The tank water storage vats should be equal to a day’s capacity and should be filled in rotation in order that agitation will be going on only in the vat being filled or the vat being emptied, and that the water in the balance of the storage vats will be permitted to be held under a settling process in the meantime, which is as follows: Heat the water to a temperature of 180° to 185° F., and then allow it to drop down to 170° F., carrying it at this temperature from eight to twenty-four hours, according to the length of time it is possible to carry the water for settling purposes, up to 36 hours is an advantage. =Collecting Grease.=--In the surface boxes spoken of in preceding chapter, it is very essential to agitate the tankage in the surface vats and keep it under steam long enough for some of the grease to rise to the top of the surface boxes and be skimmed off before the tank water is run into the storage vats. Skim the grease very carefully from the top of the water in any vat that is to be used for evaporation; then pump the water to the evaporator supply tank. After the water is pumped out of the basin, it will be necessary to remove the bottom of settlings each time after the tank is emptied. =Settlings in Bottom of Vat.=--Pump the bottoms from the holding tanks into a surface box from which these settings are passed on to the press cheeses with pressed tankage, so that the fibrous material will collect the sludge. It is absolutely necessary that these settlings be taken out of the storage vats each time the vats are emptied in order to keep them sweet and to prevent the tank water from souring as it certainly will, unless cleaned at least twice a week. It is much better to clean them every time. =Testing Tank Water.=--Tank water is tested with a standard hydrometer for liquids heavier than water, and as stated previously, any water indicating ¹⁄₂° on the Beaume scale, is collected. A sample of the tank water is cooled to 150° F. Drop a hydrometer into the water and take reading at a point on a level with the water; a hydrometer reading from 1 to 20, is used for this purpose. If hydrometer reads 2.00, the amount of the solids in the water is estimated at 3.83 per cent as shown on the table for figuring tank water. If the hydrometer should read 2.50, the estimated solids is one-half of the difference between solids shown on table at 2° Beaume and 3° Beaume added to solids at 2° Beaume which is 4.80 per cent. =Evaporating Tank Water.=--The evaporating of tank water, it is proposed to do in the most economical manner possible, and advantage is taken of the fact that as the pressure is lowered the boiling point is reduced. For example, water at atmospheric pressure and sea level boils at 212° F., and water under a twenty-eight inch vacuum boils at 100° F. Advantage is taken of these physical facts in the evaporating of tank water.