2.1. Wastewater and Microalgae species
The microalgae selected in this study was Microcystis sp .. The effluent from anaerobic fermentation tank of a pig factory in Fengxian District, Shanghai was used as the culture substrate. Under the microscope, the main characteristics of Microcystis sp.cultured in wastewater were chlorophyll-a (Chla) of 2533 mg/m3, and the density of algae was about 2×109 cells/mL. The size of algal cells was 10~400 μm, and they were clumped.
2.2 The device of microbubble air flotation test
The microbubble air flotation equipment is composed of microbubble generating device and air floating column. Among them, the microbubble generation device is composed of gas-liquid mixing pump (Nikuni, 20 FPD04Z, flow rate: 0.9 m3/h), the water inlet tank (plexiglas, length × width × height =200×200×500 mm3, thickness =5 mm) and the dissolved gas tank (stainless steel cylindrical tank, diameter =110 mm, height =810 mm,). The air floating column (length × width × height =100×30×500 mm3) is made of plexiglas with the thickness of 3 mm. It has good light transmittance and is convenient for dynamic observation of microbubbles. Each part of the test device is connected by PVC pipe (φ=25mm), and the pressure reducing valve is installed between the microbubble generation device and the air floating column. The design of flexible pipe joint is adopted among the above three parts, and the distance between the pressure reducing valve and the air floating column is adjusted by changing the length of the release pipe. The whole test device is also equipped with pressure gauge, gas gauge and flowmeter, which show the dissolved air pressure, inlet air flow rate and inlet water flow rate, respectively. The flow chart of the test device is shown in Figure 1, with the red arrow indicating the flow direction.