Characteristics and applications of biochars derived from wastewater solids

https://doi.org/10.1016/j.rser.2018.02.040Get rights and content

Abstract

Pyrolysis is a thermochemical decomposition process that can be used to generate pyrolysis gas (py-gas), bio-oil, and biochar as well as energy from biomass. Biomass from agricultural waste and other plant-based materials has been the predominant pyrolysis research focus. Water resource recovery facilities also produce biomass, referred to as wastewater solids, that could be a viable pyrolysis feedstock. Water resource recovery facilities are central collection and production sites for wastewater solids. While the utilization of biochar from a variety of biomass types has been extensively studied, the utilization of wastewater biochars has not been reviewed in detail. This review compares the characteristics of wastewater biochars to more conventional biochars and reviews specific applications of wastewater biochar. Wastewater biochar is a potential candidate to sorb nutrients or organic contaminants from contaminated wastewater streams. While biochar has been used as a beneficial soil amendment for agricultural applications, specific research on wastewater biochar is lacking and represents a critical knowledge gap. Based on the studies reviewed, if biochar is applied to land it will contain less organic micropollutant mass than conventional wastewater solids, and polycyclic aromatic hydrocarbons are not likely to be a concern if pyrolysis is conducted above 700 °C. Wastewater biochar is likely to serve as a better catalyst to convert bio-oil to py-gas than other conventional biochars because of the inherently higher metal (e.g., Ca and Fe) content. The use of wastewater biochar alone as a fuel is also discussed. Finally, an integrated wastewater treatment process that produces and uses wastewater biochar for a variety of food, energy, and water (FEW) applications is proposed.

Introduction

Typical water resource recovery facilities (WRRFs), formerly referred to as wastewater treatment plants, treat wastewater from homes and industries, producing treated water and residual wastewater solids that are rich in organic content. These facilities are currently energy intensive operations, but a new paradigm has emerged viewing WRRFs as community assets that could recover energy and generate value-added products from wastewater [1], [2]. Influent wastewater is rich in carbon, nutrients, and heat, all of which are potentially valuable resources [3]. The nutrients can be recovered as a fertilizer product, e.g. struvite, and used for agricultural purposes [4]. The organics have inherent energy content that can be recovered on-site. The wastewater solids, in particular, represent a potentially valuable energy source.

The United States Environmental Protection Agency (USEPA) estimates that approximately eight million dry tons of wastewater solids are produced each year in the United States alone [5]. Wastewater solids are either land applied as a soil conditioner and nutrient source, landfilled, or incinerated. WRRFs do not capture the inherent energy content from the organic matter of wastewater solids that are used as a soil conditioner or landfilled. Additionally, wastewater solids contain micropollutants, i.e., the organic chemicals derived from consumer products that are released to sewers after use, including antimicrobials, pharmaceuticals, personal care products, hormones, and more [6]. Due to the presence of micropollutants, the long-term environmental and public health impacts of land applying wastewater solids have caused concerns to be raised in recent years [7]. For these reasons, alternative wastewater solids handling methods are being considered to recover energy while generating valuable products [8].

Pyrolysis is the process whereby biomass, such as wastewater solids, is heated between approximately 400 and 900 °C in the absence of oxygen [9], [10]. Pyrolysis produces solid, liquid, and gas products. The solid product, biochar, is similar to charcoal. The liquid can consists of multiple phases: including non-aqueous phases often referred to as bio-oil, and an aqueous phase that is sometimes called aqueous pyrolysis liquid. The gas product, referred to as py-gas, consists of H2, CH4, CO, CO2 along with lower concentrations of hydrocarbons including C2H6, C2H4, and C3H8 [11], [12]. Py-gas is a relatively clean-burning fuel that can be used on-site at WRRFs for energy recovery. The bio-oil also has a high energy content, but contains water, organic acids and oxygenated organics that make it corrosive for combustion; therefore, bio-oil typically requires processing before use. The biochar, as reviewed in this paper, has a wide array of potential applications as a sorbent, soil amendment, energy source, or catalyst [13], [14], [15], [16]. It may be most valuable for WRRF operators to optimize pyrolysis parameters to increase py-gas yield and decrease liquid yields because they require further processing. Slow pyrolysis (defined as pyrolysis with a heating rate less than 100 °C/min) yields more biochar and py-gas than fast pyrolysis (defined as pyrolysis with a heating rate greater than 300 °C/min), and fast pyrolysis typically yields more liquid products [17], [18]. Therefore, the focus of this review is on biochars derived from slow pyrolysis of wastewater solids.

