Introduction
For recirculating aquaculture systems (RAS) to operate with minimum water consumption, an in-line denitrifying biofilter plays a key role, as nitrate is the limiting factor for good water quality. This study focuses on limiting the water consumption for RAS technology to a minimum by optimizing the process of in-line denitrification in RAS. This is crucial if landbased farming shall represent 30% of all aquaculture production by 2030, as water is becoming scarce [1]. An optimized process of in-line denitrification will enable RAS facilities to be designed for nutrient sensitive areas and/or areas with limited water availability, such as landlocked countries or arid areas. Currently, denitrification in RAS is commonly chosen as end-of the pipe treatment to reduce nitrate discharge, and application of in-line denitrification is limited.
Denitrification is a sequential process, where nitrate is reduced into inert N2 using organic carbon as an electron donor under anoxic conditions. Denitrification can be achieved by heterotrophic, autotrophic or mixotrophic bacteria, depending on their energy source being organic or inorganic compounds, or both [2]. In denitrification, oxygen inhibits the activity of enzymes involved in the nitrate reduction [3]. Therefore, oxygen must be depleted from the inlet water. In wastewater treatment plants, oxygen is removed by extending hydraulic retention time (HRT). However, this is not feasible in RAS due to the requirement of high flowrates and relatively faster processes within the system. Furthermore, heterotrophic denitrification requires an external carbon source. In RAS farms, methanol is often used as a carbon source, however the HSE regulations associated with methanol use causes some practical challenges. While acetic acid would be a cheaper and safer option for yielding a higher denitrification rate, although yielding more sludge [4]. In RAS, any accumulation of sludge may increase the risk of H2S under anaerobic conditions. Besides determining the sludge yield, carbon sources could regulate the microbial community composition of denitrifying biofilters [5]; Carlson et al. [6], showed that selective carbon sources are more likely to direct microbial nitrate respiration towards specific end products such as dinitrogen (N2), ammonium (NH4+), or intermediate nitrogen oxides (NO2−, NO, N2O). Thus, the paradigm that higher concentrations of carbon will always favour dissimilatory nitrate reduction to ammonium (DNRA, nitrate ammonification) over denitrification is no longer valid. While denitrification removes nitrate as N2 into the atmosphere, DNRA converts nitrate into ammonium, thus maintaining nitrogen in the RAS system.
Therefore, it is crucial to identify and characterize a carbon source best suited for in-line denitrification in RAS, as the requirements differ substantially to those required by end-of the pipe or in wastewater treatment. Important parameters are toxicity, low sludge yield, favouring N2 as the end products of nitrate respiration, and an adequate nitrate removal efficiency (NRE). Results of this study will identify an efficient method for treating flows from large-scale RAS to create optimum conditions for in-line denitrification and keeping the unit as compact as possible.
Objectives
The main goal of this project is to create optimum conditions for in-line denitrification in large scale RAS with minimum water discharge. To achieve this goal, following objectives are identified: a) identifying an optimum carbon source specific for in-line denitrification and b) identifying the optimal and most feasible process for oxygen removal with high flow rates. Finally, c) the optimized parameters will be combined to confirm industrial application.
Material and methods
To identify an optimum carbon source, three different sole carbon sources will be tested at lab scale fixed bed biofilm reactors (FBBR), operated in continuous flow mode over a period of two weeks. Biofilm carriers adapted to denitrification will be taken from previous studies and water will be acquired from RAS farms. Water samples will be taken in regular intervals, filtered, and analysed for NO3-, NO2-, NH4+, total nitrogen and chemical oxygen demand (COD) by Hach-Lange test kits. Sludge samples will be analysed for total suspended solids (TSS) and volatile suspended solids (VSS) by standard procedures. Based on nitrate removal rate (NRR), nitrate removal efficiency (NRE) and sludge yield, an optimum carbon source will be selected.
To identify an optimum process for oxygen removal in the inlet of in-line denitrification of large-scale RAS, two mechanical methods (vacuum degasser, bubbling N2 gas) and one biogenic method (depletion with carbonaceous bacteria) will be tested at lab scale to remove oxygen from circulating RAS water. For this, 14L FBBR bioreactors will be operated in continuous mode and rigged accordingly. The oxygen removal efficiency (ε) will be measured by an oxygen probe measuring the dissolved oxygen (DO) at the inlet-and outlet over a time period of one week. Based on the results, the method with highest efficiency will be chosen for a combined experiment.
Depending on the initial results, the processes will be combined accordingly: Two 14L FBBR will be operated in continuous flow mode, while FBBR1 will serve as buffer tank for oxygen removal, while FBBR 2 containing the denitrification process will be supplemented with the identified carbon source. Measurements of nitrogenous species, COD, TSS, VSS and DO will be taken as described above. A control experiment will be run without oxygen removal and with acetate as a carbon source.
Conclusions
For RAS to become sustainable, a significant reduction of nitrogenous compounds in-line of the water treatment system is essential. With the preliminary study at a lab-scale, we can effectively investigate the effect of selected optimization factors under well-controlled conditions to advance our mechanistic understanding. This applies both for selected carbon sources on microbial in-line denitrification and for oxygen removal strategies. This proof of concept forms an important basis/decision gate for successful implementation in a full-scale RAS.
References
[1] Precision Aquaculture Market worth $794 million by 2026 (marketsandmarkets.com), 16.05.2023. [2] Di Capua, F., et al., Chem Eng J 362 (2019): 922-937. [3] Kampschreur, M. J., et al., Water Res 43.17 (2009): 4093-4103. [4] Cherchi, C., et al., Water Environ Res 81.8 (2009): 788-799. [5] Sun, Y., et al., SpringerPlus 5 (2016): 1-16. [6] Carlson, H.K., et al., ISME J 14.8 (2020): 2034-2045.
Acknowledgements
This study is supported by the RFF Rogaland with the project number 344757- OptiDeNit - Optimized In-Line Denitrification for Recirculating Aquaculture Systems (RAS).