Oxygen Issues

1. Oxygen Issues Dr. Joe M. Fox MARI-5432

2. Learning Objectives Oxygen and aeration Ozone as a disinfectant.

3. Aeration Most aquatic systems employ some means of oxygenating holding tanks Source can be purified O2 or ambient air, depending upon needs Typically a three-step proces: transfer of O2 in gaseous form to the gas-liquid interface transfer across the gas-liquid interface transfer away from the interface into the liquid

4. Aeration Step 1 is accomplished by a combination of diffusion and convective currents in the gas, rapid process. Step 2 (transfer across interface) is a matter of diffusion, slow rate-limiting process. Step 3 (movement of oxygen into the liquid) is accomplished by convection. dC/dt = KL (A/V)(Cs � C) where KL = oxygen transfer coefficient (cm/h), A = area of gas-liquid interface (cm2), V = volume of water (cm3), Cs = saturation concentration of oxygen in liquid, C = oxygen concentration in liquid at any point in time.

5. Diffusion Increased temperature = increased diffusion rate Dirty water, water with chemicals tends to reduce KL (transfer coefficient) As A/V increases, diffusion rate increases (Kl)(A/V) increases 1.56% for each degree C increase in water temp

6. Aerator Types Four basic types: gravity, surface, diffuser, turbine Aerators use an energy input to increase the liquid surface area available for oxygen transfer or to ensure that low DO water is brought into contact with oxygen Mixing increases surface area and concentration gradient ? transfer Also used to keep suspended particles in suspension.

7. Gravity Aerators Utilize the energy released when water loses altitude to increase air-water surface area, increasing DO concentration. Turbulent motion of streams, waterfalls achieves this effect. Used in large applications, tanks at different levels, weirs situated on ponds, raceways, etc. E = 100 (Cb � Ca)/(Cs � Ca) E = aeration effectiveness (%), Ca = DO conc above aerator (mg/L), Cb = DO conc below aerator (mg/L), Cs = saturation DO conc at given temp (mg/L)

9. Lattice Aerators Lattice aerators (same as trickling filters) have greatest efficiency Benefit: no energy other than gravity Spaces between trays, media type are critical Increased space between trays = increased efficiency to a point

10. Surface Aerators Agitate or break-up water surface resulting in larger oxygen transfer rates Examples: pump spraying water into the air, nozzle aerators

11. Transfer Rate Depends on: depth of submergence rotor speed rotor diameter power input per unit area or volume characteristics of liquid being aerated liquid tank dimensions, shape oxygen concentration gradient aerator design

12. Diffuser Aerators inject air or oxygen into a body of water in the form of bubbles oxygen transfer from bubbles to water via diffusion bubbles rise in water column bubble size very important (smaller diameter is better) size of bubble determined by diffuser

13. Diffuser Aerators Oxygen transfer depends on oxygen concentration gradient between bubble and surrounding liquid, percentage saturation of water around bubble, retention time of bubbles in water, bubble size, gas flow rate

14. U-tube Diffusers Consists of a U-shaped tube in which water travels down one leg of the U and up and out the other Air or pure oxygen is injected into the water through a diffuser at the inlet Downward water velocity >>> rise velocity increased gas/water contact time increases saturation constant due to higher pressure at the bottom

15. U-tube Aerators DOo = 20 + 0.76(DO)t + 295(AWR) + 0.14d � 2.50(DO)i(AWR) DOo = DO at U-tube outlet (% sat), DOi = DO at U-tube inlet (% sat), AWR = air-water ratio as a decimal fraction, d = U-tube depth

16. AERATION DISTRIBUTION SYSTEM Aeration typically provided via large impeller-type blowers Blowers located distant from aquatic system due to noise/vibration Distribution system relatively similar to that of seawater system, but has to deal with condensation Incoming air is filtered to 1 �M via intake filters (prior to blower) Air from blower is often hot due to back pressure (cooled via immersion into water)

