Environmental Comparison of Biochar and Activated Carbon for

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Environmental Comparison of Biochar and Activated Carbon for Tertiary Wastewater Treatment Kyle Andrew Thompson, Kyle Koyu Shimabuku, Joshua P. Kearns, Detlef R. U. Knappe, R. Scott Summers, and Sherri Michelle Cook Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 22 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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Environmental Science & Technology

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Environmental comparison of biochar and activated carbon for tertiary wastewater

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treatment

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Kyle A. Thompson,1 Kyle K. Shimabuku,1 Joshua P. Kearns,1,2 Detlef R. U. Knappe,2 R. Scott

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Summers,1 and Sherri M. Cook*,1

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*

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7317, address: 4001 Discovery Drive, 607 UCB, Boulder, CO 80309

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Corresponding author email: [email protected], phone: 303-735-7288, fax: 303-492-

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Department of Civil, Environmental, and Architectural Engineering, University of Colorado

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Boulder, Boulder, CO 80309 2

Department of Civil, Construction, and Environmental Engineering, North Carolina State

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University, Raleigh, NC 27695 Table of Content Art

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ACS Paragon Plus Environment


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Abstract

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Micropollutants in wastewater present environmental and human health challenges. Powdered

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activated carbon (PAC) can effectively remove organic micropollutants, but PAC production is

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energy intensive and expensive. Biochar adsorbents can cost less and sequester carbon; however,

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net benefits depend on biochar production conditions and treatment capabilities. Here, life cycle

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assessment was used to compare 10 environmental impacts from the production and use of wood

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biochar, biosolids biochar, and coal-derived PAC to remove sulfamethoxazole from wastewater.

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Moderate capacity wood biochar had environmental benefits in four categories (smog, global

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warming, respiratory effects, non-carcinogenics) linked to energy recovery and carbon

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sequestration, and environmental impacts worse than PAC in two categories (eutrophication,

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carcinogenics). Low capacity wood biochar had even larger benefits for global warming,

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respiratory effects, and non-carcinogenics, but exhibited worse impacts than PAC in five

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categories due to larger biochar dose requirements to reach the treatment objective. Biosolids

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biochar had the worst relative environmental performance due to energy use for biosolids drying

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and the need for supplemental adsorbent. Overall, moderate capacity wood biochar is an

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environmentally superior alternative to coal-based PAC for micropollutant removal from

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wastewater, and its use can offset a wastewater facility’s carbon footprint.

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Introduction

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Wastewater treatment facilities (WWTFs) seek to reduce the negative impacts of organic

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micropollutants (e.g., antibiotics and endocrine disrupting compounds) on aquatic life and

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downstream drinking water quality by removing these compounds during treatment.1,2,3

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Adsorption with powdered activated carbon (PAC) is an effective treatment option4 for tertiary

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wastewater treatment that has shown lower environmental impacts than other options (e.g.,

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reverse osmosis, ozone/ultraviolet-light oxidation).5 However, PAC also has negative

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environmental impacts, especially because it is commonly generated from non-renewable coal

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and requires energy-intensive thermal activation to develop adsorption properties.6 A potentially

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lower cost and environmentally friendlier alternative is biochar, which is carbonized biomass not

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subjected to further physical or chemical activation.

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Biochar adsorbents have demonstrated sorption capacity for agrichemicals,7,8 pharmaceuticals

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and personal care products,9,10 and endocrine disrupting compounds.11 Biochar ($350-$1,200 per

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tonne12) costs less than PAC ($1,100-$1,700 per tonne13), on a mass basis, and can have

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environmental benefits including energy co-production,14-16 carbon sequestration,14-20 and bio-

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waste valorization (e.g., by using yard,15,17 food,16,21 and agricultural wastes22 and biosolids16,23

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as feedstocks). The adsorption capacity of biochars for organic micropollutants, though, ranges

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from negligible to similar to that of PAC depending on solution characteristics, precursor

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material, and biochar production conditions.23-26 Since adsorption capacity determines the mass

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of adsorbent needed for treatment, it also influences cost and environmental impacts. To date, the

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relationships between biochar adsorption capacity, cost, and environmental impacts, as well as

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the comparison with prevalent adsorbents such as PAC have not been quantified.

