How compost is made: Calgary’s Green Cart Program

Dinner is over. All the food has been eaten and leftovers are in the fridge. But what about those items that can’t be eaten like bones and corn cobs? They go in the green cart. What about this used paper napkin? That goes in too – and it’s not just food scraps that can go into your green cart. Grass clippings, small tree branches, pet waste, cat litter and food soiled paper like napkins, tissues and paper towels are all suitable for composting. In fact the introduction of the green cart has meant an almost 50% reduction in the amount of residential material ending up in the landfill. Instead of sending food and yard waste to the landfill where it doesn’t break down into anything useful we are making it into a valuable resource. And, all we need is a plastic bag? Actually, you don’t use a plastic bag. Put items directly in your green cart or place them in compostable bags. Look for these logos on the box. Putting the wrong things like plastic in the cart creates contamination which means more time and effort to produce quality compost. Now..Where does it all go? The contents of your green cart end up at Calgary’s composting facility. Let’s take a look inside. Here it goes through a series of specialized systems from the tipping floor and composting to screening and storage. The end result is nutrient-rich compost. Let’s take a closer look at the whole process. It all starts at the tipping floor where an average of 400 truckloads a week of material will be unloaded. On the tipping floor large pieces of contamination are removed and material is shredded into smaller pieces that will compost faster. Once it’s shredded front-end loaders and conveyors deposit it into one of 18 huge vessels. Each vessel is 7 metres high 10 metres wide and 54 metres long and filled to about 2/3 capacity, so there is enough space to circulate air and maintain high temperature. Does everything at the facility come from green carts? Well not all of it Biosolids which are nutrient rich resources that enhance plant growth and improve soil water and nutrient retention also come to the facility from wastewater treatment. Biosolids get mixed with woodchips to optimize compostability before they’re taken to separate vessels. Biosolids follow the same process as the material from your green cart but the two streams are kept entirely separate and end up as two distinct kinds of compost. What happens in the next step.. Inside the vessels? Once in the vessels the materials naturally warm up and decomposers get to work. Decomposers are things like bacteria and fungi which thrive in warm temperatures and also need oxygen and moisture to do the work of composting. The entire work in the vessel takes 21 days and is closely monitored for temperature oxygen and moisture. Outside air is pumped up through the floor and moisture is added from above, so the decomposers can break it down. Water is recycled from all areas of the facility to keep the compost moist and minimize fresh water use. Reaching a temperature of 55 degrees celsius ensures that harmful bacteria are eliminated. Composting sounds complicated, but it really is nature at work. We create and maintain optimal conditions to allow the composting process to happen naturally. It’s similar to what happens in a backyard composter, but our process can handle even more items and a lot quicker too! How do they remove things that aren’t compostable? When the active stage of the process is done and the material is removed from the vessel it goes through a screening process which removes most things that don’t belong like plastics, rocks and metals. Wow! The way they screen out all that stuff is pretty cool. Materials that are still too big like large pieces of wood get sent back to step 1 to be mixed with fresh green card or bio solids material What happens after all the screening is done? Then all the good stuff goes to the curing building for three weeks. The curing process allows the decomposers to finish their job and the compost can cool down so it’s safe to use on plants. Now, it’s time to take the final product onto the storage building. How do we know it’s safe to use? Before anything leaves the Calgary composting facility samples are tested for nutrients, organic matter, pathogens, trace metals and foreign matter like plastic and glass to ensure the highest standards of safety and quality. And it’s not just quality compost we’re responsible for. We also want to be good neighbors and controlling odors is part of that. The facility has a large filter that draws air from the active composting and curing phases and cleans it before it’s allowed out of the facility That’s it! In just 60 days yesterday’s food scraps become today’s compost, ready to be put to good use in our community. When we put the right things in our green cart we can ensure a high quality product while keeping valuable food and yard waste out of the landfill. This amazing program is a real asset to Calgarians. I see that! I had no idea what happens with the stuff in my green cart, it’s great to know I can make a big difference just by using my cart correctly. Absolutely! You’re helping us reach our goal of keeping waste out of landfills and making a positive difference today and for our future.

5 thoughts on “How compost is made: Calgary’s Green Cart Program

  1. My initial thought is pet waste would not be good in compost that is later used to grow food. Is it ok because it is "diluted" with so much other product. Or pet waste is just the same as cow manure?

