Feasibility of Produced Water Treatment and Reuse – Case Study of a GTL Plant

Abstract

In a GTL plant located in Qatar, produced water (PW) is considered for treatment and reuse in the existing onsite Effluent Treatment Plant (ETP).The onsite ETP deploys both conventional and advanced water treatment technologies including flocculation flotation unit, biotreatment, membrane filtration (submerged Ultrafiltration (sUF) and Reverse Osmosis (RO) units), evaporation and crystallization processes, to produce water for reuse within the plant mainly for cooling and power generation among many other uses.Current PW disposal is in deep wells. PW reuse was considered an economically viable option for the plant due to its quality (TDS Ͻ 4000 mg/L).A test work was conducted to determine the feasibility to route the PW to existing onsite ETP. Firstly, the PW was chemically analysed for key contaminants including hydrocarbons (condensate), heavy metals and naturally occurring elements, Total Dissolved Solids (TDS) etc.Hydrate inhibitor is normally dosed to the PW during the winter season while Corrosion Inhibitor (CI) is dosed continuously throughout the year. This results in two distinct streams, i.e. a ЉWinter PW streamЉ (Stream A) and a ЉSummer PW streamЉ (Stream B).The potential impact of Stream A and Stream B must be evaluated before routing to onsite ETP is considered feasible.Based on sample analysis (pH, conductivity, COD, TOC, ion chromatography, metals scan, toxicity tests and 28 days COD die-away test), it is known that both Streams A and B have the potential to adversely affect the daily performance of the onsite biotreater, with additional risk of sUF/RO membrane fouling (scaling).The following have been considered in the study:a. Hydraulic load of PW to the onsite ETP a. Quality of Streams A and B, in terms of i. Chemical Oxygen Demand ii. TDS which is Ͻ4000 mg/L (0.4% wt) iii. Presence of scaling components (Ca, Mg and Ba) iv. Heavy metals in PW Stream A is considered first for pre-treatment in an onsite Wet Air Oxidation (WAO) unit to destroy hydrate inhibitor, while Stream B will be stored temporarily in a holding tank to ensure complete condensate separation, prior to routing of both pre-treated effluent to the onsite ETP.Experimental test work conducted over a period of 9 months using two bench bioreactors operated at similar hydraulic and solids residence times as the onsite ETP showed COD levels are reduced to levels that meet the specification for further polishing in downstream UF and RO units. Test work suggests PW (Streams A and B) to be compatible with onsite ETP.

Biotechnology for Gas-to-Liquid (GTL) Wastewater Treatment: A Review

“…A case study was reported by Onwusogh [117], where catalytic wet air oxidation (cWAO) was applied as a pretreatment stage in GTL from GTL plant located in Qatar. The unit was placed before an activated sludge unit and used to study the removal of COD with special attention to the kinetic hydrate inhibitors (KHI).…”

“…Wastewater from typical GTL plant generally contains a high concentration of dissolved solids, since the produced cooling water from the blowdown system contains inorganic salts. The total organic compounds are generally measured collectively as COD; besides, GTL wastewater contains number of inorganic compounds including metals, chloride, sulphate, acetate, bicarbonate and dissolved gases such as H 2 S and CO 2 [117]. The contaminants that are present in GTL wastewater vary according to the GTL process unit.…”