Effluent Water Quality Improvement

The Labuan Crude Oil Terminal LCOT is the receiving plant for all crude production from 5 major oil fields offshore state of Sabah in Malaysia. Two pipelines, one in the north and another from the south deliver crude to the Terminal. The wet crude of water content of 5 to 40 percent arrives fully stabilized. The Terminal stores and prepares the crude for export via Single Buoy Mooring system line.

The primary facilities of the Terminal are:

• 4 Crude Oil Storage Tanks totaling 1.64 million barrels storage capacity,

• 4 Free Water Knock-Out Vessels (FWKO)

• An Effluent Treatment Plant (ETP)

• An Emulsion and Wax Treatment Plant (EWTP) and

• Export Pumps and Custody Transfer Meters.

Figure 1 schematically represents the primary facilities in the Terminal.

LCOT has been experiencing wide fluctuations in its water treatment performance. In order to meet legislated average effluent discharge quality limit of 100 mg/1 and the company's target of average 40 mg/1 in 1993 reducing to 10 mg/1 by 1995, several initiatives have been instituted. This paper presents an overview of the efforts to date.

Oily water from FWKO vessels, oil storage tanks, EWTP, closed and open drains flows directly to the ETP (see figure 1). The treated water is sent directly to an API Separator and flows by gravity to the Corrugated Plate Interceptor(CPI). Water from the CPI is pumped to the effluent buffer tanks for feed into an induced air flotation unit (WEMCO). Water stream from the flotation unit drains to a holding basin before being discharged via an ocean outfall line, some 100 metres from shore. Oil scum is skimmed from the API, CPI and holding basin and pumped to EWTP for further treatment.

With the effluent discharge quality above 100 mg/1 and increased volume of produced water arriving at the Terminal, the remedial efforts taken involved the following:

• Search for suitable treatment chemicals in terms of effectiveness and cost,

• Modification of 2 units of the FWKO vessels

• Hydrocyclone trials

• Induced Gas flotation trials

• Centrifuge and Decanter trials

• Performance Review of FWKO and ETP

• Operator Awareness

Both demulsifier and deoiler chemical trials were conducted. Following laboratory tests,. 3 demulsifiers and 4 deoiler chemicals were selected for field trials. The results of the field trials were used to select appropriate chemicals for usage. So far, chemicals selected have performed effectively.

The FWKO vessels are about 60 feet long and 13 feet internal diameter equipped with inlet distributor and two diffuser plates in series to distribute the flow. Sand build up and wax deposits bridged the holes of diffuser plates thus increasing turbulence in the vessels, short circuiting the flow and reducing residence time. Installation of sand sparging facilities and relocation of level displacer to increase residence time of the water phase significantly improved the effluent quality from a range of 600 - 3800 mg/1 to an average of 80 mg/1 (based on infra red spectrometer measurement).

Tests using hydrocyclones to clean the water routed from the FWKO vessels were conducted. Results indicated that with 100 to 400 mg/1 influent, the hydrocyclone would effectively treat water and produce effluent water within the 100 mg/1 limit. However, it was found difficult to be within the 40 mg/1 limit without the use of demulsifier chemicals.

Induced gas/air flotation method was next selected for trials. To determine its effectiveness, two trials using two different makes of equipment, were conducted. On both occasions, the IGFs were found not effectively treating the effluent water or no better than the existing facilities.

With the disappointing results of the Hydrocyclone and IGF trials, the use of centrifuges and decanters to treat the minority contaminated water stream from emulsion storage tank, EWTP, was recommended. Initial trials indicated that effluent water quality could be improved. Following the first testing, the company decided to conduct further testing to assess the efficiency/effectiveness of the centrifuges in improving the overall Terminal effluent discharge quality. The results indicated an average efficiency of 61 to 83 percent and limited to handling capacity of 10 cubic metres per hour which is only 6 percent of the Terminal total treated effluent volume. It was concluded that improvement to the effluent discharge quality was insignificant. The decanter trialed had many mechanical problems resulting in discontinuation of trial.

With unsuccessful trials using technology to improve the effluent discharge quality to the desired level, we commenced reviewing the performance of the FWKO and ETP.

It was found that the vessel efficiencies depended on the vessel pressure and on the water drain LICV opening. Against this are the environmental and process constraints of limiting the oil-in water content of the discharged water. The optimum operating mode was a normal operating pressure of 45 psig, with LICV at 35 percent open and at interface level at as close to 0.7 of the internal diameter as operationally possible.

Results indicated that the oil removal efficiency of the Wax Trap is low. The variance of residence times between actual and theoretical is slightly over 20 percent indicating short circuiting and turbulence, made worse by the splashing at the inlet.

Monitoring of the CPI showed that the overall system efficiency was low - (11.7 %) with an average flow rate of 50.000 bpd. Residence time was estimated at 10 minutes. With higher flow rate, the oil-in-water content of the effluent was noted higher than the influent, suggesting that oil is trapped/recirculating in some stagnant space and is partially flushed out. Clogged corrugated plates accounted for some stagnant space. Regular cleaning programme was instituted.

The original specification of the induced air flotation unit required it to discharge process water into the Holding Basin with oil-in-water content of less than 10 mg/1. Review of results showed that such a high efficiency cannot be achieved. Nevertheless, this unit, as an integral part of ETP, has the highest oil removal efficiency compared to the Wax Trap or the CPI. The Induced Air Flotation Unit is not performing to its maximum efficiency partly because more than 70% of the oil droplets in the influent water consist of droplets whose sizes are 15 microns or less and partly wide fluctuations in the effluent water quality due to fluctuation in flow rates and ‘dirty’ influent from minority streams.

Operators at LCOT were made aware of the need to progressively reduce the oil-in-water effluent. The awareness drive was done through regular meetings, reviews and seminars.

Hydrocyclones, IGF and Centrifuges were found ineffective. With modifications to the FWKO, chemicals and actions taken under the performance review of both FWKO and ETP, we have managed to reduce our oil in water discharge to within 23 mg/1.