Denitrification performance and bacterial ecological network of a reactor using biodegradable poly(3-hydroxybutyrate-co-3-hydroxyvalerate) as an electron donor for nitrate removal from aquaculture wastewater

Nitrate accumulation is a common phenomenon in aquaculture that can lead to eutrophication of surrounding water bodies. This study used poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) as a carbon source and substrate and performed a microbial co-occurrence network ecological analysis to elucidate the denitrification processes in two packed-bed reactors with different salinities. The denitrification rate reached maximum values of 0.438 and 0.446 kg m-3 d-1 in reactor I (salinity 0 parts per thousand) and reactor II (salinity 20 parts per thousand), respectively. Although ammonia was formed in both systems based on dissimilation nitrate reduction to ammonia (DNRA), the concentration was very low (2.47 +/- 1.99 and 2.84 +/- 1.79 mg L-1); moreover, the nitrite content was average (1.01 +/- 0.87 and 0.96 +/- 0.86 mg L-1). These results suggested that denitrification dominated in both reactors. PHBV generally presented a sta-ble release of DOC, although a sharp increase was observed in the start-up period of reactor II. 16S rRNA results showed that reactor I had richer microbial diversity than reactor II. Among the top ten taxa, Betaproteobacteria was the dominant class in reactor I while Gammaproteobacteria was the dominant class in reactor II. In the stable period, Thauera and Denitromonas was the most abundant genera in reactor I and reactor II, respectively. In addition, the bac-terial co-occurrence network showed that reactor I had a more complex node and edge network and faster start-up time compared to reactor II; however, reactor II had a more stable nitrogen removal capacity. Higher expression of NorB and NosZ genes in reactor II indicated higher efficient denitrification in seawater system. The SEM and FTIR showed bacterial development and materials surface erosion. These findings verified the denitrification performance and niche differences between freshwater and seawater environments.

Automated summary

This study used poly(3-hydroxybutyrate-co-3-hydroxyvalerate) as a carbon source to investigate denitrification processes in two reactors with different salinities. The denitrification rate reached maximum values in both reactors, indicating efficient denitrification. The microbial diversity and dominant bacterial classes differed between the two reactors. Additionally, the bacterial co-occurrence network and gene expression analysis provided insights into the performance and niche differences between fresh and seawater environments.