国产麻豆

Diverting materials can provide beneficial reuse and bring renewable products into the market.
By Debra Darby, CCP, Peter Klaassen, PE, and Tom Bilgri, PE

There has been a marked uptick in interest in establishing anaerobic digesters at sites with a steady supply of organic materials, including farms with abundant animal manure and crop residues, wastewater treatment plants (WWTPs), food production facilities, and landfills. Stand-alone digesters constructed to accept organics from multiple businesses are also being considered. Existing facilities would do well to consider how they can manage organics, so it is diverted from landfills and properly reused. Operators could bring waste in as part of an existing facility and not lose the tonnage that would otherwise be lost if food waste was diverted. Read on for an overview of key site considerations for siting anaerobic digestion facilities, from federal guidelines, state regulations, and permitting, to feedstock analysis, site design and layout, costs and revenue, water requirements, and digestate disposal.

Organics Management and Waste Reduction
There is a growing understanding that solid waste is a resource, as well as a paradigm shift to a more circular infrastructure. Organics has become the next solid waste commodity, as some developers are discovering new end markets for renewables, including high quality compost, biofuels, energy, green chemistry, and bioplastics. In addition to regulatory movement towards organics diversion to reduce disposal, there is a recognition that diverting organic materials can provide beneficial reuse and bring renewable products back into the market. One method is anaerobic digestion (AD), a natural process in which microorganisms break down organic materials, such as food scraps, grease, and biosolids. Figure 1 illustrates these varied sources and available end products.

 

 

 

The key factors driving the need for organics diversion and processing infrastructure for AD facilities include limited available remaining disposal capacity (in some areas; particularly in the Northeast), regulatory requirements and incentives, as well as the availability of feedstock and end markets.

Overview of Regulatory Approach to Reducing the Environmental Impacts of Food Waste
Discussion of considerations for siting anaerobic digestors must begin with an understanding of the complete range of regulatory issues, federal guidelines (including state level environmental justice regulations), changes to the existing food waste hierarchy, consideration of the role of food waste in climate change, and individual state organics bans.

Regulations
Table 1, provides a brief overview of federal regulations affecting siting AD facilities.1 Key areas of consideration include the National Ambient Air Quality Standards (NAAQS); Resource Conservation & Recovery Act (RCRA); and National Pollutant Discharge Elimination System (NPDES).

 

 

Changes to the Solid Waste Hierarchy
Another key aspect of the regulatory approach is the most recent shift from the standard solid waste hierarchy to the wasted food scale shown in Figure 2. The topic is quite broad; in this article we are focusing on the sliver that includes compost or anaerobic digestion and beneficial use of digestate/biosolids.

 

 

Links Between Food Waste and Climate Change
The regulatory sphere also considers the direct link between wasted food and climate change. EPA studies have found that about 58 percent of landfill methane emissions is from landfilled food waste and between 50 to 70 percent of methane generated by landfilled food waste is captured by LFG collection systems. The rest of the methane escapes to the atmosphere; the environmental impacts of these methane releases is anywhere from 25 to 80 times greater than C02 released into the atmosphere. That is why the EPA, along with the USDA and the FDA, has developed objectives accelerating the prevention of food loss and food waste and goals for recycling the remainder of organic waste materials across the entire supply.

The interagency alignment shown on Table 2 calls for a 50 percent reduction of food loss and waste by 2030 (this is known as the 50×30 goal). Unfortunately, it is an open secret that there is insufficient organics infrastructure and planning to meet the national recycling 50 x30 goal as reflected in state and local government planning processes.

 

Environmental Justice Issues
Another aspect of the regulatory framework that affects our understanding of the use of siting anaerobic digestion facilities for food waste is the general issue of environmental justice, or EJ. A growing list of (primarily Northeastern) states have implemented EJ policies and regulations, including Connecticut, Massachusetts, New Jersey, New York, North Carolina, and Pennsylvania.

