Complete Guide To Safe Bund Dewatering: How To Empty A Secondary Containment Bund Legally

Emptying a secondary containment bund requires separating accumulated rainwater from hazardous hydrocarbons or chemical residues before discharge. Environmental protection standards like PPG 2 and SPCC rules dictate that bund water must never be discharged if oil concentrations exceed 5 milligrams per liter (5 ppm). This technical guide outlines the precise steps, equipment configurations, and regulatory criteria required to safely pump, filter, and drain a bunded area without risking severe environmental fines.

Pre-Dewatering Assessment and Equipment Configuration

Before initiating any dewatering procedure, you must assess the structural material of the bund (masonry, reinforced concrete, or prefabricated polyethylene) and identify the specific hazardous liquids stored within the primary tank. Rainwater naturally accumulates in uncovered bunds, reducing the net containment capacity. According to industry standards such as BS EN 14015 and regional environmental regulations, a bund must maintain a capacity of at least 110% of the largest tank's volume, or 25% of the total aggregate volume stored, whichever is greater. Standing water directly compromises this safety margin and must be managed systematically.

Operational Readiness Checklist

• Essential Gear and Equipment:
  
  
  • ATEX-certified submersible or centrifugal pump (required if handling flammable Class I/II liquids like petrol, kerosene, or aviation fuel).
  • Hydrocarbon-absorbing filtration cartridges or a portable coalescing plate separator.
  • Oil-to-water interface probe or visual sampling bailer.
  • Heavy-duty oil-only absorbent booms, pads, and non-sparking hand skimmers.
  • Calibrated oil-in-water test kit (photometric or colorimetric type).
  • Personal Protective Equipment (PPE): Chemical-resistant nitrile gloves (minimum 15 mil thickness), safety goggles, steel-toed boots, and static-dissipative high-visibility clothing.
• Mandatory Prerequisite Knowledge and Compliance Standards:
  
  
  • Familiarity with UK Environment Agency Pollution Prevention Guidelines (specifically PPG 2 and PPG 26) or US EPA Spill Prevention, Control, and Countermeasure (SPCC) rule 40 CFR Part 112.
  • Understanding of local wastewater discharge permits (consent to discharge to foul sewer or surface waters).
  • A completed Point-of-Work Risk Assessment (POWRA) and Hot Work / Cold Work permit if operating in designated Zone 1 or Zone 2 hazardous areas.
• Estimated Benchmarks:
  
  
  • Duration: 1 to 3 hours for standard rainfall volumes (up to 5,000 liters) using manual setup; continuous for automated systems.
  • Budget: $150 to $400 for basic consumables (skimmers, absorbent pads, manual cartridge filters) or $3,500 to $8,000 for automated, sensor-controlled bund dewatering systems (e.g., BundGuard).

Step-by-Step Bund Dewatering Execution

Successfully emptying a bund without causing environmental pollution requires a phased approach that isolates free-phase product, filters dissolved hydrocarbons, and monitors effluent quality in real-time.

Step 1: Conduct a Multi-Layer Liquid Level Assessment

Never activate a pump without analyzing the contents of the bund. Visually inspect the surface of the standing water for a visible oil sheen, rainbow discoloration, or thick sludge layers. Use a transparent bailer or an electronic oil-water interface probe to measure the exact depth of the floating product layer (light non-aqueous phase liquids, or LNAPLs) relative to the water column below.

Warning: If the surface hydrocarbon layer exceeds 5 millimeters in thickness, do not attempt to pump the water immediately. Pumping water out when a heavy product layer is present risks drawing pure oil into your pump intake, saturating your filtration media instantly and causing an environmental bypass release.

Step 2: Remove Free-Phase Floating Hydrocarbons

Before transferring any water, you must recover the bulk of the floating product. Deploy oil-only absorbent booms around the perimeter of the bund to corral the oil. Use oil-only absorbent pads—which repel water and selectively absorb hydrocarbons—directly on the surface of the liquid. For larger volumes of free product, use a manual, hand-operated diaphragm pump with a floating skimmer attachment to draw off the top layer of oil into a designated hazardous waste recovery drum. Continue this recovery until only a faint iridescent sheen or clear water remains.

Step 3: Configure the Filtration and Dewatering System

Position your dewatering pump inside the bund. If you are using a submersible pump, place it on a raised, solid surface such as a concrete paver or a specialized plastic sump bucket. Do not drop the pump directly onto the floor of the bund, as this will stir up sediment, organic debris, and heavy sludges, which will rapidly plug your downstream filters.

Connect the discharge hose of the pump to a multi-stage filtration system. The system must consist of:

1. A particulate pre-filter (typically a 50-micron bag filter) to capture suspended solids and silt.
2. An oil-retention cartridge containing specialized hydrocarbon-reactive polymers or granular activated carbon (GAC) that binds dissolved oils on contact.

Pro-Tip: Ensure the discharge line is equipped with a calibrated inline flow meter and a sampling port immediately downstream of the final filtration stage. This allows you to measure cumulative discharge volume and take water samples to verify regulatory compliance before the water leaves your containment site.

