December 2018

Water Management

Advanced biological treatment removes benzene, phenol from refinery wastewater

Historically, US regulators have not required benzene and phenol monitoring for wastewater from oil refining, petrochemical processing, coking or coal gasification operations.

Dale, C., Ekenberg, M., Wenta, R., Veolia Water Americas

Historically, US regulators have not required benzene and phenol monitoring for wastewater from oil refining, petrochemical processing, coking or coal gasification operations. However, a new dynamic has come into play. In the Marcellus shale, lower-priced natural gas at just under $3/MMBtu is prompting industry to construct ethane cracker plants that refine natural gas for usable products, such as polyethylene. The thinking, according to industry sources, is that proximity to the natural gas source is a better business model compared with the Texas Gulf Coast, especially after Hurricane Harvey shut down refineries in late summer of 2017.

With the development of the Marcellus shale came requests for proposals for wastewater treatment at newly constructed ethane crackers in the region that have very low benzene and phenol limits. Going forward, other new projects along the Ohio River watershed are expected to have similar requirements. In time, these new, stricter benzene and phenol effluent regulations for greenfield facilities could potentially spread throughout the US.

This insight provided the impetus to launch a study to monitor the capability of moving bed biofilm reactor (MBBR) technology to achieve very low limits for benzene and phenol under normal design conditions for complete chemical oxygen demand (COD) removal.

New capabilities in proven technology

MBBR technology has been proven as an effective biological treatment in a variety of applications since its invention in the mid-1980s. The technology utilizes polyethylene carriers (or media) to create a large, protected surface on which biofilm can attach. Bacteria required for treatment of a variety of organics develop on the carriers. The continuous motion of the carriers optimizes diffusion in and out of the biofilm, enhancing process kinetics. It also ensures continuous sloughing of excess biomass, facilitating process operation, and it can achieve the results of more conventional biological processes in a much smaller footprint.

The company that invented MBBR continues to conduct research and development to discover new ways to apply the technology to meet the water treatment needs of industry and municipalities. The company also monitors the performance of existing facilities to evaluate the ability of the process to meet more stringent discharge limits. In response to new demand in the marketplace for technology to achieve low benzene and phenol limits, a study was initiated to determine the effectiveness of MBBR technology to remove these constituents from refinery wastewater.

Benzene and phenol removal in full-scale MBBRs

Samples were collected at three refineries where MBBR wastewater treatment systems are in operation. Two were US refineries, while the other is in Lund, Sweden. The plants have been in operation for many years. Grab samples were collected at these facilities to evaluate the removal efficiency of benzene and phenol under normal operating conditions, even though these parameters are not regulated at those sites. In addition, laboratory studies were conducted at the R&D facility with spiked samples to learn the upper limit for the influent benzene and phenol concentrations while still achieving an effluent concentration of 10 µg/l.

FIG. 1. This proprietary MBBR carrier removed benzene from influent concentrations to below the detection limit of 4.4 μg/L, while reducing pure phenol to below 10 μg/L on most of the samples from a US refinery wastewater treatment plant.
FIG. 1. This proprietary MBBR carrier removed benzene from influent concentrations to below the detection limit of 4.4 μg/L, while reducing pure phenol to below 10 μg/L on most of the samples from a US refinery wastewater treatment plant.

Refinery 1. One US refinery wastewater treatment plant has two MBBRs operating in parallel, with a total reactor volume of 4,554 m3. The reactors have a fill rate of 26% of carrier media,1 with a surface area of 500 m2/m3, as shown in Fig. 1.

The MBBRs, installed downstream of a nonmechanized deoiling tank, can achieve combined removal of COD and nitrification in a single stage. The applied surface area loading rate (SALR) is 1.5 g–9.8 g total COD (TCOD)/m2/d.

Feed and effluent samples were collected 10 times over 5 mos. A third-party laboratory analyzed the samples for benzene and phenol, COD and hexane-extractable material (HEM), and the TSS concentration was analyzed in-house. Samples for soluble chemical oxygen demand (SCOD) were filtered using 0.45-µm paper.

Despite exposure to fluctuating levels of HEM (oil and grease), the MBBR performed well. HEM in the MBBR effluent was consistently below 10 mg HEM/l with an influent concentration ranging from 7 mg/l up to 270 mg/l. The MBBR reduced SCOD to between 4 mg/l and 66 mg/l (average 56 mg/l) from influent concentrations of 168 mg/l–567 mg/l (average 369 mg/l). The MBBR removed benzene to below the detection limit of 4.4 µg/l from influent concentrations of 4,700 µg/l–14,000 µg/l, while reducing total recoverable phenols to 10 µg/l–30 µg/l from influent concentrations of 540 µg/l–4,900 µg/l. Pure phenol was reduced below 10 µg/l in samples containing 540 µg/l–1,500 µg/l.

