Seawater aquaculture needs to control the impacts of ammonia, nitrate and nitrite on the cultivated animals because high-level inorganic nitrogen combined with pathogenic microorganisms can significantly reduce shrimp seafood harvest. Non-ionic ammonia in seawater can produce toxic effects on the hepatopancreas of shrimps, and significantly reduce the survival rate and growth performance of Penaeus prawns (Han et al., 2017; Kim et al., 2005). When nitrite, an intermediate product from the nitrification of ammonia nitrogen (Lin et al., 1996), exceeds 1 mg/L in rearing seawater, the growth rate of shrimp decreases significantly (Siikavuopio et al., 2006). In the later stage of intensive shrimp aquaculture, nitrite concentrations can be as high as 20 mg/L (Han et al., 2017), which can lead to mass mortality of shrimp in severe cases. This happens because the high concentrations of nitrite can greatly weaken the immunities of the shrimp over time, making them more susceptible to the infection by pathogenic microorganisms, such as Vibrio.
Inorganic nitrogen species in rearing seawater usually come from the breakdown of protein-rich feeds. Nitrogen in feeds partitioned into animal body and wastes such as leftover feeds, excreta, aquatic animal remains and other nitrogenous matter which will be converted into ammonia nitrogen by bacteria (Rijn, 1999). It was estimated that 40–90% of the total nitrogenous excreta waste is converted to inorganic nitrogen in the form of ammonia in the case of biofilter mariculture system (Tovar et al., 2000). Summerfelt (2006) reported 60–70% of the nitrogen in feed used in coastal aquaculture is discharged into nearby water bodies. Studies have also shown that ammonia and nitrite at relatively high concentrations in rearing water can act synergistically, resulting in greater toxicity and higher mortality for the shrimp (Dominic et al., 2010). To avoid the accumulation of ammonia and nitrite to toxic levels, aquaculture systems is inevitably needed to dilute with fresh seawater and accordingly the same volume of rearing water will be drained into adjacent bays. This is the reason why most of the world’s marine aquacultures are always located at the coast. However, more and more countries are legislatively banning untreated discharges of aquaculture wastewater. If goes unchecked, the negative impacts caused by the accumulation of toxic inorganic nitrogen species and disease-causing pathogens will present a major challenge to achieve a sustainable aquaculture (Ahmad et al., 2022).
At present, a number of strategies have been deployed to remove the nitrogenous wastes from the rearing water. For example, biological filtration is widely used to remove ammonia from the rearing water. However, this process still results in pollutant accumulation in the aquaculture facilities as it basically transforms ammonia into nitrite and nitrate through the activity of the autotrophic bacteria that live in the water (Davidson et al., 2009; Gómez et al., 2019). Another method that also have great potential for the removal of inorganic nitrogen in an aquaculture system is electrochemical approach. In addition, electrochemical approach also be used to disinfect the water (Zhang et al., 2018). It is generally acknowledged that indirect electrooxidation is cost-effective for disinfection and effective removal of inorganic nitrogen species (oxidation of ammonia, nitrite and nitrate into nitrogen gas) from the seawater.
When seawater is subjected to electrochemical processes, oxidants are produced from the inherent chlorine ions in the seawater. Seawater is a natural electrolyte because of its abundant ions. Seawater with a salinity of at least 30‰ has a conductivity as high as 30–50 mS/cm (Hsu et al., 2015). The chloride ions in the seawater serves as a source of oxidative radicals during electrolysis. The electrooxidation of seawater pollutants comprises two processes, direct oxidation and indirect oxidation. The pollutants (e.g., ammonia, nitrite and nitrate) are adsorbed onto the anode surface and then degraded by anodic direct oxidation. During indirect electrochemical oxidation, chloride ions are oxidized to chlorine (Eq. 1) and the excessive chlorine in the seawater is then converted to hypochlorous acid (HClO) (Eq. 2) at the anode surface. HClO can convert ammonia to nitrogen gas and nitrate (Zhang et al., 2018), with nitrogen gas being the major product (Qing et al., 2021) (Eq. 3,4), while Cl− is released into the electrolyte (Eq. 5).
2Cl−→ Cl2 + 2e− (1)
Cl2(aq) + H2O → HOC1 + C1− + H+ (2)
2NH4+ + 3HOCl → N2 + 3H2O + 5H+ + 3Cl− (3)
4HOCl + NH4+ → NO3− + H2O + 6H+ + 4C1− (4)
NO2− + HOCl→ NO3− + Cl− (5)
Many studies have reported the parameters, including current density, surface area and spacing distance between anode and cathode, of electrochemical reactors for the purification of aquaculture seawater. For example, boron-doped diamond anode with a surface area of 70 cm2 and spacing of 0.8 cm from the graphite cathode can achieve a 90% removal rate for the inorganic nitrogen in the wastewater of the fish cultivating tank (Diaz et al., 2011). The current densities ranging from 5 mA/cm2 to 60 mA/cm2 all appear to be effective at removing the ammonia from the seawater (Diaz et al., 2011; Ruan et al., 2016). However most of the electrochemical experiments were conducted in laboratories with small reactors of the size of 2 L or less and magnetic stirrers are commonly used. When scaling up to pilot scales and commercial production, the real aquaculture water as the electrolyte will not be homogeneous because the chemical composition in bulk solution is different from the vicinities of the electrodes as the electrolysis proceeds. The energy cost of the electrochemical reactor will disproportionately increase when applying at real aquaculture farms.
Efficiencies for electrooxidation technology to remove inorganic nitrogen and sterilize pathogenic bacteria in rearing seawater have been well validated (Cano et al., 2011; Lopez-Galvez et al., 2012; Jeong et al., 2009; Särkkä et al., 2015). However, energy cost and residual chlorine as a byproduct of the electrolytic process are still two major problems unsolved in extensive application of the electrochemical approach in seawater aquaculture. In this study, a 50 L reactor, shrimp tanks of 500 L, smart switches, and a cellphone APP were integrated and used in an electrochemical recirculating aquaculture system (ERAS). The system aimed to (i) test the effects of the precise timing of the ERAS on the removal of inorganic nitrogen from the rearing water and its disinfection by electrochemical process; and (ii) validate the parameters of the ERAS for residual chlorine control and energy conservaton. It is expected to provide evidence that electrolysis operation is capable to realize the multiple goals of stable aquaculture seawater quality, minimized residual chlorine threats and reasonable energy consumption.