Technology Development and Strong Alliances: Treating Wastewater with Biodigesters

by: Jaime Martí Herrero (CIMNE; Universidad Regional Amazónica Ikiam)

Domestic low-cost biodigesters are commonly known as a technology for producing biogas. But their basic working principle is the anaerobic digestion of organic matter diluted in water. Can low-cost biodigesters be used for treating wastewater?

This blog narrates the fascinating journey of a technology development, which aimed at deploying the potential of low-cost tubular biodigesters for treating wastewater from different sectors and contexts. This journey was supported by WISIONS of sustainability at several stages. However, the journey also illustrates the importance of building alliances with diverse actors and programs in order to achieve impactful results.

What was the starting point of this journey?

Low-cost biodigesters, also known as tubular biodigesters, have been developed in Latin America based on a simple yet powerful technology. The systems, designed primarily for small family farms, transformed manure from cows and pigs into biogas for cooking and fertilizer for crops. They were capable of providing around 1,000-1,200 liters of biogas per day – enough for approximately four hours of cooking.

The development of tubular biodigesters began in the mid-1980s in Colombia, and the technology quickly spread to other Latin American countries. The second decade of the 2000s saw experiments with new substrates – including coffee waste in Central America, slaughterhouse waste in the Andean region and Mexico, and milk whey in Colombia – which expanded the range of situations where tubular biodigesters could be applied.

The alliance between academia, NGOs, and local municipalities helped further refine the technology. Through research, these collaborations identified the limits and the optimal conditions for implementing biodigesters in different settings, improving livelihoods in the respective communities.

But what are tubular biodigesters?

Tubular biodigesters are elongated reactors made from plastic – typically greenhouse plastic, PVC, or polyethylene geomembrane. Today, most biodigesters are constructed from geomembrane materials, which are more durable and effective.

A typical tubular biodigester maintains a length-to-diameter ratio of between 5 and 10. The influent enters, and the effluent exits through PVC pipes at each end. The microorganisms inside the biodigester produce gas (biogas) which is collected through an outlet on top of the reactor. In some designs, a sludge outlet is added at the bottom, which facilitate the removal of sediments from the bottom of the biodigester.

Scheme of a tubular digester (figure: Jaime Martí Herrero)

Previous experience with biodigesters and wastewater

Treating wastewater with biodigesters presents unique challenges due to the high dilution levels of the organic matter contained in the water. This has a direct impact on the size of the systems, because biodigesters of larger volumes are needed in order to treat the rather small portion of organic matter contained in the wastewater.

Addressing these challenges required new strategies. One solution involved adding solid elements inside the biodigester (for instance plastic bottles without its base) that increase the surface area in which the microorganisms that “digest” the organic matter can proliferate. In this way the efficiency of the anaerobic digestion process can be increased. Another approach involved connecting multiple biodigesters in series. In this way the period in which the organic matter is retained in the anaerobic digestion process increases, what in turn enhances biogas production.

One alternative to improve the anaerobic digestion is to introduce cut PET bottles inside the biodigester, in order to increase the inner surface able that is going to be colonized by microorganism (seen below).

photo: Jaime Martí Herrero

Other alternative to improve the anaerobic digestion in biodigester is to divide the system in several stages, as in the picture below. So instead of one big digester is better to have multiple smaller and sequential digesters.

photo: Jaime Martí Herrero

These strategies had been tested successfully in small pilot systems treating slaughterhouse wastewater. By 2016 the main challenges consisted in scaling-up those pilot experiences and in bringing the technology to solve wastewater treatment needs of real-life situations.

Initial focus on scaling-up the technology

In 2016, a project funded by WISIONS, began to develop and validate this technology on a larger scale. In this first phase, the focus was on partially treating wastewater from the metropolitan slaughterhouse in Quito, Ecuador. Four 100 m³ biodigesters, connected in series, were installed to treat 40 m³ of wastewater daily. The original plan foresaw the use of the produced biogas to replace a fraction of the diesel that is fired for heating process water.

