Optimization of dissolved air flotation technique in harvesting microalgae from treated wastewater without flocculants addition

Abstract

Harvesting is a crucial step in biofuel production from microalgae. The expense of harvesting operation is estimated to be 20%-30% of the total biofuels production cost. Dissolved air flotation (DAF) is a well known technology for separating solid particles from liquid streams such as wastewater. This technology has been introduced for microalgae harvesting in the past few years. However, the high operating cost has been a hurdle to commercialization of this technology for microalgae dewatering. High power consumption for air compression and high cost of chemical coagulants are two main cost items. In this study, the DAF process was optimized without the use of chemical coagulants. It was found that saturator could be running at the low pressure of 3 atm to reduce the power consumption to 0.0025 kW h/m 3 that was a reasonable value compared to similar DAF units that used chemical coagulants. Statistical analysis revealed that concentration of algal solution was the most significant factor to gain maximum removal efficiency. In this study, high concentration of algal solution along with low flow rates of air-saturated water contributed to the unprecedented removal efficiency of 91% without flocculation. V C 2015 AIP Publishing LLC.

Harvesting Marine Microalgae Nannochloropsis sp. using Dissolved Air Flotation (DAF) Technique

“…Data collected from the same study also reported that low power consumption (0.0025 kWh/m 3 ) was achievable when no flocculant was added during the pilotscale DAF. Hence, it was able to reduce the operating cost (Niaghi et al 2015). However, these studies (Niaghi et al 2015;Wiley et al 2009) focused on algal removal from water (considered wastewater) with lower concentration than the present study, as flocculant was utilised to increase the particle size of Nannochloropsis sp.…”

“…The former was proven by Wiley et al (2009) where they reported that DAF required high energy (7.6 kWh/m 3 ) compared to the energy consumption of suspended air flotation. In another study, optimal saturator pressure at 3 atm (3.04 bar) appeared sufficient to achieve removal efficiency greater than 90% for algae concentration higher or equal to 500 mg/L (Niaghi et al 2015). Data collected from the same study also reported that low power consumption (0.0025 kWh/m 3 ) was achievable when no flocculant was added during the pilotscale DAF.…”

“…Hence, the finding further proved the probability of collision between bubbles and particles at higher saturator pressure, similar to this study (Gulden et al 2020). The concentration of microalgae within the culture is the most significant factor for maximum harvesting efficiency (Niaghi et al 2015). The initial microalgal culture volumes attempted in this study was to imitate the effect of different initial culture volumes tested in a previous report (Zhang et al 2014).…”

A review on microalgae cultivation and harvesting, and their biomass extraction processing using ionic liquids

“…Dissolved air flotation system generates air bubbles by the means of saturating the culture with compressed air and then discharging the culture at atmospheric condition. This method has been widely used in wastewater treatment but is hindered by high cost due to power consumption and usage of chemicals [68]. Dispersed air flotation on the other hand uses a sparger to generate air bubble which in turn have lower energy demand when compared to dissolved air flotation [69].…”

Physico‐Chemical Processes

“…Dissolved air flotation (DAF) is one of the conventional technologies for separating solid particles from liquid streams. In harvesting microalgae from the treated wastewater, DAF process was selected to avoid using chemical coagulants (Niaghi et al, 2015). High Ethanol could be used to regenerate the used adsorbent.…”