Activated sludge denitrification in marine recirculating aquaculture system effluent using external and internal carbon sources
Introduction
Environmental control and increased biosecurity concerns have incentivized the construction of land-based recirculation aquaculture systems (RAS) for smolt phases and grow-out cycles of marine fishes (Dalsgaard et al., 2013; Martins et al., 2010). The number of production units with the capacities of 1000 up to 90,000 tons of fish/y is increasing (Dalsgaard, 2017). However, implementing the European Marine Strategy Framework Directive (MSFD) and the Water Framework Directive (WFD) forces aquaculture industry to reduce nutrient discharge into natural water bodies. Thus, cost-efficient end-of pipe treatment technologies are needed to reduce nitrogen (N) in saline effluent discharges (Borja et al., 2010; WFD, 2000).
In order to increase water reuse and avoid toxic conditions for fish in RAS, biofilters with nitrifying microbes oxidize ammonium (NH4+) into nitrate (NO3−), which accumulates and needs to be removed before RAS effluents are discharged into the environment (Hamlin et al., 2008; Suhr et al., 2013; van Rijn et al., 2006). Heterotrophic denitrification, the main NO3- removal technology applied in RAS, is a sequential process with four enzymatic steps, including reduction of nitrate to nitrite (NO2), nitric oxide (NO), nitrous oxide (N2O), and finally, to nitrogen gas (N2) (Henze et al., 1997). Denitrifying bacteria use a wide spectrum of organic carbon sources that can be obtained either commercially (external carbon sources) or from RAS effluent (internal carbon sources). Methanol and ethanol are the common choices of external carbon source for RAS (van Rijn et al., 2006). However, being reactive flammable alcohols, they require special standards for transport, storage, packaging, handling, and disposal (European Parliament and the Council of the European Union, 2008), which causes additional capital costs (Cherchi et al., 2009). Alternative carbon sources, such as volatile fatty acids (VFA) (e.g. acetate, propionate, butyrate), obtained either commercially or produced from the fermentation of fish waste offer a promising approach for an effective reduction in the operational costs of end-of-pipe treatment (Letelier-Gordo et al., 2017; van Rijn et al., 1995).
Media-laden reactors are the most popular technology in aquaculture denitrification in brackish or marine water conditions (Balderston and Sieburth, 1976; Grguric et al., 2000; Gutierrez-Wing et al., 2012; Honda et al., 1993). However, channeling, clogging, and increased pressure drops due to the organic matter accumulation are common, when media-laden reactors are used for the denitrification process (Balderston and Sieburth, 1976; Müller-Belecke et al., 2013; Sauthier et al., 1998). This might reduce the effective denitrification capacity of the reactor, requiring frequent back-washing in order to sustain continuous operation. Organic matter accumulation is especially problematic in marine systems, since the anaerobic conditions will promote the production of toxic hydrogen sulfide (H2S) (Letelier-Gordo et al., 2020). Therefore, alternative solutions, with simple construction and operation, high denitrification rates, and a low footprint, for media-laden denitrification reactors are required in marine land-based facilities.
The use of bacterial flocs or flocculent bacteria (e.g. activated sludge and granules) is globally the most common treatment technology for biological nitrogen (N) and organic matter removal in municipal and industrial wastewater treatment (Henze et al., 1997, 2008; Tchobanoglous et al., 2002). Unlike media-laden reactors, flocculent bacteria live suspended inside the reactors without the need for plastic carrier elements. Depending on the type of flocs developed, high denitrification rates can be achieved (12−14 kg NO3−-N/m3 reactor d) (Klapwijk et al., 1981; Letelier-Gordo and Martin Herreros, 2019).
A fed batch reactor (FBR) has a simple fill-and-draw setup (a cycle of fill, react, settle, and discharge) with a small footprint and is easy to construct and operate in e.g. an existing buffer tank. It has a long reliable operation age and a strong tolerance towards variations in the flow and substrate (Strous et al., 1998). In municipal wastewater treatment, FBR has been broadly studied (Wang et al., 2010), but the applicability of FBR for N removal in marine RAS, or in aquaculture in general, is not currently known.
To evaluate the applicability of this technology for RAS, the main objectives of this study were to: 1) define the best reaction time for denitrification using fed batch reactor and 2) measure the activated sludge denitrification capacity using external (acetate, propionate, and ethanol) and internal carbons sources (RAS fish waste and RAS fermented fish waste).
Section snippets
Experimental setup
The experimental work was divided into two phases. In phase I, the effect of different FBR operational cycle times (OCT; 2, 4, and 6 h) on the denitrification rate was studied, using acetate as a carbon source. In phase II, the denitrification capacity of the FBR was evaluated using different external and internal carbon sources.
Reactors
The FBR system consisted of six 10 L glass bottles (ISO 4796, PYREX, USA), where the filling and discharge volume was 8 L (80% of volume reactor), and 2 L were left for
Operational conditions in FBR (phase I)
The OCT significantly affected both nitrate (ANOVA, F2,9 = 46.8, P < 0.001) and nitrite (F2,9 = 25.2, P < 0.001) removal rates, when the activated sludge was fed with acetate. The highest denitrification rates (98.7 ± 3.4 mg NO3−-N/(h g biomass); 93.2 ± 13.6 mg NOx−-N/(h g biomass)) were obtained on 2 h cycle conditions. Denitrification rates decreased as the operational time increased to 6 h (Fig. 2), but there was no significant difference in NOx− removal rates (pairwise comparisons, P>0.05)
Conclusions
This study demonstrates that FBR could be a viable option for end-of-pipe N removal in marine land-based RAS. Among the five carbon sources tested, acetate had the highest N removal rate, resulting in a smaller reactor volume needed. Furthermore, no accumulation of TAN, PO43−, or S2- was observed when using external carbon sources, except a pronounced sulfide production found when using ethanol. Using fish organic waste for N removal will save operational costs, becoming a cost-efficient option
CRediT authorship contribution statement
Carlos O. Letelier-Gordo: Conceptualization, Methodology, Writing - original draft, Writing - review & editing. Xiaoyu Huang: Investigation, Writing - original draft. Sanni L. Aalto: Formal analysis, Writing - review & editing, Visualization. Per Bovbjerg Pedersen: Resources, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The study was funded by BONUS (Art 185), funded jointly by the EU and national funding institutions of Finland (Academy of Finland), Sweden (Vinnova), and Denmark (Innovation Fund Denmark IFD). The technical skills and invaluable assistance of Ulla Sproegel, Brian Møller, Ole Madvig Larsen, and Rasmus Frydenlund Jensen (DTU Aqua) is highly appreciated.
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