Marsh. — Sewage and Fish Culture 173 



tion is likewise in solution. It is this organic portion, 

 whether suspended or dissolved, which makes sewage 

 a hygienic or aesthetic oifense, or both, as the case may be. 



The sewage is now diluted with a considerable volume 

 of clean water and distributed to shallow ponds of one 

 or two acres in size, of which the best shape is somewhat 

 rectangular and about twice as long as wide. The ponds 

 convert a proportion of sewage representing about 6,000 

 of population, or that of about 800 persons per acre of 

 pond area, which therefore covers between seven and 

 eight acres. The sewage is let into the ponds at many 

 separate places arond one end, a thorough distribution 

 being of the first importance. The depth varies from 

 twelve inches at the edges to twenty inches in the middle 

 and three feet or more at the outlet. A slow current 

 sets lengthwise of the pond and practically complete 

 purification must be obtained before the outlet is reached. 

 The borders of the ponds must be clear of trees or 

 bushes hindering the free access of sunlight. In the 

 ponds themselves certain water plants are provided — 

 sweet-flag and manna-grass (Glyceria) near the inflow; 

 Glyceria, Ceratophyllum and Myriophyllum in deeper 

 parts, while undesirable plants are removed. The duck- 

 weed (Lemna) for instance led to such interference with 

 the growth of algae and consequent oxygenation by 

 blanketing the surface that young ducks were introduced 

 to feed upon the Lemna. They were thus reared upon 

 the products of the ponds and when fattened just before 

 marketing contributed appreciably to the profits of 

 operation. They were moreover of some use in aerating 

 the water. 



The ponds are first prepared by supplying them from 

 other waters with large quantities of Crustacea, insect 

 larvae (Chironomus abundant), mussels and snails. 

 Cyclops and Daphnia and related groups are very abund- 

 ant and important in this stock of minute life; in fact 

 pits are used for breeding these small forms. The young 

 fishes which it is intended to rear are introduced in such 

 numbers as appear suited to the fish cultural capacity 

 of the ponds. These are then maintained for two or 



