Nutrient management of wastewater from aquaculture farms has been the focus of significant research over the past ten years.
A summary of options studied by Queensland Government researchers in the past decade
On this page:
- Mariculture wastewater research
- Freshwater wastewater research
Prawn farming and other mariculture activities are economically important agricultural practices in Australia. Seafood farming is considered vital for continued food security and general health and wellbeing as the human population increases. Since these broad-scale farming activities have the potential to cause adverse environmental impacts, they are managed as environmentally relevant activities, which include licensed wastewater releases. The nutrient use efficiencies within these farming systems can directly affect their profitability, productivity and environmental footprint. Compliance with discharge regulations designed for environmental sustainability is likely to be one of the most limiting factors facing the expansion of the Australian mariculture industry. Therefore, cost-effective wastewater treatment and recirculation technologies are one of the highest priorities for industry-based research and development.
Several years of multidisciplinary research in the mid to late 1990s regarding the nature and management of these man-made ecosystems has provided a greatly improved understanding of the composition of effluents and the ecological processes and nutrient cycling that occur in the farms and in adjacent waterways receiving their discharge. This work has generated a large number of research papers (1) and has led to the standardised use, in Australia, of sedimentation basins to mitigate water flows and provide primary treatment of effluent prior to release or recirculation.
Following this important work, DPI&F (now DEEDI) conducted a range of studies investigating ways to further beneficially treat mariculture effluent and sequester nutrients in profitable forms. The Queensland Government's Aquaculture Industry Development Initiative (AIDI 2002-04) (2) has supported this work, and a range of state and federal government-funded projects since then have made incremental steps towards viable innovative solutions. Much of this work has been undertaken or based at the Bribie Island Research Centre (BIRC) in southern Queensland, but several projects have also been based at the Northern Fisheries Centre in north Queensland.In considering this large body of information it is also important to recognise that no two mariculture farms are the same. Different physical, biological, geographic, hydrodynamic, climatic and economic factors will drive decisions made by individual farmers regarding their preferred approach to waste nutrient minimisation. By investigating a wide range of potential options, and drawing attention to the perceived advantages and disadvantages of particular approaches, it is anticipated that a group of best options will begin to emerge from industry-based evaluations, as driven by the profitability and environmental sustainability that is essential for successful mariculture businesses in Australia.
Discharge settlement basins are now routinely used by industry in Australia. Several studies have assessed the effects of stocking detritivores into these settlement basins to produce harvestable biomass to improve farm profitability and help assimilate accumulated organic matter. Organic matter accumulation can become problematic late in the cropping cycle because its decomposition leads to higher nutrient discharge.
Banana prawns were shown to grow quickly in these unfed farm settlement systems and consumer acceptability assessments showed that they were of high quality and suitable for human consumption (3). At low densities (~5 m-2) the prawns did not adversely affect the levels of nutrients in discharges, but at higher densities they caused an elevation of nutrients in the water column due to their swimming action disturbing settlement processes (4). Vertical artificial substrates were also investigated as a way to baffle water flows and to provide better settlement conditions whilst enhancing the surface area where prawns feed for higher nutrient sequestration. However, this did not reduce nutrient levels or increase the growth of prawns in this particular study (4). Since low-density cultivation of banana prawns in settlement ponds did not appear to greatly affect the nutrient discharge, regulatory bodies considered that this practice was an acceptable commercial activity and several farms adopted the practice to utilise waste nutrients and increase their productivity.
Mixed-species (polycultures) are also known to fill more ecological niche space in the pond and better utilise available nutrient sources. The potential to combine fish species with banana prawns in settlement ponds was therefore investigated. Sea mullet were shown to successfully coexist with banana prawns at a range of life stages in fed and unfed environments, suggesting that this could be a viable polyculture. Rabbit fish, on the other hand, which were thought to be primarily herbivorous, demonstrated a tendency to prey on banana prawns, thus preventing this particular polyculture (5).
