The Larger Impacts of Biochar from Biosolids: CO2 Reductions Using The BEAM Model, The Elimination of Contaminants of Emerging Concern, and the Creation of a Superior Product for Land Application
Introduction
Sustainability, toxicity, and increased regulations are forcing wastewater treatment plants (WWTP) to reevaluate the way they manage and dispose of the solid residuals derived from their processing. The long-held practices of our industry have left many of us unsure of how to best prepare for the necessary shifts in our management strategies. Simultaneously, misconceptions of emerging technologies are slowing the progress of these necessary shifts. To help move the industry toward a more progressive, cleaner future, this essay is published with the intention of dispelling any misconceptions about the process of creating biochar from biosolids at an industrial scale, shed light on the myriad benefits of this management strategy, explain the sustainable life cycle of this approach compared to land application of biosolids (proven through a BEAM Model study), and dispute the perceived negatives concerning its agricultural applications.
It goes without saying that, as a drying and pyrolysis technology company, we have a vested interest in biosolids biochar succeeding. We do create pyrolysis technologies and the success of this material creates success for our efforts. We do not seek to make any claims directly supporting our technology in this essay without the backing of internal and independent rigorous testing, third party scientific studies, and insights from Dr. Jorge Paz-Ferreiro, a scientist who has been researching agricultural behaviors of biosolids, biosolids biochar, and NPK fertilizer amended soils for over a decade. Through this essay, we seek to provide undeniable evidence that creating biochar from biosolids is one of the safest and most sustainable ways to manage the material to date. We encourage the readers of this essay to review our research as well as that of which has been conducted by others in the field and develop an informed decision based upon this information.
With over 30 installations globally and nearly ten years of first-hand experience with these processes, we have gained valuable knowledge of the political, economic, and scientific landscape of pyrolysis technology globally. As the first, and currently only, operational full-scale biosolids pyrolysis system in the US, we have by necessity become experts on every aspect of the regulations, process, inputs, and outputs of our technology and of competing technologies. It is with this experience and knowledge that we feel uniquely positioned to explain the important concerns about biosolids land application and the benefits of biosolids biochar below.
Hazards of Biosolids Use in the US
There are many positive impacts that come from land applying biosolids. It is certainly an advantageous practice when compared to the harmful environmental effects of landfilling or incinerating the tremendous amount of sludge we produce every year. It returns valuable nutrients to the soil and can improve overall soil conditions. There are, however, many disadvantages to this practice that have come to the attention of society and our industry in recent years. This essay will assume that its readers understand the benefits of land application of biosolids and will instead focus its attention on illuminating some of the disadvantages of this practice, both in general and in comparison to land applying biochar made from biosolids.
Since our wastewater is a reflection of society, the primary danger in land applying biosolids comes from a group of chemicals/materials that tend to concentrate at our wastewater treatment plants known as contaminants of emerging concern (CEC’s). CEC’s include PFAS, pharmaceuticals, personal care products, and microplastics, all of which have been deemed ‘forever chemicals’ by their pervasive ability to bioaccumulate in our ecosystems and in our own bodies. As noted by the EPA, these chemicals present risks to the health of our planet, our ecosystems, and to our species directly (EPA 2016)¹. Human health impacts from CECs include low infant birth weights, impacted immune systems, cancer, thyroid hormone disruption, and more. The risk of these chemicals is particularly alarming as research has now shown that almost the entirety of our global population has been exposed to many of them. Our bodies absorb these chemicals, such as PFOA and PFOS, accumulating them over time through our food, water, and immediate material context (EPA 2016)¹. Although EPA has an on-going process to continually evaluate the quality of biosolids and the risks associated with CECs, the regulatory review process is an immense undertaking and exceptionally far behind.
Wastewater facilities do not generate CECs, but are a unique bottleneck for them. Therefore biosolids are uniquely contaminated with these chemicals. By applying biosolids to land that our food grows on, our water runs through, and our children play on, we compound our own exposure to these bioaccumulating chemicals while radically disrupting ecosystems (EPA 2014)². Rather than a dangerous point of contamination, we feel that the unique context that WWTPs provide for the accumulation of CEC’s presents a responsibility and opportunity to rid our ecosystem and population of these contaminants.
