I. Executive Summary

In April 1998, a red tide caused by Gyrodinium aureolum posed serious damages to the fish cultivating industry in Hong Kong SAR and Shenzhen, PRC. The total fishkill of this red tide incident was more than five hundred tons with the total economic loss greater than 20 million US dollars.

A study was carried out as an immediate rescue effort to the red tide. An ozone injection system, which generated the disinfecting agent by lighting and electrical charges, was tested to elucidate the possible effects of using ozone in killing harmful dinoflagellates. In an experiment, ozone was generated at the rate of not more than lgm per cubic meter per hour and was diffused into the red tide-contaminated water pumped up from the fish cultivation zone. It was discovered that after one minute treatment of the ozone, 53% of the dinoflagellates in sea water was killed. All (6800cell/ml) dinoflagellates were effectively killed within 12 minutes of ozone treatment. Besides, the dissolved oxygen content in the contaminated seawater increased from 7.6 mg/l to 17.4 mg/l. The seven fishes cultivated in the experimental water were found unaffected and maintained their healthy conditions after ozone injection.

The ozone injection system was also tested in situ a fish cultivation zone. A cage of 2m (L) x 2m (W) x 2m(H) was immersed in the contaminated seawater for cultivation of commercial fishes and ozone treatment. The cage was surrounded by a nylon cloth to enclose the ozone injection boundary and to separate the treated seawater with outside contaminated waters. As recorded, after 15 minutes of ozone treatment all the red tide causative dinoflagellates in the surface layer were killed and the level of dissolved oxygen significantly increased from 0.03mg/l to 3.Smg/l. The health conditions of the fishes in the cage were generally unaffected.

The economic aspects of ozone injection was assessed and compared with other technologies in tackling red tide. As shown, the ozone injection technology is shown of practical values in terms of its cost-effectiveness in killing algal cells and abilities in increasing the dissolved oxygen in water. Ozone may also oxidize the algal available ammonium into nitrates thus reducing the risk of red tide outbreak.
 
 

II. Introduction of Ozone and Applications

Ozone is triatomic oxygen (O₃), and is a colorless gas. It is created when oxygen is exposed to either a high-voltage electrical discharger or ultraviolet radiation in the earth's stratosphere. It is unstable in nature, ozone decomposes very quickly into normal molecular oxygen (O₂), leaving no residue. This makes ozone environmentally attractive in many applications. In water, ozone is 3000 times faster than chlorine as a disinfectant. Ozone kills viruses and potential pathogenic microbiological organism such as bacteria. In addition, ozone oxidizes organic and inorganic compounds, removes color- and odor-causing compounds and neutralizes surfaces charges on suspended solids and to control algae and other aquatic growth.

Ozone is the second most powerful oxidizing agent known. Ozone is a disinfectant and biocide. Ozone kills viruses and potential pathogenic microbiological organisms such as bacteria. In addition, ozone oxidizes organic and inorganic compounds, removes pesticides and odor-causing compounds and neutralizes surface charges on food. The following discusses how ozone works:

A. Disinfection

The oxidation strength of ozone is 1.51 times of chlorine. Consequently, ozone 5 oxidation rates are much faster than chlorine's. Unlike some of the other oxidants, ozone also disinfects and inactivates viruses.

The method in which ozone kills microorganisms is thorough as no limits have been discovered in the volume or species that can be eliminated. Contrary to the halogens, ozone allows no reversible inhibitor effect on intracellular enzymes.

In a study where bacterial species are identified from soiled hospital sheets and terry cloth items, E. coli is found to be one of the most active pathogens. Research found that "ozone kills E. coli 3125 times faster than chlorine, which means that contact time is of little consequence for ozone sterilization.

B. Algae Control

Algae generate odor naturally in warm temperature and absorb organic compounds in water. Ozone oxidizes the organic components and completely eliminates algae in the water.

III. Pilot Systems in Fish farm, San Mum Tsai, Tai Po

A harmful red tide happened in Hong Kong recently. More than 500 tons of fishes were killed in the fish cultivation zone. To tackle algae bloom, a joint study, headed by Dr. Ho Kin Chung, Program Leader in Environmental Studies of The Open University of Hong Kong, E Technologies Ltd. and Marine Resource Technologies. was carried out from April 17 to 19, 998 with an aim to test ozone for red tide control.

