Hazardous air pollutants are heavily regulated and control technologies can be expensive and generate high carbon footprints. This article outlines new wastewater emissions protocols that are providing a cost effective approach with reduced environmental impact.

For industries with a need to reduce biodegradable hazardous air pollutants, the environmental restrictions are stringent. The United States Environmental Protection Agency’s (USEPA) National Emission Standards for Hazardous Air Pollutants (NESHAP) compliance regulations, require 95% or greater removal of regulated hazardous air pollutants (HAPs) in off-gas from storage vessels, process vents and wastewater equipment. The Benzene Waste Operations NESHAP (BWON) regulations for refineries require 98% benzene removal, or 95% total volatile organic carbon (VOC) removal.

Previously, the only control technologies routinely acceptable to the USEPA were vapour phase adsorption and thermal incineration, including flares, which are expensive both in terms of initial capital and operationally, and generate high carbon footprints. The regulations do, however, allow the implementation of an alternative control device (ACD), which is subject to 98% benzene or 95% VOC removal and specified operating and monitoring conditions.

Innovative protocols have been developed by ENVIRON and Marathon Petroleum Corporation (MPC) in Garyville, Louisiana, to certify an ACD that is cost effective and has a minimal carbon footprint.

This benzene biodegradation system was approved in writing by the State of Louisiana and accepted by the USEPA Research Triangle, North Carolina. One of these integrated systems is now approved as enhanced biodegradation units (EBU) and ACDs for treatment of benzene under BWON criteria.

Following these approvals there is potential for any properly designed and operated activated sludge system to be qualified as an ACD when developed following these protocols.

Case history

Marathon Petroleum Corporation (MPC) Garyville, Louisiana Refinery This project demonstrated the development of protocols for the certification of an ACD for benzene biodestruction using an existing activated sludge system.

Constructed in 1976, the MPC Garyville refinery was the most recent grass roots refinery built in the United States. It currently complies with the six megagrams/year (Mg/yr) ‘treat to target’ option under BWON (section 342e – 2i), which requires: • Total benzene mass in all uncontrolled aqueous wastes are less than 6Mg/yr benzene • All organic wastes are controlled

MPC’s new source review (NSR) global settlement also requires the facility to comply with the end of line sampling plan, with less than 4.8 Mg/yr uncontrolled benzene. The facility installed its first innovative system in 2005 that integrates bioreactor and clarifier, to supplement three older existing external clarifier activated sludge trains.

The MPC Garyville major expansion (GME) project was completed in May 2009 and increased capacity from 185,000 to 370,000 barrels per day (bpd). Wastewater treatment plant (WWTP) capacity increased from 477 m3/hr to 1,432 m3/hr.

The facility added a new parallel WWTP train, including a new: • API – oil/water separator • IGF – induced gas floatation • Closed circuit cooling tower • Integrated activated sludge system, installed in May 2009 to handle the entire wastewater expansion

MPC Garyville had already installed an activated carbon system when the refinery was approached regarding a payback on the carbon, using biodestruction in the existing integrated biosystem.

Integrated systems

An integral system is a conventional activated sludge system where the secondary clarifier is integrated into the bioreactor. The configuration is ideal for maximum nitrification-denitrification and a substantial number of new or retrofitted refineries and petrochemical-organic chemical plants have selected this activated sludge configuration.

The deeper bioreactors and low surface area indicated some integral systems could also be utilised effectively for high efficiency treatment for regulated biodegradable VOC constituents. MPC Garyville therefore offered support to evaluate the feasibility of certifying one of these such integral systems as an ACD for the biotreatment of benzene, subject to BWON regulations.

Project drivers

Wastewater gaseous emission considerations apply to regulated VOC emissions from wastewater treatment related processes, such as API separators, dissolved air and induced air flotation processes, uncovered tanks, and include sumps and wet well emissions. The fugitive off-gas emissions at the MPC Garyville refinery WWTP were two API separators, associated sumps, and the dissolved and induced air floatation processes. Current control of these emissions is very energy intensive and responsible for substantial greenhouse emissions using traditional control technologies.

Regulatory drivers

Under BWON regulations, an ACD must be used to reduce benzene emissions from these sources by 98%. Current approved controls are thermal oxidisers and vapour-phase activated carbon. The regulations also allow for an ACD, provided that certain performance and operating guidelines are followed.