While CEC’s are of significant concern and garner the bulk of headline coverage, the impact from repeat application of heavy metals on our soils and into our ecosystems caused by their presence in biosolids should not be forgotten. While the US has adopted a risk-based approach to setting allowable heavy metals concentrations in soils treated with biosolids, European standards are considered more protective since they rely primarily on a regulatory approach based on “no net degradation.” In other words, European standards focus on not increasing the existing soil concentration of heavy metals. Although EPA metals content compliance can be achieved without pyrolysis, we are able to greatly decrease the release of heavy metals in soils amended with biosolids biochar, an important and vital distinction between the behavior of the two materials.
Well summarized by Sally Brown in this writeup (Metals in Biosolids, Sally Brown, University of Washington)³, the EPA limits on heavy metals in biosolids are extremely high compared to today’s achievable levels across most municipal wastewater plants. While this makes compliance easy for WWTPs, it fails to encourage our industry to further reduce heavy metal levels. The below table from a study titled Biochar from Biosolids Pyrolysis: A Review (Paz-Ferreiro et. al., 2018)⁴ describes acceptable levels of heavy metals in the European Union and the US, shedding light on the significantly high acceptability of heavy metals in the US compared to other countries. In many cases described in the table America allows biosolids containing as much as over 4 times the amount of heavy metals than the allowances of the European Union, and in no case are the US limits the more stringent ones. While the US has made strides in regulating toxicity over the many decades since we have become aware of the effects of industrial contaminants, we can clearly see a better and achievable metric to rise to.
Not only does biochar from biosolids show decreased levels of some metals as compared to biosolids, but it also significantly decreases any potential plant uptake of heavy metals. After pyrolysis, metals that remain in the biochar are immobilized, due to some processes including precipitation and adsorption in the surface of this material, and are much less bioavailable¹⁰. This would result in much less risk of leaching into groundwater and being taken up by plants when compared to biosolids, as shown by a study conducted in 2012 (Méndez et. al., 2012)⁵. A second study (Méndez et. al., 2017)⁶ shows that even when metals content is increased in biochar from biosolids, metals uptake by plants decreases as compared to soils amended with biosolids, not to mention the study finding a mix of peat and biosolids biochar more than doubling plant mass and extending root length. In short, much of the metals that are present are much less bioavailable in biosolids biochar as compared to biosolids land application, meaning that ecosystems and food crops will reduce the uptake of these metals. Bioforcetech conducted a study internally to confirm the extensive literature on this topic. The table below shows the results of leaching of heavy metals from biosolids biochar with both the TCLP (Federal test and limits) and the STLC (California test and limits). The results of this testing show that biosolids biochar allows extremely low amounts of heavy metal leachate, and the accompanying research explains the ability of the material to hold on to these metals and to prevent migration into ground and surface waters.
Agronomics
Particularly in the United States, in recent years researchers have been largely in support of applying biosolids directly to agricultural crops because of the benefits visible in the plant and soil health. These researchers have limited the scope of their studies and have failed to consider the larger environmental and human health implications discussed earlier in this essay. Focusing on studies investigating factors like the growth rate or the bioavailability of nutrients contained in the material while ignoring the human health and environmental risks that land application of biosolids poses does a disservice to the industry. A further disservice is done by not focusing on the benefits of biosolids biochar as compared to biosolids when utilized as a soil amendment. While the benefits of growth rate/soil fertility in biosolids amended soil do outweigh unamended soils, the following research will show that the benefits of applying biochar from biosolids to soils far exceed that of applying biosolids. This research, coupled with the proven removal of CEC’s from biosolids through pyrolysis, makes biochar from biosolids a preferred choice for land application.
It is true that there is a comparatively small number of studies comparing crop yield in soils amended with biosolids and with biochar from biosolids. However small the pool of study, the data strongly confirms that biosolids biochar can lead to nearly the same increase in crop yields as conventional fertilizers (Hossain et al., 201⁰¹¹; de Figueiredo et al., 202⁰¹³), feat biosolids alone cannot achieve. Biochar made from biosolids has been shown to outperform biosolids, used in place of peat in the case of the referenced study, as a component in growing media (Méndez et al., 2017)⁴. In this study, soils with biosolids biochar resulted in as much as 10 times more microbial activity, increased plant yield, and reduced plant uptake of heavy metals.
Further studies found similar benefits including less CO2 emissions from soil (Gascó et al., 2012)¹², improved soil microbial activity (Paz-Ferreiro et al., 2012)¹⁴, and reduced heavy metal mobility (Méndez et al., 2012)⁵.
