Campaign Blog

This is a series of posts from participants in the campaign.  The posts shown do not reflect the adopted view of the campaign, and our met to offer background from the various participants.  Interested in posting, please contact the campaign. Happy reading!

Our Campaign’s Rationale Not To Support Plasma Arc

posted Apr 20, 2011, 7:11 AM by Bob Wenzlau   [ updated Apr 25, 2011, 8:51 PM ]

Our campaign has focused on biologically-based strategies for the organic waste streams.  However, in several public forums and in online posts our campaign is challenged to embrace Plasma Arc.  


First and foremost, this is a moot discussion as the Initiative will not specify the technology other than representing a environmental improvement over existing methods. However, our campaign has been committed to biological methods - and in particular anaerobic processes, and not methods like Plasma Arc.

The proponents of Plasma Arc are strong in their belief, and it seemed reasonable to develop why our campaign will not be supporting Plasma Arc.

Let’s first start out with what is Plasma Arc?

Plasma arc gasification is a waste treatment technology that uses electrical energy and the high temperatures created by an electric arc gasifier. This arc breaks down waste primarily into elemental gas and solid waste (slag), in a device called a plasma converter. The process has been intended to be a net generator of electricity, depending upon the composition of input wastes, and to reduce the volumes of waste being sent to landfill sites. (Wikipedia)


Our campaign has many voices. We are fortunate to have assembled a Technology Committee, and the following represents our collected objections to this technology in Palo Alto and applied to our organic residues.
  • Plasma Arc Would Be Inefficient on Palo Alto’s Feedstocks.  The  Compost Blue Ribbon Task Force heard presentations during the course of its research that Plasma Arc works best on dry feedstocks.  We even observed considerable pre-processing to dry materials for introduction to the plasma arc. That our “feedstock” is very moist caused the Task Force not to pursue plasma arc technology.  Two principal feedstocks in Palo Alto are biosolids and foodwaste, and each are approximately 70 percent moisture. Peat International, a Plasma Arc vendor reports that feedstocks at 30 percent as requiring considerably more electrical energy to destroy.  
  • Plasma Arc is Difficult to Permit. No commercial plasma arc gasification facilities exist in the United States. The air permitting requirements for plasma arc are un-tested, and therefore pose a risk.  Many environmental organizations site in opposition to plasma arc, and one could anticipate their opposition during permit hearings. In contrast,  currently the State of California is establishing a special permit for anaerobic digestion technologies to expedite the State approval process for new anaerobic digestion facilities.
  • Plasma Arc Produces More Greenhouse Gases than Anaerobic Digestion. All the carbon that processed in a Plasma Arc is converted to carbon dioxide, where in an anaerobic digestion process a considerable amount of the carbon stays as a sequestered woody material.  An incentive of plasma arc seems to be the generation of inert slag, the slag volume is insignificant to the original waste.  
  • Precautionary Principle. The precautionary principle invites that if an action or policy has a suspected risk of causing harm to the public or to the environment, in the absence of scientific consensus that the action or policy is harmful, the burden of proof that it is not harmful falls on those taking the action.
Plasma arc can reasonably be suspected of posing a risk of harm from emissions, and a choice to move forward seemingly conflicts with the precautionary principal's tenets. The identified emissions from staged incinerators (inclusive of plasma arc) include particulate matter, volatile organic compounds (VOCs), heavy metals, dioxins, sulfur dioxide, carbon monoxide, mercury, carbon dioxide and furans. Even small amounts of some of these toxins can be harmful to human health and the environment. Mercury, for example, is a powerful and widespread neurotoxin that impairs motor, sensory and cognitive functions. Dioxin is the most potent carcinogen known to humankind—to which there is no known safe level of exposure. Health impacts of dioxin include cancer, disrupted sexual development, birth defects, immune system damage, behavioral disorders and altered sex ratios.
  • Plasma Arc is Incineration and Our  Community Consensus is to Move Away From Incineration.  Multiple definitions from the USEPA as well as the  European Union include plasma arc as a form of incineration.  For example  U.S. EPA Hazardous Waste Regulations (40 CFR 260.10) states that “Incinerator” means any enclosed device that: (1) Uses controlled flame combustion and neither meets the criteria for classification as a boiler, sludge dryer, or carbon regeneration unit, nor is listed as an industrial furnace; or (2) meets the definition of infrared incinerator or plasma arc incinerator. There is strong agreement within our campaign to shut down the current incinerator, and plasma arc as a form of incineration, and would conflict with the campaign’s tenants.
  • Plasma Arc Does Not Generate Compost.  Our campaign began with a desire to retain local compost production.  While the approach has evolved to incorporate anaerobic digestion, the process still generates the desired output: a finished compost.  Plasma arc generates slag - definitely not the sought compost.

