Membrane bioreactors for wastewater treatment
Processing opportunity crudes enables higher profits but introduces bigger challenges in effluent treatment
BRIAN ARNTSEN, STEPHEN KATZ and WAJAHAT SYED
GE Water & Process Technologies
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As refineries continue to process opportunity crudes to increase profitability, they often operate the desalter more as an extraction vessel, removing many more contaminants than just salt; for example, metals, solids, and tramp amines are pushed from the crude oil into the desalter brine and onto the wastewater treatment plant. Significant variation in crude quality leads directly to high variation in brine quality fed to the wastewater treatment plant. At the same time, stricter wastewater regulations and water scarcity in some regions of the world continue to be the trend. The combination of high variation in input contaminant quality and tighter effluent requirements makes wastewater train operation much more difficult and unpredictable, increasing operational risk.
Membrane bioreactor (MBR) technology combines biological treatment, clarification and tertiary treatment into one step. At the core of the MBR is an ultrafiltration membrane which provides a barrier to solids and pathogens and frees the process from the restrictions set by the settling dynamics of a conventional clarifier. The MBR has a much smaller plant footprint and enables an inherently more robust design and operation. The improved effluent quality of MBR makes it suitable for several direct reuse applications like cooling system make-up water and better process control.1 In addition, the MBR does not require chemical addition for solids removal, leading to less sludge generation. In refinery applications, where there can be variability and toxicity of influent streams, MBR technology, as compared to conventional treatment technologies, offers the ability to deal with these types of upsets without impacting system operation and effluent quality.
The GE refinery MBR solution can also incorporate GE’s MACarrier, a specialised carbon based biological carrier that promotes nitrification and enhances the removal of recalcitrant COD and other toxic compounds. Its successful integration with GE ZeeWeed ultrafiltration membrane, prolongs biodegradation times, creates a healthier biology, and reduces effluent recalcitrant COD by more than 50%. The ability of the MACarrier to be regenerated while present in the bioreactor reduces additional operating costs.
Due to these advantages, MBR technology has been, and continues to be, applied extensively in industrial and municipal applications, growing exponentially over the last two decades. It is expected that high demand for MBR systems will continue over the next decade due to increasingly stringent regulations and the huge demand in water reuse applications.2
More than 1000 full-scale Zee-Weed MBRs are installed around the globe, with more than 30 in refinery and petrochemical wastewater application, several of which have been in operation for more than five years. The following case studies highlight the MBR’s adoption in refinery applications and show its ability to meet stringent effluent requirements and reuse standards.
Marine River Terminal, Kentucky
This American petroleum refining, marketing and transportation company, at the Marine River Terminal, located in Catlettsburg, Kentucky, services barges that carry petroleum to various refineries.
In 2001, the company began experiencing problems meeting the City of Ashland’s discharge criteria for wastewater generated at barge treatment operations at the Marine River Terminal. The wastewater treatment system in place at the time consisted of an equalisation tank, followed by a dissolved air flotation (DAF) unit. This wastewater was very difficult to treat as it contained solids, oil and grease, and aromatic hydrocarbon, including benzene, toluene, ethylbenzene and xylene (BTEX), metals, BOD and occasionally arsenic. Due to the nature of the wastewater, it was clear that further treatment was required. In 2002, a treatability study showed that using reinforced, immersed, hollow-fibre ultrafiltration membranes in an MBR process would meet their required effluent targets. In addition to COD, BOD and total suspended solids removal, the system removed BTEX compounds and heavy metals to acceptable levels. The company went ahead with the construction of a 50000 gal/d (189 m3/d) full-scale MBR system to treat this difficult wastewater. The plant was commissioned in less than one year and since start-up successfully pretreats the Marine Terminal wastewater, discharging the effluent into the City of Ashland’s municipal wastewater treatment system.
Raw water from the facility is pumped to a grit removal system to remove heavy solids. Then it goes through an oil-water separator before entering the bioreactor. Mixed liquor from the bioreactor overflows to the membrane tank where filtration is achieved by drawing water to the inside of the membrane fibre using suction created by permeate pumps. Recirculation pumps take the remainder of the flow back to the bioreactor to maintain the desired solids concentration. Sludge waste is sent from the recirculation line to a filter press, where it is thickened and then hauled offsite to a non-hazardous solid waste disposal facility.
The system performance is detailed in Table 1. On average, the treatment plant has achieved greater than 99% BOD removal and 95% COD removal. BTEX removal has been greater than 98%. Oil and grease is also monitored on a regular basis by the hexane extractable test method. The MBR system is responsible for degrading these compounds, which are primarily emulsified oils, below detectable levels. The influent wastewater also contains free oil that is primarily removed in the oil coalescer upstream of the MBR.3
Refinery in São Paulo, Brazil
This refinery, located in São José dos Campos, São Paulo State, increased its capacity to 251000 b/d to produce gasoline, diesel fuel, jet fuel, LPG and sulphur. As part of its modernisation and expansion, an upgrade to the treatment facility was proposed whereby the majority of the low salinity wastewater is treated in the existing conventional activated sludge plant, while the more challenging saline stream, originated from the refinery’s desalter, is treated by a new (MBR) plant based on the ability of the process to meet more stringent discharge requirements. A complete schematic of the treatment system is shown in Figure 1.
The existing conventional activated sludge plant consists of an API oil water separation, constant level equalisation tank, dissolved air flotation, parallel biological reactors and final clarifiers. A fraction of the treated wastewater is then sent for tertiary treatment for solids removal and blended with fresh surface water before being sent to the water treatment plant. The exceeding flow that does not go to the water treatment plant is blended with the MBR permeate before discharge.
The saline stream (brine) is collected in a buffer tank and passes through heat exchangers before entering the MBR treatment system. The portion of the low salinity wastewater that is not treated in the conventional system is mixed with the brine prior to treatment in the API separator and dissolved air floatation (DAF) unit. A polishing step, after the DAF, consists of Nutshell filters that were designed to protect MBR membranes from potential oil carryover by reducing oil and grease concentrations to below 5 mg/l. Subsequently, the wastewater is sent to constant level equalisation tanks with a total hydraulic retention time (HRT) of 12 hours. The outlet from the equalisation tanks does not cascade to the bioreactor system to avoid any release of excess oil after the primary treatment which could find quiescent conditions for its retention within the equalisation tank as free oil.
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