Comparing the Carbon Footprint of Conventional Gas-Fired Thermal Sludge Drying to Solar Sludge Drying

The high, and ever-rising costs associated with disposal of sewage sludge containing high moisture content have prompted waste water treatment plants to search for technologies capable of consistently drying sludge to 75% dry solids or greater. A commonly accepted technology in the United States for this has been gas-fired thermal drying. Although this type of dryer has proven to produce a high quality end product, literature has shown they require 70–100kWh of electrical energy plus 2.7 – 3.4 million BTU's of thermal energy per ton of water evaporated. The thermal energy consumption is typically generated by burning a fossil fuel like natural gas which leads to high levels of CO2 production. Using CO2 emission numbers generated by the US Department of Energy for electrical generation and for burning natural gas, it can be calculated that, on average, gas-fired thermal dryers produce 0.24 tons of CO2 per ton of water evaporated from sewage sludge.An energy efficient sludge drying technology that has been popular for many years in Europe and has been growing in popularity in the United States is solar sludge drying. Since its introduction in Europe in the early 1990's, the technology has established a world-wide installation base of nearly 200 installations. Literature has shown that solar sludge dryers only consume 20–40kWh of electrical energy per ton of water evaporated from sewage sludge. The thermal energy needed for evaporation of water is provided by the sun, which produces energy that is free for all to use and produces no CO2. This leads to a 90% reduction in CO2 emissions when compared to gas-fired thermal dryers, or production of 0.02 tons of CO2 per ton of water evaporated.Test results from two solar sludge drying trials conducted at an 8,500 ft2, full-scale, solar dryer installation in California during the months of April - June 2009 proved that the technology is capable of producing a very dry, safe and quality end product like that of a gas-fired thermal dryers. In Trial 1, 210 yd3 of sludge were loaded in to the dryer at 17.8% dry solids, reached 75% dry solids in 18 days and a maximum dry solids concentration of approximately 90% in 20 days. In Trial 2, 210 yd3 of sludge were loaded in to the dryer at 14.7% dry solids, reached 75% dry solids in 14 days and a maximum dry solids concentration of approximately 90% in 23 days. In both trials, pathogen levels were reduced to those required by the EPA for Class A biosolid classification.