55. One reason that solid waste management is viewed as a possible arena for private sector participation is that the economies of scale are not pronounced. This is in contrast to the case of water, electricity, and telecommunications that have such significant economies of scale that they are often regarded as natural monopolies (56). In solid waste management, there are economies of scale to a limited extent, as follows:

• Collection in low-income areas. Low-income areas are commonly characterized by narrow or steeply graded roads (or both) that are accessible only by relatively small vehicles of about 2-tonne payload capacity that are able to make 2 trips for each daily shift (4 tonnes per day), or by communal container vehicles of about 3-tonne payload capacity able to make 5 trips for each daily shift (15 tonnes per day). Assuming a daily neighborhood waste generation rate of about 0.35 kilograms per capita in low-income residential areas, 1 vehicle can serve about 10,000 to 40,000 residents, respectively.

• Collection in high-income areas. High-income areas are commonly characterized by roads of moderate width and grade that are readily accessed by compaction vehicles of about 6-tonne payload capacity able to make 2 trips for each daily shift (12 tonnes per day). Assuming a daily neighborhood waste generation rate of about 0.60 kilograms per capita in high-income residential areas, 1 vehicle can serve about 20,000 residents.

• Transfer systems. Transfer station design is based on the use of large-capacity hauling vehicles (tractor trucks with trailers) that have a payload capacity of about 20 tonnes and are able to make at least 4 trips for each daily shift (80 tonnes per day). Assuming a citywide waste generation rate of about 0.70 kilograms per capita per day, 1 vehicle can serve about 1 15,000 residents. In systems that use compaction devices to fill the trailer trucks, one stationary compactor moves about 60 tonnes per hour, or 480 tonnes per day. Using this same analysis, l stationary compactor can serve about 685,000 residents.

• Sanitary landfill. Sanitary landfills rely on bulldozers as their main piece of equipment for spreading and grading refuse and for daily soil cover. One bulldozer of 200 horsepower can handle about 400 tonnes per day. Assuming a daily, citywide waste generation rate of about 0.70 kilograms per capita, 1 bulldozer can serve about 570,000 residents.

• Composting systems need be no more complicated than the manual sorting of non compostables from incoming waste, followed by the mechanized turning of windrow piles with a wheeled loader or windrow turning machine and the screening of compost product with a portable trommel screen. If the composting operation is performed at a site adjacent to the sanitary landfill operation, the wheeled loader used for the excavation of soil cover at the landfill can be shared with the compost operation. If the composting operation is at a separate location, thus requiring dedicated equipment, 1 wheeled loader of about 170 horsepower would handle about 200 tonnes per day. Assuming each windrow pile is turned once weekly over a 7week period, l wheeled loader would handle an incoming waste load of 200 tonnes per day, or serve about 285,000 residents.

• Waste-to-energy. Waste-to-energy incineration systems are not technically viable for most developing countries, because the refuse, on an as received basis (wet basis), is not sufficiently high in calorific value to sustain incineration. Refuse of least 1,300 kilocalorie per kilogram of "lower heating value" needs to exist on a year-round basis for incineration without supplemental fuel. If waste to-energy incineration is viable, the frequency and duration of downtime for maintenance require 100 percent standby capacity. A waste-to-energy incinerator needs to operate continuously, on a 24-hour basis, at no less than 5 tonnes per hour per unit. As a result, the smallest viable waste to-energy incineration system would consist of one 120 tonnes per unit per day, plus one standby, which would serve about 170,000 residents.

56. Based on studies of costs for refuse collection in the United States, no economies of scale are thought to exist for communities greater than 50,000 people (56). Only for transfer, disposal, and resource recovery systems are there economies of scale to handle more than 200 tonnes per day, or 150,000 residents based on the United States' higher waste generation rate.