Measuring your Solar CITIES 5 gallon Water Jug Biodigester temperature and gas production: A STEM activity

Once you have innoculated your Solar CITIES "biojugestor" you can start to do some real and real fun science!  

Actually, you can start the science without innoculating, that is, without having a fully functional biodigester, because learning how to take temperature data for the thermal mass of a digester can be done with plain water. Since it normally takes between 3 to 5 weeks for the microbes to settle in to your digester and the methanogens among them to start making the methane gas, and we are sure you are as impatient as we are to get into the scientific research methodology that puts the S, T and M (Science, Technology and Math) into the Engineering project you started when you built the 5 gallon water jug digester, we want to be able to jump right in and start outfitting your tank with temperature probes.
We will also explore here the simplest way that  you can capture the gas that will be produced once your digester is digesting and how to measure that gas.  You can start learning how to measure gas before the methane is produced by using air as a proxy and your lungs to blow into the tube to simulate filling the gas tank (which we make out of a collapsable water jug used for camping).
Once you know how to get and record the measurements using the Environmental sensors and the Arduino code your system will be set up for automatic recording once you've innoculated it and it actually does start producing gas!

What you will need: 


1 Arduino Uno Microcontroller with USB cable

1 Arduino SD card shield with built in RTC (real time clock)

 CR1220 3V Battery for Real time Clock



Prototype shield with mini breadboard and jumper cables



(NOTE: If these aren't available for some reason you can order the following two items separately:)


1 mini breadboard


Package of different colored Arduino jumper cable (header pin) wires to connect screw terminal to arduino board




3 DS18B20 waterproof temperature probes



1 14.7 Kilohm resistor (color code yellow, purple, red).



Screw terminals to connect probes to arduino pin wires (we use these so we don't have to solder)




Foldable 15 liter water jug


1/2 inch by 5/8 ich clear vinyl plastic tubing (for connecting gas holder to biodigester)



Ultrasonic Ping Sensor


3 female to male jumper wires


10 feet (3m) of Cat 5 Cable (to extend connection from probe to arduino board).

Wire stripper



We use three temperature probes so that we can get ambient temperature, top tank temperature and bottom tank temperature.  We want two temperature readings from our digesters because the water inside is generally unmixed and since hot water rises and cold water falls they tend to stratify.  Biodigestion occurs best in warm regions and the microbes hibernate in cold regions of the tank so we want to know how cold it is getting at the bottom (where most of the sludge and microbes accumulate) and how hot it is getting at the top and use that information to design better ways for the microbes and food particles we feed the digester to stay in the warmer areas. 

STEM Task:  See if YOU can come up with ways to improve the internal design of your water jugestor to encourage microbes to live and eat and fart in the hottest parts of the tank!

To hook up the temperature probes to the ardunio you should follow this diagram:(NOTE, for our code sketch we will be using digital pin 8 instead of digital pin 2 shown in this diagram).

You can place the resistor in the bread board, though I tend to use it to bridge the positive wire and signal wire at the  screw terminals instead.
NOTE: You only need one resistor for all three temperature probes, so I tend to bundle the wires from the probe into screw terminals (3 to 1) and then insert my resistor between the positive and signal wire and extend to the arduino with jumper wires.



Use the female to male jumpers to connect the three pins of the sensor to screw terminals and then strip three wires from the ends of a 10 foot piece of CAT 5 cable and use screw terminals to connect them to the jumper wires.  On the other side of the CAT 5 cable use screw terminals to connect jumper wires to the arduino board as shown:

To measure the volume of the expandable/collapsable water jug (which we are using like a balloon to store the gas) we mount the Ping sensor under a table or chair and place the jug under that.

Final measurements: pH

pH paper


Besides measuring temperature and gas volume, we need to know the pH because acidity and alkalinity have extreme effects on the methanogens (they really should be kept in a pH neutral environment; they eat acids but too much overwhelms them!).

You can measure pH with arduino sensors and with electronic pH meters, but the cheapest and simplest way is to do a simple litmus test using pH paper.  You don't get continuous data this way, and you will have to write the values down instead of recording them electronically, but our experience is that trying to data log pH is more hassle than it is worth!


With this equipment you should be well on the road to doing real citizen science, STEM environmental sensing and get some great data.  

Good luck and enjoy!

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