Introduction
To provide capacity for the increasing production volumes of Atlantic salmon smolt and post-smolt in Norway, land-based recirculating aquaculture systems undergo rapid technological development and upscaling. To meet economic and environmental expectations, fresh-water usage and emissions of NO3-N and PO4-P must be minimized. The zero-water concept (ZWC, AKVA Group, Norway) allows the removal of P by precipitation, and the removal of N by heterotrophic microbial denitrification. While the precipitation of P is an instant and easy to control chemical reaction, the stable control of the anoxic microbiological denitrification is more complex. To achieve a full performance of the facility at an early stage, protocols for rapid start-up and operational management for the denitrification must be followed. While literature describes start-up and operation of lab-based reactors, protocols for the start-up at commercial scale and (low) temperatures around ~13°C are lacking (Oboody, 2017). To assist RAS operators with these challenges, we provide a protocol that allows for the stable denitrification within 10 weeks in maturation.
The protocol was applied at a commercial smolt production facility in Mid Norway. This study reports the performance of the denitrification bioreactor during start-up under controlled conditions, as defined by AKVA group’s standard operating procedure (SOP). Solutions for minimizing the risk for H2S formation are given.
Materials and Methods
The RAS bioreactor is part of a smolt production facility in Mid-Norway. The system is characterized by the following key performance indicators: Max feed rate = 3.2t/d, operating temperature = 13-16°C, max salinity = 6ppt. The removal of solids is achieved by mechanical drum filters, the nitrification by fixed bed bioreactors with semi-automated cleaning technology (SAC, bio-media is circulated by water injectors), ozone is used to decrease turbidity, and degassing of ozone and CO2 is achieved in a degassing tank.
The denitrification fixed-bed-bioreactor has the following measures: l × w × h = 12.7 × 2.2 × 4.3m = 120 m3 total volume. The relative volume of the fixed-bed bio-media account for 50 %, providing 60 m3 of total bio-media volume. Multiplied with the specific surface area of the bio-media (800 m2/m3), a total surface area of 48,000 m2 is available. The maximum flow rate is 100 m3/h, accounting for a hydraulic retention time (HRT) of ~45 min and the treatment of ~35 % RAS-volume/d. Intake water for the denitrification bioreactor originates mainly from the fish tanks, and to smaller extend from rinsing water from the drum filters. The fixed-bed bio-media are connected to a semi automated cleaning system, consisting of 6 water injector pumps which facilitate continuous circulation of the bio-media. The start-up of the denitrification bioreactor was on 14th of Jan 2022. To create low oxygen concentrations, the HRT was was increased from 45 min to ~3h. To provide for chemical oxygen demand (COD), methanol was injected via an peristaltic pump to achieve a target ratio of NO3-N:COD = 1:4.5 (stoichiometric optimum for complete denitrification with methanol). Given a background COD of ~100 mg/l, and a NO3-N concentration of ~70 mg/l, 200 mg/l COD was supplemented. Water samples at the inflow and outflow of the reactor were analysed for dissolved oxygen, NH4-N, NO2-N, NO3-N and COD twice daily. The first sampling took place during the first 10 days after start-up (14-20 Jan 2022), at high stocking densities (~180t total biomass) and feed input (~2.6 t/d), so elevated levels of nitrate are to be expected. A second sampling was taken approx. 1-month after start-up (08-11 Feb 2022). This sampling followed a vaccination event, with high water exchange and low nitrate levels to be expected. A 3rd sampling was taken 68 days after start-up (23 Mar 2022). This event followed a cleaning of the reactor 4 days earlier.
Results
On day 1 after start-up, oxygen values in the outflow of the reactor remained well < 1mg/l. The average level of NO3-N entering the reactor was ~67 mg/l. On day 1, about 10% of the NO3-N entering the reactor was removed. The removal increased to 26% relative to the input on day 10 after start-up (Figure 1).
A 2nd sampling was carried out on day 27 after start-up. NO3-N levels entering the reactor were ~32 mg/l and decreased to ~10 mg/l in the outflow, so by ~69% relative to the inflow values. A 3rd sampling was carried out on day 68 after start-up, NO3-N levels entering the reactor were ~65 mg/l and decreased to ~10 mg/l, so by ~85% relative to the inflow values.
We expect that under decreased HRT (45 min/h), nitrate-N levels will be maintained below 75 mg/l, with substantially less water reuse.
Discussion and Conclusions
We found, that when HRT and COD are adjusted according to our recommended values, the denitrification increases rapidly after just a few days. According to project partners (R. Netzer, 2022), denitrifying microorganisms are detectable also inside the nitrification bioreactor. Therefore, facilitating optimum conditions for these bacteria inside the denitrification bioreactor will result in rapid colonization and reactor performance, hence conversion of NO3 to N2 gas. This was obvious in the increase of nitrate removal over the first 10 days of sampling.
The second sampling at low nitrate levels (day 27) revealed that this process is also strong at low levels of nitrate (~30 mg/l NO3-N).
Almost complete denitrification was detected at day 68 post-start (3rd sampling). We therefore expect that at the 4th sampling in May 2022, when stocking densities, feed input and nitrate levels are high, and HRT at normal operational levels (45min/h), the performance of the reactor will allow maintaining nitrate levels below the maximum tolerable level of 75 mg/l NO3-N.
The installation of an ORP-monitoring system (connected to SCADA) allows the precise adjustment of COD to achieve good denitrification, but also to decrease the risk of H2S formation.
References:
OBOODI, Ali; FATHI, Ehsan; NEZAMMAHALLEH, Hassan. Development of a Denitrifying Biofilter with Short Start-up Period. Journal of Biology and Today’s World, 2017, 6. Jg., Nr. 4, S. 75-82.
NETZER, Roman. Personal communication. SINTEF, Trondheim 2022.