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VOLATILE ORGANIC COMPOUND EMISSION 

CONTROLS FOR PETROLEUM REFINERY 

SECONDARY WASTEWATER SEPARATORS 

by 

PEDCo Environmental, Inc. 

1006 N. Bowen Road 

Arlington, Texas 76012 

Contract No. 68-02-3512 

Task Order No. 43 

Project Officer 

Eileen Glen 

U.S. ENVIRONMENTAL PROTECTION AGENCY 

REGION I II 

PHILADELPHIA, PENNSYLVANIA 19106 

December 1983 · '.~: 

' ''. ·' .'.) 
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CONTENTS 

Summary 

1. Introduction 

1.1 Source Description and Types of VOC Emissions 

2. 

3. 

4. 

Emission Control Techniques 

Cost Analysis 

3.1 Model plants 

3.2 Assumptions 

3.3 Costs 

3.4 Cost-effectiveness 

Regulatory Analysis 

References 

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SUMMARY 

A petroleum refinery wastewater separator removes and recovers hydrocarbons from refinery wastewater. In a primary separator, this is effected 

through density differences. Sometimes a second stage of treatment, or secondary separator, is required to comply with air and water regulations. A 

separator is considered 11 secondary11 when all entering wastewater streams have 

received prior treatment that has removed at least 90 percent of the hydrocarbons. 

Covering the separator is the most practical control method for both 

primary and secondary separators. According to AP-42, covering primary separators can reduce emissions by an estimated 95 percent. Estimates also indicate that covering secondary covers will reduce VOC Emissions by 90 percent. 

Process modifications and improved process control also can reduce VOC emissions by reducing the quantity of hydrocarbons entering the plant wastewater 

stream. 

The estimated control cost of retrofitting a cover onto a secondary 

wastewater separator is $225 per ton of VOC controlled with hydrocarbon recovery credits. Because very few refineries have secondary separators there is 

limited data on control costs, emission reductions, and hydrocarbon credits. 

Therefore, no VOC emission controls for secondary wastewater separators at 

petroleum refineries are presented. 

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1. INTRODUCTION 

Over the past several years, the Office of Air Quality Planning and 

Standards (OAQPS) of the Environmental Protection Agency (EPA), has developed 

a series of Control Techniques Guidelines (CTGs) for volatile organic compounds (VOCs) to assist state and local agencies in developing regulations for 

VOC control. Although, the CTGs cover major VOC source categories from an 

overall nationwide perspective, several source categories that are not covered 

by CTG documents are major contributors to the ozone problem within given 

areas. 

Air pollution control agencies in the Philadelphia Air Quality Control 

Region (AQCR) have requested guidance in determining whether VOC controls are 

available for non-CTG sources and information for use in developing appropriate regulations. Secondary wastewater separators at petroleum refineries 

comprise one such VOC source category. 

1.1 SOURCE DESCRIPTION AND TYPES OF VOC EMISSIONS 

A petroleum refinery wastewater separator is a device that separates 

organic material hydrocarbons from refinery wastewater. The two kinds of 

wastewater separators used are corrugated plate interceptors (CPis) and American Petroleum Institute (API) separators. The smaller CPI, which is controlled by a fixed-roof cover, is effective for initial oil-water separation. The 

API separator has a floating-roof cover. 

Contaminated wastewater originates from several sources in petroleum 

refineries, including leaks, spills, the cooling and flushing of pump and 

compressor seals sampling, equipment cleaning, and rain runoff. Although some 

organic contaminants that are immiscible with water can be separated rather 

easily, other organic constituents may dissolve or form an emulsion that 

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remains suspended in the water. Chemicals (e.g., ferric chloride) are frequently added to wastewater streams to achieve better separation of water and 

hydrocarbons. 

Primary wastewater separation is often the only treatment required at a 

refinery. Separation is effected through differences in the densities of 

water and hydrocarbons. The oil that coalesces and floats on the surface of 

the water and the sludge that settles to the bottom of the separator are 

removed. Because the recovered hydrocarbons are pr9cessed in the refinery, 

primary wastewater separation is economically advantageous. 1 

Floating hydrocarbons in wastewater streams have a high surface-to-volume 

ratio that enhances evaporation and VOC emissions. According to AP-42, uncontrolled emission factors for primary oil/water separators are 5 lb/103 gallon 