Step 4: Execute the Pumping Operation and Monitor Effluent

Slowly engage the pump, keeping the flow rate within the manufacturer's specified limits for your filtration media (typically between 20 to 50 liters per minute for portable cartridges).

Throughout the pumping process, perform visual inspections of the discharge point. Check for any foaming, discoloration, or oil sheens. Collect a sample from the sampling port every 15 minutes and run a quick field test using an oil-in-water comparator. If the testing indicates hydrocarbon levels approaching 5 ppm (or your local permit limit), shut down the pump immediately to replace the saturated filter media. Stop the pump when the water level reaches approximately 50 millimeters from the bottom of the bund to avoid drawing in bottom sediment and sludge.

Step 5: Decommission, Document, and Dispose of Waste

Once the target water level is reached, disconnect the pump and drain the hoses back into the bund or a collection container to prevent localized spills. Collect all saturated absorbent pads, booms, and used filter cartridges, and place them into heavy-duty, UN-approved hazardous waste bags.

Document the entire procedure in your site's environmental logbook. Record the date, total volume of water discharged, the starting and ending oil concentrations, and the disposal routing of the saturated filters. Any recovered free-phase oil and spent filters must be disposed of via a licensed hazardous waste contractor under a registered Waste Transfer Note (WTN) or Hazardous Waste Consignment Note.

Bund Dewatering Methods and Equipment Specifications

Selecting the correct extraction method depends on the scale of your storage facility, the frequency of rainfall, and the specific chemical characteristics of the stored product. Use the table below to compare the operational parameters of primary dewatering methods.

Common Site Dewatering Failures and Corrective Field Actions

Even with rigorous planning, physical components can fail due to chemical degradation, physical clogging, or environmental extremes. The following scenarios represent common field issues and their systematic remedies.

• Problem: Hydrocarbon filtration cartridges clog or restrict flow within minutes of starting the pump.

  
  
  
  • Root Cause: High concentrations of suspended solids (silt, algae, or organic leaf litter) or heavy, emulsified oil-water mixtures are blocking the physical pores of the filter media before the chemical polymer can react.
  • Actionable Fix: Shut down the pump. Install a high-capacity, 100-micron washbag pre-filter upstream of the hydrocarbon cartridge to trap sediment. If emulsified oil is present, route the liquid through a gravity settling tank or weir tank prior to filtration to allow the phases to stratify.

• Problem: The automated dewatering system pump runs continuously but fails to discharge water.

  
  
  
  • Root Cause: Air lock in the pump chamber, a blocked suction intake screen, or a false positive reading from the system's optical oil sensor due to biofilm buildup or debris wrapping around the probe.
  • Actionable Fix: Turn off the power supply. Inspect the pump inlet screen and clear any debris. Clean the face of the optical sensor using isopropyl alcohol and a soft microfiber cloth to remove oily films or biological growth, then prime the pump manually and restart the system.

• Problem: The discharged water contains a faint hydrocarbon odor or a light sheen, despite passing through new filters.

  
  
  
  • Root Cause: Fluid velocity is too high, leading to "channeling" where water bypasses the active filter media, or the stored chemical has a lower surface tension (such as solvents or detergents) that has emulsified the oil, rendering standard physical coalescers ineffective.
  • Actionable Fix: Reduce the pump flow rate by partially closing the discharge valve (do not restrict the suction side) to increase the contact residence time inside the filter housing. If detergents or emulsifiers are present, switch to an organoclay-based media cartridge specifically designed to break chemical emulsions.

Frequently Asked Questions

Can you pump rainwater from a bund directly onto the ground?

No, you cannot pump rainwater directly onto the ground or into storm drains unless you have verified that the water is completely free of contaminants (typically less than 5 ppm of oil/fuel) and have received permission or a discharge consent from your local environmental regulator. Even clean-looking water can contain dissolved hydrocarbons or toxic metals that will pollute local aquifers and surface water.

How often should a storage tank bund be emptied of water?

A bund should be emptied whenever accumulated rainwater occupies more than 10% of its designed containment capacity. Leaving water in a bund reduces the physical volume available to capture a catastrophic leak from the primary tank, which directly violates environmental safety regulations and increases your risk of catastrophic failure.

What is the difference between an oil-water separator and a bund dewatering system?

An oil-water separator is designed to handle continuous flows of wastewater containing low levels of hydrocarbons (such as runoff from a forecourt or washing bay). A bund dewatering system is specifically designed for batch operations, utilizing smart sensors to distinguish between clean water and pure oil, automatically shutting down the pump to prevent discharging pure chemical product if a primary tank leak occurs.

Do I need ATEX-approved equipment for all bund-emptying procedures?

You must use ATEX-rated, intrinsically safe pumps and electronics if the liquid stored in the primary tank has a flashpoint below 60°C, such as petrol, kerosene, or certain solvents. Using non-ATEX equipment in these environments creates an ignition risk from electrical arcing, which could result in a catastrophic fire or explosion.

Professional Environmental Containment Solutions

If your facility requires compliant, heavy-duty bund management systems or specialized dewatering equipment, contact our engineering team today. We provide bespoke environmental containment solutions, automated dewatering pumps, and high-capacity filtration media designed to keep your operations running within full regulatory compliance.