Valorising nutrient-rich digestate: Dilution, settlement and membrane filtration processing for optimisation as a waste-based media for microalgal cultivation.

Digestate produced from the anaerobic digestion of food and farm waste is primarily returned to land as a biofertiliser for crops, with its potential to generate value through alternative processing methods at present under explored. In this work, valorisation of a digestate resulting from the treatment of kitchen and food waste was investigated, using dilution, settlement and membrane processing technology. Processed digestate was subsequently tested as a nutrient source for the cultivation of Chlorella vulgaris, up to pilot-scale (800L). Dilution of digestate down to 2.5% increased settlement rate and induced release of valuable compounds for fertiliser usage such as nitrogen and phosphorus. Settlement, as a partial processing of digestate offered a physical separation of liquid and solid fractions at a low cost. Membrane filtration demonstrated efficient segregation of nutrients, with micro-filtration recovering 92.38% of phosphorus and the combination of micro-filtration, ultra-filtration, and nano-filtration recovering a total of 94.35% of nitrogen from digestate. Nano-filtered and micro-filtered digestates at low concentrations were suitable substrates to support growth of Chlorella vulgaris. At pilot-scale, the microalgae grew successfully for 28 days with a maximum growth rate of 0.62 day-1 and dry weight of 0.86  g⋅L-1. Decline in culture growth beyond 28 days was presumably linked to ammonium and heavy metal accumulation in the cultivation medium. Processed digestate provided a suitable nutrient source for successful microalgal cultivation at pilot-scale, evidencing potential to convert excess nutrients into biomass, generating value from excess digestate and providing additional markets to the anaerobic digestion sector.

Light Intensity and Nitrogen Concentration Impact on the Biomass and Phycoerythrin Production by Porphyridium purpureum.

Several factors have the potential to influence microalgae growth. In the present study, nitrogen concentration and light intensity were evaluated in order to obtain high biomass production and high phycoerythrin accumulation from Porphyridium purpureum. The range of nitrogen concentrations evaluated in the culture medium was 0.075-0.450 g L-1 and light intensities ranged between 30 and 100 µmol m-2 s-1. Surprisingly, low nitrogen concentration and high light intensity resulted in high biomass yield and phycoerythrin accumulation. Thus, the best biomass productivity (0.386 g L-1 d-1) and biomass yield (5.403 g L-1) were achieved with NaNO3 at 0.075 g L-1 and 100 µmol m-2 s-1. In addition, phycoerythrin production was improved to obtain a concentration of 14.66 mg L-1 (2.71 mg g-1 of phycoerythrin over dry weight). The results of the present study indicate that it is possible to significantly improve biomass and pigment production in Porphyridium purpureum by limiting nitrogen concentration and light intensity.

Formulation and utilisation of spent anaerobic digestate fluids for the growth and product formation of single cell algal cultures in heterotrophic and autotrophic conditions.

Spent anaerobically digested effluents of agricultural origin were collected and treated using membrane filtration to achieve three-large particle free-nutrient streams of N:P ratios of 16.53, 3.78 and 14.22. Three algal species were grown on these streams, achieving good levels of bioremediation of digester fluids simultaneously with biomass and associated end product formation. Nannochloropsis oceanica and Scenedesmus quadricuada, where proven highly effective in remediating the streams achieving ammonia and phosphate reduction over 60% while for Schizochytrium limacinum SR21 these serve as an ideal production medium for lipids and biomass reaching 16.70w/w% and 1.42gL-1 correspondingly. These processes thus provide treatment of sludge, avoiding the disposal problems by land spreading. The solid components are nutrient depleted but rich in organic matter as a soil enhancer, while the fluids rich in nutrients can be efficiently utilised for growth to generate high value materials of microalgae facilitating water reclamation.

Determination of volumetric gas-liquid mass transfer coefficient of carbon monoxide in a batch cultivation system using kinetic simulations.