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Previous life cycle assessments (LCAs) have identified important biochar properties that affect

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environmental performance. The feedstock moisture content, energy content, and alternative uses

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and production conditions have been found to influence overall environmental performance.16

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Many studies quantified only global warming impact or net energy production,15-18,20,27 so

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analysis using a broader set of environmental impacts will help to further identify influential

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properties and environmental trade-offs. In addition, most biochar LCAs have focused on the

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application of biochar as an agricultural soil amendment and energy generation co-product,14-

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20,27-29

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engineered wastewater treatment applications.

but no studies to-date have assessed comparative life cycle impacts of using biochar in

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Future requirements for micropollutant removal from wastewater effluents are expected in

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coming years,3,30 and the implementation of new treatment capabilities must be balanced against

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overall environmental protection (e.g., minimizing energy requirements and associated air

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pollution of new treatment technologies). The objective of this study was to quantify relative

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environmental impacts of using biochar adsorbents for tertiary wastewater treatment made from

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wood and biosolids compared to coal-derived PAC. LCA methodology was used to assess

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impacts associated with adsorbent generation, use, and disposal for removal of sulfamethoxazole

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(SMX), a prevalent human and livestock antibiotic, from wastewater effluent. Uncertainty and

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sensitivity analyses were conducted to determine the results’ sensitivity to modeling

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assumptions. This study aims to elucidate the most effective ways to reduce environmental

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impacts of and to identify environmental tradeoffs between adsorbents used for micropollutant

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control in tertiary wastewater treatment. In addition, this study aims to assist in the selection of

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environmentally preferred adsorbents from the perspective of feedstock selection and production

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conditions. 4


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Methods

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The production and use of PAC, wood biochar, and biosolids biochar for tertiary wastewater

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treatment was evaluated using a comparative LCA methodology following the ISO 14040

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framework.31 The main processes in each scenario and the LCA system boundary are

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summarized in Figure 1. The functional unit was 75% removal of sulfamethoxazole (SMX) from

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47,300 m3/day (12.5 MGD) of secondary wastewater effluent over 40 years. The 75% SMX

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removal target was chosen as a representative goal because: (i) SMX has a common occurrence

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in wastewater and in surface water at concentrations shown to endanger aquatic ecosystems.32-34

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(ii) For the sorbent doses used in this study, 75% SMX removal is expected for any SMX

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concentration typically found in wastewater effluent.23 (iii) Emerging regulations on

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micropollutants are considering 80% as a general target,3 and SMX has a relatively low tendency

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for adsorption compared to other organic micropollutants present in wastewater effluent;35,36

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therefore, 75% SMX removal is expected to result in greater than 75% removal of other

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micropollutants, which is representative of proposed removal targets. The Boulder, Colorado

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WWTF provided data on biosolids quantity, composition, and fate,37 and its location was used

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for hauling distances.

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Figure 1. Process flow diagram of the main processes in each of the PAC, wood biochar, and biosolids biochar scenarios for tertiary wastewater treatment.

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The adsorbent dose necessary to achieve 75% SMX uptake (dose75) from wastewater effluent

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over a 60-minute contact time was experimentally determined in previous bench-scale work.23 In

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that previous study, SMX adsorption was quantified at initial concentrations ranging from 50

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ng/L to 1 mg/L, and SMX proportional removal was independent of concentration at or below 10

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ug/L.23 Typical SMX concentrations in wastewater effluent are ≤0.178 µg/L in China,38 ≤2.00

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µg/L in Germany,39 and ≤3.25 µg/L in the USA.40 The dose75 for the commercial bituminous

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coal-based PAC was 70 mg/L.23 Using wood and biosolids as feedstocks, biochars were

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generated using different pyrolysis conditions and classified based on their experimentally

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determined adsorption capacities (i.e., dose75) relative to PAC: low capacity (600 mg/L23), and

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moderate capacity (150 mg/L23). While these biochars had adsorption capacities lower than PAC,

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they have high adsorption capacities compared to many other biochars.23 The low capacity wood 6


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biochar was generated in a full-scale pyrolysis facility where pine wood chips were exposed to a

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temperature gradient from 400 to 1200˚C.41 The moderate capacity wood biochar was produced

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from pine wood pellets in a 1-gallon top-lit updraft gasifier under high draft (850˚C)

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conditions.23 The properties of the moderate capacity biosolids biochar were estimated using data

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from biosolids and wood biochars. Pyrolysis mass yield and elemental composition of each

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biochar were based on measurements from a previous study23 (see Table S2). Bench-scale batch

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reactors were used to experimentally determine the aluminum sulfate (alum) dose required to

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remove an adsorbent from solution and achieve a final turbidity less than 1 NTU (ASTM

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D2035).42 A 10 mg/L alum dose was sufficient for all adsorbents. The apparent density of each

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adsorbent was determined using a tapped apparent density standard method.43 Table S2 describes

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each adsorbents’ properties.

