August, 1907 



AMERICAN HOMES AND GARDENS 



311 



The Purification of Sewage and Factory Waste 



By E. Boullanger, of the Pasteur Institute at Lille 



S^HE methods which have been devised in 

 recent years for purifying and rendering 

 sewage and the waste water of factories in- 

 nocuous, before allowing them to flow into 

 streams to poison fish and possibly human 

 beings, may be classified in three groups: 

 purely chemical, purely biological and 

 mixed. In the last group, part of the organic matter is pre- 

 cipitated by chemical means, and the water is then further 

 purified by biological methods. 



The principal reagents employed in chemical purification 

 are ferric sulphate, ferric chloride, calcium permanganate, 

 lime and chloride of lime. When any of these substances is 

 added, in suitable proportion, to sewage water, a precipitate 

 is formed which entangles and carries down with it all matter 

 in suspension, leaving the water clear and partially purified. 

 As an example of these chemical methods we may take the 

 process of purification with ferric sulphate, which was re- 

 cently studied at the experimental station at La Madeleine- 

 les-Lille. The sewage, 

 flowing continuously 

 into a cistern, receives, 

 through an adjustable 

 inlet cock, a solution of 

 ferric sulphate in a pro 

 portion determined by a 

 preliminary experiment, 

 and is then pumped 

 into an elevated clearing 

 basin, which overflows 

 into a second basin. 

 The solid matter ac- 

 cumulates on the bot- 

 toms of the basins, 

 chiefly of the upper one, 

 and the clear water 

 flows off from the top of 

 the lower basin. When 

 the basins have become 

 filled with soft mud 

 they are emptied by 

 opening sluices, and the 

 mud is passed through a filter press, which converts it into 

 cakes containing fifty per cent, of water. Chemical analysis 

 of the water after purification shows that from forty to sixty 

 per cent, of the soluble organic matter has been precipitated. 

 The water, therefore, is only about half purified. 



The advantages of the chemical method are the following: 

 LInlike the biological method, it is applicable to water rich 

 in organic matter or in antiseptic substances. In some cases 

 it permits the extraction of fats and nitrogenous substances 

 of commercial value. Finally, it does not require an ex- 

 tensive plant. 



On the other hand, the chemical method offers many 

 inconveniences. It is very difficult to regulate the quantity 

 of the reagent so as to secure satisfactory purification because 

 the proportion shoulci vary with the chemical composition of 

 the sewage or waste, and this may vary from minute to 

 minute. At best, the purification is incomplete, and the 

 water still contains organic matter which may become offen- 

 sive. The outlay for reagents, even the cheapest of them, 

 is heavy, because of the great volume of water to be treated. 



Septic Foss, Showing Sand Chamber 



Finally, the dehydration of the precipitate by filtering and 

 pressing is laborious and costly, and it is often impossible to 

 dispose of the product as its value as an agricultural fertilizer 

 is uncertain. Every city that has tried chemical methods has 

 encountered these difficulties, so that, despite their seductive 

 simplicity, attention has been turned to methods which do 

 not produce bulky and worthless waste products. 



Lhe first employed of the biological processes were irriga- 

 tion and intermittent filtration. In the irrigation method the 

 water to be purified is spread over cultivated ground. Here 

 it leaves its organic matter, which becomes converted, through 

 the agency of bacteria, into nitrates, water and gaseous prod- 

 ucts. This method has been adopted for the purification of 

 the sewage of Paris. It requires a very porous soil of such 

 chemical composition that it will eftectively attract and ex- 

 tract the organic matter which is dissolved in the water. 

 The soil must also be well aerated, and consequently the irri- 

 gation must be intermittent and not so copious as to drown 

 the land. Finally, the purified water must be removed from 



the subsoil by a good 

 system of drainage. 

 When all these condi- 

 tions are fulfilled excel- 

 lent results are obtained. 

 The irrigated land is 

 usually planted with 

 \cgetables in order to 

 utilize and remove the 

 nitrates, which are 

 the ultimate result of 

 the decomposition of the 

 nitrogenous matter by 

 bacteria. 



The irrigation proc- 

 ess, when conducted 

 with care, furnishes per- 

 fectly pure water, but it 

 is attended with certain 

 serious inconveniences. 

 It is not always possible 

 to find, near a city, a soil 

 of the requisite depth 

 The method requires vast plots of land 

 The best soil purifies only 



and permeability 

 and extensive and costly piping 



about one quart of sewage per square foot per day, and the 

 usual rate Is less than one-third of this. Consequently a plot 

 of about 800 acres would be required for the purification of 

 the sewage of a city of 100,000 inhabitants, amounting to 

 350,000 cubic feet per day. The method is therefore im- 

 practicable In many cases, owing to the lack or great cost 

 of suitable land. 



If the sewage is allowed to flow intermittently upon beds 

 of coarse sand, a much greater volume of water can be puri- 

 fied. This fact is the basis of the method of purification by 

 intermittent filtration, which has been studied chiefly at the 

 experimental station at Lawrence, Mass. The sewage flows 

 at regular intervals upon beds of sand, six or seven feet 

 thick, in which bacteria rapidly develop and destroy all or- 

 ganic matter. The intermittent irrigation is necessary in 

 order to admit air to the beds between the floodlngs. This 

 method can be employed only on sandy soils. In some cases 

 crops are raised on the filter beds. The results obtained by 



