Chapple. — On the Disposal of Sewage. 521 



moisture. Underdrainage by laying porous pipes at a depth 

 of about 6ft. is an important because it is an expensive 

 matter. A loose friable surface-soil, with a subsoil of coarse 

 gravel, can be worked without underdrainage, as in Croydon. 

 But whenever the soil for a considerable depth is favourable 

 to the retention of water underdrainage is necessary, for it 

 must be remembered that sewage must pass through, not over, 

 the soil in order to fulfil sanitary requirements. Many farms 

 have no underdrainage, and in some, with very impervious 

 clayey subsoil, the managers are content to allow the 

 sewage to flow over the land, finding that after about a mile 

 of flow it is comparatively pure, and the amount of water 

 small in quantity. But though underdrainage is neces- 

 sary to allow of the rapid relief of the land from moisture, 

 and the consequent aeration of the soil, it is found that, 

 except in rainy weather, there is not much flow in the 

 affluent drain-pipes, sometimes the whole quantity of water 

 passing upwards through the vegetation of the farm by evapo- 

 ration. 



Pure sandy soil is 'an extremely suitable one for sewage 

 farming, and the most luxuriant crops are very soon produced, 

 while the cost of levelling and draining is usually less than for 

 most others. A gentle slope is necessary for the purpose of 

 irrigation, while what is known as the "ridge and furrow " 

 system is adopted for the growth of vegetables, and a system 

 of gentle slopes, with grips, for the growth of grass-crops. 

 Vegetables are planted on the tops of the ridges, and the 

 furrows convey the sewage at a slow pace. The area 

 required where the dry-weather flow varies from 20 to 40 

 gallons per head is 1 acre for every hundred of the population. 

 If "intermittent downward filtration" alone be adopted for 

 the disposal of sewage, 1 acre for every thousand of the 

 population is sufficient. 



It is advisable, whenever much rain-water is admitted to 

 the sewers, to have a small area of well-prepared filter-beds, 

 for downward filtration in wet weather, for the disposal of any 

 excess of sewage, or when it is not required for the purposes 

 of irrigation . 



In a properly-laid-out farm tanks receive the sewage at 

 the outfall when it issues from the main sewer, and here the 

 larger solids are retained and collected and dug into the soil 

 with town ashes and refuse, making a very superior and 

 profitable manure. A main open carrier, formed generally of 

 concrete or of split pipes, conveys the sewage from the tanks 

 through the centre of the farm. Grips in the land, or on the 

 tops of ridges, strike out at right-angles from this main carrier, 

 and opposite these grips stoppers are placed by the workmen 

 every now and then, causing an overflow of the sewage, and 