The potential for using bivalves to treat prawn pond effluent was also reviewed in terms of their water filtration mechanisms and efficiencies, their nutrient cycling, and other biological attributes that would affect their integration with prawn production (6). That work also assessed the potential in southern Queensland of a number of native bivalve species, and showed that a well-known edible cockle (Anadara trapezia) could adequately tolerate the silt-laden conditions that would prevail in settlement basins. However, due to the potentially high bacterial levels and uncertain algal backgrounds in the growth environment, bivalves appear unsuited to wastewater remediation because depuration would likely be necessary after harvest from water treatment systems to ensure that the shellfish were a safe food-grade product. Additionally, the ecological effects of bivalve co-culture would also need to be considered, particularly from a nutrient cycling point of view, since intensive bivalve cultures are known to cause benthic eutrophication and sequester only a small proportion of material that they filter from the water.
In recognition of the preference by some farmers for mechanical devices that can be used to treat effluent, a series of AIDI studies were also conducted to assess both experimental and commercially available systems. Some of these studies could not be published due to either technical uncertainties or proprietary information, but none of the systems studied seemed to offer the nutrient-removal efficacies and volume-handling capacities necessary to be industrially relevant. Various mechanisms utilising foam fractionation seemed at the time to offer the most promise, and the best potential to scale to relevant levels. One of these, the SKIM by IFREMER, was subjected to rigorous testing at BIRC. This technology was shown to facilitate moderate removal of phosphorus and suspended solids, but it did not significantly reduce levels of ammonia, nitrogen or chlorophyll (7). In particular, this study drew attention to the potential to combine mechanical pre-treatment, which removes undesirable materials in the effluent stream (e.g. inorganic silt), with downstream biological treatments that could produce valuable secondary crops to offset the overall costs of power and materials depreciation, or to be profitable in their own right. In this regard, the SKIM system is used overseas to assist in the depuration of oysters before sale for human consumption, and thus could find application in integrated shellfish filtration systems in the future.
In association with AIDI studies, work at BIRC between 2000 and 2004 also focused on the optimisation of nutrient recoveries in secondary crops grown within settlement basins in compartmentalised polycultures. Theoretical models for nutrient sequestration in constructed aquatic ecosystems utilising the natural properties of selected fish, prawns, algae and bacteria were developed in PhD studies (8, 9). That work found that sea mullet could only assimilate low percentages (2-3%) of the nitrogen entering these settlement basins, and that they also resuspended settled material causing high levels of turbidity. Artificial substrates reduced this resuspension of particulates, and banana prawns were shown to assimilate substrate biofilms to a greater degree than mullet (10). This grazing on biofilms by the prawns substantially increased bacterial growth rates, but again nutrient assimilation was low considering the amounts that were available. Polyculture of mullet and rabbit fish improved the overall N retention in fish biomass but this effect was relatively small (11). Much greater assimilation (22-25% of incoming N) was demonstrated in this work by opportunistic macroalgae (e.g. Enteromorpha sp.). Rabbit fish were shown to act as in-pond harvesters of macroalgal material and mullet were shown to reduce the algae's surface growth.
Further preliminary studies at BIRC through University of the Sunshine Coast student projects looked specifically towards the culture of selected macroalgae (e.g. Ulva sp.; Gracilaria sp.) in mariculture wastewater streams to absorb nutrients and create harvestable material. However, this work demonstrated that there were several physical and biological problems associated with this approach. The periodic proliferation of microalgae in culture ponds often meant that nutrients were not continuously available for the seaweeds. Amphipods also regularly invaded seaweed cultures, greatly diminishing their biomass and usefulness as a nutrient sink. Variable, and sometimes high, levels of turbidity in the effluent also appeared to limit the levels of light available to the seaweeds for photosynthesis, and the accumulation of silt and other particulate matter on the plant surfaces further challenged plant growth causing senescence. Even when apparently favourable conditions did occur, the selected species often became overgrown by weed species (e.g. Enteromorpha sp.) creating contamination issues. Despite the wide recognition that plant uptake is a basic means to sequester dissolved nutrients from wastewater, in our work it has so far proved unreliable for application to the conditions within broad-scale pond mariculture systems.
