The gas transfer efficiency can be improved by using smaller bubbles through the use of sintered porous stones or pipes, resulting in larger gas-liquid interface area compared to larger bubbles . As shown in Figure 13.Continuing research is underway to determine the optimum CO2 to water ratio for efficient algae growth. However, initial values for values for saturation region sharply vary from author to author–from 0.1% to 10% . The pump size and number of pits for a open raceway pond is very dependent on many factors and should also take into account energy consumption.The device supplying the CO2 optimizes the purification of the gas used as sources of CO2, adjusting its flow and design system to achieve high gas purification and therefore, nft hydroponic system a greater utilization of the CO2 supplied, with minimum energy consumption.Harvesting cost may contribute 20% – 30% to the total cost of algaebiomass . Determining the most suitable harvesting system requires consideration of many different conditions. The harvesting method depends on the species, on the cell density, and often also the cultural conditions.

The most suitable harvesting system should also take into considering the energy use and availability for the particular conditions. Algae can be harvested by centrifugation, flocculation or froth floatation .Flocculation is a chemical process relying on the interaction of negatively charged particles resulting in larger particles that can be collected much more easily. Flocculation involves the aggregation of destabilized particles and the precipitation products formed by one or more coagulants into larger particles known as flocculant particles or, more commonly, “flocs” . This process may also assist in the dewatering stage of algaebiomass production.Centrifugation is an effective way of harvesting algae but can result in high energy use due to the need to use electrically operated mechanical devises known as decanters. Centrifugal processes rely on the generation of a centrifugal force which acts radially and accelerates the movement and separation of particles based on density difference between the particle and the surrounding medium.Froth Flotation is often used in waste water treatment as a separation method of unwanted contaminates, but now offers a method for removing desired algae particles. Flotation is a separation process based on air or gas bubbles adhering to the particles, which are then carried to the liquid surface where they can be separated, usually skimming . This process also requires the use of electrical aerators which can be energy use extensive.

Many other forms of algae harvesting have been trialed with some success,some of which are used in combination with flocculation, centrifugation or froth flotation. In this case a combination of flocculation and centrifugation shall be proposed as the most reliable method of harvesting. The harvesting elements of ten used in large-scale production first include natural settling of the culture followed by a primary harvesting step using flocculation while secondary harvesting uses a centrifuge . Chemical flocculation is a method of reducing harvesting cost by minimizing the energy required in the centrifugation process due to the flocs being more dense and responding more readily to centrifugal forces. The resultant flocs are generally easier to recover by the traditional solid –liquid separation techniques. Different species of algae react differently to the chemicals used for flocculation requiring the some alterations in chemical types and ratios. All algae strains do not respond the same to a particular chemical,hydroponic nft system so tailoring will be required to fit the organism being harvested . Care must be taken when choosing chemical for flocculation as negative effects may occur as a result of remaining residue.

Negative impacts have been countered in a number of ways,including the use of degradable biopolymers as flocculants  and electro flocculation where no flocculant is directly added.Inorganic and organic flocculants have been trialed in the microalgae harvesting process with indications that organic flocculants have less negative effects. It is reported that these biodegradable polymers do not contaminate the biomass as much as inorganic coagulants and that cationic polymers are superior to anionic and neutralones. Centrifugation combined with some flocculation is currently the most common and reliable form of microalgae harvesting but requires a lot of energy and expensive equipment. It is flexible and can run continuously to handle huge volumes to scale .Decanter type centrifugation devices are proving to be efficient and effective.The decanter or scroll centrifuge is one of the most promising centrifugal devices for the recovery of microalgae as they can operate continuously, have high capacity and lower maintenance requirements .