by Nick C. Parker, PhD
In part II of this series we explored the sources of energy and distinguished between non-renewable and renewable supplies. Biomass in the form of municipal solid waste (MSW) and the sludge produced by wastewater treatment plants was identified as a renewable energy source which is expanding even more rapidly than population growth.
On average, each person in the US produces about 4.6 lbs of MSW/day. In Colorado, the waste production was 1.7 lbs/day in 2006. If individual production of MSW remains constant as population grows, the volume of MSW produced annually will double by 2050, if not by 2030. MSW and sludge are the only sources of biomass that expand unintentionally as population grows.
What’s in MSW? Typically, about 40 to 60% is organic and the remainder is inorganic. Organic constituents include wood, paper, food wastes, fabrics, plastics, and even the tar component of asphalt. Organics, by definition, contain the element carbon. All products made from petroleum are actually organic. Plastics made from petroleum won’t necessarily decompose to produce compost, but with time and exposure to ultraviolet sunlight and heat, plastics will decompose. Similarly coal and even diamonds are organic in origin and certainly won’t make good compost.
The renewable organics such as wood, paper, food waste, ad agricultural residues will make compost. These organics can also be blended with sludge to produce a high grade compost. Two cities in the US, Pinetop, AZ and Sevierville, TN, now combine MSW and sludge to produce compost. In Canada, Vancouver has a very large operation to produce compost from MSW and sludge. The benefits of these operations include minimizing the load of landfills, extracting value from waste, producing a product for both agricultural and horticultural use and is, in many cases, minimizing trips, and therefore transportation costs, to landfills.
The inorganics such as metals, stone and concrete can be reclaimed and recycled. Today the aluminum in cans is recycled about seven times per year. In some areas, used tires are recycled into rubber mats and even constituents of road surface materials. Stone, concrete, asphalt, bricks and other inorganics are recycled into paving materials. Glass is recycled to make more glass and is also crushed to produce a mulch used in landscaping. Ferrous materials, such as tin cans, steel and iron are easily reclaimed and recycled. Electronic components contain a variety of high-value metals and components such as antimony, arsenic, copper, cadmium, lead, zinc and tantalum. Many of these elements are hazardous and other are of very high value, both monetarily and environmentally. For example, the metal tantalum, derived from columbite-tantalite ore, is mined in Africa…destroying the habitat of the mountain gorilla. Recycling cell phones, of which there are 500 million in the US, with 100 million new ones added annually, reduces the demand for columbite-tantalite ore and helps preserve gorilla habitat. Tantalum is also used in DVD players, computers, and game consoles.
Dickson Despommier, a microbiologist at Columbia University, has proposed urban farms to be located in skyscrapers. These urban farms could supply the food needs for 50,000 people consuming 2,000 calories/day. He proposed to use sewage water and a series of hydroponic plants to extract nutrients, producing clean water, fruits and vegetables. Additional high-quality water could be captured by condensing water vapor placed in the atmosphere by evapotranspiration of plants. This volume of water collected from each urban farm would supply the needs of 50,000 people.
In 2005, US residents, businesses and institutions produced over 245 million tons of MSW, or about 4.5 lbs of waste per person per day. By comparison, MSW per person was only 2.7 lbs per day in 1960 and the total quantity generated in the US was only just over 88 million tons. The total volume generated doubled in just about 30 years. The bulk of the waste is paper, followed by yard trimmings and plastics (Figure 1).
Figure 1. 2005 Total Waste Generation - 245 Million Tons (before recycling).
The rate of recycling has increased from about 6.4% in 1960 to 32.1% in 2005. Auto batteries have a recycling rate of 99% with the next highest rate of 62% for steel cans. Yard trimmings are recycled at 61.9% and paper at 50%. All other components of MSW are recycled at less than 50% (Figure 2).
Figure 2. Recycling Rates of Selected Materials.
These figures and data obtained from EPA reflect only MSW and do not include construction and demolition debris, agricultural residues, forestry residues, petroleum contaminated soil, coal combustion ash and similar other large volume wastes.
By comparison to the US, Colorado produced over 8.2 million tons of wastes in 2006 – up from 4.7 million tons in 1995. The solid waste user fee has remained about constant over this period, being $3.2 million in 1995 and increasing to only $3.7 million in 2006. Additional costs not reflected in these figures has been the increasing reliance on foreign oil used to fuel trucks transporting solids to the landfills.
In the next part of this series we will explore alternative ways to extract value from biomass, including MSW and sludge.
Nick C. Parker, PhD, is Chief Scientist for Global Scientific, Inc, a Lubbock, Texas-based consulting company that specializes in commercialization of research and development products for sustainability. Dr. Parker can be contacted at "Contact us" on this website.