17. AERATION DISTRIBUTION SYSTEM Pipe extending from blowers is often constructed of metal due to high heat Ozone can be injected into pneumatic flow for disinfection of distribution lines or for in-tank treatment Aeration diffusion is typically via airstones or microbubble diffusors (e.g., plastipore tubing, Schramm Bioweave) Very little hatchery aeration is via compressor due to large volume required

18. Calculating Aeration Required Most aquatic systems operate with shallow-depth tanks Pressure requirement is small, but volume requirement is large Pressure typically recorded in psi (lbs. per square inch) and volume in cubic ft (cf) Flow rate is measured in cfs for most blower installations

19. Which Delivery System is Best? Difference between delivery systems is primarily a function of pressure Step 1: identify pressure required Depends on water pressure at depth of diffusor, piping/tubing friction loss and diffusor resistance to air flow For example: if most tanks in your hatchery are 36� deep (36 x .04 = 1.44 psi), your distribution pipes are low-restriction (total 0.15 psi) and your air diffusors are low-resistance (0.25 psi), total pressure needed is less than 2 psi. Note: most blowers operate efficiently up to 4 psi (this is the system you need!)

20. Aeration Criteria for Aquatic Systems Step 2: what is the total air flow you need? Most hatcheries use airstones as diffusors Most diffusors require about 0.5 cfm flow rate at most system water depths (cfm increases with decreased pore size) With 500 airstones (possible) in a system, you could need 500 cfm at 2 psi This equates to two 10 hp blower running simultaneously!

22. Oxygen and Model Ecosystems Most aquatic organisms require an oxygenated environment Some submerged aquatic plants can actually store oxygen in their tissues Very simple organisms (e.g., bacteria) will remove oxygen from the water directly through the cell wall Others (e.g., fish) have specialized organs and blood pigments for carrying oxygen Those adapted to an anaerobic environment include yeasts, protozoans, fungi (what you don't want) Hence, the need for aeration or oxygenation in aquatic systems

23. Oxygen and Model Ecosystems In microcosms and mesocosms where all aspects of the environment are simulated, adequate light for photosynthesis is provided in order to generate baseline levels of oxygen. In aquaria, managers often attempt to maximize equivalence between conditions in the natural environment and the aquarium Difficulty: scaling and inadequate ratios between water surface and water volume don't allow this In aquaria, display is optimized�volume is small and biomass density is high. Feeding is typically in excess of wild equivalents Hard to simulate an adequate oxygen environment by simple aeration.

24. Oxygen and Model Ecosystems Newer methods of trickle filtration certainly help, but concomitantly prevent supersaturation. Can be achieved using liquid or bottled O2, but this approach is expensive and potentially dangerous. Model ecosystems are, thus, typically designed to optimize photosynthesis.

25. Simulating Nature Most natural reef systems achieve supersaturation on a daily basis Due to excess photosynthesis and wave action Source of water is also typically at oxygen saturation (approx. 6.5 mg/L) Waves smooth out metabolic activity highs and lows Aquarists simulate this environment by maintaining a night-to-day D.O. range of 5.5 - 8.5 mg/L

26. Simulating Nature In order to evaluate oxygen concentration in the aquarium system during the dark Measurement made at or near saturation to reduce influence of exchange with the atmosphere Respiration rate averages about 2 g O2/m2/hr in aquaria, only about 50% of a natural reef! At higher respiration rates, you must elevate photosynthesis to avoid increases in CO2 High CO2 depresses pH, reducing carbonate formation

27. Simulating Nature Shallow reef aquarium simulations should be supersatured for 8-10 hr/day Example: Starting with an ambient DO concentration of 6.5 mg/L, a 60-gal aquarium with 0.5 m2 reef surface area having a dark consumption rate of 3 g O2/h would have all of its oxygen consumed in one hour! Situation mitigated by wave action bubbling or use of a trickling filter.

