Prawn Culture: Traditional and Progressive Needs
Fertilizer may be added to these ponds to promote an algal bloom, and the prawns feed primarily on the natural pond community that becomes established. In semi-intensive culture ponds, prawns are stocked at a density of 20 to 50 postlarvae per square meter. This is the most commonly used density in Australian prawn farms. Commercial feeds are added to the ponds to supplement the prawn diet. The higher prawn densities used in intensive and super-intensive cultures (50 to 150 postlarvae per square meter) greatly deplete natural food in the pond, hence, daily commercial feed applications are further increased. This increase in feed can reduce pond water quality.
Successful prawn culture requires maintenance of pond water quality conducive to prawn growth. Common water quality concerns for prawn farmers include low oxygen, and high ammonia. Low oxygen occurs when respiration rates in the pond (algae, bacteria, protozoa, zooplankton and prawns) exceed oxygen production through algal photosynthesis. Respiration rates are driven by the amount of degradable organic matter in the pond, which originates largely from algal growth and feed applications. Supplemental oxygen is generally added to semi-intensive and intensive culture systems by mechanical mixing and aeration of the pond water. High ammonia is derived from the decay of feed and pond organisms. Traditional prawn farms maintain water quality through daily water exchange with the adjacent estuary, which typically has much lower nutrients and algal levels and higher oxygen levels than the culture pond. There are, however, two primary issues that are gradually causing the global prawn farming industry to change its traditional management strategy of daily water exchange. These issues are environmental degradation of estuaries due to effluent releases from aquaculture and other industries, and the potential transfer of crop-destroying prawn pathogens to and from the estuary.
Successful prawn culture with no water exchange has been practiced in the United States for over a decade, and more recently in South and Central American prawn ponds. In these no exchange systems, water quality is generally not as good as in systems with daily water exchange, but acceptable prawn growth rates and production are maintained. While the ability to grow Australian prawn species in similar no exchange systems has not been confidently determined, the need to reduce water exchange and develop water treatment strategies for Australian prawn ponds is recognized by both the aquaculture industry and environmental regulators.
In Queensland, licensing requirements for aquaculture have become increasingly stringent over the last several years, partly in response to a variety of concerns raised by local community and conservation groups. Recently, these organizations have sought to prevent any further licensing of new farms in some regions of Queensland. While legislation for stricter aquaculture regulations is pending in Queensland, the Federal government has reiterated advocacy group concerns through enactment the Environment Protection and Biodiversity Conservation Act 1999 (EPBC). This legislation requires approval of the Environment Minister for activities that are likely to have "national environmental significance," defined as activities that may affect the Commonwealth marine properties, World Heritage and Ramsar wetland properties, threatened communities and species, etc. Large-scale aquaculture operations are specifically listed as a type of activity that may trigger the EPBC. In anticipation of further licensing requirements for prawn farms, CSIRO Marine Research is working with farmers to test management strategies that may greatly reduce pond discharges.
One of the strategies that CSIRO Marine Research is testing is the use of foam fractionation to assist in the maintenance of acceptable pond water quality with reduced rates of water exchange. In cooperation with Living Oceans International, Gold Coast Marine Aquaculture has installed a foam fractionation system to process water from one of its prawn ponds. Researchers in the Aquaculture and Biotechnology program of CSIRO Marine Research have been monitoring the effectiveness of this system. Foam fractionation, also known as protein skimming, is a technique that utilizes the tendency of polar molecules or charged particles to concentrate at an air-water interface. It works by injecting fine bubbles into the water column and then collecting the resulting foam, with its concentrated load of particles. These particles include both non-living dissolved and particulate organic matter, in addition to bacteria and phytoplankton. This technology is well-established in wastewater treatment and in aquaria at all scales, from small home fish tanks to large public facilities; but has not been applied to commercial aquaculture ponds. Photos of a foam fractionation unit being installed on a prawn farm and discharge from this unit are shown on the last page of this report.
Removal of organic matter from prawn ponds by foam fractionation may allow farmers to reduce the amount of water exchange needed for maintenance of acceptable water quality. Ultimately, recirculation of water through a water treatment regime such as foam fractionation may facilitate long-term prawn culture with no water exchange. However, regardless of whether water passed through a foam fractionation unit is discharged or recycled into the ponds, the high level of active organic matter in the foam must be reduced prior to release to the receiving water.
Wastewater Treatment Methods
The organically enriched wastewater produced in the foam fractionation demonstration project is roughly similar to municipal wastewater with respect to the amount of readily degradable organic matter. A basic goal of primary wastewater treatment is to reduce the amount of readily degradable organic matter through bacterial digestion. This process results in the production of carbon dioxide and depletion of oxygen levels in the water. While treatment regimes for wastewater varies among municipalities, there are several basic steps. Initially, larger material that easily settles or floats is removed. The wastewater then undergoes aerobic digestion in a continuously mixed system with air injection as needed to maintain water dissolved oxygen levels of at least 2.5 mg/L. After several hours of aerobic digestion, many treatment regimes have a period of stagnation to allow anaerobic digestion. Additional strategies to promote waste decomposition that are applied at some facilities includes enzyme additions and ozonation. The objective of these supplemental techniques is to chemically break down organic matter to produce organics that are more readily digested by bacteria.
