Storm-water ponds are the closest alternative source of water for many people living in urban areas. In an emergency this source of water may be all that is available to you. Eventually any water you have stored will be consumed and the water in a storm-water pond may be the difference between life and death. With the proper treatment your local storm-water pond can be a great backup source of drinking water.
Storm-water presents a unique set of challenges during treatment. Because storm-water ponds collect surface water, the water is exposed to all the contaminants on the ground in the catchment area. This includes but is not limited to pesticides and fertilizers applied to lawns, motor oil and gasoline leaking from vehicles and litter like cigarette buts. It all ends up is the storm-water pond. Those chemicals are already in storm-water ponds on a normal day. During an emergency there may be additional contamination from sewage runoff from an overloaded or broken sewage system. The water in the pond will also contain all the microorganisms like ecoli, giardia and cryptosporidium normally in surface water. Any one of these will make you very sick if you get infected with them.
Finally, there will be high levels of nitrates in storm-water ponds. Too much nitrates consumed by young children can cause blue baby syndrome.
The first step in treating water from a storm-water pond is straining. Straining the water through a cloth or loose sand filter will remove large particles (ones you could pick up with your fingers). Remove as much of the suspended particles from the water as you can. Straining the water first will extend the life of your proper water filter.
If you have a clarifying agent like aluminum sulfate, this is the best time to add it to the water. It will make contaminants too small to be filtered become attracted to each other and become significantly larger. Larger particles are easier to remove from the water. Let the water sit still for at least 30 minutes without disturbing it. All the newly formed large particles (called floc) will sink to the bottom. When you take the water from this container, make sure you leave the majority of the settled material at the bottom of the container.
The next step is to filter the water. Filter the water even if it looks clear, the human eye is five times too weak to detect dangerous levels of particles. Filter the water at least once through an activated carbon filter. Activated carbon is known to remove many different chemicals from water including pesticides, chlorine and fluoride. Activated carbon is not the same as charcoal. Charcoal is similar, it can remove toxins from water but it is nowhere near the efficiency of activated carbon.
The third step is oxidation. Oxidation will help with disinfection as most disinfectant chemicals are also oxidizers. Chemicals like sodium hypochlorite and potassium permanganate are both oxidizers and disinfectants. Oxidation will break down many of the remaining contaminants and inactivate many of the remaining bacteria. Keep adding the oxidizer/disinfectant till you can detect a residual after 20 minutes. The 20 minutes is the minimum you should wait for a gallon of water. Wait longer for larger volumes. This is because oxidation is a chemical reaction that isn’t instant. It needs time to complete the reaction.
The fourth step is to filter the water again. Filtering again is necessary because the disinfection/oxidation step will create some potentially carcinogenic byproducts. We filter before oxidation to minimize the amount of chlorine (or other chemical) and to limit the possibility of forming dangerous byproducts. We filter the second time to remove any byproducts that have been formed.
The final step is to boil the water. This will help with disinfection, but the main goal of boiling at this point is to remove any volatile chemicals. Any chemical with a boiling point lower than water will be removed after boiling.
A note about disinfection. If all of these steps are followed there is no need for a step dedicated for disinfection. Between the oxidation and the boiling of the water any microorganisms will be inactivated. If you are storing the water for a long time then add some sodium hypochlorite for a residual disinfectant. The residual disinfectant will prevent the water from becoming recontaminated before you drink it.
One additional possible step is to aerate the water. Ponds are frequently stagnant. Stagnant water is green with algae, it smells bad and tastes worse. After the water is made potable, transfer the water back and forth between two glasses. This adds oxygen to the water and will make the water taste better.
This may seem like a lot of work for something as small as a storm-water pond. What I have described are the basic steps to turn the potentially toxic water in the pond into clean and safe drinking water.