SCERP Project Number: W-ll
Principal Investigator: Sam Ghosh
University of Utah
1. Objectives of the project:
The objective of this project is to develop an appropriate and innovative bioremediation technology for the stabilization and detoxification of mixed conventional and hazardous, liquid and solid wastes of residential and industrial origin with simultaneous recovery of renewable energy as methane or electric power.
An innovative bioremediation system is being tested in our laboratory
using a bench-scale pilot plant consisting of a plug-flow anaerobic reactor
(digester), a solid-liquid separator, and a "nutrient-film" photosynthetic
process for final treatment and production of an acceptable effluent. The
pilot plant is charged with a simulated organic waste, and benzene, toluene,
and xylene (BTX), which are common hazardous wastes in the U.S.-Mexican
border area. The end products of this bioremediation system are a stabilized
and detoxified liquid effluent, and fuel gases that are used as such or
utilized to generate electric power. It is anticipated that the bioremediation
system will be field tested in the final phase of the project.
2. Activities/results/milestones:
* Develop a "direct-injection" gas-chromatographic (GC) procedure for the detection, analysis, and quantification of benzene, toluene, and ortho-xylene (O-xylene)
* Test the performance of the plug-flow anaerobic reactor with mixed conventional and hazardous pollutants
* Test the performance of the custom-designed anaerobic settler as to its ability to effect additional removal of soluble and solid pollutants
* Study biodegradation of benzene, toluene, and xylene (BTX) in the reactor and the anaerobic settler
* Complete installation of the nutrient-film system designed to provide final biological treatment of the anaerobic reactor effluent
* Grow Bermuda grass to be used in the nutrient-film system.
ANALYTICAL PROCEDURE: A GC Procedure for analysis of the hazardous feeds (BTX). A direct-injection GC procedure was developed for quantitative analysis of benzene, toluene, O- xylene in aqueous samples. A computer-operated Perkin-Elmer GC equipped with a FID was used to analyze feed and reactor samples for BTX. Chromatographic separation of the three hazardous substances was accomplished by injecting centrifuged samples through a guard column and into a glass capillary column. Appropriate temperature programming was used to effect separation of the chromatographic peaks representing the three hazardous substances. Calibration curves were developed to determine BTX concentrations in liquid- and gas-phase samples. A GC procedure was also developed to detect and quantify phenol, an intermediate of catabolic pathways utilized in microbial breakdown of BTX.
ANAEROBIC REACTOR PERFORMANCE: The anaerobic reactor was operated at a mesophilic temperature of 35 deg C with continual feeding of a synthetic organic substrate simulating conventional solid and liquid pollutants. The composition of the system feed was reported in an earlier quarterly report. The feed was withdrawn from a mechanically mixed, refrigerated feed reservoir, and delivered to the digester by a timer-operated pump. This quarter, a Teflon-coated aluminum floating cover was installed in the feed reservoir to minimize volatilization of BTX included in the process feed. It may be recalled that the anaerobic bioreactor was operated at an HRT of 16 days at the end of last quarter. This quarter, the reactor HRT was decreased in steps from 16 days to 15.2 days to 13.3 days to 11.8 days to 10.8 days. The feed concentration expressed in terms of total COD was about 11, 000 mg/l excluding the COD of benzene, toluene, and/or xylene. The COD loading rate increased in inverse proportion to the decrease in HRT. Bottom sludge collected from the reactor floor near the effluent end was recirculated to the reactor inlet at a flow rate equal to 20% of the influent feed rate to enhance and accelerate the bioconversion process. Toluene was included in the reactor feed since September 25, 1992. Toluene concentration in the feed varied between 49.2 and 50.3 mg/l. About three percent of the toluene escaped into the gas phase, and 97% of the added toluene was available for biodegradation. The bioreactor design may be modified later to eliminate or significantly reduce the loss of toluene or other volatile hazardous substances into the gas phase.
After completion of steady-state operation and beginning on December 17, benzene and xylene were included in the feed along with toluene. Benzene and xylene concentrations were increased gradually from a low level of 10 mg/l to 50 mg/l during the last two weeks of this quarter; benzene concentration was further increased to 100 mg/l. Digester performance was not significantly affected by inclusion of benzene and xylene in the feed.