Work towards zero discharge and on-farm recirculation technologies has also been undertaken in north Queensland. Water qualities and production levels in flow-through and recirculated ponds were compared in a study in 2001 (12). Similar commercial management regimes were applied except that the recirculated ponds received their intake water from a treatment pond rather than the adjacent river. The treatment pond consisted of a settlement channel and bioremediation area where partitions made from HDPE plastic guided water through a network of vertical shadecloth panels which harboured natural benthic organisms. Aeration was provided in the bioremediation area. The grow-out to treatment-pond surface area ratio in the experiment was 2.01:1.3 (in hectares), which means that a water treatment area equivalent to about 65% of the culture pond area would be needed to implement this approach. The results suggested that this recirculation design could produce high-quality prawns and improve the farm's environmental performance. Higher levels of denitrification (gaseous losses) and lower proportions of nitrogen in the water column were apparent in the recirculated system, and this coincided with higher levels of nitrogen in the sediment. The accumulation of sludge and remineralisation of organic matter to form ammonia in the recirculated system were causes for concern with the particular approach that was taken. Increasing salinities from evaporation were also identified as of concern for fully recirculated systems, potentially causing slow growth or, in extreme cases, toxic effects.
This work followed a Churchill Fellowship study tour by the senior researcher that sought to identify and investigate overseas developments in the design and operation of pond-based recirculating culture systems for prawns and fish (13). A broad range of facilities in several leading countries, and discussions with many scientists provided an up-to-date account for consideration by the Australian industry. Project work in north Queensland has also been undertaken to investigate the use of constructed mangrove forests to provide low-maintenance effluent treatment basins (14). The thrust behind that work was to assess the nutrient uptake by the mangroves and effluent retention times necessary for effective wastewater treatment. Mangrove timbers would also be a potentially profitable by-product of this approach. However, the long period that the mangroves took to become established was a difficulty that saw the project stall before meaningful data could be generated.
The advent of zero-exchange biofloc systems where carbon to nitrogen ratios are adjusted with soluble carbon sources (e.g. molasses) has also been supported by several projects at BIRC. Culture systems growing species that can utilise bioflocs as a food source can benefit from this approach since waste nutrients that can sometimes be toxic (e.g. ammonia) are continuously removed, recycled and converted into available protein (biofloc), which is taken up by the stock in the culture ponds. Experimental and larger pond-scale work conducted at BIRC sought to diversify the crop by investigating the co-culture of sand whiting and sea mullet with banana prawns, thereby combining the nutrient sequestration attributes of both bioflocs and polycultures. Experimental work investigated the dosing rates of molasses based on cited literature and the nitrogen contents of feeds (15). Radio-isotope-labelled ammonia assessed the uptake of bioflocs into these target species. As expected, mullet (a well-known consumer of bacterial biomass) and prawns outperformed the whiting in terms of their biofloc uptake, so the trial aimed to feed the whiting directly with artificial feeds but allow the biofloc to feed the prawns. With this approach prawn growth was supported almost entirely with nutrients from the metabolic waste products of the fish. Final production (without any water exchange) was in the order of 12 tonnes ha-1 of whiting over a 10-month production period (16). Unfortunately, no prawns were harvested due to their predation by the fish, but this nevertheless contributed to the high fish biomass that was produced. The whiting were only small when harvested but proved to be of high quality and very suitable for butterfly fillets. In this situation the prawns grew on the nutrients in the biofloc, which was less available to the whiting, but the whiting eventually benefited from the biofloc system through their consumption of the prawns.
