Periodic Reporting for period 1 - AlgaeFLOAT (Developing nanocellulose-based recyclable flocculants for flotation harvesting of microalgae)
Reporting period: 2022-03-01 to 2024-02-29
In Europe, the market demand for biomass is expected to increase to greater than €25 billion by 2050, mainly due to increased use of biomass for production of energy and chemicals. European initiatives such as the Green Deal and Blue Biotechnology aim to diversify and extend our biomass resources. Microalgae are an attractive novel biomass feedstock to complement agricultural and forestry biomass production. However, the small size of microalgae cells (1-10 μm) and low biomass concentration in the liquid culture medium (~1 g·L-1) complicate the harvesting of microalgal biomass using conventional solid-liquid separation technologies such as centrifugation or membrane filtration. It is widely believed that the harvesting of microalgal biomass could be better facilitated by aggregating small individual cells into larger aggregates using chemical flocculants via the flocculation process. Nonetheless, challenges remain, including contamination of the microalgal biomass with synthetic chemical flocculants and the separation of the flocculated biomass from the culture broth.
In this project, the experienced researcher (ER; Dr. Narasinga Rao) aimed to combine flocculation using a bio-based and recyclable flocculant with conventional and the novel activated-bubble (advanced) dissolved air flotation (DAF) processes as a sustainable microalgal harvesting technology. To do this, the ER built on and further improved flocculants based on non-toxic polymers and cellulose nanocrystals (CNCs) developed by Profs. Koenraad Muylaert and Wim Thielemans at KU Leuven, Belgium. The second aim was to create a pH-responsive flocculant that can be removed from the biomass and recycled after harvesting, thus avoiding contamination of the harvested biomass. The ER combined these flocculants with his expertise on conventional and novel advanced DAF processes to develop an efficient and sustainable technology to concentrate microalgae.
In this project, the experienced researcher (ER; Dr. Narasinga Rao) aimed to combine flocculation using a bio-based and recyclable flocculant with conventional and the novel activated-bubble (advanced) dissolved air flotation (DAF) processes as a sustainable microalgal harvesting technology. To do this, the ER built on and further improved flocculants based on non-toxic polymers and cellulose nanocrystals (CNCs) developed by Profs. Koenraad Muylaert and Wim Thielemans at KU Leuven, Belgium. The second aim was to create a pH-responsive flocculant that can be removed from the biomass and recycled after harvesting, thus avoiding contamination of the harvested biomass. The ER combined these flocculants with his expertise on conventional and novel advanced DAF processes to develop an efficient and sustainable technology to concentrate microalgae.
Several non-toxic and novel polymeric and cellulose-based nanomaterial flocculants were synthesised. Of these, two pDMAEMA polymers – high molecular weight (~150 kDa) and a low molecular weight (~9 kDa) were used in the first phase of the project for the flotation harvesting of algae. The operating conditions and protocols of the dissolved air flotation process were optimised by using polymers synthesised in Objective – 1. Following the DAF optimisation, several types of cellulose nanocrystal (CNCs) flocculants were synthesised – CNC-MIM (CNCs modified with imidazolium groups), CNC-PYR (CNCs modified with pyridinium groups). It is of note that different variations of the CNC-PYR were attempted including, variations of the positive and negative charges by TEMPO oxidising the CNCs prior to the modification with the pyridinium groups. Novel CNCs which were modified with glycine, dimethyl glycine, trimethyl glycine were also synthesised to be evaluated as flocculants for harvesting microalgae. Ongoing work includes the modification and evaluation of cellulose nanofibrils (CNFs), which are fibres of cellulose that are in the order of microns rather than < 300 nm which are commonly observed in traditional CNCs. This ongoing work is being carried out by PhD student co-supervised by the researcher.
The appropriate dimensions of the flotation jars were evaluated to optimise efficiency while decreasing the volume of the culture suspension used. In the same project, the repeatability of the experiments was evaluated to ensure that the results obtained were statistically reproducible. In another project, DAF was contrasted against sedimentation as a part of a two-stage harvesting-dewatering treatment train. A techno-economic assessment was undertaken to evaluate the benefits of DAF over sedimentation for microalgal harvesting. CNC-based flocculants were also evaluated for harvesting freshwater and marine microalgae. Previous studies had indicated that CNCs form small flocs when harvesting marine microalgae species that are grown in seawater. These small flocs settle very slowly (several hours) or do no settle at all, and hence, DAF was evaluated as an alternative method to harvest these small flocs.
