Removal efficiency of nitrogen salts by Chaetomorpha maxima in aquacultural wastewater

Abstract: The high efficiency in removing nitrogen salt is the key to insure the operation of recirculating aquacultural systems. The current study focused on the efficiency and the characteristic of Chaetomorpha maxima clones (hereinafter referred to as Chaetomorpha maxima) to remove the ammonia, the nitrite and the nitrate in seawater. The absorption efficiency of Chaetomorpha maxima to nutrients is mainly affected by the enzyme activity of algae and the concentration of nutrients in water. The absorption of NO3--N and NO2--N by Chaetomorpha maxima is more limited to its enzyme activity. The absorption of NH4+-N depends on the concentration of NH4+-N in water. Within a certain concentration (NH4+-N: 0-15 mg/L, NO2--N: 0-3 mg/L, NO3--N: 0-15 mg/L), the absorption rate of Chaetomorpha maxima on nitrogen salt suppressed with the reduction of nitrogen concentration in the water. The absorption rate of ammonia nitrogen by algae changed greatly. The final absorption rate of the 5, 10 mg/L group was close to 0, and that of the 15 mg/L group was 20×103 mg/(g·h). While the absorption rate of nitrite and nitrate is relatively stable. The final absorption rate of three groups of NO3--N is 20-30 mg/(g·h), and that of NO2--N was 2-4 mg/L. When the concentrations of three nitrogen salts were same (3 mg/L), Chaetomorpha maxima first absorbed NH4+-N, and the absorption rate on NH4+-N was notably higher than on NO2--N and NO3--N. There was no significant difference between NO2--N and NO3--N. However, when the NO2--N content was too low, the algae will preferentially absorb NO3--N. When ratio (g/L) of the fresh weight of Chaetomorpha maxima to water volume was 10∶1, the removal rates on NH4+-N, NO2--N and NO3--N of Chaetomorpha maxima in the artificial aquacultural wastewater (NH4+-N: 3.0±0.5 mg/L, NO2--N: 0.1±0.05 mg/L, NO3--N: 3.0±0.5 mg/L, PO43--P: 0.2±0.5 mg/L）were 94.3%, 100% and 82.2% in 15 hours. Although the removal rate of NH4+-N by Chaetomorpha maxima was high, it could not replace microorganism as the main unit of NH4+-N removal completely. In order to obtain sufficient ammonia NH4+-N as electron acceptor for microbial, the algae unit was suggested to be located behind the microbial unit. The practical application of algae treatment unit should consider comprehensively both the algae treatment efficiency and water renewal rate. In the actual production, the ratio between the fresh algae quality and the water volume can be adjusted according to the concentration of NO3--N in the water, so as to improve the removal rate of nitrate. The mass concentrations of NO3--N, NH4+-N, NO2--N in recirculating aquacultural systems were generally at 0-10, 0-5, 0-2 mg/L. Within the range of nitrogen salt concentration in circulating water system, three kinds of nitrogen salts will not cause adverse effects on algae. This study could provide data for the application of Chaetomorpha maxima clones in the treatment of aquacultural wastewater and promoted the utilization of aquaculture wastewater as a resource.