(Adopted: 05/07/76; CARB Ex. Ord. G-73: 02/01/77; Readopted: 

07/25/77; Amended: 08/24/94; Amended: 06/23/14)

MDAQMD Rule 464 464-1

Oil-Water Separators 

RULE 464 

Oil-Water Separators 

(A) General 

(1) Purpose 

(a) The purpose of this rule is to reduce emissions of Volatile Organic 

Compounds (VOCs) from Oil-Water Separators. 

(2) Applicability 

(a) This rule applies to Oil-Water Separators including Air Flotation Units as 

defined in this rule, and process units or containers used to store skimmed 

oil or tar from Oil-Water Separators. 

(B) Definitions 

For the purposes of this rule only, the following definitions shall apply: 

(1) “Air Flotation Unit” – Equipment used to remove suspended matter, both oil and 

solid, from water by dissolving air under pressure and then releasing the air at 

atmospheric pressure in a tank or basin. 

(2) “Air Pollution Control Officer” (APCO) – The person appointed to the position of Air 

Pollution Control Officer of the District pursuant to the provisions of California Health 

& Safety Code §40750 and his or her designee. 

(3) “California Air Resources Board” (CARB) – The California State Air Resources 

Board, the Executive Officer of CARB and his or her authorized representative, 

the powers and duties of which are described in Part 2 of Division 26 of the 

California Health & Safety Code (commencing with Section 39500). 

(4) “Control Device” – Any device for reducing emissions of VOC to the atmosphere. 

(5) “Effluent Water” – Any wastewater generated as a byproduct of industrial 

processes and containing dissolved, particulate organic materials. Consists of a 

mixture of water with a petroleum product, including but not limited to the 

following: gasoline, kerosene, distillate fuel oils, residual fuel oils and lubricants. 

(6) “Fixed Cover” – Any cover made out of metal(s), polymer(s) or other material, and 

installed in a permanent position over the liquid. 
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Oil-Water Separators 

(7) “Floating Cover” – Any cover made out of metal(s), polymer(s) or other material, 

which is in contact with a liquid surface at all times. 

(8) “Forebay” – That section of a gravity-type separator which (a) receives the 

untreated, contaminated Effluent Water from the preseparator flume, and (b) acts as 

a header which distributes the influent to the separator channels. 

(9) “Fugitive Vapor Leak” (Leak) – The detection of 500 ppm or greater above 

background (expressed as methane), measured at the interface of the component 

using an appropriate hydrocarbon analyzer according to the procedures specified in 

EPA Method 21.

(10) “Non-Contact Water Cooling Systems” – Any system which involves the cooling 

of organic vapors via coolant injected through piping. There is no contact between 

the cooling fluid and the vapors being cooled. 

(11) “Oil-Water Separator” – Any device or piece of equipment, which utilizes the 

difference in density between oil or petroleum products and water to remove the oil 

or associated chemicals from the water, or any device, such as a flocculation tank, 

clarifier, etc. that removes petroleum-derived compounds from wastewater. 

(12) “Operator” – Includes, but is not limited to, any person who owns, leases, 

supervises, or operates a facility and/or equipment. 

(13) “Organic Materials” – Chemical compounds of carbon, excluding carbon 

monoxide, carbon dioxide, metallic carbides, metallic carbonates and ammonium 

carbonate. 

(14) “Organic Vapors” – Chemical compounds of carbon, excluding carbon monoxide, 

carbon dioxide, metallic carbides, metallic carbonates and ammonium carbonate in 

their gaseous state. 

(15) “Overall Control Efficiency” – The product of the capture efficiency multiplied by 

the control efficiency; the weight per unit time of VOC removed by a control 

device divided by the weight per unit time of VOC emitted by an emission source, 

expressed as a percentage. 

(16) “Petroleum Products” – Any crude oil or oil distillate derived from tar sands, shale 

or coal, including, but not limited to gasoline, kerosene, distillate fuel oils, residual 

fuel oils and lubricants. 

(17) “Reid Vapor Pressure” – The absolute vapor pressure of volatile crude oil and 

volatile nonviscous petroleum liquids, except liquified petroleum gases, as 

determined by ASTM D 323-89. 

(18) “United States Environmental Protection Agency” (USEPA) – The United States 

Environmental Protection Agency, the Administrator of the USEPA and his or her 

authorized representative. 
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Oil-Water Separators 

(19) “Vapor Recovery System” – A vapor-gathering system capable of collecting VOC 

vapors and gases emitted during the operation of equipment. 

(20) “Volatile Organic Compound’ (VOC) – Any volatile compound containing at least 

one atom of carbon, excluding methane, carbon monoxide, carbon dioxide, 

carbonic acid, metallic carbides or carbonates, and ammonium carbonate, and those 

compounds listed in 40 CFR 51.100(s). 

(C) General Requirements 

(1) Fugitive Vapor Control Devices 

(a) A person shall not use any Oil-Water Separator subject to the provisions of 

Section (A)(2), unless it has been equipped with one of the following vapor 

loss control devices, except when gauging or sampling is taking place: 

(i) A Fixed Cover with all openings sealed and totally enclosing the 

liquid contents of the compartment, except for such breathing vents 

as are structurally necessary; or 

(ii) A Floating Cover or double-deck type cover, equipped with closure 

seals that have no holes or tears, installed and maintained so that 

gaps between the compartment wall and seal shall not exceed 

one-eighth inch for an accumulative length of 97 percent of the 

perimeter of the tank, and shall not exceed one-half inch for an 

accumulative length of the remaining three percent of the tank. No 

gap between the compartment wall and the seal shall exceed 

one-half inch; or 

(iii) Route all vapors to a Control Device with an Overall Control 

Efficiency (collection and control efficiencies) of at least 95 percent 

by weight of VOCs, measured according to the test method 

specified in Section (G)(3). 

