**9. Harvesting of algae**

Typical plant gate selling prices/production costs are well above \$100/kg from such systems. Consequently, biofuels production based entirely on photobioreactor systems is generally

Algae pumped with nutrient rich water through plastic and borosilicate tube, exposed to sunlight called photobioreactor (PBR). Algae biomass produces using carbon dioxide and light by the process of photosynthesis and nutrient from wastewater in artificial environment not in natural environment. Using photobioreactor, algae easily grow on the land which is not arable such as desert and even ocean surface also. PBR is more productive and controlled but

Table 10 makes a comparison between PBR and ponds for several culture conditions and growth parameters. Comparison of performances achieved by PBRs and open ponds may not be easy, as the evaluation depends on several factors, among which the algal species cultivated and the method adopted to compute productivity. There are three parameters commonly used to evaluate productivity in algae production units: Volumetric productivity (VP): productivity per unit reactor volume (expressed as g/L d).Areal productivity (AP): productivity per unit of ground area occupied by the reactor (expressed as g/m2 d). Illuminated surface productivity

d).

(ISP): productivity per unit of reactor illuminated surface area (expressed as g/m2

considered unlikely to be commercially viable.

122 Biofuels - Status and Perspective

more costly and difficult than open pond system.

**Figure 6.** Photobioreactor for large scale algae production [122]

Algae harvesting overcome to get desired algae product that is fuel. Harvesting method use for algae harvesting depends upon type of algae. There is number of algae harvesting method but some of most common is Flocculation, Centrifugation and Microorganism. There are some issues related to algae harvesting that should carried out before harvesting Process to be done such as the water content should be within desired limit, algae must be in paste form before processing. Size of microalgae cells increase by Flocculation so that sedimentation will be easily done with large cells particle. Chemical flocculation and centrifugation is useful in high density algae because using certain chemical such as alum, lime and aluminum sulphate will coagulate and precipate the cell down or float to the surface. This method is very high costly because of the large amount of the chemical used in this process.

Algal harvesting consists of biomass recovery from the culture medium that may contribute to 20–30% of the total biomass production cost [47]. In order to remove large quantities of water and process large algal biomass volumes, a suitable harvesting method may involve one or more steps and be achieved in several physical, chemical, or biological ways, in order to perform the desired solid–liquid separation. Experience has demonstrated that albeit a universal harvesting method does not exist, this is still an active area for research, being possible to develop an appropriate and economical harvesting system for any algal species.

Most common harvesting methods include sedimentation, centrifugation, filtration, ultrafiltration, sometimes with an additional flocculation step or with a combination of floccula‐ tion–flotation. Flocculation is used to aggregate the microalgal cells to increase the effective particle size and hence ease sedimentation, centrifugal recovery, and filtration [47]. Weissman and Goebel [298] studied four primary harvesting methods for the purpose of biofuels production: microstraining, belt filtering, flotation with float collection, and sedimentation. These methods discriminate on a size and density basis in performing the biomass separation. Microstrainers are an attractive harvesting method because of their mechanical simplicity and availability in large unit sizes. The recent availability of very fine mesh polyester screens has revived interest in their use for microalgae harvesting.

Subsequent studies concluded that it would be necessary to flocculate the cells prior to microstraining. Filter presses operating under pressure or vacuum can be used to recover large quantities of biomass, but for some applications filtration can be relatively slow which may be unsatisfactory. Also filtration is better suited for large microalgae such as Coelastrum probo‐ scideum and S. platensis but cannot recover organisms with smaller dimensions such Scene‐ desmus, Dunaliella, or Chlorella [47]. Alternatively, membrane microfiltration and ultra-


**Table 11.** A comparison of open and closed large-scale culture systems for microalgae [115].

filtration are other possible alternatives to conventional filtration for recovering algal biomass, which are more suitable for fragile cells and small scale production processes. Furthermore these filtration processes are more expensive especially because of the need for membrane replacement and pumping.

Richmond [96] suggested one main criterion for selecting a proper harvesting procedure, which is the desired product quality. In one hand for low value products, gravity sedimenta‐ tion may be used, possibly enhanced by flocculation. Sedimentation tanks or settling ponds are also possible, e.g. to recover biomass from sewage-based processes. In other hand for highvalue products, to recover high quality algae such as for food or aquaculture applications, it is often recommended to use continuously operating centrifuges that can process large volumes of biomass.

Albeit at considerable cost, centrifuges are suitable to rapidly concentrate any type of micro‐ organisms, which remain fully contained during recovery. Additionally, these devices can be easily cleaned or sterilized to effectively avoid bacterial contamination or fouling of raw product.

Another basic criterion for selecting the harvesting procedure is its potential to adjust the density or the acceptable level of moisture in the resulting concentrate right to the optimum subsequent process [47, 96]. Gravity sedimented sludge is generally more diluted than centrifugally recovered biomass, which substantially influence the economics of product recovery further downstream. Since costs of thermal drying are much higher than those of mechanical dewatering, in order to reduce the overall production cost, a concentrate with higher solids content is required after harvest to easy biomass dehydration (e.g. in a drum drying).

In this case a combination of methods can also be used, e.g. a pre-concentration with a mechanical dewatering step such as microstrainer, filtration, or centrifugation and then, a postconcentration by means of a screw centrifuge or a thermal drying. After separation from the culture medium algal biomass (5–15% dry weight) must be quickly processed lest it should get spoiled in only a few hours in a hot climate.


**Table 12.** Comparison between various Harvesting Techniques [93]