Wastewater solids are an emerging biomass source of interest for pyrolysis, in part, because they are centrally produced in urban locations. Therefore, one of the most energy intensive components for biochar generation, i.e., biomass collection in a central location, has already been completed. From this logistical standpoint wastewater solids represent a potentially practical and easily accessible biomass stream to produce biochar via pyrolysis. Biochar derived from wastewater solids, referred to hereafter as wastewater biochar, however, has not been studied to the same extent as other biochars, nor has wastewater biochar been comprehensively reviewed. It is important to understand how wastewater biochars differ relative to other commonly studied biochars. The goal of this review is therefore to describe the characteristics of wastewater biochars relative to other biochars, current and future biochar uses, and research needs. The specific objectives of this review paper are to: i) determine how basic properties of wastewater biochar properties differ from other biochars ii) identify the appropriate uses of wastewater biochar for sorption, iii) establish the benefit of wastewater biochar as a soil amendment, iv) determine toxic hazards related to land applying wastewater biochar v) establish the role of wastewater biochar as a catalyst and vi) determine the feasibility of energy recovery from wastewater biochar.

Section snippets

Basic properties of wastewater biochars compared to other biochars

Wastewater biochars have a lower concentration of carbon (C) than other biomass-derived biochars (Table 1). This is not surprising considering that wastewater solids are comprised of organic and inorganic solids whereas biochars derived from other biomass streams such as switchgrass are composed primarily of organic matter. Wastewater biochars, on the other hand, typically have higher concentrations of nitrogen (N), phosphorus (P), and potassium (K), i.e., essential nutrients for plant growth.

Nutrients removal

Biochar derived from a wide range of feedstocks, including wastewater solids, can adsorb nutrients in the form of ammonium and phosphate. Table 3 summarizes research regarding biochars produced from different feedstocks and at different temperatures and washing/preconditioning protocols to adsorb ammonium or phosphate. Among the biochars reviewed, wastewater biochar had intermediate to high ammonium adsorption capacity and high phosphate adsorption capacity.

Surface area, surface chemistry, and

Wastewater biochar as a soil amendment

Wastewater biochars have been investigated as soil amendments to improve growth of a variety of plants, including fruiting plants, grasses [14] and rice as well as garlic [88] and lettuce [89]. Wastewater biochars have been shown to increase the growth rate of peppers [90] and tomatoes [30]. A number of grasses have also been shown to benefit from wastewater biochar soil application, including bentgrass [91], Kentucky bluegrass [14] and ryegrass [92].

It is important to consider the type of

Toxicity evaluation of heavy metals

Some biochars contain heavy metals and organic contaminants such as polycyclic aromatic hydrocarbons (PAHs) so they may pose negative impacts to the ecological environment. Therefore, the bioaccumulation and mobility of these potential pollutants is of great concern during land application of biochar.

Previous research indicated that wastewater biochars likely have heavy metals below concentrations of concern, but they should be tested to ensure that levels are safe. In general, the heavy metal

Wastewater biochar as a catalyst for thermochemical conversions

Biochar is an effective catalyst for tar cracking, i.e., converting bio-oil constituents into py-gas. Gasification is a process that converts fossil fuel or renewable carbonaceous feedstock into energetic product gas. Tars are the condensable organic fraction of the gasification byproducts and are largely high molecular weight (i.e. larger than benzene) aromatic hydrocarbons [118]. Tars are difficult to destroy and handle, leading to clogging problems in the gasification process. Mani et. al.