The four sequential biodigesters of 100m3 each, in the Quito slaughterhouse working at full capacity (photo: Jaime Martí Herrero)

Unfortunately, the project was halted before the objectives were achieved. The main difficulties emerged when the management of the slaughterhouse was changed. In contrary to the previous management – who actively supported the project conception and design – the new management team didn’t show interest for the project and the implementation process lacked the crucial support from all the instances of the slaughterhouse. Despite these setbacks, the experience brought valuable lessons, for instance:

• Installing large tubular biodigesters (100 m³) is challenging due to the weight of the geomembrane material. Smaller reactors in series are recommended to facilitate easier installation.
• Even in secure locations, acts of vandalism can occur, indicating the need for controlled access and improved physical protection.

After this bittersweet experience, the project was explored different possible scenarios where carry on with the research. Finally, it was redirected to focus on domestic wastewater treatment in collaboration with the Universidad Regional Amazónica Ikiam, which showed compromise with the project.

Shifting the focus to deploying the technology for domestic wastewater treatment

The new setting for this effort was the Universidad Regional Amazónica Ikiam, located in the Ecuadorian Amazon, where the university invested in earth movements and installing of fences to harbor the project. A system of three biodigesters with a total volume of 20 m³ was implemented to treat wastewater from two university buildings. Unlike the slaughterhouse project, the goal here was not to produce biogas but rather to treat wastewater as an alternative to traditional treatment systems, such as unmonitored septic tanks, which do not a real treatment, or the activated sludge systems, which are energy and maintenance intensive processes.

Three sequential tubular biodigesters installed at the Universidad regional Amazónica Ikiam for the treatment of university wastewater (photo: Jaime Martí Herrero)

This system has been operational since late 2018, with the only required maintenance being occasional mowing of the grass in the area. The biodigesters have been processing between 7 to 10 m3 of wastewater per day, resulting in very short retention times of less than 3 days. Despite this, the biodigesters have successfully reduced the organic load, measured as Chemical Oxygen Demand (COD), by 50%. During this process, approximately 600 liters of biogas are produced daily, though it is not currently being utilized. After passing through the biodigesters, the wastewater is fed into a wetland, which currently functions like a gravel filter as it lacks plants. The wetland is also covered to mitigate the impact of heavy rainfall in the region. Within the wetland, the organic load (COD) decreases by an additional 65%. Overall, the combined system of biodigester and wetland achieves a COD reduction of over 80%. Important results from this experience are presented a recently published article.

Subsurface flow wetland installed at the Universidad Regional Amazónica Ikiam, as a complement to tubular biodigesters, for the treatment of university wastewater (photo: Jaime Martí Herrero)

This successful experience has provided valuable lessons:

• The biodigester and wetland system effectively treats domestic wastewater with very short retention times, without the need for agitation or external devices. The combination of these two components (biodigester and wetland) makes the system robust, providing consistent treated water quality despite variations in input flow.
• Over the past six years, the system has required no significant maintenance. The biodigesters have not become saturated with organic matter, but the wetland is beginning to show signs of clogging, which indicates that the pebbles will need to be cleaned soon.

Applying the technology to tackling sanitation needs of rural communities

After the successful results at Universidad Regional Amazónica Ikiam, the next step in 2020 was to apply the technology for improving the sanitation service of rural communities. The first community to benefit was Huamaurco, a Kichwa community of 35 families that, until then, had no access to drinking water or toilets in any of their homes. In collaboration with several partners – GreenEmpowerment, ENGIM, Universidad Regional Amazónica Ikiam, CIMNE, and the Huamaurco community itself – an integrated water project was developed. The goal was to address both the lack of access to safe drinking water and the challenge of effectively returning treated water to the environment.

The drinking water system included a water catchment and a system of sand filters with a chlorinator to make the water drinkable. More than 2 km of buried pipes were laid to bring water to each family. A water meter was installed for each family. A water board was formed, and they set their own fees to pay their own technicians and pay for materials and supplies to maintain the water system.