There are, however, some potential down-sides to this biofloc approach, which are in need of further economic and biological evaluation. From work overseas it is known that some forms of floc can cause off-flavours and black gill conditions in prawns, and the experiences at BIRC demonstrated that levels of sludge production can be high, creating disposal issues at the end of crops. Although water discharge from these systems can be avoided so that nutrient discharge is largely non-existent for a large part of the production season, the final draining of ponds containing high nutrient and suspended-solids loads often exceeds maximum environmental discharge limits, necessitating special management arrangements. There is also high biological oxygen demand created in these culture systems and therefore larger amounts of power are needed to drive additional aeration devices for favourable culture conditions (although this may be offset somewhat by reduced pumping costs). The benefits and risks of this new heterotrophic approach are the subject of current research projects being operated by the industry at some of the leading prawn farms in Australia.
With much of the previous wastewater treatment research in perspective, investigations into polychaete-assisted sand filters (PASF) began at BIRC in 2005. This concept involves constructed sand filtration beds where marine polychaete worms are stocked and cultured to help facilitate the broad-scale filtration process. The cultured polychaetes help to keep the sand filter clean and functional, and in turn provide a harvestable high-value secondary crop (17, 18). Each square metre of sand in PASF systems has been experimentally shown capable of treating up to 1500 L of prawn pond water per day, which means that an area equivalent to about 10% of the culture pond area is needed to treat all the water from a farm operating on the limited flow-through model. At these treatment rates the sand filtration bed can more than halve suspended solids and chlorophyll levels in the wastewater. It has been shown to convert much of the particulate nitrogen and phosphorus into dissolved forms that are readily available for plant uptake. Recently completed research (19) which studied this system in commercial prawn and fish farms at much larger scale has also demonstrated significant removal rates of both nitrogen and phosphorus. Collectively, these results demonstrate the best suspended solids, chlorophyll and macronutrient removal capacities so far reported for any mariculture wastewater treatment methodology to date.The method generally uses a low-maintenance regime that is controlled by pumps on timers. The sand that has been used is bedding sand, which is commonly used in the construction industry. It has been shown capable of producing 300-400 g of worm biomass per m2
during the term of a prawn crop. The harvested worms are suitable for use as fishing bait and for fish and crustacean broodstock feeds, and are therefore considered a valuable addition to a farm's produce, whether for off-farm sale or internal use in their hatcheries. Since this is a new field of aquaculture in Australia, there has been a need to develop many of the basic support technologies and procedures from a low knowledge base. Additionally, as the methods are scaled up to handle larger quantities of water, new challenges are becoming evident, including the potential for bird predation and protection from heavy rainfall. Results so far suggest that marine worms can be reliably produced in this way whilst also favourably treating mariculture wastewaters. Future research and development in this area is looking towards further scale-up of methods to provide industrial capacity, optimisations for better effluent treatment, and basic studies of nutrient cycling and biological processes that occur in these enhanced benthic systems.
Developments associated with polychaete-assisted sand filter technologies have recently led us to re-examine the potential for culture of seaweeds in integrated systems, particularly because the filtered water produced by the sand-worm beds is very suitable for algal growth. Many of the earlier problems associated with turbid waters carrying particulates and fouling organisms are addressed by this type of pre-filtration arrangement, suggesting that the very productive capacities of algae may now be worth pursuing. Theoretical calculations (provided below) based on the amounts of nutrients that are typically present in pond-based mariculture discharges, and the contents and growth rates possible for species of seaweeds that have been extensively studied, suggest that vast quantities of seaweed could be produced from waste nutrients at mariculture farms.
Theoretical maximum seaweed production capacities based on nitrogen in integrated pond mariculture systems
Simplified weekly seaweed culture regime (assumes weekly doubling of Ulva biomass: after Neori et al. 1991).
- Stock 1.5 kg wet weight (ww) m-2 # After 1 week harvest 3 kg ww m-2 # Restock 1.5 kg ww m-2.
- Gives 1.5 kg m-2 ww production of Ulva per week.
Nitrogen (N) input and output in broad-scale pond wastewater (assumes the feeding of 10 000 kg of stock per week in a 1-hectare pond; back calculation of N budget after Funge-Smith and Briggs, 1998)
- Feeding 1% body weight per day = 100 kg feed d-1 = 700 kg feed per week.
- If feed contains 40% protein = feeding 280 kg protein per week ÷ standard N conversion factor (6.25), gives 44.8 kg N in feed per week.