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Adsorbent Scenarios. Six main scenarios are described below: low-impact PAC, high-impact

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PAC, moderate capacity wood biochar, low capacity wood biochar, moderate capacity biosolids

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biochar supplemented with low-impact PAC, and moderate capacity biosolids biochar

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supplemented with moderate capacity wood biochar. Early in the analysis it was found that the

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mass of biochar generated from biosolids would be insufficient to meet the 75% SMX removal

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objective and therefore biosolids biochar would need to be supplemented with other adsorbents.

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The comparative LCA system boundary (Figure 1) does not include activities and processes

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common to all scenarios (e.g., the production of secondary wastewater effluent and dewatered

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biosolids). Life cycle stages included raw material acquisition, production, use, and hauling, but

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not end-of-life impacts (e.g., emissions from a landfill). For each scenario, the amounts and types

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of materials and energy required to achieve the functional unit were quantified and used to

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estimate life cycle emissions with data from the US-EI v.2.244 and Agri-footprint45 life cycle 7



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inventory (LCI) databases. All emissions were translated into ten environmental impact

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categories using the EPA’s Tool for the Reduction and Assessment of Chemical and Other

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Environmental Impacts (TRACI).46

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depletion (kg CFC-11 equivalent), global warming (kg CO2 equivalent), smog (kg O3

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equivalent), acidification (kg SO2 equivalent), eutrophication (kg N equivalent), carcinogenics

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(comparative toxic units, CTU), non carcinogenics (CTU), respiratory effects (kg PM2.5

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equivalent), ecotoxicity (CTU), and fossil fuel depletion (MJ surplus).

The 10 TRACI midpoint impact categories are: ozone

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PAC Scenarios. Life cycle impacts of PAC were estimated for the generation, hauling, and

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storage of PAC before dosing and the removal and landfilling of spent PAC. Emissions due to

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coal-derived PAC generation were estimated using Agri-footprint,45 which accounted for coal

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extraction and hauling, water and energy production, and direct emissions of carbon dioxide and

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water vapor.45 PAC production at four locations (California, Kentucky, Pennsylvania, and

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Texas47) was considered. For each, state-specific electricity production mixes and semi-trailer

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truck hauling distances to the Boulder WWTF were used. Of the four locations, Kentucky

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resulted in the highest impacts overall, so this location was used for the “high-impact” PAC

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scenario. California had the lowest impacts and was used for the “low-impact” PAC scenario.

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The number of silos needed for adsorbent storage and associated mass of galvanized sheet steel

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were based on typical PAC silo dimensions.48,49 Each silo had a continuously operating air

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fluidizer system to keep the PAC dry and friable.48 Electricity requirements for air fluidizers

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were estimated from commercially available systems.50 After PAC was dosed and SMX uptake

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was achieved, the spent PAC was removed from the effluent using alum coagulation and settling.

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The infrastructure and energy requirements of dosing, coagulation, and settling of all adsorbents

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were assumed to be the same (e.g., all required the same alum dose). The settled adsorbent was 8


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dewatered using a stainless steel belt filter press, and commercially available units51 were used to

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estimate energy and infrastructure requirements. Truck emissions from hauling spent adsorbent

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to a landfill were based on mass and a distance (19.6 km between Boulder WWTF and nearest

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landfill).

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Wood Biochar Scenarios. Life cycle impacts of wood biochar were estimated for the

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generation, hauling, and storage of wood biochar before dosing and the removal and landfilling

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of spent wood biochar. Biochar generation (wood chip generation, drying, and pyrolysis) had the

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same system boundary as PAC generation. Wood chip generation LCI data accounted for forest

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harvesting, hauling, and chipping.44 The electrical energy requirements for wood chip pyrolysis,

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based on a full-scale facility in Kremmling, CO (Biochar Solutions, Inc.) that co-produces

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biochar for land application and dried wood pellets for heating, was 0.54 MJ electricity per

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kilogram of non-ground biochar.41 Net thermal energy requirements, which are specific to the

