Simple Anaerobic Digester for Classroom Demonstration or Projects

For anyone interested in attempting to generate biogas (methane) as a class demonstration or student project, I’ve attached a powerpoint (Biofuels teaching energy in life science) developed for my sustainable energy course, which I also used to present at a town fair when I taught in Vermont. The organic waste remains contained, so odors were not a problem.  Also, no known pathogens can grow in the anaerobic environment of the sealed digester, so health risks are quite low.  Use proper precautions as one would in any lab environment (gloves, lab apron, safety glasses) when loading and unloading the digester.  In fact, this technology is used on farms, with one of many benefits being the treated fibrous waste can be used as bedding for farm animals.  Studies have demonstrated that this type of livestock bedding actually reduces infections among the livestock, compared to the use of sand or sawdust as is typical.  Pretty amazing!

The very last slide shows a pic of the design I had the most success with.  I’ll insert it here for convenience:

digester

I used a 5gal Poland spring water bottle.  Only drawback is its difficult to fill and even more so to empty. (A food grade 5 gallon bucket with lid might be an improvement.)  I used goat droppings as my source of methanogenic bacteria because the goat pellets can pass through a large funnel during filling.  I also added quick oats as a source of carbon and nitrogen to feed the microbes (you want a high C/N ratio, which is why human waste streams in the USA can pose problems, as we Americans eat a lot of protein).  Ideally the system should be stirred constantly, but I had success by shaking the bottle three time/day.  A Mylar balloon is impermeable to the CO2/CH4 gas mixture so its ideal as a gas collection vessel.  Rather than attach the filled balloon to a bunsen burner, I would attach the balloon via tygon tubing to a glass pasteur pipette.  Its small opening would allow for a small, pencil tipped size flame, so one full ballon would last all day to demonstrate to all my classes.

These simple, desktop scale classroom systems generally do fail after a few days as the pH  drops below 7.  This is because acid producing bacteria grow much more rapidly than the methanogens.  Ironically acetic acid is the “preferred food” of most methanogenic bacteria, but they are also very sensitive to pH.  So as their food supply becomes overabundant, pH drops, and the population collapses.

This is why its best to start with a lot of manure and relatively less oatmeal.  Thus encouraging a large population of methanogens relative to the acid producing bacteria.  Also, larger volume bioreactors are generally more stable over time than smaller reactors.  I have not had very good luck with buckets smaller than 5 gallons.

A big water bath heated with an aquarium heater works to maintain the constant ~98F temp for “mesophyllic” methanogenic bacteria (the type that live in the gut of a cow, horse, or goat.   However, thermophyllic (higher temp) digesters have been used successfully to treat hospital waste, with the added benefit that the higher temperatures not only drastically reduces the total amount of biomass needed for eventual disposal, but is even more effective at sanitizing the final waste product.  The mesophyllic digesters are more commonly used due to lower cost (less energy needed to maintain mesophyllic temperatures), for sewage treatment (example: Deer Island, Boston) and livestock operations.  Its still a very much underutilized technology hear in the USA.

Lesson Overview of New High School Analysis of Computer Energy Use

Here is the lesson overview for a whole class open inquiry project completed in 2015 following completion of a new high school construction project.  We used the building as a learning laboratory, which helped our school earn points toward green building certification (LEED–Leadership in Energy and Environmental Design).  A copy of the final project and summary of our results appear in the previous post.

Essential Question:

What opportunities exist in the new Greenfield High School to reduce electric power consumption, which will result in cost savings and decreased greenhouse gas emissions?

Standards:

SCI.9-12.E.II.SIS2.5.a – making observations

SCI.9-12.E.II.SIS2.5.c – collecting data or evidence in an organized way

SCI.9-12.E.II.SIS2.6 – [Learning Standard] – Properly use instruments, equipment, and materials (e.g., scales, probeware, meter sticks, microscopes, computers) including set-up, calibration (if required), technique, maintenance, and storage.

SCI.9-12.E.II.SIS3.5 – [Learning Standard] – Present relationships between and among variables in appropriate forms.

SCI.9-12.E.II.SIS3.5.a – Represent data and relationships between variables in charts and graphs.

SCI.9-12.E.II.SIS3.5.b – Use appropriate technology (such as graphing software, etc.) and other tools.

SCI.9-12.E.II.SIS3.1 – [Learning Standard] – Use mathematical operations to analyze and interpret data results.

SCI.9-12.E.III.9.2 – [Learning Standard] – Use appropriate metric/standard international (SI) units of measurement. (KWH)

SCI.9-12.E.III.1 – [Learning Standard] – Construct and use tables and graphs to interpret data sets.

SCI.9-12.E.III.2 – [Learning Standard] – Solve algebraic expressions.

 Overview of Lesson Plan Objectives:

This is a multi-day whole class project designed to provide students with an authentic experience in applied building science. Student’s will identify a problem associated with school energy consumption, develop a plan to document power consumption using data loggers, organize and analyze data in tables and graphs, and project energy and cost savings using mathematical methods such as dimensional analysis. The results of their project will be presented to the superintendent of schools, as well as the general public at the school’s spring open house. The results of this project will also be submitted for review toward earning points in the LEED for new construction certification process. LEED, or Leadership in Energy & Environmental Design, is a green building certification program that recognizes best-in-class building strategies and practices. To receive LEED certification, building projects satisfy prerequisites and earn points to achieve different levels of certification. “Using the building as a learning tool” qualifies for LEED accreditation points.

 Activities:

  1. Initial building walkthrough to identify sources of electric power consumption and opportunities for data collection and potential savings.
  2. Data collection using appropriate tools (Watts-Up! Professional Power meters).
  3. Data Analysis, calculations, and organization using excel spreadsheets.
  4. Poster presentation at school open house. Summary of results and recommendations mailed to Greenfield Public School’s central office.

Materials needed and other supporting resources:

  • Watts Up-Pro Data Loggers (x6)
  • Text: “Residential Energy Efficiency–cost savings and comfort for existing buildings” by John Krigger and Chris Dorsi.
  • Microsoft Excel
  • Microsoft Publisher

 Assessment (Summative):Each student working alone or as a group of two will be responsible for the analysis, calculations, and organization of data associated with power consumption from some aspect of the new Greenfield High School Building (The initial building walk though resulted in the identification of computer energy use as the project focus. Since the students played a direct role in problem identification, the project meets the definition of Open Inquiry, the central practice of the scientist.)

Rubrics:

21st Century Learning Expectations

  • A2 Think critically and effectively to solve problems
  • A4 Use relevent technology
  • C10 Engaged citizens

Point system grading:

Data Graphs:

Title—1 point

Axis labels—2 points

Appropriate scale on axes—2 points

Presentation of data—3 points

Data Tables and Calculations:

Calculations including Dimensional Analysis: 24 points

Total points 32 points