My application of using sourdough ferment as a model for the activated sludge process is for an elective, wastewater treatment course taken by seniors and graduate students.  The technical aim is for students to see, smell, and taste sourdough ferment (well, to taste the resulting bread!) to better be able to describe the various inputs, outputs, and variables described in the mass balance relationship, and to feel comfortable writing mass balance equations.

The EM aim is for students to explore connections between and be curious about what they're learning in the classroom with real-world applications of microbiology that they interact with every day.  While many of these applications are "hidden", these biotechnologies play an outsized role in modern society from treating municipal and industrial wastewaters to crafting vaccines, medicines, and foods that we rely on every day.  Students use the sourdough ferment model to learn about the mass balance equations used in describing wastewater treatment, and they consider the role microbiology plays in various other ways.  Students also receive a sample starter that they are asked to nurture, observe, and even create with.

While the concepts can be expanded, this card highlights the EM foci of curiosity and creating connections.  There can also be a component of persisting through failure if students are asked to bake with their sourdough ferment and/or keep a journal of their sourdough journey for a given period of time. However, these items are optional and students can learn from the model regardless.

This activity can be a fairly quick demonstration in class, but it does take prep time so you'll need to plan ahead.  If you have a sourdough starter available to you, then the prep time is much reduced, but if not, it's easy to grow one--however give yourself a week to 10 days to prepare. 

In this card, I propose two approaches for using the sourdough ferment model.  The first is to use it to set up the mass balance equations and derive the design equations for the activated sludge process.  The second approach is to share the analogies for the sourdough ferment and the activated sludge models after the students have already derived the overarching equations. Either approach provides a valuable model for the students.

For the demonstration there are a few things to prepare, including obtaining/creating a sourdough starter and baking bread ahead of time.  But, it's a lot of fun and the students get to sense with more than their eyes and ears!

• Sourdough ferment (see description below on creating your own!)
• All-purpose flour
• Water
• Clean spoons
• Clean ziplock bags
• Handouts (see attached)
• Sourdough loaf with condiments and appropriate utensils, cutting board, a nice bread knife, lots of napkins

Preparing the sourdough ferment:

You can create your own sourdough ferment with your own local yeast with just some pre-planning.  It is simple, I promise!  Many recipes and blogs give very detailed and complex directions, but I have successfully grown my own sourdough ferment with minimal complications and time.

First, mix together wheat-based flour (whatever you have--I typically use all-purpose) and water, in approximately equal amounts.  I used to weigh to make sure they were equal amounts, but no need to be that precise.  Make a liquid-y paste.  Then, let it sit.  Mix it a couple times a day (as in, when you go into the kitchen in the morning, and same in the evening; any extra mixing is great, but not required). 

After about a week, you can start wasting your ferment, and adding new flour and water in equal amounts.  After about 10-days to two weeks, you now have a starter!  Every day (or even every two days if you're just too busy) waste some starter, and add new flour and water in equal amounts.  If you need to take a break, you can simply stick it in the fridge.  Just feed it equal amounts of flour and water every three months or so (you can waste some as needed so it's not overflowing).

You did it! :)

Baking bread:

There are many recipes for making sourdough bread and so many of them are very lovely.  Because I am busy, my go-to is a no-knead version published by NYT Cooking, which adapts Jim Lahey's no-knead sourdough bread recipe.  Essentially, you need an active sourdough starter and ~22 hours.  I start my bread the night before, and get up early to finish it with a final 2-hr rise before baking (the baking step itself takes ~50 min).  In the summer when I don't want to heat up my kitchen first thing, I start the dough in the AM and bake in the evening.  No multiple rises and mixes.  No "this step then that step with this flour folded in with water at this temperature, next a slight variation 20 min later"...I don't have time for that.  Jim Lahey's recipe is no-nonsense and delivers a trusty sourdough loaf every time.  Pro tips: invest in a kitchen scale for reliable baking, and get yourself a nice dutch oven--it is required for a crusty loaf.

Students struggle with situating all of the variables involved in describing the complexity of a complete mix reactor and their placement in the mass balance equations as we design wastewater treatment systems.  We have two boundaries, one around the full system, one around the settling tank.  This activity is particularly situated to help students conceptualize the mass balance set up around the whole system.  This model would be even more applicable to a sequencing batch reactor setup, and I could envision resurrecting the model when teaching SBRs as well.

In addition, this activity can help students make connections between the wastewater systems they are learning about and upon which they rely on every day and other useful applications of biotechnology that they may not initially recognize.  By thinking outside the box with this activity, students can make connections between what they're learning and many other biotechnology applications.  

Moreover, I have found that (at least some) students are motivated to care for their sourdough starter and even bake sourdough bread.  Students describe these "responsibilities" as a journey--the first bakings may not work, but as the students problem-solve and persist in their baking art or try other concoctions to make with their sourdough discard, they do become successful.  In this way, the sourdough ferment and bread analogy extends beyond the classroom: biotechnology is a little less predictable than applications of other sciences, yet it can be understood and harnessed with the ability to create (yummy, in this case!) value.  We can study it, model it, and test it to yield incredible services to society like wastewater treatment, bread, vaccine and medicine development, libations, and the list goes on.

Bread day!

Bread day means mass balance day! This is the day to formally introduce the mass balance equations that underpin the design equations for wastewater design.  Students want to conceptualize these ideas because they want to see how the systems work, and using a real product that they can create from scratch themselves and take care of every day transforms these concepts into ones they can internalize and connect with other material and ideas.  In addition, students are curious about if they can successfully produce sourdough bread at home later, and through iterating, they can persist and achieve the ability to be successful aficionados and producers of sourdough bread and other sourdough discard spin-offs.

Step 1: Bake some sourdough bread.  I use a simple no-knead recipe by Jim Leahy.  Also bring in starter and all the materials listed above.

Step 2: Introduce the mass balance on the reactors using the sourdough ferment! Students should help identify what is the substrate (BOD in WW treatment = flour in sourdough ferment), the analogy to the return activated sludge (settled sludge in WW treatment = what we don't discard in the ferment), the analogy to the waste activated sludge (in both cases, what's wasted), etc.--see attached handout.  When "wasting" the ferment, you can aliquot a Tablespoon or so into clean, ziplock bags for students to take home.  Instructions for taking care of the sourdough starter are included on the handout.

In this class we use Metcalf and Eddy's Wastewater Engineering: Treatment and Resource Recovery 5th Edition excellent book as our course textbook, and we also refer to Mackenzie L. Davis' Water and Wastewater Engineering: Design Principles and Practice, Second Edition.