Recently, more and more refineries are adopting MBR for wastewater treatment, but not many reports with full-scale data are available in public domain. However, based on the available anecdotal information on full-scale MBR along with lab-scale data, it is apparent that removal efficiencies of hydrocarbons, suspended solids, BOD/COD are very satisfactory while concerns are still remaining on the reliability of membrane under certain circumstances.
Both refinery wastewater and produced water contain salts, petroleum hydrocarbons, oil and grease, suspended solids,etc., but refinery wastewater is typically much less concentrated than produced water because it is mainly from desalting process, where crude oil is contacted with large quantity of clean water to strip salt out to meet the crude oil specification for distillation tower as shown in Fig. 1 (Wikipedia, 2013). Wastewater is also produced from distillation, thermal cracking, catalytic conversion, etc.
Fig. 1. Flow diagram of crude oil processing (Wikipedia, 2013)
The amount and quality of wastewater produced from refining process widely vary depending on the crude oil quality and processes used to refine crude oil. Wastewater volume varies at 0.4-1.6 times the volume of crude oil (Diya’uddeen, 2011). The qualities of wastewater of four different refinery wastewater are shown in Table 1. Nitrogen and phosphorous may be supplemented, if BOD:TKN:TP is significantly deviated from 100:5:1.
Just like for produced water, removing free heavy oil drops that are not emulsified is crucial for the success of MBR, which can cause not only short term performance loss, but also long-term membrane integrity issue by compromising the physical property of the polymeric material. API (American Petroleum Institute) separator is commonly used to remove floating oil and precipitating solids simultaneously from water. Its design is very similar to the rectangular clarifiers used for non-oily wastewater, but skimmers and scrappers design and operation are optimized for removing oily wastewater. Slanted parallel plates can be also inserted to save footprints just like those for municipal wastewater.
Membrane fouling appears more significant in refinery MBR than in municipal MBR, which might be caused by the combination of high salt concentration, high oil and grease contents, more dispersed flocs, non-ideal nutrient balance, potentially toxic compounds to certain microorganisms, etc. Daily average design flux ranges 10-20 LMH (or 6-12 gfd), which is lower than that for municipal MBR, i.e. 25 LMH (or 15 gfd).
Treatment efficiencies are inline with those of produced water discussed here. Petroleum hydrocarbon removal is generally very high provided operating condition is reasonable including nutrient balance, hydraulic retention time, solids retention time, etc. Especially BTEX (benzene, toluene, ethylbenzene, xylene) can be removed below 1 µg/L regardless of the concentration in influent under practical condition.
Table 1. Petroleum refinery wastewater quality (Ishak, 2012)
Fig. 2. API separator (Wikipedia, 2013)
© Seong Hoon Yoon