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low capacity wood biochar produced at the full-scale facility (because energy production is

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dependent on pyrolysis conditions) was -24 MJ thermal energy per kilogram non-ground biochar

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due heat recovery from pyrolysis gas combustion beyond drying energy requirements.41 Energy

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production from the moderate capacity biochar was estimated as the difference between the

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feedstock (wood52) and biochar (calculated from elemental composition53) thermal energies,

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multiplied by the efficiency factor estimated from full-scale data. Energy recovered from

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pyrolysis gas offsets energy generated from wood chip combustion at the full-scale facility,41 and

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the model used this same energy offset. Direct air emissions of treated pyrolysis gas were

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estimated using modeling14 and measured data for wood biochar pyrolysis.29,54,55 The modeling

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data was most representative of the Colorado full-scale facility since it was for a large-scale

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wood biochar facility that had air emission regulations and treatment by thermal oxidation and 9



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cyclones. The measured data were from various small-scale, low-cost technologies that only

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captured and combusted a proportion of the pyrolysis gas and did not treat the exhaust with

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cyclones, so these data were used in the uncertainty assessment to evaluate worst-case air

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emissions scenarios (see Tables S4 an S5). The energy requirements of grinding wood biochar to

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a size fraction comparable to PAC (45 µm) were based on commercially available activated

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carbon grinders.56 The ground wood biochar was hauled 185 km from the full-scale facility to the

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Boulder WWTF by truck. The same assumptions and methods as the PAC scenarios were used to

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calculate adsorbent storage, dosing, coagulation, dewatering, and disposal hauling.

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Biosolids Biochar Scenarios. Life cycle impacts of biosolids biochar were estimated for the

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generation, hauling, and storage of biosolids biochar and any supplemental adsorbent; the

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removal and landfilling of spent adsorbent; and fertilizer production due to biosolids diversion

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from land application. Biosolids biochar generation was assumed to be at the WWTF and

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included biosolids drying (from 77.4% to 8% moisture content), pyrolysis, and grinding.

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Electricity requirements of pyrolysis were based on data from the full-scale wood facility (0.54

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MJ/kg non-ground biochar).41 Thermal energy requirements were based on data for biosolids

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drying in the Biosolids Emissions Assessment Model57 and were met using energy recovered

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from pyrolysis gas and then by natural gas (if needed). Energy recovery was estimated using the

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same methods as moderate capacity wood biochar with values for typical biosolids58 and

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biosolids biochar (calculated from elemental composition53) thermal energies. Additional energy

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required was assumed to be supplied by natural gas. Direct pyrolysis emissions were based on a

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full-scale biosolids pyrolysis facility’s emissions of treated and combusted pyrolysis gas.58

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Biosolids biochar grinding, storage, dosing, coagulation, dewatering, and disposal hauling were

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calculated using the same assumptions and methods as the wood biochar scenarios. 10


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The diversion of biosolids from land application (fertilizer) to biochar in these scenarios

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required the production of substitute fertilizers. Fertilizer quantities were based on biosolids

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content of plant available nitrogen and phosphorus.37,59 The mass of supplemental adsorbent was

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based on the biosolids biochar mass and each adsorbents’ dose. The impacts of supplemental

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adsorbents were based on this mass and the calculations described for each type (i.e., PAC or

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wood biochar scenarios). For storage, biosolids and wood biochars were stored together, whereas

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biosolids biochar and PAC were stored separately because of their different adsorption

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capacities.

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Uncertainty and Sensitivity Analysis. The aggregate impact of uncertainty in assumed values

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on the ten TRACI categories was estimated using a Monte Carlo analysis with the software

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Crystal BallTM. There were 24 uncertainty parameters (Tables S3 and S5) that represent main

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assumptions about storage systems, and biochar properties, and pyrolysis conditions, and

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pyrolysis gas air emissions. Each uncertainty parameter was assigned plausible maximum and

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minimum values based on literature values or typical WWTF operations and was characterized

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with a uniform probability distribution due to the lack of data to justify assigning any other

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distribution. The impact categories’ uncertainty ranges represent the 25th and 75th percentiles of

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100,000 Monte Carlo simulations. If a correlation coefficient’s magnitude was greater than 0.8

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(|ρ|>0.8)), then that impact category was defined as sensitive to that uncertainty parameter.

