Until within recent years all the water used by the furnace was returned to the sewer so little polluted that it required no special treatment, the water having been used almost exclusively for cooling and condensing purposes, but this is no longer true since the introduction of the gas washer. The wet scrubbing of gas requires a large quantity of water both for cooling and cleaning purposes, and the water discharged from these systems is exceedingly foul, containing the dust from the gas, and also much of the fume which we have been accustomed to consider separate from the dust. In most cases to return this foul water through the sewer system to the original source of supply would be to invite legal action, and it must accordingly be settled so as to remove the greater proportion of its suspended matter. Even after receiving careful settlement it is still quite foul and should not be turned into the main sewer system without careful examination to see that it will not cause the filling up of the latter either by precipitation of suspended material, or by the formation of a deposit of dissolved matter upon the sewer walls.

Where very large quantities of waste water are to be handled brick or concrete sewers are used practically the same as in regular city sewer work, but the branches from these or the main sewer at smaller plants may be of different material. Generally terra cotta pipe is used from about two feet in diameter down, but in many cases square wooden boxes of heavy plank, strongly spiked together, are used very satisfactorily since they are relatively inexpensive, are very durable if located so that they stay wet all the time, and are not liable to be broken by light shocks and heat, as are terra cotta sewers. It is probably safe to say that in nine cases out of ten the sewer system does not receive as much attention in the designing of plants as it should, and is put in as an afterthought, with the result of costing more, and not improbably being worse located than it might have been had it received earlier consideration. Above all, care should be taken to design small sewers so they can be cleaned by punching without digging them up.

In many cases also where terra cotta is used for secondary permanent sewers it is probable that the greater strength and durability of cast-iron pipe of light section would more than justify its increased cost, since difficulties with a sewer, while they are not very frequent, are inconvenient and expensive to the last degree. Some miserable sewer buried under the ground and never thought of once a year, whose failure to carry off the waste water results in flooding everything, may necessitate the shutting down of the plant for hours in order to cut off or reduce the flow of water while it is being repaired.

One point in the design of the sewer system relative to the plant as a whole is worthy of note. In plants located in northern climates much trouble is sometimes experienced from anchor ice forming on the screen below the water level and shutting off the supply of water. The main waste sewer is normally located to discharge below the pump intake, as is absolutely right and proper, but where this discharge consists of practically clean water which has been used only for cooling purposes the sewer may well be provided with an auxiliary discharge just above the main intake screen of the pumping plant, whereby some of the warm water returning may be discharged at the river bank so as to warm the entering water a little above the freezing point and so serve to cut off or prevent the formation of anchor ice with all its attendant annoyances. These are minor points in the consideration of the blast-furnace plant as a whole, but they are apt to be extremely expensive ones unless given careful thought in advance.