A mathematical model of microbial kinetics was introduced to predict the overall volumetric gas-liquid mass transfer coefficient (kLa) of carbon monoxide (CO) in a batch cultivation system. The cell concentration (X), acetate concentration (Cace), headspace gas (Nco and [Formula: see text] ), dissolved CO concentration in the fermentation medium (Cco), and mass transfer rate (R) were simulated using a variety of kLa values. The simulated results showed excellent agreement with the experimental data for a kLa of 13/hr. The Cco values decreased with increase in cultivation times, whereas the maximum mass transfer rate was achieved at the mid-log phase due to vigorous microbial CO consumption rate higher than R. The model suggested in this study may be applied to a variety of microbial systems involving gaseous substrates.

Utilising light-emitting diodes of specific narrow wavelengths for the optimization and co-production of multiple high-value compounds in Porphyridium purpureum.

The effect of specific narrow light-emitting diode (LED) wavelengths (red, green, blue) and a combination of LED wavelengths (red, green and blue - RGB) on biomass composition produced by Porphyridium purpureum is studied. Phycobiliprotein, fatty acids, exopolysaccharides, pigment content, and the main macromolecules composition were analysed to determine the effect of wavelength on multiple compounds of commercial interest. The results demonstrate that green light plays a significant role in the growth of rhodophyta, due to phycobiliproteins being able to harvest green wavelengths where chlorophyll pigments absorb poorly. However, under multi-chromatic LED wavelengths, P. purpureum biomass accumulated the highest yield of valuable products such as eicosapentaenoic acid (∼2.9% DW), zeaxanthin (∼586µgg-1DW), ß-carotene (397µgg-1DW), exopolysaccharides (2.05g/L-1), and phycobiliproteins (∼4.8% DW). This increased accumulation is likely to be the combination of both photo-adaption and photo-protection, under the combined specific wavelengths employed.

Moving towards sustainable resources: Recovery and fractionation of nutrients from dairy manure digestate using membranes.

The fractionation of nitrogen (as ammonia/ammonium) and phosphorus (as phosphate ions) present in the dairy manure digestate was investigated using a nanofiltration membrane NF270. The filtration and separation efficiencies were correlated to pH across the range 3 < pH < 11. Filtration at pH 11 enabled higher permeate flux of 125-150 LMH at 20 bar, however rejection of ammonia was high at 30-36% and phosphate was 96.4-97.2%. At pH 3 and pH 7, electrostatic charge effects led to higher permeation of ammonium and thus more efficient separation of nitrogen. The rejection of phosphorus was relatively constant at any given pH and determined as 83% at pH 3, 97% at pH 7 and 95% at pH 11. The fractionation of nitrogen and phosphorus from complex aqueous solutions was demonstrated to be highly dependent on the charge of the membrane and ionic speciation. Solutions rich in nitrogen (as ammonia/ammonium) were obtained with almost no phosphorus present (<1 ppm) whilst the purification of the PO4-P was achieved by series of diafiltration (DF) operations which further separated the nitrogen. The separation of nutrients benefited from an advantageous membrane process with potential added value for a wide range of industries. The analysis of the process economics for a membrane based plant illustrates that the recovery of nutrients, particularly NH3-N, may be commercially feasible when compared to manufactured anhydrous NH3.

Microbial synthesis gas utilization and ways to resolve kinetic and mass-transfer limitations.

Microbial conversion of syngas to energy-dense biofuels and valuable chemicals is a potential technology for the efficient utilization of fossils (e.g., coal) and renewable resources (e.g., lignocellulosic biomass) in an environmentally friendly manner. However, gas-liquid mass transfer and kinetic limitations are still major constraints that limit the widespread adoption and successful commercialization of the technology. This review paper provides rationales for syngas bioconversion and summarizes the reaction limited conditions along with the possible strategies to overcome these challenges. Mass transfer and economic performances of various reactor configurations are compared, and an ideal case for optimum bioreactor operation is presented. Overall, the challenges with the bioprocessing steps are highlighted, and potential solutions are suggested. Future research directions are provided and a conceptual design for a membrane-based syngas biorefinery is proposed.