State Organics Bans and Diversion Mandates
State organics bans and diversion mandates are a further regulatory consideration. As states and municipalities aim to implement zero waste strategies, organics is one of the primary materials to divert from landfills. Even for locations that have addressed recycling or made other progress, collection and processing of organics is often a largely untapped area.

We know legislation is forcing the discussion of organics recycling in many states. Planning, education, programs, and infrastructure are needed to implement change and more of the country is looking to pivot. The map in Figure 3,聽 shows the status of state organics bans and diversion mandates, which are currently focused in the Northeast and West.

 

 

Promoting Renewable Energy Sources with Less Carbon Intensity
AD produces biogas, a renewable energy source that has far less carbon intensity than fossil fuel production. Whereas production of biogas through use of food waste pulls carbon out of the environment, fossil-based natural gas production is a very greenhouse gas (GHG)-intensive process that releases methane during the extraction process and from pipeline leaks. The rest is combusted, releasing CO2, adding to the GHGs already in atmosphere. Figure 4 compares the carbon intensity of fuels, clearly demonstrating the carbon intensity benefits. Food waste makes low carbon fuel (the green circle on the slightly obscured label refers to Biomethane CNG).

 

 

Technology overview
Facility siting considerations are affected by the AD technology selected. The following is an overview of available AD technologies, with a discussion of associated logistical and cost factors. The selection of the AD technology is often predicated on the type of organics that are collected and processed.

Dry AD
Dry AD is typically used for material with a high solids content, most commonly for source separated organics (SSO), including food waste. It is also more adaptable to woody materials, including leaf and yard waste. The substrate is 20 to 50 percent total solids; the higher solids content equates to higher transport efficiencies in comparison to wet systems where 90 percent or more of the feedstock transported is simply water. Dry technologies tend to be more adaptable to residential materials as co-mingled food waste and yard waste debris can be accepted. This digestion process is quite effective at killing most bacteria.

Two key dry AD technologies are used (see Photo 1). In the garage style batch process (top photo) organics decompose in a chamber, producing gas that gets stored, and then extracted and cleaned for later use. Most AD facilities in the U.S. and Canada are dry garage style.

 

Plug flow digesters (bottom photo) use a continuous (input/output) style that takes organic waste through a chamber where it is churned, and moisture is extracted. This method takes 20 to 30 days to extract the maximum amount of gas. Plug flow tends to offer more flexibility in the feedstocks it accepts.

Dry AD typically features higher capital cost (CAPEX), since depackaging and post processing is necessary to remove contamination and get to a consistent particle size. A mechanism to clean the organic waste at the front end or back end is critical. There have been numerous instances when systems lacking proper depackaging have failed in both the U.S. and Canada.

Dry AD technologies do however have some cost reduction advantages. For example, there is no need to implement dewatering in post processing; instead, the material (the digestate) can be delivered to a composting facility as a feedstock and/or may be spread directly to farm fields where land application is allowed. Dry AD facility operators are not producing liquid that needs treatment and are not paying dewatering costs if required by legislation.

Wet AD
Wet AD (Figure 5) increases the amount and types of organic materials that can be brought in for processing and produces more gas than dry systems. It is used for low solids sources, such as those from WWTPs. Wet AD uses a biological process similar to dry technologies, but the substrate is a slurry, usually with less than 15 percent total solids by mass.

 

Wet AD can use available digester capacity at the WWTP (but can also be constructed exclusive of WWTP), and it increases biogas quality and quantity that could be sold and/or used to supplement energy use at the plant. It is becoming more acceptable to co-digest bioslurry with food waste as the energy created can help with operational energy needs and potentially provide electricity/incentive to taxpayer. WWTPs have learned their lesson from systems failures, so now most have a robust preprocessing component to remove as much contamination as possible.

Wet AD can be used for co-digestion of wastewater treatment residuals (biosolids) with SSO from municipal solid waste. Food waste and biosolids can be used as feedstocks, but yard waste would not be accepted. Leaf and yard waste is not easily digested in wet AD processes. In California, there has been a large increase in the number of applications that incorporate food waste into an existing WWTP, which might eliminate the need to build a separate digester.