- Feed often represents 78% of N input to ponds, so total N input for a pond holding 10,000 kg of stock is in the order of 57 kg N per week.
- If 17% of input N is typically lost in water exchange (not including final drain of pond), then 9.7 kg N is discharged in wastewater per week when the pond is carrying its maximum load.
Seaweed contents (based on unpublished DEEDI results)
- If Ulva contains 90% water, 1.5 kg ww = 0.15 kg dry matter.
- If Ulva contains 4% N by dry matter, 1.5 kg ww of seaweed = 0.006 kg N.
- From above culture regime, Ulva can provide an assimilation rate of 6 g of N per m2 per week.
Seaweed production potential (if we assume that all N in discharge water is available for plant growth ie: 9.7 kg N per week, and that all of this N can be sequestered in Ulva biomass each week)
- From above calculations, Ulva can sequester 6 g of N per m2 of culture area per week.
- So the amount of culture area required for Ulva to sequester the weekly N discharge of 9.7 kg N each week is 9.7÷ 0.006 = 1616 m2, where 2424 kg of Ulva biomass is produced each week.
These estimates suggest that an area equivalent to 16% of the culture ponds would be sufficient to sequester all the nitrogen that is typically discharged from shrimp ponds operating on the limited flow-through model, and in doing so over two tonnes of seaweed could be produced on a weekly basis from the discharge of each one-hectare culture pond. However, many assumptions are made to generate these projections, including a consistently high prawn/fish stock carrying capacity and consistent seaweed growth rates. In reality, given that rainfall events and cloudy weather would intermittently slow the growth of seaweed, and that early in the fish/prawn production season much lower levels of nutrients would be available in the discharge water, seaweed production averaged over the season would be much lower. Additionally, to sequester all of the nutrients from the farm in this way, it would be necessary to mitigate the flows from final pond drains during or after stock harvests, and release this water through the nutrient collection systems at manageable rates. The amounts of suspended sediments and nutrients in drain-harvest water from shrimp ponds can be high, even in conventional flow-through systems (10% of total N budget: Funge-Smith and Briggs, 1998), so alternative ways of dealing with this are also needed to completely eliminate outgoing nutrients.
This seaweed-based approach would also rely heavily on the ability to maintain vigorous growth in reseeded and standing crops and to reliably start new batches at will, just as hatchery operators routinely do with microalgae. To ensure this aspect of seed production does not potentially impede progress in this direction in the future, and to lay the foundations for the controlled culture of macroalgae in wastewater remediation systems, BIRC has fostered MSc studies through the University of the Sunshine Coast. The most recent study investigated induced reproduction methods for several commonly occurring species of the genus Ulva (20). It involved three naturally occurring species at BIRC that seem to form a complex that repeatedly co-inhabits wastewater streams, or which flourish in succession over the year. In particular, this complex seems to grow very well in the stream of filtered water from sand-worm beds, so this also formed part of the study. The results showed that all three species could be cultured separately in the laboratory using artificial fertilisers, where weekly doubling of biomass was achieved. Sporulation (33% of cells) was then induced by fragmentation of the thalli from these controlled cultures.
All of this research and developmental work has been undertaken in full recognition of the need for good farm management practices that facilitate the high health of stock and underpin optimal in-pond utilisation of feeds and other nutrient inputs. The production in 2006 of an easy-to-read prawn farming manual (21) has made the collective experiences of some of Australia's leading industry specialists more accessible and provides a backdrop for farmers to measure and compare their production methods and better guide their individual development programs.
Smarter demand-driven and automated feeding technologies, and state-of-the-art real-time water quality monitoring systems are in particular focus for improving farm-based culture practices at present. These developments have an important role to play in improving the efficiencies of feed uptake for improved productivity, and in understanding the dynamic pond ecosystems, which have an underlying effect on pond water and effluent qualities. It is expected that the improved feed utilisation and pond management practices that these technologies offer will minimise the nutrients that wastewater remediation methods need to deal with, thereby alleviating the problem at its source.
