Perhaps you could combine an off-the-shelf commercial dishwasher with:
1 - a solar water heater (might as well have this work on the main hot water lines and not only the one the dishwasher uses)
2 - a "greywater" system which basically filters some of the larger materials out of the waste dish water and then has the remaining water feed an irrigation system, say to the trees / gardens around the market (if there are some)
You could try making this water available to market merchants to, say, water their plants -- but this could be tricky since you'd have to pressurize the water somehow (like have a barrel stored on the roof) and fend off health inspectors who might worry that recycled human germs would make their way on lettuce seedlings, etc.
(Ideally, it would be best to use the greywater to flush the market's toilets, since these are likely the worst clean water wasters! But this would definitely be experimental!)
You'd have to choose the dishwater detergent well if you have this system, since you don't want most commercial/toxic ones ("Nature Clean" is a brand I'm familiar with here in the east that is greywater-friendly).
Beyond this, more extensive solar proposals and water-recycling systems might be approaching experimental technologies.
I would guess that simply having a dishwashing system would be considered "radical" by some at the market and so you'll want to start off with more tried-and-tested technologies.
Good luck! - Eric (ewmiller - at - yorku.ca)