WRRCTMR No.44 Nitrogen Removal in the Operation of the Mililani Sewage Treatment Plant

Abstract

It had been observed over a one-year period, January 1972 to January 1973, that the Mililani Sewage Treatment Plant (STP), located in central Oahu, Hawaii, discharged in its effluent only approximately 30% of the total nitrogen it received in its predominantly domestic raw sewage influent. During the period that high nitrogen removal rates were observed, the STP was operated as a secondary plant with raw sewage, after comminution and aerated grit removal, going directly to the "Rapid Bloc" activated sludge unit. Sludge stabilization was by aerobic digestion. During the fall of 1973, a primary sedimentation tank and an anaerobic digester, which was to replace the aerobic digester, were added to the components of the STP. The plant was designed for an average waste water flow of 0.93 mgd capacity before modification and 1.81 mgd after modification. Inasmuch as the relatively high removal rate (about 70% in 1972) rivals present expensive advanced waste water operations that were designed specifically for nitrogen removal, and considering the difficulty and expense of removing significant quantities of nitrogen in waste water, a study was initiated for a one-year period, 1 July 1973 to 30 June 1974, in an attempt to determine the cause for such a high nitrogen removal rate both before (Phase I) and after (Phase II) modifications to the STP occurred in the faU of 1973. The waste water flow during Phase I, 9 July to 6 August 1973, averaged 0.665 mgd with an overall total nitrogen removal of about 54%. The major nitrogen loss, speculated to be by means of gaseous ammonia to the atmosphere, apparently occurred in the aeration unit, settling tank, and aerobic digester. During Phase II studies, January through June 1974, the mean monthly flow increased to a range of 0.838 mgd to 0.904 mgd; however, the mean total nitrogen loss decreased to 29%, a range that is typical for conventional secondary activated sludge operations. In both Phases I and II, ammonia nitrogen was the predominant form showing major losses.