In the graph below from a study published 2020 (de Figueiredo et. al., 2020)⁷, it is clear that biosolids biochar performs incredibly well as a soil amendment for the increased production of plants. It is also of note that the NPK group, a test plot using full synthetic fertilizers, had fertilizer applied to the land every year during a four years span while the biochar was only applied in the first and second year. Despite the lower frequency of application, the biochar still performed at levels well above a control plot and without any statistically significant difference compared to NPK fertilized soils.
Oftentimes the higher levels of nitrogen in biosolids as compared to biochar is seen as a benefit, assuming that plants will have more nitrogen available from biosolids amended soils. This, however, might not be the case. Biochars from biosolids have a limiting effect, which could result in more plant N availability. Research in multiple studies (Paz-Ferreiro et. al., 201⁸⁴. Célio de Figueiredo et. al., 202⁰⁸) has shown that biochar made from biosolids can improve nitrogen use efficiency, even two years after ceasing amendment. This demonstrates that biosolids biochar is a highly beneficial alternative to synthetic and fossil fuel-intensive NPK fertilizers, and can help diminish the environmental problems (mainly N leaching) associated with these synthetics. Further sustainability aspects of the production and use of biosolids biochar are covered below.
Sustainability of Biochar from Biosolids: BEAM Model Study
Because the parameters in which pyrolysis is undergone can have such an influence on the resulting product, we will speak about our own biochar, OurCarbon™, in detail with our own testing and experience and will reference studies of biosolids biochar in general to communicate the possibilities of other pyrolysis technologies. We find it important to note that undergoing pyrolysis of biosolids does not always yield the same biochar as end products can vary in material makeup, hydrocarbon content, and more. Pyrolysis and biochar are two general terms that will need tighter definitions with terms that describe differentiation in quality, content, and end-use applications as the technology scales and more material is produced.
OurCarbon™, the name of the biochar that we produce from our technology, has been tested by third-party labs in contract with both Bioforcetech and to outside parties including the United States Environmental Protection Agency for the presence of CECs and other contaminants. We are proud to report that internally contracted testing and externally conducted EPA testing over the past two years confirm our technology reduces all PFAS, PFOA, PFOS, and other CECs to non-detectable levels in our biochar, effectively eliminating these compounds.
Regarding heavy metals, OurCarbon is considered Excellent Quality (EQ) by the EPA and has metals contents below acceptable levels for land application in both the US and European Union. Although a few metals, such as mercury, volatilize in our system and are of undetectable levels in OurCarbon™. However, they are captured in our various filters and scrubbers that have allowed us to achieve operating permits in the Bay Area Air Quality Management District, one of the most stringent air quality regulators in the world.
Regarding carbon footprint, a BEAM model study conducted by Northern Tilth in March of 2021 found that the production of biochar from biosolids through the BFT system results in a reduction of over 10 tons of CO2e per ton of OurCarbon produced This major reduction in greenhouse gas emissions is comprised primarily of elimination of fugitive methane emissions from landfilling, the use of recovered energy in our process, and the fixing of volatile carbons in place during pyrolysis. The superior carbon sequestration benefits from the agronomic use of biochar and the ability to replace other fossil fuel intensive materials with biochar like OurCarbon add further emission prevention potential. The study also concludes that our complete system, even before product application, is a lower carbon footprint option compared to both direct land application and composting of biosolids. It is of note that the environmental impact of disposal practices outlined in the BEAM model LCA are representative of recent regulations rather than current practices which in many cases do not yet meet these new regulations. We appreciate the conservative estimates and regulatory reliance of the BEAM model and are encouraged by the results of a study performed with such a stringent set of parameters.
Generally, the drying and pyrolyzing of biosolids are seen as processes that require large amounts of external energy, making them carbon-intensive. This reality can discourage WWTPs from installing drying or pyrolysis systems for fear of increased utility bills and carbon footprints. While this is true for many drying and pyrolysis technologies proposed and on the market, this is not the only way to create biochar from biosolids. In contrast, biodrying and pyrolysis utilized in tandem can eliminate the environmental impact associated with these tasks, rendering the entire process energy neutral. The BFT system serves as an example of this net-zero technology.