One of the various meetings held with the Kichwa community of Huamaurco to implement the integral water project, which includes bathrooms, biodigesters and wetlands for each family (photo: Jaime Martí Herrero)

Once the drinking water system was set, the project proceeded with the construction of toilets, biodigesters and wetlands. Basic toilets were built that included a shower, sink, and toilet. The greywater from the shower and sink was directed straight to a wetland, while the blackwater from the toilet was sent to two sequential biodigesters, 1m³ each. The effluent from these biodigesters then flowed into the wetland. After treatment in the wetland, the water was infiltrated into the soil to prevent human contact and avoid contamination of nearby water sources.

Bathrooms constructed in the Kichwa community of Huamaurco (photo: Jaime Martí Herrero)

Both sequential tubular biodigester sum a capacity of 2 m³, and the wetland was a subsurface flow wetland covering 1.5 m2. Thirty-five of these systems were installed—one per family. This meant that 70 tubular biodigesters had to be constructed (two per family). Although each biodigester was small, the work required was substantial. This experience highlighted the need to simplify future projects by using a single, larger biodigester per family to reduce the workload.

Two biodigester systems and a wetland. Left: system being installed, right: system already working (photo: Jaime Martí Herrero)

Two years after installation, an evaluation of the systems was conducted to assess their performance. The results showed that the biodigesters effectively reduced pollution by around 75%, but the treated effluent did not meet Ecuador’s water quality standards. The Chemical Oxygen Demand (COD) at the outlet of the dual biodigester system was around 400 mg/l, whereas regulations require 200 mg/l. The wetlands, which were expected to further reduce COD to acceptable discharge levels, managed to lower COD to only 230 mg/l. This indicated that while the biodigesters were performing well, the wetlands were not achieving the expected level of efficiency.

Another aspect evaluated was system durability. Within two years of installation, three systems had experienced accidents: one was damaged by a heavy fruit (Patas muyu) falling from a tree, and two others had their geomembranes cut by machetes during weed clearing. These incidents highlighted a significant vulnerability in the systems. The systems still work, but the biogas produced is not captured, and is not possible to flare it. The lesson learned here is the need to protect the biodigesters or to use materials that can withstand such incidents, such as rigid polyethylene water tanks.

Impact of the project

This project has extended far longer than any of us – whether the implementers or the funder such as WISIONS – initially anticipated, pandemic or not. Yet, it has also taken us into unexplored territories. What began as a search for a solution to treat slaughterhouse wastewater ended up offering a breakthrough in decentralized domestic wastewater treatment in the Amazon. This outcome speaks to the dynamism of science and technological development.

The project successfully facilitated the transfer of practical technology from the university to local communities, bridging research with real-world application. It also fostered collaboration among a diverse group of stakeholders working on water solutions in the Amazon region of Ecuador, leading to the proposal of integrated water projects. This holistic approach addressed not only the challenge of delivering safe drinking water from nature to families, but also how to responsibly return treated wastewater back to nature.

The results of the monitoring of the biodigester and wetland systems in Huamaurco were presented at the World Congress on Anaerobic Digestion, organized by the International Water Association (IWA) in June 2024 in Istanbul, Turkey. With over 700 participants in attendance, the interest sparked by the practical and straightforward nature of our technology led to us being awarded the best oral presentation of the Congress. This recognition has brought visibility to our modest yet effective technological solutions within the global academic community of anaerobic digestion. It is a significant achievement for Latin America and the Caribbean, and especially for RedBioLAC. Also, the results of a survey over decentralized domestic wastewater systems in the Ecuadorian Amazon, that consider Huamaurco systems, have been send for publication in the Bioresource Technology Journal.

Final words

Developing a project like this – with all its changes, unexpected events, successes, failures, and even a pandemic – while consistently having the support of WISIONS, has been an incredible privilege for me as a researcher. This kind of slow science, which evolves technology for the people and with the people, requires these rare opportunities. I am deeply grateful to WISIONS for enabling this process, which has not only reached a successful conclusion but has also opened up new avenues for research and technology transfer.