FISHERIES RESEARCH AND DEVELOPMENT CORPORATION

{ The FRDC is a rural research and development corporation within the Commonwealth Government portfolio of Agriculture, Fisheries and Forestry - Australia (AFFA). }

Research Results 95/162
Prawn farm effluent: composition, origin and treatment

· Construct nutrient and suspended solid budgets for representative prawn farms

· Assess alternative methods of pond effluent prevention and treatment

NON TECHNICAL SUMMARY
Prawn farming is an expanding, high-value primary industry in coastal areas of Australia. Currently there are approximately 500 ha of farm ponds. The majority of prawn farms are in Queensland, but there are also farms in NSW, NT and WA with plans for expansion of the industry in all these states. Current production is 2,200 t valued at $45M with predictions that the number of hectares of prawn ponds will double over the next decade.

The relatively small Australian prawn farming industry has developed in the wake of a very large, rapidly expanding prawn farming industry in Southeast Asia, South America and Central America where poor environmental management practices have caused widespread public concern. In comparison to these countries, the high level of community awareness and strict environmental regulations in Australia has ensured that the industry has developed under close scrutiny of environmental regulators and other government agencies. However, as the industry has developed, the need for scientifically rigorous information on the environmental management of prawn farming has emerged. Accordingly, the Australian prawn framing industry, environmental regulators and marine research community have devoted a high level of resources, relative to the size and value of the industry, to collaborative scientific research on the environmental management of prawn farming.

The focus of the environmental management research has been principally determined by the priorities identified by key stakeholders. The priority issues addressed in this study were first identified in a series of regional workshops held in Cairns, Townsville and Brisbane in 1996. The workshop participants included representatives from industry, research, and government primary industry and environmental protection agencies. The outcome of these workshops was the development of a nationally coordinated study of the environmental management of prawn farming in Australia.

Effective environmental management of prawn farms requires a detailed understanding of the pond ecosystem, feed and sediment management practices, the composition of pond effluent, the effectiveness of effluent treatment systems and the fate of effluent discharged into receiving environments. The nationally coordinated research program addressed each of these issues, but with different levels of intensity and resources depending on the relative priorities at the time. The principal research approach was to conduct field studies at representative farms in parallel with targeted experiments in controlled laboratory conditions.

The nationally coordinated research program was funded by the Cooperative Research Centre for Aquaculture (CRC), the Fisheries Research and Development Corporation (FRDC), and an environmental research levy paid by all Australian prawn farmers. In broad terms the three main research components of the overall program were: pond management; effluent management; and the impacts of effluent on coastal waters. This report describes the effluent management component, which was funded by FRDC but integrated into the main body of the CRC research. The component on pond management is described in the CRC for Aquaculture Final Report (Preston et al. 2001), and the component on the impacts of effluent on coastal waters is described the FRDC Final Report 97/212 (Trott & Alongi 2000) and in the CRC for Aquaculture Final Report.

The purpose of the effluent management component (this study) was to determine the origin and composition of pond effluent from commercial prawn farms and to assess the effectiveness of effluent treatment ponds. A major part of this study was the quantification of whole farm, whole season budgets for total suspended solids (TSS), total nitrogen (TN) and total phosphorus (TP) for three representative farms. Two of the farms used a flow-through system to supply water to their ponds and to discharge effluent into the receiving environment. In contrast, the third farm used a system of effluent treatment ponds prior to recirculation or discharge. As the project progressed, we were also able to assess the performance of a newly constructed effluent treatment pond at a fourth farm.

The results of our study confirmed previous observations that untreated prawn pond effluent contains elevated levels of TSS, TN and TP compared to the intake. Most of the TSS (60% to 90%) was inorganic, originating principally from eroded material from the pond floor and banks. Most of the nitrogen in untreated effluent originated from the feeds added to the production ponds. Only 22% of the nitrogen in the feeds was converted to prawns. The largest proportion (57%) was in the discharge water with 14% remaining in the pond sediment. Almost all (> 90%) of the total phosphorus in untreated effluent was in the particulate form. The sources of total phosphorus included feeds, phytoplankton and detritus.

There was a high level of variation in effluent TSS, TN and TP loads over short time periods and between farms. This variability adds considerable complexity to the task of setting regulatory limits on discharge loads and in designing waste management systems to meet the permitted discharge loads. However, the results of this study have, for the first time, provided sufficiently accurate data on prawn farm discharges to permit comparisons with other sources of nutrients and suspended solids discharged into the same receiving waters. The results have also provided a quantitative basis for setting permissible discharge loads from prawn farms, and for assessing the accuracy of effluent sampling strategies. These data are currently being incorporated into revised government prawn farming licensing processes.

One of the major achievements of this project has been to develop and assess the use of settlement ponds to treat pond effluent prior to recirculation into production ponds or discharge into adjacent waterways. The results of our trials showed that settlement ponds reduced the net TSS load by 60%, TP load by 30% and TN load by 20%. Although pond effluent treatment technology is at an early stage of development, settlement ponds are now being used to assist farmers to meet the effluent discharge standards set by regulators.

All new farms, or expansions of existing farms, now require the use of effluent treatment systems to meet effluent discharge standards. So far, most of the focus on effluent treatment has been on minimizing environmental impacts. However, the use of treatment ponds also provides the opportunity to recapture waste nutrients prior to discharge or recirculation. Field studies and tank trials have demonstrated that effluent nutrients can be successfully converted to secondary cash crops such as seaweeds, bivalves and fish (Lin, 1995; Jones & Preston, 1999, Jones et al. 2001). Further research is needed to develop cost-effective techniques for recapturing waste nutrients from prawn pond effluent.

In summary, the results of this research achieved the objectives of determining the origin and composition of prawn pond effluent, constructing nutrient and suspended solid budgets for representative prawn farms and assessing the effectiveness of effluent treatment ponds. The results have been disseminated via a series of regional workshops culminating in a national workshop on the “Environmental Management of Shrimp Farming in Australia”, held in Brisbane in May 2000. Communication of the project results will continue via: scientific publications; publication of the final report of the National Workshop on the Environmental Management of Prawn Farming in Australia; industry workshops; and participation in a new initiative by the Standing Committee on Fisheries and Aquaculture (SCFA), to apply Environmentally Sustainable Development principles to the Australian prawn farming industry.

Decisions are currently being made about the most economically and environmentally sustainable forms of primary industry in coastal regions. The results of our research are contributed to the process of providing a solid scientific basis for ensuring that the prawn farming industry is well placed in the future to meet these challenges. However, there is considerable work to be done to ensure the sustainable development the industry. In particular, further investment into the development and implementation of integrated waste management has significant potential to improve the economic and environmental performance of the industry (Burford et al. 2001).