A trial at a refinery in Saudi Arabia has led to the development of a MBR-based technology to help with the reuse of wastewater from the petroleum industry. This article examines the trial results from the membrane evaluation of processing this tough to treat industrial water.
By William Cunningham
Wastewater from oil-processing facilities presents a number of challenges for owners and operators contemplating reuse of treated wastewater. The oily wastewater often has a high concentration of dissolved solids that must be removed before reuse as boiler feed water, cooling water and utility water.
Difficult-to-degrade (refractory) organic compounds are also present in refinery wastewater and these compounds must first be removed to protect reverse osmosis (RO) equipment designed to remove the dissolved solids.
Typical biological oxidation methods used to treat these wastewaters often provide inadequate removal of oils and difficult-to-degrade organic compounds, resulting in organic fouling problems in the downstream ultrafiltration (UF) and RO membrane filtration systems.
Wastewater treatment in the oil and gas industry
The petroleum industry traditionally uses very large quantities of water. Refineries, for instance, discharge approximately one-half to one barrel of wastewater for every barrel of crude oil that is processed.
It is becoming increasingly difficult to reduce that figure further because water is an essential component in the refining process. The crude oil distillation tower at the heart of most modern refineries depends on water as a heat exchange mechanism. Water is also used to provide steam as a power source and for washing or desalting crude.
It should come as no surprise that meeting this fresh water requirement through a RO desalination process, and transporting this water to the refineries, is becoming cost-prohibitive for companies like Saudi Arabian oil company - Saudi Aramco.
The company's policy of minimizing the extraction of groundwater and the use (and associated expense) of desalinated seawater for process water use, led them to actively investigate sustainable methods and technologies that will allow wastewater reuse, either as cooling water makeup and/or boiler feed water. Typically, RO treatment of biologically treated effluent is required for reuse. However this leads to excessive organic fouling of RO membranes due to recalcitrant COD remaining in the treated effluent.
An investigation into combining activated carbon with microbiology led to the development of the EcoRight System from Saudi Aramco and Siemens Water Solutions. The system relies on Siemens' Petro Membrane Bioreactor (MBR) technology, modified to allow the aeration tank mixed liquor suspended solids (MLSS) to operate with suspended granular activated carbon (GAC).
Siemens contracted with Saudi Aramco to perform a pilot study using the system to treat wastewater from a Saudi Aramco refinery. The pilot study scope included an evaluation of RO filtration of the system effluent to determine if the system produced longer RO run times by minimizing membrane fouling and reducing clean-in-place (CIP) requirements.
The EcoRight pilot unit was operated over a 319 day period. Even after extraordinary variable discharges of oil and grease and COD loading, and frequent power interruptions, the system effluent complied with all discharge standards, indicating that this technology has the capacity to process upsets and quickly return to normal operating conditions.
Recalcitrant COD and Extracellular Polymeric Substances (EPS), which typically foul UF and RO membranes, were successfully removed. This was demonstrated by nearly eight months of system operation, and more than two and a half months of reverse osmosis operation without requiring CIP procedures.
No damage to the system membrane fibres was observed during the membrane autopsy conducted at the end of the study, indicating that:
• The screen system was effective at keeping the granular carbon in the aeration tank and preventing contact with the membrane fibers in the MOS tank
• The ability to keep the granular carbon segregated from the membrane fibres increases membrane life over powdered carbon MBR systems by eliminating carbon-related membrane abrasion potential
• Bio-regeneration of the granular activated carbon likely contributed to reduced activated carbon consumption.
Treated effluent use
The pilot study results have demonstrated the system to be a cost-effective solution for treating refinery wastewater targeted for reuse.
Membrane abrasion is eliminated by separating the granular activated carbon from microbiology prior to the membrane tank.
By using a very stable system, it results in lower contaminant concentrations in the treated effluent, allowing the wastewater to be economically and efficiently sent for further treatment for reuse, or to provide a high quality water to meet most discharge requirements. In most cases, the treated effluent can be reused, or fed directly to RO equipment for reuse as boiler feed or cooling water, all while meeting stringent environmental discharge requirements.
Development of the system by Saudi Aramco and Siemens has provided a new opportunity to recover and reuse an existing water supply to reduce the impact on desalination facilities and the local fresh water supply.
William J. Cunningham, P.E. is the manager of biological processes and products for the water solutions business unit of Siemens Energy's Oil and Gas Sector. Email: [email protected]