Enhanced nitrogen removal and energy saving in a microalgal-bacterial consortium treating real municipal wastewater

The optimization of total nitrogen removal from municipal wastewater was investigated in a photo-sequencing batch reactor (PSBR) operated with a mixed microalgal-bacterial consortium spontaneously acclimatized to real wastewater. No external aeration was provided in the PSBR to reduce energy consumption: oxygen was only supplied by the microalgal photosynthesis. The enhancement of total nitrogen removal was obtained through: (1) feeding of wastewater in the dark phase to provide readily biodegradable COD when oxygen was not produced, promoting denitrification; (2) intermittent use of the mixer to favor simultaneous nitrification-denitrification inside the dense flocs and to obtain 41% of energy saving with respect to continuous mixing. Efficient COD removal (86±2%) was observed, obtaining average effluent concentrations of 37 mg/L and 22 mg/L of total COD and soluble COD, respectively. TKN removal was 97±3%, with an average effluent concentration of 0.5±0.7 mg NH4 +-N/L. Assimilation of nitrogen by heterotrophic bacteria accounted only for 20% of TKN removal, whilst the major part of TKN was nitrified. In particular, nitrification rate was 1.9 mgN L-1 h-1 (specific rate 2.4 mgN gTSS-1 h-1), measured at dissolved oxygen near zero, when the oxygen demand surpassed the oxygen produced by photosynthesis. Total nitrogen of 6.3±4.4 mgN/L was obtained in the effluent after PSBR optimization.