removal of dissolved hydrogen sulfide and a reduction in bod in waste systems treated with sodium nitrate or calcium nitrate is believed to occur for the reasons described below . the presence of dissolved hydrogen sulfide in sewage occurs as a result of a lack of dissolved oxygen . the addition of nitrate ions no 3 provides an oxygen source for certain bacteria already present in the waste or sewage to thrive . the bacteria that grow as a result of the nitrate oxygen utilize the dissolved hydrogen sulfide as part of their metabolism . the dissolved hydrogen sulfide contains sulfur which the bacteria also require in their metabolism . it is theorized that the biochemical reaction which occurs has the following half reactions : 120 2 o 2 + 5h 2 s → 5so 4 2 − + 4h 2 o + 2h + based upon the above it is calculated that 2 . 4 parts of nitrate oxygen ( no 3 — 0 ) ( no 3 — o ) are necessary to remove 1 part of dissolved sulfide ( s 2 − ): 8   moles   no 3 - 5   moles   h 2  s × 48   lb   oxygen / mole   no 3 - 32   lb   sulfide / mole   h 2  s this ratio of oxygen to sulfide has been confirmed in both bench and field tests . the source of nitrate to accomplish the sulfide removal is not critical , and both aqueous solutions of sodium nitrate and calcium nitrate have been used successfully . this reaction is biochemical and it will not occur within a sterile solution , i . e ., naturally occurring bacteria in sewage must be present . additionally , the sulfide removal is not instantaneous ; tests have shown that an “ incubation ” period of 24 - 48 hours is necessary to culture the bacteria and thereafter 3 - 12 hours for ongoing sulfide removal . it is believed , however , that the incubation period may extend from about 8 to about 96 hours , and the ongoing removal period from about 1 . 5 to about 20 hours , depending on conditions . the promotion of biological activity via nitrate addition as described also achieves a reduction in sewage bod due to the utilization of organic matter in the metabolism described . with reference to the figure , sodium nitrate was added to a sewer system in jacksonville , florida at a master pump station , or feed point b , upstream of a second master pump station comprising a monitoring point a . the feed point b was at a point removed from an intersection c of the feed line and main sewage line , as indicated in the figure . the treated sewage continued to a downstream waste water treatment plant in jacksonville , indicated as point d . average detention times ( based on average daily flows , line sizes and lengths are as follows : in terms of the description provided above , the b → c distance and retention time of 7 hours constitutes the incubation period , coupled with the distance c → a and associated retention time of 3 . 3 hours comprises a total of 10 . 3 hours from addition of the nitrate station at point b to the monitoring at point a , thereby permitting sufficient time for the bacteria to culture . the following table shows the change in dissolved hydrogen sulfide at point a , with addition of nitrate occurring at point b . during the period of time , the average daily h 2 s at point b was 25 - 30 ppm . it is readily apparent from the above chart that significant reduction in h 2 s was achieved over a nine day period of time , commencing about 24 hours after the addition of the sodium nitrate , with maximum reductions occurring after 48 hours . subjective sampling also indicated a significant reduction in sewage odors other than hydrogen sulfide . it was also found that sewage bod was also reduced or indicated as in the following table : it will thus be appreciated that the present invention provides for the removal of significant amounts of existing dissolved hydrogen sulfide and a corresponding reduction in sewage bod . by properly feeding sodium nitrate into the sewage or waste , odor and corrosion problems can also be substantially eliminated . while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment , it is to be understood that the invention is not to be limited to the disclosed embodiment , but on the contrary , is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims .