Abstract
The optimization of total nitrogen removal from municipal wastewater was investigated in a laboratory-scale 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 achieved 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 achieve 41% 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, the nitrification rate was 1.9 mgN L-1 h-1 (specific rate 2.4 mgN gTSS-1 h-1), measured with dissolved oxygen near zero, when the oxygen demand was higher than the oxygen produced by photosynthesis. Total nitrogen of 6.3 ± 4.4 mgN/L was measured in the effluent after PSBR optimization.
Highlights
The treatment of real wastewater with microalgal–bacterial consortia has gained increasing attention in the last few years because the synergistic effects of microalgae and heterotrophic bacteria can be exploited for carbon and nitrogen removal through nitrification–denitrification, ensuring a significant energy saving (Su et al ; Zhang et al ; He et al ; Abinandan & Shanthakumar ; Wang et al b; Arcila & Buitrón ; Quijano et al )
Considering the annual energy consumption by the air supply in aerated tanks in the range of 4.7–28 kWh PEÀ1 yÀ1 (Foladori et al ), a medium-size wastewater treatment plant (WWTP) (20,000 population equivalent, PE) may consume [94–560] MWh/y, which corresponds to an annual cost of 11,000–64,000 €/y
The chemical parameters chemical oxygen demand (COD), soluble COD (sCOD), total Kjeldahl nitrogen (TKN), NH4þ-N, NO2À-N, NO3À-N, total phosphorus (TP), PO43À-P and total suspended solids (TSS) were analyzed in the influent and effluent wastewater according to Standard Methods (APHA )
Summary
The treatment of real wastewater with microalgal–bacterial consortia has gained increasing attention in the last few years because the synergistic effects of microalgae and heterotrophic bacteria can be exploited for carbon and nitrogen removal through nitrification–denitrification, ensuring a significant energy saving (Su et al ; Zhang et al ; He et al ; Abinandan & Shanthakumar ; Wang et al b; Arcila & Buitrón ; Quijano et al ). This energy consumption, represents up to half of the energy expenditure of a wastewater treatment plant (WWTP) based on activated sludge. Considering the annual energy consumption by the air supply in aerated tanks in the range of 4.7–28 kWh PEÀ1 yÀ1 (Foladori et al ), a medium-size WWTP (20,000 population equivalent, PE) may consume [94–560] MWh/y, which corresponds to an annual cost of 11,000–64,000 €/y (average EU electricity price for industrial consumers of 0.114 €/kWh; Eurostat ). This paper contributes to the optimization of a PSBR, in terms of enhancement of nitrogen removal, reduction of the time of treatment, and energy saving where possible, improving the global sustainability of the system
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