Testing of Two-Stage Biofiltration Unit for Mitigation of VOC Emissions
Volatile organic compounds (VOCs) present in crude oil can be released to the atmosphere from storage tanks, waste waters, and equipment leaks. A pilot-scale sequential biotrickling/biofiltration (BTF/BF) unit was designed and tested as an environmentally safe and economical treatment procedure for removal of VOCs from a wastewater sump at a Texas refinery. This pilot study can be used to design and optimize the performance of a bio-oxidation unit under actual refinery operating conditions.
Biological-treatment technology has developed rapidly in the US over the past 30 years. It is an environmentally friendly and cost-effective method of mitigating VOC emissions to the atmosphere that relies on the natural characteristics and abilities of microorganisms. Low operational cost and low energy requirements make this method economically and environmentally attractive to many industries. Production of only nonhazardous byproducts such as carbon dioxide, water vapor, and biomass is another advantage of this method as compared with conventional treatment processes.
A typical BF system usually consists of natural organic packing media such as peat, soil, or compost, which must be kept damp by periodic spraying. The biofilm surrounding the filter media uses the contaminants as a carbon and energy source in the presence of nutrients. Another type of biological system commonly used for VOC removal is BTFs. In contrast to BFs, inorganic inert packing media, which does not consist of any inherent microorganisms or nutrients, is used in BTFs. In addition to the biofilm attached to the packing media, microorganisms suspended in the recirculating liquid also can help in degradation of VOCs in BTFs.
Site and Pilot-Scale-System Setup. Approximately 60 different wastewater streams were directed to this sump from which two streams, BTX-7 and HYD-23, were continuous, with the largest drainage coming from a benzene column reboiler. The condensate vaporization of the wastewater in the sump resulted in VOC emissions.
For treating VOC emissions from the wastewater sump, two bioreactor vessels were designed and constructed from Plexiglas. In the first vessel (BTF), crossflow plastic media was used to obtain an optimal gas/biofilm interface as well as durable reactor operation. For the second unit, engineered compost-based media was selected.
An induced draft fan located at the outlet of the sequential BTF/BF unit was used to pull VOC vapors from the wastewater sump to the treatment system. Using the valve installed above the induced fan, the air-flow rate could be changed. Air with a flow rate of 16 m3/hr was introduced to the unit, corresponding to a total retention time of 90 seconds. A three-way valve was installed at the outlet of the fan as a safety precaution. The irrigation system comprised a water pump and a nutrient-solution tank that continuously sprayed the BTF media.
Analytical and Monitoring Techniques. Before beginning the pilot test, a VOC characterization was performed by collecting and analyzing air samples from the headspace of the wastewater sump. Monitoring of the pilot-scale unit included analyzing the concentration of total VOCs continuously at the inlet, middle, and outlet of the unit using photoionization detectors (PIDs). Air-sampling ports were mounted at different heights of the column walls to analyze air samples. An average of the PID instantaneous readings in isobutylene equivalent parts-per-million (ppm) concentrations provided an estimate of the total VOC concentration at each location of the unit.
To determine the empty bed retention time (EBRT) in each unit, a handheld anemometer was used to measure the air-flow rate at different sampling ports of the unit and adjust the air-flow rate if needed. The water-flow rate was monitored using a digital flowmeter installed on the water pipeline and can be controlled using a ball valve.
Results and Discussion
Characterization of Emissions From Wastewater Sump. The wastewater sump at the refinery operated full-time, receiving several streams from the refining-process units and facilities. Hence, it became an emission source of various organic and inorganic compounds. High and fast flow rate of streams to the sump, and water turbulence, caused vaporization of VOCs and pushed them into the air from the water. In this study, eight VOCs belonging to C6 to C8 compounds were detected and identified at the headspace of the wastewater sump.
The detected VOCs can be classified into two main groups, including aromatics and alkanes. An average concentration of total VOCs was 6,857 parts-per-million volume (ppmv), which was the sum of the concentration of individual VOCs. The most-abundant detected aromatic species was benzene, followed by p-xylene. The average concentration of benzene inside the wastewater sump was 5,405 ppmv and accounted for 78 and 92% of total VOCs and the total aromatics, respectively. Benzene was expected to be the dominant VOC at the sump because the benzene column reboiler constituted the largest drain rate to the sump. Hence, benzene was considered as a marker of VOCs at the wastewater sump and the main target for biofiltration. Toluene, with a concentration of 164 ppm, had the lowest concentration among the aromatics.
