value was 150 s.sup.1, 450 s.sup.1 or 650 s.sup.1 as shown in FIG. 2(b) where the value of G.sub.R.Math.T.sub.R is on the abscissa, an increase in the value of G.sub.R.Math.T.sub.R on which the turbidity depends, and the decrease in the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m, are approximately proportionally related.

[0060] Even when the G.sub.R value is 1500 s.sup.1, the degree of reduction in the concentration of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m was greater compared to other cases, but the increase in the value of G.sub.R.Math.T.sub.R was still in approximately the proportional relationship up until the T.sub.R value reached approximately 3.5 minutes.

[0061] Such a proportional relationship indeed grounds on a reason that the turbidity of sedimentation-treated water depends on the remaining amount of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m, similar to the value of G.sub.R.Math.T.sub.R in [Formula 8].

[0062] Such a dependence is understood by standing on a fact that the amount of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m is clearly a relatively greater amount than flocs with sizes exceeding 1.0 m, and since their particle sizes are smaller, they significantly influence the turbidity due to their major effect on light scattering during measurement of the turbidity.

[0063] After setting the necessary prescribed level for the concentration of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m at the final stage of the rapid agitation in the process 1 of the basic construction (1), based on the aforementioned proportional relationship, the G.sub.R0 value as the initial G.sub.R value allowing this reference value to be satisfied is set to 150 s.sup.1 to 450 s.sup.1, which is employed as a feasible G.sub.R value for almost all purification plants, and the initial T.sub.R value is set to 5 minutes, as an usual upper limit for the rapid agitation time.

[0064] In contrast, in the process 1 of the basic construction (2), after having set the necessary prescribed level based on the turbidity in the final stage of the rapid agitation in a straightforward manner instead, an initial value G.sub.R0 is set as the same G.sub.R value as the basic construction (1), and 5 minutes is set as the upper limit.

[0065] In the process 1 of the basic constructions (1) and (2), the minimum G.sub.R1 value is set with 75 s.sup.1 as a lower limit and the T.sub.R1 value is set with 10 minutes as a lower limit, with G.sub.R1.Math.T.sub.R1=G.sub.R0.Math.5 minutes being satisfied.

[0066] The reason for setting the lower limit for the T.sub.R1 value to be 10 minutes is to ensure a more moderate agitation state by setting a time of 10 minutes or longer, where 10 minutes is assumed and set as the upper limit for the rapid agitation time in the prior invention already mentioned.

[0067] Setting the G.sub.R1 value to be smaller than the G.sub.R0 value and setting a maximum T.sub.R1 value to be at least twice the 5 minutes that is the upper limit of the normal state of use, is based on rule of thumb that when using a rapid agitation time that has been lengthened due to moderate rapid agitation intensity, a formula G.sub.R0.Math.T.sub.R0=G.sub.R1.Math.T.sub.R1 holds and the turbidity decreases even with the same value of G.sub.R.Math.T.sub.R.

[0068] Actually, when comparing the G.sub.R value of 450 s.sup.1 and the T.sub.R value of 5 minutes with the G.sub.R value of 150 s.sup.1 and the T.sub.R value of 15 minutes, using the raw water obtained from the Yodo river as the water to be treated in the rapid agitation tank 1, despite a fact that both values of G.sub.R.Math.T.sub.R were equivalent at 225060=135,000 as shown in a respective graphs of FIG. 3, the turbidity at the stage where the rapid agitation had been completed was 0.64 degree with 450 s.sup.15 minutes while 0.45 degree with 150 s.sup.115 minutes, indicating considerable improvement in the turbidity in the latter case compared to the former case.

[0069] The reason for this improvement is understood to be that in the latter case, the smaller G.sub.R value and larger T.sub.R value exhibited the same function as slow agitation, lowering the remaining amount of the micro flocs with particle sizes of 15 m or smaller and resulting in the aggregation of the micro flocs with particle sizes of greater than 15 m.

[0070] Incidentally, in graphs of FIG. 4 and FIG. 5 based on a testing rapid agitation tank, the G.sub.R value of greater than 1000 s.sup.1 may result in breakup of larger micro flocs with particle sizes of 15 m or greater, as explained above under Background Art.

[0071] Conversely, no such breakup occurred when the G.sub.R value was equal to or below a prescribed numerical value, i.e. equal to or below 1000 s.sup.1, even in the testing rapid agitation tank, thus providing support that the aggregation is promoted.

