On my rodi unit there is a screw u can turn to control the flow of the waste water ? What is it for it is a second hand unit tempted to just buy a new unit from bulkreef cause the unit is slow takes a day to flow ten gallons
What brand and model? Any photos? Is it a needle valve on the waste line that you can adjust? Most systems are non adjustable or some better systems have a capillary tube you can trim when new to set the waste ratio.
If you are looking for a new system I would highly recommend Spectrapure or Buckeye Hydro, both are better performers than BRS systems at a lower cost. Love BRS chemicals but their RO/DI is just average.
Aquatic Reef Systems has been out of business for 10 years or more. They were a good company and I used to buy a lot of my supplies from them but there were some family health problems so they folded and the customers were picked up by PurelyH2o. That is another vendor I often recommend along with the first two.
Even if you replaced the filters and membrane your housings and fittings are old enough I would suggest a new system as plastic gets brittle and degrades over time.
You would adjust it until your waste ratio is 4:1. Use a measuring cup and a clock or watch and measure your waste flow for exactly one minute then do the same for the treated flow and adjust the waste so it is as close to exactly 4 times the treated flow as possible.
Use only cold water to feed the RO with the faucet or feed water valve wide open to get the highest pressure and if you have softened water it is better for the RO membrane.
The next thing is you need a handheld TDS meter to monitor the TDS to see if the membrane is functioning or not. If it is good it should be removing around 96-98% of the tap water TDS. If it is much less than that the membrane is fouled and no good.