Planning a wastewater treatment plant with sustainable power generation in San Salvador

A Wastewater Treatment Plant Becomes a Power Plant

Builders and operators of wastewater treatment plants are increasingly designing such plants to be energy self-sufficient, but rarely do they focus on marketing electricity. The state-owned power generation company Comisión Ejecutiva Hidroeléctrica del Río Lempa is doing things differently. Not only does it want to build a wastewater treatment plant in San Salvador that is self-sufficient in terms of energy costs, but it is also thinking one step ahead. Optimal use of the power generation options provided by sewage gas, solar photovoltaics and hydropower at the site of this new treatment plant will allow for an electricity yield that exceeds the wastewater treatment plant’s own power requirement by a factor of six. This innovative concept creates a source of income that could also solve the problem of a lack of funding for wastewater treatment plants in many other countries.

Some 1.75 million people live in the metropolitan region of San Salvador in El Salvador, and their untreated wastewater ends up in the nearby waters, including the Acelhuate River. This is also a challenge for the state-owned power generation company, Comisión Ejecutiva Hidroeléctrica del Río Lempa (CEL), as it operates three cascade hydropower plants whose reservoirs are partially filled by the highly polluted flow of the Acelhuate River. This wastewater treatment plant would not only solve the problems that pollution, suspended matter, and objects cause during operation of the hydropower plants, but would also significantly reduce water pollution in the San Salvador area. On top of solving an environmental problem, CEL also wants to generate electricity on the plant site.

Wastewater Treatment Plant with Multiple Benefits

Generating electricity in a wastewater treatment plant is nothing new. Other operators of wastewater treatment plants also use the sewage gas produced in the fermentation process for this purpose. In most cases, however, the focus is on covering part of the plant’s own electricity needs. CEL, on the other hand, is aiming for a high energy yield in order to be able to feed as much green electricity as possible into the grid. The planned wastewater treatment plant, which is to be built in the municipality of San Salvador East, is therefore most probably unique in the world, as it would:

  1. purify 70 percent of the wastewater of a large city such as San Salvador,
  2. enable smoother operation of the existing hydropower plants,
  3. generate almost six times as much electricity as is needed to operate it by combining various technologies, and also
  4. generate sewage sludge that can be used as biosolids or fertilizer.

The innovative power generation concept is based on a combination of a sewage gas plant, solar photovoltaic plant, and small hydropower plant so as to make optimum use of the available renewable resources.

Key Data of the Overall Plant in San Salvador

  • 3.2 km-long, partially underground wastewater collector with seven bridges and a rated diameter of 1.2 m

  • Wastewater treatment with anaerobic biological process (UASB), trickling filter, and a flow rate of approx. 100,000 m³/day

  • Biogas production and storage

  • Own power requirement of approx. 0.8 MW

  • Electricity generation
    a) Biogas plant (1.7 MW)
    b) Solar photovoltaic plant (3.17 MWp)
    c) Small hydropower plant (0.56 MW)

Gaining Over Four Megawatts of Electrical Power

The high level of power generation is partly thanks to the climatic conditions and the geographical features with large differences in altitude in El Salvador: Due to the optimal temperatures, the production of sewage gas is high all year round and does not require any external heat input. The volume of sewage gas will therefore enable power production of 1.7 MW (with a high plant factor of more than 80 percent). The conditions are just as good for the photovoltaic plant, which will cover a large area of the wastewater treatment plant site and is expected to deliver a peak electrical output of 3.17 MW, while a small hydropower plant will contribute a further 0.56 MW. If the plant’s own power requirement of 0.8 MW is deducted, a peak electrical output of 4.63 MW remains to be fed into the power grid, thus providing a further contribution to sustainable power generation in El Salvador.

wastewater collector

The wastewater collector between the city of San Salvador and the plant runs through a very hilly region. It will be alternately laid underground or carried by bridges to cross valleys or ravines.

Far-Sighted Planning: Fichtner’s Team Presents Alternatives

When planning the wastewater treatment and power generation plants, Fichtner first reviewed the existing feasibility study prepared by CEL and optimized the concept to then prepare the preliminary, basic and detailed design for permitting in the next step. Although the boundary conditions were predetermined, we thought “outside the box” to achieve the best result. For example, Fichtner’s hydropower experts realized that the intended location for the water turbine was not ideal. A location 40 meters downstream offers better conditions for a small hydropower plant, as it is not only more accessible but also promises a significantly higher energy yield. So the team submitted an alternative proposal as part of the planning work, which was gratefully received by CEL. The project made swift progress, not least thanks to the excellent communication between the teams of experts at CEL and Fichtner. And thanks in part to Fichtner’s multidisciplinary team, a holistic and optimized design was developed. In the meantime, the plant design has been in place for several months and Fichtner has been contracted to support CEL in the tendering and contract award process.

Joining Forces in an Innovative Project

Projects such as the San Salvador wastewater treatment and power plant require a multidisciplinary team and excellent cooperation to achieve a holistically optimized design. The following Fichtner experts were involved in this case:

  • Project coordinators

  • Experts in wastewater collectors, wastewater treatment and biogas utilization

  • Specialists in hydraulics and hydropower

  • Experts in structural engineering and hydraulic engineering

  • Specialists in solar photovoltaic systems

  • Electrical, automation, civil and structural engineers

Blueprint for Wastewater Treatment Plants in Other Regions of the World

The concept of a combined wastewater treatment and power plant could also be of great benefit in other cities in El Salvador and in other regions of the world. Due to the high energy costs of wastewater treatment plant operation, there is often a lack of funding – not only for constructing the plant itself, but also for its operation and maintenance. However, if power generation at the wastewater treatment plant becomes a source of income over and above the plant’s own power requirement, the financial situation becomes much more favorable. In further considerations, Fichtner has recognized that many locations offer good conditions for a combination of sewage gas production, solar photovoltaics and hydropower. The resulting potential income could be of interest to both wastewater treatment plant operators and financiers, as the project in San Salvador demonstrates. The first international investors have already expressed their interest in the project. This plant concept could therefore improve wastewater pollution in many places around the world.

Francisco Mendoza Parissi on his role as project director:

“My two main tasks in the wastewater treatment plant project in San Salvador were coordination and communication. As a native Mexican, I speak the official language of El Salvador – Spanish – fluently. It was therefore easy for me to talk to our client CEL. So I spent a lot of time on site in direct contact with the client and coordinated our multidisciplinary teams from there. This was particularly advantageous during the Covid pandemic, when traveling was difficult and entailed additional risks for those involved. The digital collaboration between the engineering teams, which was still unusual at the time, quickly became routine. I believe that the well-engineered plant design is proof of our effective teamwork both internally and also together with CEL. It would be my pleasure to oversee the upcoming construction of this combined wastewater treatment and power generation plant in El Salvador. In my view, the concept of the San Salvador wastewater treatment plant can serve as a model elsewhere globally and provide dual benefits there too, namely reducing environmental pollution and at the same time generating climate-friendly electricity.”

May 2024


Francisco Mendoza Parissi

Regional Director Latin America
for Water Supply and Sanitation