Advocates for plasma arc will assert that the footprint of the technology is small, and thereby might make the Initiative's request for land not be pertinent.  Our campaign would maintain that the extra land is reasonable to avoid use of an incineration technology. Furthermore, advocates of plasma arc will claim this is not “incineration” as the first state of the process is low oxygen and an “arc” is used rather than a flame.  The campaign believes that government agencies adequately vetted this issue, and that plasma arc is by definition a form of incineration. Advocates will also speak to the lower cost of plasma arc.  Plasma arc works optimally with dry feedstocks, and as moisture increases the energy inputs to dry the feedstocks are needed.  Ultimately, the costs of plasma arc are not clear, and given other considerations warrant no further consideration by our campaign.

Re-Imagining Wastewater as a Resource

posted Apr 15, 2011, 4:35 PM by Bob Wenzlau   [ updated Apr 16, 2011, 8:48 PM ]

Stanford University's Environmental Engineering program was invited to make recommendations toward Palo Alto's Long Term Waste Water Planning. A presentation by Dr. Craig Criddle of Stanford University was pivotal as it lays out a strategy whereby wastewater systems actually return green energy rather than being large energy consumers.  That's important, as the water pollution control plant is often a city's single largest energy consumer in electricity and gas.

How is this possible? In one respect it is simple, and applies the same methodology we are asking for the organic solids Palo Alto generates - its leaves, food waste and yard trimmings.  The wastewater, like the organic solids, would be treated anaerobically to yield energy, rather than the current aerobic/incineration process that requires enormous resource inputs.  Recall that the anaerobic process generates methane to be used as a "green" energy source, while aerobic processes consume oxygen and the energy yielded actually goes toward generating biomass (or sludge) that we now incinerate. A part of the current "aerobic" process is the incineration of the biosolids - an incinerator targeted for elimination by this campaign.

One of my Stanford classmates David Carmein, the Director of Design at Accio Energy, Inc., noted hat carbon dioxide is the product of the aerobic reactants, and that no useful work is harnessed in the process. "Each person's organic waste stream is part of their carbon load. It's nice to have a pathway, chemically and organizationally, that holds the potential to harness the stored energy and offset carbon use elsewhere."

As the wastewater treatment plant of the future emerges, and our campaign for anaerobic treatment of the organic solids succeeds, we will arrive at an integrated wastewater and organic treatment system that yields the City of Palo Alto significantly more energy. The new approach eliminates major energy consumption associated with the current methodologies. I liken this to merging two silos - the silo of "trash" with the silo "waste water" -- what emerges is a more appropriate view point - that all organics (solid or liquid) should be treated in an aligned and coordinated fashion. Reducing energy usage is a critical element to climate change.  Now not all the technology Stanford identifies is conventional, but much of it is, and allows implementation without fear of failure.  Dr. Criddle rightly identifies an approach of "baby steps" - moving and implementing the changes incrementally.  These "baby steps" should start now.

This emerging vision reinforces the confidence the campaign holds in the siting of our compost facility adjacent to the current wastewater plant. (Recall the Blue Ribbon Compost Task Force attempted to find another piece of land at the airport side of the water pollution control plant, and were thwarted by airport advocates.)  . In this vision, the same operator of the anaerobic digestion process for the liquids would be the same operator to manage the anaerobic processes for the solid organics -- an efficiency that would enhance operational effectiveness and lower overhead. Biologically combined operations generate advantageous synergies. Food waste appears to catalyze anaerobic processes for wastewater solids.  Conveniently the woody organics in yard trimmings provide a binder for the digested biosolids from the treatment of wastewater, and aid their final disposition as compost. The logic for this integration is compelling, and would certainly be economically validated as well. 

The remarks by Dr. Criddle, and joined by Dr. Perry McCarty reinforce confidence in the path this campaign has taken identifying land next to the water pollution control plant.  We will need land to accomodate the requirements of organics management, be they derived from waste water or placed into a compost bin for pickup.  We will need land to re-invent our waste water treatment systems -- one cannot turn it off to put in new anaerobic systems, instead one must build next to the old, and as such land would be needed.  We were short-sighted to let the land next to the waste water treatment plant be inadvertently converted to parkland, and we are wise to return it back for developing innovative sustainable methods.  

