Nitrogen removal and reuse in land-based aquaculture

Land-based prawn and barramundi farms produce large volumes of dilute wastewater containing both nitrogen rich suspended solids and dissolved organic and inorganic nitrogen. Settlement ponds are used to treat aquaculture wastewater by removing total suspended solids (TSS) through settling. Transformation of soluble nitrogen is also facilitated by the microbial community in the sediments of the settlement ponds but the prevailing transformation pathways and rate processes are largely unknown in these systems. Denitrification and anaerobic ammonium oxidation (anammox) are two transformation pathways which permanently remove fixed nitrogen from the system by converting it to gaseous nitrogen (N₂). Potential rates of denitrification and anammox were measured in the sediments of four settlement ponds using isotope tracer techniques in homogenised sediment. N₂ was produced in all ponds, although potential rates were low (0-7.07 nmol N cm⁻³ h⁻¹), relative to other aquatic systems. Denitrification was the main driver of N₂ production, with anammox only detected in two of the four ponds. Potential N₂ production rate did not correlate with any of the measured sediment variables (total organic carbon, total nitrogen, iron, manganese, sulphur and phosphorous) and was not stimulated by the addition of an exogenous carbon source. A simple mass balance model demonstrated that only 2.5% of added (through wastewater inputs) fixed nitrogen was removed in these settlement ponds through denitrification and anammox.\ud \ud Denitrification and anammox are outcompeted in some tropical ecosystems by transformation pathways which retain nitrogen within the system. Manipulative intact core experiments were conducted using sediment collected from one settlement pond to elucidate the entire suite of soluble nitrogen transformation pathways and to ascertain the potential role of competing pathways in limiting N₂ production. Indeed denitrification was slower 39 ± 9 µmol m⁻² h⁻¹ than nitrate (NO₃⁻) uptake (89 ± 63 µmol m⁻² h⁻¹). Denitrification also occurred at slightly lower rates than dissimilatory nitrate reduction to ammonium (DNRA). Additional retention pathways of dissolved organic nitrogen (DON) and ammonium (NH₄⁺) uptake (747 ± 40 and 22 ± 22 µmol m⁻² h⁻¹, respectively) and release (20 ± 3 and 12 ± 2 mmol m⁻² h⁻¹, respectively) were also rapid. Understanding the transformation of DON in aquaculture settlement ponds is particularly important as it is the dominant nitrogen species in the dissolved fraction but has rarely been studied. Following the rapid uptake and release of DON, it was subsequently transformed to NH₄⁺ (remineralisation) and to NO₃⁻ (nitrification) and a small proportion (0.7%) was transformed to N₂ after 17 h, indicating that DON removal occurred, albeit at slow rates. Taken together, results from the homogenised sediment experiments and the intact core experiments indicate that the majority of the added nitrogen is conserved within a settlement pond system and that sludge removal is essential to prevent water quality degradation through mineralisation and subsequent release of soluble nitrogen.\ud \ud Accordingly, the potential of enhancing wastewater treatment by capturing and converting nitrogen rich TSS to a secondary product was investigated. TSS were characterised and subsequently harvested. Particle sizes ranged from 0.04-563 µm with the majority of particles residing in the 11-20µm size fraction. Microalgae constituted a large portion of the TSS (26.1 ± 2.7%), and the nitrogen and carbon content of the TSS was high (3.9 ± 0.3% and 20.2 ± 1.8%, respectively). The microalgal community was comprised predominantly of cyanobacteria and diatoms and was rich in fatty acids (28.5-42.0 mg FAME g⁻¹ DW of TSS). 60% of the TSS were captured during harvest using an Evodos (centrifugal force). Diatoms were selectively removed with cyanobacteria and chlorophytes remaining in the water post processing. TSS were pyrolysed and the resulting biochar was high in nitrogen (2.5-3.5%) and potassium (1.4- 2.0%). However, carbon content, cation exchange capacity and surface area were moderate to low. It was estimated that biochar production, on a large prawn farm (~100 ha) could capture and reuse 940 tonnes of waste TSS per annum. This equates to annual sequestration of 226 tonnes of carbon and 28 tonnes of nitrogen.\ud \ud This thesis culminates with a review to identify technologies originally developed for the treatment of municipal wastewaters and intensive recirculating aquaculture systems which could be transferred to land-based aquaculture systems to enhance wastewater treatment. I present a conceptual model with recommendations for treatment steps which focuses on value adding outputs. Initially wastewater should be treated in a set of deep anaerobic ponds that can be easily managed and desludged. Resulting sludge (from anaerobic ponds and from culture ponds) should be digested anaerobically and power generation through methane conversion is possible. Nonsettled colloidal and supracolloidal solids and dissolved nutrients can then be removed through biological treatment in algal treatment ponds. Algal cultivation has potential to produce 146 tonnes of valuable biomass per annum and available conversion options include pyrolysis to biochar, inclusion into aquaculture feeds, application as fertilisers, and refining to biofuel or bioenergy. Constructed wetlands should be used as a polishing stage to assimilate residual waste nutrients into biomass or convert them to N₂, with concomitant benefits in the form of ecosystem services.\ud \ud This study is the first to evaluate tropical settlement ponds in terms of soluble nitrogen cycling. It provides evidence to support upgrades to farm management and system design, with a focus on optimising nutrient cycling to enhance sustainability and increase profit margins

