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Tonkolili Iron Ore Project – Engineered Wetland Treatment Design

Tonkolili Iron Ore Project  
passive treatment system design
Office: London 
Customer: African Minerals Limited (AML) 
Location: Sierra Leone, Africa 
Timeframe: 2010 - 2011 
AML required an appropriate passive/semi-passive treatment system to reduce the contaminant load in leachate being produced at a proposed onsite Municipal Solid Waste (MSW) landfill for discharge into a controlled open watercourse.

The treatment system’s development required both a rigorous scientific and pragmatic engineering approach to solve the difficult site problems including, limited hydrological and chemical data, extreme precipitation and hydrological regimes, as well as a requirement to use locally sourced construction materials.

Specific design objectives included:

  • Minimal financial outlay for construction
  • Minimal operating costs
  • Minimal maintenance and daily management
  • Reduced contaminant load to meet 95-percentile discharge requirements
  • Operate efficiently under the highly variable hydrologic regime

Project Services

Based on available data, WorleyParsons developed a design model for a 3-stage passive treatment system. This consisted of:

  • A primary waste stabilization pond with excess storm water retention
  • Optional forced aeration rock drains
  • Horizontal subsurface flow reed beds to remove waste


The design met all primary customer objectives and provided a simple and cost-effective option for the onsite treatment of wastewater arising from the mining operation.

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