Microalgal bacterial floc properties are improved by a balanced inorganic/organic carbon ratio.

Microalgal bacterial floc (MaB-floc) reactors have been suggested as a more sustainable secondary wastewater treatment. We investigated whether MaB-flocs could be used as tertiary treatment. Tertiary influent has a high inorganic/organic carbon ratio, depending on the efficiency of the secondary treatment. In this study, the effect of this inorganic/organic carbon ratio on the MaB-flocs performance was determined, using three sequencing batch photobioreactors. The MaB-flocs were fed with synthetic wastewater containing 84, 42, and 0 mg L(-1) C-KHCO(3) supplemented with 0, 42, 84 mg L(-1) C-sucrose, respectively, representing inorganic versus organic carbon. Bicarbonate significantly decreased the autotrophic index of the MaB-flocs and resulted in poorly settling flocs. Moreover, sole bicarbonate addition led to a high pH of 9.5 and significant lower nitrogen removal efficiencies. Sucrose without bicarbonate resulted in good settling MaB-flocs, high nitrogen removal efficiencies and neutral pH levels. Despite the lower chlorophyll a content of the biomass and the lower in situ oxygen concentration, 92-96% of the soluble COD-sucrose was removed. This study shows that the inorganic/organic carbon ratio of the wastewater is of major importance and that organic carbon is requisite to guarantee a good performance of the MaB-flocs for wastewater treatment.

Converting tequila vinasse diluted with tequila process water into microalgae-yeast flocs and dischargeable effluent.

During tequila production from agave, wastewaters are produced, such as dark-colored vinasse. To add value to this vinasse, microalgae-yeast biomass was produced on vinasse diluted with tequila process water (first rinsing water of agave syrup production). In batch experiments, a vinasse concentration of 10 %v/v resulted in the highest biomass productivity, pH and microalgae growth compared to 20 and 30 %v/v. To ease harvesting, microalgae-yeast flocs (MaY-flocs) were developed in a sequencing batch reactor (SBR). A MaY-floc SBR was run with diluted vinasse (10 %v/v) enriched to 76 mg N-TA L-1, resulting in a doubled biomass productivity (49.5 ± 8.3 mg VSS L-1 day-1) of MaY-flocs compared to the best batch reactor performance. Based on response surface experiments, enrichment to 150 mg N-TA L-1 and 5.9 %v/v vinasse are recommended. The MaY-floc SBR system is a promising, novel technology to treat tequila wastewaters while producing settleable MaY-floc biomass of interest to aquaculture.

Economic feasibility of microalgal bacterial floc production for wastewater treatment and biomass valorization: A detailed up-to-date analysis of up-scaled pilot results.

The economic potential of outdoor microalgal bacterial floc (MaB-floc) raceway ponds as wastewater treatment technology and bioresource of biomass for fertilizer, shrimp feed, phycobiliproteins and biogas in Northwest Europe is assessed. This assessment is based on cost data provided by industry experts, on experimental data obtained from pilot-scale outdoor MaB-floc ponds treating aquaculture and food-industry effluents, and from different biomass valorization tests. MaB-floc ponds exhibit a cost-performance of EUR 0.25-0.50m-3 wastewater which is similar to conventional wastewater treatment technologies. The production cost of MaB-flocs in aquaculture and food industry effluent is EUR 5.29 and 8.07kg-1TSS, respectively. Capital costs and pond mixing costs are the major expenses. Commercializing MaB-flocs as aquaculture feed generates substantial revenues, but the largest profit potential lies in production of high-purity phycobiliproteins from MaB-flocs. These results highlight the large economic potential of MaB-floc technology, and justify its further development.

Microalgal bacterial flocs treating paper mill effluent: A sunlight-based approach for removing carbon, nitrogen, phosphorus, and calcium.

Treatment of upflow anaerobic sludge blanket (UASB) effluent from a paper mill in aerated activated sludge reactors involves high aeration costs. Moreover, this calcium-rich effluent leads to problematic scale formation. Therefore, a novel strategy for the aerobic treatment of paper mill UASB effluent in microalgal bacterial floc sequencing batch reactors (MaB-floc SBRs) is proposed, in which oxygen is provided via photosynthesis, and calcium is removed via bio-mineralization. Based on the results of batch experiments in the course of this study, a MaB-floc SBR was operated at an initial neutral pH. This SBR removed 58±21% organic carbon, 27±8% inorganic carbon, 77±5% nitrogen, 73±2% phosphorus, and 27±11% calcium. MaB-flocs contained 10±3% calcium, including biologically-influenced calcite crystals. The removal of calcium and inorganic carbon by MaB-flocs significantly decreased when inhibiting extracellular carbonic anhydrase (CA), an enzyme that catalyses the hydration and dehydration of CO2. This study demonstrates the potential of MaB-floc SBRs for the alternative treatment of calcium-rich paper mill effluent, and highlights the importance of extracellular CA in this treatment process.

