How To Empty A Bund: Safe And Compliant Secondary Containment Dewatering

Emptying a bund requires isolating and removing accumulated rainwater while preventing any leaked hydrocarbons or chemicals from contaminating the surrounding environment. Operators must use oil-water separators, specialized sump pumps, or vacuum tankers to ensure the discharge contains less than 15 parts per million (ppm) of oil, adhering to strict environmental standards such as CIRIA C736 or EPA SPCC rules. Always verify water purity using a visual test, oil-sensitive test strips, or laboratory analysis before discharging to surface water or sewers.

Pre-Operation Planning & Equipment Checklist

A bund (or secondary containment dyke) is designed to retain 110% of the capacity of the largest storage tank within its perimeter, or 25% of the total aggregate volume of all tanks inside, whichever is greater. When rainwater accumulates, this critical safety margin is compromised. However, because the water inside a bund can easily mix with leaked fuel, oil, or chemical residue, it cannot simply be drained or pumped directly into surface water drains without risk of severe environmental penalties.

Before initiating the dewatering process, you must assess the chemical makeup of the liquid and select the appropriate extraction method based on the degree of contamination.

Essential Equipment & Materials Checklist

• Pumping System: ATEX-rated submersible pump (mandatory if pumping flammable substances like petrol, kerosene, or diesel) or a manual hand-operated siphon pump for minor volumes.
• Filtration Media: Hydrocarbon-absorbent filters, inline oil-water separators, or oil-absorbent socks/booms.
• Testing Equipment: Hydrocarbon detection test strips, oil-indicating paste, or oil-in-water analysis kits.
• Containment & Disposal Vessels: UN-approved 205-litre steel or plastic drums, or intermediate bulk containers (IBCs) for contaminated liquid and sludge storage.
• Personal Protective Equipment (PPE): Nitrile gloves (minimum 0.4mm thickness), chemical splash goggles, anti-static safety boots, and high-visibility coveralls.
• Spill Kit: Minimum 20-litre oil/chemical spill response kit placed adjacent to the work area.

Regulatory and Operational Benchmarks

• Compliance Frameworks: CIRIA C736 (UK/Europe), US EPA SPCC (40 CFR Part 112), and local environmental protection regulations.
• Maximum Allowable Discharge Limit: 15 ppm (parts per million) of oil-in-water for standard industrial discharge; 5 ppm or zero-detection in highly sensitive aquatic environments.
• Estimated Duration: 1 to 3 hours for manual/pumped filtration; 1 hour for professional vacuum tanker extraction.
• Estimated Budget: $150 to $400 for basic filtration consumables and pump rental; $800 to $2,500 for professional vacuum tanker services and hazardous waste disposal certification.

Step-by-Step Bund Dewatering Execution

Successfully clearing a bund of liquid without violating environmental laws requires a systematic approach. Follow this step-by-step procedure to ensure safety, regulatory compliance, and mechanical efficiency.

Step 1: Pre-Dewatering Assessment and Testing

Before touching any equipment, you must identify the nature of the liquid inside the bund. Rainwater will often sit on the bottom, while lighter-than-water hydrocarbons (like diesel or heating oil) will float on top.

1. Visual Inspection: Look for a rainbow-like iridescent sheen on the water surface, which indicates hydrocarbon contamination. Observe if there are dark sludge deposits or unusual coloration indicating chemical spills.
2. Depth Measurement: Use a dipstick coated with water-finding paste and oil-indicating paste. Lower it to the bottom of the sump or low point of the bund. The color change on the pastes will indicate the exact boundary level between the water layer and any oil layer.
3. Chemical Testing: Dip an oil-sensitive test strip into the water layer. If the strip changes color, the water contains dissolved or emulsified hydrocarbons exceeding permissible discharge limits and cannot be pumped directly to the ground or surface drains without advanced filtration.

Warning: Never assume a clear liquid is safe to discharge. Soluble chemicals, glycol from cooling systems, and highly acidic or alkaline liquids do not always display a surface sheen. If in doubt, sample the liquid for laboratory pH and Chemical Oxygen Demand (COD) testing.

