Dewatering of microalgae using flocculation and electrocoagulation
2017-02-06T05:10:46Z (GMT) by
Sustainable development and environmental protection has become a major global concern. Biofuels are an alternative transport fuel that can reduce the consumption of non renewable carbon resources and greenhouse gas emissions. Biodiesel is a type of biofuel that is made from renewable lipid containing biological resources and is non toxic. Current feedstocks for biodiesel production include vegetable oils and animal fats, which are vital components of the human food chain, and this makes them less suitable for biofuel production. Lipid-containing microalgal biomass is another feedstock for biodiesel production and has the potential to replace the conventional sources. Microalgae could have many advantages over traditional feedstocks, such as fast photosynthetic growth rates and high lipid content, and these have triggered interest in microalgae bioprospecting. However, one of the main disadvantages of microalgae-to-biofuel process engineering is the high energy consumption and operational costs associated with the culture dewatering stage for biomass creation. This is due to the small size of microalgae cells and the extremely dilute nature of the microalgae culture even at the late exponential phase of the cultivation process. In order to make biodiesel production from microalgae sources commercially viable, the dewatering step must be technologically and economically effective. Current dewatering technologies such as centrifugal recovery and filtration are highly energy intensive. Cell aggregation induced by either flocculation or coagulation is another commonly used dewatering technique that has the advantage of using less energy under optimum conditions. These techniques have found numerous applications in industrial wastewater treatment, and hence could have a potential application for microalgae removal. The work conducted in this research investigated two dewatering techniques for microalgae under the umbrella of bioseparation by induced cell aggregation; flocculation and electrocoagulation. A series of batch experiments were carried out under varying experimental conditions using two marine microalgae species; Chlorococcum sp. and Tetraselmis sp. The key assessment criterion, the microalgae recovery, was quantified by determining the amount of microalgae recovered, that is, the amount of microalgae that settled due to flocculation or floated due to electrocoagulation. Two types of flocculants were used in this research; polyelectrolytes and aluminium sulphate (alum). All flocculants were able to achieve successful microalgae flocculation to varying degrees. Contrary to literature, the results from this research showed that anionic and non-ionic polyelectrolytes were able to adequately flocculate marine microalgae. It was also found that alum flocculation was possible at doses that were comparable to those used for freshwater microalgae flocculation, where previous literature had stated that the doses required for marine microalgae flocculation required were in the range of 5 to 10 times larger. The polyelectrolyte flocculants were able to achieve up to 90 % recovery at doses between 2 to 10 mg/L. Alum was able to achieve up to 99 % recovery at doses under 100 mg/L. Flocculation recovery was seen to increase with pH, and the zeta potential showed that the microalgae become more electropositive with decreasing pH. The recovery of microalgae was also seen to increase with increasing temperature. Electrocoagulation was carried out with two sacrificial anode materials; aluminium and ferritic stainless steel type 430, and a carbon inert anode. The maximum recovery obtained was 99, 90 and 38 % for electrocoagulation with the stainless steel 430, aluminium and carbon anodes, respectively. In order of efficiency, the optimum anode was aluminium, stainless steel 430 and then carbon. The microalgae recovery increased with increasing applied voltage and electrocoagulation time. The valencies of dissolved metal ions from the anode were investigated for multivalent species and confirmed with a metallic and ionic mass balance of the dissolution and electrocoagulation process. The pH of the solution did not appear to have a significant effect on recovery. The zeta potential of the microalgae after electrocoagulation was seen to become more electropositive with increasing experimental run time. An increase in temperature increased recovery whilst a reduction in salinity resulted in reduced microalgal recovery. The theory of the mechanism of electrocoagulation was proven with the results of the experimental work conducted in this research. Evidence was shown of the three main steps involved in the electrocoagulation mechanism; coagulation, charge neutralisation and flotation. The results demonstrated that coagulation of microalgae occurs in the bulk of solution, and not at a specific location such as in the vicinity of an electrode. The rate limiting step of the electrocoagulation process was found to be the rate of coagulation of microalgae cells, induced by the binding of metal ions onto the microalgae surface. A mathematical model was developed in order to predict the recovery of microalgae at predetermined electrocoagulation conditions such as the applied current, time, microalgae species and electrode material. The mathematical model was able to accurately predict microalgae recovery for all microalgae systems. This model made it possible to optimise the electrocoagulation process for conditions that required a low energy demand and accomplished high microalgae recovery. These optimum conditions enabled a comparison between alum flocculation and the aluminium anode electrocoagulation. Such a comparison is interesting as the two processes have a similar flocculation mechanism. A techno-economic and carbon assessment was performed on the unit operation of microalgal culture dewatering operation as part of the microalgal biodiesel production process. This involved estimating the operation costs, carbon dioxide emissions and energy consumption. Three different dewatering technologies were investigated; centrifugation, electrocoagulation/centrifugation and alum flocculation/centrifugation. The analysis showed that both electrocoagulation and flocculation require significantly less energy to dewater in comparison to centrifugation. However, when the energy to produce the raw materials was taken into account, electrocoagulation was found require a greater energy demand. The operational cost of continual replacement of the anodic material significantly increased the overall economics of electrocoagulation compared to alum flocculation. In terms of energy requirements, carbon dioxide emissions and overall costs, alum flocculation showed to be the most promising dewatering technique with genuine potential to be used as the main dewatering technology in microalgae sourced biodiesel production. Future work may involve the investigation of electrocoagulation as a continuous process and also the combination of alum flocculation and electrocoagulation as a single dewatering method.