During this phase of the work, a new area of research was also investigated – the use of DAF for restoration of lakes infested by cyanobacterial blooms. Cyanobacteria are blue-green algae which are a nuisance in recreational and shallow lakes. They also contain valuable pigments and metabolites which can be extracted post-separation. These cyanobacteria also contain gas vacuoles or air pockets which give them buoyancy. So, the DAF process which uses microbubbles takes advantage of cyanobacteria’s natural buoyancy to float them.
For reversibility, the following approaches were proposed. CNCs were modified with imidazolium groups, pyridinium, glycine, dimethyl and trimethyl glycine groups. The CNCs modified by the imidazolium groups showed pH responsive nature but did not detach effectively. So, the next step to TEMPO oxidise the CNCs prior to modifying them with the imidazolium groups is being undertaken. This gives enough negative charges on the molecule to repel the cells. The results of these were not reproducible. The glycine and trimethyl glycine modifications did not show any pH responsive nor flocculating activity. The dimethyl glycine modification in contrast showed good pH responsiveness at a ~ pH of 7-8. However, these flocculants did not show any floc formation during jar testing. It is hypothesised due to poor grafting caused non-stoichiometric amounts of reagents used.
The appropriate dimensions of the flotation jars were evaluated to optimise efficiency while decreasing the volume of the culture suspension used. In the same project, the repeatability of the experiments was evaluated to ensure that the results obtained were statistically reproducible. In another project, DAF was contrasted against sedimentation as a part of a two-stage harvesting-dewatering treatment train. A techno-economic assessment was undertaken to evaluate the benefits of DAF over sedimentation for microalgal harvesting. CNC-based flocculants were also evaluated for harvesting freshwater and marine microalgae. Previous studies had indicated that CNCs form small flocs when harvesting marine microalgae species that are grown in seawater. These small flocs settle very slowly (several hours) or do no settle at all, and hence, DAF was evaluated as an alternative method to harvest these small flocs.
During this phase of the work, a new area of research was also investigated – the use of DAF for restoration of lakes infested by cyanobacterial blooms. Cyanobacteria are blue-green algae which are a nuisance in recreational and shallow lakes. They also contain valuable pigments and metabolites which can be extracted post-separation. These cyanobacteria also contain gas vacuoles or air pockets which give them buoyancy. So, the DAF process which uses microbubbles takes advantage of cyanobacteria’s natural buoyancy to float them.
For reversibility, the following approaches were proposed. CNCs were modified with imidazolium groups, pyridinium, glycine, dimethyl and trimethyl glycine groups. The CNCs modified by the imidazolium groups showed pH responsive nature but did not detach effectively. So, the next step to TEMPO oxidise the CNCs prior to modifying them with the imidazolium groups is being undertaken. This gives enough negative charges on the molecule to repel the cells. The results of these were not reproducible. The glycine and trimethyl glycine modifications did not show any pH responsive nor flocculating activity. The dimethyl glycine modification in contrast showed good pH responsiveness at a ~ pH of 7-8. However, these flocculants did not show any floc formation during jar testing. It is hypothesised due to poor grafting caused non-stoichiometric amounts of reagents used.
Simple gravity sedimentation to separate the microalgal biomass from the culture medium was a relatively slow process and resulted in a microalgal sludge with a large water content. Alternatively, flotation processes emerged as a more effective technology that resulted in a much faster separation, more concentrated sludge with low water content and consequently, reduced sludge dewatering chemical demand. In conventional DAF, the flocculant was first mixed into the microalgal broth after which the flocculated suspension was mixed with a pressurised air-saturated recycle stream to generate μm-sized air bubbles that attach to the flocs and rise to the surface, where they are concentrated in a thin surface layer that can be skimmed off. In a novel adaptation of DAF developed by the ER, known as the activated-bubble DAF (advanced DAF), the flocculant was dosed in the pressurised recycle stream, thereby creating positively charged bubbles that can bind directly to microalgal cells. The advanced DAF was shown to reduce chemical demand and generated a concentrate with a lower water content compared to the conventional DAF process. The ER for the first time combined the novel flocculants he developed with conventional and advanced DAF. Over the year, the expected outcomes are the development of a pH-responsive flocculant. This work is currently ongoing and being undertaken by the student co-supervised by the ER. This project will achieve critical breakthroughs in reducing costs and improving sustainability of microalgal harvesting which is essential to expedite commercialisation of microalgae biomass production. Moreover, CNC-based recyclable flocculants may have applications in other industries such as treatment of mining waste, drinking water and wastewater treatment, which could be examined in spin-off projects.