Holding Tanks

Recommended Practices for Non-Hazardous, 
Industrial Wastewater Holding Tanks

Non-hazardous, industrial wastewater holding tanks are stationary devices constructed of non-earthen materials (e.g., fiberglass, steel or plastic) that are used to accumulate and store non-hazardous, industrial wastewater. These holding tanks do not discharge wastewater to surface or ground water or onto the surface of the ground, but are designed and constructed to facilitate collection of the wastewater for ultimate disposal at another site.

Non-hazardous, industrial wastewater is generally characterized as having components by either type or at concentrations which you would not find in normal household or sanitary wastewater. Non-hazardous, industrial wastewater includes wash or process water contaminated with minor amounts of fuels, oil or antifreeze; with diluted, non-household cleaning solvents and solutions; or with almost any chemical used in an industrial process. Although termed "non-hazardous," this industrial wastewater still has the potential to contaminate ground water if not disposed of properly. By state law, it is illegal to discharge this wastewater directly into septic systems or surface waters, or to discharge this wastewater to floor drains which empty into dry wells, septic systems, surface water or on top of the ground. In unsewered areas, connecting floor drains or otherwise capturing non-hazardous, industrial wastewater in a holding tank is the only environmentally-acceptable and legal option.

Industrial wastewater becomes hazardous waste when one or more of its components can be characterized as such, according to 06-096 CMR Chapter 850, "Identification of Hazardous Wastes." Wastewater that meets the definition of "waste oil" or "hazardous waste" is regulated by DEP's Bureau of Remediation and Waste Management. It is very important that business owners are knowledgeable about their wastewater and the substances that may be in it. Storage of hazardous waste in underground tanks is prohibited by Maine law and storage of waste oil in underground tanks is strictly regulated by the DEP. If you have any reason to believe your wastewater could be characterized as waste oil or hazardous waste, you are advised to contact the Bureau of Remediation and Waste Management prior to installing your holding tank.

Choosing and Installing the Proper Holding Tank  top

Holding tanks are available in many different shapes, sizes and materials. The DEP recommends considering the following points when choosing and installing a holding tank for non-hazardous wastewater.

  • Make sure the holding tank is compatible with the materials it will hold. For example, a cement holding tank is suitable for most truck garages but unsuitable for an industrial facility with acidic wastewater or with wastewater containing gasoline or solvents. Fiberglass tanks are generally not compatible with wastewater which may contain acetone, as found at autobody shops. Holding tank manufacturers and vendors have additional information on which type of tank or lining is appropriate for storing wastewater of certain characteristics.
  • Size the holding tank appropriately. The smaller the holding tank, the more frequent the pumping schedule. However, an oversized tank and infrequent pumping schedule can cause odors to form in the stagnant wastewater. Holding tanks ranging in size from 1,000 gallons to 2,000 gallons are most common, however, tanks can be as large as 10,000 gallons. When deciding on size, the best starting point is to estimate the average amount of wastewater generated per day and multiply it by the preferred number of days between pumping.

    Holding tanks are being used increasingly for capturing wastewater from floor drains in motor vehicle maintenance areas (i.e., automotive and auto body repair shops; aircraft, outboard engine, snowmobile, lawnmower, and ATV repair shops; etc.). For garage owners and operators, the DEP has derived the following chart based on industry practice.

Common Holding Tank Size for Garages Servicing

Number of Garage Bays

Cars and Light Trucks

Large Truck and
Heavy Equipment


500 gallons

1,000 gallons


1,000 gallons

1,500 gallons


1,500 gallons

2,000 gallons


2,000 gallons

2,500 gallons or greater

  • Install an alarm system to indicate when the holding tank nears full. An audio and visual alarm is best and should be located in a prominent location. The alarm system should be activated when the wastewater level in the holding tank reaches 75% of capacity. Otherwise, liquid levels should be measured manually ("sticking" the holding tank) and at least monthly during the first few months of installation to gauge rate of fill. One advantage of translucent, aboveground holding tanks is that alarms are unnecessary as the wastewater levels can be determined by visual inspection.
  • For below ground tanks, it is important that the holding tank be accessible at ground level. Generally, this is accomplished using a catch basin riser and access cover. The holding tank also needs to be "tight" ensuring that it does not leak though its sides, bottom, seams or top and there is no entrance for storm water, surface or ground water. Below ground tanks in vehicle traffic areas must withstand appropriate loading and the holding tank foundation must be able to both support the holding tank when it is full of industrial wastewater and prevent uplift when it is empty. Adhering to proper backfill standards will minimize any potential shifting or cracking of the tank. Steel piping is also not recommended.
  • Above ground tanks should be kept within a spill containment area that is 110% capacity of the holding tank. The above ground holding tank should be secure and the containment area should be impervious to contain leaks and spills.
  • Some municipalities and LUPC require permitting of the holding tank prior to installation. Check with local code enforcement or planning authorities prior to holding tank installation and obtain necessary local approvals.
  • Finally, begin planning for disposing of the wastewater before the holding tank is installed. See below, Disposal of Non-Hazardous, Industrial Wastewater, for disposal information and begin making contact with laboratories, haulers and disposal facilities.
Diagram of underground wastewater holding tank

