The paper discusses efficiency issues in the public provision of environmental services, in particular waste water treatment. It is shown that in the face of increasing, respectively decreasing returns to scale the zero profit constraint of a cost minimizing public firm induces underinvestment, respectively overinvestment in public capacity compared with efficient allocation between public purification and effluent control by private polluters. X-inefficiency of the public firm counter-acts the inefficiency in allocation arising from overinvestment, and it reinforces the inefficiency in allocation in case of underinvestment in public purification capacity. As subsidy can bring down the user's charge imposed on sources, but it will also increase X-inefficiency. The subsidy counteracts underinvestment but reinforces overinvestment in public capacity.