Evaluation of thermophilic anaerobic digestion processes for full-scale Class A biosolids disinfection at Hyperion Treatment Plant.

This paper describes 5 phases of full-scale testing at the City of Los Angeles Hyperion Treatment Plant (HTP) for producing Class A biosolids (U.S. EPA Part 503 Biosolids Rule) by thermophilic anaerobic digestion. Phases I and II were tests with a two-stage continuous-batch process in a thermophilic battery of six digesters and a designated post-digestion train that was isolated from mesophilic operations. These tests demonstrated that digester outflow biosolids met the Class A limits for fecal coliforms and Salmonella sp. However, fecal coliform densities sharply increased during post-digestion. The recurrence was possibly related to a combination of a large drop of the biosolids temperature after the dewatering centrifuges and contamination of thermophilically digested biosolids from mesophilic operations. Phase III was conducted after insulation and electrical heat-tracing of the post-digestion train to maintain a biosolids temperature throughout post-digestion at about the same level as in the digester outflow. Biosolids monitoring at the last points of plant control (silos at Truck Loading Facility and farm for land application) indicated that fecal coliform recurrence was prevented. After completing the conversion of HTP to thermophilic operation, certification tests of Phases IV and V demonstrated Class A compliance of a two-stage continuous-batch process under Alternatives 1 and 3 of the Part 503 Biosolids Rule, respectively. HTP received the permit for Class A (indeed exceptional quality) biosolids land application in Kern County, California, in December 2002 under Alternative 3. Since 2003, HTP has consistently complied with the federal and local standards for Class A biosolids, indicating that Class A limits can be met under conditions less stringent than defined by the Alternative 1 time-temperature requirement for batch treatment.

Thermophilic-anaerobic digestion to produce class A biosolids: initial full-scale studies at Hyperion Treatment Plant.

The highest quality of biosolids is called exceptional quality. To qualify for this classification, biosolids must comply with three criteria: (1) metal concentrations, (2) vector-attraction reduction, and (3) the Class A pathogen-density requirements. The City of Los Angeles Bureau of Sanitation Hyperion Treatment Plant (HTP) (Playa del Rey, California) meets the first two requirements. Thus, the objective of this study was to ensure that HTP's biosolids production would meet the Class A pathogen-reduction requirements following the time-temperature regimen for batch processing (U.S. EPA, 1993; Subsection 32, Alternative 1). Because regulations require the pathogen limits to be met at the last point of plant control, biosolids sampling was not limited to immediately after the digesters, i.e., the digester outflows. The sampling extended to several locations in HTP's postdigestion train, in particular, the last points of plant control, i.e., the truck loading facility and the farm for land application. A two-stage, thermophilic-continuous-batch process, consisting of a battery of six egg-shaped digesters, was established in late 2001 for phase I of this study and modified in early 2002 for phase II. As the biosolids were discharged from the second-stage digesters, the Salmonella sp. (pathogen) and fecal-coliform (indicator) densities were well below the limits for Class A biosolids, even though the second-stage-digester temperatures were a few degrees below the temperature required by Alternative 1. Salmonella sp. densities remained below the Class A limit at all postdigestion sampling locations. Fecal-coliform densities were also below the Class A limit at postdigestion-sampling locations, except the truck-loading facility (phases I and II) and the farm for final use of the biosolids (phase II). Although federal regulations require one of the limits for either fecal coliforms or Salmonella sp. to be met, local regulations in Kern County, California, where the biosolids are land-applied, require compliance with both bacterial limits. Additional work identified dewatering, cooling of biosolids after the dewatering centrifuges, and contamination as possible factors in the rise in density of fecal coliforms. These results provided the basis for the full conversion of HTP to the Los Angeles continuous-batch, thermophilic-anaerobic-digestion process. During later phases of testing, this process was demonstrated to produce fully disinfected biosolids at the farm for land application.

Effects of transient temperature increases on odor production from thermophilic anaerobic digestion.

The City of Los Angeles, Bureau of Sanitation, has implemented thermophilic anaerobic sludge digestion at the Hyperion and Terminal Island Treatment Plants (HTP and TITP). A two-stage continuous-batch process was established at HTP, while a single-stage sequencing batch process was established at TITP. This was to evaluate compliance with the Class A pathogen reduction requirements of U.S. EPA 40 CFR Part 503. A rapid increase of the digester temperature at TITP from 57.5 to 65.5 degrees C caused an increase of the volatile fatty acid to alkalinity ratio, a decline in digester performance, and an elevated production of methyl mercaptan and hydrogen sulfide. A rapid increase of the digester temperature at HTP from 54 to 58 degrees C caused an elevated production of methyl mercaptan, but the effect on the volatile fatty acid to alkalinity ratio and digester performance was insignificant. It is likely that these effects observed at TITP and HTP were transient responses to rapid changes in temperature.

Solving fecal coliform growth/reactivation in biosolids during full-scale post-digestion processes.

