Summary: A new way to dispose of NYC's sewage biosolids can save money and counter the climate crisis at the same time - but needs support from NYC and NY State agencies and legislators

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more detailed version of this summary article here.

To respond to the climate crisis, we'll have to switch from fossil fuels to renewable energy, but that won't be enough.  We'll also have to remove massive amounts of carbon dioxide from the atmosphere as quickly as possible. Part of the strategy for removing the necessary 700 gigatons of carbon is planting trees - about three trillion

Not just carbon emission reductions, but carbon capture

Another part is to keep the carbon in biomass from decomposing and returning to the atmosphere by capturing it in forms that are stable over the long term. Some approaches to carbon sequestration, like capturing CO2 directly from power plant exhausts, 
are unproven and expensive. 

Capturing carbon has a long history - and many uses

One approach has a very long history.  Before Europeans came, the indigenous people of the South American Amazon created the high-carbon terra prieta soils, that are unusually fertile to this day, by controlled burning of biomass. A modern version of this approach can be widely applied in both rural and urban areas.  Biochar production is listed as one of the
top 100 policy responses to the climate crisis by Project Drawdown.

When organic material is pyrolyzed - heated in a low-oxygen environment -  instead of catching on fire, it turns into a form of charcoal called biochar.  In this form, carbon can be stable for thousands of years.  Ideally, massive biochar production will be part of a Green New Deal package of emergency actions.  Until then, we have to raise awareness of the many other benefits of biochar to make its production popular.

Biochar has many potential uses

- It can be a soil amendment for farming, a compost accelerator, and a substitute for peat or vermiculite in potting soil;
- It's chemically similar to activated carbon, and can be used as a filter for water and exhausts;
- it can be mixed with asphalt, concrete and steel, improving their performance and properties;
- it can be mixed into packaging materials, and used in a host of commercial products;
- many more uses are documented in
Burn: Using Fire to Cool the Earth.

Solving waste disposal problems

Turning biomass into biochar reduces its volume, which can help with the many types of biomass we struggle to dispose of: crop waste, seaweed washed up on beaches, trees destroyed by drought or disease, manure, and sewage sludge.   

The City has been looking into new ways of handling its massive solid waste streams that minimize what has to be sent to landfills. A significant fraction of NYC's solid waste is sewage sludge, which after being processed at the City's wastewater treatment plants, is called biosolids.

NYC seeking new ways to dispose of its sewage biosolids

NYC generates roughly 1,400 tons per day of biosolids, and pays contractors about $50 million a year to ship it to faraway landfills. 

NYC DEP has been researching biosolids alternatives for years.  I've been in regular contact with a DEP staff member working on the issue.  Some of DEP's planning is available to the public, but most is not. 


In 2016, DEP reported it was studying four main options for biosolids disposal, after landfilling.

- applying it to land; 
- anaerobic digestion; 
- incineration / waste to energy; 
- and pyrolysis / gasification - the heating in a low-oxygen environment - which can yield biochar. 

While NYC biosolids could in theory be applied directly to land, in practice the market is very limited. Anaerobic digestion is already a key part of how NYC handles sewage sludge.

The two other approaches are both forms of thermal processing, but there are significant differences. 

With incineration, the waste is heated in the presence of oxygen, so it catches on fire and burns. There are about 86 waste to energy facilities around the US, which combine incineration of mixed municipal solid waste with some materials recovery.  Some of NYC's MSW already goes to the Covanta WTE in Newark, NJ.

With pyrolysis, the waste is heated in a low- or no-oxygen environment. Both types of thermal processing will sharply reduce waste volume, lowering disposal and transportation costs. The cost per ton, energy recovery, and yields of solid and gaseous end products will all vary depending on the types of  equipment, feedstock, and operating conditions. 

The crucial difference is that gasification / pyrolysis systems can be set to produce a solid end product that's not just ash, but stable carbon.  Biochar made from wood has a higher percentage of carbon that biochar from sewage sludge.

By 2021, it became clear from public DEP documents and staff statements that it had completely dropped incineration as an option - leaving the thermal processing methods of gasification / pyrolysis as the default preferred choices.

There's a pilot municipal biosolids pyrolysis plant for NYC to consider - nearby in New Jersey

Incineration has been around for a long time.  On the other hand, the technology for pyrolysis is new, making this a new disposal option for cities. Stockholm, Sweden has been turning municipal green waste into biochar for several years.
Some cities in India have begun pyrolyzing their sewage sludge. 

NYC DEP is not able to fund, site and build its own G/P facility within NYC.  It will have to develop partnerships with one or more vendors of this technology to set up nearby facilities at which an increasing percentage of NYC's biosolids waste stream can be thermally processed. 

Fortunately for NYC, there is a pilot project for this technology nearby.  Aries Clean Energy has built a pyrolysis facility in Linden, NJ that will convert 400 tons per day of NJ municipal biosolids to ash containing 5-10% carbon by weight. All of the ash will be taken by a large concrete manufacturer.




             Rendering of the Aries facility in Linden, NJ

Still, these new thermal processing techniques face very serious obstacles.  Without public advocacy and public education for G/P, projects using those technologies will be opposed, and NYC will run into a bottleneck for disposal of its biosolids.

.- Hardly anyone has ever heard of G/P or knows what these technologies are, let alone their importance as a climate response. 

- The Community Leadership and Climate Protection Act, which was passed to set up climate planning in New York State, specifically and erroneously lumps incineration and pyrolysis together, excluding both from being considered carbon control measures.

- Many people - such as environmental justice advocates and government officials - consider G/P just another name for  incineration, a harmful technology to be opposed

- When Aries Clean Technologies tried to site a second biosolids pyrolysis plant in Newark, NJ, an environmental justice group assumed it was an incinerator, refused to examine evidence it was not, and launched a campaign that killed the project.

Much public education and advocacy for G/P is needed - but who is stepping up to offer it?

NYC DEP expects the City to increasingly rely on G/P facilities that have not yet been built.  For this option to be available, City and State officials and agencies should work to build public awareness and support for these technologies, and remove roadblocks like the pyrolysis exclusion in the CLCPA. We have to educate our elected officials about how carbonizing waste streams supports the City's climate plans. Then, we have to get them to push NYC DEP and NYS Department of Environmental Conservation to take a stand in support of gasification / pyrolysis of organic wastes.

See a
more detailed version of this summary article here.