338 CITY MILK SUPPLY 



(1) digestion in the septic tank followed by filtration; and (2) treatment 

 in the Imhoff tank. Creamery sewage differs from ordinary sewage in 

 that it contains considerable fat and much casein, which make it prone to 

 clog, and lactic acid which in tanks has a tendency to restrain bacterial 

 decomposition. Besides the butterfat it carries also the machinery 

 oil so that it is well to interpose a grease trap on the pipe line between the 

 plant and tank to catch this fat and also lumps of casein, that do not de- 

 compose easily. Where a tank is used, waste acid should be kept out of 

 the sewage because it interferes with bacterial decomposition. The clear 

 water that is used for cooling should also be kept out of the sewage partly 

 because it is cold and, therefore, tends to retard bacterial action but chiefly 

 because it greatly increases the volume of the sewage to be handled and 

 so increases the cost of treatment by making larger works necessary. 



The tanks should be large enough to hold the sewage for 3 days and 

 should be provided with a dosing chamber for continuous discharge onto 

 the filter beds interferes with their action. Sometimes the addition in 

 the dosing chamber of chloride of lime at the rate of 1 to 5 Ib. per 1,000 

 gal. is advisable an hour before running the tank effluent onto the filter 

 beds for in this way foul odors are checked. 



The filter beds are built of properly selected sand or crushed stone that 

 is underdrained with open vitrified tile. The surfaces should be kept clean 

 and when they become clogged the beds should be raked lightly. In 

 time the upper surface of the sand has to be removed and replaced. The 

 effluent of the filter beds may be disposed of by running it into a stream, 

 or more economically by subsurface irrigation through open-joint tile 

 or by surface irrigation. 



The Imhoff tank is constructed and operated as such tanks usually are 

 and needs no special description. However, it should be noted that owing 

 to the development of acid fermentation it may be necessary to add a 

 small quantity of lime to the waste in the tank. If a highly purified efflu- 

 ent is desired it may be advisable to filter the effluent from the Imhoff 

 tank. Otherwise, it can be discharged into a stream or onto land. 



Holding Milk at Low Temperature. Dairymen who operate their 

 routes from the farm generally hold the milk till it is wanted for delivery 

 in tanks of spring or of well water. Such water as it issues from the 

 ground usually has a temperature of between 50 and 55F. so that ordi- 

 narily by its use milk cannot be cooled to as low a temperature as desirable, 

 viz., 45F. Somewhat better results can be obtained with ice water. 

 Small milk plants use ice bunkers to cool their storage rooms and larger 

 ones use the gravity brine system. The big plants use mechanical re- 

 frigeration generally with ammonia as the refrigerant. Refrigeration 

 is a highly important part of the milk business but it cannot be properly 

 developed in detail here. Those readers who are interested in the sub- 

 ject are referred to Bowen's bulletin on the subject. 