Instead of utilizing external energy to dry and pyrolyze a feedstock, the BFT system leverages the energy within the material itself. The BioDryer cultivates thermophilic bacteria in biosolids that generate enough heat to evaporate the bulk of the water in each batch with only warm air as an input. Second, the P-Series Pyrolysis utilizes syngas as it forms to generate renewable energy. After the syngas is oxidized at Lamba 1.05 in the flameless combustion chamber (FLOX®), the exhaust gases are passed through the annular space between the central tube and the outer casing of the pyro-reactor, allowing for a self-sustained 24/7 heating system with no external energy required. Any excess thermal energy is directed back to the BioDryer to add even more efficiency to the system: nothing is wasted.
To reinforce the beneficial life cycle of biosolids pyrolysis in general, please refer to this 2018 study (Patel et. al., 201⁸⁹) which concludes that pyrolyzing biosolids at a moisture content of less than 50% yields an energy positive life cycle, meaning that it produces more energy than it requires. The BFT BioDryer brings biosolids from as low as 17% solids to over 55% solids passively before the material is sent through pyrolysis. Even using conventional methods of pyrolysis that incorporate external energy sources, a method the BFT system uses as sparingly as possible, a 2014 study (Miller Robbie et. al., 2014)¹⁵ concludes that adding pyrolysis to a WWTP management stream reduces greenhouse gas emissions by 26%.
Economics/Feasibility of Pyrolysis Technology
The economics of current land application practices, landfilling, and incineration of biosolids are not moving in favor of either management method. Increased tipping fees at landfills, longstanding public concerns with CECs present in typically treated biosolids, and increased environmental regulation are leading to a correlated increase in disposal cost with a decrease in physical locations available for disposal. Europe is already seeing tipping fees many times larger than ours, in the range of €120 — €200 per wet ton. It is only a matter of time before the US follows.
The economic incentive of utilizing a biodrying and pyrolysis system together in a wastewater treatment context goes beyond eliminating tipping fees and transportation costs. Biosolids can be digested or undigested before entering the drying phase, eliminating the need for purchasing or upgrading a digester system and allowing plants flexibility if digesters are down for maintenance. By utilizing the energy produced in the pyrolysis process and circulating it back to the dryer the system uses net-zero energy, drastically reducing electricity and heat energy costs. At the same time, the pyrolysis system generates valuable biochar which can be sold for a profit. The BFT system offers a free handling service, organizing transportation and sales of the material while sharing a portion of the profits back with the municipality operating the technology. What was once a cost is now a value.
Conclusion
While the toxicity of biosolids is not to the fault of the biosolids industry, it is a reality that our growing scientific understanding of the impact of these contaminants will increasingly force our industry to reckon with it. The only two ways to clean the wastewater stream of these contaminants is to wait until changes are made in the many industries and contexts that add contaminants to the wastewater stream or to take the initiative to create cleaner outputs ourselves. Although governments should and must address source control to eliminate CECs from our wastewater, this is a tremendous undertaking and will take an immense amount of time. But without cleaner inputs immediately — an impossible ask — dry biosolids will continue to contain CECs for decades to come. Therefore the practical reality is that, without some form of advanced treatment of biosolids, CECs will continue to make their way back into the environment, our food supplies and the world’s population. At present the only proven way to eliminate PFAS and other CECs from this cycle is through advanced thermal processes such as pyrolysis. Furthermore, continuing to engage in practices with increasing costs and stricter regulation every year is not a sustainable economic strategy for the management of biosolids. The industry must adapt to this new understanding.
We see WWTPs, as accumulation points for both contaminants and resources, as an opportunity to separate the two, eliminate the former, and leverage the latter. The result of our labors as an industry can either continue to proliferate contaminants into our environment or create a cleaner world for future generations.
