SANITATION 



75 



Sewage farms require careful management, and frost and heavy or 

 continuous rainfall interfere with the satisfactory working. The 

 crops are most commonly used for feeding to cattle and pigs. 



Intermittent Dowmvard Filtration. With this system the sew- 

 age is conveyed over ground so prepared that it acts as a filter. 

 Sand, chalk or clay soils are unsuitable, and a porous loam or marl 

 containing hydrated ferrous oxide and alumina are the most suit- 

 able. As with the sewage farming method, tile-pipes are laid about 

 6 feet below the surface to carry off the effluent. Sections of the 

 land, or filter-beds, are saturated in turn with the sewage, thus the 

 term intermittent. 



Crops are grown on the land much in the same way as in sewage 

 farming, but they are of secondary importance. By filtration the 

 coarser material is arrested and the nitrifying organisms in the soil 

 convert the ammonia into nitrites and nitrates. Aeration of the 

 soil also oxidises the organic matter, and if the soil is suitable there 

 is a chemical action on the sewage. The amount of land required 

 for the filter-beds is about 1 acre per 1000 head of population when 

 crude sewage is filtered and about 1 acre per 5000 if a proportion 

 of the suspended matter is previously removed. This method is 

 very efficacious, but the original cost of preparing the beds is large 

 and suitable ground is difficult to obtain. 



Precipitation or Chemical Treatment. This process consists of 

 the precipitation of the suspended matter by the addition of pre- 

 cipitants such as lime, lime and alum, or lime and sulphate of iron, 

 or magnesium chloride. Lime by itself is unsatisfactory as such a 

 large quantity has to be used and the resultant effluent is strongly 

 alkaline. Large quantities of deposit or sludge are produced by this 

 method of treating sewage, which is pressed through filters to remove 

 the water, and the material is then sold as native guano for manure. 



Electrolytic Method. With this method the sewage flows past 

 cast-iron plates, through which is passed an electric current. The 

 chlorides in the water and sewage are split up into chlorine and 

 oxygen. Hypochlorous acid is formed which oxidises the organic 

 matter. Hydrated ferrous oxide is also formed from the iron 

 plates, which oxidises as well as precipitates the organic matter. 



Bacteriolytic or Biologic Method. The object of this method 

 is to bring about self-destruction of the sewage by favouring the 

 action of bacteria. The anaerobic organisms digest and liquefy the 

 solid organic matter, producing soluble nitrogenous compounds, 

 fatty acids, ammonia and other gases. The aerobic organisms 

 cause the formation of nitrites and nitrates, carbon dioxide and 

 water. 



