Municipal Solid Waste Composting: Biological Processing

ZHENGZHOU, China - June 29, 2017 - PRLog -- Composting is in essence a biological process. This fact is sometimes obscured by the wealth of sophisticated technical options available. But proper design and management must be based on the needs of microorganisms if the process is to be a success. Neglect or misunderstanding of biological process control can lead to serious problems at composting facilities, and has contributed to facility closings.

Carbon and Nitrogen
Carbon and nitrogen are the two most important elements in the composting process, as one or the other is normally a limiting factor. Carbon serves primarily as an energy source for the microorganisms, while a small fraction of the carbon is incorporated in their cells. Nitrogen is critical for microbial population growth, as it is a constituent of protein which forms over 50 percent of dry bacterial cell mass. If nitrogen is limiting, microbial populations will remain small and it will take longer to decompose the available carbon. Excess nitrogen, beyond the microbial requirements, is often lost from the system as ammonia gas or other mobile nitrogen compounds and can cause odors or other environmental problems. While the typically recommended C:N ratios for composting MSW are 25:1 to 40:1 by weight, these ratios may need to be altered to compensate for varying degrees of biological availability. While wood chips have a high C:N ratio, most of the carbon in a large wood chip will not be available to microorganisms during the time frame of typical composting processes.

Decomposition slows dramatically in mixtures under 40 to 45 percent moisture, which can lead facility operators to prematurely assume compost is stabilized and ready to sell. A minimum moisture content of 50 to 55 percent is usually recommended for high rate composting of MSW. MSW collection programs which include paper are often drier than this, and water or sludge should be added to bring moisture into the optimum range. The heat and airflow generated during composting evaporate significant amounts of water and tend to dry the material out. During the active composting phase, additional water usually needs to be added to prevent premature drying and incomplete stabilization. MSW compost mixtures usually start at about 52 percent moisture and dry to about 37 percent moisture prior to final screening and marketing.

Oxygen and Temperature
The remaining key environmental parameters, oxygen and temperature, are linked by the decomposition process. Both fluctuate in response to microbial activity, which consumes oxygen and generates heat. Oxygen and temperature are also linked by a common mechanism of control: aeration. Aeration both resupplies oxygen as it is depleted and carries away excess heat. This dual purpose makes aeration management a central feature of biological processing, as it controls these two parameters.

Inadequate oxygen levels lead to the growth of anaerobic microorganisms which can produce odorous compounds. While adequate oxygen can minimize these odors, it is important to note that anaerobic pockets will exist in a heterogeneous material like MSW, and some odors including ammonia and some organic compounds can be generated even under generally aerobic conditions. Thus, while proper oxygen supply can minimize odors, it may not completely eliminate them. And since some of the odors causing problems at MSW composting facilities originate at the tipping floor with raw waste prior to composting and aeration, most MSW composting facilities are likely to require odor treatment to maintain good neighbor relations. Odor treatment options include biological, chemical, and thermal technologies.

Just a few of the many pre-processing steps described in Fact Sheet 1 are normally adequate to provide reasonably aerobic conditions as the feedstock enters the composting system, but additional oxygen must quickly be supplied. Rapidly decomposing wastes can use up the oxygen introduced by turning within a matter of minutes. Oxygen concentrations in the large pores must normally be at least 12-14 percent (ideally 16-17 percent) to allow adequate diffusion into large particles and water filled pores. Most MSW composting systems (http://compost-turner.net/composting-technologies/municip...) used a forced aeration system with blowers and distribution pipes to supply oxygen during the initial phases of active composting. Passive diffusion and natural convection help supply oxygen to windrow systems between turning events.

Biological Process Control
Most composting systems reconcile these trade-offs between reaction rate, pathogen reduction, and odor generation by an attempt to control temperatures to a narrow range near 55 to 60°C (131-140°F). To maintain this temperature range, heat gains from microbial activity need to be balanced against heat losses, which occur primarily through evaporation of moisture and heating the aeration air. Temperature, like oxygen supply, is usually managed by an aeration system: the same air which supplies oxygen can carry away excess heat. During most of the active composting stage, several times as much air is needed to remove heat as to supply oxygen, so maintaining proper temperatures will usually also keep oxygen levels in the proper range. Forced aeration systems are commonly used during this stage of composting, using temperature sensors to control blowers. Eventually, as readily available compounds are decomposed, the rate of composting slows, with less oxygen required and less heat generated. This slower stage is called curing, and while it requires less management than active composting, it is important to compost quality,

Windrow composting (http://compost-turner.net/composting-technologies/windrow...) usually relies on natural convection and diffusion for oxygen supply. Pile size and turning frequency are used to balance heat loss in managing temperature control