While complete mineralization of organic matter to carbon dioxide and water is often faster with aerobic digestion, some types of organic matter can only be digested under anaerobic conditions. Thus, it is common to cycle wastewater through both aerobic and anaerobic conditions to achieve digestion of more types of organics. When anaerobic digestion is applied, a succesive stage of aerobic digestion is used to further digest by-products of anaerobic digestion and produce a final effluent water that has acceptable oxygen levels for discharge. Municipal wastewater treatment facilities run batches of wastewater through this aerobic/anaerobic/aerobic cycle within a single container, called a single batch reactor (SBR). A common SBR treatment time for municipal waste in a freshwater system is around 15 hours. Wastewater may then be passed to a settlement system for clarification or further (i.e., secondary) treatment, depending upon the facilitys discharge requirements.
While municipal wastewater treatment methods can suggest strategies for the treatment of prawn wastewater, optimal methods details may differ due to the different types of common organic matter in these systems. Another significant difference between prawn pond waste and municipal wastewater is that municipal wastewater is freshwater (i.e., not salty), while prawn pond wastewater is seawater. Marine salt contains high levels of sulfate, which, under anaerobic conditions, is used by sulfate reducing bacteria. This group of bacteria typically plays a key role in organic matter decomposition in anaerobic marine systems, but are relatively uncommon in freshwater systems. Thus, the rates of organic matter decomposition may vary between freshwater and marine systems as a result of the different types of dominant bacteria. Consequently, while municipal wastewater treatment regimes can be used to suggest treatment strategies for marine wastewater, the optimal digestion times may vary regardless of whether or not the proportion of decomposition desired is the same.
CSIRO/SCDNR Test Treatment of Prawn Pond Foam
Three types of experiments were conducted to indicate the following:
Treatment time for a bioreactor cycling from aerobic to anaerobic to aerobic conditions.
Comparison of digestion in bioreactors with continuous aeration, cycling aeration and stagnation (i.e., anaerobic), and continuous stagnation.
The effect of initial UV treatment of wastewater on bacterial digestion. (Note: This is a previously untested application of UV that was made possible through collaboration with Natural Flow Water Treatment Systems.)
Replicate bioreators were constructed from PVC pipe (length: 80 cm; diameter: 10 cm). Bioreactors were filled with approximately 5.5 L wastewater from a prawn pond foam fractionation unit, with four replicate bioreactors per treatment. To ensure stable temperature, all bioreactors were incubated in a single tank filled with water that was maintained at approximately 33 C.
The extent of wastewater digestion was indicated through measurements of the relative amount of readily biodegradable organic matter as determined by oxygen consumption rates. Changes in the quantity and type of organic matter were also determined by measurement of changes in the forms of nitrogen. In prawn ponds, organic nitrogen corresponds with organic carbon, providing a measurement of organic matter. In bioreactors, inorganic nitrogen is produced as a result of bacterial decomposition of organic matter.
An initial experiment compared the amount of decomposition that occurred in reactors cycling between aerobic and anaerobic conditions for a total of 18, 30, 42 hours. Significantly more decomposition was indicated with longer digestion times. Total digestion times in excess of a couple days were seen unlikely to be practical from a commercial standpoint, and thus were not tested.
A second experiment tested the amount of decomposition in bioreactors that were either continuously aerated, continuously stagnant, or cycled between these states. While determination of nitrogen levels in this experiment is ongoing, oxygen consumption rates suggest that the greatest amount of digestion occurred in the continuously aerated treatment, and the lowest amount of digestion occurred in the stagnant, anaerobic treatment. This finding contrasts with typical municipal wastewater treatment, in which cycling between aerobic and anaerobic states is preferred. This may be the result of the differences in the types of organic matter in these systems. This experiment was conducted twice, using wastewater from the foam fractionation unit running at settings that produce wastewater with different concentrations of organic matter. Preliminary conclusions were the same for each trial.
A final experiment testing the potential of initial UV treatment to affect decomposition rates has been completed. Sample analysis is currently underway. Oxygen consumption rates do not suggest large differences in the relative biodegradability of organic matter between treatments at the UV doses tested.
The digestion strategies tested at CSIRO with waste from a commercial prawn pond foam fractionation unit will be repeated with wastewater from super-intensive prawn culture systems at the Waddell Mariculture Center in Bluffton, SC, USA. Additional prawn wastewater treatment methods such as enzyme additions and ozonation may also tested. The results of this research will assist both CSIRO and the South Carolina Department of Natural Resouces is designing treatment methods for prawn pond waste.
CSIRO Marine Research
Cleveland, QLD, Australia