What is the best range for TDS in potable water?

Hello everyone. In our project area in Nepal we have a community water supply system. We found out that the TDS level in the water is 400. I have researched it on the Internet and I am confused by the permissible range of TDS for drinking water. Can someone tell me what the permissible range of TDS for drinking water is? Is low TDS in water safe for drinking water? If the TDS is higher, how should we treat the water? We need a cost-effective solution to treat the water.  Thank you.