Biological Drying Systems
Biological drying is the reduction of water content from a high-moisture organic material through the internal heating of that material by biological oxidation. From all outward appearances, ‘biological drying’ and ‘composting’ are identical processes and the terms are sometimes used interchangeably. The fundamental difference is in the quality control over the biological oxidation process and the intended use of the final material. In a nutshell, ‘biological drying’ is the low-end composting process. Like composting, the processing objective in biological drying is to create an environment in which natural microbial activity feeds on biologically-available organic constituents within the material. In biological drying, the goal is to produce adequate heat energy over a time period sufficient to evaporate much of the bound and unbound moisture in the material.
Like composting, the biologically-available organic content is also reduced in the drying process and, if continued for a sufficiently long time, the organic content can be reduced to a level of biological stability making the material nearly inert. This material may then be stored for long periods of time without appreciable heating or methane production.
Unlike composting, which usually has the goal of producing a high-quality product with nearly complete pathogen and weed seed kill and a high level of biological stability, the objective of biological drying or ‘biological stabilization’ is to produce a relatively inert material that may be used as an industrial fuel, disposed in a landfill without significant contribution to greenhouse gas production, or used as a soil amendment on lands not used for food production.
The decision to biologically dry, rather than compost, a high-moisture organic material is often driven by the lack of a suitable bulking material required for either adequate porosity or carbon addition necessary to produce a consistent, high-quality compost. In some cases, adequate supplies of suitable bulking material is not available; in other cases, the delivered-cost of bulking materials cannot be supported by the economics of the facility.
The biological drying process, most commonly applied to animal wastes or biosolids from wastewater treatment plants, simply enables the high-moisture material to achieve elevated temperatures at which escaping water vapor can be transported out of the material under forced aeration. Additional raw material may be added to the process either intermittently or continuously at a rate that allows the biological oxidation process to be sustained but not under conditions that are considered most desirable to achieve a finished product that is uniformly pathogen free or at the higher stability (maturity) levels required for the most beneficial impact as a soil amendment in commercial agricultural operations.
While biological drying facilities require most of the same components as a compost facility, the material throughput may be higher, the control system may be less sophisticated, and the processing cost (on an equal weight basis) is generally lower for biologically dried material than for a higher-value compost.