Water for consumption in a city may be taken directly from a river and waste water from the city returned to the river. If this is repeated at each large centre from the upper reaches of a river to its mouth, as is the case in the River Thames, for example, then the same water will be treated and reused several times. Water engineers are charged with the duty of ensuring that the water supplied is non-toxic and of a sufficiently high standard for human consumption and that the waste waters are treated to remove pollutants to bring the water to an acceptable quality for return to the natural river.