To demonstrate the RDVF filtration process for algae-dewater- ing, a mathematical model was developed to model the filtrate water transport through the algal cake-layer as well as the under- neath filter medium。 The transport resistances of algal cake-layer and filter medium were correlated to their physical properties。 Correlating the cake-layer resistance to its characteristic parame- ters serves to make the model fundamental, straightforward and readily applicable for simulation。 Algae are a soft material, and compression of the algal cake-layer in the filtration process must be taken into consideration in this model, in order to properly sim- ulate the observed behaviors of the algal cake-layer。 A value was determined for the equivalent pore size in the cake-layer, which was a parameter required for modeling the algal cake-layer dewatering。

Process economics is an important measure characterizing the viability of a technology。 Literature survey shows that little work has been carried out on the cost  analysis  of  algae  dewatering [20]。 To determine costs of algae-dewatering with the filter, the process capital and operational energy costs were evaluated。 Pro- cess optimization was performed to demonstrate the potential of reducing this dewatering cost by identifying the best operating conditions。 Analysis of the global dewatering cost, which consisted

of energy, capital and installation, infrastructure, labor and mainte-

through the filter medium to the  low pressure  zone  through use of a vacuum pump。 Algae are thus held and accumulated on the external surface of the filter to form a dynamic algal cake-layer。

The filtrate water passing through the filter and the drain piping is first collected in the vacuum receiver and is finally moved out by a water pump。 The algal cake-layer keeps building up on the filter surface until it is carried out of the feed tank as shown in Fig。 1。 The water contained in the algal cake-layer is removed in the subse- quent algae-dewatering zone under a controlled vacuum pressure during the portion of a drum revolution between exiting the liquid region and prior to arriving at a scraping blade, which removes it from the drum surface。 This dewatered  algal  cake  is  peeled off the drum filter surface and collected for   drying。

3。 Modeling of the constant pressure filtration of centrifuge- harvested algae

3。1。 Transport of filtrate water through the algal    cake-layer

Modeling the dynamic formation of algal cake-layer on the filter surface is a challenge。 Some assumptions should first be made, including (i) neglecting the effects of tangential shear induced by filter rotation in the feed tank, on the formation of algal cake-layer atop the filter surface, (ii) neglecting the effects of mixers in the feed tank, and (iii) assuming all the algae moving toward the filter surface with the water in the filtration process are retained, and build up on the filter surface  to  form a  uniform algal cake-layer on the filter surface。

According to Darcy’s law, the flow-rate of the filtrate can be written as [21–23],

dV A · Dp1 1

nance, suggests that the proportion of costs can be significantly reduced through appropriate scale-up。 Thus, scenarios for   further

reducing this dewatering cost were also explored by considering overall process scale。

2。 An overview of algae-dewatering  process

Algae-dewatering using a rotary drum vacuum filter is illus- trated in Fig。 1。 The rotary drum is partially submerged in the har- vested algal feed。 Water in the algae feed is drawn into the drum