HARVESTING MICROALGAL CELLS BY CONTINUOUS DISSOLVE AIR FLOTATION

Cell biomass harvesting is one of the key segments when culturing microalgae in large scale. Although many traditional methods such as centrifuging, filtration, flocculation and sedimentation can be applied to collect microalgal cells from culturing medium, their recovery efficiencies are all relative low due to low concentration and density of microalgal cells. Dissolved air flotation (DAF) is a novel solid-liquid separation technique, which is characterized of inducing fine gas bubbles mechanicallg to form bubble-cell aggregates, lowing density of agglomerates and forcing them to liquid surface thus realizing cells harvesting process. In the past decades, DAF has been applying to many biochemical separation fields such as enrichment of fermentation product, but there are little reports of its utilizations in microalgae harvesting. This paper focused mainly upon the feasibility of harvesting microalgal cell biomass by continuous dissolved air flotation with Spirulina pltensis as model organism, and the influences of a series of operation conditions such as recycle ratio, feeding concentration, saturation pressure and detention time on harvesting efficiency were evaluated in order to provide useful guide for microalgae culturing in industrial scale. The experiment system consisted of air compressor,saturation tank and flotation tower. The saturation tank,which was made of stainless steel, has a volume of 50L, and its safety operation pressure was in the range of 0.1—0.6 MPa.The flotation tank made of acryl glass had a diameter of 3.2 cm and a length of 50.0 cm. Harvesting Spirulina pltensis by DAF under continuous operation mode was performed as following firstly adjusting the flux rate of peristaltic pump and needle valve respectively according to set recycle ratio, after which simultaneously inducing saturated water and pre-flocculated cell suspension from the bottom and middle part of the flotation tank respectively. The concentrated and diluted cell suspensions were withdrawn from the top and bottom of flotation tower separately,the cell concentration of them were assayed until it reached steady state. The main results obtained from experiments were: ① Good flocculation behavior, which is beneficial to cell recovery, could be deduced by adjusting the pH of the algal suspension to 11.5—12.5. This method to flocculate alagl cell was much superior to commonly used cationic polyelectrolyte. Flocculation percentage of Spirulina cells reached 80% after 30 min of treatment by pH adjustment, which corresponded to the result of treatment under 100 mg/L of Al3+for 60 min. ② The recovery of spirulina cell increased with the augment of recycle ratio, saturation pressure and detention time. When recycle ratio was elevated from 0.06 to 0.22, harvesting efficiency increased from 2% to 34% after 30 min of treatment. When saturation pressure was raised from 0.25 MPa to 0.35 MPa, cell recovery ascended from 26% to 36% after 30 min of treatment. ③ When feeding concentration was 0.799 and 0.211 (OD560), Cell recovery was 14% and 36% respectively after 30 min of treatment. This phenomenon demonstrated that the diluter the feeding concentration was, the high the harvesting efficiency could be, which suggesting that the DAF would be more effective when it is applied to dilute solid-liquid systems. ④ A macro-dynamic model that could well fit the experimental data was proposed.