SEWAGE TREATMENT — KEEFER 381 



often so pronounced that it is impossible to get the sewage to flow 

 through the filter bed. At times it is necessary to remove the filtering 

 material and wash it. As this process is quite expensive, cheaper ways 

 of correcting the difficulty have been devised. One of these is to add 

 a small quantity of chlorine to the sewage being applied to the filter 

 bed. 



REDUCTION OF BIOCHEMICAL OXYGEN DEMAND 



Chlorine can be used to reduce the biochemical oxygen demand of 

 sewage. This reduction is probably caused by the oxidative action of 

 chlorine. Chlorine reacts with nitrogenous bodies to produce chlo- 

 rine substitution compounds, some of which are probably useless as 

 bacterial food and are therefore less putrescible. Every pound of 

 chlorine added to sewage is capable of reducing the biochemical oxygen 

 demand about 2 to 2.5 pounds. Using chlorine to reduce the biochemi- 

 cal oxygen demand is not a routine procedure, because the expense is 

 considerable and other cheaper methods of sewage treatment are 

 available. 



DISINFECTION OF SEWAGE 



Where sewage effluents are discharged into watercourses that are 

 used for bathing, for the culture of shellfish, or for sources of water 

 supply, chlorine is often used to disinfect the effluent. The amount 

 required will depend upon the degree of treatment the sewage has re- 

 ceived; sewage that has undergone complete treatment will require 

 less chlorine than that partially treated. Dosages vary from 2 or 3 

 parts per million up to 25 to 30 parts per million. When sewage 

 effluents are disinfected with chlorine, it is essential that the chlorine 

 be thoroughly mixed with the sewage and be maintained in contact 

 with it about 15 to 30 minutes. If the proper operating procedures are 

 followed, chlorine will kill more than 99 percent of the bacteria. 



CONTROL OF AQUATIC LIFE 



Where sewage effluents are discharged into some watercourses, both 

 the organic and inorganic matter serve as a source of nutriment for 

 aquatic life such as fungi and algae. These growths may become very 

 prolific and produce nuisances. Chlorine has been applied in a num- 

 ber of instances to inhibit these biological growths. 



TREATMENT OF SLUDGE 



SLUDGE DIGESTION 



The sewage solids, which collect in sedimentation tanks, contain 90 

 percent or more of water and from about 60 to 80 percent of organic 

 matter on the dry solids basis. Within a few hours after sludge is 

 drawn from sedimentation tanks, it becomes highly odorous. Many 

 ways of treating and disposing of it are available. One method fre- 

 quently adopted is to store it in tanks, called sludge digesters, where 



