SEWAGE 



34! 



capable of purifying 60,000 gallons of sewage per 

 acre per day, ana produce an effluent as chemically 

 pure as most drinking-waters. The sewage, how- 

 ever, used in these experiments is much more 

 dilute than sewage in England, as 100 gallons 

 represent the sewage of a single individual per 

 day. Other modes of nitration have been also very 

 successfully used for the purification of sewage for 

 instance, a sand filter in which there is introduced 

 a layer of mineral substance composed of magnetic 

 oxiae of iron combined with carbon, or what is 

 now called polarite, but which was called by Mr 

 Spencer, its discoverer, magnetic carbide of iron. 

 ] t is now manufactured by carbonising in a retort 

 the materials composing a bed of shale found in 

 the coal-measures of South Wales. This material, 

 like Spencer's magnetic carbide of iron, is shown 

 to have remarkable properties in purifying sewage 

 or other liquids containing organic substances. 

 Filters of this character, however, require constant 

 cleansing, as however perfectly a chemical process 

 may be applied, sewage still contains a certain 

 amount of flocculent matter which tends to clog 

 the surface of the filter-bed. The area of a polarite 

 filter required for the purification of sewage after 

 chemical treatment is comparatively small, as a 

 superficial yard may be trusted to purify in a pro- 

 perly prepared and aerated filter 500 gallons per day. 



Chemical Treatment, used either separately or in 

 combination with both natural and artificial filtra- 

 tion, or in connection with some sewage-irrigation 

 works, requires a certain amount of tank space, so 

 arranged as to secure the precipitation of matters 

 separated from the sewage. As a rule, sewage is 

 alkaline, and if it is treated with further alkali in 

 excess, such as with lime, it tends to coagulate 

 certain albuminous substances present ; also the 

 lime tends to combine with the carbonic acid con- 

 tained in the sewage, or held in excess in the waters 

 which go to make up the sewage. The conse- 

 quence is that a carbonate of lime is precipitated 

 at a flocculent deposit, forming a sort of net, which 

 entangles and drags down other suspended im- 

 purities to the bottom of the precipitating tank. 



In other cases both an alkaline ana an acid 

 chemical are used. It should be noted that there 

 is hardly an earthy salt that has not been used 

 in connection with the processes of precipitating 

 sewage ; the salts of alumina, iron, lime, magnesia, 

 potash, soda, silica, zinc have all been used, either 

 by themselves or in combination with each other. 

 When an alkali and an acid salt are used for pre- 

 cipitating sewage, such as lime and sulphate of 

 alumina, the lime should l>e first added as a milk 

 of lime to the alkaline sewage, which tends to 

 increase its alkalinity. The sulphate of alumina 

 dissolved in water or sewage is subsequently added, 

 and the alumina itself is precipitated as an insoluble 

 hyd rated oxide of alumina, which drags down 

 impurities with it ; while the lime combines with 

 the sulphuric acid of the sulphate of alumina and 

 forms a sulphate of lime, which goes away as 

 -olutinn in the effluent, so that the total solids in 

 solution in the effluent are in excess of those in the 

 sewage. Of all the precipitating processes the 

 lime process ia the only one in which there is less 

 solid matter in solution in the effluent than in the 

 original sewage, and, combining cheapness with 

 efficiency, more work is got out of it for a given 

 expenditure than by any other process. Lime 

 effluents, however, unless passed over or through 

 land or artificial filters, are as destructive to fish 

 life as decomposing sewage, and therefore should 

 not be tamed direct into any stream in which 

 injury is likely to arise to the fisheries. 



Newriijt Sludge is the semi-liquid substance that 

 is deposited in tanks, whether by mere sedimenta- 

 tion in preparing the sewage for its application to 



land, or by its chemical treatment and clarifi- 

 cation ; and the disposal of this sludge is often 

 a difficult problem. If not already in a state of 

 decomposition, it is very likely soon to be highly 

 offensive from that cause, and if exposed in an 

 inhabited neighbourhood would soon prove to be 

 an intolerable nuisance. Sludge is a difficult 

 material to handle, as when it leaves the tanks 

 not less than 90 per cent, of it is water. In some 

 instances it is pumped direct on to land and at 

 once covered over with soil ; in others it is left on 

 the surface of the land, not without risk of nuis- 

 ance, until a large part of its moisture has either 

 evaporated or filtered into the ground, when it is 

 dug in. In some cases the sludge is mixed with 

 other refuse of towns, and burned in destructors. In 

 one case it is taken by steam hopper barges out to sea 

 and cast away, as being the least expensive method 

 of its disposal. By far the most effectual way of 

 disposing of the sludge is to pump it into filter- 

 presses. In this way it is rendered portable, and 

 becomes free from nuisance, as sufficient water does 

 not remain in the mass to render it offensive and 

 liable to decomposition, and what does remain is 

 i soon partly evaporated. By pressing, about five and 

 a half tons of crude sewage direct from the tanks 

 i are reduced to one ton of pressed sludge, containing 

 about 50 per cent, of moisture. Pressed sludge is 

 just about as valuable as farm-yard manure, and 

 it- sale in some places realises something, and 

 partly defrays the cost of pressing. 



London sewage is discharged by two outfalls 

 viz. Barking on the north and Crossness on the 

 south, into the river Thames, which divides the 

 metropolis into two distinct drainage areas. The 

 sewage on the north side is chemically treated and 

 sludge removed before the clarified effluent is dis- 

 charged into the Thames. On the south side of 

 the Thames similar sewage-works were constructed 

 in the years 18P2-94. On the north side of the 

 Thames there is a population of about 2,900,000, 

 and the dry-weather now of the sewage is roughly 

 estimated at about 100,000,000 gallons per day. 

 The sewage first receives lime in the form of lime- 

 water at the rate of 3 - 7 grains of lime per gallon of 

 sewage, and subsequently lime sulphate at the rate 

 of 1 grain per gallon of sewage. In hot weather, 

 however, the sewage receives further treatment, 

 and a small quantity of permanganate of soda is 

 applied to the sewage, usually about 1000 tons of 

 permanganate )>eing used in the course of a season. 

 The sludge produced at the existing sewage-works 

 at Barking, on the north side of the Thames, ia 

 about 21,000 tons per week, of which 91 per cent, 

 is water. After getting rid of a portion of the 

 water, the remainder is pumped into steam hopper 

 ships specially constructed, and is conveyed down 

 the Thames and out to sea, where it is discharged, 

 this mode of disposal being found the cheapest 

 method. 



For many years the sewerage systems of American 

 cities were modelled on Kuropenn methods ; but 

 experience showed that the conditions on which 

 these were based as to rainfall, for instance 

 differed so much from those of America, that of late 

 the special needs of each particular case have been 

 more carefully studied. Chicago, Memphis, and 

 various summer-resorts may be mentioned as cases 

 where local conditions have largely modified the 

 methods of sewerage employed. 



House-drainage. However perfectly the sewers 

 of a town may be constructed, however safely the 

 sewage may be disposed of, yet if care has not 

 been taken in the design and construction of the 

 works necessary for the drainage of each individual 

 house very little sanitary benefit may accrue. And 

 in any case direct evils are almost certain to follow 

 bad house-drainage work. All house-drains, while 