Minimizing the energy requirement of dewatering scenedesmus sp. by microfiltration: performance, costs, and feasibility.

The harvesting of the microalgae Scenedesmus species using a 200 L pilot-scale microfiltration system was investigated and critically assessed. The energy requirement was determined and correlated to the different operating parameters, such as transmembrane pressure (ΔP), membrane area, temperature, and initial biomass concentration. A filtration model was developed and showed a strong correlation with experimental data up to 20.0 g of dry cell weight (DCW)/L. The non-optimized filtration system had an energy requirement of 2.23 kWh/m(3) with an associated cost of $0.282/kg of microalgae. The investigation into the influence of the operating parameters and scale-up effects showed that the energy requirement could be substantially reduced to 0.90 kWh/m(3) and $0.058/kg of microalgae harvested. Maintenance costs associated with cleaning were estimated to be 0.23 kWh or $0.029/batch of microalgae processed. Dependent upon the operating conditions, harvesting may represent 6-45% of the energy embedded in the microalgae with a carbon footprint of 0.74-1.67 kg of CO2/kg of microalgae. Microfiltration was demonstrated to be a feasible microalgae harvesting technology allowing for more than 99% volume reduction. The energy requirement and associated carbon footprint of microalgae harvesting reported here do not forfeit the need for an industrial-scale study; however, the information provided presents a more realistic approximation than the literature reported to date.

Selection for fitness at the individual or population levels: modelling effects of genetic modifications in microalgae on productivity and environmental safety.

A mechanistic model of microalgae is used to explore the implications of modifying microalgal chlorophyll content and photosynthetic efficiency with an aim to optimising commercial biomass production. The models show the potential for a 10 fold increase in microalgae productivity in genetically modified versus unmodified configurations, while also enabling the use of bioreactors of greater optical depth operating at lower dilution rates. Analysis suggests that natural selection of a trait benefiting the individual (high Chl:C(max), i.e., high antennae size) conflicts with artificial selection of a trait (low Chl:C(max)) of most benefit to production at the population level. The implication is that GM strains rather than strains selected from nature will be most beneficial for commercial algal biofuels production. Further, escaped GM algae populations may, depending on the specific nature of the modification, be quickly out-competed by the natural forms because individually a high Chl:C is beneficial in low light environments. However, it remains possible that changes in biochemical composition associated with genetic modification of photosystem competence, or with other selection processes to enhance commercial gain, may adversely affect the value of such organisms as prey for zooplankton, leading to the unwanted generation of future harmful algae.

Use of an industrial grade medium and medium enhancing effects on high cell density CO fermentation by Eubacterium limosum KIST612.

Studies were made on the composition of the growth medium to increase the cell concentration in a cell-recycled continuous culture (Eubacterium limosum KIST612) with carbon monoxide as a sole energy source using phosphate-buffered basal medium (PBBM) and modified PBBM. One of major limiting factors in PBBM might be nitrogen during the high cell density culture. This limitation could be overcome by increasing of inorganic nitrogen or yeast extract concentration in the medium. Anaerobic digester fluid, which could replace the organic nitrogen in PBBM, was used to develop an industrial grade medium for conversion of CO to multi-carbon compound.

Performance assessment of malolactic fermenting bacteria Oenococcus oeni and Lactobacillus brevis in continuous culture.