Bioremediation research has also been undertaken by Queensland Government scientists in several freshwater environments over the past decade. This has generally included assessments of the potential for native vegetation to sequester available nutrients, and for native fish to be used as additional nutrient sinks and bio-control agents for problematic or noxious weeds.
Integrated water quality management studies in the Burdekin Irrigation Area included a series of trials to determine the ecological relationships between aquatic weeds and herbivorous fish that occurred in the area (22). Three fish species were deployed in experimental mesocosms where their biomass changes and the nutrient levels in the water column provided insight into their ecology and nutrient cycling in disturbed habitats. Mullet had the most pronounced effect on weed growth, markedly reducing the prevalence of filamentous algae through ingestion and physical disturbance. However, bird predation on the fish in open waters reduced the potential of this treatment at large scale. The suitability of the available native fish species to control some of the most problematic weeds (e.g. water hyacinth) was considered to be low compared with exotic species used for this purpose in other parts of the world (e.g. grass carp).
Further pilot-scale work was also undertaken in the Burdekin region to investigate the recovery of nutrients from secondary-treated municipal wastewater (23). Duckweed, jade perch and sea mullet combinations were evaluated using different water retention times (3.5-10.4 days) and system configurations. Results showed that the longer retention times were capable of sequestering the highest levels of nutrients (up to 70.3% N and 13.6% P) and that duckweed was the main nutrient sink. It doubled its biomass every six days and provided a weekly harvest of 1.23 kg m-2 (equivalent to 98 g dry weight). When dried it contained 6.7% N and 1.3% P, and did not contain hazardous levels of any toxic substances that are known to occur in secondary treated sewage. Jade perch actively consumed the duckweed in the wastewater treatment system, progressively gaining weight over the 102-day trial (0.7 g d-1).
This prompted a number of further pilot studies using duckweed as a removable nutrient sink. In particular it grew prolifically in the settlement pond of a low-salinity inland prawn farm, offering nutrient recovery in a form that could be easily used to feed other livestock. Problems experienced in that work included the unanticipated need for regular manual removal of duckweed biomass from the ponds. Since the weed forms mats on the water surface, mechanical devices have been developed which achieve this for large areas where it is harvested commercially for use as stock feed. These and many other virtues of duckweed-based wastewater treatment systems were highlighted by these studies and a subsequent review (24).
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- 'The environmental management of prawn farming in Queensland - world's best practice', CSIRO, October 2009.
- Palmer, PJ (ed.) 2005, Wastewater remediation options for prawn farms - Aquaculture Industry Development Initiative 2002-04, Queensland Department of Primary Industries and Fisheries project report no. QO04018.
- Palmer, PJ, Erler, D, Burke, MJ, Lobegeiger, R, Morrison, C, Bell, G and Knibb, W 2005, Growing banana prawns Penaeus merguiensis (de Man) in prawn farm settlement ponds to utilise and help remove waste nutrients, in 'Wastewater remediation options for prawn farms - Aquaculture Industry Development Initiative 2002-04', Queensland Department of Primary Industries and Fisheries project report no. QO04018, pp. 5-24.
- Palmer, PJ, Erler, D, Morrison, C and Rutherford, BW 2005, Nutrient levels in experimental tanks supplied with prawn pond effluent: the effect of artificial substrate and different densities of the banana prawn Penaeus merguiensis (de Man), in 'Wastewater remediation options for prawn farms - Aquaculture Industry Development Initiative 2002-04, Queensland Department of Primary Industries and Fisheries project report no. QO04018, pp. 25-39.
- Palmer, PJ, Morrison, C, Rutherford, BW and Pledger, BKD 2005, Investigations into the potential for mixed cultures of banana prawns Penaeus merguiensis, sea mullet Mugil cephalus and rabbitfish Siganus nebulosus for bioremediation of aquaculture waste in 'Wastewater remediation options for prawn farms - Aquaculture Industry Development Initiative 2002-04', Queensland Department of Primary Industries and Fisheries project report no. QO04018, pp. 40-9.