The vision we see in Palo Alto will and already is being extended elsewhere.  San Jose is incorporating the same mixed organics management in a project at their wastewater treatment plant. However, Palo Alto's vision can be more compelling as traditional aerobic treatment is replaced by energy yielding anaerobic processes. That is one of the important contributions Palo Alto brings to our region.

Therefore, please enjoy Craig's slides as he steps through the anaerobically based wastewater treatment plant (begin at slide 39), or watch a video cast of related remarks. We all appreciate Stanford's research capabilities and skills as a leader in environmental engineering -- we are lucky to be able to heed their input.

Palo Alto Green Energy and Compost Initiative Qualifies for November Ballot

posted Mar 24, 2011, 2:23 PM by Bob Wenzlau

NEWS RELEASE
March 24, 2011

Proponents of the Palo Alto Green Energy and Compost Initiative received word yesterday that their measure has qualified for the November ballot.  Donna Grider, Palo Alto's City Clerk, notified the campaign that the Santa Clara County Registrar of Voters tallied 5,128 valid signatures out of the 6,023 submitted.  4,356 valid signatures were needed to qualify the Initiative for the ballot.

The measure will ask voters to repurpose 10 acres of the 126-acre City landfill for a biological conversion facility that would turn the City's 60,000 tons per year of organic waste into green energy and compost once the landfill closes, which is expected to happen in the next year.  Otherwise, the entire landfill would be added to Byxbee Park, although no dedicated funds currently exist for that project.  10 acres is roughly 8% of the 126-acre landfill.

"We're excited, but not surprised," said Carolyn Curtis, the campaign's volunteer signature gathering coordinator.  "We had a tremendous team of more than 60 volunteers who dedicated hundreds of hours to collecting signatures, and we got great results.  Only 2% of respondents refused to sign our petition because they disagreed with it."

"This is the first activist effort like this I've ever been involved in, and I've ended up with great admiration and respect for everyone who has done the research, spent the time, and followed through until results were achieved," said Lois Fowkes, a long-time Palo Alto resident and signature gatherer for the petition.

The campaign received other good news on Monday night at a City Council study session when the consultant for a feasibility study on the project acknowledged that building an anaerobic digester in Palo Alto would be cheaper than the alternatives of continuing to incinerate sewage sludge and trucking food and yard waste to Gilroy and San Jose if the City pursued public versus private financing for the project.  The consultant also acknowledged that the feasibility study missed a number of costs for the off-site alternatives that will be included in the final draft of the study that is expected to come back to Council in June.  Big-ticket items include the cost of rebuilding the City's sludge incinerators, assigning a price on greenhouse gas emissions, and applying a contingency to the Gilroy and San Jose alternatives as was done for the Palo Alto option.

Cedric de La Beaujardiere, former Co-Chair of the City's Blue Ribbon Task Force on Composting and a proponent of the Green Energy and Compost Initiative, calculated that once these costs are included, building an anaerobic digestion facility in Palo Alto would save the City between $30 and $38 million over the first 20 years, and considerably more after that.

"We used the consultant's financial model and plugged in what we felt were conservative numbers to see what would happen if we included the cost of building new incinerators, a modest price on carbon dioxide emissions and a 15% contingency on the San Jose project," said de La Beaujardiere.  "The results showed that the Palo Alto option would save the City between $1.5 and $2 million per year.  And that's only for the first 20 years.  After that, the facility would be paid off, and the City would save considerably more money.  The price per ton for processing our organic waste would drop from $106 to $65 per ton.  Compare that to the $118 per ton it would cost to truck our waste elsewhere and continue incinerating our sewage sludge."

The preliminary feasibility study also concluded that, compared to trucking away the City's food and yard wastes and continuing to incinerate its sewage sludge, building an anaerobic digester in Palo Alto would reduce the City's carbon dioxide emissions by 12,000 tons per year, the equivalent of taking 1,600 cars off the road.  Since all the options would stop sending food waste to landfills, the anaerobic digester's total reductions from current practices are closer to 20,000 tons per year.

"Imagine that, we could make tremendous progress toward reaching our Climate Protection Plan goals while saving tens of millions of dollars," said de La Beaujardiere."