Reactor operation this quarter can be divided into three segments: transient-state operation during which time the bioreactor HRT was reduced from 16 days to 13.3 days; steady- state operation at an HRT of 13.3 days; and transient-state operation at reduced HRTs of 11.8 days and 10.7 days. The one- liter capacity anaerobic settler was put in operation five days after completion of the steady-state run at an HRT of 13.3 days. Digester performance during the steady-state period is summarized in Tables 1 and 2. Data presented in these tables show excellent bioreactor performance in terms of effluent quality and efficiency of biodegradation of conventional pollutants (COD, solids, and protein) and toluene. Data presented in Figures 2 through 4 and Figures 6 through 8 show that the conventional pollutants (COD and solids) were removed within the first one-third of the reactor. In contrast, most of the toluene removal occurred within the first two-thirds of the reactor (Figure 5). These data indicate that the bioreactor was underloaded in terms of conventional pollutants, and that bioreactor design may be controlled by the kinetics of toluene biodegradation.
Bioreactor performance (including removals in the anaerobic settler)
under unsteady-state conditions at an HRT of 10.7 days as shown in Tables
3 through 6 indicated 96% removal of benzene, 91% removal efficiencies
of toluene, 98% removal of xylene, 98% removal of total suspended solids
(TSS), 99% removal of volatile suspended solids (VSS), and 97.5% removal
of total COD. During this transient-state operation, the reactor effluents
contained phenol in concentrations of 5.2 and 5.6 mg/l; detection of this
intermediate of the catabolic pathways of BTX provided evidence that the
observed removals were attributable to biodegradation of these hazardous
substances. The collected information showed that the anaerobic bioreactor
had excellent capabilities to stabilize conventional pollutants and eliminate
BTX.
INSTALLATION OF THE NUTRIENT- FILM("THIN-FILM") SYSTEM. Installation of the nutrient-film system was continued this quarter. A 400-watt daylight halide lamp was installed to serve as the light for growth of the selected biomass, Bermuda Grass (Cynodon dactylon). This lamp emits light with a spectrum similar to that of natural sunlight. The nutrient- film system would be operated with settled anaerobic bioreactor effluents emanating from an anaerobic settler which was made operational this quarter. Effluents from the nutrient-film system would be recycled to the inlet end to dilute the system feed. A 1.7-GPM centrifugal pump with a magnetic-drive was acquired and installed this quarter to be able to recirculate nutrient-film system effluents. The timer- operated pump has a ceramic shaft and Viton O-ring gaskets, and is of stainless steel construction so that it is resistant to attack by BTX. The growth trays supporting the Bermuda grass will be covered with clear glass covers to prevent any possible escape of BTX in the laboratory environment. Design of these covers was completed this quarter.
Bermuda grass was grown by placing the seeds on a sponge pad wetted by a nutrient solution. Table 7 presents the composition of the synthetic nutrient solution used to grow grass. Germination of the seeds occurred, but the rate of growth of the seedlings was very slow even after three to four weeks of incubation. Since good grass growth was not obtained by this growing technique, the nutrient solution was recirculated to continuously wet the sponge pad in an attempt to accelerate seedling growth. In addition, a plastic cover was placed over the sponge bed to increase the humidity of the growth environment. The modified growing condition was beneficial; the grass seedlings exhibited more rapid growth.
The temperature of the growth environment was 30 deg C with the halide light off and 35 deg C with the light on; this temperature range was above the minimum Bermuda grass- growth temperature of 21.1 deg C, and it included the optimum growth temperature of 35 deg C.
Operation of the nutrient-film system will be started after the grasses
grow to a height of about 12 inches. The grass will be harvested when they
reach a height of about 24 inches. The harvested grass will be included
in the anaerobic bioreactor feed to simulate recommended operating practice
in the U. S.-Mexican border area.
3. If appropriate, identify any difficulties encountered and remedial actions taken:
As discussed above, difficulties were encountered in growing Bermuda
grass, but remedial measures were taken to overcome these problems.
Last updated 7/1/99