1.1 What are Biosolids?
Biosolids are a major by-product of primary and secondary treatment of residential and industrial effluent in wastewater treatment plants (WWTPs) (Toronto Water 2004; NEBRA 2008; Sewage Treatment 2010); once produced, biosolids are either sent to disposal (including incineration and landfilling), or are land-applied. An otherwise waste product of a burgeoning global urban population, the potential use of land-applying biosolids to amend agricultural soils, among other uses in land restoration (including mine tailings) (CCME 2013), presents an opportunity for using renewable resources within the transition to sustainable development. Biosolids have been, and are increasingly, land-applied to supplement soils with macronutrients (i.e., N, P, K) and organic matter. Moreover, biosolids provide an inexpensive source of essential fertilizer to farmers, support plant growth, improve crop yield, improve soil structure, and reduce soil erosion (Butt and Nuutinen 1998).
Land-applying human wastes as a means of efficient disposal, as well as the recycling of nutrients, has been used by the United States and Canada for over 50 years (Synagro 2002; O’Connor et al. 2005; Pepper et al. 2006; City of Toronto 2009). However, as a human waste product, biosolids can also contain unwanted contaminants or pathogens from industrial and residential wastes (Reilly 2001; Kinney et al. 2006; Kinney et al. 2008; Wu et al. 2008; Clark et al. 2010; CCME 2013; Lapen et al. 2018). Indeed, the perceived toxicity from contaminants and heavy metals to ecosystems has raised the concerns of many in the public arena (LeBlanc 2007; Vyhnak 2008; City of Toronto 2009) and over the years, these concerns have outweighed the many known benefits to soil and crop fertility. Public opinion, rather than technical problems, have often driven opposition in land-application programs by most government agencies (USEPA 2000; Imrie 2013; Lucas 2020); thus, the use of biosolids have not seen their full potential. Indeed, if deemed “sustainable”, the land-application of biosolids would typify true recycling: where nutrients from the soil, which are incorporated into the biomass of crops and then consumed, could be, without recourse for potential detriment to the natural environment, returned to farmed land. To address the public’s uncertainty regarding biosolids, there is a need to determine whether biosolids exhibit detrimental impact on the ecosystem.
1.2 History of Biosolids Research
To date, most research on the potential toxicity of the land-application of biosolids has focused on the identification, through chemical analysis, of discrete chemicals or classes of chemicals. There exists a huge database identifying the compounds potentially available in biosolids; there are 86,000 + chemicals on the United States Toxic Substance Control Act Inventory (USEPA 2021). Additionally, several pathogens of concern including bacteria, viruses, protozoa, and helminths have been identified in WWTPs; thus, impact assessment, rather than compound or pathogen identification, is the only scientifically-sound strategy to determine the sustainability of land-application (see Rogers and Smith 2007 for review). To initially address this conundrum, a comprehensive risk assessment study of emerging substances of concern (i.e. ESOCs) and pathogenic substances in biosolids was initiated (McCarthy and Loyo 2015). Additionally, a comprehensive literature review has elucidated that since the early 2000s, biosolids toxicity tests have continued to investigate individual chemicals through ‘spiking’ tests. Toxicity testing by spiking biosolids grew from prior concerns regarding toxic sediments (Burton Jr 1991). From earlier studies, spiking experiments in biosolids saw select concentrations of chemicals in a substrate raised to, or more often exceeding, environmentally-relevant levels for the purpose of testing potential toxicity. Testing with spiked contaminants has led to several scientific debates over the decades, with Samoiloff (1989) arguing that testing single chemicals in toxicity experiments is irrelevant, as compounds are not present in isolated concentrations in the natural environment. Not only are parent compounds of concern, but also the biodegradative products from microbial or photolytic breakdown. Additionally, Cairns Jr. and Mount (1990) argued that the use of chemical analysis of single compounds was irrelevant for toxicity testing, as organisms – through no dearth of endpoints – will exhibit responses differently to single compounds compared to the chemical mixtures that are traditionally found in their environments. More recently, Crouau et al. (2002) summarized the shortcomings of discrete chemical toxicity tests in stating that they are i) too expensive to conduct for all potential pollutants, ii) provide no information about bioavailability, and iii), do not account for the significant number of possible antagonistic or synergistic reactions. Additionally, Johnson and Sumpter (2016) quote Schoettger of the USEPA: “the US scientific community does not have the time, research facilities, trained personnel, experimental animals, nor financial resources to provide the additional data needed for comfortable predictions of the possible environmental effects of a broad spectrum of chemical contaminants”. A priori to these concerns, single-chemical studies have the potential to be problematic in biosolids research. Moreover, the testing of individual compounds serves as a form of confirmation bias as researchers are deciding which chemical to test for potential toxicity. In spiking biosolids, the research question changes from one of investigating the fate and impact of land-applying biosolids to the more costly testing of discrete substances on organisms, using land application solely as an exposure pathway.