As for alkanes, an average concentration was 990 ppm, with the highest concentration attributed to pentane, 3-methyl-, followed by heptane. All detected alkanes had branched structures and the most abundant alkanes were C6, accounting for 55% of total alkanes. The percentage value of alkanes was 14.46%, with aromatics taking up approximately 85.54% of the total VOCs by volume. Generally, numerous VOCs can be found in typical petroleum refinery waste water. The quartet of benzene, toluene, ethylbenzene, and xylene, commonly known as BTEX, is the most-common compound, followed by phenol, styrene, hexane, and naphthalene.
Pollutants with high Henry’s constant and low solubility are more difficult to remove by biodegradation process because of their unfavorable gas/liquid proportion. Thus, on the basis of the characterization results, it could have been predicted that the biodegradation process could successfully eliminate the main elements of the wastewater sump. However, a high biodegradation rate could not be sustained for more hydrophobic VOCs, such as detected alkanes, in the wastewater sump as a result of the low concentration of pollutants in the liquid biofilm.
Performance of the Pilot-Scale Unit. The pilot-scale unit was set up at the refinery to treat VOC emissions from the wastewater sump from 2 July to 11 July 2014. The total concentration of VOCs at the wastewater sump ranged from 66 to 15,000 ppm. The average was 1,618 ppm, which corresponds to 204 g/m3.hr as an average inlet loading rate. The daily average outlet concentration of VOCs was started from 1,443 ppm on Day 1 and decreased to 5 ppm on the last day of the pilot test.
In general, the total VOC concentration at the wastewater sump was highly variable according to the PID data. Fig. 1 shows instant variations in VOC concentrations on a typical day. Intermittent pattern and variability in concentration of total VOCs can be attributed to the fluctuations in temperature, production process, flow rate, and concentration of inlet streams to the BTEX wastewater sump. Over the course of the pilot test, the temperature in the sump ranged from 36°C (average on Day 6) to 48°C (average on Day 1). When the temperature of the water in the sump was increased, the vapor pressure of the liquid increased which, in turn, caused higher emission of VOCs to the air.
Apparently, the highly variable loading rate did not lead to poor performance of the pilot unit, demonstrating the robustness of this technology for industrial emissions. Despite fluctuations in the inlet concentration of VOCs to the system, the removal efficiency (RE) was satisfactory and quite stable during the entire experimental period. This demonstrated not only the high capacity of microflora to withstand high variations in VOC concentration, but also the relative resistance of the refinery waste water to high concentrations of toxic VOCs such as BTEX.
The maximum detected concentration of total inlet VOCs was 15,000 ppm, which was measured during the startup period. Early performance of the BTF/BF unit on Day 1, following inoculation with refinery waste water, showed 75% RE for total VOCs. To assure an adequate supply of microorganisms capable of degrading VOCs, the system was inoculated continuously for 3 days by spraying the inoculum over the media in both units. The RE exceeding 75% confirmed progressive adaption of microbial population in the wastewater to the contaminated VOCs during the startup period.
The RE gradually increased, reaching an average 86% VOC removal within 7 days of operation, including startup and system operation. This pilot test was only conducted for 1 week. The continuous increase in the RE of the system showed that, if the test were to be continued for a few more months (at least 6 months), a higher average RE could be achieved because a thicker biofilm could grow on the surface of the packing media.
On Day 6, an average inlet concentration of total VOCs to the BTF/BF unit was 84 ppm, increasing to 168 ppm on Day 7. In spite of an increase in the concentration of influent to the unit, the system responded well to this change by sustaining the RE at more than 90%. With increasing VOC loading rate, the elimination capacity (EC) increased linearly up to an organic loading rate of 200 g/m3.hr. Hence, when the VOC loading rate is less than 200 g/m3.hr, nearly 100% RE could be achieved. Beyond 200 g/m3.hr, an increase in EC was slower.
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