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Results and Discussion

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Wood Biochar to PAC Comparison. Environmental impacts of PAC and wood biochar are

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compared in Figure 2 for the 10 TRACI impact categories. Each category represents the

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magnitude and type of environmental or human health impacts that are based on the types and

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quantities of chemicals released into the environment as a result of the processes, materials, and 11



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energy used throughout each adsorbent’s life cycle. The results show that the production and use

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of moderate capacity wood biochar for SMX removal results in environmental impacts that are

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higher than low-impact PAC but lower than high-impact PAC in two categories (eutrophication,

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carcinogenics); environmental impacts that are lower than low-impact PAC in four categories

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(ecotoxicity, acidification, ozone depletion, fossil fuel depletion); and environmental net benefits

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in four categories (smog, global warming, respiratory effects, and non carcinogenics) that were

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not realized with low-impact PAC. Low capacity wood biochar had larger environmental

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impacts than both PAC scenarios in five categories (eutrophication, carcinogenics, ecotoxicity,

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acidification, ozone depletion) and lower impacts in the other half of the categories (fossil fuel

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depletion, smog, global warming, respiratory effects, and non carcinogenics). For low capacity

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wood biochar, three categories exhibited environmental benefits (global warming, respiratory

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effects, and non carcinogenics). The wood biochars had similar trends in each impact category,

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and the magnitude of environmental impacts or benefits scaled with adsorbent quantity (i.e.,

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adsorption capacity) (Figure 2). Moderate capacity wood biochar had the lowest overall

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environmental impacts. It exhibited lower environmental impacts than high-impact PAC in all

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ten categories, and eight out of ten categories compared with low-impact PAC.

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Figure 2. Relative environmental impacts for four scenarios: low-impact PAC, high-impact PAC, moderate capacity wood biochar, and low capacity wood biochar. All emission factors (i.e., impacts) are normalized to low-impact PAC. Negative emission factors represent an environmental benefit; positive emission factors represent a negative environmental impact.

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Wood Biochar. The wood biochars had environmental benefits and lower environmental

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impacts than PAC primarily due to carbon sequestration and energy production during pyrolysis

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(Figure 3). The estimated net amount of carbon sequestration for moderate capacity wood

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biochar was 0.57 kg CO2 equivalent (eq.) per kg dry feedstock and for low capacity wood

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biochar was 0.67 kg CO2 eq./kg dry feedstock. Both values are comparable to values reported by

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other studies on the production and use of biochar for land application, which reported a range of

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0.07 to 1.25 kg CO2 eq./kg dry feedstock.14,16,17,19,20,60 Low capacity wood biochar resulted in

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greater carbon sequestration than moderate capacity wood biochar primarily because a larger

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mass of feedstock was converted to biochar to satisfy the larger dose75 requirement.

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The estimated energy recovered during the production of moderate capacity wood biochar was

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8.6 MJ heat/kg dry feedstock and 7.5 MJ heat/kg dry feedstock for low capacity wood biochar.

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The energy produced as a percent of feedstock heat content was 44% and 38% for moderate and

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low capacity wood biochars, respectively, which is similar to another study’s value of 37%.17 In

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this LCA, the environmental benefit of the energy produced during pyrolysis is due to the offset

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of energy produced from wood chip combustion, which was the protocol used at the full-scale

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wood biochar and wood pellet co-production facility before the installation of pyrolysis energy

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recovery infrastructure. This energy replacement resulted in a net environmental benefit for

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moderate capacity wood biochar in three impact categories (smog, respiratory effects, and non-

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carcinogenics) and contributed to net environmental benefits in the global warming category.

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When considering replacing heat from natural gas instead of wood chips, moderate capacity

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wood biochar still had lower environmental impacts compared to PAC (see Figure S12).

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In addition to energy production and carbon sequestration, there were several activities in the

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wood biochar scenarios that resulted in harmful environmental impacts. Figure 3 shows the

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contribution of different life cycle processes’ impacts to the overall smog impact. The smog

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category was selected for display since it had trends similar to and was representative of the

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other nine impact categories (see Figures S2 to S11). For moderate capacity wood biochar, the

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largest contributor to negative smog impacts was wood biochar generation, mostly due to direct

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air emissions from pyrolysis gas combustion, wood chip generation, and electricity use. The

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second largest contributor to smog was silo storage of wood biochar at the WWTP, due mostly to

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electricity consumption for air fluidization. The impact from this electricity consumption was

