 In this video, we trace what happens to human sewage. More than a third of the world's population does not have access to toilets that ensures human waste is kept separate from human contact. In consequence, millions of people die every year from water-borne diseases. Where they do have access to toilets, most people have adopted the flush system where clean purified water is used to carry human feces and urine away for treatment. Before we examine what happens at these treatment works, let's look at a more sensible way of building a toilet, which does not waste so much water, but still returns the nutrients back to the soil, but without any harmful pathogens. That's microorganisms which cause disease. A composting toilet does just this. Currently, they're used where there is no water supply or sewage disposal, but if they replaced the flush system worldwide, huge amounts of energy in water would be saved. They work with aerobic bacteria, that is those that need oxygen, and you end up with the fertilizer that is pathogen-free and ready to be returned to the soil. In flush toilets, the human waste is mixed with fresh water and sent down the sewers to the sewage works. The main task is to separate out the waste and the pathogens from the water, so clean water can enter the river again. Here are the stages. First of all, screens take out large objects that may have fallen into the sewers. Then the sewage flows slowly to allow grit and gravel to settle. The solids suspended in the water, mostly feces, settle out in sedimentation tanks, and are pumped into anaerobic digesters, where bacteria turn some of the carbon into methane, which is burnt to provide electricity to drive the pumps and the machinery. Remaining sludge is dried and normally can be used as a valuable fertilizer. However, if industrial waste is diverted into the domestic sewers, the sludge may be contaminated by heavy metals and other pollutants, and this valuable fertilizer has to be dumped in landfill. The water is still full of pathogenic bacteria and needs further treatment. First air is bubbled through the water, promoting the growth of aerobic bacteria. Then the water trickles over stones, where other animals feed on the bacteria. Further sediment tanks capture more sludge, and finally, the water passes slowly through reed beds, where any remaining pathogens are killed by the oxidizing conditions, and the water rejoins the river, or is pumped out to sea. So, to summarize, less than two-thirds of people on this planet have access to a safe toilet. Most of these use the flush system, which is very wasteful of water. This water has to be purified at the sewage works before it can be put back into the rivers, and the human waste, now free of pathogens, can be used to fertilize the land, as long as it has not been contaminated by industrial waste.