Step 2: Isolation and Site Preparation

Ensure the dewatering operation does not create new environmental hazards.

1. Stop Tank Activity: Suspend any refueling, chemical transfers, or maintenance tasks on the primary storage tanks within the bund during the dewatering process.
2. Position the Spill Kit: Deploy oil-absorbent booms around the external perimeter of the pump discharge zone to capture any accidental bypass.
3. Establish ATEX Boundaries: If the bund houses flammable liquids, ensure all electrical equipment, generators, and pumps are positioned outside the designated hazardous zone (typically 3 meters from the bund wall) unless they are certified ATEX Zone 1 or Zone 2 compliant.

Step 3: Removing Surface Hydrocarbons (Phase Separation)

If your inspection reveals a floating layer of oil or fuel, this must be removed before pumping the water beneath it. Pumping from the bottom first can cause vortexing, which mixes the oil and water into a difficult-to-treat emulsion.

1. Lay Absorbent Booms: Place oil-only absorbent booms or pads directly onto the surface of the liquid. These materials are hydrophobic, meaning they repel water while actively absorbing hydrocarbons.
2. Skim the Surface: Use a manual hand pump or a floating skimmer attachment connected to a recovery drum to draw off the concentrated top layer of oil.
3. Store Recovered Product: Transfer this recovered oil directly into designated hazardous waste drums. Label these drums immediately with the correct waste codes (e.g., EWC 13 02 08 for waste engine, gear, and lubricating oils).

Step 4: Pumping and Filtration Setup

Once the bulk of the surface oil is removed, you can begin the water evacuation process using an inline filtration setup.

1. Elevate the Pump Intake: Place the submersible pump inside a mesh strainer basket or suspend it approximately 100mm to 150mm above the floor of the bund. This prevents the pump from sucking up heavy bottom silt, leaves, and dense hydrocarbon sludge, which would instantly clog your filter cartridges.
2. Connect the Inline Filter: Run the discharge hose from the pump outlet into a multi-stage filtration system. This system should consist of a particulate pre-filter (to remove suspended solids) followed by a granular activated carbon (GAC) filter or a specialized coalescing media filter designed to capture dissolved hydrocarbons.
3. Route the Discharge: Direct the final output hose from the filter system to the designated discharge point (such as a foul sewer or surface water drain), provided you have a valid discharge permit from your local utility provider or environmental agency.

Pro-Tip: Install an automatic shut-off valve or a secondary oil-sensing probe at the end of the filtration line. If the filter media becomes saturated and hydrocarbons begin to slip through, the probe will instantly cut power to the pump, preventing an accidental pollution incident.

Step 5: Final Sludge and Silt Clearout

After the bulk of the water has been pumped out, a dense layer of silt, organic debris, and heavy hydrocarbons will remain on the bund floor.

1. Manual Scraping: Use non-sparking plastic or brass shovels to scrape the remaining sludge into a central collection sump within the bund.
2. Containerization: Scoop the sludge into heavy-duty, UN-approved plastic open-top drums. Do not mix this sludge with municipal waste or general site rubbish.
3. Wipe Down: Clean the internal walls and floor of the bund using oil-absorbent wipes. Avoid using detergents or degreasing agents to wash down the bund unless you plan to vacuum up 100% of the wash water, as surfactants emulsify oils and render standard gravity oil-water separators completely useless.

Step 6: Recording and Compliance Documentation

Every bund dewatering event must be fully documented to prove compliance during regulatory audits.

1. Log the Details: Record the date, name of the operator, estimated volume of water discharged (in liters or cubic meters), and the final hydrocarbon test readings.
2. Retain Waste Consignment Notes: If hazardous waste (sludge, oil-saturated filters, or oily water) was removed from the site, secure a hazardous waste consignment note (or equivalent regional manifest) signed by the registered waste carrier. Keep these documents on-site for a minimum of three years.

