Characteristics and applications of biochars derived from wastewater solids
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
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