Reverse Osmosis

The answer would need to be based on the initial water source being put through the RO system. (Both for yield and impact on septic system)

If you're using an RO system, I presume that you already have a water analysis that has caused you to take that decission and this should be directed to the manufacturer for their response.

No, the waste water is not harmful to your septic system.

You will waste around 3-12 gallons per gallon you produce. Higher quality feed water lowers waste water output. The manufacturers play down this number, expect at least 6 on well water and closer to 3 on most city water. I have however heard of 12 on city water.

The number of filter stages affects water quality. I recommend a minimum of 4 stages; I use a 5 stage system.

its called "Brine" and its fine for a septic, its exactly like the water out of your tap. sending it through an RO system doesnt change it, it separates larger molecules from smaller ones, the brine is just a concentrate of the stuff in your water that you could do without. an RO filter adds nothing to the water. you wouldn't want to drink the brine but no, it wont harm your septic.

Do you want to drink it, use it in your aquarium, or grow pot with it?

Frankly, my opinion is that residential city tap water does not need RO filtration.

Filter your drinking water with a particle filter. Wash with what comes out of the tap.

If you are using untreated well water, then, maybe, RO is called for.

The separated water or filtered water simply gets rejoined with the reject water in a septic tank. There is generally no increase in ionization from raw water to R/O water once both are returned to a septic system. Bear in mind that any water in sewage will likely gain in ionization from other sources. I am assuming this is a simple low output unit designed for a single point of use.

If the water is whole house R/O, you may want to get an approval from the health board or whoever approves usage. You may be asked to dispose of the reject water in a holding tank or other method. That authority should be able to verify proper disposal. A recent thread in CR4 was based on not allowing more sodium in the discharge water and there may be specific septic use guidelines for your area. Hopefully you will know (accredited test) about the major ions or whatever specific parameter you are trying to remove.

If the septic system receives the RO reject as well as the used RO permeate water, then there will be no net effect on the septic system; all it is doing is collecting and re-blending the waters in that case.

I agree to a point. it is fine for small systems that require small amounts of water to use. But suppose you are treating whole house water and the raw water is salty to start, and you have a R/O system designed to deliver 300 gallons per day with 90% rejection. That means in order to deliver the treated water you may be required to pump and treat 3000 gallons of water. 2700 gallons will be directed to the septic system over normal use and may not be able to handle the load or the level of salts in the reject. The user should contact the local authorities to determine if his discharge is acceptable. I am assuming that is the reason for RO treatment in the first place but there could be a metal like lead or cadmium or fluoride or arsenic or some other parameter that is being removed. Perhaps that comes an issue during a building permit and it may be cheaper to use water from a better source.

The reject water in such a case would be 100%/90% as concentrated as the feed water, and will recombine with the permeate following its use to yield the feedwater again with the substances coming from the used permeate. If the septic system can carry on with the feed water, then the combined stream will be merely the feed water with the used permeate.

90% reject would be unusually high even for seawater RO.

Without a raw water analysis from the original poster one can only speculate on the levels of minerals. In such a case, it might be cheaper to use bottled water for consumption, save the feed water for flushing the loo, and do away with the RO plant altogether, perhaps.

GA. I am getting too old to think straight and was not using RO as my basis for high reject. Thanks. I guess that may work for a dilution factor. You are right the volume of water produced and recombined will still be OK for most systems.

There is some faulty reasoning in all this somewhere. If you look at a "normal" home RO system, they are designed to recover 50% of the inlet water. Average feed concentration under steady-state conditions will be 1.5 X source, and the permeate with new membranes will be <2% of the average feed concentration, depending on the composition of the dissolved solids. Thus if the source water is 1500 ppm TDS (really bad tasting water, fairly brackish) and the system recovers 50%, then average feed is 2250 ppm TDS, and the typical permeate (product) will be < 45 ppm TDS, which is well into the good drinking water range. Usually the RO concentration outlet tube will have a tiny orifice plug inserted into the tubing as a flow restriction.

