There is generally some water wastage in using RO water purifiers. I am also using the RO water purifier for the last ten or more years. By experience, we know how much may be the wastewater coming out when we operate the machine to fill the output storage tank. So we keep a large vessel at the wastewater outlet and collect he full quantity and use it for various purposes like vessel washing, floor mopping, and other miscellaneous use.
The wastewater quantity depends on the inlet water quality and output water standard kept. Suppose your water output standard is set to say 50 ppm-level. Now if the input water TDS is 100ppm, the purification load is less that is, TDS has to be brought down from 100 to 50. But if the input water TDS is 300 then it takes more load and generates more water to arrive at 50 ppm.
If your RO water purifier has a TDS control valve/switch /screw, then you can adjust yourself by trial and error or getting guidance from the RO technician. RO technician will have the TDS measuring meter and he can tell you the TDS level of input and output water. Some technicians set the output level TDS to very low levels like 10 or 15. If it is so, please ask them to change it around 40-50. This will give good taste to the water. Low TDS gives some bitter taste to water.
You can also avoid more wastage by scheduling the power 'On" time to the optimum. From your experience, you know what will be a day's consumption of purified water. Fill the storage tank at one stretch. Keep a large vessel at the waste outlet end. Collect the wastewater in the vessel and use it for purposes like watering potted plants, mopping the floor, washing vessels, etc. This will save water waste.
After the output storage tank is full, power off the machine. Power it again only when the storage level is low. Again fill it one stretch as earlier.
Nowadays' as per guidelines RO water purifier manufacturer has come with 'Zero water waste' purifiers. You can buy them. Or you can contact the manufacturer if he can upgrade your existing purifier to a zero-waste grade by some addition or technical tweaks.
Call the technician. Let him do servicing. Ask him to check the RO filter.Ask hime to check and adjust the TDS levels.