Algae Harvesting With a High Rate Water Processing Technology: Dissolved Air Flotation

The algae-to-biofuel industry requires a cost effective means for separating a relatively dilute algae suspension from a large flow of water. Ideally, algae recovery should be economical and result in a thick algae concentrate with minimal extraneous and inert materials that would detract from the value of the resource. To a large extent, the economy of microalgae production depends on the method employed for harvesting and concentrating the algae. This paper discusses some of the possible algae separation techniques with an overview of the advantages and disadvantages of each. Emphasis is placed on the dissolved air flotation process because it still remains the most effective means of separating a relatively low concentration of algae from a large body of water. Some of the important design features of dissolved air flotation that promote effective algae separation and harvesting, as well as thickening of the separated algae, are discussed. The efficiency of recovery is a significant issue because it relates to the economics of the separation technique. However, it is not necessarily of overriding importance since the water from the separation unit can be recycled back to the algae growth units. The efficiency of recovery and operation of the separation technique depends to a large extent on the means by which the algae is conditioned. The paper discusses various inorganic and organic chemicals for algae coagulation, to facilitate algae separation by dissolved air flotation, or by other means. Inorganic coagulants tend to be the most effective, although some natural and synthetic polymers are effective. Chitosan is a natural polymer that does not detract from the animal feed option for the co-product from algae biofuel processing, although it does introduce significant quantities of inert material. Other organic polymers that are GRAS certified should be used preferentially to those products that are not certified. Source: WEFTEC 2009 Proceedings