36 U. S. BUREAU OF FISHERIES. 



to the head of the tank through a 6-inch pipe controlled by a gate 

 valve. It enters an end compartment in the tank 4 feet wide and 20 

 feet long, from which it flows into a second compartment 10 feet 

 wide and 70 feet long, returning through a similar compartment and 

 discharging into a compartment 5 feet wide and 70 feet long. Near 

 the lower end of this compartment is a sand filter bed 20 feet long. 

 The water passes down through the gravel and sand and is conducted 

 to the discharge chamber through five lines of 3-inch tile. These 

 drains are about 12 inches in length, with broken joints about | inch 

 apart, and they extend into bulkheads in either end of the compart- 

 ment. The hatchery supply pipe is extended into this compartment 

 and receives only water that has passed down through the filter bed, 

 and since there is a fall of approximately 12 feet between the filter 

 and the hatchery supply trough, this pipe carries a satisfactory 

 head. 



Arrangements are provided for cleaning the sand and gravel used 

 as a filtering medium. To reach the filter bed, the water passes over 

 a cement bulkhead built across the compartment. Near the bottom 

 of this bulkhead is a 6-inch valve whereby the water is admitted for 

 cleaning the filter. The water used for cleaning passes up through 

 the sand bed and discharges through a gate valve in the lower end 

 into the waste ditch. Wliile the water is passing up through the filter 

 bed a rake, made by driving spikes into a 2-incli plank, is moved 

 back and forth, by means of a sprocket and chain, over the surface 

 of the sand. This rake is manipulated easily by one man operating 

 a crank on the outside. This action loosens the sediment contained in 

 the sand and the flow of water carries it into the waste ditch. About 

 10 minutes' work is required, twice each day, to keep the filter in 

 perfect working order. 



To assist the percipitation of sediment and to relieve the sand 

 filter an alum dropping device was installed between the spring 

 and the settling tank. The alum solution is lifted by means of a 

 hand pump to an elevated tank of 60 gallons' capacity; after re- 

 maining in this tank for 24 hours it is drawn off by means of a 

 valve into a tank at a lower elevation. This gives a clear solution 

 for use in the water supply. From the second tank the solution 

 passes through a brass pipe to a copper-lined closet tank, in the 

 bottom of which a -J-inch sight-feed oil dropper is fitted. The 

 float in the closet tank maintains a uniform level therein, and the 

 solution is constantly discharged through the oil dropper into the 

 water supply pipe. A constant rate of flow through the oil dropper 

 is maintained — 60 gallons per 24 hours— but the strength of the 

 solution varies in proportion to the turbidity of the water. In ex- 

 treme cases a mixture of 1 part of alum to 50,000 parts of water 

 will supply the hatchery with practically clear water. From this 

 mixture the solution is reduced to as low as 1 part of alum to 

 120,000 parts of water. As a rule, 4 pounds of sulphate of alumina 

 per 24 hours, applied as described above, to each 20 gallons of water 

 per minute used in the hatchery is sufficient to clarify the muddiest 

 water. The untreated water carries in solution sufficient alkali to 

 react completely with the small amount of sulphate of alumina 

 used, leaving a sufficient amount of alkaline nitrate to prevent any 

 " after coagulation " in the filtered water. It is doubtful if any of 

 the alum is carried into the hatching troughs. Experiments con- 

 ducted in Washington demonstrated that neither the eggs nor young 



