Organics: Compost vs. Landfill
It is a question that comes up time and again – why is composting better than landfilling organics? I went to RRS President and organics team member JD Lindeberg to find out the answer.
M: If we landfill food/yard waste, it decomposes and generates greenhouse gases – that’s bad. But if we compost it, isn’t it still decomposing and generating greenhouse gases?
JDL: Good question because this is the crux of the issue: methane versus carbon dioxide. I’m going to give a bit of a chemistry lesson, so bear with me. You see, different greenhouse gases have more heat holding capability in the atmosphere. Methane (CH4) can hold 25 times more heat than carbon dioxide (CO2). Side note, nitrous oxide (N2O) is almost 300 times worse than CO2 – think about that the next time you visit the dentist.
Landfill gas is comprised of roughly 50% CO2 and 50% CH4. The methane is developed due to the anaerobic decomposition – lack of oxygen – that takes place in a landfill. Whereas a compost pile decomposes aerobically – with oxygen – producing mainly CO2. This depends upon the types and ratio of material included in the compost (i.e., food, manure, yard waste), and how often the pile is turned or use of another method of oxygen introduction.
So, if you believe that your carbon is going to decay anyway (a safe bet for food or yard waste), you’d like it to do so into CO2, not CH4.
M: But what about the gas collection systems at landfills?
JDL: The basic system collects the landfill gas and is then flared (burned) or is converted for energy use by removing all contaminants. The US EPA Landfill Methane Outreach Program estimates that 60%-90% of the CH4 emitted from landfills can be captured dependent upon the system and its effectiveness. But recalling that CH4 is 25 times worse than CO2, you need to collect about 95% of the landfill gas to simply break even.
However, I have reason to believe that in fact the collection efficiency of CH4 derived from food waste is quite a bit worse than 60 – 90%. I’ve seen studies that suggest 90% of the decay of food waste occurs in the first two to four weeks of landfilling, a period of time long before the gas collection for most landfills can be implemented. Effectively that means that much of the CH4 generated from our food waste goes uncollected to the atmosphere.
That said, I believe that every landfill should construct a landfill gas collection system immediately. Every molecule of CH4 that we collect before emission benefits us all.
M: What choices do municipalities, businesses, schools and site operators have when it comes to composting?
JDL: There are several systems to choose from that are based on a number of variables including type and volume of feedstock, available space and budget.
- Open Pile: This is what it sounds like – a large pile of organic material. But you must include a bulking agent to allow airflow through the pile to ensure aerobic
- Windrow: Long rows or piles, called windrows, of material are created and turned periodically with a windrow turner.
- Static Pile: Organic material is placed in a windrow, but unlike the windrow it remains unturned throughout the composting process. Often piles are covered and aerated from beneath.
- In-Vessel: Organic material is placed into a drum-shaped vessel to allow for rotation and control of temperature, airflow and moisture.
- Vermicomposting: This system uses worms – yes, you heard correctly. The worms break down and aerate the material, creating compost.
In any composting situation you must understand the nutrient balance to ensure the proper carbon to nitrogen ratio (approximately 30:1), which means understanding your feedstock. Achieving proper moisture and temperature is also critical to a successful compost operation. And remember the key is oxygen – if oxygen is not circulating than you have the same environment as a landfill and methane can begin to generate.