Evaluation of four dietary protein sources for use in microbound diets fed to megalopae of the blue swimmer crab, Portunus pelagicus

Introduction of formulated diet particles for larval culture will simplify hatchery protocols and promote consistent survival, while decreasing total operating costs. The blue swimmer crab, Portunus pelagicus, is a commercially important species with substantial aquaculture potentials in the Indo-Pacific region. Feeding trials aimed at investigating protein nutrition of P. pelagicus larvae were undertaken with the megalopae instar. Microbound diets (MBD) with four different dietary protein sources (fish meal, squid meal, krill meal and soybean meal) were formulated and fed to newly molted megalopae until they metamorphosed to the first crab stage. Thirty megalopae were reared individually for each dietary treatment, with the addition of two control treatments of megalopae fed live Artemia nauplii and an unfed treatment.\ud \ud Survival was high for all fed treatments (73.3–93.3%), while total mortality was recorded for the unfed control. Megalopae fed the fish meal based MBD demonstrated higher survival than that of megalopea fed live Artemia, suggesting that MBD have the potential to completely replace live food for the culture of P. pelagicus megalopae without adversely affecting their survival. These results also suggest that P. pelagicus megalopae are able to adapt to a range of dietary protein sources. For megalopae fed live Artemia nauplii, mean development time was significantly shorter (4.0 ± 0.4 days) and the mean carapace width and dry weight of newly settled first stage crabs were significantly greater, (3.1 ± 0.0 mm, 1.3 ± 0.1 mg, respectively) than megalopae in all other fed treatments. Among MBD treatments, no significant difference in mean development time was detected, however, the mean carapace width of crabs that metamorphosed from megalopae fed fish meal based MBD (2.8 ± 0.0 mm) was larger than those fed squid meal based MBD (2.6 ±0.0 mm) and the mean dry weight of individuals fed fish meal based MBD (0.87 ± 0.04 mg) was also significantly heavier than those fed soybean meal based MBD (0.72 ± 0.04 mg). The outcome of this research provides valuable information regarding protein requirements of P. pelagicus megalopae and will aid in optimising formulated diet particles for potential use in commercial hatcheries

Sediment microbial community analysis: establishing impacts of aquaculture on a tropical mangrove ecosystem

