ADDING VALUE TO PIKEPERCH CULTURE WASTEWATER VIA MICROALGAL BACTERIAL FLOCS: FROM CONCEPT TO OUTDOOR POND
Introduction
During the past century, major efforts have been undertaken to remove nutrients from aquaculture wastewater to obtain a satisfactory effluent quality that does not result in eutrophication of the natural recipient ecosystems and/or that enables water recycling (Crab et al., 2007). Nowadays, global warming and depletion of resources such as fossil energy, fresh water and phosphorous, mandates urgent efforts to redesign conventional wastewater treatment systems towards energy efficiency and nutrient recovery. Microalgae could play a key role in this redesign. Being photosynthetic microorganisms, they consume CO2, lower the need for mechanical aeration by providing oxygen, scavenge resources from the wastewater and convert solar energy into biomass. To optimise the pH of microalgal reactors, flue gas can be injected (Van Den Hende et al., 2012). A key factor determining the economic viability of microalgae systems for wastewater treatment is the cost-effective separation of microalgae from the treated wastewater (Udom et al., 2012). In this regard, a novel approach based on bioflocculation was developed. Microalgal bacterial flocs in sequencing batch reactors (MaB-floc SBRs) settle by gravity (Van Den Hende et al., 2011). After this MaB-floc settling during the night phase, biomass-free effluent can be discharged. In this way, no expensive biomass removal techniques are needed for effluent discharge.
On lab-scale, MaB-flocs SBRs showed promising results for the treatment of pikeperch (Sander lucioperca L.) culture wastewater and concomitant microalgal biomass production (Van Den Hende et al., 2014). To assess the technical feasibility of outdoor MaB-flocs SBRs for treatment of pikeperch wastewater in Northwest Europe, MaB-floc SBRs were up-scaled from indoor lab-scale reactors (4L, 40L, 400L) to an outdoor raceway pond (12m3) (Fig. 1). Moreover, the potential of MaB-flocs as pigmentation-enhancing ingredient of diets of Pacific white shrimp (Litopenaeus vannamei Boone 1931) was evaluated.
Material and methods
Wastewater from indoor pikeperch culture (Inagro, Belgium) was treated in indoor lab-scale reactors of 4L, 40L and 400L and in an outdoor raceway pond of 12m3. All reactors were operated as SBR with a hydraulic retention time of 2-8 days. Outdoors, flue gas containing 5% CO2 was injected in the pond when the reactor pH reached a value above 9. Harvesting of MaB-flocs consisted of 2 steps: (1) concentration by settling and (2) dewatering by press filtering at 150-250 µm. The key factors governing the technical potential of MaB-floc SBRs were evaluated: wastewater treatment (nutrient removal, effluent quality, need for flue gas sparging), MaB-floc characteristics (floc settling, chlorophyll content, ash content), biomass productivity and MaB-floc harvesting.
Considering the high ash content of the harvested MaB-flocs, the latter were included for in diets of shrimp at 2-4-6-8%. These diets were continuously fed to juvenile Pacific white shrimp (Litopenaeus vannamei Boone 1931) cultured in a hybrid recirculating aquaculture system (AFT and Crevetec, The Netherlands). Shrimp survival, weight gain, size distribution, food conversion ratio (FCR), proximate composition and fatty acid profile of shrimp tails, and pigmentation of shrimp tails upon cooking were evaluated.
Results and discussion
Scale-up of MaB-flocs SBRs from lab to outdoor pond decreased the nutrient removal efficiencies with a factor 1-3 and the biomass productivities with a factor 1-13 to 33 ton total suspended solids ha-1 pond area year-1. Current discharge norms were met, except for nitrite. Outdoors, flue gas sparging was needed to decrease the effluent pH. Both settling and ash content of MaB-flocs strongly increased during summer in the outdoor pond. At lab-scale, MaB-flocs were dominated by the cyanobacterium Phormidium sp., while outdoors the microalga Ulothrix sp. dominated the MaB-flocs. Bioflocculation enabled successful harvesting by gravity settling and dewatering at 150-250 µm, resulting in high MaB-floc recoveries of 98.8±0.9% and a MaB-floc cake of 42.9±8.7% dry matter.
The shrimp diet modifications did not affect the shrimp survival, weight gain, size distribution and FCR, nor did they affect the proximate composition and fatty acid profile of the raw shrimp muscle. However, increasing the amount of MaB-flocs in shrimp diets significantly increased the pigmentation (redness and yellowness) of cooked shrimp tails.
Conclusion
MaB-floc SBRs seem promising for treatment of pikeperch culture wastewater in Northwest Europe, except for nitrite removal. The produced MaB-flocs can substitute 8% of the diet ingredients of Pacific white shrimp while enhancing its pigmentation Future research should focus on nitrogen removal of MaB-flocs SBRs and screen increased levels of MaB-flocs in shrimp diets.
References
Crab, R., Y. Avnimelech, T. Defoirdt, P. Bossier, and Verstraete, W., 2007. Nitrogen removal techniques in aquaculture for a sustainable production. Aquaculture 270: 1-14.
Udom, I., B.H. Zaribaf, T. Halfhide, B. Gillie, O. Dalrymple, Q. Zhang, and S.J. Ergas, 2012. Harvesting of microalgae grown on wastewater. Bioresource Technology 139: 101-106.
Van Den Hende S., E. Carré, E. Cocaud, V. Beelen, N. Boon, and H. Vervaeren. 2014. Treatment of industrial wastewaters by microalgal bacterial flocs in sequencing batch reactors. Bioresource Technology 161: 245-254.
Van Den Hende S., H. Vervaeren, and N. Boon, 2012. Flue gas compounds and microalgae: (Bio-)chemical interactions leading to biotechnological opportunities. Biotechnology Advances 30: 1404-1424.
Van Den Hende S., H. Vervaeren, H. Savey, G. Maes, and N. Boon, 2011. Microalgal bacterial flocs are improved by a balanced inorganic/organic carbon ratio. Biotechnology and Bioengineering 108: 549-558.
