Wednesday, August 2, 2017

Can NYC's 1,400 tons per day of sewage waste be turned into biochar?

NYC is lowering its carbon emissions, mostly through energy efficiency and conservation measures, and to a lesser degree, by adding more solar and wind power.  There’s an entirely different avenue that so far, NYC hasn’t ventured into: carbon sequestration. 

Carbon can be sequestered, or captured for long-term storage, before it can escape into the atmosphere as carbon dioxide, which is what usually happens when fossil fuels or organic solids are burned or decompose.

The most widely discussed methods of carbon sequestration propose removing CO2 from exhaust gases at fossil fuel burning power stations, and storing it in underground reservoirs, but these are unproven as well as costly.  Proven natural methods, such as reforestation and raising carbon content of soil through regenerative agriculture practices, don’t directly apply to urban environments. 

However, it’s possible for NYC to sequester the carbon in sewage sludge, one of its biggest municipal solid waste streams, by turning sludge into a type of charcoal called biochar that can be reused or disposed of locally – potentially saving many millions a year in dumping fees.

When organic solids are heated in the absence of oxygen, instead of catching on fire, they turn into charcoal.  Most charcoal is burned as a fuel, but when made to meet certain standards, for a wide range of specific applications, it’s referred to as biochar

Production of biochar is a known way to stabilize carbon for hundreds or thousands of years.  It’s one of the top 100 climate change solutions researched by scientists of the Drawdown Project.

When added to soil, biochar improves agricultural productivity and water retention.  The structure of wood is preserved at the microscopic level, providing habitat for beneficial soil microbes and fungi.

This was common practice in South America’s Amazon region before the European arrival, as layers of unusually dark and fertile soils with high charcoal content attest.  

Scientific study of biochar has revealed many potential industrial uses, such as in water filters and building materials. 

Many forms of organic waste in NYC, such as paper, cardboard, yard waste and leaves are already being recycled.  Food waste is increasingly being collected for composting.    However, the sewage waste biosolids left over at the end of the City’s water treatment process are more difficult to dispose of.

NYC’s Department of Environmental Protection manages the City’s 14 wastewater treatment plants, which together treat 1.3 billion gallons of wastewater daily.  Methane, the main constituent of natural gas, is always produced in the anaerobic phase of sewage treatment, and when waste decomposes in landfills.  It’s increasingly common for methane to be captured and burned at these locations to heat water into steam, run a turbine, and produce electricity.  Not only does this harvest a lucrative resource, it also avoids serious harm:  methane escaping into the atmosphere has over 25 times the greenhouse gas impact as carbon dioxide.  There’s a major initiative underway to collect methane at all 14 of the City’s plants and to put all of it to beneficial use.

Whether or not gas is extracted from sewage, there’s still a lot of sludge at the end of the process. NYC produces 1,400 tons per day of biosolids. Until it was legally prohibited in 1988, the City was dumping it in the ocean.

As the City’s 2006 Solid Waste Management Plan and its Biosolids Management appendix explain, biosolids can be used as fertilizer when spread on farms, parks, lawns, golf courses, and cemeteries, or it can be less usefully dumped in landfills. 

According to anecdotal reports from DEP staff, NYC biosolids are free of metal contamination or biological risk.  They’re even safe to use as fertilizer for growing food for human consumption.  However, because of the availability of other inexpensive fertilizers, the market for sewage biosolids may be limited.  NYC is paying contractors to take virtually all of NYC’s biosolids into landfills.

At $108 per wet ton, the City is now paying $151,200 per day in hauling fees, or about $55,188,000 per year. 

NYC may be better off turning its biosolids into biochar.  Relatively low carbon organic solids like sewage sludge or food waste must be mixed with woody biomass, high in both carbon and lignin, before it can be used as a good feedstock for biochar. 

Next questions for exploring biochar as an option for NYC

Before the operating costs of using this approach for NYC’s biosolids can be estimated, agency staff will have to consult with biochar scientists and other experts and stakeholders to assess what feedstocks are needed to mix with it, and what are available; the capacity of various biochar systems, their purchase cost and costs of operation.

Even if turning some or all of the City’s biosolids to biochar would cost more than the $55 million annually in shipping costs paid now, other benefits (in addition to carbon sequestration) may outweigh the extra cost.

Potential feedstocks

The NYC Parks Department, the Department of Transportation, and Con Edison are constantly pruning and removing trees within the City. How big is NYC’s supply of waste wood chips, and where it is going now?

How much wood is in NYC’s construction and demolition waste stream, and how much is being productively reused, or is going into landfills?  
Even if construction waste wood is chemically contaminated, charring it before landfilling would permanently sequester much of its carbon content and reduce its volume. Some contaminants can be rendered chemically inert or biologically unavailable after the material they’re in is turned into biochar.

August updates: According to the NYC Department of Sanitation's 2013 Waste Characterization Study, the City produces
3.25 million tons of waste annually.  Debris from construction and demolition has stayed the same as a percentage of aggregate discards between 2005 and 2013.

In 2013, treated / contaminated wood made up 1.3% of that total, and 0.8% was untreated lumber, pallets and crates. That may be 42,000 tons of contaminated wood and 26,000 tons of untreated wood.

Staff at NYC Parks Department report their inventory of wood chips ranges from anywhere from 35,000 - 70,000 cubic yards annually.  

Biochar production systems

NYC will have to research which of the many manufacturers of biochar production systems have a successful track record with municipal sewage biosolids, will be best suited for a pilot project, and can potentially scale up to NYC-sized volumes of sludge.

The International Biochar Initiative, the US Biochar Initiative, and the Ithaka Institute are biochar advocacy groups that can provide guidance among vendors.

Combined heat and power plants increase the energy efficiency of the fossil fuels they burn both by generating electricity and creating steam or hot water at the same time.  Can heat from existing in-city power generation facilities also be used simultaneously to produce biochar? Does this mean locating biochar ovens at power plants, and shipping waste for processing? Or if biochar ovens are best located at sewage treatment plants, can captured methane be burned for some or all of the heat they need?

Uses of biochar

In a project recognized by the European Mayors Challenge and C40 Cities, the City of Stockholm has started turning plant waste into biochar, and using it as a soil amendment for the city’s trees. 

The Ithaka Institute cites 55 uses for biochar. Some biochar processes can yield chemical byproducts that can be sold profitably to industrial users. 

If NYC can demonstrate a financially viable pathway for charring sewage biosolids, the model could be applied globally, boosting NYC’s role as a leader in climate response - and multiplying our carbon sequestration impacts.

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