An on site ozone generation system was installed in a fish farm in Tai Po. Experiments were carried out in an enclosed plastic water basket on April 17, 1998. During the experiments, Ozone was injected into sea water for disinfecting red tide organism. The dissolved oxygen (DO) content and active/passive algae cells counts per millimeter were measured and analyzed. To test the effect of ozone on fishes, seven fishes were put into the basket to see whatever health conditions of fishes were affected by ozone.
 

IV. Results of Pilot Tests

On April 17, 1998, the experiment was conducted in a 250 liter plastic water basket. The basket was filled with red tide contaminated sea water collected at San Mum Tsai, Tai Po. At the beginning of the experiment, seven fishes including Sea Bream, Grouper, Rabbit Fish were cultivated in the tank for assessment of the possible effects of ozone on fishes.

Dissolved oxygen contents were continuously measured by a YSI Do Meter. Algae cell were enumerated under microscope with samples being taken from the tank at fixed intervals. Conventional aeration was also conducted to compare with the results of ozone injection. Ozone was first generated by an ozone system and was introduced into the basket. The performance data are as follow:

A. Water Sample Treated with Conventional Aeration by a Diffuser

The following table shows the data obtained:

Figure 1: Algae Counts with Conventional Aeration

It is quite obvious that conventional aeration does not have significant effect in killing algae cells and in improving dissolved oxygen.

B. Water Sample Treated with Ozone

The water quality and algae data in the ozone treated water are shown below:

Figure 2: Algae Counts with Ozone Injection

From Table 2 and Figure 2, it was found that ozone is extremely effective in killing red tide organisms with most of the algae cells being inactive within a short period. Within the first minute, the active cell decreased to 46%. In the 9th minutes, active algae cell dropped to only 2%. All algae cells were inactivated after 12 minutes of ozone injection. At the same time, DO was found increased from 7.6 to 17.4 mg/l. The results indicated that ozone injection was able to increase dissolved oxygen content in sea water. During the entire experimental period, all seven fishes did not show any adverse condition and were pretty much alive at the end of this experiment.

C. Water Treated with Ozone at Low Dissolved Oxygen Level

On Sunday, April 19, 1998, red tide gradually dissipated at San Mum Tsai. However, many fishes died in that morning due to de-oxygenation. Monitoring results showed that the average DO level was only 0.07 mg/l which was far below the water quality objective in Inner Tolo Harbor.

Another experiment was conducted in a 2 m x 2 m x 2 m open-bottom cage. Such a cage was bordered by "red-blue-white" nylon bag which was able to enclose the fish cultivation area and to limit the exchange of the internal water with outside contaminated sea water.

As observed, nearly all red tide organisms were inactive during that morning at San Mum Tsai. Hence, we decided to measure only the dissolved oxygen content for the sea water samples with conventional aeration and direct ozone injection. As observed, no obvious change in DO content for the sea water treated with conventional aeration. The DO contents in this aeration treatment varied from 0.03 mg/l to 2.02 with average of 1.31 mg/l.

Table 3: Sea Water Treated With Ozone at Low DO Level

Time	Time Elapse (Min)	DO (ppm)
11:22	0	0.68
11:23	1	0.43
11:24	2	1.20
11:25	3	0.68
11:26	4	2.16
11:27	5	3.22
11:28	6	2.90
11:29	7	3.39
11:30	8	2.76
11:31	9	2.93
11:32	10	2.15
11:33	11	3.50
11:34	12	3.25
11:35	13	3.71
11:36	14	3.18

However, ozone injection has a significant result on oxygen improvement. The results are shown in Table 3.

As noted, the DO content increased to 2.16 mg/l at the 4th minutes with the overall average equal to 3.16 mg/l after treatment by ozone for 10 minutes. The fishes in the cage survived with a healthy condition throughout the ozone injection process.
 
 

V. Conclusion

The results in present experiments are very promising in. (1) verifying that ozone is very effective in killing red tide causative organism and (2) proving ozone injection be able to increase oxygen supply in sea water. The preliminary experiments on situ San Mum Tsai also shows that ozone injection at low concentration has no adverse effect on cultivated fishes.

The practical application of ozone to the fish cultivation zones and its effect on the aquatic environment need further studies. We would like to invite Hong Kong government and representatives from Fishmen Association to participate in further large scale testing. From past experience of ozone applications in swimming pool and sewage treatment processes, it is reasonable to believe that ozone injection will be a useful weapon in killing red tide organisms, improving water quality, reducing the organic components (especially ammonium) in sea water and so improving the competitiveness of fish cultivating industry in Hong Kong.