(b) Any oil-water separator subject to this rule shall provide the following 

vapor loss control device: 

(i) A Fixed Cover for all Forebays, such that no liquid surface is 

exposed to the atmosphere. 

(c) Skimmed oil or tar removed from Oil-Water Separators shall be either 

charged to process units with feed or transferred to a container with a 

control system with at least 90 percent control efficiency by weight. A 

Control Device must be under District permit. 

(2) Requirements for Covers 

(a) Covers for oil-water separators shall meet the following requirements: 
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Oil-Water Separators 

(i) The cover material shall be impermeable to VOCs, and free from 

holes or openings. 

(ii) Any gauging or sampling devices on the compartment cover shall be 

covered. The latter cover shall be kept closed, with no visible gaps 

between the cover and the compartment, except when the sampling 

device is being used. 

(iii) Hatches on covers shall be kept closed and free of gaps, except 

when opened for inspection, maintenance or repair. 

(iv) The perimeter of a cover, except for a Floating Cover, shall form a 

seal free of gaps with the foundation to which it is installed. 

(3) Fugitive Vapor Leak Monitoring 

(a) When an instrument reading of 500 parts per million (ppm) or greater is 

measured, a leak has been detected and the reading shall constitute a 

violation of this rule.

(D) Exemptions 

(1) The provisions of this rule shall not apply to:

(a) Segregated storm water runoff drain systems or to non-contact cooling 

water systems, where applicable. 

(b) Any system which collects and processes effluent or process water 

contaminated with oil or other petroleum products and recovers less than 

760 liters (201 gallons) a day of oil or other petroleum products 

(c) Oils, tars and petroleum products with a Reid Vapor Pressure of less than 25 

mm Hg (0.5 pound per square inch). 

(E) Monitoring 

(1) Monitoring for Fugitive Vapor Leaks shall be performed on a monthly basis and in 

accordance to test method specified in Section (G)(1). Monitoring records shall be 

kept on file as prescribed in Section (F)(1). 

(2) Monitoring of the control device shall be performed on an annual basis and in 

accordance to the test method specified in Section (G)(4). 

(F) Record Keeping 

(1) A log of the monthly leak inspection shall be kept on file at the facility. The log 

shall record, at a minimum, the following information: 

(a) Date of the inspection. 
MDAQMD Rule 464 464-5

Oil-Water Separators 

(b) Documentation of all written or machine recorded operator inspections, 

VOC measurements including corresponding background levels, source 

tests, repairs, replacements, and reinspection records.

(c) Leak determination method (shall be in accordance to the test method 

specified in Section (G)(1), using an appropriate hydrocarbon analyzer). 

(d) Corrective action (date of leak repair and a written justification for any 

repair interval in excess of 15 calendar days). 

(e) Inspector's name and signature. 

(2) Any person using an emission control device/system pursuant to Section (C)(1) as a 

means of complying with provisions of this rule shall maintain records of key 

system operating and maintenance data for the purpose of demonstrating 

continuous compliance during periods of emission producing activities. The data 

shall be recorded in a manner as prescribed by the District. 

(3) Any facility claiming exemption pursuant to Sections (D)(1)(a) or (D)(1)(b) of this 

rule shall keep records to substantiate the claimed exempt status. 

(4) Any record required or produced pursuant to this rule shall be retained on site for a 

minimum of five (5) years and shall be made available to the APCO upon request.

(G) Test Methods for Compliance Verification 

A violation determined by any one of these test methods shall constitute a violation of the 

rule: 

(1) Fugitive Vapor Leaks - Detection of VOCs - EPA Method 21 shall be used to 

determine compliance with this rule in regards to fugitive or VOC leaks. 

Instrument shall be calibrated with Method 21 using zero air (less than 10 parts per 

million (ppm) of hydrocarbon in air) with a mixture of methane or n-hexane. 

(2) Determination of Reid Vapor Pressure - Shall be determined by measuring the Reid 

Vapor pressure in accordance with Test Method for Vapor Pressure for Petroleum 

Products (Reid Method), ASTM D 323-82 (April 8, 1987). 

(3) Control Device Efficiency - Determining the destruction or removal efficiency of a 

control device shall be: 

(a) For systems utilizing add-on control equipment, EPA Method 25 or 25A, as 

applicable and analysis of halogenated exempt compounds shall be by ARB 

Method 22. 

(b) For incinerators or catalytic incinerators, EPA Method 25, unless the 

concentration of VOC in the outlet stream is below 50 ppm as carbon, in 
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Oil-Water Separators 

which case EPA Method 25A shall be used. 

(c) Where add-on control equipment is utilized, collection efficiency shall be 

determined by EPA document “Model Regulatory Language for Capture 

Efficiency Testing,” August 3, 1990. 

(4) Any applicable alternative test method may be used so long as such method has 

been approved by USEPA, CARB and the APCO. 

[SIP: See SIP Table at 

http://www.mdaqmd.ca.gov/Modules/ShowDocument.aspx?documentid=45]