The world is currently facing the prospect of a severe global shortage of fresh water alongside finite energy resources and the development of energy-efficient desalination methods is of paramount ...importance to solve these complex problems. In this paper the basic principles of freeze concentration processes are presented. Even though the process has the advantage of low energy usage and high concentration factors, only lab and pilot scale studies have been conducted in the desalination industry and application of the process has been limited due to the dominance of more traditional thermal and membrane technologies. Finally, the paper looks at the future applications for freeze concentration and discusses the possibility of application to high saline brine wastes in hybrid-technology.
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•Review of processes for desalination by freeze concentration•Developments in freeze concentration methodologies in food, wastewater and desalination industries•RO-freeze desalination hybrid process
Nanofiltration technology has come a long way since first inception in the late 1980s. Research activity in this area covers a great many topics and the aim of this review is to quantify the level ...interest in each of these areas. The number of annual publications directly related to nanofiltration technology has been harvested from ScienceDirect since 2007. This quantification of research has shown that interest in nanofiltration technology has grown over the past decade, particularly over the past five years. The primary journals reporting articles on nanofiltration are the Journal of Membrane Science, Desalination and Separation and Purification Technology, although articles have been spread across a further 139 journals. Unsurprisingly, the major topics of interest have been water processing, membrane fabrication and membrane surface modification. There has been clear growth in the areas of organic solvent nanofiltration, pharmaceutical and biological applications, design and economics of nanofiltration processes and review articles. Nanofiltration modelling has received less support over the period reviewed and has experienced a steady decline.
Clearly the overall growing trend in nanofiltration research indicates that the technology remains popular and this interest should materialise into further applications for a robust and sustainable future.
The future of microalgae as a sustainable feedstock for biofuel and other products is still uncertain. Although productivity and environmental benefits surpass that of many other types of feedstock, ...the associated costs with production and downstream processing hinder this type of technology. The microalgae biorefinery approach addresses many of these issues in which upstream and downstream processes are important. Upstream technologies associated to nutrient recovery from waste effluents have been reviewed and discussed. Potentially, waste-derived nutrients will enable the formulation of optimal growth media from wastewater at lower costs. Microalgae dewatering is still seen as a major burden. A thorough review of the associated membrane processes in the literature has highlighted lack of consistency in terms of the influence of pore size and membrane materials. Moreover, only very few pilot-scale cost estimates could be found. The fractionation of microalgae products is perhaps the less developed area in the context of a microalgae biorefinery. Membrane filtration for the recovery of lipids, proteins and carbohydrates from microalgae is still an infant technology and major developments are expected to take place within the next few years. This review highlights the achievements, potential and future challenges of integrating membrane technology into microalgae-based biorefineries.
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•Green light plays a significant role in the growth of Porphyridium purpureum.•Multi-chromatic LED wavelengths accumulated the highest yields of valuable products.•Photo-adaption and ...photo-protection are suspected to boost product yield.•Specific wavelengths can increase product ratios increasing biomass value.
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), β-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.
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•Dilution, settlement, and membrane filtration to valorise digestate.•Digestate optimised as a waste-based medium for Chlorella vulgaris cultivation.•Mechanical treatment separated ...liquid and solid fractions and recovered nutrients.•C. vulgaris performed better with low concentrations of processed digestate (2.5%)•C. vulgaris growth on digestate is scalable and converts excess nutrients to biomass.
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.
In this paper we investigate the potential for using suspension crystallisation for the production of clean drinking water from a seawater source. The experimental results show that the produced ...water from the suspension crystallisation plant not only meets water quality standards but is comparable in ionic composition to premier bottled water from around the globe. The experimental results obtained from a pilot scale suspension crystallisation unit showed that the achievable water recovery was around 41% and the salt rejection ratio reached over 99%, which is comparable with most desalination technologies. Moreover, a hybrid RO-Freeze plant has been proposed that is capable of significantly increasing the potable product water that could be achieved by RO alone (~400% increase), while simultaneously concentrating the RO brine (used as feed water) producing a super brine of ~13wt%. While there is a cost to this additional process in terms of capital and energy that must be quantified, the obvious increase in water harvest and reduction in residual brine quantity lead to a very attractive desalination process. If the energy demands are acceptable, then this technology could lead to a more sustainable water future.
•Suspension crystallisation has been successfully demonstrated as a desalination technology.•The product water generated is of drinking water standards.•A hybrid RO-Freeze process has been proposed that increases water recovery by 400%.•Desalination brines can easily be concentrated to the order of 13wt%.
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 (<1ppm) 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.
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•NF270 membrane was used for the fractionation of nutrients from manure digestate.•Membrane flux was highest at pH 11 but nutrient separation was more effective at pH 7.•P was mostly retained however the permeation of N was highly dependent on the pH.•94.2% of N was obtained as a series of permeates by DF strategies.•Nutrient recovery may be economically feasible under certain conditions.
Cultivating microalgae requires a nitrogen and phosphorous feed source. Anaerobic digestion waste (digestate) provides a cheap sustainable feedstock for these materials. Previous studies have ...successfully demonstrated the feasibility of nutrient recovery and subsequent algae growth. There is now a need to fully characterise digestate filtration to improve our understanding of this process prior to its commercialisation. In this work, digestate filtration is characterised at bench scale using frontal (dead-end) filtration and a 100 kDa MWCO ultrafiltration membrane. Our experiments demonstrated rapid cake formation causing significant flux decline. The steady-state permeate flux for digestate was 2.4 to 4.8 L m−2 h−1, a reduction of ~90% compared to clean water flux. The specific cake resistance was ~1015 m kg−1 and the compressibility index 1.07. A series of four filtration and cleaning cycles showed 90% flux recovery following a clean water wash. Digestate filtration was then evaluated at a commercial scale using crossflow and the KOCH ABCOR® tubular membrane (100 kDa MWCO). The results were similar to those at the bench scale, i.e., rapid initial fouling leading to a period of steady-state flux (approximately 7 L m−2 h−1). The commercial membrane was flushed with water and diluted bleach after each use, and a digestate permeate flux decline of only 4.8% over a 12-month active use period was observed. The present research provides bench scale characterisation and demonstrates the commercial scale operation of anaerobic digestate filtration using ultrafiltration. The overall filtration performance was excellent, and the process can now be scaled to any operational capacity.
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/m3 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/m3 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.
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