No abstract

The digestion process at the Hyperion Treatment Plant received Class A Exceptional Quality (EQ) Biosolids certification from the EPA, Region 9 on December 27, 2002. Currently, the feed to the digesters is a blend of Waste Activated Solids thickened by centrifuge to 6% and nonthickened Primary Solids. Investigations are underway to determine if the use of Thickened Primary Sludge can be employed to optimize the process. Potential benefits are twofold. Firstly, a reduction of the water content being sent to the digestion process will dramatically reduce the mass heated to thermophilic temperatures. This is significant as the energy needed to raise the temperature from Mesophilic to Thermophilic is approximately three times more at a cost of $300,000 per month. Secondly, the concentrated biosolids being digested effectively increases much needed digester volume necessary to achieve Class A Biosolids through Alternative 1-Time/Temperature. In effect this will increase operational reliability, as more units will be available for maintenance, standby duty or future treatment needs.To that end, the City engaged in large scale pilot testing to determine the viability of primary sludge thickening through the use of centrifugation. During the pilot study, centrifuges from two vendors were tested. Both centrifuges were able to thicken primary sludge to as high as thirteen (±13%) percent total solids concentration. Throughout the tests, both solids recovery and centrate concentrations were found to be satisfactory.Based on this preliminary information, it became apparent that a second set of pilot tests was necessary. These tests were aimed at determining the practical limit of primary solids dewatering relative to the operation of the thermophilic anaerobic digestion process. The investigation was aimed at the determination of two possible queries; determine the maximum (%TS) concentration of biosolids allowable to pump, and determine the maximum concentration of biosolids allowable prior to inhibiting the digestion process.

In 1999, the Environmental Engineering Division (EED), Bureau of Engineering, City of Los Angeles, designed and constructed a pilot facility at the Hyperion Treatment Plant. The purpose of the facility was to pilot test various advanced digestion scenarios in an effort to advance the digestion process at the Hyperion Treatment Plant. The outstanding feature of the facility is that it thoroughly removes itself from the typical bench scale testing approach by offering full-scale data under real plant conditions without risk to the plant's full scale solids processing system and it activities.The Hyperion Treatment Plant's solids handling processing was converted to Class A Biosolids production in 2002. This conversion has brought forth some interesting queries with regard to temperature and hydraulic retention time as they relate to the time/temperature regime. Two studies were completed to address those queries; a Temperature Stress test and a Hydraulic Retention Time Stress test. As the results of these studies have already been reported, the pilot facility continued these studies as they would related to a more recent difficulty, odors. In the early stages of conversion, Hyperion experienced very high odor levels due to mercaptan concentrations in the off gas. The previous tests and their current setup provided available digestion scenarios as the means to investigate the phenomena. Digester temperature was carefully controlled and performance closely monitored. Again, the observations from these studies will be a part of this paper where data indicates mercaptan levels to decrease with control of temperature.