Special Note: Those looking to implement a residential curbside organic waste program and are considering AD, might be more successful with a dry AD program. While wet AD technology creates more biogas, the technology is not designed to receive yard waste and woody materials and creates more odors than dry technology.

Chemicals of Emerging Concern
One issue becoming central to the discussion on the use of AD is per- and polyfluoroalkyl substances (PFAS), which are introduced into biosolids and SSO by products and packaging materials and accumulate in biosolids and digestate. When biosolids and/or digestate are land-applied, PFAS are taken up by crops, livestock, wildlife; migrates to surface- and groundwater; and accumulates in soils.

Wet co-digestion is taking place at many existing WWTPs. There has been concern about whether the digestate that comes out of these plants can be land-applied. If PFAS levels are too high, operators may have to do something with the digestate, and/or there might be a need for another destruction technology. PFAS are not removed by wastewater treatment or AD. Incineration of biosolids is presently an option with efficiency being studied and showing promise. Wet AD may be more attractive for PFAS reduction.

Siting Considerations
Successfully siting an AD facility requires attention to location and transportation logistics, waste characteristics and sources, capital costs (CAPEX), operational costs (OPEX), and end-markets/revenues.

Location
Developers need to evaluate proximity to the source of the organic waste feedstock, the natural gas pipeline or electric grid, the end use of digestate, and sensitive receptors. Proximity to feedstocks is a key concern because of its impact on transportation costs. Facilities should ideally be close to where the organic waste is generated; hauling long distances can be costly and increases the possibility of odor issues. Also of concern is proximity to end-markets, including potential markets for digestate and nearness to a natural gas pipeline. If a pipeline is not nearby, developers must factor in the cost of building one, including the permitting time that will be necessary. They may also consider use of a 鈥渧irtual pipeline,鈥� i.e., transporting fuel from the treatment facility to the injection/usage point via over the road transport trailers.
‘Proximity to sensitive receptors is another key factor. Developers of facilities must pay particular attention to addressing odor (from both delivery of organics and actual processing) and traffic issues (from feedstock and material leaving the facility) early in the process. Community and stakeholder engagement early in the siting process is critical; a community is more likely to accept the facility if the public is informed and developers work through the potential concerns. See the earlier discussion on EJ issues.

Costs and revenues
Dry AD processes typically have higher CAPEX than Wet AD and generate less gas. Dry and Wet AD have similar OPEX, in terms of labor, maintenance, and other capital items installed. Developers should prepare a pro forma showing the system lifetime (15 to 20 years) and include likely revenue that offsets the negative outflow. Tipping fees for municipal solid waste (MSW), industrial/commercial/institutional (ICI) waste, and organics from schools, hotels, grocery stores, and casinos vary considerably. Renewable natural gas (RNG) or electrical revenues may also offset costs, including incentives from utilities to create green electricity.

Figure 6 is an example of costs and revenue for a hypothetical 100 ton per day (TPD) wet AD system co-located at a landfill facility. In this scenario, the operator could bring waste in as part of an existing facility and not lose the tonnage that would otherwise be lost if food waste was diverted.

 

With $30 to $40 million in CAPEX, an annual OPEX of $1.3 to 1.8 million, and Renewable Identification Number (RINs) credits worth about $2.40 per D3 RIN, the facility could generate about $144 million in net revenue over 20 years if the biogas is cleaned and injected into a pipeline as 鈥済reen鈥� RNG. The digester generates about 700 scfm of clean biogas at 60 to 65 percent methane. As an added benefit, the owner could use the generated biogas in an existing gas utilization project rather than create a new stand-alone AD-RNG project, further reducing the potential CAPEX and OPEX. Biogas generation from AD makes up what is missing from the organics as part of the tonnage in the landfill and the owner is still maintaining the organics fraction of the MSW as an in-bound waste stream. (Note: The value of RINs in the U.S. may change significantly, which would have an impact on the economics.)