- Palmer, PJ and Rutherford, BW 2005, Bivalves for the remediation of prawn farm effluent: identification of some potentially useful species in Southern Queensland, in 'Wastewater remediation options for prawn farms - Aquaculture Industry Development Initiative 2002-04', Queensland Department of Primary Industries and Fisheries project report no. QO04018, pp. 50-74.
- Palmer, PJ, Morrison, C, Willett, DJ and Rutherford, BW 2005, Testing the effectiveness of the SKIM foam fractionator to reduce nutrient levels in prawn farm effluent, in 'Wastewater remediation options for prawn farms - Aquaculture Industry Development Initiative 2002-04' Queensland Department of Primary Industries and Fisheries project report no. QO04018, pp. 75-93.
- Erler, D 2000, Bioremediation of aquaculture waste, Queensland Aquaculture News, issue 16, p. 3.
- Erler, D 2003, 'Nitrogen removal in a prawn farm effluent treatment system containing secondary crops and artificial substrate', PhD thesis, University of the Sunshine Coast, Queensland.
- Erler, D, Pollard, PC and Knibb, W 2004, Effects of secondary crops on bacterial growth and nitrogen removal in shrimp farm effluent treatment systems, Aquacultural Engineering, vol. 30, pp. 103-14.
- Erler, D, Pollard, P, Duncan, P and Knibb, W 2004, Treatment of shrimp farm effluent with omnivorous finfish and artificial substrates, Aquaculture Research, vol. 35, pp. 816-27.
- Robertson, C, Burford, MA and Johnston, A 2003, 'Recirculation prawn farming project', final report, NHT project no. 717511, Queensland Department of Primary Industries and Fisheries project report no. QO03014.
- Robertson, C 2001, 'International advances in prawn farm recirculation technology: design principles and efficiency of treatment systems', Queensland Department of Primary Industries project report no. QO01008.
- Robertson, C 2000, Constructed mangrove wetland project, Queensland Aquaculture News, issue 16, p. 2.
- Willett, D and Morrison, C 2006, Using molasses to control inorganic nitrogen and pH in aquaculture ponds, Queensland Aquaculture News, issue 28, pp. 6-7.
- Willett, D and Palmer, P 2008, 'Integrated pond culture for improved production and environmental performance', Oral presentation given at the World Aquaculture Society Conference entitled 'Innovation in a global market', Brisbane, Queensland 3-6 August 2008.
- Palmer, PJ 2008, 'Polychaete-assisted sand filters - prawn farm wastewater remediation trial', National Landcare Program innovation grant no. 60945, technical report, 61 pp.
- Palmer, PJ 2010, Polychaete-assisted sand filters, Aquaculture, vol. 306, pp. 369-77.
- Palmer, PJ 2011, 'Commercial application of polychaete sand filters for wastewater remediation and broodstock feeds', Landcare sustainable practices grant no. SEQC1418, technical report, 34 pp.
- Pettett, P 2009, 'Preliminary investigation into the induction of reproduction in Ulva spp. in Southeast Queensland for mass cultivation purposes', MSc thesis, University of the Sunshine Coast.
- Robertson, C (ed.) 2006, 'Australian prawn farming manual - Health management for profit', Department of Primary Industries and Fisheries, Queensland.
- Willett, D, Erler, D, Rutherford, B and Knibb, W 2003, 'Role of native fish in integrated aquatic weed and water quality management within the Burdekin Irrigation Area', final report to the South Burdekin Water Board.
- Willett, D, Rutherford, B, Morrison, C and Knibb, W 2003, 'Tertiary treatment of Ayr municipal wastewater using bioremediation: a pilot-scale study', final report to the Burdekin Shire Council and the Burdekin Rangeland Reef Initiative.
- Willett, D 2005, 'Duckweed-based wastewater treatment systems: design aspects and integrated reuse options for Queensland conditions', Queensland Department of Primary Industries and Fisheries project report no. QI05019.
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