Recycling nutrients from soy sauce wastewater to culture value-added Spirulina maxima

Soy sauce is one of the most common condiments in East Asian cuisines. Its manufacturing process generates a large amount of organic-rich soy sauce wastewater, which is difficult to treat through traditional processes. In this study, Spirulina NCU-Sm was used to treat soy sauce wastewater and simultaneously produce value-added biomass. The results showed that NCU-Sm grew well in 100% (undiluted) raw soy sauce wastewater (RSW). The maximum biomass yield of NCU-Sm reached 1.984 g/L, and the removal efficiencies of NH₄⁺-N, TN, COD, chromaticity, and salinity in RSW reached 93.86, 81.76, 84.08, 40.93, and 63.08%, respectively. High COD removal efficiency in 100% RSW proved NCU-Sm grew in mixotrophic mode. pH and light spectrum, two major factors affecting growth of soy sauce wastewater-grown Spirulina NCU-Sm, were further studied. When the initial pH of RSW was adjusted to 8, NCU-Sm biomass yield was increased by 11.11%, and the contents of protein, chlorophyll-a, and carotenoid were increased by 8.69, 37.14, 40.40%, and reached 65.57%, 7.57 and 2.78 mg/g, respectively. The removal efficiencies of NH₄⁺-N, TN, and chromaticity in RSW with pH 8 were improved to 99.34, 88.29, 43.18%, respectively. Red light enhanced the photosynthetic oxygen release efficiency of NCU-Sm and increased the carotenoid content of NCU-Sm by 117.95%, reaching 5.95 mg/g. Blue light was adverse to the growth and reduced the protein content of NCU-Sm but significantly increased its chlorophyll-a content to up to 10.99 mg/g. These results suggest that cultivation of Spirulina in soy sauce wastewater serves dual roles in soy sauce wastewater treatment, as well as value-added algal biomass (such as high-quality protein and natural pigments raw materials) production.