<div><p>Harvesting is one of the key challenges to determine the feasibility of producing biodiesel from algae. This paper presents experimental results for a cost-effective system to harvest <i>Chaetoceros calcitrans</i>, using natural sedimentation, flocculation, and inducing pH. No efficient sedimentation of microalgal cells was observed only by gravity. By alkalinity-induced flocculation, at a pH value of 9.51, 86% recovery of the cells was achieved with a sedimentation rate of 125 cm/h and a concentration factor (CF) of 4 (volume/volume (v/v)) in 10 min. The maximum photochemical quantum yield of photosystem II (<i>F</i><sub><i>v</i></sub>/<i>F</i><sub>m</sub>) of concentrated cells was almost the same as fresh culture (0.621). Commercial flocculants, aluminium sulphate and poly-aluminium chloride (PAC), were also successful in harvesting the studied algal cells. Optimum concentration of aluminium sulphate (AS) could be concluded as 10 ppm with 87.6% recovery and 7.10 CF (v/v) in 30 min for cost-efficient harvesting, whereas for PAC it was 20 ppm with 74% recovery and 6.6 CF (v/v). <i>F</i><sub><i>v</i></sub>/<i>F</i><sub>m</sub> yields of concentrated cells with AS and PAC showed a 1% reduction compared to fresh culture. Mg<sup>+2</sup> was the triggering ion for alkalinity-induced flocculation in the conditions studied. The rheology behaviour of the concentrated cells was Newtonian with values between 2.2 × 10<sup>−3</sup> and 2.3 × 10<sup>−3</sup> Pa s at 30°C.</p></div