  2. It is so important to understand that sewer sludge aka biosolids is NOT just human excrement – it is a concentration of all domestic and industrial pollutants that go down drains and sewers. It has some good stuff in it, which plants can use, but a huge load of thousands of other contaminants. Please read what independent scientists have to say on this issue –

    Prof. Murray McBride, Cornell University – "Agricultural soils are a unique and valuable resource. Protecting agricultural soils requires anticipating and avoiding potential harms since once contaminated with persistent pollutants, the damage will remain for the foreseeable future." … "Is it reasonable to conclude that there is little or no risk of land-applying a material (biosolids) containing unknown concentrations of thousands of chemicals with undetermined toxicities?"

    Prof. Jordan Peccia- Yale University – "Land application is often accompanied by strong odors, and biosolids contain heavy metals, hazardous organic chemicals, microbial pathogens, and antibiotic resistant bacteria … A recent study found more than 27 different forms of human viruses in the sewage sludges of five large U.S. cities, ranging from Adenovirus to Corona virus … Metals and organic chemicals that resist biological mineralization can sorb to solid particles and also accumulate in sludge. These include polybrominated flame retardants, pharmaceuticals like Prozac and Tagamet, human hormones such as estrogen, antibiotics, narcotics including cocaine, and the metabolites of these compounds."

    Dr. Sierra Rayne – "The science doesn't support the disposal of sewage sludge across the landscape. The supposed benefits are more than offset by the risks to human and environmental health. As scientists, we have been watching the issue with increasing concern. An unimaginably large number of chemical and biological contaminants exist in these materials, and they persist in the product up to, and after, land disposal. Scientific investigations have identified only a tiny fraction of the total contaminant load. We cannot even say with any degree of confidence what the true range of contaminant risk is from the sludge … Governments are playing Russian roulette with sewage sludge. Over time, there is a high probability this game will be lost at the public's expense."

    Brian Bienkowski (Scientific American – May 12, 2014) – "Sewage sludge used as fertilizer on farms can leave traces of prescription drugs and household chemicals deep in the soil, according to a new study by federal scientists. The findings suggest that the widespread use of biosolids could contaminate groundwater near farms with a variety of chemicals, including anti-depressants such as Prozac and hormone-disrupting compounds in antibacterial soaps …The researchers looked for 57 “emerging” contaminants that are increasingly showing up in the environment. Ten were detected in the soil at depths between 7 and 50 inches 18 months after the treated sludge was applied. None was in the field’s soil beforehand… Other studies have found hormones, detergents, fragrances, drugs, disinfectants, and plasticizers in treated sludge used as fertilizer. But this is the first study to show how they can persist and move in soil."

    Dr. Richard Honour – ""Few in any governments appreciate that nearly all chronic diseases are caused by long-term exposure to low levels of environmental contaminants and pollutants. We should be trying to minimize this exposure, not amplifying it. It is time to end land disposal of Toxic Sewer sludge, and look at cleaner, greener alternatives – gasification / pyrolysis."

    Let's get on the right side of history, and use this waste resource to make energy. It is time to stop covering Mother Earth with our cities' toxic sewage.

    Prominent Scientists and Universities outline the Dangers of Biosolids –

    1. Canadian Scientists –

    2. UK Scientists –

    3. USA experts say –

    4. USA – on regulatory failures and risks –

    5. Overview of the issues – by Dr. Thomas Maler

    6. Ten Toxic Truths By Professor Marc Cohen –



    9. Documentary –

    10. Interview with Sewage Sludge Fighting HERO … Dr. Caroline Snyder –

    11. UK Scientists –

  3. If this expensive compost is so great, why aren’t the lab results made easily accessible to the citizens of Calgary?

    I still resent the fact that we we’re forced to pay for an extra cart(eyesore) and the city sells that compost to farmers and other agri-businesses, without returning some of that profit back to us.

    Those carts decorate most driveways (disgusting) and stink btw.

    Lower our property taxes if you really care about your citizens.

  4. Great video City of Calgary! I appreciate the thorough and simple explanation of the processes in this one, as well as the preceding one "Too Good To Waste" . I work as an Energy Manager in BC, and would be interested to get in touch with your production team. We would like to create something similar for the work we are doing. Thanks!

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