[0072] Moreover, judging from the concentration N of the micro flocs and the flocs from the general formula of [Formula 3] and the approximation of [Formula 8], the improvement in the turbidity can be attributed to a larger mean volume of the micro flocs or the floc particles per unit volume, on which the N depends.

[0073] The lower limit for the T.sub.R1 value is 10 minutes and the upper limit for the G.sub.R0 value is 450 s.sup.1.

[0074] Therefore, the upper limit for the G.sub.R1 value, calculated from 4505=G.sub.R110, is 225 s.sup.1.

[0075] Since the upper limit for the G.sub.R0 value is 450 s.sup.1 while the lower limit for the G.sub.R1 value is 75 s.sup.1, the upper limit for the T.sub.R1 value, calculated from 4505=75T.sub.R1, is 30 minutes.

[0076] In the basic constructions (1) and (2), for selection of the G.sub.R value and T.sub.R value in prescribed ranges, it is essential to set the maximum G.sub.R value and the minimum T.sub.R value on the assumption of 10 minutes as the lower limit for the T.sub.R value.

[0077] In the process 2 of the basic construction (1), within 5 minutes after starting a rapid agitation operation, after having detected the G.sub.R2 value which is the upper limit for the G.sub.R value where there is no decrease in the concentration of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m at the stage where the rapid agitation step has been completed, the maximum G.sub.R2 value and the minimum T.sub.R2 value are set by the relational expression: G.sub.R25 minutes=G.sub.R2.Math.T.sub.R2, while in the process 2 of the basic construction (2), within 5 minutes after starting the rapid agitation operation, after having detected the G.sub.R2 value which is the upper limit for the G.sub.R value where there is no increase in the turbidity at the stage where the rapid agitation step has been completed, the maximum G.sub.R2 value and the minimum T.sub.R2 value are set by the relational expression: G.sub.R25 minutes=G.sub.R2.Math.T.sub.R2.

[0078] The reason for setting these G.sub.R2 value upper limits means that detection of the upper limit at which the concentration of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m does not decrease (for the basic construction (1)) or detection of the upper limit at which the turbidity does not decrease (for the basic construction (2)) at the stage after completion of the rapid agitation step, after having passed the normal upper limit for the rapid agitation time, i.e. 5 minutes, means that both cases set the upper limit for the G.sub.R value that avoids breakup of the larger micro flocs with particle sizes of 15 m or greater.

[0079] In these cases, setting the maximum G.sub.R2 value as G.sub.R2=G.sub.R2 (5/T.sub.R2) by the aforementioned relational expression results in a situation in which, under the normal state of use, the breakup of the larger micro flocs naturally cannot take place, even with the maximum G.sub.R2 value based on the proportion of (5/T.sub.R2) with respect to G.sub.R2 as the upper limit for the G.sub.R value that avoids breakup of the larger micro flocs.

[0080] In addition, the turbidity at the final stage of the rapid agitation step can be further lowered with G.sub.R2T.sub.R2, compared to G.sub.R25 minutes.

[0081] Explaining concretely, even though the G.sub.R2 value is usually well above 450 s.sup.1, the turbidity at the stage where the rapid agitation step has been completed still falls when the G.sub.R value has been decreased and the T.sub.R value has been increased with the same value of G.sub.R.Math.T.sub.R, even if the G.sub.R value is 450 s.sup.1 or greater, similar to the comparison between G.sub.R05 minutes and G.sub.R1T.sub.R1.

[0082] Actually, when 1500 s.sup.11.5 minutes and 450 s.sup.15 minutes in the graph shown in FIG. 2(a) are compared, even though both have equal G.sub.R.Math.T.sub.R values of 135,000, the remaining amount of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m is 400,000/mL in the former while 150,000/mL in the latter, demonstrating that the latter clearly has fewer.

[0083] In the case shown in the graph of FIG. 2(a), when the G.sub.R value is 1500 s.sup.1, the micro flocs and the fine suspended particles in the remaining state with particle sizes of 0.5 to 1.0 m switches to an increase at the stage where the T.sub.R value is 3.5 minutes.

[0084] A cause for this increasing state comes from breakup of the larger micro flocs, and also necessarily indicates increased turbidity as well.