(0.6 kg/103 liter) of wastewater or 200 lb/10~ bbl (0.6 kg/10 3 liter) of 

refinery feed. 2 

The April 1, 1981, AP-42 does not give an emission factor for secondary 

wastewater separators. 2 An American Petroleum Institute publication estimates 

potential uncontrolled VOC emissions to be about 10 percent of that sustained 

in primary wastewater separators, which would be 0.5 lb/103 gallon (0.06 

kg/103 liter) of wastewater processed or 20 lb/103 bbl of refinery feed (0.06 

kg/103 1 iter). 1 

Because no definition currently exists for secondary separators, for the 

draft regulations separators will be defined as being 11 secondary11 when all 

entering streams have received prior gravity separation or equivalent treatment that has removed at least 90 percent of the hydrocarbons. If any stream 

entering the separator has not received this prior treatment, the separator 

should be considered a primary separator. According to AP-42 estimates, a 

covered primary separator gives 95 percent control. 2 

A refinery having an API separator with multiple bays in series may 

define the first bay as a primary separator, the second bay as a secondary 

separator, etc. This is appropriate only if the first bay is sized to recover 

at least 90 percent of the oil entering the separator in the first bay. 

Otherwise, the whole unit should be considered a primary wastewater separator. 

A published estimate of the oil recovery from a primary separator at a 100,000 

bbl/day refinery is 4,200 to 21,000 gallons/day (15,900 to 79,500 liters per 

day). 1 

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2. EMISSION CONTROL TECHNIQUES 

Primary wastewater separators can achieve reasonable control of VOC 

emissions if the separator is covered to minimize the amount of oily water exposed to the atmosphere. The cover may be either a sealed solid cover or one 

that floats on the liquid. According to AP-42, covered separators and/or 

vapor recovery systems achieve 95 percent control and reduce VOC emission 

rates to 0.2 lb/103 gallon (0.024 kg/10 3 liter) of wastewater or 10 lb/103 bbl 

(0.03 kg/103 liter) of refinery feed. 2 At least 80 percent of the petroleum 

refineries currently cover their primary wastewater separators. 3 

Additional control may be obtained by reducing the quantity of hydrocarbons that enter the plant wastewater treatment facility. Such reduction may 

be possible through improved process control or modifications that reduce the 

number of leaks, spills, and other upsets. A stringent fugitive leak emission 

control program can result in up to a 50 percent reduction of VOCs. Adding a 

CPI to the wastewater system can result in reductions of up to 90 percent in 

VOC emissions. 

Covering the secondary separator is the most practical control method. 

Although no controlled VOC emission rate for secondary wastewater separators 

was found, it is estimated that the controlled VOC emission rate will be 10 

percent of the uncontrolled rate for secondary separators, or 0.05 lb/103 

gallon (0.006 kg/10 3 liter) of wastewater processed. Although AP-42 indicates 

that 95 percent control is achievable for primary separators, 2 only 90 percent 

control is achieved in most secondary separators because the more volatile, 

easily separated and recoverable hydrocarbons have already been removed. 

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3. COST ANALYSIS 

This section defines the model plant, explains the assumptions made, and 

presents a cost analysis for control of VOC from secondary separators at 

refineries. Cost-effectiveness is expressed in terms of the annual control 

cost in dollars divided by the annual tons of VOC controlled. 

3.1 MODEL PLANTS 

Petroleum refineries vary considerably in size, design, age, feedstocks, 

and product mixes. This report presents an evaluation of retrofit controls to 

an existing secondary wastewater separator. Because most retrofits have 

enough unique characteristics to make each one a special case, attempting to 

cover every situation with a model plant would be impractical. Therefore, 

this report considers a single case of retrofitting a cover onto a secondary 

wastewater separator at on oil refinery with a capacity of 150,000 barrel/day 

(23.8 million liters/day). 

3.2 ASSUMPTIONS 

The annual costs for all control options include a capital recovery 

charge and an allowance for property taxes, insurance, and adminstrative 

costs. A 1977 EPA report discusses costs for primary wastewater separator 

covers. 4 Costs, which are expressed as a percentage of total capital costs, 

are: 

Capital recovery factor (10-year life and 10% 

interest rate) 

Property taxes, insurance, and administrative costs 

Operating and maintenance costs 

16.27 

4.00 

10.00 

Since the equipment for covering primary and secondary separators is identical, the same cost factors were used for the latter. 

