Combustible Gas Monitoring System Guidelines


The Sanitation Districts' Combustible Gas Monitoring System Policy is established under the provisions of Section 414(c) of the Sanitation Districts' Wastewater Ordinance and requires all significant potential dischargers of flammable substances to install, operate and maintain an adequate combustible gas monitoring system. The purpose of this policy is to provide the Sanitation Districts and Industry with a method of early detection of any discharge of flammable substances so that preventive measures can be taken to avoid loss of life, damage to the Sanitation Districts' sewerage system, and damage to public and/or private property.


All industrial wastewater dischargers subject to this policy shall submit legible engineering drawings of the proposed combustible gas monitoring system for Sanitation Districts' review and approval prior to construction. The drawings shall indicate locations, dimensions, elevations, types and details of the detector/sensor head assembly and control unit, details of upstream and downstream piping, the means of diverting flow from the sewer to an appropriate holding vessel or pond, and the location, dimensions, details, and capacity of the holding vessel or pond. Manufacturer's specifications, catalogue cuts and data sheets shall be included with the required drawings for any manufactured equipment to be installed as part of the combustible gas monitoring system. Failure to submit plans or install a required combustible gas monitoring system will result in the initiation of enforcement action against the company.

The combustible gas monitoring system to be installed shall meet these basic minimum requirements:

  • The system shall be continuous and fixed (permanent rather than portable) and shall be installed near the company's approved flow monitoring location (where applicable).
  • The system shall have an indicator as well as an automatic continuous recorder capable of maintaining a permanent record of readings (i.e., chart recorder).
  • The system shall be equipped with a two-stage alarm system that is adjustable. The upper alarm level must be set at 20% LEL (Lower Explosive Limit).
  • The system shall be calibrated for methane detection.
  • The system shall incorporate a means of diverting flow from the sewer to an appropriate holding vessel or pond during periods when the combustible level is 20% LEL or more.

In addition to these requirements, it is highly recommended that the system have the capability of being operated on both regular and standby AC/DC power sources such that the system will operate during power outages. The control unit for the combustible gas detection meter should be located where the alarm will be heard and acted upon promptly (i.e., control room).


Two-Stage Alarm System

The two-stage alarm system of the gas detection system shall be adjustable as previously mentioned. The upper alarm level must be set at 20% LEL. When this level is reached, the company must cease or divert flow from the sewer to an appropriate holding vessel or pond until such time that the combustible level falls below 20% LEL.

NOTIFICATION: If the combustible level reaches > 20% LEL and diversion has failed, the company must notify:

  • the local Fire Department, and
  • the Sanitation Districts at (562) 908-4288, extension 2907, during office hours or (562) 437-6520 during non-office hours.

The lower alarm level setting will be left to the judgment of the company as it will depend upon the average percent LEL background level. However, the lower alarm level setting must not exceed 15% LEL. It is anticipated that the selected initial warning level will give the company sufficient time to investigate and correct any situation so that it is unlikely the 20% LEL alarm level will be reached.

Detector/Sensor Head Assembly Location

The detector/sensor head assembly (SHA) should be located at the site of maximum probable gas or vapor concentration. The SHA should be mounted, facing downward, on anti-corrosive and rigid material. The control unit should be mounted in a location that is free from shock and vibration and must be easily accessible for maintenance and calibration.

The SHA location shall be at a point in the final wastewater flow where sufficient agitation is provided to simulate the maximum possible sewer turbulence conditions that may be encountered. If no existing location meets with the Sanitation Districts' approval, then a sampling area with such simulated sewer agitation (i.e., the installation of an air bubbler) must be provided by the company.

Since the filament in the SHA is catalytically active, the sensor will become inactive when it is covered by a film of moisture condensate. Therefore, the SHA should be equipped with a moisture filter and situated such that the moisture contained in the evolved gas is removed. Continuous flow of evolved gas must be provided to sustain a continuous workable system.

Combustible Gas Detection Systems are designed to detect combustible gases and vapors in air. They are not capable of measuring the percentage of gases or vapors in steam or inert atmospheres, due to the lack of oxygen necessary to support combustion in the filament chamber. Concentration of oxygen in the compartment where the SHA is located must exceed the minimum level required to support combustion in the sensor head.

If the company has more than one Industrial Wastewater Discharge Permit for the same situs address, and those wastewaters having the potential of containing flammable substances are not discharged through the same pretreatment equipment, a separate detector head assembly unit and recorder will be required for each outfall.

Chart Recorder

Because chart recorders are normally designed for positive readings and do not reflect reductions in electrical resistance, the base line (zero percent combustible gas level) on the chart recorder should be moved upwards [i.e., to the ten percent (10%) LEL value line]. This action will serve to detect any negative drifts that may occur and facilitate correction of the problem during routine inspection and maintenance. The Industrial Wastewater Discharge Permit number and the base line level must be posted at the recorder location.


To insure proper operation and continued accuracy of the combustible gas monitoring system, the Sanitation Districts require that all companies with such systems have a program of regular maintenance and calibration in accordance with Section 414 of the Sanitation Districts' Wastewater Ordinance.

A regular maintenance and calibration schedule shall be established by the company based upon the manufacturer's specifications and recommendations for the type and model selected. As a general rule, the zero check/ adjustment shall be done on a more frequent basis than the span check/ adjustment (i.e., once per week versus twice per month).

All companies that have installed combustible gas monitoring systems must keep accurate records of any maintenance and calibrations done on the system. The records should be maintained for a period of three (3) years, and copies should be regularly submitted to the Sanitation Districts in accordance with the following requirements:

  • Maintenance Records - A record of all maintenance performed on the combustible gas monitoring system must be kept and retained for a period of three (3) years. The Combustible Gas Monitoring System Maintenance Record form or a similar form should be used for this purpose. The zero level of the meter and chart should be checked each time the recorder chart is changed to insure proper operation. Any maintenance which is performed more often than twice per month may simply be listed as "routine" on the form.

The maintenance record form should be kept near the combustible gas meter control unit, or another location easily accessible for inspection by representatives of the Sanitation Districts.

  • Calibration Check - The span check/adjustment should be performed on a regular basis (no less than twice per month) to insure proper operation and continued accuracy. The calibrations performed should be recorded on the Combustible Gas Detection Meter Calibration Check Record form. This form should be kept with the maintenance record.
  • Information Submittal Frequency - Companies will be required to submit copies of the calibration check and the maintenance record forms to the Sanitation Districts semi-annually on January 31 and July 31.
  • Calibration of the Combustible Gas Meter After an Upper Alarm Level Incident - In addition to the regular calibration schedule, if the upper alarm level (20% LEL) is reached, the combustible gas meter must be recalibrated to insure the continued proper operation of the unit.

Significant potential discharger is defined under this policy as:

  • All petroleum refineries
  • Gasoline storage/transfer facilities, chemical manufacturing plants, and oil and gas extraction facilities having industrial wastewater discharges of 25,000 gallons or more on any one day
  • Any other facility that, upon evaluation with respect to wastewater-producing operations, discharge flow volume, type and quantity of materials being used, stored, or produced, is determined to be a potential discharger of flammable substances.

Flammable substances is defined under this policy as: any gasoline, benzene, naptha, solvent, fuel oil or any liquid, solid, or gas that would cause or tend to cause flammable or explosive conditions to result in the sewerage system.