Oxygenation

Oxygen sources differ between pond and other types of holding systems. Possible sources of oxygen in a pond are the production by aquatic plants, includingphytoplankton, mechanical reaeration, and reaeration across the water surface with minimum contributions by the water exchange, with the relative importance of the sources depending on the type of production system being used. In raceways and tanks, oxygen sources are limited to the water supply and some form of mechanical reaeration.

Oxygen Production by Phytoplankton. Phytoplankton produce oxygen through photosynthesis. The rate of oxygen production by phytoplankton is very difficult to quantify but conceptually may be expressed as

DOPHYTO = DOequi x PHYTO X MAX

where DOPHYTO is the oxygen production rate by phytoplankton (g m~3 h-1), DOequi is the oxygen production per unit of phytoplankton biomass growth (g g~*), PHYTO is the phytoplankton concentration (g m~3), MAX is the maximum phytoplankton growth rate under optimum conditions (h_1), NUTRI is the nutrient limitation on growth (0-1), LIGHT is the light limitation on growth (0-1), and TEMP is the temperature limitation on growth (0-1).

A multitude of expressions have been proposed for the various limitation terms (eg, Ref. 15).

The overall effect of phytoplankton on oxygen concentration in a pond is further complicated by their respiration at night. This combination of oxygen production by phytoplankton during the day and consumption at night by phytoplankton, fish, and other organisms results in wide variations in dissolved oxygen over diurnal cycles (8). Maintaining a balance between the phytoplankton production and consumption terms in ponds becomes the primary goal of water quality management in most pond production systems.

A new production system has been developed recently that makes use of the water-treating and oxygen-producing capabilities of ponds. The system has been called the Partitioned Aquaculture System (PAS) (16) and consists of a shallow pond used for water treatment and oxygen production, and a small raceway in which fish are held. Water is continuously recirculated between the raceway and the pond. A secondary fish crop is raised in a section of the raceway downstream from the primary fish crop. The species to be raised as the secondary crop is selected such that it has the ability to filter the water and remove particulates such as algae and uneaten feed from the primary crop. These new systems are still in the experimental stage but offer great promise given their high production rates per unit area and the fact that the water is recycled and no effluent is produced.

Oxygen Production by Mechanical Aeration. Oxygen transfer into the water takes place as a result of a driving force equal to the difference between the saturation concentration and the actual concentration found in solution, a mass-transfer coefficient, and the area of contact between the liquid and gas phases. The mass-transfer coefficient and the area of contact are normally combined into an overall gas-transfer coefficient, Kha, that may be determined experimentally. The equation describing the rate of oxygen transfer can be written as

where DO^s is the dissolved oxygen transfer rate (g h ~ r), DOSat is the saturation concentration of dissolved oxygen (g m~3), DO is the dissolved oxygen concentration in solution (g m"3), Kha is the overall gas-transfer coefficient (h"1), and V is the volume over which the aeration is taking place (m3).

Saturation concentration is determined by the composition of the gas phase, by temperature, by atmospheric pressure, and by the presence of dissolved substances in the water (eg, salinity) (17). Although more complete estimation procedures are available that consider the factors just mentioned (17), saturation concentration for fresh water as a function of temperature may be expressed as a regression equation (18):

DOSat = 14.652 - 0.41022 X TEMP + 0.007991 X TEMP2 - 0.000077774 X TEMP3 (11)

where TEMP is the water temperature (°C).

Equation 12 and tables of dissolved oxygen saturation are normally prepared for an atmosphere of 20.9% oxygen. Oxygen enrichment of the atmosphere will result in a corresponding increase in saturation dissolved oxygen concentration. Enrichment may be achieved by aerating with "pure" oxygen.

An important difference between aeration systems used for aquaculture and those used in water and wastewater treatment is that the "driving force" (DOSat — DO) in aquaculture tends to be lower, reducing the rate of oxygen transfer for a given aerator. The lower driving force is caused by the requirement of most fish for dissolved oxygen concentrations above 3 to 4 g m-3, compared with the usual limit of just above zero in wastewater treatment.

Common types of aerators used in aquaculture may be classified as surface, gravity, and diffuser types (5,9,19). Surface aerators spray water into the air or beat the water, increasing the turbulence and area of contact between the water and air. Gravity aerators rely on the fall of water for aeration. Different types of structures such as plates, baffles, and so on may be used in gravity aerators to increase transfer rates. A particularly useful and common gravity aerator is the packed column aerator (PCA), where water is introduced at the top, and air or another gas is introduced at the bottom of a column filled with a medium designed to maximize turbulence and transfer area. Design equations for the use of PCAs in aquaculture have been proposed by Hackney and Colt (20). In diffuse aeration, gas bubbles are introduced into the water, and transfer takes place between the bubble and the water. The simplest type of diffuse aerator is an airstone where bubbles are released at the bottom of a tank or pond. This type of aerator tends to be inefficient in conventional aquaculture systems due to the shallow water depths used and the resulting short contact times between the bubble and the water. Variations on the basic concept of diffuse aeration have been developed, including some systems designed to be used as part of a pipeline and that pressurize and trap bubbles allowing for longer contact times and, in some cases, complete dissolution of the bubbles into the liquid (5,9).

The use of pure oxygen in aquaculture operations has become commonplace over the last few years, especially for intensive systems that include some form of water reuse. Pure oxygen systems rely on either liquid bottled oxygen or on the on-site production of oxygen. The use of pure oxygen makes possible the addition of large amounts of oxygen to the water, increasing fish biomass held in a given water supply.

Oxygen Production by Surface Reaeration. The transfer of oxygen by reaeration is important in ponds where it can result in substantial net gains or losses of oxygen depending on whether the dissolved oxygen concentration is below or above saturation. Estimates of reaeration rates may be obtained from the following (21):

where DOReaer is the reaeration rate (g m~3 h-1),

^Reaer = 0.03 X l>°"5 - 0.0132 X Uw + 0.0015

uw is the wind velocity (m s_1), and D is the pond depth (m).

An implicit assumption in equation 13 is that conditions in the pond are uniform and there is a minimum of stratification.

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