Food Waste Dryer vs. Aerobic Digester

  • What’s The Difference?
  • Operational Considerations
  • Which One Is Better For Me?

These machines are both types of food waste dewatering equipment, designed to reduce the weight and volume of food waste before disposal and removal from site for recycling. But they work in different ways and there are important operational factors you need to consider before buying or leasing one. So let’s dive in.

What’s The Difference In The Process?

‘Aerobic’ means ‘with oxygen’. An aerobic digester is essentially a type of rapid composter. It reduces the volume and weight of food waste by enzymes which are added to the food waste and turning it in a warm air stream (around 50-60°C). Enzymes are a substance that speeds up chemical reactions in cells, in this case in the decomposition of food, in which large nutrient molecules (such as proteins, carbohydrates, and fats) are broken down into smaller molecules. The food waste with the added enzymes is gradually turned over, exposing the waste to oxygen in the warm air and the process ‘digests’ the food waste, effectively speeding up the decomposition, removing the water content and turning the food waste into a dryish mulch which is about 20-30% of the original weight of food waste.

Composting machines are aerobic digesters. Traditional composting machines use a carbon source to feed natural bacteria present in food waste, typically over a period of about six weeks. Aerobic digesters speed up this decay, shortening the timescale to between a day and a week, depending on the type of food waste.

A food waste dryer uses applied heat to evaporate the water content of food waste (average 80% by weight). The food waste is turned over in this higher temperature (70-85°C) and the water content rises as steam within the closed unit and is recirculated through a condenser which turns the steam back into water which is discharged to a normal drain. The solid matter is reduced to a sterile, dry powder which is typically 15-20% of the original weight of food waste.

Operational Considerations

  1. Batch loading vs continuous loading: Food waste dryers are ‘batch loaders’ which means you fill them up and turn them on to start the drying cycle. You cannot add any other food waste until the cycle has finished which is typically 12 – 16 hours later, depending on how full it is filled. Aerobic digesters are ‘continuous loaders’ which means that you can load new waste while there is still decomposing waste inside. This difference has several operational consequences:
  1. Cycle time: Dryers reduce the weight and volume of food waste much more quickly. The cycle time is 12-16 hours as opposed to at least 24 hours for an aerobic digester. The dryer is therefore more precise in reducing food waste to a state where 99% of the water is removed, whereas only some of the food waste in an aerobic digester will be fully digested in 24 hours, and much of it will need longer to process.
  1. Effective capacity: Importantly, ALL of the food waste will be processed in a dryer in the cycle time of 16 hours, whereas maybe only ONE THIRD of the food waste in an aerobic digester will be processed in 24 hours. Typically, it takes at least 72 hours for full ‘digestion’ as some types of food waste decompose quicker than others.
  1. Smells: Because a food waste dryer is a closed unit, no smells from drying food waste escape. When the discharge door is opened 12-16 hours later to empty the machine, the dried powder is virtually odourless. Because it is a continuous loader, the loading door on an aerobic digester is constantly being opened for new food waste to be added. Every time this happens, the smell of decomposing food waste escapes, which can be unpleasant. While the end product, after the full digestion has little smell, except up close, the decomposing matter still in the process is smelly.
  1. Hygiene: A food waste dryer sterilizes food waste as it is heated to 70-80°C for several hours. (Sterilization requires at least 30 minutes exposure at 70°) An aerobic digester only reaches 60°C and maybe lower than this as the door is being opened continuously to add more food waste.
  1. Power: Food waste dryers use more power in order to reach a higher temperature to sterilize food waste but are on for a shorter time as the cycle is only 12-16 hours. Aerobic digesters use less power but are on 24/7.

Which Machine Is Better For Me? 

The operational considerations above are a key factor in making the decision.

Smell:

You should ask yourself whether having the smell of decomposing food waste in your kitchen from an aerobic digester is acceptable. If not, then you should consider an alterative location which will mean moving the food waste to the machine to process rather than processing it in the kitchen where the food waste is generated. Food waste dryers produce no smell.

Hygiene:

Do you want a hygienic solution to your food waste disposal requirement? Dryers sterilize food waste (the temperature reaches the required temperature within a couple of hours of the cycle starting). E-coli and salmonella are guaranteed to be killed in the process. By contrast aerobic digesters decompose food waste over a 24-72 hour timescale and the final residue still contains bacteria.

Effective capacity:

Do you want to be able to empty the machine completely and refill it, knowing that you have processed all the food waste loaded? You’ll do this with a dryer but only about one third of the capacity of the aerobic digester will become available each day. Many customers prefer the precision of knowing that all their food waste has been processed in the 12-16 hour cycle in the dryer rather than the imprecision of the aerobic digester.

Kitchen operating hours:

Do you want a machine available all the time to load food waste whenever it arises? An aerobic digester is a continuous loading machine (provided there is capacity within it) so in theory you can continue to load food waste as it arises. However, you should be aware that every time you open the loading door smells will escape and you will not be sure how long what you’ve loaded will take to process.

As a dryer is a batch-loaded machine, once it is full and has been turned on, it is no longer available to fill until the process is finished. Typically, this isn’t much of an inconvenience as the machine will have finished drying the next morning as most users start the drying cycle in the evening when the machine is full. Customers with long kitchen opening hours (say, 7am to 9pm) often choose to split their capacity over two smaller machines so one is drying while the other is available to fill.

For more information about the Eco-Smart Food Waste Dryer, how it can reduce your disposal costs, improve your food waste processing and ensure a hygienic operation, go to this page on our website or contact us for a no-obligation discussion.

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