For each of the 5 filtration pressures ranging from 10 to 60 kPa shown in Fig。 7, an optimized cycle time and a minimized dewater- ing cost can be determined。 Plotting these minimized dewatering costs and the optimized cycle times with respect to the corre- sponding filtration pressure helps identify the optimum operating conditions and the minimum dewatering cost。 As shown in Fig。 8, the minimum dewatering cost of 0。93 US$/h can be achieved by operating the filter at the filtration pressure of 20 kPa, and at the rotation cycle of 40 s。

A contours plot for dewatering cost levels covering a wide range of the operating conditions and the major range of the simulated dewatering cost from 0。93 to 1。30 US$/h are shown in Fig。   9。

Two cost-sensitive areas A and B were found in the domain of operating conditions。 Operating the filter in area A comes with great energy expense from the greater rotational resistance experi- enced by the filter。 While operating the filter in area B, with a low filtration driving force, much more capital investment for the pro- cessing is required, rendering the dewatering process less cost- effective。 In view of these considerations, the filter should operate with at least 10 kPa of filtration pressure, and with a rotation cycle upwards of 20 s。

This identified cost-sensitivity is also valuable for offering bet- ter control over the operating variables。 For such dewatering pro- cesses, it is very difficult to maintain the  operating  conditions such as the vacuum pressure completely constant。 Normally, the pressure would fluctuate in the process within control tolerance。 Fig。 9。 suggests that a positive deviation relative to the optimum operating conditions (p > popt。 in the pressure, and T > Topt。 in the cycle time) would incur less increase in dewatering cost。 A more favorable control scheme can be formulated with this cost- sensitivity understanding。

Table 3

A report of process techno-economics。

Item Amount Unit

Feed 100 m3/d

Algae conc。 20 w/w%

Capital  investment 54。73 kUS$

Equipment 23。69 kUS$

Installation 14。21 kUS$

Infrastructure 11。85 kUS$

Project life-span 20 yr

Capital  depreciation 2。74 kUS$/yr

Power 5。59 kW

Energy consumption 176。3 GJ/yr

Energy cost 2。45 kUS$/yr

Labor 3。0 kUS$/yr

Maintenance 0。25 kUS$/yr

Annual  dewatering cost 8。23 kUS$/yr

5。6。 Techno-economics  of  the algae-dewatering

The list of optimal parameters arrived at through techno-eco- nomic evaluation of the algae-dewatering process are summarized in Table 3。 For processing 100 m3 of algae feed daily, the total cap- ital investment for the project is 54。73 kUS$ with a process life- span    of    20 years,    and    the    annual    capital    depreciation   is