Energy recovery from wastewater biochar

As a reduced carbonaceous material, wastewater biochar can be used for energy generation or fuels production. Combustion of wastewater biochar [134], [135], or co-combustion with a fuel like coal [135], [136], [137], can supply process heat or contribute to powering a steam cycle [137]. Gasification or co-gasification of wastewater biochar with steam and a limited amount of oxygen can be used to produce syngas [138], [139], [140], [141], a mixture of H2 and CO, that can be combusted for energy

Conclusions related to the objectives of the review

Wastewater biochar is chemically different from other biochars and has many potential value-added applications, as noted in the objectives of this review.

Objective 1. Determine how basic properties of wastewater biochar properties differ from other biochars. In general wastewater biochar has a lower C content than other biochars stemming from biomass primarily because wastewater is composed of both organic and inorganic solids. Wastewater biochar also typically has a higher H to C ratio, as

References (162)

  • E. Agrafioti et al.

    Biochar production by sewage sludge pyrolysis

    J Anal Appl Pyrolysis

    (2013)
  • A. Méndez et al.

    Effects of sewage sludge biochar on plant metal availability after application to a Mediterranean soil

    Chemosphere

    (2012)
  • M. Inguanzo et al.

    On the pyrolysis of sewage sludge: the influence of pyrolysis conditions on solid, liquid and gas fractions

    J Anal Appl Pyrolysis

    (2002)
  • A. Méndez et al.

    Physicochemical and agronomic properties of biochar from sewage sludge pyrolysed at different temperatures

    J Anal Appl Pyrolysis

    (2013)
  • H. Lu et al.

    Relative distribution of Pb 2+ sorption mechanisms by sludge-derived biochar

    Water Res

    (2012)
  • M.K. Hossain et al.

    Thermal characterisation of the products of wastewater sludge pyrolysis

    J Anal Appl Pyrolysis

    (2009)
  • K.K. Shimabuku et al.

    Biochar sorbents for sulfamethoxazole removal from surface water, stormwater, and wastewater effluent

    Water Res

    (2016)
  • K. Lu et al.

    Effect of bamboo and rice straw biochars on the bioavailability of Cd, Cu, Pb and Zn to Sedum plumbizincicola

    Agric Ecosyst Environ

    (2014)
  • J. Park et al.

    Slow pyrolysis of rice straw: analysis of products properties, carbon and energy yields

    Bioresour Technol

    (2014)
  • F. Tinwala et al.

    Intermediate pyrolysis of agro-industrial biomasses in bench-scale pyrolyser: product yields and its characterization

    Bioresour Technol

    (2015)
  • Y. Lin et al.

    Water extractable organic carbon in untreated and chemical treated biochars

    Chemosphere

    (2012)
  • S. Gul et al.

    Physico-chemical properties and microbial responses in biochar-amended soils: mechanisms and future directions

    Agric Ecosyst Environ

    (2015)
  • K. Cantrell et al.

    Impact of pyrolysis temperature and manure source on physicochemical characteristics of biochar

    Bioresource

    (2012)
  • W. Song et al.

    Journal of Analytical and Applied Pyrolysis Quality variations of poultry litter biochar generated at different pyrolysis temperatures

    J Anal Appl Pyrolysis

    (2012)
  • J. Tang et al.

    Characteristics of biochar and its application in remediation of contaminated soil

    J Biosci Bioeng

    (2013)
  • S. Kizito et al.

    Evaluation of slow pyrolyzed wood and rice husks biochar for adsorption of ammonium nitrogen from piggery manure anaerobic digestate slurry

    Sci Total Environ

    (2015)
  • J. Tian et al.

