A lot of heat exchangers run at about 10 degree temperature difference. Subtract 10 degrees from 40 degrees to allow a temperature gradient and it will probably become futile to try to extract any heat.
About the only practical use may be to preheat cold air entering the building.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Can also be used as a heat source for a heat pump; more efficient to use a heat pump than an electric resistance heater. For example, at many power plants the wastewater stream can be eliminated by use of a ZLD zero liquid discharge system. The electric heaters needed for that system's falling film evaporator and crystalyzer are inefficient if configured as resistance heaters, but not such a bad penalty if configured as heat pumps.
Yes. It depends on the simultaneous heating and cooling loads of a facility. Neutralized waste water can be filtered and heat extracted by plate and frame heat exchanger. Common uses are to preheat well water and/or makeup air.