Separation of fine organic particles by a low-pressure hydrocyclone (LPH)

• The separation performance of a low-pressure hydrocyclone was tested at the low pressure from 1.38 to 5.56 kPa.
• The separation performance was affected by inflow rate, bypass ratio and feed concentration.
• The maximum separation efficiency predicted by the RSM model was provided.

The separation performance of a low-pressure hydrocyclone was tested using fine organic particles from 1 to 700 μm. The dimensions of the low-pressure hydrocyclone were an inflow diameter of 30 mm, a cylinder length of 575 mm, an overflow diameter of 60 mm, an underflow diameter of 50 mm, a cylinder diameter of 335 mm and a cone angle of 68°. The low-pressure hydrocyclone was operated with a lower inlet pressure (average 1.38–5.56 kPa) that could be maintained under water level differences that ranged from 17.5 to 53.5 cm between the water surface of the feeding mass cylinder and the middle of the inlet pipe of the low-pressure hydrocyclone. By varying the inflow rate, underflow ratio and feed concentration, the separation performance of the low-pressure hydrocyclone was affected. The separation performances were determined from total separation efficiency and grade efficiency. Separation performances were determined according to the different inflow rates of 400, 600, 800 and 1000 ml s−1 and their respective underflow ratios that ranged from 5% to 30%. The maximum total separation efficiencies for each inflow rate were 41%, 46% and 46% at 400, 800 and 1000 ml s−1 inflow rates, respectively, and at underflow rates of 30% of the inflow rates. In addition, a total separation efficiency of 46% was employed at 600 ml s−1 of inflow rate and with an underflow rate of 25% its inflow rate. As the feed concentration increased from 25 to 150 mg l−1, the separation performances were gradually decreased. For the fine particles ranging 1–200 μm, the grade efficiency was higher at the higher inflow rate (higher than 600 ml s−1) and higher underflow rate. However, for the coarse particles ranging 400–700 μm, the grade efficiency was higher at the lower inflow rate (lower than 600 ml s−1) and higher underflow rate. The cut-point (d50) values ranged from 30 to 200 μm for a feed size range of 1–700 μm. The Response Surface Method (RSM) model predicted an optimum operating inflow rate and underflow ratio of 721 ml s−1 of inflow rate and 30%, respectively, for the low-pressure hydrocyclone at a maximum total separation efficiency. Based on these findings, further design and operating adaptation of low-pressure hydrocyclones used for fine solids removal in recirculating aquaculture systems is expected.