FEEDING FISH 219 



elements that can be applied to advantage, or with safety, as the case 

 may be. An excess of certain nutrients may result in actual harm 

 (poisoning). It is known, however, that the potash salts are the only 

 ones that are dangerous in this respect. Even excesses of nitrogen and 

 phosphorus salts do no harm, while such an eventuality is partly guarded 

 against by the high cost of these elements. 



Sewage, also, can be used with the best success for the fertilization 

 of fish ponds. This practice is attended with the additional virtue of 

 saving artificial manures and assisting in water purification (Hofer, 

 Strell and R. Demoll). 



To avoid putrefaction processes, sewage used for this purpose should 

 previously be clarified and thus freed from 50 to 60 per cent of sediment 

 and suspended matter. 



Any method of sewage disposal which does not require retention of 

 the sewage in the settling tanks longer than 24 hours is suitable for 

 this purpose, provided that the settling tanks are so constructed that 

 daily removal (or removal at two or three day intervals at the most) 

 is practicable. This is necessary to prevent putrefaction of the pond 

 water. In the pond fish industry at Strassburg, where sewage is utilized 

 as the fertilizing element, the preliminary clarification is conducted suc- 

 cessively with a coarse rake, a Geiger screen paddle wheel, and a Neu- 

 stadt settling basin 12.5 meters long and 3.6 meters wide with a capacity 

 of about 100 cubic meters. A similar plant in Amberg employs 3 Kremer 

 apparatuses for each 10,000 inhabitants. 



After the sewage has been subjected to the processes above outlined 

 it is diluted with about two parts of pure water from streams. Thus 

 diluted, the sewage is discharged into the ponds by means of a number 

 of wooden conduits. In Strassburg, these conduits are arranged horse- 

 shoe fashion around approximately one-third of the front shore line of 

 each pond. They are placed about 10 meters apart and discharge their 

 contents through fan-shaped distributors about 2^ meters long. To 

 favor oxygenation the distributors are so arranged that the water is 

 discharged in the form of cascades or against so-called splashboards. 

 On hot summer nights and during sultry periods preceding storms the 

 diluted sewage supply is shut off and fresh water is admitted according 

 to needs. 



A water area of one hectare (2.471 acres) is required for the sewage 

 from every 2,000 population, estimating 100 liters per inhabitant per day. 

 The rate of discharge into the fishponds is so regulated that the sewage 

 water remains in the ponds about 6 days. During this time, even in 

 winter, the water is purified to a degree that approaches ordinary river 

 water. The nitrogenous substances present in the "clarified" sewage 

 water are reduced to the extent of from 60 to 70 per cent and the 

 bacteria from 85 to 92 per cent. 



The ponds in Strassburg are 100 to 150 meters long, 40 to 50 meters 

 wide and 0.5 meter deep. Before they are filled with water they are 



