Technology Innovations Deliver Greater Output and More Sustainable Operation From Anaerobic Digesters

Upgrades to accommodate a variety of organic wastes and optimize the process can help communities curb greenhouse gases while creating reliable revenue streams.

Technology Innovations Deliver Greater Output and More Sustainable Operation From Anaerobic Digesters

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The U.S. EPA reports that organic food waste and yard trimmings accounted for 28% of the total mixed solid waste in the country in 2019.

When landfilled, these organics break down and release carbon dioxide and methane gas, a greenhouse gas 25 times more potent than carbon dioxide. Capturing and diverting these materials at the source can substantially reduce GHG emissions from landfills.

Instead of being landfilled, these materials can be anaerobically digested along with biosolids at wastewater treatment plants. Of the roughly 1,200 anaerobic digesters in operation in the U.S., many are significantly underloaded. By making use of that capacity, communities can curb GHG emissions and generate revenue.

Impact on climate

The Intergovernmental Panel on Climate Change says up to 2.245 metric tons of equivalent CO2 GHG emissions can be reduced for every metric ton of organic waste diverted from landfills, mainly by eliminating methane releases.

Wastewater treatment plants can further reduce emissions by converting that waste into biogas, offsetting the use of fossil fuel natural gas. GHG emissions can be further reduced by applying the digested biosolids to cropland, offsetting the use of synthetic fertilizer.  

With electricity prices relatively low, municipalities and plant owners are seeing strong financial returns by upgrading biogas to renewable natural gas, or RNG, for injection into a commercial natural gas pipeline. Plants that go this route can sell this gas directly and generate credits in the form of renewable identification numbers.

These RINs are sold to parties such as refiners and refined fuel importers who need them for compliance reasons. RINs serve as currency in the Renewable Fuel Standard program. Because RINs are traded on an open market, their value is determined by demand. Based on current market pricing, plants that generate RNG from organic food waste and yard scraps can recoup six to eight times the equivalent price value of natural gas on the RIN market.

Optimizing capacity

An effective way for plants to get the most for their digesters is to invest in infrastructure to optimize the digestion process. One pretreatment option is biological hydrolysis, which conditions sludge to accelerate the efficiency with which it can be digested.

Installed in front of the digester, this retrofit solution can be integrated without taking the process offline. By shortening the time that sludge resides in the digester, biological hydrolysis in effect increases digester volume, allowing plants to codigest with materials like FOG, liquid waste and organic slurries.

A more extensive upgrade includes biowaste separation — an integrated, multistep process that removes contaminants and grit and maximizes organics capture. This mechanical solution produces a high-quality slurry that is easily digestible, reducing downstream operational issues and improving digester performance. Plants that adopt this technology can realize revenue in the form of tipping fees for receiving waste.

The ultimate step is the technology to convert biogas to RNG for pipeline injection.

Processes at work

Experience from three projects completed and under development shows how handling organic waste in these ways can reduce GHG emissions while creating reliable revenue streams.

1. Retrofit

This project includes a series of technology and digester upgrades at a municipal wastewater treatment plant in the southern part of England and generates more than 36,000 dry metric tons of solids per year. Previously, >span class="s6">roughly half the total volume and required lime stabilization to treat the remainder.

In phase 1, a biological hydrolysis system was added, enabling the existing digesters to treat more than 80% of the combined volume. Two years later in phase 2, four sludge holding tanks were repurposed as digesters, enabling the entire load to be digested. This led to the production of enough renewable energy to cover the plant’s entire energy demand.

Finally, in phase 3, the facility added food waste reception, pretreatment and pasteurization; and two digesters were dedicated to food waste. Today, both sludge and source-separated organics, or SSO, are treated in dedicated digesters, and the plant generates more than 13 MW of renewable energy as RNG, renewable electricity and heat.

2. New construction

In this project in eastern Canada, a stand-alone, dedicated anaerobic digestion facility will be constructed to manage and process 60,000 metric tons of residentially collected food waste. The facility will be equipped with technologies to convert the organic waste into biomethane for injection into a local gas network.

3. Pretreatment plus RNG

This project at a wastewater treatment plant in eastern Canada will add pretreatment technology to intensify the digestion process in an existing anaerobic digestion system. The project will allow 24,000 metric tons of SSO such as household food scraps, expired groceries, restaurant waste, chicken viscera, fish products and bread/dough to be imported for codigestion with biosolids. The biogas will be upgraded to RNG for pipeline injection.

The actual and potential GHG emission reductions for each project are shown below.

As shown in these examples, anaerobic digestion can help communities reduce GHG emissions, meet waste diversion goals and reduce the carbon footprint of waste management.

About the author

Michael Theodoulou (michael.theodoulou@suez.com) is a senior product manager with SUEZ Water Technologies & Solutions.   



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