Nitrogen removal and reuse in land-based aquaculture

Castine, Sarah (2013) Nitrogen removal and reuse in land-based aquaculture. PhD thesis, James Cook University.

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Abstract

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.

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.

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.

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.

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.

Item ID: 28268
Item Type: Thesis (PhD)
Keywords: algal biomass, anaerobic ammonium oxidation, anammox, aquaculture, biochar, biosolids, dentrification, fixed nitrogen, land based aquaculture, nitrogen removal, settlement ponds, suspended nitrogen, waste byproducts, waste sediments, wastewater treatment
Date Deposited: 08 Aug 2013 04:09
FoR Codes: 07 AGRICULTURAL AND VETERINARY SCIENCES > 0704 Fisheries Sciences > 070401 Aquaculture @ 100%
SEO Codes: 83 ANIMAL PRODUCTION AND ANIMAL PRIMARY PRODUCTS > 8301 Fisheries - Aquaculture > 830199 Fisheries - Aquaculture not elsewhere classified @ 100%
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