1Civil Engineering Department, 2Chemical Engineering Department, 3Core University Research Resource Laboratory New Mexico State University, Las Cruces, New Mexico, USA

ENERGY-EFFICIENT HARVESTING OF MICROALGAEC. Kukutla1, N. Nirmalakhandan1, F.Montelya1, T. Muppaneni2, P. Cooke3, S. Deng3

INTRODUCTIONSeveral studies had determined that microalgal-derived fuels can be a potential alternate renewable resource for fossil fuels. Advantages of microalgae over other fuel crops include their fast growth rate, high lipid content, high yields on a lower input of resources, and carbon dioxide sequestering capacity. However several challenges are yet to be overcome to make the algae-to-fuel pathway economically feasible.

One specific challenge is the harvesting step that is energy-intensive, accounting for 20 – 30% cost of the end product. Energy consumption for harvesting unit mass of dry biomass depends on the algal species, cultivation conditions, input concentrations, the desired concentrate quality, and the harvesting technology. Most of the commercial algal-based industries use centrifuge or filtration to harvest microalgae because of their ability for high recovery of biomass from the feed. Although centrifuge works effectively, its high energy consumption and maintenance only permits its use in high value based end products. This study suggests that bioflocculation followed by hydrothermal liquefaction, can be a more energy-efficient approach than current technologies in the algal biomass-to-fuel pathway.

Centrifugation is the most common harvesting technology in use today. According to literature reports, energy consumption of disk stack centrifuges, for example, can range from 8 to 16 kJ/g of dry biomass harvested (Ref 1). Considering the higher heating value of the harvested biomass (typically ~26 kJ/g), the energy requirement of current harvesting technologies would drive the net energy ratios of algal fuels to fall below 1, rendering the algal biomass-to-fuel pathway unsustainable (Ref 2). Even though the high value of the traditional algal products had justified the use of energy-intensive harvesting technologies in the past, for algal fuels to become affordable and sustainable, energy-efficient and cost-effective harvesting technologies with minimal material and energy demands have to be deployed.

Ref 1: Salim S., Bosma, R., Vermue, M. H., Wijffels, R. H., Harvesting of microalgae by bio-flocculation, J Applied Phycology (2011) 23: 849-855.

Ref 2: Abdelaziz, A. E.; Leite, G. B.; Hallenbeck, P. C., Addressing the challenges for sustainable production of algal biofuels: II. Harvesting and conversion to biofuels. Environ Technol 2013, 34, (13-16), 1807-36.

BACKGROUND

OBJECTIVE OF THIS STUDYEvaluating the feasibility of harvesting Chlorella sorokiniana from the growth media using low energy technique and extracting energy from the harvested material without any further dewatering via hydrothermal liquefaction. This study hypothesized that bioflocculation followed by hydrothermal liquefaction can lead to net energy ratios (ER) higher than 1 when the target algal strain is mixed with an atuoflocculating strain in appropriate ratio, without any change in culture conditions.

MATERIALS AND METHODSBioflocculation involves the use of auto-flocculating algal strains to enhance gravity settling of non-flocculant microalgae. As this approach does not use any chemical flocculants or external input of energy, it is energy- and cost-efficient. Additionally, the settled biomass does not require any pre-treatment prior to oil extraction; and, the supernatant can be reused in the cultivation step.

In this experiment, Coelastrella (10-16 μm in diameter) an autoflocculating, rapidly settling strain (AFRS) is used to enhance the settling of the non-flocculating, slowly settling strain (NFRS), Chlorella Sorokiniana (3 - 6 μm in diameter).

Settling studies were performed in 250 mL graduate cylinder where both strains were allowed to settle under respective volume proportions. All the experiments were conducted for 1 hour, taking OD readings at 30 minutes interval.

MICROSCOPIC IMAGES OF BIOFLOCCULATED MATERIAL

AFRS: Coelastrella (10 – 16 μm)

NFSS: C.Sorokaniana (3 – 6 μm)

¿+¿Bio-flocculated and

settled material

One of the mechanisms to separate biosolids from the bulk culture medium is to drive the biosolids away from the medium under the action of one or more external forces. Examples of such forces can include gravity, buoyancy, inertia, centrifugal, or electrostatic. Considering gravitational settling as a potential low-energy harvesting technology, where the external forces are due to gravity and buoyancy, the settling velocity Vt (m/s) of biosolids can be estimated from (1)where, is the density of biosolids (kg/m3) and D their equivalent volume-diameter (m); m is the viscosity of the culture medium (N-s/m2); is the density of the medium (kg/m3). For typical algal cells (D ~5 µm), settling velocity of 0.1 m/d calculated from Equation (1) is too small for practical application of gravitational settling due to the negligible difference in density between the cells and the medium ( = 1,030 to 1,230 kg/m3 vs. = 1,000 kg/m3). This theoretical consideration indicates that increasing the diameter of biosolids, for example, by flocculation can improve gravitational settling.

EXPERIMENTAL RESULTS OF BIOFLOCCULATION

THEORETICAL BACKGROUND ENERGY RATIO (ER) FOR CURRENT METHOD VS. BIOFLOCCULATION FOLLOWED BY HTL

Energy input = Energy for harvesting + Energy for HTL Energy output = Energy equivalent of biocrude from HTL

Energy equivalent of biocrude from HTL

Energy for harvesting + Energy for HTLEnergy ratio =

For current methods = 8 kJ/g For bioflocculation ~ 0

EXPERIMENTAL RESULTS FOR CALCULATING ER

27% 44%

These results indicate that bioflocculation followed by hydrothermal processing of the harvested biomass has the potential for energy-positive and sustainable algal biofuel production. The energy-advantage is higher when biochar is included in the analysis providing the motivation to develop technologies capable of utilizing the energy content of biochar to make the algal biomass-to-biofuel pathway more efficient.

CONCLUSION

ACKNOWLEDGEMENTThis study was supported in part by the NSF Engineering Research Center: ReNUWIT award # EEC 1028968; NSF NM EPSCOR award # IIA-1301346; US Department of Agriculture BGREEN; US Department of Energy: REAP project and the Ed & Harold Foreman Endowed Chair.

Figure 1. Relationship between effective volume-diameter, specific gravity, and settling velocity of NFSS strain a, AFRS strain b, and the combination a+b, settling under gravity

Figure 2. Removal efficiency at different AFRS:NFSS volume ratios as a function of settling time. AFRS strain: Coelastrella; NFSS strain: C. sorokiniana

Figure 3. Scanning Electron Microscopic images of individual and flocculated material .