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

Abstract

A membrane photobioreactor (MPBR) plant was operated continuously for 3 years to evaluate the separate effects of different factors, including: biomass and hydraulic retention times (BRT, HRT), light path (Lp), nitrification rate (NOxR), nutrient loading rates (NLR, PLR) and others. The overall effect of all these parameters, which influence MPBR performance had not previously been assessed. The multivariate projection approach chosen for this study provided a good description of the collected data and facilitated their visualisation and interpretation. Forty variables used to control and assess MPBR performance were evaluated during three years of continuous outdoor operation by means of principal component analysis (PCA) and partial least squares (PLS) analysis. The PCA identified the photobioreactor light path as the factor with the largest influence on data variability. Other important factors were: nitrogen and phosphorus recovery rates (NRR, PRR), biomass productivity (BP), optical density of 680 nm (OD680), ammonium and phosphorus effluent concentration (NH 4 , P), HRT, BRT, air flow rate (F air) and nitrogen and phosphorus loading rates (NLR and PLR). The MPBR performance could be adequately estimated by a PLS model based on all the recorded variables, but this estimation 2 worsened appreciably when only the controlled variables (Lp, F air , HRT and BRT) were used as predictors, which underlines the importance of the non-controlled variables on MPBR performance. The microalgae cultivation process could thus only be partially controlled by the design and operating variables. A high nitrification rate was found to be inadvisable, since it showed an inverse correlation with NRR. In this respect, temperature and microalgae biomass concentration appeared to be the main factors to mitigate nitrifying bacteria activity.

H2 production by photofermentation in an innovative plate‐type photobioreactor with meandering channels

“…It has been suggested that the thickness of FPPBR should be limited to 5 cm to allow efficient light penetration (Adessi & De Philippis, 2014; Basak et al, 2014; Marsullo et al, 2015). However, a 10 cm‐thick, 10 L FPPBR was used for laboratory growth of PNSB (Lu et al, 2020) and FPPBR of 10 cm thickness (230 L volume) and 25 cm thickness (550 L volume) were used for long‐term outdoor operation (Gonzalez‐Camejo et al, 2020). On the other hand, a thin‐film FPPBR, with a cross‐section of 61.2 m 2 and a thickness of 3.26 cm, allowed scale‐up to 2000 L for mass cultivation of microalgae (Yan et al, 2016).…”

Production of microalgal external organic matter in a Chlorella-dominated culture: influence of temperature and stress factors

“…However, after day 10, the ambient conditions changed (temperature increased round 5ºC and solar PAR suffered a significant reduction) and probably favoured nitrifying bacteria growth. 16 In addition, the culture was expected to be under ammonium-limited conditions, since NH 4 -N concentration was under 10 mg N•L -1 . 55 This situation made the nitrification rate (NOxR) (which measures the nitrate and nitrite produced through nitrification and is used as an indicator of nitrifying bacteria activity 16,56 increase during Period A to a maximum of 9.3 mg N•L -1 •d -1 (Fig.…”

“…In fact, light and competition with nitrifying bacteria have been reported to be key factors in microalgae cultivation systems. 63,16,38,64 Hence, the higher normalised EOM in the culture might not have been the main factor in the lower microalgae cultivation performance observed by the end of both Periods A and B. It will thus be necessary to monitor the system for longer operating periods and to relate all the possible factors which influence nutrient recovery and biomass productivity to properly assess the weight of each individual factor on MPBR performance.…”

“…In this respect, microalgae have appeared as a suitable option for wastewater remediation7-9 as they are able to reduce the nutrient content of these AnMBR effluents.10,11 In addition, microalgae biomass can serve as a renewable source of biofuels, biofertilisers and other valuable products.12-15 From all the microalgae reported in the literature, the green microalgae Chlorella is one of the genus that have shown higher adaptability to wastewater. 16,17,7 To cultivate microalgae under outdoor conditions, membrane photobioreactors (MPBRs), which consists of the combination of closed PBRs and membrane filtration,18 have appeared as promising technology.10 PBRs are designed to attain high photosynthetic efficiencies, biomass productivities and nutrient removal rates,19 while membrane filtration enables to operate the system at lower hydraulic retention time (HRT), hence reducing the surface area needed to cultivate microalgae.20,11 Filtration entails membrane fouling due to the accumulation of microalgae biomass on the membrane (cake-layer) and the partial block of the internal pores, 21-23 which reduces the filtration efficiency and increases the energy consumption of the process.24,25 It must be noted that membrane fouling can be more severe due to the release of microalgal external organic matter (EOM) into the medium since it can intensify the cake layer formation or the blockage to the membrane pores.21,26-28 To remove reversible fouling, backflushing and air spargin are usually employed. 29 However, the higher attachment of foulants caused by EOM decreases membrane filtration efficiency due to either too frequent back-flushing stages or unsustainable values of specific air demand (SAD) of the membrane.30 Moreover, irreversible fouling can only be removed by chemical cleaning,31 which is non-desirable since excessive use of reagents deteriorates the membrane.…”

“…From all possible factors, temperature variations can be of great interest in outdoor large-scale microalgae cultivation applications due to the variable conditions microalgae are exposed to.38,39 In addition, the activity of nitrifying bacteria in a microalgae culture has been reported to affect microalgae performance. 16 Nevertheless,the influence of microalgae stress due to nitrification on EOM production has not been evaluated previously. Apart from affecting membrane filtration, EOM increases the organic matter concentration of wastewater,40 which can hinder microalgae activity by favouring the growth of microalgae-competing organisms such as heterotrophic bacteria and grazers.41,23 Bacteria can also produce compounds harmful to microalgae such as toxins,32 while grazers devour the microalgae cells,42 meaning that EOM production can affect the robustness of the microalgae culture.…”