You might want to try a ceramic material, if you like you could mail us the parameters and dimensions, we could have a look. As long as there is little fluid.
Engineering@ceratec.nl
www.ceratec.nl
It depends strongly on what kind of waste water it is, when there are not too many or to large solids in the fluid, no problems should be expected witd a standard bearing cartridge, the coupling is made up of non wearing parts, so the bearing will be the bottleneck.
You might want to try: formatec in the netherlands, i know for a fact that they have produced more than one product made of black zirconia. One of the products was a letter opener, very nice peace of work!!
www.formatec.nl