Bund Dewatering Methods & Equipment Specifications

Choosing the right dewatering method depends on water volume, contamination level, and site infrastructure. The table below compares the primary industry-standard methods.

Common Site Failures & Field Fixes

Water-Oil Emulsion Bypasses Standard Filter Systems

• Root Cause: The use of high-shear centrifugal pumps agitates the mixture of oil and water, turning it into a milky emulsion. Standard gravity separators and particulate filters cannot separate emulsified droplets because they are chemically bound to the water molecules.
• Actionable Fix: Switch to a low-shear, positive-displacement pump (such as a double-diaphragm pump or peristaltic pump) which transfers the liquid gently without blending it. If emulsification has already occurred, you must treat the liquid with a chemical demulsifier (coagulant) before filtration, or vacuum-extract the entire volume for treatment at an industrial waste facility.

Pump Burns Out Due to Heavy Sediment

• Root Cause: Placing the submersible pump directly on the floor of the bund causes it to draw in sand, gravel, leaves, and thick hydrocarbon sludge, blocking the impeller and overheating the motor.
• Actionable Fix: Elevate the pump. Place the pump inside an empty, clean plastic bucket that has had numerous 10mm holes drilled into its upper half. This acts as an effective, low-cost pre-filter cage. Alternatively, suspend the pump from a tripod so that the intake sits at least 150mm above the bottom sediment layer.

Environmental Agency Audit Failure

• Root Cause: Site personnel discharged water that appeared "clean" to the naked eye but actually contained dissolved hydrocarbons or glycol exceeding regional limits, or failed to produce written verification records of the discharge.
• Actionable Fix: Implement a formal, mandatory "Permit to Pump" system on-site. No pump can be switched on until a supervisor signs off on a physical checklist that includes a visual check, a hydrocarbon test strip reading, and confirmation of where the water will discharge. Keep this logbook in the primary site office.

Frequently Asked Questions

Can I install a gravity drain valve in a bund wall to let rainwater out?

No, modern environmental regulations (such as CIRIA C736) strongly discourage or outright ban the installation of gravity drain valves or drain plugs through a bund wall. Valves can be left open accidentally, corrode over time, or fail mechanically, which would allow a catastrophic tank spill to escape directly into the surrounding environment. All water removal must be performed using over-the-wall pumping methods or manual extraction.

What is the legal limit for oil-in-water discharge from a bund?

In most industrial regions, the legal limit for discharging water into surface systems (such as rivers, streams, or storm drains) is 15 parts per million (ppm) of Total Petroleum Hydrocarbons (TPH). In highly sensitive environmental areas or drinking water catchment zones, this limit is often reduced to 5 ppm or "no visible sheen." If discharging to a municipal foul sewer, you must obtain a Trade Effluent Consent from your local water authority, which will specify acceptable chemical limits.

How often should a bund be emptied of rainwater?

A bund should be checked weekly and emptied as soon as rainwater accumulation exceeds 10% of its containment volume. If water is allowed to build up, the bund’s effective holding capacity is severely reduced. For example, if a bund is 30% full of rainwater, it can no longer hold the mandatory 110% volume of the primary storage tank in the event of a catastrophic tank failure, leaving your facility in direct breach of environmental safety regulations.

What should I do if a chemical bund contains a major product leak?

Do not attempt to pump or filter a major chemical leak using standard dewatering equipment. Immediately isolate the source of the leak using your site’s emergency containment procedures. Once the leak is stopped, contact a registered hazardous waste disposal contractor equipped with specialized chemical-resistant vacuum tankers to pump out the concentrated chemical. Keep all site personnel clear of the area, and consult the Safety Data Sheet (SDS) of the spilled chemical to determine correct chemical-resistant PPE requirements.

Upgrade Your Secondary Containment Protection

Ensure your facility remains fully compliant with environmental regulations by implementing a professional, automated bund maintenance program. Contact our environmental engineering specialists today to upgrade your secondary containment infrastructure with advanced, automated dewatering systems.