[0085] Explaining in more detail for the cause of this breakup of the larger micro flocs, FIG. 6, similar to FIG. 5, shows the state of change in the number of particles exceeding a particle size of 30 m, and a STR value (Suction Time Ratio: a ratio as an index represented by T.sub.S/T.sub.V, when distilled water with the same temperature and the same amount as the water to be treated has been drawn into the same filter paper at the same drawing level, where the drawing time of the water to be treated is T.sub.S and the supply time for the distilled water is T.sub.V), after setting the G.sub.R value to 1500 s.sup.1, in a testing rapid agitation tank used to collect data for FIG. 4.

[0086] The state of decrease in the larger micro flocs exceeding a particle size of 30 m is identical to the state shown in FIG. 5, and the STR value successively decreases with increasing T.sub.R value.

[0087] This decrease in the STR value supports an interpretation that, according to the general solution and approximation of the Smoluchowski equation shown as [Formula 3] and [Formula 8], the collision efficiency decreases due to the effect of an inorganic coagulant and breakup of the larger micro flocs takes place associated with the rapid agitation, while the decrease creates a situation in which it is not possible to provide replenishment by formation of the larger micro flocs in approximately equal amount as the breakup.

[0088] Therefore, the 1500 s.sup.1 cannot be considered as a detected value for G.sub.R2.

[0089] In the graph of FIG. 4 where the T.sub.R value is 5 minutes, the turbidity tends to increase with a G.sub.R value of 1500, but the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m are decreased, which is a different state than represented in the graph of FIG. 2(a).

[0090] The main causes for this difference are that the concentration of the suspended particles, and particularly the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m, contained in the water to be treated is clearly greater in the graph of FIG. 2(a) than in FIG. 4, and that in the case of the graph of FIG. 2(a), a slow agitation step does not come afterward, whereas it does come afterwards in the case of the graph in FIG. 4.

[0091] In the case shown by the graph of FIG. 2(a), when the G.sub.R value is 650 s.sup.1, the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m decreases only very slightly, and therefore the turbidity does not increase, with the rapid agitation treatment for 5 minutes, such that 650 s.sup.1 can be considered as the detected value for the upper limit G.sub.R2 value.

[0092] When the lower limit of 10 minutes is set as the minimum T.sub.R2 value, it is possible to set 650/2 (650/2 (S.sup.1)=325 s.sup.1 as the maximum G.sub.R2 value for the rapid agitation shown in the graph of FIG. 2(a).

[0093] In the operating conditions in the rapid agitation tank 1 by the process 3 of the basic constructions (1) and (2) , the G.sub.R3 value and the T.sub.R3 value are selected to satisfy G.sub.R1G.sub.R3G.sub.R2 and T.sub.R2T.sub.R3T.sub.R1, and to satisfy G.sub.R3.Math.T.sub.R3600,000.

[0094] In above selection, the G.sub.R3 value is in the range between the minimum G.sub.R1 and the maximum G.sub.R2 value while the T.sub.R3 value is in the range between the lower limit for the T.sub.R value, i.e. T.sub.R2, and the maximum T.sub.R1 value, but such selection allows a wide numerical range to be obtained for appropriate values of G.sub.R and T.sub.R.

[0095] The reason for the inequality for the G.sub.R3.Math.T.sub.R3 value is that the basic constructions (1) and (2) are based on a major assumption that the value of G.sub.R3.Math.T.sub.R3 is selected so as to be the same as the value of G.sub.R.Math.T.sub.R in normal rapid agitation, making it essential to establish G.sub.R3.Math.T.sub.R320005 minutes60=600,000.

[0096] In contrast, the lower limit for the value of G.sub.R.Math.T.sub.R in the normal rapid agitation is 150 (s.sup.1)1 minute60=9000, but the lower limit for the G.sub.R3.Math.T.sub.R3 value is 751060=45000, which is clearly larger than the above-mentioned lower limit of 9000, and therefore the conditions for the lower limit do not necessarily need to be set.

[0097] The following is the reason that selection of the G.sub.R3 value and the T.sub.R3 value in the process 3 allows energy consumption to be reduced per rapid agitation unit.

[0098] As is already explained, the following formula:

P=G.sub.R.sup.2[Formula 9]

(where is the viscosity coefficient)
holds between the energy P required for rapid agitation per unit time and unit volume in the rapid agitation tank 1 per unit time and unit volume, and the G.sub.R value.