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greatest in the eutrophication and carcinogenics categories (Figures S2 and S3). Since moderate

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capacity wood biochar required over twice as much adsorbent mass as PAC, wood biochar had

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larger storage and disposal impacts than PAC for all impact categories. Also, the larger doses for

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lower adsorption capacity biochars can increase costs and make materials handling more

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burdensome. The third largest contributor to the smog impact was hauling adsorbent from the

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generation site to the WWTF. The impact of hauling to the WWTF is four times smaller for

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moderate capacity wood biochar than for PAC because the shorter hauling distance (185 km

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from the biochar production facility compared to 1664 km from the PAC manufacturer). Overall,

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for the smog impact category, moderate capacity wood biochar exhibits a net environmental

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benefit due to energy recovery.

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Figure 3. Contribution of life cycle processes to the smog impact category for 5 scenarios: highimpact PAC, low-impact PAC, moderate capacity wood biochar, moderate capacity biosolids and wood biochars (MCBB+MCWB), and moderate capacity biosolids biochar supplemented with low-impact PAC (MCBB+low-impact PAC). Adsorbent disposal includes dewatering and landfill hauling. All values are normalized to low-impact PAC.

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PAC. Results for the two PAC scenarios are summarized in Figure 3. PAC generation and

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hauling to the WWTF were the two largest contributors to negative impacts. Low-impact PAC

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had a lower smog impact because of the lower emissions from electricity consumption during

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PAC generation (i.e., electricity production was cleaner in California than in Kentucky) and the

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shorter hauling distance to Colorado from California (1664 km) than from Kentucky (2118 km).

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The third and fourth largest contributors to smog were adsorbent storage and disposal. Both PAC

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scenarios required the same dose of PAC, so each has the same environmental impacts from

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storage and disposal. Overall, the PAC scenarios exhibited negative environmental impacts in all

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categories. The most effective ways found to reduce environmental impacts from these results

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were to decrease net energy use during PAC generation and to reduce hauling. In addition, future 15



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research could focus on a diversity of PACs (e.g., in terms of feedstock and activation method)

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to identify additional approaches for improving environmental performance. For example, while

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coal-based PACs are the most common,61 biomass-based activated carbons (e.g., wood-based or

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coconut-based PACs) might exhibit some similar benefits to wood-biochar (e.g., carbon

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sequestration). However, the systematic comparison of these sorbents requires further research to

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determine adsorption capacity and production factors. In this study, coal-based PAC exhibited

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worse overall environmental performance compared to moderate capacity wood biochar.

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However, PAC alone was found to be a better environmental option than a scenario employing

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biosolids biochar supplemented with PAC, as described below (Figures 3 and S1).

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Biosolids Biochar. Hybrid scenarios (i.e., supplementing biosolids biochar with PAC or wood

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biochar) were evaluated because of biosolids feedstock limitations. The quantity of biosolids

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produced at the Boulder WWTF would generate enough biochar to treat up to one-sixth of

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effluent for 75% SMX removal. Overall, hybrid scenarios involving moderate capacity biosolids

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biochar resulted in larger environmental impacts than the low-impact PAC or moderate capacity

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wood biochar scenarios (see Figures 3 and S1). This trend was mostly due to biosolids biochar

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generation (especially the energy required for drying) and the artificial fertilizer production

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needed to replace land applied biosolids (Figure 3).

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Biosolids drying required 14.9 MJ heat/kg dry biosolids to get from 77% to 8% moisture

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content by mass. Comparatively, wood chip drying required 0.39 MJ heat/kg dry wood chips to

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get to 8% moisture content, based on full-scale data. Large energy requirements due to the high

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moisture content of biosolids feedstock has also been noted by other researchers.16 There is no

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environmental benefit for net energy recovery in the biosolids biochar and PAC hybrid scenario

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because all of the energy produced during pyrolysis (12.2 MJ heat/kg dry biosolids) was needed 16


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for biosolids drying. For biosolids biochar, the energy produced as a percent of feedstock heat

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content was 65%, which is similar to another study’s range of 60% to 80%.62 There is an energy

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recovery benefit for the wood and biosolids biochar hybrid scenario because pyrolysis heat

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energy in excess of needs for wood chip drying offset wood chip combustion. This extra energy

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could not be used for biosolids drying since wood-based and biosolids-based biochars would be

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generated at different locations.

