Lecture 7 Outline
The Compost Heap
Household Waste Type Distribution
Organic material
Humus
dead bacteria, plants, and animals decaying
the "tasty" (easily degraded) materials are eaten first
the leftovers are humic acids
lignin is found in plant cells walls, especially in woody materials
lignin is not readily biodegraded and leaves behind a complex poly-chicken wire product
Soil structure
Clay plates bind to humic materials
these then bind with bacteria and fungi forming aggregates
roots and fungi then loosely hold these aggregates together forming crumbs
these soil crumbs (macroaggregates) have pores and channels that run through them
liquids and gases can thus infiltrate the crumbs
microbes, protists and such live within these pores and channels
Gross soil structure
soil is divided into horizontal layers called horizons that differ in soil type
groundwater runs in channels between layers, often above bedrock
in a vertical column of soil, different transport processes occur
What kind of things will move in and out of soil columns?
gases (CO2, O2, methane etc.), liquids (water, gasoline), microbes, sunlight
water can dissolve minerals from the soil into the groundwater stream
toxic substances can also be transported into the groundwater
in a vertical soil column there is also a division between aerobic and anaerobic at a certain depth
What types of reactions do we expect in the anaerobic zone?
reductive reactions like denitrification
the depth at which anaerobosis takes over depends on certain factors
soaking the soil with water forces out the gas and raises the anaerobic/aerobic dividing line
Composting
the composting process
oxygen, water and microbes are needed to degrade organic trash
the composting mixture produces humus, water, and carbon dioxide
showed breakdown of household wastes
showed examples of windrows and aerated piles
emphasized that the compost needs to be kept aerobic (or else it stinks)
Where is a compost pile the hottest?
in the center
microbial activity produces heat since the oxidative reactions are analogous to fire and release energy
the center of a good compost pile may reach 50-60 C
only thermophilic microbes can live at these temperatures
initially a compost pile contains all types of microbes, but the mesophiles predominate
as the pile heats up, thermophiles are selectively enriched in the population and succeed the mesophiles as the dominant organisms
eventually the thermophiles use up the available oxygen and the pile cools
then we either aerate (turn) the pile or the pile becomes anaerobic and good composting stops
if we aerate, the thermophiles again heat up the pile and degrade materials
this cycle can be repeated many times
finally the pile cools down because all the "tasty" stuff is gone
mesophiles and fungi become the dominant organisms again
this process is known as curing
in the end, yummy compost for your garden
