Extraction of bio-flocculant from okra using hydrothermal and microwave extraction methods combined with a techno-economic assessment

Lee, Chai Siah (2017) Extraction of bio-flocculant from okra using hydrothermal and microwave extraction methods combined with a techno-economic assessment. PhD thesis, University of Nottingham.

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Abstract

Since the usage of chemical flocculants especially polyacrylamides is closely related with environmental pollution and health hazards, synthesis of environmental friendly and economic viable bio-flocculants that exhibit high flocculating efficiency is highly desirable. The production of natural bio-flocculants extracted from plant is urgently needed as an alternative to chemical flocculants due to their inherent low toxicity, biodegradability and low environmental footprint.

In this study, a plant-based bio-flocculant was extracted from Hibiscus/Abelmoschus esculentus (okra) with conventional hydrothermal extraction (CHE) and microwave assisted extraction (MAE) processes by using water as solvent. The aims of this work were to produce the bio-flocculant through environmentally friendly and economically feasible process, optimise its extraction yield, optimise its flocculating and sludge dewatering properties, and also minimise its production cost in order to be comparable to chemical flocculants (polyacrylamides) from perspectives of quality and cost. The extraction efficiencies of bio-flocculants were justified by the optimised yields, the flocculating abilities were evaluated by the removal of suspended solids (SS) and turbidity after flocculation process and sludge volume index (SVI) whereas the sludge dewatering abilities were assessed by SS removal after filtration and water recovery.

Single factor experimental design was employed to study the effects of extraction temperature, time, solvent loading, and agitation speed and particle size on yield, flocculating and dewatering properties of bio-flocculants. The influence of extraction parameters to yield and the reliability of the experimental data were verified by analysing the single factor experimental results with response surface methodology. Results showed that extraction yields were significantly affected by extraction temperature and time and solvent loading. Prolonged extraction (few hours) at high temperature (60-90 ˚C) decreased the yields of bio-flocculants extracted with CHE method. Conversely, extraction at high temperature was favourable for MAE method to raise the yield due to short extraction time in minutes.

Flocculating abilities of bio-flocculants were not significantly affected by the extraction conditions whilst the sludge dewatering abilities were mainly influenced by the temperature and particle size. High extraction temperature at ≥ 70 ˚C and the smallest okra particle size at 1mm were the crucial conditions for extraction of bio-flocculants with high dewatering abilities which showed >95% of SS removal after filtration and ≥75% of water recovery.

The optimised yield of CHE at 25.9% was obtained at 50 ˚C, 2 hours, solvent loading of 2.5 w/w and agitation at 200 rpm. On the other hand, microwave-extracted bio-flocculants were best extracted at 90 ˚C, 10 minutes, and solvent loading of 3.5 w/w and okra particle size at 1mm to get the optimised yield at 48.7%. These results revealed that the extraction yield has been enhanced by 87.8% by using MAE method and the extraction time was markedly reduced from hours to minutes.

This study uses a combination of empirical observations and an analysis of mass transfer behaviour to yield new insights into the mechanism of MAE. Enhancements in extraction rate and yield achieved by microwave extraction were observed experimentally compared with hydrothermal extraction at temperatures in excess of 50 ˚C, however at lower temperatures there was no observable difference between the two processes. A step-change in extraction yield between microwave and hydrothermal processes was shown to be caused by selective heating. A temperature gradient of the order of 1 ˚C is sufficient to reduce the water chemical potential within the plant cell structure, which causes diffusion of water solvent into plant cell such that internal plant cell pressures can increase to the point where disruption occurs. These findings demonstrate the need to operate microwave extraction processes at a temperature that enables selective heating, and a newly-proposed mass transfer phenomenon that could have wider positive implications for extraction and leaching processes.

The bio-flocculants extracted with both methods were applied in flocculation and sludge dewatering without pH alteration and addition of coagulant. Efficient flocculating abilities were attained with >99% of SS and turbidity removal and <10 mg/L of SVI. However, microwave extracted bio-flocculants were shown to exhibit higher dewatering abilities than hydrothermal extracted bio-flocculants. The bio-flocculants displayed the results of >95% SS removal after filtration and 75% water recovery during sludge dewatering at dosage of 30 mg/L, and was shown to be comparable to or even better than polyacrylamides due to achievement of higher water recovery at the same dosage. Dried bio-flocculants were found to have higher dewatering abilities than aqueous bio-flocculants, probably because drying at low temperature (40 ˚C) has minimised the moisture content and extended the shelf life of bio-flocculants. Biopolymer bridging was preliminary predicted as the plausible bio-flocculation mechanism.

The economic feasibility of production of industrial scale bio-flocculant was investigated and the production process was modelled by using SuperPro Designer v9.0 simulation software. The MAE process in continuous mode was verified to be more economically viable than CHE process either in batch or continuous mode if the production scale was beyond 15 tonne/year. Sensitivity analysis for continuous microwave process was conducted and the results indicated the strong influences of annual production and extraction yield followed by raw material okra price on the unit production cost. Continuous microwave process with extraction conditions at 90 ˚C, 10 minutes and solvent loading of 3.5 w/w was identified as the optimised scheme for production of industrial scale bio-flocculant at the lowest production cost. The estimated selling price of aqueous and dried bio-flocculants for an annual production of 220 tonne/year were 41 and 52 $/kg respectively, and was shown to be lower than food grade bio-flocculants but higher than polyacrylamides.

This work has clearly showed that okra bio-flocculant could offer a feasible and sustainable alternative to synthetic flocculants for water treatment and sludge dewatering applications due to its high efficiency in flocculating and dewatering, and can be extracted using only water as a solvent, minimising the environmental footprint of the extraction process.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Robinson, John
Binner, Eleanor
Gomes, Rachel L.
Chong, Mei Fong
Keywords: bio-flocculant, coagulation, flocculation, sludge dewatering, wastewater treatment, microwave-assisted extraction, conventional extraction, microwave selective heating, economic analysis, scale-up
Subjects: T Technology > TD Environmental technology. Sanitary engineering
Faculties/Schools: UNMC Malaysia Campus > Faculty of Engineering > Department of Chemical and Environmental Engineering
Item ID: 43270
Depositing User: LEE, CHAI SIAH
Date Deposited: 21 Nov 2017 02:48
Last Modified: 24 Nov 2017 12:55
URI: http://eprints.nottingham.ac.uk/id/eprint/43270

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