Fecal coliform recurrence has been observed at the City of Los Angeles Hyperion Treatment Plant during pilot-scale experiments with a designated thermophilic battery of six anaerobic digesters, while other digesters were still at a mesophilic temperature. Several lab and full-scale experiments indicated the following possible causes of the growth/reactivation of fecal coliforms in post-digestion: a) contamination of thermophilically digested biosolids with mesophilically digested biosolids; b) a large drop in the biosolids temperature between the centrifuges and silos, which could have allowed the reactivation and/or growth of fecal coliforms. These were resolved by the full plant conversion to thermophilic anaerobic digestion and design modifications of the post-digestion train.

Odor and volatile organic compound removal from wastewater treatment plant headworks ventilation air using a biofilter.

Laboratory-scale experiments and field studies were performed to evaluate the feasibility of biofilters for sequential removal of hydrogen sulfide and volatile organic compounds (VOCs) from wastewater treatment plant waste air. The biofilter was designed for spatially separated removal of pollutants to mitigate the effects of acid production resulting from hydrogen sulfide oxidation. The inlet section of the upflow units was designated for hydrogen sulfide removal and the second section was designated for VOC removal. Complete removal of hydrogen sulfide (H2S) and methyl tert-butyl ether (MTBE) was accomplished at loading rates of 8.3 g H2S/(m3 x h) (15-second empty bed retention time [EBRT]) and 33 g MTBE/(m3 x h) (60-second EBRT), respectively. In field studies performed at the Hyperion Treatment Plant in Los Angeles, California, excellent removal of hydrogen sulfide, moderate removal of nonchlorinated VOCs such as toluene and benzene, and poor removal of chlorinated VOCs were observed in treating the headworks waste air. During spiking experiments on the headworks waste air, the percentage removals were similar to the unspiked removals when nonchlorinated VOCs were spiked; however, feeding high concentrations of chlorinated VOCs reduced the removal percentages for all VOCs. Thus, biofilters offer a distinct advantage over chemical scrubbers currently used at publicly owned treatment works in that they not only remove odor and hydrogen sulfide efficiently at low cost, but also reduce overall toxicity by partially removing VOCs and avoiding the use of hazardous chemicals.

Changing mesophilic wastewater sludge digestion into thermophilic operation at Terminal Island Treatment Plant.

This paper describes the progress up to June 2000 for thermophilic digestion of wastewater sludge at the Los Angeles, California, Bureau of Sanitation's Terminal Island Treatment Plant. The development of the microorganism culture has followed a course similar to that seen at other successful plants for establishment of a stable, well-balanced thermophilic culture in a large digester, but at an accelerated pace. This study began with rapid heating, increasing the temperature of the 4500 m3 (1.2 mil. gal) digester to the target temperature of 55 degrees C at approximately 3 degrees C/d. A method of feeding to maximize the rate of culture development was used as feeding accelerated to approximately 400 m3/d (0.1 mgd). An initial rise of acid concentration (primarily acetate) was seen. Within two weeks, acid concentration declined and stabilized, indicating that acidogenic and methanogenic microbial communities came into balance. Coliform data indicate that digester disinfection was stably effective from the middle of April. The salmonella tests done to date satisfy the U.S. Environmental Protection Agency (U.S. EPA) class A specification. Testing with helminth ova and enteric viruses before and after the digester shows satisfaction of class A standard for those organisms. The present combination of low volatile fatty acids and low hydrogen sulfide is good news for odor control. The data show increases in volatile solids destruction and estimated gas production, compared with the previous mesophilic operation; however, large uncertainties have been calculated from the data. As the digester is now operating successfully at the current feed rate, there seems to be no barriers to processing the entire sludge production of the plant. Other results indicate that the U.S. EPA requirements for exceptional quality class A biosolids are likely to be achieved.

Odor and volatile organic compound treatment by biotrickling filters: pilot-scale studies at hyperion treatment plant.

A pilot-scale biotrickling filter was installed at the Hyperion Treatment Plant in Los Angeles, California, to study hydrogen sulfide (odor) and volatile organic compound (VOC) removal from headworks waste air. The performance of the reactor was continuously monitored during a 10-month period. At an average empty bed gas residence time of 24 seconds, 10 to 50 ppm of hydrogen sulfide was consistently removed at greater than 98% efficiency, corresponding to an average volumetric elimination capacity of 5.2 g/m3 x h. Concentration profiles over the height of the reactor indicated nearly complete removal in the first section of the reactor, suggesting that elimination capacities up to 30 g/m3 x h could be obtained. The odor reduction (as dilution to threshold) was 98%, which correlated with the efficiency of removal of hydrogen sulfide as the primary pollutant. Volatile organic compounds were present at concentrations up to 225 ppb. Moderate but significant removal of toluene and benzene was observed when the biotrickling filter was operated with pH control to neutralize sulfuric acid production from hydrogen sulfide oxidation. Xylenes and chlorinated VOCs were not removed regardless of experimental conditions in the reactor. The results led to the conclusion that VOC removal is the limiting process in biotrickling filters for the simultaneous removal of hydrogen sulfide and VOCs at publicly owned treatment works.