Alternatively, the fuel from the AD-RNG process could be used onsite to fuel collection vehicles, potentially offsetting the use of about 1.2 million DGE in fossil fuels.

Pay Attention to Existing and Emerging RNG Markets
In the U.S., there are three potential markets: the transport fuel market (dominates RIN, most lucrative), voluntary market (lower revenue), and emerging market (for example, hydrogen). In California, the landscape is ruled by the Renewable Identification Number/Low Carbon Fuel Standard (RIN/LCFS) market, as well as the quest for fuels with lower carbon intensity (CI), since RNG has a cleaner footprint than fossil fuel energy. The approximate value of the gas is six to 10 times the natural gas (NG) value (about $32/million metric British thermal units (MMBtu)).

To get a full picture of RNG markets, it is important to understand Canadian federal and provincial policies. Key drivers are currently British Columbia and Quebec (with Ontario poised to join in the future), where gas providers are subject to a renewable gas mandate to switch to RNG (5 percent by 2026 and 20 percent by 2030). These mandates are forcing gas companies to buy credits from outside (for example, from the U.S.). This is one reason the Northeast U.S. may be considered good for siting AD facilities to feed the RNG pipeline. With Canada projects, developers get a long term (10 to 15 years) contract, rather than the daily rates received in the U.S. The value of this RNG is currently approximately $24/MMBtu, lower than current U.S. rates, but likely to rise because of the pressure to implement the mandates. U.S. projects can be tapped to Canadian credits if there is a pipeline connection.

Summing it Up鈥擳he Future is Now
There are management options for food waste other than landfills. These options promote a circular economy and can use the nutrient value of the food waste. The environmental impact of food waste is compounded when it goes to waste and is disposed of in landfills (generating methane emissions), given the significant resource inputs needed to produce and deliver food to consumers.

AD facilities can play an important role in waste reduction and reuse. To make it work, developers should work with a qualified consultant to conduct their due diligence on facility specifics like site design, road access, utilities, and receptors; undertake facility planning; prepare a feedstock market analysis; and obtain equipment evaluations and recommendations. Consider civil and facility design and permitting issues and, perhaps most importantly, undertake stakeholder engagement.

Existing facilities should consider the changes coming to the industry. (They are here!) Organics infrastructure is needed, and developers should leverage opportunities becoming available based on pricing with RNG and with utilities to create end-markets and new revenue streams. | WA

Debra Darby, a U.S. Composting Council Certified Composting Professional (CCP), is a client manager with 18 years of organics experience in the solid waste industry. Debra has an extensive background working with organics and compostable materials, implementing organics management infrastructure, including composting and anaerobic digestion systems. She leads Tetra Tech鈥檚 Solid Waste East organics management initiative and serves on the Board of the Compost Research and Education Foundation (CREF). She can be reached at [email protected].

Peter Klaasen, P.Eng, MBA, is the Vice President for the Solid Waste Management Practice in Ontario and Manitoba. He is a senior environmental professional, with more than 35 years of solid waste experience. Peter has undertaken numerous organics projects, with a focus on aerobic and anaerobic process technologies, and their respective final products. He can be reached at [email protected].

Thomas Bilgri, P.E., manages Tetra Tech鈥檚 Biogas Engineering Services group, with more than 30 years of landfill gas (LFG) management system design, engineering, and permitting experience. Thomas has conducted assessments of LFG production for landfill facilities throughout the world and has been engaged in the development of beneficial-use projects, including direct sale, electrical generation, compressed natural gas (RNG) and vehicle fuel (R-CNG) production facilities. He can be reached at [email protected].

Note
Emissions are not directly measured, but are based on landfill operators鈥� reported methane generation, collection rates and oxidized methane. EPA鈥檚 analysis relied predominately on existing widely used EPA models, and data sources, including Greenhouse Gas Reporting Program (GHGRP), GHG inventory, LMOP database, WARM and LandGEM models. National estimates of methane emissions from food waste have not been previously quantified by EPA, which notes that there is no other known peer-reviewed national reference point.