References
1) EPA 2016: PFOA & PFOS Drinking Water Health Advisories https://www.epa.gov/sites/production/files/2016-06/documents/drinkingwaterhealthadvisories_pfoa_pfos_updated_5.31.16.pdf
2) EPA 2014 : Columbia River Strategy for Measuring, Documenting and Reducing Chemicals of Emerging Concern, https://www.epa.gov/sites/production/files/2014-07/documents/columbia-river-cec-strategy-july2014.pdf
3) Metals in Biosolids, Sally Brown, University of Washington. http://faculty.washington.edu/slb/docs/basics/Metals_in_biosolids.pdf
4) Biochar from Biosolids Pyrolysis: A Review. Jorge Paz-Ferreiro , Aurora Nieto , Ana Méndez , Matthew Peter James Askeland and Gabriel Gascó. https://maba.memberclicks.net/assets/research-updates/April_2021/1%20-%20Paz-Ferreiro2018-Biochar%20from%20Biosolids%20Pyroly.pdf
5) Effects of sewage sludge biochar on plant metal availability after application to a Mediterranean soil: A. Méndez, A. Gómez, J. Paz-Ferreiro, G. Gascó. 2012. https://coek.info/pdf-effects-of-sewage-sludge-biochar-on-plant-metal-availability-after-application-t.html
6) The effect of sewage sludge biochar on peat-based growing media: A. Méndez, E. Cárdenas-Aguiar, J. Paz-Ferreiro, C. Plaza & G. Gascó. 2017
https://www.tandfonline.com/doi/full/10.1080/01448765.2016.1185645?scroll=top&needAccess=true
7) Direct and residual effect of biochar derived from biosolids on soil phosphorus pools: A four-year field assessment: Cícero Célio de Figueiredo Thamires Dutra Pinheiro, Luiz Eduardo Zacanaro de Oliveira, Alyson Silva de Araujo, Thais Rodrigues Coser, Jorge Paz-Ferreiro. 2020.
https://maba.memberclicks.net/assets/research-updates/April_2021/4%20-%20Char%20and%20P.pdf
8) Sewage sludge biochar increases nitrogen fertilizer recovery: Evidence from a 15N tracer field study: Cícero Célio de Figueiredo, Éllen Griza Wickert, Helen Cristina, Vieira Neves, Thais Rodrigues Coser, Jorge Paz‐Ferreiro. 2020.
https://onlinelibrary.wiley.com/doi/10.1111/sum.12672
9) Transformation of biosolids to biochar: A case study:Savankumar Patel, Sazal Kundu, Jorge Paz‐Ferreiro, Aravind Surapaneni, Leon Fouche, Pobitra Halder, Adi Setiawan, Kalpit Shah. 2018.
https://aiche.onlinelibrary.wiley.com/doi/abs/10.1002/ep.13113
10) Characteristics of leachate from pyrolysis residue of sewage sludge. Laboratory of Solid Waste Disposal Engineering, Graduate School of Engineering, Hokkaido University, Kita, Nishi, Kita-ku, Sapporo 060–8628, Japan
https://www.sciencedirect.com/science/article/abs/pii/S0045653507003165
11) Influence of pyrolysis temperature on production and nutrient properties of wastewater sludge biochar: Mustafa K Hossain, Vladimir Strezov, K Yin Chan, Artur Ziolkowski, Peter F Nelson. 2010 https://pubmed.ncbi.nlm.nih.gov/20870338/
12) Thermal analysis of soil amended with sewage sludge and biochar from sewage sludge pyrolysis: G. Gasco, J Paz-Ferreiro, A Mendéz. 2012 https://www.researchgate.net/publication/226086064_Thermal_analysis_of_soil_amended_with_sewage_sludge_and_biochar_from_sewage_sludge_pyrolysis
13) Direct and residual effect of biochar derived from biosolids on soil phosphorus pools: A four-year field assessment: Cícero Célio de Figueiredo, Thamires Dutra Pinheiro, Luiz Eduardo Zacanaro de Oliveira, Alyson Silva de Araujo, Thais Rodrigues Coser, Jorge Paz-Ferreiro
https://pubmed.ncbi.nlm.nih.gov/32540669/
14) The Effect of Biochar and Its Interaction with the Earthworm Pontoscolex corethrurus on Soil Microbial Community Structure in Tropical Soils: Jorge Paz Ferreiro, Liang Chengfei, Shenglei Fu, Ana Méndez. 2012 https://www.researchgate.net/publication/276170761_The_Effect_of_Biochar_and_Its_Interaction_with_the_Earthworm_Pontoscolex_corethrurus_on_Soil_Microbial_Community_Structure_in_Tropical_Soils
15) Life cycle energy and greenhouse gas assessment of the co-production of biosolids and biochar for land application: Leslie Miller-Robbie, Bridget A.Ulrich, Dotti F. Ramey, Kathryn S. Spencer, Skuyler P. Herzog, Tzahi Y.Cath, Jennifer R. Stokes, Christopher P. Higgins.
