METHOD FOR TREATING BALLAST WATER

Abstract

This method entails: collecting in advance untreated ballast water to which a chlorine-based active substance has not been added; measuring in advance the turbidity of the untreated ballast water; and adding a chlorine-based active substance with an adding amount determined on the basis of the turbidity. The amount of the chlorine-based active substance to be added is set according to the turbidity such that the concentration of total residual oxidants (TRO) is 0.5-3 mg/L (asCl.sub.2) when the ballast water is discharged.

Claims

1. A method for treating ballast water by adding a chlorine-based active substance for a bactericidal treatment on aqueous microorganisms in the ballast water when supplying obtained ballast water to a ballast tank, wherein turbidity of untreated ballast water not added with any chlorine-based active substance is measured in advance, and a chlorine-based active substance with an adding amount determined based on the turbidity is added to neutralize the ballast water.

2. The method for treating ballast water according to claim 1, wherein a total residual oxidizing substance concentration (TRO) in the ballast water is 0.5 to 3 mg/L (asCl.sub.2) at the time of discharging.

3. The method for treating ballast water according to claim 1, wherein the chlorine-based active substance is added to attain 2 to 14 mg/L (asCl.sub.2) when a value of the turbidity is less than 10 NTU, the chlorine-based active substance is added to attain 2 to 30 mg/L (asCl.sub.2) when 10 NTU or more but less than 50 NTU, and the chlorine-based active substance is added to attain 18 to 30 mg/L (asCl.sub.2) when 50 NTU or more.

4. The method for treating ballast water according to claim 3, wherein when a value of the turbidity is 10 NTU or more but less than 50 NTU, a concentration of a chlorine-based active substance to be added is determined so as to satisfy
C=0.4X+a  (1) (In the formula, ‘C’ is a concentration of chlorine-based active substance to be added, ‘X’ is turbidity and ‘a’ is 2 to 10).

5. The method for treating ballast water according to claim 1, wherein the chlorine-based active substance is one or more kinds selected from dichloroisocyanuric acid, trichloroisocyanuric acid and hypochlorite.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0022] FIG. 1 A graph showing a relation between turbidity and an adding concentration of sodium hypochlorite (chlorine-based active substance) in the treatment method of ballast water according to an embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

[0023] Below, the treatment method of ballast water of the present invention will be explained in detail based on an embodiment.

[0024] The treatment method of ballast water of the present embodiment is to determine an adding amount of a chlorine-based active substance for a bactericidal treatment on aqueous microorganisms in ballast water when supplying the ballast water taken from a water inlet, wherein untreated ballast water not added with any chlorine-based active substance is obtained in advance, turbidity of the unreacted ballast water is measured in advance, a chlorine-based active substance with an adding amount determined based on the turbidity is added to neutralize the ballast water at discharging. Here, as a chlorine-based active substance, one or more kinds selected from dichloroisocyanuric acid, trichloroisocyanuric acid and hypochlorites may be used and particularly hypochlorite, such as sodium hypochlorite, is preferable because bactericidal property thereof is excellent and calculation in a logarithm equation using a total residual oxidizing substance concentration, which will be explained later on, approximates to an actual measurement value to a certain degree.

[0025] Note that a total residual oxidizing substance concentration indicates TRO (Total Residual Oxidants) and includes other oxidizing components generated by an oxidizing chlorine concentration as a result of adding a chlorine-based active substance and a reaction with the oxidizing chlorine. The total residual oxidizing substance concentration can be measured at a normal temperature by using a market-available high-precision TRO meter using DPD absorptiometry.

[0026] An adding amount of the chlorine-based active substance is set in accordance with turbidity so that the total residual oxidizing substance concentration (TRO) after neutralizing the ballast water becomes 0.5 to 3 mg/L (asCl.sub.2) at the time of discharging. When the total residual oxidizing substance concentration (TRO) is less than 0.5 mg/L, it is difficult to reduce harmful planktons and bacteria, etc. to a reference value or lower or it leads to repopulation of bacteria and hatch of plankton eggs. On the other hand, when exceeding 3 mg/L, no further bactericidal effect on harmful planktons and bacteria, etc. can be obtained and, moreover, a neutralizer amount necessary for neutralizing increases or environmental burden increases when discharging, which are not preferable.