Specifically, Title 40: Protection of Environment: 40 CFR section 61.340 also states ‘other’ control devices can be used provided that certain conditions are met, including that:

• The device shall recover or control the organic emissions vented to it with an efficiency of 95% weight or greater, or shall recover or control the benzene emissions vented to it with an efficiency of 98% weight or greater

• The owner or operator shall develop test data and design information that documents the control will achieve an emission control efficiency of either 95% or greater for organic compounds, or 98% or greater for benzene

Furthermore, the owner or operator shall identify:

• The critical operating parameters that affect the emission control performance of the device

• The range of values of these operating parameters, which ensure the emission control efficiency specified in paragraph (a)(2)(iv)(A) of the aforementioned regulation is maintained during operation of the device

• How these operating parameters will be monitored to ensure the proper operation and maintenance of the device

Economic and sustainability drivers

This approach is based on utilising existing biotreatment facilities for biodegradation of benzene and other regulated VOCs, rather than more expensive activated carbon and thermal oxidation. The capital costs for implementation will be very low, simply piping and proper conveyance from the sources to the biotreatment facility.

Table 1 presents the economics of the common control devices and the preliminary ACD projections for MPC Garyville.

Table 2 presents the impact of energy and carbon footprint on the currently approved control devices by US EPA.

Ideally, the ACD should be an existing biotreatment facility, and more specifically, an activated sludge system.

In order to achieve the required 98% benzene biodestruction, the ACD must be configured to thoroughly mix and adequately aerate, disperse and transfer the benzene from the gas phase to the liquid phase for microbial assimilation.

This objective is evaluated by developing a site-specific predictive model, which allows evaluation of significant parameters to determine the most effective operation of the system. Additionally, each system must be qualified using site-specific criteria.

Project approach

Due to the unique nature of this project and the request to use a modification of USEPA’s methodology to demonstrate a reliable and realistic benzene biodegradation rate, it was decided to coordinate and interface with both the USEPA office of air quality planning and standards – research triangle park, North Carolina, and the Louisiana Department of Environmental Quality (LDEQ).

The basic project approach included:

1. A preliminary feasibility estimate of performance and economics of utilising the existing activated sludge system as an ACD.

2. Confirmation that the integrated activated sludge process at Garyville is thoroughly mixed and aerated properly in an enhanced biodegradation unit (EBU).

3. Development of a site-specific model for accurately predicting benzene biodegradation.

4. Development of a site-specific Henry’s Law coefficient and benzene biodegradation rate.

5. Full scale demonstration of 98% benzene biodegradation.

Items 1 and 2 have been discussed above. Items 3, 4 and 5 are discussed below.

Benzene biodegradation model Several basic data needs are required when developing the site-specific predictive model. Major variables include the benzene biodegradation rate, air flow, biomass concentrations, potential benzene injection locations into integrated systems and benzene loadings.

Other significant variables include: • Air distribution in zones • Depth of bioreactor • Aeration tank surface area • Temperature • Hydraulic flow rate and chemical oxygen demand (COD) loading • Sizing, dimensions and configuration of bio unit • Maximum operating criteria of bio unit • Mass balance of benzene in emissions and water

Considering the data input, it is possible to develop the site-specific model to predict the percentage of benzene removal by selecting design parameters, assuming benzene biodegradation rate and calculating the percentage of benzene removal at the assumed biodegradation rate.

Based on the raw data and system operating variables, the specific modelling variables and the assumed benzene biodegradation rate, it is also possible to evaluate influences of parameters on the system performance regarding benzene biodegradation and stripping. Modelling results are then generated using assumed benzene biodegradation rates.

The preliminary modelling results are then examined for limitations of the system and desired operational flexibility. The benzene destruction efficiency can be improved by lowering the airflow rate, which reduces stripping, and/or by increasing the biomass concentration, which increases the mass rate of benzene biodestruction. The WWTP operators did not, however, want to vary from current operational protocols, so it was decided that a benzene biodestruction rate of greater than 6.5 would be required in order to maintain current operational criteria, e.g. airflow of 3,200 standard cubic feet per minute (SCFM) and mixed liquor suspended solids (MLSS) of 3,000 mg/L.

Preliminary modelling results showed that a minimum benzene biodegradation rate of 6.5 L/g MLVSS-hr is required to achieve 98% gaseous benzene removal in the current operating conditions. If the air flow rate is <6 L/gm-hr, a system will require lower air flow rate and/or higher mixed liquor volatile suspended solids (MLVSS) concentrations; however, neither option is desirable for operational protocol on site. Operators wished to remain at 3,200 SCFM and 3,000 gm/L MLVSS.

Henry’s law and benzene biodegradation

The USEPA offers five experimental methods for determining the fraction or rate of benzene biodegradation. Due to the very low inlet and assumed outlet benzene concentrations, the laboratory BOX test method was chosen.

