2o8 MADRAS FISHERIES BULLETIN VOL. XIII, 



If chloride of lime (bleaching powder) is available, a small 

 quantity added to the tank fluid and to the sludge will prevent 

 all nuisance while not affecting the value of the fluid to the 

 trees, especially if a corresponding amount of soda ash (sodium 

 carbonate) be added, in which case sodium hypochlorite will be 

 formed which will entirely deodorize the fluid and, /;/ so doing, be 

 converted to sodium chloride {commo)i salt) which is beneficial to 

 coconuts. 



If the supernatant fluid is first passed through a filter, it would 

 probably be wholly inoffensive when used for irrigation, since, as 

 shown below under " septic tanks," the operation of the filter is to 

 bring nitrogenous matters into innocuous forms. Such filter is 

 preferably an open masonry tank, sunk in the ground, and may be 

 of any area convenient for the quantity of water to be filtered, and 

 about 6 feet deep. A perforated false bottom is placed about I foot 

 above the true bottom, and over this a mass of hard cinders, broken 

 coke, broken brick, etc., not larger than will pass a two-inches ring. 

 The fiuid from the depositing tank should be baled or pumped into 

 this tank through a pipe entering the filter below the false bottom, 

 so that the fluid may pass upwards through the filter and flow off 

 from the surface into a cistern whence it can be readily baled, etc., 

 to the trees. In this way it would be easy to avoid all real nuisance 

 and at the same time to utilize the waste waters profitably. 



If trees or crops are not available for utilizing the treated water, 

 it must, after treatment as above, be sent into deep sand pits which 

 should be daily covered over with fresh sand. 



lOI. An alternative plan is to establish "septic " tanks. These 

 " tanks " take advantage of bacterial action to break up organic 

 matters, such as are contained in excreta, tanners' " waste," 

 distillery and brewery waste fluids, etc., and thereby reduce them to 

 innocuous and inoffensive forms. The bacteria required for the 

 purpose are largely contained in the waste matters themselves, and 

 the usual method is just to pass them into a " septic " tank which 

 is air tight and utilizes the anaerobic bacteria which cause putre- 

 faction ; in this tank the solid matters break down and liquefy in 

 a few hours, whence these septic (putrefactive) tanks are often 

 called " liquefying " tanks. Thence the fluid passes to the 

 "filter," an open tank of masonry filled with layers or masses of 

 coke, cinders, fragments of well-burnt brick, etc., etc., as described 

 in the preceding paragraph ; here the aerobic bacteria play their 



