Effect of pistachio shell as a carbon source to regulate C/N on simultaneous removal of nitrogen and phosphorus from wastewater.

Acid-pretreated pistachio shells were used as carbon sources to investigate the effects of carbon source dosage on simultaneous nitrogen and phosphorus removal under different carbon/nitrogen (C/N) ratios (7, 9, and 11). Results showed that C/N was positively correlated with mixed liquor suspended solids (MLSS) (R = 0.998, p < 0.01) and f value (R = 0.975, p < 0.05). Moreover, it was negatively correlated with the sludge volume index (SVI) (R =  - 0.959, p < 0.05). C/N was also significantly negatively related to chemical oxygen demand removal rate (R =  - 0.986, p < 0.05) and positively related to ammonia nitrogen (NH-N), total nitrogen (TN), and total phosphorus (TP) removal rate (p < 0.05), the correlation coefficients were 0.992, 0.990 and 0.994, respectively. In the reactor with C/N of 11, the MLSS concentration and f value were the highest, the SVI was the lowest, and the removal efficiencies of NH-N (85.49 % ± 1.96 %), TN (84.19 % ± 1.42 %) and TP (94.10 % ± 1.67 %) were the highest. Furthermore, the relative abundance of denitrifying bacteria was the highest in the reactor. The abundance of nitrifying bacteria and phosphorus-removal bacteria was also relatively high.