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Setting the precedent with Kigali’s sewerage network

For most capital cities in the world, the centralised sewerage network extends as the city grows. And for most international development projects, the government is reacting to a clear crisis, rather than proactively mitigating a growing problem.

Our brief was to create a sewerage network that will meet Kigali’s needs today and in 45 years’ time. This will be the first network of its kind, so it was important to the Government of Rwanda we gave reassurance that the treatment process used would be complete and proven. The design was therefore conceived to build confidence as well as a major public health infrastructure.

Sebastien Mellot

Rwanda country manager

From the start, therefore, the new centralised sewerage network for the Rwandan capital Kigali was always going to be out of the ordinary.

How could we retrofit a sewer network and add a future-proof wastewater treatment plant in a large fast-growing city, which has never had a centralised sanitation system, and so prevent pollution and health risk in a sustainable way?


Kigali is experiencing worrying levels of pollution both to the soil and the Nyabugogo river, caused by the one million-strong population relying exclusively on septic tanks and latrines for sanitation. Discharge is largely uncontrolled, resulting in potentially harmful effluence of pathogens or other infectious agents entering the water supply – a scenario that will only increase as rapid population growth pushes pressure on the land to saturation point.

Government regulations state that any business that is more than three storeys needs to have an approved individual wastewater facility, and this has slowed down development as these systems can represent 7% of the capex cost. The case for making a proper sewerage system for Kigali was therefore very strong, economically, environmentally and in terms of public health.

Kigali is not alone in East Africa in needing this retrofitted sewerage system. However, as the first to commission it in the region, the project inevitably faced risks in terms of investor confidence and scarce local skills. Setting a precedent is always harder than following one. Added to this, the steep topography of the city brought its own engineering challenges in the land of a thousand hills.


We were appointed by the European Investment Bank (EIB), initially as technical advisor and then as design consultant, to develop the country’s first urban waterborne sewer network and associated wastewater treatment plant. Our brief was to develop engineering designs and tender documents for the first phase of the sewerage and wastewater treatment works.

A combination of aerial surveys, GIS and legwork created a detailed map that we overlaid on the city’s urban development masterplan. This “high tech, low tech” approach ensured the sustainability of the sewers by taking into consideration the existing roads and buildings, and also any future ones. Our designs limited the need for pumping to reduce energy use and costs. We also minimised social impacts by reducing the need for relocation of local dwellers or additional resettlements.

Maintaining the right flow velocity is critical for moderating maintenance costs and ensuring efficient delivery of effluence – a challenge that becomes magnified in such hilly terrain. We therefore used a series of backdrop manholes to achieve the right drop in height. Sewerage modelling software also helped generate the right sizing and elevation profiles to meet demand in potential areas of low or high usage.

Not knowing the load to be treated could have forced us to be conservative. However, we used this challenge as an opportunity to design a treatment process that would allow additional treatments for different reuse of sludge varieties. These included a digester for the production of methane as a potential energy source, as well as treatments to ensure dried sludges are suitable for agriculture use.


Our mission was to prepare a sustainable and bankable project for EIB that would help the Government of Rwanda to implement its economic development vision. Beyond satisfying the client and beneficiaries, our designs unlocked finance to enable the implementation of a project that’s critical for public health, environment preservation, and the economic development of the capital city. We have designed a treatment process that is flexible enough to guarantee full compliance with local standards for wastewater treatment over the next 45 years. The initial 850ha-network will service the central financial district, with in-built capacity to extend the footprint to 2,300ha.

In addition, our extra care and effort in preparing very high standard tender documents, which are both clear and prescriptive, will allow the EIB and the government to proactively mitigate risks for the financiers. Likewise, they were able to attract potential contractors and development investors who might perceive this project as a high risk, simply because there’s never been one like it in Rwanda – or similar cities - before.

Due to the high number of unknowns, we carefully looked at adding more detail than usual. Therefore, instructions to contractors allowed the client to monitor works, commission at every step and so prevent contractors from abusing the client’s inexperience and reliance on third parties.

Adhering to UK standards saved money and build capacity

The technical experience in wastewater treatment engineering is not available in the region. Combined with a limited budget, the government insisted we deliver a project with UK standards in Rwanda, with UK-based engineers who would build Rwandan stakeholders capacity, particularly around factors associated with climate change. We addressed this challenge by:

  • Creating a virtual model of the sewer area through detailed mapping to allow computer based hydraulic modelling before site verification to reduce time and costs
  • Appointing a regionally based project leader who ensured continuous and structured communication between the steering committee and the design teams in the UK for a seamless project delivery

Our philosophy was to use the remoteness of our teams as an advantage for offering global experts’ inputs and the benefits of our proficiency in digital technologies as drivers for cost efficiency and sustainability. This approach brought additional outcomes beyond the project, as our design philosophy was integrated into the new Rwandan water and sanitation policy, published in March 2017.

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