The impact of organic loading from a tropical finfish cage farm was investigated by comparing the microbial community in surface sediments (0-8.5 cm) at sites under fish cages and on a gradient away from the farm lease area. Both total bacteria and Vibrio sp., enumerated by epifluorescence microscopy and TCBS plate counts respectively, were more abundant at cage sites than at the control sites away from the cages. Total bacteria ranged from 1.1 x 10(7) cells g WW(-1) at cage sites to 1.9 x 10(6) cells g WW(-1) at sites away from the cages and Vibrio sp. abundance ranged from 1.6 x 10(5) CFU g WW(-1) at cage sites to 5.3 x 10(2) FU WW(-1) at sites away from the cages. Bacterial 16S rRNA gene clone libraries demonstrated highly diverse microbial communities with similar patterns at the phyla and sub-class affiliation level for sites under the cages and sites away from the cages. However, small shifts in relative abundance of 16S rRNA gene sequences of key microbial groups were observed for sites under the cages. For example, 16S rRNA sequences related to epsilon-Proteobacteria and Vibrio sp. were present at cage sites and absent at sites away from the cages. In addition, a higher relative abundance of sequences related to the delta-Proteobacteria were observed at cage sites compared to sites away from the cages. This study is the first to investigate impacts of finfish cage aquaculture on the microbial community of sediments under fish cages in tropical marine ecosystems and demonstrates that changes in microbial abundance and shifts in sedimentary bacterial communities may be potentially useful indicators of organic perturbations in these environments. (c) 2009 Elsevier B.V. All rights reserved

Algal bioproducts derived from suspended solids in intensive land-based aquaculture

Land-based aquaculture produces suspended solids in culture pond and settlement pond waters that could be harvested as a bioresource. Suspended solids were quantified, characterised and harvested from these two sources to assess their suitability for conversion to bioproducts. The suspended solids of settlement ponds were less concentrated (87.6 ± 24.7 mg L⁻¹) than those of culture ponds (131.8 ± 8.8 mg L⁻¹), but had a higher concentration of microalgae (27.5 ± 4.0%) and consequently higher particulate organic carbon (24.8 ± 4.7%) and particulate nitrogen (4.0 ± 0.8%). The microalgal community also differed between sources with a higher concentration of fatty acids in the biomass from settlement ponds. Consequently, biochar produced from biomass harvested from settlement ponds was higher in organic carbon and nitrogen, with a lower cation exchange capacity. In conclusion, we characterised a renewable and potentially valuable bioresource for algal bioproducts derived from suspended solids in intensive land-based aquaculture

Wastewater treatment for land-based aquaculture: improvements and value-adding alternatives in model systems from Australia

Settlement ponds are used to remove particulate and dissolved nutrients in Australian land-based aquaculture wastewater. At best, marine and brackish water settlement ponds reduce total suspended solids by 60%, but their efficiency is inconsistent. Functional improvements to nutrient removal systems are essential to provide uniform and predictable treatment of flow-through aquaculture wastewater. Furthermore, environmental regulation of discharge from intensive systems in Australia is increasing, providing the impetus to upgrade rudimentary single-step settlement pond systems. We characterise technologies used for land-based aquaculture wastewater treatment prior to discharge from shrimp systems in Australia. We identify opportunities to integrate technologies developed for the treatment of municipal wastewaters and intensive recirculating aquaculture systems, and use these to develop a model system for intensive shrimp farm wastewater. The first stage is the reduction of solids through the use of deep anaerobic ponds, which are tailored to dilute saline wastewater. Non-settled colloidal and supracolloidal solids can subsequently be removed through trapping in a sand bed filter and biological transformation to dissolved inorganic nitrogen or N2. The resulting dissolved nutrients can be treated in a 3-stage algal treatment system by assimilation into harvestable biomass, and finally constructed wetlands polish wastewater through further trapping of particulates, and transformation of dissolved nitrogen. Given that upgrading wastewater treatment facilities is costly, we highlight options that have the potential to offset nutrient treatment costs, such as the use of algal biomass for food or energy products, and the recycling of nitrogen and phosphorus via pyrolysis creating products such as biochar and biofuel

Wastewater treatment for land-based aquaculture: improvements and value-adding alternatives in model systems from Australia