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Recovered hydrocarbon credits are based on a price of $23.70/barrel and a 

density of 8 lb/gallon. The 1977 ~PA report used a value equivalent to that 

of light crude oil (about $32 per barrel). 4 Because the .lighter, more valuable, and more easily recovered hydrocarbons have already been recovered in 

the primary separator, however, it is assumed that the material recovered in a 

secondary separator would be heavier or would have to be recovered from water 

emulsions. This would make it more difficult to process and therefore less 

valuable than the material recovered from the primary separator. The hydrocarbons recovered from a secondary separator are assumed to have a value 

approximately equivalent to that of No. 6 fuel oil ($23. 70 per barrel). 5 

3.3 COSTS 

The total capital cost for covering an existing secondary wastewater 

separator at a refinery producing 150,000 barrels/day is estimated to be 

$270,000. This is based on the average cost for installing covers on three 

API separators in 1982 at a refinery producing 430,000 barrels/day (personal 

communication from C. V. Rice, Amoco Oil Company, February 15, 1983). Costs 

could be higher, however, when evaluated on a site-specific basis. For example, the cost for covering an API separator at a refinery producing 380,000 

barrels/day was $2 million. Installed costs were estimated to be $75 per 

square foot up to 1000 square feet of covered surface and $35 per square foot 

for up to 12,000 square feet. The cost of installing a 55,000-ft2 cover at a 

refinery producing 380,000 barrels/day was $36.37 per ft 2 (personal communication from B. Roney, Petrex Company, February 16, 1983). 

Annual credits for recovered hydrocarbons are estimated to be $31,400. 

This is based on the AP-42 controlled emission factor of 10 pounds per 1000 

barrels of feed for a primary separator 90 percent control in the secondary 

separator, 330 days of operation per year, and a value of $23.70 per barrel 

for the recovered hydrocarbons. 

Table 1 shows capital and annual costs for retrofitting a cover on a 

secondary wastewater separator. These represent typical costs; actual costs 

will vary on a site-specific basis. Estimated net annual costs are $50,300 

with hydrocarbon recovery credits. 

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3.4 COST-EFFECTIVENESS 

Based on net annual costs of $50,300 to control 223 tons of VOC emissions 

or $225 per ton of VOC emissions controlled, covering a secondary wastewater 

separator may be considered cost-effective from a control agency point of 

view, as these costs are comparable to VOC control for other source categories 

(e.g., pneumatic rubber tires). 

TABLE 1. CONTROL SYSTEM COSTS FOR 

SECONDARY WASTEWATER SEPARATORSa 

(September 1982 dollars) 

Annual capital recovery factor (~0 year 

life, 10 percent interest rate) 

Taxes, insurance, andbadministrative 

costs (four percent) 

Operating andbmaintenance costs 

(10 percent) 

Total annual cost 

Credit for recovered hydrocarbons 

Net annual cost 

Tons of VOC controlled 

Cost per ton of VOC controlled 

$43,900 

10,800 

27,000 

81,700 

31,400 

$50,300 

223 

$225 

aCapital costs including installation are $270,000. 

bReference 4. 

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4. REGULATORY ANALYSIS 

In this report, a wastewater separator is defined as "secondary" when all 

entering streams have received prior gravity separation or equivalent treatment that has removed at least 90 percent of the VOC. If any stream entering 

the separator has not received prior treatment, the separator is considered a 

primary separator and subject to all applicable regulations for primary separators. 

Regulation of secondary wastewater separators is not feasible. Although 

a cost of $225 per ton of VOC controlled is often considered acceptable, the 

number of refineries with covered secondary separators is not large enough to 

support regulation of secondary wastewater separators. 

Estimates of VOC emissions from secondary separators are based on AP-42 

emission factors for primary separators 2 and an API estimate that emissions 

from secondary separators would be about 10 percent of that from primary 

separatori. 1 In AP-42, the reliability of emission factors is valued on a 

scale of A to E and 1 to 40 points. The emission factors for primary wastewater separators received a rating of D (below average) and a numerical rating 

of between 6 and 15 points. A probable range of these emission factors was 

not estimated. Up to 20 points could have been awarded for measured emission 

data, up to 10 points for process data, and up to 10 points for engineering 

analyses. Hence, VOC emission rates from secondary wastewater separators are 

only estimates. Because the cost of covering a secondary separator is sitespecific,1'4 both the costs and benefits of this VOC emission control are 

doubtful. If field testing confirms the control costs, emission reductions, 

and recovered hydrocarbon credits, a regulation might be feasible. 

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REFERENCES 

1. American Petroleum Institute. Hydrocarbon Emissions from Refineries. 

Publication No. 928, July 1973. 

2. U.S. Environmental Protection Agency. Compilation of Air Pollution 

Emission Factors. 3rd Ed., including Supplements 1-12. Section 9.1. 

AP-42, April 1, 1981. 

3. U.S. Environmental Protection Agency. Assessment of Atmospheric Emissions from Petroleum Refining. Vol. I. EPA-600/2-80-075A, April 1980. 

4. U.S. Environmental Protection Agency. Control of Refinery Vacuum Producing Systems, Wastewater Separators and Process Unit Turnarounds. EPA 

450/2-77-025 (OAQPS No. 1.2-081), October 1977. 

5. Value of Residual Oil Relative to That of Crude Oil. Hydrocarbon Processing, October 1982. p. 21. 

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