LAUSR

Abstract After removing H2S from natural gas in offshore oil and gas installations, triazine-based spent scavenger solution remains as a wastewater stream, which in some cases is discharged untreated into the sea. Three nanofiltration (NF) membranes (NF270, NF99HF, and DL), as well as a reverse osmosis (RO) membrane (XLE), were used for TOC reduction from spent and unspent scavenger (SUS) wastewater. The NF270 membrane reduced the TOC of SUS wastewater (61.5 g/L) by 65% while having a superior permeate flux compared to the other studied membranes and thus was subjected to elaborated study. In particular, this membrane showed a separation between the unreacted/unspent scavenger triazine (1,3,5-tris(2-hydroxy-ethyl)hexahydro-s-triazine, HET) and the main spent scavenger reaction product (5-(2-hydroxyethyl)hexahydro-1,3,5-dithiazine, DTZ) as it removed HET by 71% versus zero removal of DTZ. DTZ is a polar and hydrophobic molecule and consequently passes readily through the membrane despite an expected removal value of 50% based on a simple size exclusion pore flow model. The NF270 permeate was then used as a feed for RO filtration by the XLE membrane to further reduce the TOC of effluent stream by 98% (1.1 g/L). Lastly, both membranes did not demonstrate remarkable fouling when they were tested in 24 h preliminary lab-scale fouling experiments with a constant-concentration feed. This paper is the first published study on SUS wastewater treatment using membrane technology proving its applicability for total organic carbon (TOC) reduction and simultaneously provides a promising route for further investigations on recovery of unreacted scavenger from spent scavenger compound.