Growth and Quality of Plecoglossus Altivelis (Ayu Fish) Cultured from a Land-based Aquaculture Fed by Secondary Treated Municipal Wastewater in a Tokyo Wastewater Treatment Plant
Introduction
High proportion of treated wastewater discharged to river water in Tokyo makes it become an important role in creating a good water environment (>90% in Kanda River, 50-60% in Tama River downstream).
Resources (water, heat, etc.) from treated municipal wastewater (TWW) have been utilizing for the benefit of local communities (agriculture, renewable energy, etc.). However, effects of TWW on aquatic organisms (such as bioaccumulation of heavy metals (HM) and untreated volatile organic compounds) are not well-known.
Objective
The study aims to evaluate effects of TWW on growth and quality of Ayu fish (Plecoglossusaltivelis) by conducting an aquacultural experiment using a flow-through land-based system with secondary TWW disinfected by ultraviolet.
Experimental description
Fig. 1 Ayu growing system
Table 1 Experimental batch information
Table 2 Water management in Ayu growing system
Result and discussion
Effects of TWW on fish growth
Winter Ayu growth (12.6 ±1.3 cm, 28.0 ±10.1 g, 2nd batch) was significantly higher than that in summer experiment (11.1 ±0.9 cm, 24.0 ±5.3 g, 2nd batch) after 3-month rearing (p<0.05) (Fig. 2).
The number of dead fish in summer batch had a significantly positive correlation with NH4-N and unionized ammonia (NH3-N)* concentrations (Fig. 3).
Fig. 2-1 Ayu growth in 1st batchFig. 2-2 Ayu growth in 2nd batchFig. 3-1 Relationship between number of died Ayu and NH4/NH3 concentrations in TWW (1st batch)Fig. 3-2 Relationship between number of died Ayu and NH4/NH3 concentrations in TWW (2nd batch)
The off-flavor volatile organic compounds in winter experiment were higher than those in summer experiment (Table 3).
These off-flavor volatile organic compounds were undetectable in the fish due to detection limit of the analysis.
On the other hand, cucumber-like aromas concentrations were ranged 3.9 – 37.1 ng/g in the Ayu flesh (Table 4).
Table 3 Earthy/musty odor concentrations in culture water
Table 4 Odorant concentrations in Ayu grown with TWW
Nutritional composition of Ayu
Protein, vitamins (B1, B2, C) and fatty acid contents in TWW Ayu were comparable with those in wild ayu and commonly cultured ayu (source: https://fooddb.mext.go.jp/ ).
There were higher bioaccumulation of Na in TWW Ayu compared with those in the reference Ayu.
Table 5 Nutritional compositions of Ayu grown with TWW
Bioaccumulation of heavy metals in Ayu
No harmful bioaccumulation of Cr, As, Pb, Cd and Hg in Ayu grown in this study.
Table 6 Heavy metal(loid) concentrations in Ayu grown with TWW
a: 1st value is after 3-month culture、b: 2nd value is after 4-month culture [1] FAO. Compilation of legal limits for hazardous substances in fish and fishery products (CLL) in Japan. [2] Official Journal of the European Union. COMMISSION REGULATION (EC) No 1881/2006. [3,4,5,6] FAO. CLL in Germany, India, Netherlands, New Zealand, respectively.
Conclusion
We succeeded in raising Ayu fish with secondary TWW in a wastewater treatment plant.
Ayu growth and aroma contents were higher in the winter experiment when there was less effect of water temperature and NH4.
Nutritional contents in TWW Ayu were comparable with those in wild and cultured Ayu.
No harmful HM bioaccumulation was found.
Off-flavor odors were found in the water but these compounds were undetectable in the fish.
Wastewater from the Ayu growing tank is treated at the wastewater treatment plant. This success opens up the prospect to develop a novel recirculating fish farming model (new land-based aquaculture model) at wastewater treatment plants to utilize the treated wastewater as well as the advanced wastewater treatment technologies.
東京大学 下水道システムイノベーション研究室