[0099] Therefore, the following formula holds for the energy consumption, when also taking into account the rapid agitation time for the rapid agitation, with selection of the G.sub.R3 value and T.sub.R3 value in process 3:

[00004] $\begin{matrix} \begin{matrix} P .Math. T_{R .Math. .Math. 3} = .Math. .Math. .Math. G_{R .Math. .Math. 3}^{2} .Math. T_{R .Math. .Math. 3} \\ = .Math. (G_{R .Math. .Math. 3} .Math. T_{R .Math. .Math. 3}) .Math. G_{R .Math. .Math. 3} . \end{matrix} & [Formula .Math. .Math. 10] \end{matrix}$

[0100] In the normal rapid agitation state, where the numerical value for the rapid agitation intensity is set in the range of 150 s.sup.1 to 2000 s.sup.1 and the numerical value for the rapid agitation time is set in the range of 1 minute to 5 minutes, and with the G.sub.R value and the T.sub.R value as G.sub.R and T.sub.R, respectively, so long as 150 s.sup.1G.sub.R2000 s.sup.1 holds and 1 minuteT.sub.R5 minutes holds, then 9000G.sub.R.Math.T.sub.R600,000 also holds.

[0101] Therefore, so long as the numerical range for the value of G.sub.R.Math.T.sub.R is larger than the numerical range for the value of G.sub.R3.Math.T.sub.R3, then for the G.sub.R.Math.T.sub.R value it is of course possible to set a state in which the following formula:

G.sub.R.Math.T.sub.R=G.sub.R3.Math.T.sub.R3[Equation 11]

holds between the G.sub.R3 value and the T.sub.R3 value, i.e. a state in which the concentrations N of the micro flocs and the flocs are theoretically equal at the stage where the rapid agitation step has been completed.

[0102] In this relational expression, T.sub.R5 minutes<10 minutes T.sub.R3 holds, and therefore G.sub.R>G.sub.R3 holds.

[0103] Therefore, the following equality holds:

P.Math.T.sub.R3=(G.sub.R.Math.T.sub.R)G.sub.R3

custom-character (G.sub.R.Math.T.sub.R)G.sub.R.[Formula 12]

[0104] As is also clear from the inequalities shown above, with the G.sub.R3 value and the T.sub.R3 value selected by the process 3 of the basic constructions (1) and (2), it is possible to produce a state with smaller energy consumption for the rapid agitation, compared to normal use with the G.sub.R value and the T.sub.R value that realize the same concentration N.

[0105] By comparing the G.sub.R value of 1500 s.sup.1 and the other numerical values in FIG. 2(a), it is also clearly seen that, in a state of the normal rapid agitation, when the G.sub.R value has exceeded a prescribed limit within the rapid agitation time of no longer than 5 minutes, this can result in an increased remaining amount of the micro flocs and the fine suspended particles with particle sizes of 0.5 to 1.0 m at the final stage of the rapid agitation due to breakup of the larger micro flocs, and therefore the increased turbidity, but such increase associated with breakup is impossible with the basic constructions (1) and (2) where the G.sub.R3 value and the T.sub.R3 value are selected by the process 3.

[0106] Furthermore, apart from presence or absence of the aforementioned breakup of the larger micro flocs, the turbidity at the stage where the rapid agitation step has been completed decreases compared to the normal state of use even with the same value of G.sub.R.Math.T.sub.R, due to selecting a small G.sub.R value and a large T.sub.R value, and specifically by a fact that G.sub.R>G.sub.R3, T.sub.R<T.sub.R3 are satisfied with respect to the G.sub.R value and the T.sub.R value that have been set in the normal state of use, as has already been explained with reference to FIG. 2(a) and FIG. 3.

[0107] Individual embodiment will now be explained.

[0108] In the basic constructions (1) and (2), it is possible to employ an embodiment according to the prior invention, having the requirement and feature of, as the final stage of the flocculation step, providing the floc-forming inclined plate 20 whose pitch width is from 5 mm or more to 50 mm or less and also using the inorganic coagulant at a stage after the micro flocculation step in a limited manner so that the turbidity of the water to be treated after passage through the inclined plate 20 is at a ratio of 4/5 or less than a ratio before the passage.

[0109] When such embodiment is employed, it is possible to achieve an effect of obtaining high-quality clear water by forming refined and highly densified micro flocs similar to the prior invention, while also decreasing the amount of sludge generated in association with use of the inorganic coagulant, in addition to achieving the effect of the present invention.

