Innovative Algae Dewatering Technology 1,*
1
1
A.R. Völkel , H. B. Hsieh , N. Chang, K. Melde , A. Kole
1
Abstract This paper describes a novel technology for dewatering of bio materials, such as algae, for biofuel production and other applications. This hydrodynamic separation (HDS) technology uses customized fluid flow patterns to focus suspended particulates into a well-defined band near a side wall of a curved channel, and where they can be separated off with a suitably designed flow splitter. The novel and innovative capability of this system is its ability to separate particles of any density, including neutrally buoyant particles such as algae and other biological and/or organic matter, from a liquid without the use of a physical barrier. Advantages of this technology over conventional practice include: small foot print, low energy requirement, rapid process, and continuous flow operation. We explored the dewatering of different algae species including Spirulina, Chlorella Vulgaris, and S. dimorphus. Larger algae or algae that naturally form larger aggregates can be concentrated very efficiently without the use of any added chemicals and harvesting efficiencies up to 97% have been demonstrated. This makes HDS very attractive for the dewatering of algae from concentrations typical for open ponds ( 90% and with more than 90% water recovery rate (Figure 2). Reducing the channel length by half reduces harvesting efficiency by about 5% to about 93%, but leads to a 50% reduction in pressure from about 20 psi to about 10 psi. Stage 1 (measured) Sample TSS [mg/l]
791
Clean effluent TSS [mg/l]
32
Concentrate TSS [mg/l]
4608
Flow rate split
80:20
Harvesting efficiency
97.3 %
Stage 1 algae
Stage 2 (estimates)
Stage 2 Concentrate
Concentrate
20% liquid vol. 97% algae mass
90 % algae mass > 9 g/l conc.
> 90 % liquid volume recycled
Output stage 1 TSS [mg/l]
4608
4608
4608
Flow rate split
50:50
60:40
70:30
Concentrate TSS [mg/l]
8967
11209
14945
Harvesting efficiency estimate [%]
97.3
97.3
97.3
Total harvesting efficiency [%]
94.7
94.7
94.7
High total harvesting efficiency can be maintained, if second stage performs similar to first stage.
Figure 2: Schematic 2-stage algae dewatering process. Top left table shows experimental results achieved using spirulina algae with a single concentration stage. Bottom right table shows concentration estimates of a second stage with different flow splits.
S. dimorphus algae Smaller algae can be concentrated with HDS devices by using a polymer, which promotes flocculation into larger aggregates. For a test with the algae species S. dimorphus, which is less than 10 mm in size, we used the bio-degradable flocculent Chitosan [Guibal 2006]. Figure 3 shows a schematic process diagram and PSD curves of the sample at different stages of this process. The aggregates are about 100
mm in size and a low pressure (low shear) HDS channel (1.1 psi) was used to avoid potential floc breakup during separation.
Coagulant addition, mixing
Algae TSS = 24 mg/L
Hydrodynamic Separator
Short Aggregation
Clean Stream TSS = 1.1 ppm
Discharge, disposal or further treatment for end use
Concentrate TSS = 47.1 ppm
Coagulant (Chitosan) plus mature flocc
Algae CFS with Chitosan by adding mature flocs Dunaliella sp 30
Raw diluted algae sample After slow mix with Chitosan and adding mature flocs Waste
25
clean Effluentstream
20
q (%)
15 10 5 0 0.01
0.1
1
10
-5 Diameter (um)
100
1000
10000
Concentrate
Clean Stream
43 NTU
1.06 NTU
47.13 ppm (mg/L)
1.11 ppm (mg/L)
Figure 3: Top: process schematics for separation experiment with S. dimorphus algae. Bottom left: PSD data for dimorphus algae: blue: raw sample; pink: after flocculent addition; yellow: concentrate stream; cyan: dilute stream. Bottom right: output from concentrate (left) and dilute (right) stream.
The separator used for this experiment had a 50:50 flow splitter, resulting in a doubling of the algae concentration, while only a few percent of the algae where lost with the dilute stream. The smaller aggregates that are left with the dilute stream grow further as can be seen by the peak at about 50 mm from the PSD measurement in Figure 3. A higher concentration factor may be achieved by using a more optimized splitter design and by cascading multiple HDS in series.
Conclusions This paper describes a novel approach for the removal of suspended solids from many water matrices. Using purely hydrodynamic forces this technology efficiently removes suspensions by concentrating them into a narrow band that is split off the main stream without the need for physical filtration. Because this is a size and not density dependent particle concentration method it is ideally suited for the dewatering of algae from concentrations typically found in algae ponds up to a few weight %. Depending on the size of the algae separation can be achieved without the extra addition of any chemicals, allowing the direct recycling of the dilute stream back into the algae ponds. Smaller algae can be addressed with the use of a suitable flocculent, such as Chitosan. Added benefits include no moving parts, high scalability, high modularity in construction, and low cost in materials and TCO. In addition, this technology can be used for water treatment processes such as activated sludge recycling in a waste water plant.
References Uduman N., et al. (2010), “Dewatering of microalgal cultures: A major bottleneck to algae-based fuels”, J. Renewable Sustainable Energy 2, 012701.
Lean, M.H., et al. (2008), “Curved Fluidic Structures to Improve Aggregation Kinetics in Municipal Water Treatment”, Proc. Water Quality Technology Conference. Lean, M.H.; et al. (2010), “Innovative system to pre-treat seawater for RO”, Proc Singapore International Water Convention. Völkel, A.R. (2011), “Innovative Technology for Selective Contaminant Removal”, Proc. TechConnect. Guibal E., et al., (2006), “Review of the Use of Chitosan for the Removal of Particulate and Dissolved Contaminants”, Separation Science and Technology, 41, 2487–2514.