World Aquaculture Magazine -December 2021

44 DECEMBER 2021 • WORLD AQUACULTURE • WWW.WA S .ORG salinity; microbial community structure diverges significantly between fresh, brackish and saltwater facilities operating under similar temperature ranges (Ahnen et al. 2019, Bakke et al. 2017, Gao et al. 2020, Hong et al. 2013). Disinfection and sterilization of equipment and culture water are often done through the oxidation of organic matter, minerals and microbes, by implementing a high Eh environment (Table 3). The pH plays a secondary role in disinfection and sterilization. Several water treatment products are either highly alkaline or acidic, exploiting rapid pH changes to perturb microbial homeostasis (Ambauen et al. 2020, Moreno-Andrés et al. 2020). Typically, these pH-dependent disinfectants range widely in their starting pH (Table 3), although it is important to not create a rapid change exceeding (+/- 0.2 units pH/ day) of the culture water to minimize animal stress or disrupt beneficial microbial communities in the system. In a RAS biofilter, the Eh is modulated by aeration to facilitate nitrification/ denitrification and improve the removal of nitrogenous waste (Goncharuk et al. 2010). The pH also plays a lesser but better-documented role as a proxy for nitrification/denitrification efficiency in the biofilter. The pH of biofilters should be maintained between 6.5 and 9.0 (Noga 2014). Although the removal of ammonia is the main metric by which biofilter health is assessed, pH variation within or even beyond this range are explained by the microbial consortium present. Rapid deviations may disrupt the established biofilter microbial consortium, resulting in community replacement by better-adapted species. The nitrification efficiency of the replacement community is not guaranteed, potentially reducing soluble waste removal efficiency (Gieseke et al. 2006). Aquaponic producers typically operate at a pH of 6.0-7.0, but are less affected by decreases in nitrification efficiency due to the significantly expanded biological surface area for nitrifying communities provided by plant roots within the hydroponic system (Anderson et al. 2019, Schwartz et al. 2019). Decreased biofilter efficiency due to an operationally lower pH further stimulates plant growth and likewise satisfies plant NH 4 + -N requirements, needed with NO 3 - -N (Abbasi et al. 2017, Errebhi and Wilcox 1990, Tabatabaei et al. 2006, Hu et al. 2021, Weil et al. 2021, Zhang et al. 2019). Fish too are more tolerate of NH 4 + -N than NH 3 -N (Noga 2014). Shifts in EC as a result of salinity change causes a temporary reduction in nitrification efficiency during freshwater to saltwater acclimatization, although this is corrected by the subsequent establishment of salt-tolerant nitrifying microbial consortia (Hong et al. 2013, Gonzalez-Silva et al. 2016, Ahnen et al. 2019, Navada et al. 2019, Gao et al. 2020). Semi-intensive and intensive aquaculture systems use mechanical filters to remove suspended solids from the water column (van Rijn 2013, Oddsson 2020, Schumann and Brinker 2020). Often, however, these filtration systems may be augmented via the manipulation of one or more physicochemical parameters. Electrocoagulation is a technique based on Eh whereby charged particles (i.e., heavy metals, soluble gases, microorganisms) are attracted to membranes acting as cathodes and anodes. In this way, highly soluble particles poorly removed by mechanical filtration or Eh and pH of recirculating aquaculture systems should be monitored regularly and corrected as needed. Eh and pH can affect the availability of nutrients, especially trace metals, for plants in aquaponic and hydroponic systems. TABLE 3. Sterilization strategies in use commercially and their target Eh environments. S t e r i l i za t i on s t ra t eg y Me chan i sm o f ac t i on Re f e r enc e Ozone Eh increase (600+ mV) Gonçalves and Gagnon 2011 UV 200 – 330,000 mWs/cm 2 Timmons et al. 2018 H 2 O 2 Can act as an oxidizing or reducing agent based on pH. Eh increases (1.5+ V) Gulaboski et al. 2019, McDonnell 2014 Localized electrical fields Low power electrical burst (< 5 V; 10 0 -10 6 Hz) Zhou et al. 2020 Peroxyacetic acid pH drop Ambauen et al. 2020, Moreno-Andrés et al. 2020 Chlorination pH drop, E h increase (700+ mV) Ambauens et al. 2020, Moreno-Andréss et al. 2020