Formulation and estimation of a correctly specified abatement cost function would be central to policy formulation regarding imposing taxes or user-fees as well as of sharing social cost in the presence of environmental pollution. Often in research, the output of an abatement activity for water pollution appears to us not to have been clearly specified. This activity is very distinct from whatever is the actual product of the plant. We propose in this paper that the output is only and exclusively the reduction in the level of the pollutant in the outflow from the plant and illustrate this with primary data for small-scale factories in Nandesari Industrial Estate in Gujarat, India. The cost function estimates provide evidence of significant scale economies and high marginal abatement cost in wastewater treatment by small-scale factories.