Good Workplace Practices for Businesses with Holding Tanks  top

  • Keep the holding tank closed at all times except when wastewater is being added or removed.
  • Label the holding tank or place a sign near the access cover with the words, "Non-Hazardous Industrial Wastewater." While the sign serves as a reminder for employees, it also alerts emergency first responders who unfamiliar with your business processes and practices.
  • If possible, conduct and document visual inspections of the holding tank and piping for leaks. Document periodic tests of the sound and visual alarms, too.
  • Implement good workplace practices, such as storing all petroleum products, solvents and hazardous materials in areas where there are no floor drains, to reduce contamination potential.
  • Train employees about spill prevention and clean-up to minimize the possibility contaminants entering the holding tank, and, in the event of a leak or spill, appropriate response measures to collect, contain and clean-up the industrial wastewater. In the event of a chemical or oil spill, report the spill to the DEP, even if the material is captured in the holding tank. DEP's Emergency Response telephone number is 1-800-482-0777.
  • For holding tanks containing wastewater with petroleum components, use oil-only absorbent materials to capture free-floating oils. Tube-shaped sump skimmers or sump socks and oil-only absorbent pads can be used to capture small amounts of oil and gasoline. And, if there is consistently more than just a sheen of petroleum in your holding tank, then you have more than housekeeping problems you have an unregistered waste oil holding tank! Just another reason to implement good workplace practices.

Disposal of Non-Hazardous, Industrial Wastewater  top

Proper disposal of non-hazardous, industrial wastewater means having the holding tank contents trucked away by a licensed septage hauler to a local wastewater treatment plant that has agreed to accept the wastewater or by a licensed hazardous waste transporter as special liquid waste.

Where available, the DEP prefers non-hazardous, industrial wastewater to be disposed of at a municipal wastewater treatment facility, provided the local sewer district or regulating authority has granted permission. The DEP cannot require a municipal wastewater treatment plant to accept non-hazardous, industrial wastewater if the local sewer district or regulating authority prohibits it. Business owners hoping to dispose of their holding tanks at the local wastewater treatment plant are encouraged to make contact with the local sewer authority prior to installation and long before pumping and disposal is needed.

When it's time to empty the holding tank, a laboratory must analyze a sample of the wastewater prior to disposal. This analysis will identify any pollutants of concern and determine the proper disposal method. If the wastewater will be transported to a local wastewater treatment plant, ask the plant operator about local laboratories and which analyses the plant prefers. Each local treatment plant has different processes and pollutant concerns and can identify which analyses are important. Holding tank owners can also contact the DEP's Bureau of Remediation and Waste Management at 207-287-2651 or e-mail deprwm@Maine.gov for assistance on how to test whether a liquid is a waste oil or hazardous waste. If the wastewater will be transported as special liquid waste, the licensed hazardous waste transporter will often include laboratory analysis in the disposal contract.

If hazardous wastes are identified in the analysis, the entire contents of the holding tank is considered hazardous waste and can only be disposed of by a licensed hazardous waste transporter to a licensed disposal facility.

Recordkeeping is an important business safeguard when disposing of any waste. The DEP recommends that the following records be kept for three years:

  • The name of the holding tank owner or operator;
  • The date of shipment;
  • The volume and description of the industrial wastewater, including any analysis of the wastewater;
  • The name, license number and vehicle registration number of the hauler or transporter; and
  • The name and address of the receiving facility and written verification from the facility that the shipment was received.

For More Information

About floor drains and non-hazardous, industrial holding tanks:

About waste oil, hazardous waste or underground storage tanks:

Underground Injection Control Program
Telephone: 207-287-3901
Fax: 207-287-7191
Phone (Bureau of Land and Water Quality): 287-3901

Bureau of Remediation and Waste Management
Telephone: 207-287-2651
Fax: 207-287-7826
E-mail: deprwm@Maine.gov

Maine Department of Environment Protection
17 State House Station, Augusta, ME 04333-0017
Web: http://www.Maine.gov/dep/