A chart of de La Beaujardiere's financial analysis is attached

Local Energy and Compost Project Can Save $4 Million to $38 Million

posted Mar 15, 2011, 12:24 PM by Bob Wenzlau   [ updated Mar 29, 2011, 12:08 AM by Cedric Pitot de La Beaujardiere ]

by Cedric de La Beaujardiere, Co-Chair, Palo Alto Blue Ribbon Task Force on Composting

An economic feasibility study indicates that through public-financing of a local Dry Anaerobic Digestion (DAD) facility, the City and rate-payers could save $4 million over a 20-year period, compared to the City's default plan, and save $8 million compared to sending our food-waste to San Jose. Furthermore, if the City received a 30% grant, we would save $19 million over the same period, or the equivalent of nearly $1 million per year.

These are some of the options being studied for what to do with the city's organic wastes (food scraps, yard trimmings, and sewage sludge) after the landfill closes next year, and in the face of needed upgrades to the City's aging sewage sludge incinerator. Currently, yard trimmings are composted in windrows at the landfill, food scraps are sent 53 miles away to Gilroy, and the sewage solids are incinerated. When the landfill closes, a 1965 ordinance says it must be converted to Byxbee Park, so the city's default plan (referred to as Case 3 in the study) is to send yard trimmings and food scraps for composting in Gilroy, and to continue to incinerate our sewage sludge. A variation on that would be to send the food-waste to a regional DAD soon to be built in San Jose (Case 2). In contrast, the local DAD facility would convert Palo Alto's organic wastes into renewable energy and compost, and reduce or greenhouse gas emissions by the equivalent of more than 11,000 metric Tons of CO2 each year, all while saving millions of dollars (referred to as Case 1a, Sensitivity Analysis #3). These net savings include debt financing for the capital construction costs.

What's more, there are reasons to believe that the savings will be even greater as the study is refined, because certain costs have not been included in the default option. These include increasing fuel costs for transportation and long-term maintenance costs of the sewage sludge incinerator past its remaining 10 year of life and to conform to likely new regulations on emissions such as mercury. Furthermore, while a 30% contingency has been applied to the local DAD option, no contingency was added to GreenWaste's rough quote of $85/ton to accept food-waste at its yet-to-be-built DAD facility in San Jose. A 30-year study-horizon would likely show even greater over-all savings for the local option.

Another local option is to use Wet Anaerobic Digestion (WAD) for our sewage and food-waste, and to compost the digestate with yard-trimmings. WAD is a proven technology for handling sewage, so it would not be subject to the high 30% contingency which was applied to the local DAD, and so would be an affordable alternative. However, compared to DAD, WAD uses more energy to move all the extra water around, so its net energy production and GHG offsets are lower.

The Palo Alto Green Energy and Compost Initiative currently being circulated would put on the ballot a vote to make 10 acres of the former landfill adjacent to the sewage treatment plant available for a facility such as the Dry Anaerobic Digester. If we get enough signatures to qualify the initiative for the ballot, and if the initiative passes, it would then be up to the City Council to choose a municipal organics management option.

When the study first came out, most people (including myself) got hung-up on the year-1 costs, and failed to notice the year-20 and 20-year total costs. Or we looked at the private financing of the facility (Base Case 1a), which made it seem like the Dry AD was more expensive, and didn't look as closely at the public financing option.

The study's Preliminary Cost Analysis Summary (http://www.cityofpaloalto.org/civica/filebank/blobdload.asp?BlobID=26102) indicates the following:


This shows that, over a 20 year period, the public financing of local Dry Anaerobic Digestion (Case 1a Sensitivity Analysis #3) saves $4 million compared to Case 3, and saves $8 million compared to Case 2. With a 30% grant (Case 1a Sensitivity Analysis #2), we save the rate-payers $19 million to $23 million, which comes out to about $1 million a year in savings through handling our wastes locally.

Phil Bobel has indicated that a Fluidized Bed is the technology that would likely be used if the sewage treatment plant were to try to keep the incinerator operational beyond its expected lifetime. He has estimated that a Fluidized Bed would cost in the “tens of millions of dollars”. From this estimate, we can expect to add about $20 million to the lifetime cost of Cases 2 and 3. If a 15% contingency is added to the estimate for processing food waste at the San Jose DAD, this adds $3.6 million to Case 2. CO2 emissions cost adders are estimated to range from $20/ton to $60/ton. If there were a conservative $20/ton CO2 emissions cost added to all the alternatives, they would cancel each other out except for the differences in emissions between them. Case 2 emits 11,796 Metric Tons more per year than Case 1a, increasing its cost over 20 years by $4.7 million. Case 3 emits 11,183 Metric Tons more per year than Case 1a, increasing its cost over 20 years by $4.5 million. In total, Case 2's cost increases by $28.3 million, and Case 3's by $24.5 million. If we then update the 20-year costs from the table above with these adjustments, the comparison becomes:


With these cost adjustments which we expect to see in the final feasibility study, the Local Dry AD option becomes the most affordable, whether privately or publicly financed. The publicly-financed Local Dry AD now saves the city and rate-payers between $30 million and $38 million over the 20 year study-horizon.

