Basic FO MBR design for greywater recycling
Okay, so the electrical wiring is not shown here, nor is the disinfecting and clean water storage.
Basically let’s start with following the flow of the water. The water comes in, and the digestion tank is the surge tank, so the water level in the tank changes quite a bit. The float switch lets the programmable logic controller know when the water level is way down there, so it can turn off the water pumps, and that’s the MBR’s empty condition. The reason you can do this sort of thing, is that when the water is removed, the MLSS of course goes way up, and would be too high in a UF MBR, but for FO it is okay. Secondly, there is not really any need for more than one digestion chamber, since the FO membrane keeps 98% of the solutes in the digestion chamber.
The other float valve is to turn off the water flow to the taps and shower if the MBR is full. If there is something like a dishwasher that could dump a lot of water in at once, you’d have to accommodate that somehow, and maybe include an overflow tube.
Then you have the aerator foot. The programmable logic controller (PLC) can turn this on and off by turning the air pump on/off.
Okay, then you have the FO module, as previously discussed. It probably can’t withstand any real pressure differential across it, so the dirty water side and the draw solution side have to be about the same pressure. To accomplish that, the FO module is mounted in it’s own container. If the water level in the container gets too low, clean water gets let in from the clean water supply line. If the level gets too high, it spills over the edge into the digestion area. And then there’s just the recirculation pump to keep dirty water flowing around the membrane module. This could maybe be accomplished with an airlift thing, too, but when the water level is low in the digestion chamber, it’s a fairly high height to airlift to.
The draw solution (DS) side has a similar level regulating mechanism, except there is not really any need to compensate for a water level that is too low because that shouldn’t ever really happen unless there was a leak in the pipes or something, though you could add a float valve if you wanted to. The water gets pumped onto one chamber, flows through the membrane, then into the second. If the level is too high in the second chamber, if overflows in to the buffer chamber.
The buffer chamber has a float switch that could either turn the RO pump on automatically, or could be monitored by the PLC.
You could optionally add some sort of mlss sensor, which tells you when it is time to empty some of the sludge. A fairly small amount of sludge is produced anyway, and control of the mlss is not very critical, so a separate sludge collection chamber is optional. You could just check the mlss manually every couple months with one of those specific gravity things things they use for checking the density of beer, or even visually.
The FO membrane should reject 97% of all sorts of stuff, and the RO membrane will again reject something like 98% of all the different contaminants, so this could end up producing pretty clean water! Any polishing steps could be optional. You should have some sort of chlorination and maybe a way to make extra sure the water is redundantly disinfected, because the RO and FO membranes can get damaged, letting bacteria through. But that would be a separate system.
Anyway, the great thing about this is how wonderfully small it is. It could be 100 liters or something without trying. It would take more power than a UF MBR system, though, no doubt about that.