Technical potential of microalgal bacterial floc raceway ponds treating food-industry effluents while producing microalgal bacterial biomass: An outdoor pilot-scale study.

To replace costly mechanical aeration by photosynthetical aeration, upflow anaerobic sludge blanket (UASB) effluent of food-industry was treated in an outdoor MaB-floc raceway pond. Photosynthetic aeration was sufficient for nitrification, but the raceway effluent quality was below current discharge limits, despite the high hydraulic retention time (HRT) of 35days. Hereafter, conventional activated sludge (CAS) effluent of food-industry was treated in this pond to recover phosphorus. The two-day HRT results in a more realistic pond area, but the phosphorus removal efficiency was low (20%). High biomass productivities were obtained, i.e. 31.3 and 24.9ton total suspended solids hapond(-1)year(-1) for UASB and CAS effluent, respectively. Bioflocculation enabled successful harvesting of CAS effluent-fed MaB-flocs by settling and filtering at 150-250µm to 22.7% total solids. To conclude, MaB-floc raceway ponds cannot be recommended as the sole treatment for these food-industry effluents, but huge potential lies in added-value biomass production.

Anaerobic digestion of microalgal bacterial flocs from a raceway pond treating aquaculture wastewater: need for a biorefinery.

An outdoor raceway pond with microalgal bacterial flocs (MaB-flocs) is a novel sunlight-based system to treat pikeperch aquaculture wastewater while producing biomass. The harvested MaB-floc biomass (33tonTSha(-1)y(-1)) needs further valorization. Therefore, the biochemical methane yield (BMY) of MaB-floc biomass was determined in batch experiments. The results show significant differences between the BMY of MaB-flocs amongst their harvest dates (128-226NLCH4kg(-1)VS), a low anaerobic digestion conversion efficiency (25.0-36.2%), a moderate chlorophyll a removal (51.5-86.9%) and a low biogas profit (<0.01€m(-3)wastewater). None of the pretreatment methods screened (freezing, thermal, microwave, ultrasonic and chlorination, flue gas sparging, and acid) can be recommended due to a low BMY improvement and/or unfavorable energy balance. Therefore, anaerobic digestion of this MaB-floc biomass should only be granted a supporting role within a biorefinery concept.

Environmental sustainability assessment of a microalgae raceway pond treating aquaculture wastewater: From up-scaling to system integration.

The environmental sustainability of aquaculture wastewater treatment by microalgal bacterial flocs (MaB-flocs) in an outdoor raceway pond was analyzed using life cycle assessment. Pikeperch aquaculture wastewater treated at pilot scale (Belgium; 28m(2)) and industrial scale (hypothetical up-scaling; 41 ponds of 245m(2)) were compared. The integration of the MaB-floc raceway pond in a broader aquaculture waste treatment system was studied, comparing the valorisation of MaB-flocs as shrimp feed and as biogas. Up-scaling improves the resource footprint of the plant (848MJex,CEENEkg(-1) MaB-floc TSS at pilot scale and 277MJex,CEENEkg(-1) MaB-floc TSS at industrial scale) as well as its carbon footprint and eutrophication potential. At industrial scale, the valorisation of MaB-flocs as shrimp feed is overall more sustainable than as biogas but improvements should be made to reduce the energy use of the MaB-floc raceway pond, especially by improving the energy-efficiency of the pond stirring system.

Sucrose preculture to simplify cryopreservation of banana meristem cultures.

A simple cryopreservation method is described for proliferating meristem cultures of banana (Musa spp.). It relies on a 2-week preculture on media containing 0.4 M sucrose followed by rapid cooling in liquid nitrogen. Different preculture media were screened for efficient protection of banana meristems against cryopreservation. Sucrose can be replaced by both fructose and glucose without significantly affecting post-thaw survival rates. A high BA concentration (0.1 mM) in the preculture medium results in less material available for cryopreservation, but does not affect cryoprotection. Culture in liquid media significantly improved post-thaw regeneration. The optimized cryopreservation protocol was applied on 36 banana accessions belonging to 8 different genomic groups. It is shown that post-thaw regeneration frequencies (ranging between 0 and 66 percent) are highly dependent on the genomic constitution of the banana cultivar.