The growth performance of malolactic fermenting bacteria Oenococcus oeni NCIMB 11648 and Lactobacillus brevis X(2) was assessed in continuous culture. O. oeni grew at a dilution rate range of 0.007 to 0.052 h(-1) in a mixture of 5:6 (g l(-1)) of glucose/fructose at an optimal pH of 4.5, and L. brevis X(2) grew at 0.010 to 0.089 h(-1) in 10 g l(-1) glucose at an optimal pH of 5.5 in a simple and safe medium. The cell dry weight, substrate uptake and product formation were monitored, as well as growth kinetics, yield parameters and fermentation balances were also evaluated under pH control conditions. A comparison of growth characteristics of two strains was made, and this showed significantly different performance. O. oeni has lower maximum specific growth rate (mu(max)=0.073 h(-1)), lower maximum cell productivity (Q (x) (max)=17.6 mg cell l(-1) h(-1)), lower maximum biomass yield (Y (x/s) (max)=7.93 g cell mol(-1) sugar) and higher maintenance coefficient (m (s)=0.45 mmol(-1) sugar g(-1) cell h(-1)) as compared with L. brevis X(2) (mu(max)=0.110 h(-1); Q (x) (max)=93.2 g(-1) cell mol(-1) glucose; Y (x/s) (max)=22.3 g cell mol(-1) glucose; m (s)=0.21 mmol(-1) glucose g(-1) cell h(-1)). These data suggest a possible more productive strategy for their combined use in maturation of cider and wine.

The measurement of Bacillus mycoides spore adhesion using atomic force microscopy, simple counting methods, and a spinning disk technique.

An atomic force microscope has been used to study the adhesion of Bacillus mycoides spores to a hydrophilic glass surface and a hydrophobic-coated glass surface. AFM images of spores attached to the hydrophobic-coated mica surface allowed the measurement of spore dimensions in an aqueous environment without desiccation. The spore exosporium was observed to be flexible and to promote the adhesion of the spore by increasing the area of spore contact with the surface. Results from counting procedures using light microscopy matched the density of spores observed on the hydrophobic-coated glass surface with AFM. However, no spores were observed on the hydrophilic glass surface with AFM, a consequence of the weaker adhesion of the spores at this surface. AFM was also used to quantify directly the interactions of B. mycoides spores at the two surfaces in an aqueous environment. The measurements used "spore probes" constructed by immobilizing a single spore at the apex of a tipless AFM cantilever. The data showed that stretching and sequential bond breaking occurred as the spores were retracted from the hydrophilic glass surface. The greatest spore adhesion was measured at the hydrophobic-coated glass surface. An attractive force on the spores was measured as the spores approached the hydrophobic-coated surface. At the hydrophilic glass surface, only repulsive forces were measured during the approach of the spores. The AFM force measurements were in qualitative agreement with the results of a hydrodynamic shear adhesion assay that used a spinning disk technique. Quantitatively, AFM measurements of adhesive force were up to 4 x 10(3) times larger than the estimates made using the spinning disk data. This is a consequence of the different types of forces applied to the spore in the different adhesion assays. AFM has provided some unique insights into the interactions of spores with surfaces. No other instrument can make such direct measurements for single microbiological cells.

Atomic Force Microscopy Study of the Adhesion of Saccharomyces cerevisiae.

An atomic force microscope (AFM) has been used to quantify directly the adhesion of metabolically active Saccharomyces cerevisiae cells at a hydrophilic mica surface, a mica surface with a hydrophobic coating, and a protein-coated mica surface in an aqueous environment. The measurements used "cell probes" constructed by immobilizing a single cell at the apex of a tipless AFM cantilever. Adhesion was quantified from force-distance data for the retraction of the cell from the surface. The data indicated stretching and sequential bond-breaking as the cell probe was retracted from all of the surfaces. Detailed studies were made for physiologically active cells, which were shown to have different adhesion properties to glutaraldehyde-treated cells. Greatest cell adhesion was measured at the hydrophobic surface. Prior adsorption of a bovine serum albumin protein layer at the hydrophilic surface did not significantly affect cell adhesion. Changes in yeast surface hydrophobicity and zeta-potential with yeast cell age were correlated with differences in adhesion. Cells from the stationary phase adhered most strongly to a mica surface. Time of surface contact was demonstrated to be important. Both the force needed to detach a cell from a hydrophilic mica surface and the length of the adhesive interaction increased after 5 min contact. The AFM cell probe technique gives unique insights into primary colonization events in biofilm formation. It will continue to aid both fundamental studies and the assessment of new procedures that are designed to lower cell adhesion at surfaces relevant to biotechnology, medicine, and dentistry Copyright 2001 Academic Press.