Comparing Operating Costs
Comparing operating costs for stainless-steel and single-use systems is more complex as can be seen in Tables 3a, 3b, and
3c. Operating costs are calculated for the first 10 years of plant operation beginning in 2010. Operating costs such as water,
personnel, and utilities should be escalated at a percentage rate that reflects local market economic conditions.
Table 3a. Rates used to estimate operating cost savings for single-use systems: Items for which costs are assumed to increase
Plant capacity (number of fermentation batches) increases over the 10-year period, which is typical for many facilities. Plant
capacity change over time is an important part of operating cost evaluation because capacity directly affects the cost of
the single-use systems. Many operating costs for conventional stainless-steel systems also increase with increasing plant
capacity (water, CIP chemicals) but some, such as the cost of routine maintenance, commissioning, validation, HVAC, and capital,
Table 3b. Rates and quantities used to estimate operating cost savings for single-use systems: Items which are assumed to
be constant or which only affect the first year.
Moreover, the cost of WFI used in 2010 is five cents per liter. In much of the published literature, the cost of WFI is taken
to be anywhere from 50 cents to one dollar per liter. Our experience is that these numbers overestimate the cost of WFI from
a bulk generation and distribution system and unfairly slant many operating cost evaluations in favor of single-use systems.
Five cents per liter is an all-in cost that includes the cost of capital, maintenance, validation, quality assurance, utilities,
and waste-water discharge. It can be argued that the incremental cost of WFI is even lower because the facility is likely
to have a bulk WFI generation and distribution system anyway to provide water for process as well as CIP of any remaining
stainless-steel equipment. In this article, however, the price of WFI covers the total capital and operating costs of the
Table 3c. Estimated operating cost savings for single-use systems
Significant savings are projected in commissioning and qualification for this alternative, which is typical for single-use
applications. We apply this as a one time savings in 2010 only. Experience is that ongoing validation costs for single-use
and stainless-steel systems are similar.
The simple scenario described above, using single-use bags for buffer hold, realizes significant labor savings as a result
of the way the single-use storage bags are used. For the stainless-steel case, each buffer was prepared for a single purification
lot. In the single-use case, buffer is prepared in larger batches and aliquoted into single-use bags for multiple purification
lots. This reduces the number of buffer preps required per week with corresponding savings in personnel, QA, CIP, and dispensing.
We take credit for savings only when we have reduced operating hours sufficiently to eliminate a full time employee (FTE).
The thinking is that saving only a few hundred hours doesn't reduce operating costs but merely lightens work schedules.
As Table 3c shows, operating cost savings for this scenario vary by year with a maximum savings of $383,000 in 2010 and a
loss of $57,000 in 2013. The high savings in 2010 reflects the one time credit for savings in commissioning and qualification.
For this alternative, savings increase as plant capacity increases, indicating the replacement cost of the single-use systems
is less than the CIP and labor costs of the stainless-steel systems.