World Aquaculture Magazine - June 2021

WWW.WA S .ORG • WORLD AQUACULTURE • JUNE 2021 49 level. These systems have multiple advantages apart from producing dry biofloc biomass as a potential source of protein in fish or shrimp diets, including recycling of effluents that can reduce the pressure of effluent discharges on coastal water bodies. This alternative feed ingredient offers aquaculture a viable option to replace costly fishmeal and conventional feed ingredients by cost- effective biofloc produced from aquaculture waste materials. The system used for treatment of effluents and its efficiency depends on 1) bioreactor or effluent treatment plant design, 2) seasonal or year-round ammonia discharges and its dynamics in reactors, 3) temperature range, 4) desired aquaculture effluent concentration for ammonia, 5) other effluent and water quality requirements and 6) costs. Biofloc production is most commonly undertaken within the culture system itself in an in-situ mode. An attempt was made, using appropriately designed production systems, to develop biofloc through an ex-situ mode. The predominance of the activated sludge systems for the production of biofloc meal has been consolidated as a cost-efficient and reliable method for biological removal of suspended solids, organic material and macronutrients nitrogen and phosphorus, using raceways and a photobioreactor. Here we present information concerning various aspects of the activated sludge system and develop procedures for optimized design and operation. Aquaculture effluent was treated using raceways and a photobioreactor at the Advanced Research Farm Facility, Madhavaram of Tamil Nadu Fisheries University. Raceways Raceways are used for biofloc production by pumping culture water or ‘used water’ from aquaculture systems. In this design, the aeration intensity is maintained at a level sufficient to avoid sludge settling to ensure the maintenance of uniform sludge suspension. This raceway design is distinguished from other activated sludge variants by the omission of a final settler or other mechanism to retain the activated sludge. Therefore, in the raceway, sludge age is always equivalent to hydraulic retention time. Although the absence of a final settler is cost effective, the cost in terms of effluent quality is high. E cologically sound management techniques are increasingly popular for the sustainable production of aquatic animals to meet rising global food demand. Biofloc- based farming is a sustainable technique where manipulation of the carbon-to-nitrogen ratio in the culture system through external application of organic carbon sources or reducing the protein content of feed stimulates biofloc production. The overall dynamic of biofloc technology results from ecological relationships that represent a trophic micro-network comprised of rotifers, ciliates, heterotrophic bacteria and microalgae (Collazos Lasso and Arisa-Castellaos 2015). The latter two groups tend to be the most abundant within the biofloc community. Research has demonstrated that in-situ biofloc culture enhances growth rate (Wasielesky et al. 2006, Crab et al. 2012) decreases feed conversion ratio and thereby feed costs (Burford et al. 2004) and stimulates digestive enzymes (Xu and Pan 2013) and positive immunological responses (Kim et al. 2014, Kumar et al. 2014). However, in-situ techniques create significant oxygen demands for microbial and algal respiration, in addition to the respiratory oxygen demand of the culture animals (Tacon et al. 2002, Burford et al. 2004). This oxygen demand requires substantial aeration, which increases costs compared to regular aquaculture systems. Further, consumption of biofloc depends on the grazing efficiency of the cultured species. Biofloc can be exploited as a feed ingredient due to a favorable nutritional profile. Moreover, in the present scenario, where fishmeal, a major ingredient in aquaculture feeds, is becoming more expensive as capture fisheries is currently overexploited or stagnated, aquaculture has been urged to investigate alternative sources of protein for aquaculture feeds. Therefore, biofloc—a source of high-quality protein, nutrients, minerals and antioxidants — is an excellent aquaculture feed ingredient as a partial or complete replacement for fishmeal. Dry biofloc biomass can be harvested after treating nutrient and organic-rich farm effluents through manipulation of the C:N ratio. Kuhn et al. (2008) generated biofloc from fish-farm effluent using a sequencing batch reactor with carbon supplementation and a membrane biological reactor without carbon supplementation. The quality and quantity of biofloc can be optimized by changing the effluent used, bioreactor methodology and/or carbon source and Design and Operation of Raceways and Photobioreactors for Ex Situ Biofloc Production S. Felix and M. Menaga ( C O N T I N U E D O N P A G E 5 0 ) FIGURE 1. Two complete biofloc production raceways.