Eyes are Opening to the Potential of Sludge

Sludge from wastewater treatment plants is often dried and landfilled. But why? It's carbon rich. It's nutrient rich. It holds endless possibility. Finally, New York City is giving it a chance.

By allowing acetogenic and methanogenic microbes to digest the sludge, biogas (methane+CO2) is produced

What are some things sludge is good for?

FERTILIZER: sludge has high levels of nitrates, phosphates, and sulfates, which are essential for crop growth. Billions of dollars are spent on these types of fertilizers every year. Note: Direct application of sludge to fields can cause contamination of ground and surface waters.

ENERGY: Through anaerobic digestion (see diagram), sludge can be converted to methane gas. Energy can also be obtained by incineration.

ALGAE/BACTERIA: Biological mechanisms of obtaining useful products, such as energy, often require the nutrients that sludge provides.

Waste Fuels an Entire Swedish City

Kristianstad

Kristianstad, a Swedish city of around 80,000, has succeeded in its effort to eliminate fossil fuel consumption. Agriculture and food processing, which generate a large amount of organic waste, dominate the city's industry. This waste is converted to biogas (primarily methane) through anaerobic digestion (AD). AD occurs in landfills, which produces some of the city's biogas. The rest is generated at the local anaerobic digestion facility, which collects waste from farms, industry, and the local wastewater treatment plant.

The resulting biogas:
1. Heats homes and provides electricity: The biogas is incinerated. Some of the heat is used to generate electricity while the rest is distributed around the city through district heating (an efficient method of providing heat to a large population).
2. Fuels vehicles: The biogas is pressurized as compressed natural gas (CNG) and used in municipal, cars, trucks, and buses.

Kristianstad's "The Best Eco-cycle"

Cost
In Germany alone, there are 5,000 biogas plants operating. How many are there in US -- a country where agriculture and livestock cover vast tracts of land? 151. These 151 facilities are mostly limited to the cattle industry and are fed solely with manure. High capital costs and lack of infrastructure are the largest hurdles facing biogas in the US. However, efforts are being made in California and Wisconsin to expand the industry.

In Kristalstad, high capital costs were heavily offset by government subsidies. Either way, biogas has a siginificant payback time. They've cut their annual fuel costs in half, saving around $3.8 million/yr.

Carbon footprint
Methane emissions from manure and landfills are reduced and redirected to energy use. This could reduce a city's greenhouse gas emissions by 2-6%. Remember though, that although waste-to-energy technology doesn't use fossil fuels, it still emits a significant amount of CO2.