It's not that often that doing the right thing, taking care of our own wastes while reducing our green house gas emissions, also saves us money. This is a great opportunity for the City that we would be foolish and irresponsible to pass up.



A Presentation on Palo Alto Green Energy Initiative to Stanford Environment and Science

posted Feb 6, 2011, 8:03 PM by Bob Wenzlau   [ updated Feb 7, 2011, 11:56 AM ]

This presentation was provided as a seminar to Stanford's Energy and Environment weekly seminar series on February 4, 2011 by Bob Wenzlau.  Click image to play or use the following link.


 

 

Using Waste, Swedish City Cuts Its Fossil Fuel Use

posted Jan 31, 2011, 10:23 PM by Bob Wenzlau

The New York Times had an article entitled "With Peels and Pig Innards, a Swedish City Forgoes Coal and Oil." This is a beautiful example of what we're trying to do through this initiative process. 

You can find the article at the following link.

"KRISTIANSTAD, Sweden — When this city vowed a decade ago to wean itself from fossil fuels, it was a lofty aspiration, like zero deaths from traffic accidents or the elimination of childhood obesity."


Responding to Misleading Statements

posted Oct 25, 2010, 1:16 PM by Bob Wenzlau   [ updated Oct 25, 2010, 5:25 PM ]

The campaign for green energy and compost is enduring confusing statements from opponents hoping to derail our local sustainability movement. These statements are misguided, and reflect a nostalgic belief that we can ship our waste resources to other communities. One such letter in the October 14, 2010 edition of the Palo Alto Daily Post was so misleading that it warrants a response.   



A View from 2009 - An Overview of the Palo Alto Challenge

posted Sep 1, 2010, 6:27 AM by Bob Wenzlau   [ updated Sep 1, 2010, 10:31 AM ]

The is an article published in 2009 in Bay Area Green.  It did not have the benefit of the Blue Ribbon Task Force results, but was my effort to frame the discussion and choices.  The Task Force settled on more modest green house gas impacts, but the forecasts I made in this article still seem reasonable to me.



 

Converting Spoiled Land to Renewable Energy - The Federal Government Can Contribute Funding

posted Aug 18, 2010, 8:43 AM by Bob Wenzlau

Our local campaign synchronizes with national policy on green energy -- take old dump sites or contaminated land and use it for the production of sustainable energy.  There is a scarcity of local projects, and our campaign will provide needed national leadership.  Fortunately the federal government can put money into these projects to offset the local expense.  Learn more at this link.

Sludge to Green

posted Apr 1, 2010, 7:16 AM by Unknown user

This entry is cross-posted from www.Econosystemics.com, an intersection of economics and natural systems philosophy. 

With almost seven billion people on the planet, a very significant fraction of the planet’s organic matter now cycles through the human-centered econosystem.  The food we eat, and much of the food we waste, goes through our toilets and garbage disposals into our sewers, and (hopefully) into a wastewater treatment plant.  If that plant performs its function well, relatively clean reclaimed water is reintroduced to our natural water systems.  What remains is a thickened sludge that contains all of the organic materials.  How we dispose of this sludge is an important ecological and economic decision.

In the United States, over half of our sewage sludge is either composted or pelletized and then applied to our national farmlands.[1]  This closed cycle is much to be desired, once concerns about heavy metal, pharmaceutical and other chemical residuals are properly addressed.  Other choices for disposal include landfills, land reclamation, or incineration.   

Incineration utilizes large quantities of natural gas to burn the sludge.  Essentially 100% of the carbon contained in the organics is released as CO2 into the atmosphere.  Because this carbon is biogenic in origin, it is not counted as human-caused GHG emissions.  The substantial CO2 emissions from the natural gas used to incinerate the sludge, however, do contribute to increased GHG concentrations in the atmosphere.  