Refinery 2. The second US refinery also has two MBBRs operating in parallel. The pretreatment system at this facility consists of gravity separation and dissolved air flotation for oil and grease removal. The pretreated effluent is pumped through a cooling tower and then discharged to an equalization tank. The equalized effluent is then pumped to the MBBR. The MBBR system is designed for partial COD removal as pretreatment to an activated sludge system. The MBBRs have an operating volume of 2,006 m3 with 50% fill of proprietary media carriers. The average applied loading rate of 32 gSCOD/m2/d is significantly higher than that applied in reactors designed for complete COD removal, since only 60% SCOD removal is required. With considerably more readily biodegradable COD available, it is not surprising that total phenols are reduced only from 3 mg/L to 0.6 mg/l.

FIG. 2. This proprietary MBBR carrier reduced influent distillable phenols to 40 μg/L–48 μg/L, while benzene could be detected only in the third sample, with a reduction to 24 μg/L in samples from a refinery wastewater treatment plant in Sweden.
FIG. 2. This proprietary MBBR carrier reduced influent distillable phenols to 40 μg/L–48 μg/L, while benzene could be detected only in the third sample, with a reduction to 24 μg/L in samples from a refinery wastewater treatment plant in Sweden.

Refinery 3. The MBBR at the Swedish refinery is applied as pretreatment to remove organic material prior to nitrification in a downstream process. To allow nitrification to occur in a trickling filter, complete removal of degradable COD is required in the MBBR. With a flow ranging from 2,830 m3/d–4,190 m3/d, the MBBR hydraulic retention time is 30 min–50 min, which is relatively short for such an application. The single MBBR, which has a volume of 95 m3, is filled with 55% of proprietary media carriers with a protected surface area of 500 m2/m3 (Fig. 2).

The wastewater, pretreated with gravity oil separation followed by sand filtration, is cooled to a range of 86°F–95°F (30°C–35°C) before entering the MBBR.1 The applied surface area loading rate (SALR) is 18 g–22 g total COD (TCOD)/m2/d.

Influent and effluent samples were collected on three occasions. The MBBR reduced influent distillable phenols to 40 µg/l–48 µg/l from 1,300 µg/l–2,000 µg/l. Benzene could be detected only in the third sample, with a reduction to 24 µg/l from 3,900 µg/l. SCOD was reduced to 50 mg/l–70 mg/l from 114 mg/l–177 mg/l.

Determining maximum organic loading rates

In addition to full-scale testing, a laboratory test was conducted to determine the maximum organic loading rates at which effluent benzene and phenol concentrations below 10 µg/L could be obtained at standard design loading rates for complete COD removal. The laboratory test was conducted in two parallel bench-scale models of two-stage MBBR processes, using a proprietary media carrier (Fig. 3).

FIG. 3. This proprietary MBBR carrier reduced phenols to 19 μg/L–79 μg/L. Benzene removal was below the detection limit of 0.5 μg/L on all samples collected from the effluent of the second reactor. Measured benzene concentration was above the detection limit twice in a laboratory test.
FIG. 3. This proprietary MBBR carrier reduced phenols to 19 μg/L–79 μg/L. Benzene removal was below the detection limit of 0.5 μg/L on all samples collected from the effluent of the second reactor. Measured benzene concentration was above the detection limit twice in a laboratory test.

The first trial was conducted with synthetic wastewater at an overall SALR between 4 g SCOD/m2/d and 14 g SCOD/m2/d, corresponding to volumetric loads between 1.5 kg SCOD/m3/d and 5 kg SCOD/m3/d. Distillable phenols were removed from 11,000 µg/l–28,000 µg/l, to below 10 µg/l. Benzene was removed to below the detection limit of 0.5 µg/l from an influent concentration of 240 µg/l–7,500 µg/l.

The second laboratory setup was operated using a refinery wastewater. The applied SALRs ranged from 3 gSCOD/m2/d–11 gSCOD/m2/d, corresponding to volumetric loads of 1 kgSCOD/m3/d–3 kgSCOD/m3/d. Phenols were reduced to 19 µg/l–79 µg/l from 5,800 µg/l–14,000 µg/l. Benzene removal was below the detection limit of 0.5 µg/l on all samples collected from the effluent of the second reactor.

The laboratory-scale trials and the data collected from the full-scale plants demonstrate that the MBBR process can consistently achieve total phenols and benzene concentrations lower than 10 µg/l under normal design conditions for complete COD removal. As refineries look to meet more stringent discharge limits, the MBBR process can offer the reliability that is required of an industrial process.

The capital cost of an MBBR system typically would be about 30% less than a conventional activated sludge system.

The study has allowed for the development of design guidelines for the removal of benzene and phenol as an optimal technological solution. HP

Literature cited

  1 Dale, C., R. Wenta, M. Ekenberg and S. Jacobsson, “Phenol and benzene removal in moving bed biofilm reactors–A review of operating data from two full-scale refineries,” 2017.

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