Turbidity (the measure of relative clarity of a liquid) is just one of the contaminants we may have to deal with, when you need to produce drinking water.
A Millbank bag is a method of rough filtration designed to remove mud, sand, dirt, silt or other suspended particulate matter such as decomposing organic material.
Before you drink any water, you will need to remove any detritus (waste or debris of any kind) from it. Then sterilise it. Also turbid water can irritate your digestive system. You don't want to have a bad tummy, diarrhea or worse later.
What a Millbank Bag does do and does not.
It is not a purification device and only clears the water depending on the quality of it. So if your water is peaty it will not remove the colour. It is only to be used for removing bigger particles of detritus.
A Millbank bag will not filter pathogenic organisms out of the water, unless those remain attached to the mud, silt or other particles.
You will need to use a purification method, either chemical sterilisation or heat it (to a roiling boil) to kill off any viruses, bacteria and protozoa that may be present.
If you find a water source that has clear water (running water for an example) and it is not turbid (dirty) there is no need to use a Millbank bag.
How to Use a Millbank Bag
Many Millbank bags have instructions attached like a tag. If they do not, it's quite simple to follow the instructions below.
Step 1: Soak
Soak the bag by squeezing it under water. You must soak the bag thoroughly. Don't just wet it, as it won't work very well. This does not have to be in clean water. You must keep it submersed and rub/massage the bag so that the fibres become wet through. This is very important. It could take hours for a few litres to run through the bag.
Step 3: Hang
Hang it up now on a suitable branch or tripod you have made. The reason for filling it to the top is that the water left on the outside of the bag must drain away first as it will still be contaminated. Some bags will have a line you can let it run down to, but if yours doesn't, letting it drain down to about 12cm from the top will be sufficient.
Allow water to run to waste down to the level of the black line. Let water run out until it gets to the line or12cm, by the time water gets down to the line, it will only be water from inside the bag that is dripping. You can then start to collect it in a container that you have ready.
Step 2: Fill
Fill up the bag with water from the source you have chosen and get ready to hang it up, you can use some paracord or some cordage through the eyelet. It will start to drip from the lowest point, but it's important not to collect any of the water at that stage. You must fill it to the top and there is a reason for this.
Step 4: Sterilise
Sterilise water in clean water bottle (not one you have used to collect the dirty water) if using tablets from sterilising source (such as chlorine, iodine or chlorine dioxide). Then follow the guide on the instructions. If you are going to boil the water, drip straight into the container you will use for boiling if you are sterilising the water this way.
Afterwards, wash any mud off the bag, and put away. If you can, give it a good clean. If the water has been heavily laden with silt or similar, the inside of the bag will need cleaning from time to time, otherwise filtering the water through the bag with become harder. Turn the bag inside out and scrub off any sediment.
If filling is slow, repeat soaking and squeezing under water. As I have said earlier, this is very important. If you can, wash it at home first. Also when you buy a new one, give it a wash before first use. But don't use any detergent or fabric softener.