Notwithstanding this concern, current studies continue to spike biosolids with higher concentrations of substances to test toxicity to terrestrial organisms. For example, Snyder et al. (2011) examined the effect of triclocarban (TCC) (an antibacterial and antifungal agent in disinfectants and soaps) on Eisenia fetida (syn E. foetida) using spiked biosolids. More recently, Jesmer et al. (2017) spiked biosolids with silver nanoparticles (AgNPs) (an antimicrobial) to test toxicity on Eisenia andrei and Folsomia candida; in addition, Velicogna (2019) tested toxicity on Eisenia andrei and Folsomia candida in spiking biosolids with copper sulfate (CuS) (a fungicide and algaecide) and copper oxide nanoparticles (nCuO) (an antimicrobial agent). While single-compound toxicity testing itself is environmentally-irrelevant, the conclusions in media are often founded on these studies, without scientifically experimenting for toxicity when land-applying biosolids at environmentally-relevant concentrations.
1.3 Behavioural Endpoints
Lethality bioassays measure mortality or survivorship; contrastingly, Taylor and Scroggins (2013) define sublethal effects as those which include the wide range of “immobility, avoidance, growth, reproduction or fertilization” responses. While behaviour has been critiqued as an endpoint for its inability to be standardized and for field verification (Little 1990; Peakall 1996; Grue et al. 2002), the responses are considered valuable as they serve as an “integration of an organism’s molecular, physiological, nervous, sensorial and muscular systems” to changes in the test environment (Raby 2013). Moreover, behavioural testing is more sensitive than lethality testing, offering insight into toxicological impacts when testing substances at environmentally-representative levels of exposure (Little and Finger 1990; Grue et al. 2002; Hellou 2011; Raby 2013). Current government protocols were developed without the inclusion of behavioural endpoints, as such measurements would require the researcher to have a sufficient knowledge of the organism’s behaviour under normal and stressed conditions. Additionally, the protocols are concerned with a perceived subjectivity in interpreting effect. The current study however examined behaviour as a sublethal toxicity endpoint and as distinct from the chronic impacts on growth and reproduction. In order to comprehensively study the ecotoxicological impact of biosolids, sublethal, chronic, and lethal endpoints were deemed necessary.
1.4 Study Overview
A review of the existing literature suggests that few studies have yet to clarify the impact of municipal biosolids as they are applied, using environmentally-relevant concentrations, to terrestrial biota (see Kinney et al. 2012; Xue et al. 2015; Coors et al. 2016 for example(s)). Thus, the current study is part of a larger research program that attempted to determine ecotoxicological impact, and thus clarify the perceived risks, of land-applying biosolids to agricultural fields. This broader study incorporated a battery of toxicity tests and endpoints and followed the potential impact of biosolids from their initial application within the terrestrial compartment, through surface and subsurface flow (as tile drainage), to the aquatic ecosystem. This current sub-study is reporting on the experiments conducted and the observations made regarding the potential impact on environmentally-relevant organisms, including two terrestrial phyla, Folsomia candida (springtails) and Lumbricus terrestris (earthworms). Subsequent testing within the biosolids test battery included four (4) terrestrial plants (Puddephatt et al. 2022). An additional objective of this study was to examine existing toxicity testing protocols.