Human Versus Machine: Compost Screening

Post by John Mulrow, PhD Candidate at University of Illinois Chicago

INTRODUCTION

Composting is a simple and traditional circular economy practice. It returns nutrients to the soil, creates economic opportunity, and can happen at a local scale. Modern industrial composting is highly dependent on heavy equipment; loaders, grinders, screeners and dump trucks are all high-powered machines, typically fueled by petroleum products. In the circular economy of the future, renewable sources of energy must power the compost industry. What role can human (metabolic) energy play in such a scenario?

Before 1850 humans and animals together provided more than 80% of the productive energy in the United States. In other words, they did most of the work. The graph below – copied (by human hand!) from a classic textbook in energy economics, Energy & Resource Quality (1986)* – depicts the relative contributions of fuel, human labor and animal energy over time in the US. Nowadays, humans and animals do so little work compared to commercial energy sources that they don’t even register on this graph after 1970.

It’s not as if we work less hours than past Americans, it’s just that our energy output is dwarfed by immense amount of energy available in modern fuels. If you spend 1 hour driving an average car at 60 miles per hour, you personally will have put about 100 kilocalories (the same as nutritional “Calories” you see on food packaging) into the task of sitting, steering and watching out for the road. At the same time, your car has burned about 2 gallons of gasoline to run the engine and move you and the car along at a high rate of speed. There are about 31,500 kcal in a gallon of gas, so we can say the engine did 63,000 kcal of work, while you did 100 kcal, or 0.002% of the total. No wonder human labor no longer registers on the graph!

METHODS

In May of 2018, Plant Chicago put this thought experiment into action when it ran an event called “Man vs. Machine.” The goal of the event was to measure the compost screening capability of humans versus modern screening machinery. Here’s the two systems we compared:

Humans with simple screener

Our compost screeners are made of lumber and 0.25-0.5″ wire mesh. Each screener requires 2-3 people to operate. A few shovels of compost are loaded onto the screen and two people shake the screen back and forth. The final product falls through into a collection container, and the stuff that’s too big – the “overs” are recycled back into the active compost piles.

Trommel screen

A trommel screen is a common piece of machinery in any composting or rock crushing/grading operation. It consists of a motor-powered mesh cylinder that turns while material is loaded into one end. The screened material is captured underneath and pulled up a ramp that creates a pile of screened compost. The overs are captured from within the cylinder and recycled. We did not have a trommel screen actually on site, but instead chose to compare human screening performance with the specs given for a Vermeer TR510 Trommel Screen.

RESULTS

We had 20 people show up to spend three hours screening compost. Let’s see what 60 person-hours can accomplish! Here are the numbers:

DISCUSSION

Kilocalories per Cubic Yard (kcal/CY) is the simplest of metrics we might chose for comparing the efficiency of human screeners to a motorized trommel screen. On this basis, machines win. The trommel screen uses about 1,300 kcal of fuel energy to screen a cubic yard of compost, while our team of human screeners collectively used about 3,400 kcal to screen the same volume. In other words, the machine is about 2.5 times as “efficient” on an immediate energy use basis.

Of course, singular metrics can distract from a more thorough treatment of the two scenarios. And this often happens in environmental arguments. A single efficiency metric is used to support a specific technology without discussing some of the tangential benefits of alternatives. How do you quantify the conversation, exercise and health value of the crew of compost screeners working together under the sun? How do you properly contextualize the economic and wellness benefits of slow versus fast compost screening? Could you ever run an economically viable compost company entirely powered by humans energy and simple tools? These may seem like fun thought-experiment questions, but they may turn out to be very important to the long term sustainability and circularity of local economies.

 

reference

Hall, C.A.S., Cleveland, C.J., and Kaufmann, R.K. Energy and resource quality: The ecology of the economic process. United States: N. p., 1986.

** calculations

Humans

people: 20

hours: 3

finished/screened: 3.5 cu yd

total processed: 3.5 = x*0.66, x = 5.3 cu yd

(1.8 cu yd per hr)

300 kcal per hour * 60 person-hrs = 18000 kcal

18000 kcal/ 5.3 cu yd = 3,400 kcal/cu yd

Machine

65 cu yd/ hr

2.4 gal/hr

35,000 kcal/gal diesel

= 84,000 kcal/hr

84,000 kcal/hr / 65 cuyd/hr = 1,300 kcal/cuyd

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