The term harvesting refers to the concentration of dilute microalgal culture sus­pension to slurry or paste containing 5% to 25%, or more, total suspended solids (TSS). This slurry can be obtained in either a one-step or two-step harvesting pro­cess. Subsequent processing of the algal paste depends on the concentration of the algal paste. Increased product concentration decreases the cost of extraction and purification, as well as the effective unit cost of biomass. Concentration of algal paste significantly influences downstream processes, including drying. Microalgae are particles that have a colloidal character in suspension. Electric repulsive inter­action between algal cells and cell interaction with the surrounding water provide stability to the algal suspension. Algal cells are usually characterized as negatively charged surfaces where the intensity of charge is a function of the species, ionic strength, and pH of the cultivation media (Taylor et al., 1998). These surface charges are helpful in the growth culture because they assist in keeping the cells in the water column so that they do not settle to the bottom of the pond, particularly in regions of the pond where the water velocity is low. However, the charges pose a challenge to the dewatering process because they eliminate the option of using a simple settling tank (or pond) for harvesting.

Harvesting and dewatering processes can be divided into two categories, namely

(1) those in which the dewatering is performed directly on the algae culture, and

(2) those involving agglomeration of the algae into macroscopic masses to facilitate the dewatering process. The former, which include centrifuges and membrane filters, avoid the complications and costs associated with the addition of coagulation and flocculation chemicals. Processes like flocculation, flotation and gravity sedimenta­tion have acceptable energy requirements but have a fairly wide range of costs asso­ciated with motors and controls.