Up-scaling aquaculture wastewater treatment by microalgal bacterial flocs: from lab reactors to an outdoor raceway pond.

Sequencing batch reactors with microalgal bacterial flocs (MaB-floc SBRs) are a novel approach for photosynthetic aerated wastewater treatment based on bioflocculation. To assess their technical potential for aquaculture wastewater treatment in Northwest Europe, MaB-floc SBRs were up-scaled from indoor photobioreactors of 4 L over 40 and 400 L to a 12 m(3) outdoor raceway pond. Scale-up decreased the nutrient removal efficiencies with a factor 1-3 and the volumetric biomass productivities with a factor 10-13. Effluents met current discharge norms, except for nitrite and nitrate. Flue gas sparging was needed to decrease the effluent pH. Outdoor MaB-flocs showed enhanced settling properties and an increased ash and chlorophyll a content. Bioflocculation enabled successful harvesting by gravity settling and dewatering by filtering at 150-250 µm. Optimisation of nitrogen removal and biomass valorisation are future challenges towards industrial implementation of MaB-floc SBRs for aquaculture wastewater treatment.

Treatment of industrial wastewaters by microalgal bacterial flocs in sequencing batch reactors.

Microalgal bacterial flocs in sequencing batch reactors (MaB-floc SBRs) represent a novel approach to wastewater treatment. In this approach, mechanical aeration is replaced by photosynthetic aeration and MaB-floc settling separates the treated wastewater from the produced biomass. However, its technical potential for industrial wastewaters needs to be shown. Therefore, wastewaters of aquaculture, manure treatment, food-processing and chemical industry were treated in MaB-floc SBRs. This treatment resulted in significantly different nutrient removal rates and effluent qualities among wastewaters. A high MaB-floc production was obtained for all wastewaters, ranging from 0.14 to 0.26g total suspended solids Lreactor(-1)day(-1). A major advantage of MaB-flocs is the harvesting via a filter press with a large pore size of 200µm, resulting in MaB-floc recoveries of 79-99% and cakes containing 12-21% dry matter. These results may contribute to evolving MaB-floc SBRs as a valuable remediation strategy, especially for aquaculture and food-processing wastewaters.

Flue gas compounds and microalgae: (bio-)chemical interactions leading to biotechnological opportunities.

Flue gases are a resource yet to be fully utilised in microalgal biotechnology, not only to moderate the anthropogenic effects on our climate, but also to steer microalgal resource management towards innovative applications of microalgal biomass compounds. These gases, both untreated and treated into current discharge standards, contain CO2, N2, H2O, O2, NOx, SOx, CxHy, CO, particulate matter, halogen acids and heavy metals. To better steer and engineer flue gas-fed microalgal cultures, all these compounds need to be considered. Therefore, here, we review (i) the chemical composition and treatment technologies of flue gas, (ii) the uptake pathways and removal of the different compounds in microalgae reactors, and (iii) the tolerance and effects on microalgae of all flue gas compounds. By emphasising the interactions between microalgae and flue gas compounds, we envisage new pathways for microalgal biomass valorisation such as enzyme production for environmental technology, novel biogas production and biosequestration of minerals. Furthermore, we highlight fundamental and applied research niches that merit further investigation.

Bioflocculation of microalgae and bacteria combined with flue gas to improve sewage treatment.

Although microalgae are promising for a cradle-to-cradle design approach of sewage treatment, their application is hampered by high harvesting costs and low C:N ratios of sewage. Therefore, the potential of microalgal bacterial flocs (MaB-flocs) was investigated for the secondary treatment of sewage supplemented with different flue gas flow rates (FGFRs) from a coal power plant. Effluent (N, P, turbidity and pH) and off gas discharge levels (NO(x), SO(x)) met the European discharge limits with a hydraulic retention time of only 0.67 days and an FGFR of 0.6Lh(-1) (0.0025 vvm). This FGFR provided sufficient carbon and resulted in removal efficiencies of 48 ± 7% CO(2), 87 ± 5% NO(x) and 99 ± 1% SO(2). MaB-flocs settled fast reaching up to a density of 19 g VSSL(-1). High biomass productivities (0.18 gL(-1)day(-1)) were obtained under a low light intensity. This successful reactor performance indicates the large potential for the industrial application of MaB-flocs for flue gas sparged sewage treatment.