Towards a better tinyhouse

Inventing to freedom?

comment that should be on another blog

leave a comment »

I tried to post this as a comment on mobile condo, but apparently it exceeds the length limit… How I do go on sometimes, but I’m too lazy to edit it, so I will post it on my own blog instead.

Hi, John,

I agree, we will see is the spirit here. There will need to be some experimenting, I daresay research, here no matter what approach is taken. I agree with your points about the advantages of dry toilets for a variety of reasons, but a bioreactor can have some advantages in greywater department too. I have some posts up about greywater MBRs… The total amount of digestible stuff (total BOD) present in a greywater system vs. the contribution by blackwater in a house is actually about 40%/60%, and it’s also otherwise not as different as you might think, mind you those people are putting different stuff down the drain including food, which is a major contributor to bod. BTW, bacteria do absorb a variety of salts when they grow, they certainly reduce the salinity a bit, but they can’t always absorb it all.

Your system as it stands is certainly worth a shot, I think, and will turn out to be simpler and cheaper if it works, and you can always add an MBR later, because you may find there are some problems that an MBR would help solve.

Just some I can think of that may or may not be an issue:
Particle filter cloggage

Disposing of the filtered particulates, will have to go down a toilet or something, the sludge from an MBR can be pasteurized and put on a garden,

Storing the particulates, they may become
septic and release some nasty smells

removal of stuff that falls into the non-ionic but very soluble (therefore difficult to adsorb) category, detergents fall into this category

frequent emptying of stored particles, or filter replacement (the MBR turns 90% of everything into co2 an water…)

But then maybe those can be solved in another way…

Btw, I have some good news for you about RO (nothing to do with the price, though I’m afraid, though I have noticed some deals on ebay, and it can be cheaper to but the part separately and put them together)

1. You can more accurately estimate the fraction of brine waste by looking at the salinity of the incoming water and the pressure your pump is outputting. The way it goes, there is the osmotic pressure (OP)acting to prevent water from crossing the membrane (in the direction we want), and of course the pump pressure (pp) is counteracting this. The water does not start to permeate until the pump pressure exceeds the osmotic pressure. Then the flow rate is determined by the resistance that the membrane poses, and the net pressure difference (PP-OP)(which is why the membranes are super thin). So as you can see, as the RO process proceeds, the salinity (and therefore OP) goes up, approaching the point where OP=PP. Obviously you never get there, but suppose you get 70% there.

Anyway, the point here is, you can think of a given RO system as concentrating the incoming water not by x percent, but up to a level of Y OP.

Your input water is much less salty than seawater. You can therefore get a much higher concentration ratio. Suppose the water coming out of a seawater unit is 70 grams/l salt. The water going into the unit was 35 grams/l, so you get a 50% waste rate. If the incoming water is 1 gram per liter, as is greywater, you could hypothetically get a 70:1 input:brine ratio, or only waste a few percent of the input water. The flow rate through the membrane will also be much higher with the same PP, because during the beginning of the process the PP-OP is nearly twice as high.

The practice of recycling almost all the water with RO is fairly common in industrial wastewater, often preceded by an MBR to get a lot of the stuff out that fouls the RO membrane(you could precede it with carbon, might help a bit), see the Marco Cremona system mentioned on my blog.

However, I have read of a system aboard a cruise ship that uses only relatively low pressure RO, and still recovers 90% of the water. Wait, I just realized that you might actually be able to save some money here with a cheaper pump. If the ingoing water is not nasty, salty seawater you maybe be able to get away with something much cheaper. And if you can allow yourself a 35:1 ratio the pump only needs t put out half as much pressure, for instance.

Mixed bed ion exchange might be a lot cheaper and energy efficient, though. You can regenerate a mixed bed, the anion and cation exchange resins have a different density so if you put it in some saltwater of the right density, one will float and the other sinks. Now that they are separate they can be regenerated, and then you mix them back together.

This is gonna get pretty long, but hey, it’s a blog poost now not just a comment, so this might be interesting to someone else, even if you have other plans.

I just thought I should mention that a TDS sensor only really senses ionic dissolved ionic stuff. Digestible material getting through the carbon will not really show up, because it is not ionic, so it doesn’t change the water’s conductivity (which is how a TDS sensor works). The overall challenge of monitoring water quality, as mentioned in a previous post, is hard. In the nasa system they accomplish this with a Total Organic Compounds (or is it carbon?) sensor. I think these are quite expensive, though.

One idea to sense for digestible material, though, might be to do exactly what they do in the laboratory to measure the BOD of a water sample: take a sample of water, and just put it in an enclosed container with a water manometer attached. If there is digestible material, the bacteria will absorb O2 in the water, and thence from the surrounding air, producing CO2 and water, and that reduces the volume of gas above the water. That is then indicated by a change in the manometer reading.

But it takes time to do that. The process could be speeded up by bubbling the air through the water, and by having more bacteria present (maybe a bit of the stuff used in water treatment systems used to keep the bacteria put because the stick to it, while the water is changed, would do). Then, you still need to have away to verify that the bacteria do not die (which would result in false negative readings), including by starvation. Maybe add some methanol every once in a while to double check it’s function. Then you could have a sort of do it yourselfer’s BOD monitor device, not quite as good as TOC, but it covers the most important base.

Actually there must be a of different ways to get a general idea of how much TOC there is. Suppose you added NaOH and let it hydrolyse what it can, then measure the Ph and compare it with how much NaOH you added, the more stuff there was to hydrolyze, the lower the ph. Wouldn’t be very sensitive, though.

Another way for bod could be with a dissolved o2 sensor, working on the same principle as the above method, but intrinsically faster, because the O2 level will drop fairly fast, and a bit simpler from the user’s standpoint. You want the membrane type, some of the other sensors release toxic stuff into the water.

A chemist might have a dozen ways to do it with various reagents, too.
I’ve read vauge whisperings but I’m not entirely sure they are right, but maybe the organic stuff would absorb UV more strongly than water, enough to be detected by piggybacking on the UV unit. Basically drill a small hole in the side of the unit and put some sort of light sensor to detect how much UV escapes. Then you have to zero the system every once and a while by passing known-clean water through it, and maybe calibrate it with some known-dirty water, too.

hm, interesting info on the nasa system phosphoric acid isn’t toxic, I think it’s added to coca cola.


Written by gregor

September 4, 2010 at 05:08

Posted in Uncategorized

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: