La Mesa Treatment Plant 1 is the largest water treatment plant in the Philippines, with a design capacity of 1,500 million litres per day (MLD). Built in the 1980s, the plant required considerable upgrades to help cope with turbidity in raw water, embrace renewable energy and increase resilience, particularly to the ever-present threat of earthquakes.

Arup undertook a feasibility study to develop effective and cost-efficient solutions for transforming La Mesa’s capabilities. These included upgrading existing components such as settlement basins, to optimise size and cost of new assets where necessary. We then undertook a seismic hazard assessment, which led to our full retrofit design for the treatment basins and administration building, making La Mesa resilient to a 1-in-2475-year seismic event.

During the transformation of the plant, Arup added photovoltaic solar panels that now cover 25% of the plant’s overall energy demands, saving operation costs and reducing carbon emissions. In order to allow the upgrades to be made with no interruption of service, Arup developed an implementation plan of staged handovers for the process.

Seismic design for structural resilience

Arup assessed the conditions of the existing structures through a systematic programme of site sampling.  We undertook a site-specific seismic hazard assessment, backed up by full on site cross-hole and downhole shear wave testing, to confirm the design conditions.  From this we carried out a full retrofit design for the treatment basins as well as the 3-storey administration and plant building. Fibre reinforcement is used for strengthening in parallel with concrete repairs and new concrete sections.  

The overall solution is designed to accommodate a 1-in-2475-year seismic event and allow the treatment plant to resume working quickly after an event.

La Mesa Water Treatment Plant now has increased water treatment capacity.
Now fully upgraded and expanded, La Mesa is equipped to resist a one-in-2,475-year seismic event, and to resume service shortly after such a major shock.


Embracing renewable energy

As part of the process requirement sections needed to be covered to create better process conditions, we included solar photo voltaic panels for power generation, maximising the value of these covers. 

Our solution maximised the generation to work within Maynilad’s regulatory constraints and allowed all energy generated to be used on site. The panels can provide 100% of the plant’s power needs during the daytime and approximately 25% of its overall energy demands, reducing operating costs and carbon emissions.

Reaching a modern standard

Modernising La Mesa to deliver a reliable service and deal effectively with the full range of weather and seismic risks has transformed confidence in the treatment plant. Digital control systems and automated processes have replaced manual operation, generating data for the client that has led to major efficiency gains.

Our team designed a sequence for the works that kept La Mesa in service throughout the upgrade. This ambitious approach to project delivery helped secure a Project of the Year award from the International Federation of Consulting Engineers.

Our work on La Mesa built on Arup’s 25-year presence in the Philippines. We provided a wide range of technical expertise, from geotechnical engineering and materials science to civil, structural, and process engineering. Our efforts have cemented the relationship with Maynilad Water Services and reshaped its approach to major projects.


Modernising La Mesa Water Treatment Plant presented an essential but daunting challenge. Arup’s experience and expertise delivered a facility that is above and beyond what we expected – without taking it offline – and changed the way we undertake other improvement projects.

Robinson Salenga Project Manager

Maynilad Water Services