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Artificial fertilizer production resulted in negative environmental impacts. Although not

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included in this model’s scope, it is important to note that biosolids and artificial fertilizers could

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have different impacts after application. For example, nutrient runoff might be higher from

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artificial fertilizer,63 and biosolids land application can imply the risk of contaminating soil and

336

adjacent waterways with heavy metals and persistent organic micropollutants.64-68

17



337 338 339 340 341 342 343 344 345 346 347

Page 18 of 26

Figure 4. Effect of uncertainty in input parameters on environmental impact categories for the low-impact PAC and moderate capacity wood biochar scenarios: Plot (a) shows the results from the Monte Carlo simulations, which included 24 uncertainty parameters (Tables S3 and S5); columns represent the uncertainty results’ mean values with error bars representing the 25th and 75th percentiles. Plots (b) and (c) show the comparison of the Monte Carlo results to results from 4 wood biochar pyrolysis gas air emissions scenarios, for the impact categories most impacted by air emissions uncertainty: smog (b) and respiratory effects (c). *Moderate capacity wood biochar scenario descriptions: uncertainty (Monte Carlo results, from a), 1) representative modeling data (large-scale facility with air emissions treatment),14 2) flame curtain kiln,54 3) Adam retort kiln,55 and 4) TLUD.29

348 349

Uncertainty. To evaluate the impact of major model assumptions on results, a Monte Carlo

350

analysis was used to quantify the uncertainty ranges for each result based on input parameter

351

uncertainty (Tables S3 and S5). Uncertainty ranges (25th to 75th percentiles of the Monte Carlo

352

analysis) for low-impact PAC and moderate capacity wood biochar are shown in Figure 4 (a). 18


Page 19 of 26


353

Since this assessment showed that the smog and respiratory effects conclusions were sensitive to

354

the air emissions data for wood biochar pyrolysis gas, the Monte Carlo uncertainty parameter

355

data was disaggregated back into the separated modeling and measured data as scenarios and

356

evaluated (Figure 4, b and c). In addition, new adsorbent scenarios were created to evaluate the

357

impacts of the WWTF location and treatment goal (see SI sections S9 and S10).

358

Uncertainty ranges for low-impact PAC were generally smaller because it had only three

359

associated uncertainty parameters, while the moderate capacity wood biochar scenario had

360

eighteen. Wood biochar results in the non-carcinogenics category had a strong correlation

361

(ρ>0.8) with pyrolysis mass yield since decreasing yield increased energy production because

362

more feedstock was pyrolyzed. This yield is impacted by pyrolysis temperature. Increasing

363

pyrolysis temperature decreases pyrolysis yield,24 which has three main effects: (i) increasing

364

pyrolysis gas (energy) production, (ii) decreasing carbon sequestration, and (iii) increasing

365

adsorption capacity,23,69 which decreases impacts from hauling and storage. Therefore, for an

366

adsorption application, higher pyrolysis temperatures should be used to have the operational (i.e.,

367

less infrastructure and materials handling) and environmental benefits (due to energy recovery)

368

of higher adsorption capacity biochars.

369

Also, wood biochar results in the smog, respiratory effects, acidification, and global warming,

370

categories were sensitive to the pyrolysis gas air emissions uncertainty. In particular, the smog

371

results are overlapping due to the large wood biochar uncertainty range, and the respiratory

372

effect uncertainty range for wood biochar shows that there could be an environmental benefit or

373

burden (Figure 4a). Figure 4b shows that wood biochar is expected to have lower smog impacts

374

than PAC, but there is one scenario that results in a significantly larger impact (Scenario 4, see

375

Table S5). To have a lower smog impact, wood biochar processes need to capture and combust 19



376

pyrolysis gas to minimize volatile organic carbon emissions. Figure 4c shows that the

377

environmental benefit of wood biochar energy recovery can be lost if particulate matter is not

378

removed from the pyrolysis gas (moderate capacity wood biochar Scenario 2). Therefore, air

379

emission control measures, such as thermal oxidation and cyclones, are important to decrease the

380

negative environmental impacts and highlight the benefits of wood biochar.

Page 20 of 26

381

The impact of the WWTF location relative to the PAC production site was evaluated by setting

382

all electricity consumption mixes to US average and delivery distance for moderate capacity

383

wood biochar and PAC were set to 20, 200, and 2000 kilometers. In these scenarios, wood

384

biochar had lower impacts than PAC in all categories for the distances of 20 and 200 km and

385

lower impacts in eight out of ten categories for the 2000 km distance (see Figures S13-15).