[0110] In order to minimize P.Math.T.sub.R as the energy consumption for the rapid agitation, the minimum G.sub.R1 value is selected as the G.sub.R3 value and the maximum T.sub.R1 value is selected as the T.sub.R3 value in the process 3.

[0111] This is because when the value of G.sub.R.Math.T.sub.R is constant, the energy consumption is proportional to the value G.sub.R of the rapid agitation intensity, as is also clear from a general formula for P.Math.T.sub.R.

[0112] If the G.sub.R3 value and the T.sub.R3 value are selected in the process 3 for each of the basic constructions (1) and (2) in a manner so as to result in the lowest turbidity, then it is possible to realize suitable aggregation and sedimentation of the water to be treated in the subsequent purification treatment.

[0113] In actual practice, in light of the comparison between 1500 s.sup.11.5 minutes and 450 s.sup.15 minutes in FIG. 2(a), and comparison between 450 s.sup.15 minutes and 150 s.sup.115 minutes in FIG. 3, selection of the lowest turbidity is interpreted as, in practice, meaning selection of the minimum G.sub.R1 value for minimizing the energy consumption P.Math.T.sub.R.

[0114] As is explained above, the numerical value that is set as the G.sub.R0 value selected within the range of 150 s.sup.1 to 450 s.sup.1 depends on a specific conditions in the rapid agitation tank.

[0115] However, complex experimentation is necessary to judge what a suitable upper limit is in practice.

[0116] In such cases, when 450 s.sup.1 is set as the initial G.sub.R0 value in the process 1 of the basic constructions (1) and (2), while in the process 3, 150 s.sup.1 is set as the G.sub.R2 value, 150 s.sup.1 is selected as the G.sub.R3 value, 15 minutes is set as the T.sub.R2 value, and 15 minutes is selected as the T.sub.R3 value, as shown in FIG. 3, this will be suitable for almost all rapid agitation tanks, and it will also be possible to both ensure satisfactory turbidity and accomplish efficient rapid agitation, and to eliminate need for the aforementioned complex experimentation.

[0117] Examples of the invention will now be described.

EXAMPLE 1

[0118] Example 1 is characterized in that as the amount of the water to be treated increases, the G.sub.R3 value increases and the T.sub.R3 value decreases, and as the amount of the water to be treated decreases, the G.sub.R3 value decreases and the T.sub.R3 value increases.

[0119] The reason of such a character is based on a fact that the increase and the decrease in the water to be treated must necessarily match the increase and the decrease in the water to be treated per unit time passing through the rapid agitation tank 1, and as a result, the peripheral speed v of the rapid agitation tank 1 must be increased or decreased to match it, as per the following formula:

[00005] $\begin{matrix} G_{R} = \sqrt{\frac{(C .Math. A .Math. v^{3})}{2 .Math. .Math. V}} & [Formula .Math. .Math. 13] \end{matrix}$

which also requires the increase or the decrease in the G.sub.R value.

[0120] Because of this feature, it is possible to maintain a function of the rapid agitation tank 1 while exhibiting the effect of the present invention, adapting to the increase and the decrease in the water to be treated.

EXAMPLE 2

[0121] Example 2 is characterized in that the amount of the inorganic coagulant used increases as the amount of the water to be treated increases, and the amount of the inorganic coagulant used decreases as the amount of the water to be treated decreases.

[0122] Adjustment of the amount of the inorganic coagulant used depending on the amount of the water to be treated is also obvious from common technical knowledge.

[0123] In addition, based on the general formula mentioned above:

N=A exp(4G.sub.RT.sub.R/)[Formula 14]

(where A: initial value of the N at the stage t=0),
the collision efficiency is kept in a constant state by adjusting the amount of the inorganic coagulant used regardless of any increase or decrease in the water to be treated, and consequently the concentration N of the micro flocs and the flocs, which reflect the turbidity, can be kept constant.

INDUSTRIAL APPLICABILITY

[0124] The present invention makes it possible to reduce the energy consumption per the rapid agitation unit, compared to the normal state of use of the rapid agitation tank, and to ensure more satisfactory turbidity, allowing it to be utilized for the coagulation sedimentation treatment of almost any type of the water to be treated.

REFERENCE SIGNS LIST

[0125] 1 Rapid agitation tank [0126] 2 Sedimentation basin [0127] 20 Inclined plate [0128] 3 Coarse grain filtration tank [0129] 4 Sand filtration tank

METHOD OF COAGULATION SEDIMENTATION PROCESS

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Abstract

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