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.

Drought in the Southwest

A few days ago, the New York Times posted an article about the 11-year drought that has been occurring in the Southwest. Lake Mead, on the Colorado River, has a water level only 15 inches above its lowest recorded minimum in 1956. From Las Vegas to Los Angeles, water prices have skyrocketed, water use restrictions have been tightened, and people are starting to feel the pain. Now, in order to relieve the stress on Lake Mead, they’re considering releasing water from Lake Powell, also on the Colorado River, so that it flows into Lake Mead. The map below is kind of hard to read but Lake Mead is at the southern tip of Nevada, and Lake Powell is in southern Utah.

Right now, 70% of water use can be attributed to outdoor activities. While some of this is legitimate, i.e. irrigation of crop growth, a lot of that is pure landscaping. Word of advice: if you want a luscious green lawn, move somewhere NOT in the desert. I know you want your yard to be just as pretty as your neighbors, but for the good of everyone, please don't fall into the tragedy of the commons. I really don’t understand the population boom in the Southwest. It’s a seemingly uninhabitable environment that we humans decided we should make habitable. The thought: why should we adjust ourselves to the climate when we can simply adjust it to fit our needs? It’s a perspective that I’m sure will change once the water crisis felt around the world finally hits America hard. The drought in the Southwest? Not bad enough.

Nationally, agriculture is responsible for 80% of our water use. What are some ways to address that? I would like to advocate vertical farming, if not simply as an effort to conserve water. A vertical farm is basically a bunch of greenhouses stacked on top of one another. They use technology like hydroponics, which requires 70% less water than conventional agriculture, and aeroponics, which uses 70% less than that. There are other benefits too. In addition to having a small physical footprint, vertical farming is indoors, which makes it relatively immune to weather variation, such as droughts and disease/pest invasion. Furthermore, biogas digesters and living machines can be employed to convert waste into usable resources, including methane for energy. Vertical farming isn't without downsides, however. The daylighting and heating needed to farm indoors requires a lot of energy. About half of the energy can be covered by biogas digestion, but the rest will need various energy sources, like solar and wind, to keep farming economical and sustainable.  I could say more, but I won’t. Instead, if you want to learn more about vertical farms, I’m going to add a long spiel about them under “Favorite Concepts”.

Hydroponics

Aeroponics

Dickson Despommier, a professor at Columbia University, is the field's all star. Here's his website.