    Nutrient release and ammonium sorption by poultry litter and wood biochars in stormwater treatment

    Sci Total Environ

    (2016)
  • C.A. Takaya et al.

    Phosphate and ammonium sorption capacity of biochar and hydrochar from different wastes

    Chemosphere

    (2016)
  • S.E. Hale et al.

    The sorption and desorption of phosphate-P, ammonium-N and nitrate-N in cacao shell and corn cob biochars

    Chemosphere

    (2013)
  • T.L. Eberhardt et al.

    Phosphate removal by refined aspen wood fiber treated with carboxymethyl cellulose and ferrous chloride

    Bioresour Technol

    (2006)
  • Y. Yao et al.

    Biochar derived from anaerobically digested sugar beet tailings: characterization and phosphate removal potential

    Bioresour Technol

    (2011)
  • S.R. Smith

    A critical review of the bioavailability and impacts of heavy metals in municipal solid waste composts compared to sewage sludge

    Environ Int

    (2009)
  • D. Mohan et al.

    Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent - A critical review

    Bioresour Technol

    (2014)
  • X. Cao et al.

    Properties of dairy-manure-derived biochar pertinent to its potential use in remediation

    Bioresour Technol

    (2010)
  • B. Chen et al.

    A novel magnetic biochar efficiently sorbs organic pollutants and phosphate

    Bioresour Technol

    (2011)
  • T. Chen et al.

    Influence of pyrolysis temperature on characteristics and heavy metal adsorptive performance of biochar derived from municipal sewage sludge

    Bioresour Technol

    (2014)
  • G. Gasco et al.

    Preparation of carbon-based adsorbents from sewage sludge pyrolysis to remove metals from water

    Desalination

    (2005)
  • M. Otero et al.

    Removal of heavy metals from aqueous solution by sewage sludge based sorbents: competitive effects

    Desalination

    (2009)
  • F. Rozada et al.

    Adsorption of heavy metals onto sewage sludge-derived materials

    Bioresour Technol

    (2008)
  • W. Zhang et al.

    Pb ( II) and Cr ( VI) sorption by biochars pyrolyzed from the municipal wastewater sludge under different heating conditions

    Bioresour Technol

    (2013)
  • H. Lu et al.

    Characterization of sewage sludge-derived biochars from different feedstocks and pyrolysis temperatures

    J Anal Appl Pyrolysis

    (2013)
  • K.M. Smith et al.

    Sewage sludge-based adsorbents: a review of their production, properties and use in water treatment applications

    Water Res

    (2009)
  • D. Mohan et al.

    Sorption of arsenic, cadmium, and lead by chars produced from fast pyrolysis of wood and bark during bio-oil production

    J Colloid Interface Sci

    (2007)
  • X. Chen et al.

    Adsorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution

    Bioresour Technol

    (2011)
  • W. Zheng et al.

    Sorption properties of greenwaste biochar for two triazine pesticides

    J Hazard Mater

    (2010)
  • K. Sun et al.

    Sorption of bisphenol A, 17α-ethinyl estradiol and phenanthrene on thermally and hydrothermally produced biochars

    Bioresour Technol

    (2011)
  • M. Ahmad et al.

    Effects of pyrolysis temperature on soybean stover- and peanut shell-derived biochar properties and TCE adsorption in water

    Bioresour Technol

    (2012)
  • N. Karakoyun et al.

    Hydrogel-Biochar composites for effective organic contaminant removal from aqueous media

    Desalination

    (2011)
  • C. Jung et al.

    Adsorption of selected endocrine disrupting compounds and pharmaceuticals on activated biochars

    J Hazard Mater

    (2013)
  • L. Yu et al.

    Preparation of adsorbents made from sewage sludges for adsorption of organic materials from wastewater

    J Hazard Mater

    (2006)
  • Cited by (75)

    View all citing articles on Scopus
    View full text