[0027] Specifically, when the turbidity value is less than 10 NTU, a chlorine-based active substance is added so as to attain 2 to 14 mg/L (asCl.sub.2), when 10 NTU or more but less than 50 NTU, the chlorine-based active substance is added so as to attain 2 to 30 mg/L (asCl.sub.2), and when 50 NTU or more, the chlorine-based active substance is added so as to attain 18 to 30 mg/L (asCl.sub.2), consequently, the total residual oxidizing substance concentration (TRO) at the time of discharging can be 0.5 to 3 mg/L (asCl.sub.2). Note that control based on turbidity as explained above may be done by using a turbidity meter.

[0028] Particularly when the turbidity value is 10 NTU or more but less than 50 NTU, by determining the concentration of the chlorine-based active substance to be added to be in a range of 2 to 30 mg/L (asCl.sub.2), which satisfies the formula (1) below

C=0.4X+a  (1)

(In the formula, ‘C’ is a concentration of added chlorine-based active substance, ‘X’ is turbidity and ‘a’ is 2 to 10.), the total residual oxidizing substance concentration (TRO) at discharging of 0.5 to 3 mg/L (asCl.sub.2) can be obtained.

[0029] Also, when the turbidity value is less than 10 NTU, by applying the maximum value of ‘a’ in the formula (1) above and determining the concentration of the adding chlorine-based active substance to be in a range of 2 to 14 mg/L (asCl.sub.2), which satisfies the formula (2) below

C.sub.2=0.4X+10  (2)

(In the formula, ‘C.sub.2’ is a concentration of a chlorine-based active substance to be added and ‘X’ is turbidity.), the total residual oxidizing substance concentration (TRO) at discharging of 0.5 to 3 mg/L (asCl.sub.2) can be obtained.

[0030] When discharging ballast water, a reductant is supplied to the ballast water to be discharged so as to reduce residual chlorine, and the residual chlorine concentration is reduced to a targeted residual chlorine concentration before discharging to the external environment. As the reductant to be supplied from a reductant supply mechanism, sodium sulfite, sodium bisulfite (sodium hydrogensulfite) and sodium thiosulfate, etc. may be used.

[0031] The present invention was explained above based on one embodiment, however, the present invention is not limited to the embodiment and includes a variety of modified embodiments. For example, the total residual oxidizing substance concentration is not limited to measurement by a TRO meter using the DPD absorptiometry and a variety of measurement means may be used as long as corresponding measurement values can be obtained.

EXAMPLES

[0032] The present invention will be explained furthermore in detail with specific examples below.

Examples 1-11 and Comparative Examples 1-4

[0033] Seawater at ports of 10 places (seawater 1 to 10) were sampled and turbidity in each seawater was measured. To the respective seawater, sodium hypochlorite was added to attain the concentrations (in terms of chlorine) shown in Table 1. Then, each seawater was sealed and left still for 2 hours at 25° C. in a dark room before measuring its total residual oxidizing substance concentration by using the DPD method and the results are shown in Table 1. Note that those with the total residual oxidizing substance concentration (TRO) within a range of 0.5 to 3 mg/L (asCl.sub.2) in Table 1 were considered as examples and others as comparative examples. In FIG. 1, those are indicated as being within the range of the thick solid lines (◯) and outside the range (). Also, a relation between turbidity and a concentration of sodium hypochlorite to be added (in terms of chlorine) in each of those examples 1 to 11 and comparative examples 1 to 4 is shown in FIG. 1, where those within the range of the thick solid lines are indicated as examples (◯) and those outside the range as comparative examples (), respectively.

