Aerated Static Pile (ASP) Composting of Medical Waste

This type of composting is more widely used in Western Europe than in the rest of the world.

This system avoids the need to mechanically mix the compost material by inserting air input devices that affect air flow through the pile. This is therefore aerobic composting, rather than anaerobic composting. Compared to other composting systems, ASP works quickly. The EPA says compost will be produced in 3 to 6 months.

Sometimes called the “rotting tunnel”, this technology is modular and even though it is a batch process (like all composting systems), it can accommodate swings in rate of waste received by closing off the tunnel into different cells. This composter accommodates animal products in the feed.

Air that has passed through the compost material is the “top stream”. Part of the top stream goes through a biofilter (for odor removal) and exits to the atmosphere.

Part of the top stream is recycled and mixed with fresh air and goes through the blower back through the aeration at the bottom of the unit.

Opposite to the air flow is the flow of liquid water, which is introduced through a sprayer at the top of the chamber. It percolates through the waste and is collected at the bottom for reuse. This water, which can be called leachate, is pumped back to the top. Fresh water is added to make up for the water that exits as vapor in the exhaust.

The variables affecting composting include oxygen, moisture content and temperature. Operators usually aim for a temperature of 60 °C. The composting process is exothermic (microbial breakdown of material yields heat) and the operators can change the air flow rate to control the temperature.