Refinery Wastewater Treatment

During the processing of oil in the refinery, large volumes of water flow through the processing units in the form of cooling water, process water, steam, equipment washings, unit hydrotests, and the like; and these streams pick up small quantities of hydrocarbons or oil from equipment and spillage. Oil-contaminated storm water flows from the curbed areas of the refinery add to this load. The objective of the treatment is to separate oil from the wastewater before this water is allowed to be discharged into the sea or any other water body, with minimum negative environmental impact. Typical tolerance limits for refinery effluent discharged into marine coastal areas is shown in Tables 9-4 and 9-5.

Water streams that need treatment is shown in Figure 9-5. Contaminated drainage and storm flows from process areas are routed to retention basins. Storm water is retained for the removal of floating oil, then discharged to the sea with no further treatment. Incidental contaminated drainage is retained for removal of floating oil and pumped to the API oil/ water separator system for additional treatment.

Retention basins are designed to collect all water transported to the contaminated sewer network. This includes contaminated storm and fire water flows as well as continuous nuisance flows from daily maintenance operations throughout the refinery. The normal operation of the basins is to provide preliminary oil removal from nuisance flows, using the belt oil skimmers, and act as a sump preparatory to pumping of these flows to API separators.

Table 9-4 Liquid Effluent Discharge to Sea*

PARAMETER

UNIT

REFINERY AVERAGE

TEMPERATURE

°C

45

PH

S.U

5.5-9.0

CHEMICAL OXYGEN DEMAND

mg/1

200

BIOLOGICAL OXYGEN DEMAND

mg/1

30

TOTAL SUSPENDED SOLIDS (TSS)

mg/1

45

OIL/GREASE

mg/1

10

PHENOLS

mg/1

1

SULFIDES

mg/1

0.5

NITROGEN AS AMMONIA

mg/1

10

PHOSPHATES - INORGANIC

mg/1

0.5

CYNIDES

mg/1

1

IRON

mg/1

2

FREE CHLORINE

mg/1

0.6

CHROMIUM AS Cr04

mg/1

0.3

ZINC

mg/1

0.001-0.05

* TYPICAL REFINERY STANDARDS.

* TYPICAL REFINERY STANDARDS.

Table 9-5 U.S. Refineries (EPA) Standards*

PARAMETER

UNITS

AVERAGE

MAXIMUM

TEMPERATURE

°C

35

. 37

PH

6-9

CHEMICAL OXYGEN DEMAND

KG/DAY

2371

4571

BIOLOGICAL OXYGEN DEMAND

KG/DAY

338

610

TOTAL SUSPENDED SOLIDS (TSS)

KG/DAY

270

427

OIL/GREASE

KG/DAY

99

185

PHENOLS

KG/DAY

2.1

4.4

SULFIDES

KG/DAY

1.8

3.8

NITROGEN AS AMMONIA

KG/DAY

136

273

PHOSPHATES - INORGANIC

CYNIDES

IRON

FREE CHLORINE

CHROMIUM AS Cr04

KG/DAY

3.3

9.5

ZINC

KG/DAY

1.7

3.8

*FOR A 180,000 BPD REFINERY ON THE U.S. GULF COAST.

*FOR A 180,000 BPD REFINERY ON THE U.S. GULF COAST.

RECOVERED OIL

RECOVERED OIL

Refinery Waste Water

SOLIO WASTE TO LAND FILL

SANITARY WASTE

SOLIO WASTE TO LAND FILL

SANITARY WASTE

Figure 9-5. Refinery aqueous effluent treatment system. CPI = corrugated plate interceptor.

Figure 9-5. Refinery aqueous effluent treatment system. CPI = corrugated plate interceptor.

Flows in excess of nuisance flows, such as contaminated rainwater and fire control runoff, is also collected in the retention basins. This flow is retained for sufficient time for preliminary oil removal using belt and rope skimmers. After this partial deoiling, the water is discharged to the sea.

In coastal refineries, the saline wastewater consists of potentially oil-contaminated once-through sea cooling water, boiler blow-down, and cooling tower blow-down. Oil-contaminated sea water is typically sea water used where the oil side of the exchanger is operating at 55 psig or greater. Potentially oil-contaminated sea water is passed through a battery of API separators. The effluent is combined with uncontaminated once-through sea cooling water in a sea water mixing basin prior to discharge to sea.

Oily wastewater generated in the refinery is segregated into two streams: one low (160-250 ppm) and one high in oil content (>250ppm). The low-oil stream is made up primarily of stripped sour water and desalter underflow. This stream requires no further treatment and is routed directly to mixing basin, where it is combined with sea cooling water prior to discharge to sea.

The high-oil stream originates from the oily water sewer, oily water pumped out of retention basins, and desalter underflows. The oil in the desalter underflows is in two forms: emulsified and free oil. All potentially high-oil-content streams are treated for the removal of oil using a corrugated plate interceptor type API oil/water separators followed by dissolved air flotation (DAF) oil/water separators. Effluent from DAF units is combined with nonoily wastewater streams and returning cooling water in the sea water mixing basin and discharged to the sea.

The sanitary sewage of the refinery is collected and treated in an aeration unit, which reduces the BOD (biological oxygen demand) and COD (chemical oxygen demand) of the influent. The biological sludge is separated and clarified aqueous effluent is chlorinated to reduce coliform bacteria before discharging this water for irrigation purposes. The sludge separated is disposed of as landfill.

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