TABLE-US-00001 TABLE 1 Concentration Seawater Turbidity of Adding TRO Example No. No. (NTU) (mg/L) (mg/L) Example 1 Seawater 1 0 4.0 1.2 Example 2 Seawater 2 1.5 12.2 2.2 Example 3 Seawater 3 2.7 9.0 1.8 Example 4 Seawater 4 3 7.0 1.8 Example 5 Seawater 4 3 11.5 1.5 Example 6 Seawater 5 4.9 12.9 2.5 Example 7 Seawater 6 6.2 10.3 2.7 Example 8 Seawater 7 11 8.5 2.5 Example 9 Seawater 8 30 15.9 1.0 Example 10 Seawater 9 40 17.6 0.6 Example 11 Seawater 60 29.9 0.9 10 Comparative Seawater 4 3 15.0 5.0 Example 1 Comparative Seawater 9 40 10.0 0.2 Example 2 Comparative Seawater 9 40 30.0 4.5 Example 3 Comparative Seawater 60 12.8 0.2 Example 4 10

[0034] As is clear from Table 1 and FIG. 1, in the examples 1 to 7, wherein the turbidity value was less than 10 NTU and the concentration of sodium hypochlorite to be added was in a range of 2 to 14 mg/L (asCl.sub.2), the total residual oxidizing substance concentration after being left still for 2 hours was in a range of 0.5 to 3 mg/L (asCl.sub.2), while in the comparative example 1, wherein the concentration of sodium hypochlorite to be added was 3 mg/L (asCl.sub.2), the total residual oxidizing substance concentration after being left still for 2 hours was high as 5 mg/L (asCl.sub.2), which was a level of requiring a large amount of neutralizer.

[0035] In the examples 8 to 10, wherein the turbidity value was 10 NTU or more and less than 50 NTU and the concentration of sodium hypochlorite to be added satisfies the formula (1) above, the total residual oxidizing substance concentration after being left still for 2 hours was in the range of 0.5 to 3 mg/L (asCl.sub.2), while in the comparative example 2 with the concentration of sodium chlorite to be added being 10 mg/L (asCl.sub.2) not satisfying the formula (1), the total residual oxidizing substance concentration after being left still for 2 hours was low as 0.2 mg/L (asCl.sub.2), which was a level of being difficult to reduce harmful planktons and bacteria, etc. to the reference value or lower. On the other hand, in the comparative example 3 with the concentration of sodium hypochlorite to be added being 30 mg/L (asCl.sub.2) not satisfying the formula (1), the total residual oxidizing substance concentration after being left still for 2 hours was high as 4.5 mg/L (asCl.sub.2), which was a level of requiring a large amount of neutralizer.

[0036] Furthermore, in the example 11, wherein the turbidity value was 50 NTU or more (60 NTU) and the concentration of sodium hypochlorite to be added was in a range of 18 to 30 mg/L (asCl.sub.2), the total residual oxidizing substance concentration after being left still for 2 hours was 0.9 mg/L (asCl.sub.2), while in the comparative example 1 with the concentration of sodium hypochlorite to be added being 12.8 mg/L (asCl.sub.2), the total residual oxidizing substance concentration after being left still for 2 hours was low as 0.2 mg/L (asCl.sub.2), which was a level hard to reduce harmful planktons and bacteria, etc. to the reference value or lower.

[0037] It was learnt from the results above that, by setting the concentration of sodium hypochlorite to be added to be within the range of the thick solid lines in FIG. 1, particularly setting the concentration of a chlorine-based active substance to be added to be 2 to 14 mg/L (asCl.sub.2) when the turbidity value is less than 10 NTU, setting the concentration of a chlorine-based active substance to be added to be in the range satisfying C=0.4X+a when the turbidity value was 10 NTU or more but less than 50 NTU and, furthermore, setting the concentration of chlorine-based active substance to be added to be in the range of 18 to 30 mg/L (asCl.sub.2) when the turbidity value is 50 TNU or more, an adding amount of the chlorine-based active substance in the ballast water treatment can be set properly without any excess or shortage.

INDUSTRIAL APPLICABILITY

[0038] According to the ballast water treatment method of the present invention, untreated ballast water before actually adding any chlorine-based active substance is obtained, turbidity of the untreated ballast water not added with any chlorine-based active substance is measured in advance and an adding amount of the chlorine-based active substance is determined in accordance with a value of the turbidity, consequently, an optimal adding amount of the chlorine-based active substance can be determined. Therefore, it is possible to optimize a loading amount of chemicals, space and facility on a ship, and a cost-competitive treatment apparatus can be provided eventually.