Improving the photobioreactor operation efficiency in the technological scheme of wastewater treatment

Abstract

Conventional process schemes of municipal sewage water treatment, advantages, and disadvantages of the methods applied when removing biogenic elements were considered. It was shown that the existing shortcomings cause additional explicit costs and difficulties when disposing of the resulting waste. Low efficiency of the removal processes themselves causing residual concentrations of biogenic elements in the treated sewage water was also shown. A process scheme for treating municipal sewage water was proposed. It includes the use of a photobioreactor of a proposed design for the removal of biogenic elements due to the metabolism of microalgae. It was experimentally shown that the use of Euglena gracilis strain for removal of phosphates in initial concentrations of 4, 7, and 14 mg/dm3 from sewage water is the most efficient way. It makes it possible to reduce these concentrations to residual 0…0.55 mg/dm3 in four days. A 3.75…5.58 times increment of microalgae biomass during this period was also shown. A mathematical model was constructed for calculating the time of staying the sewage water and microalgae mixture in a photobioreactor to achieve the required degree of removal of biogenic elements. Based on the proposed model and experimental studies, the required time of staying in the rector working area was calculated. It was shown that with the use of Euglena gracilis strain but without removal of biogenic elements at previous purification stages (process schemes including only mechanical purification), the time of mixture staying in the working zone was 37.81 hrs. With partial removal of biogenic elements at the stage of biological treatment (0.55 mg/dm3 total nitrogen, 0.91 mg/dm3 ammonium nitrogen, 0.44 mg/dm3 phosphates), this time was reduced to 26.66 hrs. It was found that the use of Euglena gracilis strain instead of Chlorella vulgaris (FC-16) in the removal of phosphates results in a 2-time increase in the process efficiency and a 50 % decrease in time of mixture staying in the working zone. Recommendations for calculating geometric parameters of photobioreactors of the proposed design were given for use in the process schemes