Читать книгу Clinical Guide to Fish Medicine - Группа авторов - Страница 127

Ozone (O₃) is a very strong oxidizer, which is useful for disinfection and water clarification but carries high risks and high equipment and operational costs. Ozone oxidizes therapeutics, color‐producing organics, ammonia, nitrite, and micro‐organisms, and it reduces turbidity through a process of microflocculation (Johnson 2000; Overby 2002). To work efficiently and safely, ozone disinfection needs:

1 Suitable ozone gas generation.

2 Appropriate gas‐to‐liquid mass transfer (mixing).

3 Adequate contact time for reaction (often ~10 minutes).

4 Destruction of residual ozone and oxidation by‐products.

Ozone is created by exposing dry O₂ to high AC voltage across a discharge gap (Figure A3.15a). Ozone must be generated on site since it is labile. Ozone is injected into a side‐stream contact chamber with system water flowing through it; this is where the oxidation occurs. The contact chamber may be a dedicated ozone chamber, trickle filter, or foam fractionator; use of a foam fractionator improves the efficiency (Figure A3.15b).

Figure A3.14 Diagram showing ultraviolet light disinfection.

Source: Image courtesy of Sarah Chen, copyright reserved.

Ozone exposed to salt water can create residual oxidative by‐products (particularly hypobromous acid and hypochlorous acid that can generate trihalomethanes), which can travel out into the main system and kill fish. Once the water flows out of the contact chamber, it should be passed through either a biological filter, and/or activated carbon, UV, or intense heat to remove residuals; these may be known as ozone destruct units. Packed column aeration is another technique for removing residuals from the water, particularly trihalomethanes. In this process, the water is trickled down a tower filled with inert material as a fan blows air upward into the tower. Most residuals are volatile and are released into the air. Ozone is short‐lived, so many residuals can also be limited by simply increasing contact time in the contact chamber. Residuals can also be reduced by creating salt mixes that are low in bromides.

Ozone needs to be carefully monitored to ensure animal and human safety. This may include:

1 Monitoring ozone generation (often 0.3–0.5 mg/h/gallon) or actual ozone dose (often 0.01–0.50 mg/L).

2 Monitoring oxidation‐reduction potential (ORP) in the contact chamber and in the aquarium (Figure A3.15c and d). This is often 700–800 mV for optimum disinfection in ozone contact chambers but should be <200–350 mV in the fish habitat. The trend in aquarium systems is to try to reduce this to avoid dosing above demand.

3 Monitoring residual oxidants, particularly total and free bromine. These should be very low in fish systems (HOBr < 0.02 mg/L).

4 Testing for ambient ozone leaks.

5 Testing turbidity.

6 Monitoring bioload in the system and changes in feeding and cleaning schedules.

7 Monitoring plastic, rubber, and metals as these deteriorate more rapidly in the presence of ozone.