Economic analysis
Cost is a key factor in the selection of alternative waste treatment systems. The total cost includes capital cost for equipment, operation (mainly energy consumption) and maintenance costs. Therefore, the cost estimation was conducted through three parts:
equipment cost; operation and maintenance costs;and revenue. Miscellaneous cost (such as installation,piping, etc.) has already been included in the capital cost. The major cost of the treatment units was previously reported for a 300-pig farm operation
(Yang et al., 1994). This was used as the basis to estimate the treatment units for the pig farms with different sizes, as shown in Table 9.
The operation and maintenance (0 & M) costs,including electricity, labor and ontingency are presented in Table 10. The maintenance costs decreased by the value of recovered gas, as energy,and nutrients (N, P), as fertilizer. The methane gas was converted into a potential energy value (i.e.7.77 Meal/m” at 30°C and one atmosphere pressure).Based on the price of natural gas in Hawaii