I am sorry this reply took so long, but we had some software problems. As you can see from the WEB page on addition instructions, we do have success in treating cesspools. I must say that they are probably the hardest to treat because of the solids, the volume of water, and the relatively small surface area for water adsorption. The best way to treat cesspools is to have them pumped. Then try to break up the bottom with a pole or something like that. If that is not possible, have the pumper use the reverse flow from his truck to agitate the bottom. This helps break up the anaerobic crust that forms on the bottom. Add two bottles of Septic-Scrub directly into the cesspool. There should be enough water to dissolve the material. The next day add two more bottles. The goal is to keep the cesspool as dry as possible, i.e. with as little water standing in it as we can maintain. Cesspools this old and in this condition probably should be pumped about every 12-18 months. Our experience shows that once we start treating a cesspool, some kind of continuous treatment is necessary. When the water levels start to rise again, additional Septic-Scrub needs to be added. I generally would suggest that the trigger point is when there is less than three day capacity left in the cesspool. If there are four people in the house, then average water consumption is generally about 50-75 gallons per day per person, or about 200-300 gallon per day. If the cesspool is round, the capacity in gallons can be calculated as follows: The radius is 1/2 times the diameter. The capacity is the radius times the radius times 3.14 times the height times 7.5. All measurements are in feet. The height should be from the bottom up to the inlet pipe. If the diameter is 6 feet and the height is 6 feet, the effective volume is about 1200 gallons. This means that when the cesspool is about half full, it should probably be treated again with about two bottles of Septic-Scrub. Please do not hesitate to call if you have any questions. Again, I am sorry this answer took so long.