You can get Alum in the spice section at your grocery store (or at least some groceries- took me visiting several to find my own).
Here is the best write-up I've found on it:
Water Treatment for Rafters
I recently took my first Grand Canyon Raft Trip. It was near the end of August and recent thunderstorms had added a lot of sediment to the river. This condition created a water supply problem for our group. The water filter system was becoming plugged too rapidly to produce the volume of water we needed. The guide was prepared with some of the correct materials, alum and lime, but was not certain of the best application methods. Earlier in my career I spent six years as an applications engineer for water treatment chemical companies. I visited dozens of water treatment plants and tested different treatment schemes using techniques that could be applied to treating river water by the bucket anywhere. Here are some principles and tips.
What determines the rate at which particles settle? The equation that describes settling of small particles is known as Stokes’ Law. Gravity pulls particles down but the viscosity of the water resists. The speed of settling is proportional to the diameter of the particle squared. That is, if you can triple the particle size the settling rate will be nine times faster.
How can we increase the particle size? The traditional treatment chemical has been aluminum sulfate, known as alum. Alum is usually thought to work through two mechanisms. Clay particles have negative surface charges that, like two south poles of a magnet, cause the particles to repel each other rather than lump together. The triple positive charge of the aluminum ions of alum act to negate this repulsion. Alum also forms a sticky aluminum hydroxide precipitate that sweeps the clay particles out of solution. Some researchers think that the precipitate formation is the most important mechanism.
How much alum should be used? It varies with the water. The concentration of particles, the type of particles and water temperature can have an effect. The best approach is to do a small test on each sample and go by experience. Before discussing test procedures we need to discuss pH and alkalinity.
Alum has a disadvantage as a water treatment chemical. As it forms the aluminum hydroxide precipitate it pulls hydroxide ions out of the water which lowers the pH.
What is pH? pH is the measure of positively charged hydrogen ions in the water and is measured on a scale of 1 to 14. A pH of 7 is considered neutral, that is the positive hydrogen ions are in balance with the negative hydroxide (OH) ions. A pH less than 7 is acidic and pH greater than 7 is basic. In natural waters the pH is usually between 6 and 9.
Why is pH important? The precipitate that makes alum an effective treatment only forms between pH 5 to 8. As the addition of alum itself lowers the pH it is entirely possible to add so much that the treatment will not work for pH reasons. Adding more is not always better. (It is also possible to overtreat by adding too much alum even if the pH is in the right range. The negative repulsive charge of the particles can be reversed, causing them to repulse as positive charges.)
What can be done about pH? In some cases the problem may be solved for you through natural Alkalinity. Alkalinity is a natural buffering system that resists pH change due to carbonates in the water. Any water that has been in contact with limestone should have some of this buffering capacity built in. If there is not enough alkalinity to hold the pH in the right range the pH can be adjusted upwards with lime. Remember that the upper limit for alum treatment is pH 8. Adding too much lime can hurt treatment also. Testing is the best approach.
TEST PROCEDURES: Equipment Needed:
2—4 oz widemouth jars
1—500 ml or 1 liter bottle
1—1 gram scooper or pre-weighed alum and lime samples
1—large spatula or spoon for mixing in 5 gal. buckets
1—1 ml syringe
1—10 ml or 25 ml syringe
This is the test procedure that I would recommend. Either pre-weigh some 1 gram samples of alum and lime or find a small scooper that will reliably give you an amount close to 1 gram. Mix the one gram with 100 ml of already clean water in a 4 oz. jar. This gives you a 1% or 10,000 ppm test solution. Add 500 ml of river water to a mixing bottle. Each 1 ml of 1% solution added to 500 ml of water will provide a dose of 20 ppm (1ml)x(10,000ppm)/(500ml)=20ppm). Start with a low dose, say 10 ppm (1/2 ml). Mix vigorously for 15 sec3. then swirl gently for about 1 min. and observe for coagulation. Increase the dose gradually until you have a good result. If the alum dose gets above 50 ppm try adding increasing lime doses. Without a pH meter you won’t know where you are and will have to go somewhat by feel. Feel free to try several tests. Eventually you will zero in on the correct dose.
To scale up to bucket sized batches you can still use your 1% solutions. Ten ml of your 1% solution will give you about a 25 ppm dose per gallon. (10ml)x(10,000 ppm)/(3785 ml/gal)=26.4 ppm.
You could thus treat 5 gal. with 50 ml of 1% solution if the dose was about 25 ppm. Scale the dosage up or down as needed.
Mixing is nearly as important as getting the dosage right. A water treatment plant will normally rapid mix for 30-60 seconds and provide slow, gentle mixing for up to an hour. The rapid mix disperses the treatment chemical so that all the water is treated with no localized overtreatment and the slow mix brings the particles together so that they will grow in size and settle out quicker. In a bucket treatment situation I would recommend at least 15 sec3. of rapid mix and 3 minutes of slow mix. The slow mix should be more back and forth across the bucket rather than swirling. Using these techniques it should be possible to produce well clarified water in less than an hour.
Are there alternatives to alum and lime treatment? Yes. Many industrial plants treat river water using Ferric Sulfate. Ferric Sulfate works across a wider pH range than alum and would normally eliminate the need for lime. There are also some organic coagulants that are collectively known as polymers. These are clear viscous liquids that do not effect pH at all and work at much lower doses. They would be tested using the same techniques as above. Mixing is particularly important when using polymers. The major suppliers of polymers include Betz Laboratories, Nalco Chemical Co. and Calgon Chemical Co. They might be willing to send you a 2 or 4 oz. sample that would last a long time.
and Occasional Rafter