At only two litres, the BOX test apparatus suggested by USEPA was considered too small in volume and it was felt that more precise rates could be achieved in a larger volume bench reactor. A modified version of the BOX apparatus was constructed and tested, then proposed to the USEPA.

In the absence of biomass, the benzene is completely stripped in 350 to 400 minutes. With biomass, the complete biodegradation of benzene is carried out in about 30 minutes.

The following conclusions were taken from the BOX test and subsequent calculation procedures:

• The K1 site-specific biodegradation rate is 29.3 L/g VSS-hr at 25.8o C. This rate must be corrected to 20o C for input into certain models

• The site-specific biodegradation rate, corrected to 20o C, is 22.6 L/g VSS-hr at 20o C

• Some models will adjust this rate to the selected temperature for full scale operating conditions

The model was calibrated with the site-specific biodegradation rate:

• Final modelling at the BEST injection location (inlet to aeration blowers) and at the temperature corrected (20o C) benzene biodegradation rate, has confidently predicted more than 99% removal of benzene in the IGF and API off-gas and sumps, without modifications to the aeration system, e.g. aeration rate, and biomass concentration

• IGF and API benzene off-gases are assumed to be injected into the inlet of the aeration blowers at 3,200 SCFM total air flow to bioreactor

• The modelled benzene removals, as a function of MLVSS and air flow rate (SCFM) at 30o C (annual average at MPC Garyville)

• The final site-specific modelling of the benzene removals, as a function of MLVSS and air flow rate (SCFM) at 30°C (annual average at MPC Garyville), shows that is presented by the upper blue curve at the top of the graph. Considering that there is little variation in benzene removal over a range of MLVSS concentrations from 3,000 mg/L to 5,000 mg/L; this means that the benzene removal from the gas stream is very operationally robust

Full scale confirmation of 98% benzene biodegradation

Although full scale verification of performance is optional before implementing capital improvements, it is prudent to understand the capacity and robustness of the site biosystem to benzene biodegradation before initiating the changes.

As full scale verification, an external core column is recommended to represent a ‘core sample’ of the full scale bioreactor and operating in parallel to the full scale bioreactor. The column has the same operating depth as full scale bioreactor and is supplied with proper pumps, compressors and benzene calibration cylinders. The major advantage of the core column is that it is possible to control all operating parameters so that performance under maximum stress conditions can be evaluated. The core column was developed and it received approval from both USEPA and the State of Louisiana.

Observations of the full scale performance testing

The full scale performance investigation required about one week in extremely hot and humid conditions. Five runs were made, each run requiring between eight to ten hours. Each consecutive run subjected the biomass to increasing benzene loadings, both in the gaseous and aqueous influent loading phases. The study plan was designed to increase vapour phase loadings of benzene until breakthrough of benzene in the reactor exhaust was measured.

The runs were successful, and it was concluded that >98% benzene biodegradation could be achieved at loadings up to 16 times design by the acclimated biomass, under the techniques of gas dispersion in the integrated system. The GC vapour phase benzene detection limit was less than two parts per billion by volume (ppbv) and the aqueous phase detection limit was <1 ppbv (µg/L). There was breakthrough of the vapour phase benzene emissions at an inlet benzene concentration of 482 ppbv, which is almost 35 times the maximum design inlet concentration. Even at this extraordinary loading, the effluent emission was only 13 ppbv for 97.2% biodegradation.

Conclusions

The BOX test and full scale confirmation testing objectives were met and benzene removal goals were achieved under maximum stress conditions at the MPC Garyville site. The methodology, protocols and apparatus developed and employed were approved by the state and federal agencies, and are compliant with 40 CFR, section 61.340, which states that:

• A bench scale BOX test batch and core column simulation full scale confirmation protocols and parameters have been developed, which delineate more realistic and reliable benzene biodegradation rates (fbio) than the EPA default rates

The biorates determined in this way are more representative of full scale conditions than the typical USEPA approach

This effort recommends modifying the protocols to improve USEPA’s approach. It included obtaining its approval of data collection, testing methodology and appropriate modelling techniques

Any properly designed and operated activated sludge system has potential to be qualified as an alternative control device when certified with the protocols herein. The approach presented here provides environmentally friendly, sustainable VOC control devices with negligible additional energy usage and a minimal carbon footprint

One such site-specific benzene biodegradation rate is 29.3 L/g VSS-hr at 26o C, or 22.6 L/gm-hr when corrected to 20o C. This provides excellent configuration and flexibility to achieve benzene removals >99% even under benzene loadings >16 times projected operating.

Published: 05th Sep 2013 in AWE International