The New Orleans water purifying establishment is now completed and is delivering to the city of New Orleans 17,000,000 gallons of pure water per day.

The plant has a capacity of 60,000,000 gallons, and this could be easily extended to supply 168,000,000 gallons per day. The method of operation is as follows:1

Water is taken from the Mississippi River, three-quarters of a mile away, through a 48-inch suction line, which is laid level and with its top two feet below extreme low water in the river, and lifted by the low-lift pumps, through the head or controlling, house, into one of the two grit reservoirs, in passing through which it deposits about 10 percent of the heavier suspended matter which it contains. From the grit reservoir the water passes through the head house again, and, in proportion to the amount of water passing and its condition, lime and sulphate of iron are added to it, after which it passes through one of the two sets of chemical mixing passages, which afford a runway back and forth and up and down of about a mile, during which any deposit is prevented by the motion of the water. This keeps the chemical solution thoroughly mixed with the water until the full chemical reaction required has taken place. Then once more the water passes through the head house, going this time to one of the two sets of settling reservoirs, through which its passage is very slow, for the express purpose of causing the deposit of the now coagulated masses of clay and precipitating lime and magnesia which were originally contained in solution in the water, as well as of the iron and lime which were added to bring about this precipitation. The result is that when the water has reached the outlet of the settling reservoirs and is ready once more to pass through the head house to the filters nearly all of the suspended matter and a large amount of the chemicals which were in solution in the river water, together with those which were added in the treatment, have been left behind in the settling reservoirs, and the remainder, about fifty parts per million out of an average of 750 parts of suspended matter contained in the raw river water, are in such changed condition that they are easily removed by the filters, which yield an effluent entirely free from suspended matter and containing less than half as much lime and magnesia and only about 1 percent as many bacteria as the river water originally contained. The resultant water is bright and sparkling in appearance and safe and desirable for every use.

The filters are of the so-called rapid type. They are, however, merely gravity sand filters, designed to handle large quantities of water and to be very easily and cheaply cleaned. In filtering, the water enters above the sand layer, passes through it and is collected by a system of drains into one effluent pipe for each of the ten filter units, which effluent pipe is automatically throttled to prevent a too rapid flow through the filters. As the suspended matter in the water accumulates in the sand layer, the latter is gradually choked up, until finally the filter will not pass as much water as is required of it with the throttle on its effluent pipe wide open. This stage is reached in from 100 to 300 hours of service under present operating conditions in New Orleans, and then the filter has to be cleaned. The process of cleaning consists in closing the inlet and outlet of the filter from its connection with the operating portion of the system and in forcing filtered water into the effluent pipe, through the drains, and up through the sand layer, at a high velocity, which stirs up the sand layer, loosens the mud and causes it to overflow through troughs placed at a higher elevation than that to which this velocity will raise the sand of which the filtering material is composed. It takes less than 10 minutes to go through the entire operation of washing a filter, and only about 1/2 to 1 percent of the filtered water is required for filter washing.

1The Journal of Industrial and Engineering Chemistry, Volume 7, No. 5, May, 1915, page 444.

The entire cost of treating and filtering the water and pumping it into the distribution system is not over two cents per 1,000 gallons, and the cost of water delivered to domestic consumers through meters by the city is now usually less than one-fourth of what these same consumers had to pay for raw river water pumped direct from the river before the city constructed and operated its own waterworks plant.

The methods of purification used in Chicago and New Orleans are typical of the two kinds of water supply for cities - viz., natural reservoirs, such as lakes and running waters such as rivers.