Chapple. — 0)1 the Disposal of Sewage. 519 



absence of nitrates and nitrites, this absence, as I have ah-eady 

 said, being the chemical indication that no oxidation or nitrifi- 

 cation has taken place in the passage of the fluid through 

 the sand. But let the process of filtration continue for twenty- 

 four hours and another sample be examined, a marked im- 

 provement in the filtered fluid is at once noticed, and chemical 

 and biological examination show that the putrescible nitrogen 

 and ammonia are diminished, and the bacteria are fewer in 

 number, while the nitrates and nitrites are largely increased. 

 This improvement keeps going on till the fluid passing through 

 the sand is as absolutely pure and sparkling as the finest 

 spring water. Chemically tested it reveals absolute freedom 

 from all organic matter and ammonia, with the presence of 

 a considerable amount of the innocent mineral compounds 

 nitrates and nitrites. Tested biologically there is a complete 

 absence of bacteria ; in short, this water is purer than most 

 drinking-water, and no one would hesitate to drink of and 

 appreciate it. This mysterious and marvellous conversion of 

 the foulest liquid into the purest w^ater will continue for 

 many days, but gradually the affluent wiU lose its purity, and 

 in time will become as noxious as the original fluid, and get 

 extremely foul. But if now the flow of the impure liquid on 

 the surfa,ce of the sand be stopped for twenty-four hours, and 

 then resumed, the affluent will again become pure as before. 

 If, further, this cessation of the flow is permitted upon alter- 

 nate days, or upon one day out of three, by other cylinders 

 being placed beside the original one, the purification of the foul 

 liquid experimented with may go on indefinitely without any 

 renewal of the sand or impairment of its efficiency. This 

 process of purification is known as " intermittent downward 

 filtration." 



If the experiment I have just described be extended by 

 planting in the surface of the sand various garden-plants they 

 will, of course, grow luxuriantly, provided other conditions of 

 warmth and light be attended to, and the nitrates and nitrites 

 hitherto present in the filtrates will diminish and almost 

 entirely disappear, the water issuing from the tap being pure 

 and sparkling as before, differing only in being free from its 

 mineral compounds. This process of purification extended to 

 agricultural land is known as" broad irrigation," or " sewage 

 farming." 



If the chemical and vital processes which have taken place 

 in the sand be examined more closely they will be found to 

 depend upon the presence and labours of the minute bacteria 

 before referred to. The particles of sand get coated with a 

 slimy material secreted by the bacteria themselves, and in 

 this they live in countless myriads, taking from the organic 

 nitrogenous matter in solution as it slowly trickles past them 



