Costa Rica – Ecological Engineering in the Tropics

MSU / UCR – December 15-29, 2012

Day 12 – Fabio Baudrit Experiment Station – Anaerobic digestión – Bioenergy engineering

Fabio Baudrit Experiment Station – Anaerobic digestion – Bioenergy engineering – Alex

The 26th of December started off as any other day, extremely early.  A quick breakfast was eatten at San Luis Ecolodge and in no time we were on the road again.  We drove for three hours down winding mountain roads until Costa Rican nature was replaced by the urbanization of San Jose.  The saddest part of the day came at the end of this road trip, with the departure of our bus driver Edgar.  He didn’t speak a word of english, but he didn’t need to.  He took care of us and was a center piece of the group.  He is missed. After a bit of refreshment at the Adventure Inn, we made the familiar trip to Fabio Research center.  The days goals were to harvest a bit of bamboo from the small patch growing in Fabio and after take a tour of the anaerobic digester and wetland let by the ever knowledgeable Dawn Reinhold.

The bamboo harvesting proved to be more of a challenge than anticipated.  It was needed for some experimentation with the floating mats we were to build the next day, and much of it had already been cut.  Just one lone stalk of bamboo was needed.  For well over twenty minutes, Dmitri and a few UCR students slashed away at the base of the forty foot behemoth with a machete   The base finally gave, but to the crowds horror was caught by another stalk and twenty to thirty more minutes of hard manual labor was needed for the bamboo stick was finally freed.  This bamboo battle left many exhausted.


Dmitri, machete, and Bamboo.

Not long after the bamboo situation was dealt with began the tour.  The first stop of the tour was the anaerobic digestor and the solar panel attachments.  The anaerobic digestor is the first of its kind in Costa Rica and will be used as a model for digestors of the same type to be built in the the future.  The model of digestor is called a CSTR, which stands for continuously stirred tank reactor.  More technical information on CSTRs will be provided near the end of this blog.  To summarize, waste is put in the digester and is broken down into gases by a variety of microorganisms.  The reaction is meant to be thermophilic, which means that a temperature of roughly 50 degrees celcius is required for optimum performance.  The solar panels located next to the digester provides this heat through heating water which subsequently heats the digester tank.  Inside the digester, a mechanism stirs the waste to increase efficiency.  All together the CSTR digester can break down its 15 ton waste load in ten days.


The anaerobic digestor (big tank) in all of its glory.


Solar panels for heating

The next step in the tour was the wetland treatment which was located across the road from the anaerobic digestor.  The wetland contains four cell that all have a different purpose.  In the order the water flows in, the cells are aligned were designed as follows; sand filter without vegetation, sand filter with vegetation, vertical flow wetland, and floating mat wetland.  All four were built to clean the remaining waste that the digester could not clean.  The idea to create four cells to clean the digester waste was meant to be a little experimental, as all four cells might not be needed to efficiently clean the water.  After the water is cleaned to an adequate level, it is designed to be recycled back into the system to eliminate the need for a constant water source.  This is also a feature that makes the whole system more sustainable.


One of the wetland cells.

After the tour was over, we departed Fabio.  Back at the Adventure Inn, we were given money for dinner and made our way out into the city for dinner.  The majority of the group split up to go to locations across San Jose.  Luckily we still had some of our UCR friends to give us rides across town to wherever we wanted.

Technical Details for Bio-digester

As I stated above, the CSTR being built at Fabio is the first CSTR to be build in Costa Rica.  Biodigesters are present but the vast majority of them are bag digesters which are less complicated and less efficient than CSTR digesters.  A bag digestor is a plug flow system, where manure and other wastes are put in the bag and flow slowly through it to the other side.  While inside, anaerobic bacteria breaks down the manure and methane and CO2 gas is released and collected in a storage area.  Essentially, only three holes are needed for a bag digester: one for waste inlet, effluent outlet, and a hole to collect gas.  Heat is typically varying in such a system and digestion is not always maintained at a constant rate.  This cause influxes in methane production and therefore influxes in power.


Example of bag digester. Waste goes in on one side and flows to other side. Methane is captured in tube.

The CSTR is considerably more consistent in its energy production.  The main difference is that there are certain controls in the design that allow for consistent temperature and waste consistency.  What makes the CSTR design in Fabio so special is not only the fact that this anaerobic digestive system is original in Costa Rica but so is the use of solar energy to maintain the heat of the internal reaction.  Alongside the use of wetlands to make finishing touch ups on the waste water, these additions make the system nearly entirely sustainable.  The solar panels operate in a passive way, meaning they don’t convert light from the sun into electricity like photo voltaic  solar panels do but instead heat water running through the cells.


To combat the inconsistency of passive solar energy, a water heater was added to maintain the needed 50 degrees centigrade for thermophilic digestion.

In comparison to the mesophilic model used at Michigan State University, the thermophilic version in Fabio is considerably more efficient.  Costa Rica’s climate allows for the use of thermophilic digestion due to consistent seasonal temperatures and more direct sunlight.  Much more energy would be needed to maintain a thermophilic digestion at MSU, especially in the winter.  Therefore a mesophilic system is more desirable, which is efficient at around 35 degrees centigrade.

whoa buddy take it easy

Maximum Daily Net Energy Produced. The blue bar is Michigan State University’s mesophilic model while the red is University of Costa Rica’s thermophilic model. The thermophilic is approximately 200% more efficient.

With the use of solar power, anaerobic digestion, and wetland treatment, the Fabio system is a very sustainable design indeed.  Future improvements still need to be made, but the design should be up and running efficiently in the near future.


One thought on “Day 12 – Fabio Baudrit Experiment Station – Anaerobic digestión – Bioenergy engineering

  1. Alex, this was a wonderful explanation of the digester. I appreciated the pictures. I’m learning a lot! Had to Google again to remind myself what methane is used for.

    Sounded like a fight to death with the lone bamboo stalk. Wish I could have watched!

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