386

However, another scenario was considered in which the WWTF was located in California near

387

the PAC production site (California WWTF scenario). For this scenario, PAC was produced with

388

California electricity and delivered to a nearby WWTF (185 km), and moderate capacity wood

389

biochar was produced with Colorado electricity, which uses more coal and fewer renewable

390

energy sources, and delivered to California (1664 km). In this scenario, moderate capacity wood

391

biochar had higher impacts in six out of ten categories than PAC (see Figure S16), although

392

moderate capacity biochar would still be environmentally beneficial in three categories. Overall,

393

for similar electricity production mixes and delivery distances, wood biochar is more

394

environmentally favorable. This analysis shows that differences in electricity production mixes

395

and delivery distances could make PAC environmentally competitive. However, there are

396

substantial environmental benefits to generating and using an adsorbent from renewable

397

materials near the point-of-use as compared to using coal-based PAC delivered from far away.

20


Page 21 of 26


398

The impact of adsorbent dose was evaluated by relaxing the treatment objective to the point at

399

which biosolids biochar alone could treat the entire WWTF effluent flow (i.e., a biochar dose of

400

25 mg/L and PAC dose of 12 mg/L); and this dose would target other more strongly adsorbing

401

micropollutants. Also, the fate of biosolids at the WWTF was changed to landfilling (instead of

402

land application) so that no scenario required artificial fertilizer production. Under these scenario

403

conditions, moderate capacity wood biochar was still the preferred adsorbent (Figure S17). The

404

environmental performance of biosolids biochar did improve given these conditions, to the point

405

of similarity with high-impact PAC. In this study, using PAC or wood biochar was found to be

406

environmentally preferable to biosolids biochar. This was particularly the case when biosolids

407

were diverted from land application, necessitating the production of artificial fertilizers. Previous

408

studies have found tradeoffs between biosolids fates in land application, energy generation, and

409

incineration,64 so future research that compares multiple biosolids fates that include multiple

410

biochar applications could also inform WWTF operation.

411

With future requirements to remove micropollutants at WWTFs expected,30 sustainable tertiary

412

treatment options are needed. PAC adsorption was previously found to be an environmentally

413

preferred tertiary wastewater treatment option.5 This study suggests that environmental

414

performance can be further improved through the use of wood biochar adsorbent. In particular,

415

wood biochar has significant environmental benefits for climate change mitigation. The

416

moderate capacity wood biochar scenario sequestered enough carbon (about 6.5 gigagrams CO2

417

eq./yr) to offset all of a WWTF’s carbon emissions from energy and chemical use (about 5.0

418

gigagrams CO2 eq./yr. for a 12.5 MGD facility, assuming 0.29 kg CO2 eq./m3 for a WWTF that

419

has organics and nutrient removal,70 SI Section S8.5) and to result in additional carbon

420

sequestration; in other words, 130% of the carbon emissions could be offset. The results of this 21



421

study present adsorption with wood biochar as a novel, environmentally sustainable way for

422

WWTFs to remove trace organic micropollutants. Further innovation should be undertaken to

423

develop low cost carbonaceous adsorbents that have high micropollutant adsorption capacities

424

and are made from renewable resources located in close proximity to treatment facilities and

425

disposal sites.

Page 22 of 26

426

Supporting Information. The Supporting Information is available free of charge on the ACS

427

Publications website at DOI: X. It includes a more detailed description of the LCA methods,

428

uncertainty assessment, adsorbent characteristics, and LCI data; additional figures for unit

429

process contributions to each impact category; and results from the alternative functional unit

430

assessment. This information is available free of charge via the Internet at http://pubs.acs.org/.

431

The authors declare no competing financial interest.

432 433

Acknowledgements

434

We thank Jonah Levine of Biochar Solutions Inc. and Cole Sigmon of the City of Boulder for

435

full-scale and reference data. The first author was funded by a National Science Foundation

436

Graduate Research Fellowship and second author by a United States Environmental Protection

437

Agency STAR Fellowship. Any opinions, findings, and conclusions or recommendations

438

expressed in this material are those of the authors and do not necessarily reflect the views of any

439

other organization.

440 441 442 443 444

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Environmental Comparison of Biochar and Activated Carbon for

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