Is there an equation for calculating the estimated cost of a municipal and industrial wastewater treatment plant, when the volumetric capacity of the plant is known?
Likewise, what is the equation for working out the area required for the plant, when capacity is known?
As such there is no equation available to give the cost of Sewage / Industrial Waste water treatment. However many efforts are made to develop the mathematical models to predicts the cost. The cost mainly depends on Biological oxygen demand, suspended solids, quality of treated waste water and the site conditions. In past I did some work on financial costing of sewage treatment plant. For further details please contact on rvsveipl@gmail.com. Prof. Rajendrakumar V Saraf, Chairman Viraj Envirozing India Pvt. Ltd. Pune, India
There is an embedded design tool here on The Water Network that will provide a free design including much moe than simple parameters that you ask for https://wwtpdesign.thewaternetwork.com/
There is a software; CAPDET Works by Hydromantis of Canada can provide cost estimate for wastewater treatment plant "accurately" for North American locations.