Optimising an outdoor membrane photobioreactor for tertiary sewage treatment

Abstract

The operation of an outdoor membrane photobioreactor plant which treated the effluent of an anaerobic membrane bioreactor was optimised. Biomass retention times of 4.5, 6, and 9 days were tested. At a biomass retention time of 4.5 days, maximum nitrogen recovery rate:light irradiance ratios, photosynthetic efficiencies and carbon biofixations of 51.7 ± 14.3 mg Nꞏmol -1 , 4.4 ± 1.6 % and 0.50 ± 0.05 kg CO2ꞏm 3 influent, respectively, were attained. Minimum membrane fouling rates were achieved when operating at the shortest biomass retention time because of the lower solid concentration and the negligible amount of cyanobacteria and protozoa.Hydraulic retention times of 3.5, 2, and 1.5 days were tested at the optimum biomass retention times of 4.5 days under non-nutrient limited conditions, showing no significant differences in the nutrient recovery rates, photosynthetic efficiencies and membrane fouling rates. However, nitrogen recovery rate:light irradiance ratios and photosynthetic efficiency significantly decreased when hydraulic retention time was 2 further shortened to 1 day, probably due to a rise in the substrate turbidity which reduced the light availability in the culture. Optimal carbon biofixations and theoretical energy recoveries from the biomass were obtained at hydraulic retention time of 3.5 days, which accounted for 0.55 ± 0.05 kg CO2ꞏm -3 influent and 0.443 ± 0.103 kWhꞏm -3 influent, respectively.

Continuous 3-year outdoor operation of a flat-panel membrane photobioreactor to treat effluent from an anaerobic membrane bioreactor

“…the sudden proliferation of algae in natural waters, which reduces water quality, increases health risks and impairs wildlife (Lau et al, 2019). Microalgae cultivation has emerged as the ideal option to avoid this problem, as it can recover nutrients from AnMBR effluents (González-Camejo et al, 2019a). It also produces valuable microalgae biomass that can be used to obtain biofuels or fertilisers, amongst other applications (Seco et al, 2018;Xu et al, 2019).…”

Preliminary data set to assess the performance of an outdoor membrane photobioreactor

“…The culture for the experiments was composed of 100 mL of microalgae taken from the MPBR plant during the continuous operation of Experiment HRT1, and 100 mL of AnMBR effluent, i.e. microalgae substrate [1]. The initial microalgae biomass concentration was measured by means of optical density of 680 nm (OD680) by a MERC Spectroquant Pharo 300 spectrophotometer, obtaining values of around 1.74–1.80, so that the shadow effect due to microalgae was not considered.…”

“…Seven respirometric tests were done at ammonium concentrations of 7, 10, 12, 14, 17, 24 and 27 mg N·L −1 . Microalgae were obtained from a sample from the MPBR plant during the continuous operation of Experiment BRT4.5 [1]. This sample was diluted with tap water in order to reduce the ammonium concentration up to 7 mg N·L −1 .…”

“…3b).

Fig. 3Evolution of optical density at 680 nm (OD680) during: a) Assay a (batch stage of Experiment BRT4.5 [1]) ( Scenedesmus -dominated) and b) Assay b (batch stage of Experiment HRT3.5 [1]) ( Chlorella -dominated).

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“…Scale bar = 20 μm. BRT Experiments in González-Camejo et al [1] a) Experiment BRT4.5: Scenedesmus and scarce density of Chlorella; b) Experiment BRT6, bright-field image showing Scenedesmus, Chlorella and scarce cyanobacteria and diatoms; c) Experiment BRT9, a mixture of Scenedesmus and Chlorella and high concentration of cyanobacteria.…”