28. Preliminary Ozone Notes Pungent and unstable gas. Tri-molecular oxygen. Most common use as disinfectant. Oxidizer: remove turbidity, algae, color, odor and taste. Has industrial and aquaculture uses. Aquaculture: both recirc and flow-through applications. Remove disease, dissolved/suspended organic compounds. Treatment prior to discharge.

29. Ozone Chemical Properties

31. Ozone Chemical Properties Ozone inorganic conversions: - Sulfides and Sulfites to Sulfates - Nitrites to Nitrates - Chlorides to Chlorine - Ferrous to Manganous ions to their insoluble ionic forms resulting in precipitates. Ozone organic conversions: - Rupturing compounds at unsaturated bonds and destroying humic acids, pesticides, phenols, and a host of other types of compounds.

32. Ozone Chemical Properties As a strong oxidant, it is very unstable in both gaseous and aqueous forms. Decomposition in water is a function of pH, ultraviolet light, ozone concentrations, presence of inhibiting compounds. Very rapid decomposition to superoxide anion (O2-) in presence of hydroxide ions, peroxides, humics.

33. Ozone Chemical Properties Can react directly with ammonia to form nitrate, but at a very slow rate. Other break-down chemicals react faster. Treatment design should therefore favor formation of radicals.

34. Ozone Oxidizing Potential Bacterial disinfection. Viral inactivation. Precipitation of metals (Fe, Mn). Decomplexing organically-bound heavy metals. Flocculent for precipitation of dissolved organics Taste, odor, color, algae control. Destruction inorganics (sulfides, nitrites). Degradation of pesticides, detergents. Nitrification enhancement.

35. Ozone Use Characteristics Powerful oxidizing agent. Very rapid reaction times. 3-5 x more effective as a disinfectant than chlorine. Short contact times. Complete reaction w/residual half-life of 20-30 min. Residual is oxygen (beneficial). Other residuals easily removed.

36. Major Disadvantages High capital equipment costs. High operating costs (corona discharge) Some residuals can be toxic to fish/shrimp. Dangerous to humans to breathe. On-site generation required. Inefficient energy: only 10% of power required generates ozone (rest is heat).

37. Ozone as a Disinfectant Usually applied at end of water treatment process. Remove most demand first by mechanical means and then apply O3. 0.4 mg/L residual kills 99.9% bacteria in drinking water in less than 4 min. Wastewaters have higher ozone demand = more difficulty establishing a residual.

38. Ozone Generation Usually by either corona discharge (electrical spark) or by low wavelength UV. Cannot be stored (pressure = heat = dissociation. Corona: passing dry air or oxygen between two surfaces having an electric potential. As oxygen passes through field, it is excited to energy level conducive to formation of ozone.

39. O3 Corona Discharge Feed gas must be dry, free of impurities (e.g., N2 in feed = nitric acid). Pure O2 = 2x more O3 than ambient air. Factors affecting output: frequency of electricity, concentration of O2 in feed gas, gap between plates, plate thickness. Air as feed: 60 g O3/kWh. Oxygen as feed: 120 g O3/kWh.

40. UV Ozone Generation Exposure of air or oxygen to UV light at 140 - 190 nm. Factors: voltage, lamp glass quality, gas used in packing lamp. Problems: cost of lamps, cheap knock-offs, health hazards. Cheap models: 1/3 O3/kWh of corona, poor lamp life. Good models: equal, but less electrical cost, 3 yr lamp life.

41. Ozone Toxicity Mutagen, oxidizing compound. Humans should avoid exposure to concentrations in air of more than 0.3 mg/L. For most species: 0.5 mg/L achieves good water quality, but high mortality of eggs and larvae (fish, bivalves). 0.1 mg/L residual had no effect on cutthroat and steelhead trout fry (Colberg et al., 1977).

42. Ozone Toxicity Toxicity may be a function of organic material on gills. Susceptibility largely result of activated species present. Some decomposition products are toxic. Check out situation first with bioassay.