What remains after incineration is a quantity of ash, which contains all the non-combustible elements of the sludge.  Most of this is inert silicates and other elements that help make up the bodies of plants and animals.   But trace heavy metals will be present as well, often in concentrations that make the ash hazardous material that must be disposed of properly in a landfill.  If concentrations are lower, the ash may be used in the production of concrete or other permanent materials.

From the point of view of removing potential toxins from the environment, incineration is a very effective technology – presuming proper incineration where only low levels of toxins are released in the combustion smokestack emissions.  From ecosystem and econosystem perspectives, however, much value is lost in incineration.  Rather than return nutrients and essential trace minerals to the soil, we break the cycle and sequester them.   Our farmlands are supplemented instead with fossil-fuel manufactured fertilizers, and are gradually leached of the trace elements necessary for healthy plants and nutritious food.   We are using fossil fuels to incinerate our biosolids, and then using more fossil fuels to make fertilizer.   This is not a sustainable solution.

Dumping sewage sludge into a landfill also breaks the natural biological cycle and sequesters the organic and inorganic elements away from the ecosystem.  Decomposition of this sludge in the landfill will also generate large amounts of methane gas, much of which will escape to the atmosphere before any methane recovery system can be put into place.   The increasing expense of landfills and transportation also makes landfills a poor choice for organics disposal. 

Higher population densities combined with a more environmentally aware populace has made Europe the leader in innovative methods for environmentally and economically effective organics recycling.   For several decades, European countries have been rapidly adopting variations of a technology known as “anaerobic digestion” to treat their wastewater sludge, increasingly combined with source-separated yard waste and the organic fraction of municipal solid waste[2]. 

Although the term “anaerobic digestion” may not be familiar to many people, all of us are intimately familiar with the process because we carry around a similar facility inside our bodies.   A wide range of bacterial microbes in the oxygen deprived (anaerobic) interior of our digestive tracts break down the complex organics of the food we eat, and feed our bodies as well as theirs with the nutrients.  They do not do a perfect job, however, and our feces still contains abundant, energy rich organic material. 

A municipal anaerobic digestion facility utilizes a different set of bacterial microbes that live at much higher temperatures than those in our bodies.   Those microbes thrive on our sewage, foodwaste, compostable paper and yard trimmings, generating biogas rich in methane (natural gas), as well as an end product of high-value compost.

The latest generation of “dry” anaerobic digestion facilities are able to process the full range of municipal organics and are most favored for new facilities, delivering two strong benefits to the econosystem:  biogas that has value as a “green” substitute for fossil natural gas, and high quality compost that restores soil health and productivity. The relatively high temperature microbial digestion process destroys all pathogens and most complex organics.  Trace heavy metals not removed in the wastewater treatment process will remain in the finished compost, but concentrations are much lower than in the highly concentrated ash from incineration. 

The EPA has set standards for heavy metals concentrations in compost, but in Europe anaerobic digestion facilities are able to meet far more stringent standards for compost from anaerobic digestion, allowing the use of anaerobic digestion compost on food crops.  Standards for copper and chromium, for example, are up to ten times more restrictive than the EPA standard.[3]  Concentrations of heavy metals and toxic molecules will of course also depend on levels in the original wastewater, and testing of compost should always be performed regularly.  By Europe’s example, however, there does not seem to be a residual toxins risk that outweighs the environmental benefits of recycling our organic wastes.  

Water extracted from the anaerobic digestate prior to final aerobic composting is returned to the wastewater treatment plant for settling, oxidization, and testing before being released into the natural watershed.   Again, Europe’s experience suggests that this digester effluent does not negatively affect the quality of water ultimately released to the natural watershed.

Combining yard clippings, compostable paper, and foodwaste with biosolids significantly increases biogas production and can improve the quality of finished compost as well, by improving the balance of carbon to nitrogen inside the digester.  However, some facilities allow for two simultaneous digestion processes, one with and one without sewage biosolids.   This may diminish the actual biological value of both compost products, but a “no sewage” product may have greater economic value as an “organic” compost with essentially no trace contaminants.

Advanced anaerobic digestion is the best technology available for cost-effectively and environmentally recycling municipal organic wastes back into the ecosystem and returning value to the econosystem.  It has not been deployed in the United States simply because until recently our economic calculations have not incorporated environmental costs and benefits.  When the values of green energy, GHG reductions and organics recycling are added into the equation, anaerobic digesters are the “natural” solution.


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