In the present scenario, it is assumed that three workers are required to work on a three-shift rotation for an industrial project for dewatering 5000 m3 algae feed daily。 The scale of this studied demonstration  project  is  50  times  smaller  than  this       referred

0 20 40 60 80 100 120 140 160

Filter rotation cycle (sec)

Fig。 3。 Dependence of algal cake-layer upon the filter rotation cycle and the filtration pressure。

72 P。  Shao  et  al。 / Chemical  Engineering  Journal  268  (2015)     67–75

Table 1

Effect of pressure on the porosity of compressible algae  cake-layer。

as the working pressure is enhanced from 10 to 60 kPa。 Overall it appears better to conduct the dewatering at a lower pressure so as to avoid significant compression of the cake-layer and avoid a level of filtration resistance which hinders   efficiency。

Filter-based algae-dewatering is a continuous process, and the cake-layer must be removed from the filter within the cake-dis- charging zone。 It is therefore required that algal cake-layer formed on the filer surface be thick enough to easily remove the cake-layer

in the algae-discharging zone。 In Fig。 3 it is shown that the algal

0 20 40 60 80 100 120 140 160

Filter rotation cycle time (sec)

cake-layer thicknesses are mostly in the range of 10–50 mm, suit- able for effective removal by the separator  blade。

5。2。 Equivalent pore size in algal cake-layer and the minimum dewatering pressure

The estimated equivalent pore size and the minimum pressure for cake-layer dewatering are illustrated in Fig。 4。 At a higher filter- ing pressure, it is found that the cake-layer pore size is narrowed with a denser algae packing, which is caused by compression, in consistency with the pressure-dependent cake-layer porosity sum- marized in Table 1。

As indicated in Fig。 4, the minimum pressure for algae-dewater- ing is lower  than  the  filtration  pressure  except  in  the  case  of 10 kPa。 Thus, the filtration pressure is high enough to extract the water out of the cake-layer, and there is no further need to increase the vacuum level, indicating the feasibility of algae dewatering by the RDVF filter。 As shown in Eqs。 (22) and (23), the equivalent pore size is proportional to the algal size, and the minimum dewatering pressure is inversely proportional to the size of algae。 This implies that a smaller algae size makes algal cake dewatering difficult: it requires a higher vacuum level to displace the water out of the net- work pores。 In this case, the cake-layer dewatering conducted at  a

100

Fig。 5。  Processing  capacity  of  the  filter  under  different  operating  conditions with

the filtration pressure, and filter rotation cycle varying from 10 to 60 kPa, and 10 to 120 s, respectively。

same vacuum level as in algae filtration obviates the complex vac- uum system design for the rotary drum filter。 In practice, a small quantity of water will be retained in the cake-layer after dewater- ing。 This residual water is about 1–2 w/w% of the wet cake, which is removed in the subsequent process of   algae-drying。