Passage Summary: Tanning leather with biological tools is cheaper overall because, even if the materials cost the same as chemicals, you save a lot of money on cleaning up the mess afterward.

Conclusion: Using biological catalysts for tanning leather is more cost-effective than using traditional chemicals.

Reasoning: While the initial costs are similar when ignoring waste, biological catalysts produce significantly less waste, and disposing of that waste is a major expense.

Analysis: The argument relies on the assumption that the 'nearly 20 percent' reduction in waste is enough to outweigh any other potential costs not mentioned. For the conclusion to be true, it must be the case that the biological process doesn't have some other hidden, massive expense that cancels out the waste-disposal savings. We are looking for a necessary assumption, so ask yourself: what would happen if the biological catalysts required a million-dollar machine to work? The argument would fall apart, meaning it requires that no such prohibitive costs exist.