 Towns and cities across North America use a similar method for treating waste water, and purifying drinking water. On this page we'll look at the methods which are used to do this, and some of the problems which might arise. Treating Waste Water Waste water is all the water that has been used by a city. Besides sewage, there is runoff from rainfall (water that flows down streets and into storm drains) which might also contain contaminants such as gasoline, oil, paint, heavy metals, pesticides, human and animal waste, and trash. Eventually it will end up in a river, lake, or the ocean, so it is usually sent to a sewage treatment plant first, to make it clean. 
Waste water enters the treatment area from town sewers (A).It is screened (B) to remove large objects. The liquid is stored in a settling tank (C) where solids suspended in the water can settle to the bottom. This sludge is collected, and bacteria are added (D), which consume organic waste. Gases produced by this process are collected (E) and sold as fuel; the remaining sludge is dried and buried, burned, or sold as fertilizer (F). Meanwhile the water has chemicals added to remove phosphates (1) and is then moved to aeration tanks (G), where oxygen is added, and bacteria consume any organic materials. A final settling tank (H) allows remaining solids to be collected (I). The water has chlorine added (2) to kill any remaining bacteria, and the water is tested (J) before being returned to the lake or river (K). Treating Drinking Water Water used by a town or city is drawn from a lake or river, but needs to be treated first to make sure it is safe to drink. The intake pipe extends far out into the lake or river, where it draws in a supply of water which is sent to a treatment plant. 
Chlorine is added (1) to kill microorganisms. Alum and lime may also be added (2). Alum causes suspended particles to clump together, making them easier to remove. Lime or carbonic acid changes the pH level, if required. The water is stirred in a large tank (3) to make the suspended solids clump together; they are then allowed to collect on the bottom of a settling tank (4). Next the water is passed through a series of filters (5), which may include sand, carbon, gravel and rocks. Sand helps remove any remaining solids, algae and silt. Carbon removes taste and odour-producing chemicals from the water by adsorption. Chlorine is added (6) to again kill algae and micro-organisms, and the water is sent to a storage tank (7) for distribution to homes.
Not all towns and cities use this method. Depending on how much money they can afford to spend, they may use some of these methods, or add others for even better results. A slightly different method being implemented in many towns is to use a marsh as a stage in the treatment. Sludge is removed as usual, and then the water is treated with bacteria to remove organic materials. But then the water is released into a series of artificial marshes, where it is further filtered and cleansed by reeds, cattails, and bacteria, as they do photosynthesis. The water from these marshes is then chlorinated. It may also be sent to fish hatcheries, where the remaining nutrients in the water provide food for fish, which create a source of food for people. As an additional bonus, the marshes and lagoons serve a wildlife sanctuary and city park, providing habitats for otters, seabirds, and other marine animals, and attracting many tourists. What Can Go Wrong: What can happen to go wrong with the water treatment process? Assuming that the facilities as described above are in place, it is still possible for untreated water can still make it to your taps. Here's how it can happen. During heavy rainfall, the sewer systems of a town may become overloaded, When this happens, runoff and sewage is diverted directly into the river, without being treated. Somewhere further along the river, a town will draw its drinking water from this highly toxic water. If the town's water treatment facility isn't working properly (perhaps because of clogged filters, or infrequent inspections) the water can remain full of chemicals or micro-organisms, which will be in the drinking water pumped to homes. If the design of a town's water treatment plant is really stupid, it will be set up to draw its drinking water from the river downstream of its own treated sewage disposal area. Hard to imagine, right? Well, this design was used by the city of North Battleford, Saskatchewan, and when the problems above occurred, people got sick and died. |