Settlement ponds are used to remove particulate and dissolved nutrients in Australian land-based aquaculture wastewater. At best, marine and brackish water settlement ponds reduce total suspended solids by 60%, but their efficiency is inconsistent. Functional improvements to nutrient removal systems are essential to provide uniform and predictable treatment of flow-through aquaculture wastewater. Furthermore, environmental regulation of discharge from intensive systems in Australia is increasing, providing the impetus to upgrade rudimentary single-step settlement pond systems. We characterise technologies used for land-based aquaculture wastewater treatment prior to discharge from shrimp systems in Australia. We identify opportunities to integrate technologies developed for the treatment of municipal wastewaters and intensive recirculating aquaculture systems, and use these to develop a model system for intensive shrimp farm wastewater. The first stage is the reduction of solids through the use of deep anaerobic ponds, which are tailored to dilute saline wastewater. Non-settled colloidal and supracolloidal solids can subsequently be removed through trapping in a sand bed filter and biological transformation to dissolved inorganic nitrogen or N2. The resulting dissolved nutrients can be treated in a 3-stage algal treatment system by assimilation into harvestable biomass, and finally constructed wetlands polish wastewater through further trapping of particulates, and transformation of dissolved nitrogen. Given that upgrading wastewater treatment facilities is costly, we highlight options that have the potential to offset nutrient treatment costs, such as the use of algal biomass for food or energy products, and the recycling of nitrogen and phosphorus via pyrolysis creating products such as biochar and biofuel

Denitrification and anammox in tropical aquaculture settlement ponds: an isotope tracer approach for evaluating N2 production

Settlement ponds are used to treat aquaculture discharge water by removing nutrients through physical (settling) and biological (microbial transformation) processes. Nutrient removal through settling has been quantified, however, the occurrence of, and potential for microbial nitrogen (N) removal is largely unknown in these systems. Therefore, isotope tracer techniques were used to measure potential rates of denitrification and anaerobic ammonium oxidation (anammox) in the sediment of settlement ponds in tropical aquaculture systems. Dinitrogen gas (N2) was produced in all ponds, although potential rates were low (0–7.07 nmol N cm−3 h−1) relative to other aquatic systems. Denitrification was the main driver of N2 production, with anammox only detected in two of the four ponds. No correlations were detected between the measured sediment variables (total organic carbon, total nitrogen, iron, manganese, sulphur and phosphorous) and denitrification or anammox. Furthermore, denitrification was not carbon limited as the addition of particulate organic matter (paired t-Test; P = 0.350, n = 3) or methanol (paired t-Test; P = 0.744, n = 3) did not stimulate production of N2. A simple mass balance model showed that only 2.5% of added fixed N was removed in the studied settlement ponds through the denitrification and anammox processes. It is recommended that settlement ponds be used in conjunction with additional technologies (i.e. constructed wetlands or biological reactors) to enhance N2 production and N removal from aquaculture wastewater

Improving productivity and environmental performance of aquaculture. Installment 5 of "Creating a sustainable food future"

Fish—including finfish and shellfish—are an important item in the human food basket, contributing 17 percent of the global animal-based protein supply in 2010. They are an especially valuable food source in developing countries, where more than 75 percent of the world’s fish consumption occurs. In addition to protein, fish contain micronutrients and long- chain omega-3 fatty acids that are essential for maternal and child health, but often deficient in the diets of the poor. However, the global supply of wild-caught fish has long peaked and is unlikely to rise again unless overexploited stocks are rehabilitated. As world fish consumption continues to grow, aquaculture (fish farming) has emerged to meet demand. Already, just under half of all fish that people consume come from aquaculture, which is one of the world’s fastest-growing animal food producing sectors. With the supply of wild-caught fish stagnant, any future increase in world fish consumption will need to be supplied by aquaculture. In a resource-constrained world, aquaculture could be an attractive option for expanding animal protein supply. Farmed finfish are similar in feed conversion efficiency to poultry, and much more efficient than beef. Filter-feeding carp and mollusks are even more efficient producers of animal protein, as they require no human-managed feeds and can improve water quality. Because the aquaculture sector is relatively young compared with terrestrial live- stock sectors, it offers great scope for technical innovation to further increase resource efficienc