RO waste %

[steveb87]

So I have found out that a 50gal a day has a v high rejection rate say 99% and a 200gal has 96% so the lower gal will produce better quality water. What I need to know is how much waste water is produced by the 50gal and the 200gal is it worth the lower rejection rate to save on water.

 

[seangee]

I can't answer the question on cost because our water supplier announced that we would get a meter a month after I installed my first system (a typical budget 50gpd unit). These supposedly have an actual rejection rate of 75%. I was able to double the throughput by putting in a 100gpd membrane with no noticeable drop in quality. Going higher gave no further improvements. The cost (in water alone) is a lot lower than buying RO from my local MAQ.

The tank system was really frustrating because of the trickle of water it produced. The under sink tank did not do much to help (15l but output dropped to a trickle after around 7l - not a significant amount when you need 200l per week). It also meant I spent most of my weekend getting next week's water change out of the tap.

Late last year I finally upgraded to a "flow through" tankless system at 400gpd. This - according to the manufacturer - produces 2l of waste for every 1l of RO. Water quality is identical, if anything slightly better, but the system has a built in flush which means the filters and membranes last longer and work more efficiently - I suspect that is how the get the lower rejection rate, they flush out less as you go and periodically do a 30 second flush to clear the membrane and filters. This made a massive difference to time and I can now fill a 25l jerry can in around 35 mins, compared to around 2 hours previously. I still collect next week's water immediately after my water change. It also takes up less space under the sink because I don't have to accomodate a tank. My tap water has a TDS of 300 and what comes out of the RO filter constently has a TDS below 10 (0 when I used the DI filter as well).

You can also get systems that use multiple membranes, typically an 800gpd unit has 2 x 400gpd membranes. They work by passing the rejected water through a second membrane. This does not reduce the quality of the output. The high rejection rate is not because the rejected water is bad, it is to flush away the muck that came out of the water from the membrane, otherwise the membranes would get fouled too quickly. I have seen home systems advertised up to 1600gpd with 4 membranes. I could not justify the cost for my use and have a single membrane system.

FWIW this is the system I got https://www.osmiowater.co.uk/catalo...al-surgery-reverse-osmosis-system/category/4/. The difference between the home and dental system is that the home system comes with a re-mineralising filter. I don't trust these because they don't say what is in it and the filter would add less minerals as it got depleted meaning the water parameters would not be consistent. The dental system has a DI filter as the final stage rather than the re-mineralising filter. Since I do not need DI I have now removed this and put a carbon post filter in its place. Not strictly neccessary but I already had a box of 10 in the garage - and I do drink the same water my fish swim in

 

[steveb87]

I can't answer the question on cost because our water supplier announced that we would get a meter a month after I installed my first system (a typical budget 50gpd unit). These supposedly have an actual rejection rate of 75%. I was able to double the throughput by putting in a 100gpd membrane with no noticeable drop in quality. Going higher gave no further improvements. The cost (in water alone) is a lot lower than buying RO from my local MAQ.

The tank system was really frustrating because of the trickle of water it produced. The under sink tank did not do much to help (15l but output dropped to a trickle after around 7l - not a significant amount when you need 200l per week). It also meant I spent most of my weekend getting next week's water change out of the tap.

Late last year I finally upgraded to a "flow through" tankless system at 400gpd. This - according to the manufacturer - produces 2l of waste for every 1l of RO. Water quality is identical, if anything slightly better, but the system has a built in flush which means the filters and membranes last longer and work more efficiently - I suspect that is how the get the lower rejection rate, they flush out less as you go and periodically do a 30 second flush to clear the membrane and filters. This made a massive difference to time and I can now fill a 25l jerry can in around 35 mins, compared to around 2 hours previously. I still collect next week's water immediately after my water change. It also takes up less space under the sink because I don't have to accomodate a tank. My tap water has a TDS of 300 and what comes out of the RO filter constently has a TDS below 10 (0 when I used the DI filter as well).

You can also get systems that use multiple membranes, typically an 800gpd unit has 2 x 400gpd membranes. They work by passing the rejected water through a second membrane. This does not reduce the quality of the output. The high rejection rate is not because the rejected water is bad, it is to flush away the muck that came out of the water from the membrane, otherwise the membranes would get fouled too quickly. I have seen home systems advertised up to 1600gpd with 4 membranes. I could not justify the cost for my use and have a single membrane system.

FWIW this is the system I got https://www.osmiowater.co.uk/catalo...al-surgery-reverse-osmosis-system/category/4/. The difference between the home and dental system is that the home system comes with a re-mineralising filter. I don't trust these because they don't say what is in it and the filter would add less minerals as it got depleted meaning the water parameters would not be consistent. The dental system has a DI filter as the final stage rather than the re-mineralising filter. Since I do not need DI I have now removed this and put a carbon post filter in its place. Not strictly neccessary but I already had a box of 10 in the garage - and I do drink the same water my fish swim in

Great reply, thanks. Thought a 200gpd might not reject as much water as a 50gpd. Going to piggy back 2 or 3 so should reduce waste. My water the last time I tested it was 777us 7.8ph and hard as nails. London water. Again when using cold water will add to the waste rate.