Hi hazic,
Far easier to duplicate what the circuit and sensors are doing yourself, then you can do what you want with the signal.
I have seen someone interface to the connections on an LCD display, but it took a long time to do, and they had no choice.
Just looking at the board, it can't be complex. TDS will be total dissolved solids, so it is looking for dissolved salts and will be measuring conductivity. If you look around the web, you'll find some resources for dissolved salt measurement. A few devices I've seen use carbon electrodes.
It's going to be far easier to reproduce what your circuit is doing, then put your own analogue signal into your own microprocessor system - far easier. That way, you'll only have to reverse engineer the relatively simple analogue measurement system. I suggest that you try a varied of resistors, or a resistor substitution box on the electrodes t get an idea of the calibration curve it uses.
Hope that helps.
Cheers, Colin