Estimating organic waste volumes for composting 

Imagine you’re the facilities manager for a large commercial building or institution, staring at a row of overflowing carts or roll-off boxes sidled up to the wall in the alley or back parking lot. 

They’re filled with crumpled paper, reams of old reports, food leftovers, used coffee pods, an avalanche of plastic bottles and aluminum cans, and pizza boxes with paper napkins stuck to the remnants of a variety of cheese toppings.    

You know all of those discards can be recycled and/or used as compost feedstocks.  Since most of that waste is compostable, diverting the organics from the landfill to composting could save money.   

The problem is, few composting operations will accept mixed waste loads.  The resulting compost is just too contaminated to use and winds up in the landfill anyway.  And if you want to develop an on-site composting project, knowing the volume/weight of compostables influences everything from sizing to siting to process selection. 

Source-separation – removing recyclables and compostables from the disposal stream at your location – is not as difficult as it sounds.  A good education program supported by a healthy dollop of (re)enforcement usually does the trick.   But before investing time, money, and brainpower in the project, you know the decision-makers will want to talk dollars and sense.  They’ll want assurances that the economics work.   

The first step is to determine how many pounds or tons of compostables are being generated each year.  Then, you can begin the planning process and start to gather cost estimates. 

Routes available for developing estimates all come with advantages and disadvantages, mostly related to things like people-power, total generation volumes and required degree of accuracy.  A web search will offer lots of ideas.  Here, we look at 3 broad categories: 

Route 1 – Develop estimates based on published norms and averages 

The easiest, fastest, and cheapest method to estimate compostable volumes is to glean “typicals” and “averages” from the web.   

The U.S. EPA says about 61% of the total MSW stream is made up of food, paper and cardboard, wood, and yard trimmings.  If your commercial or institutional stream is similar, this method could work for you.  Couple this percentage with known weight capacities of the specific receptacle in use, and the result is a baseline number you can use to calculate weekly or annual tonnage.   

Simply convert those gallons or cubic yards to pounds (based on container weight limits), divide by 2,000 to get tons, then multiply that number by .61 or 61%.  This is the estimated weight of all compostables. 

There are also sources that will provide weights based on generation rate per defined unit.  Example: 41 pounds MSW per week per household or 200 pounds per week for each fast food restaurant employee.  Again, convert pounds to tons and multiply that number by .61 for a rough estimate of compostable tonnage. 

Want to count bins or carts?  Contact your waste hauler for specific container sizes/weights or use a more generic number like 180 pounds for a 96gallon cart (from one online resource). 

Just focusing on food waste?  A cubic yard of food waste weighs about .5 tons. 

This method of estimating volumes for composting is probably best for low volume generators, because the total volume and weight of any “error” will be relatively small.  For everyone else, use this type of generic data for rough estimates only. 

You can find charts to help with weight estimates in our SlideShare title:  Estimating volumes of food waste and other organics for composting. 

Route 2  the DIY waste audit 

This method relies on statistically representative random sampling to develop a picture of the total waste stream.  There are several sample size calculators available online to help you get it right, and they come in handy for evaluating validity of other types of surveys, too. 

The following examples were calculated using the SurveyMonkey tool: 

Let’s say you counted a total of 40 trash cans in your office building.  Using a confidence level of 95% and a 5% margin of error, the calculator suggests a sample size of 37 trash cans. 

If you have a larger building with 400 trash cans, using the same confidence level and margin of error, the sample size is 197. 

Trying to pin down the generation volume of a city of 40,000?  The sample size is 381. 

Once you know how many units you need, identify a representative subset for sampling.  A human can do this, but to be totally unbiased in the choice of trash cans, let a computer randomize the list.  Using the 40-can building as an examplecreate a list of all trash can locations in a spreadsheet.  Then randomize the list Randomizing is easy … this site is just one of many with step-by-step instructions.   

Once the list is randomized, use locations 1-37 for your audit: 

  • Assemble supplies (gloves, aprons, scales, etc.) and identify helpers. 
  • Pull all 37 trash cans at the same time on the same day and move to your audit location. 
  • Provide training to any helpers who might not be able to distinguish compostable from non-compostable.
  • Separate can contents into those two piles.  If conducting a full waste stream audit, further subdivide the non-compostables into glass, metal, plastic, etc. 
  • Weigh the compostables pile and divide by 37 (sample size) to get an average weight per can.  Multiply that average by 40 cans (total building) for a daily average.  Multiply the daily average by the number of workdays per year to arrive at an annual weight and divide by 2,000 to convert pounds to tons. 
  • If you’re just doing compostables, you’re done.  Otherwise do the same calculations for each waste group you wish to audit. 

Advantages of this method are improved accuracy and the fact that audits can make good group projects.  But the audit is only as accurate as the volunteers, and auditor safety (masks, gloves, etc.) must be a top priority.  

Also consider, as an alternative to the internal DIY, the resources of a local university where a researcher, class or student may be looking for a project.  Some private companies and governmental entities also offer free audits.  Just make sure they understand the focus is compostables, not just the more traditional recyclables like plastic and glass. 

Route 3 – professional waste audits 

Sometimes, only a professional audit will do.  This will include sizeable and/or toxic waste streams where the expense of professional expertise is warranted.  Typically, these will be engineering firms and other specialists with experience in waste management. 

Professionals can charge by the hour or by the contract.  If taking this route, choose a reputable firm and make sure there is a clear set of deliverables, as well as a timeline, spelled out in the Scope of Work agreement. 

As might be expected, this option can require a healthy budget.  But on the plus side, using professionals can be more accurate than any other when estimating volumes for composting.  If composting costs less than landfilling in your region, the audit may well be a money-saver in the long-term. 

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