As to the volumes of water required, yes, it take twice the amount of water sourced as utilized, the remaining 50% goes to waste (or is diverted to irrigation purposes). The real wastage comes into play when these units are not in "service", but are idle, and there are no automatic feed shutoff valves present to stop inlet flow. No permeate is produced (the bladder tank is full), and all of the incoming flow (which is less due to no production, same pressure applied as before, and the concentrate restriction limiting rejected flow to waste.

Not very relevant but my only experience of this was on a tour of a brewery. Some of the beer they sold was alcohol free & they had an osmosis plant to extract the alcohol. The waste water was about 3% proof & quite refreshing but they were not allowed to sell it.

What a ridiculous waste that is? Why ferment the beverage if not for the alcohol?

ON the other question: RO concentrate from a simple residential system is typically about 50% recovery when the system is active, so it's about 2x concentration of whatever was in the feed water. When these systems (that are not sophisticated at all) do go idle (pressure tank is full), the feed continues to run through the system with no water recovery as permeate. This is the same as a leaky faucet just adding water to the septic system 24/7. Again, what a waste. These home systems are not really optimal toward water conservation, unless you have a plan to completely use the resource (all of the purified water the system is capable of producing), and also find an alternate route for the waste water (a flower bed??).

Who cares... Its MY well. What does not get used goes into the Septic System, and then eventually back into the well. Hopefully much later.

Gawd, some people have funny ideas of "efficiency"!

I have used two ROS on my septic system for more than twelve years. There is NO effect on the system. My well water is pretty hard, and destroys anything but copper and plastic.

The "brine" that comes off the ROS is not bad enough to do anything nasty. I drop easily 10 to 20 litres of water for every liter I get filtered. Its my well...its not a problem.

No it is not in any way going to be harmful since the RO system is for residential application only! Whatever the chemical properties of the resultant effluents or wasted water that may be discard directly to the sewer is not highly concentrated to cause any harm to the sewer. Additionally, most of the product or treated water once used will be again mixed with whatever wasted that was initially rejected by the RO unit.

You may consider too, that the sewer system was initially designed to handle and carry any used and untreated water as supplied in your locality.

An earlier attempt to post has not shown up so I'll repeat it.

There needs to be significant caution in suggesting that there will be no effect. in many situations the impact will be negligible, but there are instances where the impact can be significant and without full detail we cannot assure the OP that "all is good".

If the RO system is supplying total house water, then the following situations might be an issue. The total flow will be the normal household volume plus the reject water flow from the system.

The increased total flow through the septic might mean that detention times are not adequate to break down the "nasty" components of the waste stream, thus releasing untreated materials.

The increased flow without necessary additional nutrient loading could significantly impact the effectiveness of the system and it might actually cease to function. (Again leading to release of untreated materials.)

The increased flow could also lead to increased sludge accumulation in the system. Things like iron precipitation, meaning that more frequent maintenance is necessary (pump-out) or through neglect again failure of the system.

We run a 2ML per day system (2 micron) treating borewater. The reject water from that system is impacting the function of the local sewer treatment facility. Increased flow, increased sludge and lower nutrient levels have meant significant process changes were needed. This plant is the water source for the community that flushes to that facility so it is really a larger model of the single house unit that is hinted by the OP.

If the water is sufficiently bad that RO is necessary, then it is possible that the addition of the reject water to the septic flow can have adverse impact. Suitable qualified technical advice should be obtained.

Easy to say. Likely to be expensive. And really, why?

Fact is.. water with a certain amount of minerals comes into the house. The flow splits...becomes clean water and mineralized water. As I pee through the day, the clean water mixes back into the septic or sewer. In other words, what goes into the house comes back OUT of the house. The only extra minerals in my septic come from the salt cod I devour on a regular basis. If I don't use all the RO water, it bypasses. There is a little energy loss due to pressure head drop in my pump system through the day,...but really...it causes the pump to kick on what...once or twice extra per week?

If the RO system caused the pump to cycle once an hour...I would worry a bit. Otherwise, I don't really see why you should hire on an engineer at whatever rate to check out a system which was installed correctly in the first place.