Naturally occurring nanoparticles (NONPs) are found in the environment in concentrations orders of magnitude higher relative to engineered nanoparticles (ENPs). (Bernhardt et al. 2010, Sun et al. 2014) However, more research studies have focused on the fate and transport of ENPs because of their novel properties and their potential impact on the natural environment. (Bernhardt, et al. 2010) Additionally, NONPs have been generally studied as colloids within wastewater treatment processes where the particles may be composed of a large variety of materials (Dean 1969, Dean et al. 1967, Kowalkowski 2010, Kretzschmar et al. 1997, Kretzschmar et al. 1994, Kretzschmar and Sticher 1997, 1998, Lead and Wilkinson 2006, Prestel et al. 2005, Rickert and Hunter 1971) and sizes; (Aiken et al. 2011, Lead and Wilkinson 2006, Mohlman and Pearse 1922, Prestel, et al. 2005) this may be due to the fact that the definitions for colloids and nanomaterials overlap, and there is no established distinction between the two terms. Colloids are defined by some as small particles that are suspended and do not stabilize well, with sizes ranging from 1 nm to 1 µm. (Lead and Wilkinson 2006, Mohlman and Pearse 1922) In contrast, nanomaterial is broadly defined by the US Environmental Protection Agency (US EPA) and the European Commission (EC) as material between 1 and 100 nm in at least one dimension, (EC 2011, USEPA 2021) with recognition that there are naturally occurring nanoparticles. As a result, “colloids” and “nanoparticles” were used interchangeably in past research. In this study, we used 0.45-µm filter and the Zetasizer Nano ZS to aid our understanding of particle distribution in the wastewater, and refer to naturally occurring particles of size range 1 to 100 nm as NONPs.
Nanoparticles and colloids, depending on their physical, chemical, and biological properties, can have adverse effects on wastewater treatment and the quality of treated effluent. (Dean 1969, Dean, et al. 1967, Kowalkowski 2010, Lead and Wilkinson 2006) For example, it has been well studied that colloids can cause membrane fouling in wastewater treatment processes. (Lee et al. 2005, Lesjean et al. 2005, Meng et al. 2009, Meng et al. 2006, Safarik and Phipps Jr. 2006, 2009) In particular, many of the nano-sized suspended particles in wastewater are organic (Kowalkowski 2010, Lead and Wilkinson 2006) (e.g., protein and polysaccharide) and are thought to be part of soluble microbial products (SMP) and extracellular polymeric substances (EPS), (Wang et al. 2009) both of which have been found to cause reversible as well as permanent fouling on filtration membranes even after backwash cleaning. (Meng, et al. 2009, Safarik and Phipps Jr. 2009) This type of fouling increases the cost of treatment, either in the form of increased energy demand over time (Akhondi et al. 2014, Chua et al. 2002, Gander et al. 2000, Krzeminski et al. 2012) or in the form of the material cost of cleaning or replacing the filtering membrane. (Guo et al. 2012, Meng, et al. 2009) In addition, colloids can cause light scattering resulting in inadequate disinfection by UV and also impair the aesthetic quality of the treated effluent. (Christensen and Linden 2003, Gilboa and Friedler 2008, Hassen et al. 2000, Passantino et al. 2004) During chlorination specifically, colloidal particles may harbour and shield pathogens, (Winward et al. 2008) and if they are organic colloidal particles, they may also increase the chlorine demand of the treated effluent. (De Beer et al. 1994, Falsanisi et al. 2008, Gautheir et al. 1999) Furthermore, smaller particles have high specific surface area which provides a higher number of potential adsorption sites for many contaminants; (Kowalkowski 2010, Rudolfs and Zuber 1953) if the colloids are not removed during the treatment process, they could enhance the transport of contaminants to the environment. (Bolong et al. 2009, Brown et al. 2007, Carballa et al. 2005, Darwano et al. 2014, de Souza et al. 2017, Maskaoui and Zou 2010, Okuda et al. 2009, Paulson et al. 1984, Petrovic et al. 2004, Rimkus 1999, Senta et al. 2013, Stasinakis et al. 2013, Sumner et al. 2010, Thomas and Forster 2005, Wijayaraine and Means 1984, Winkler et al. 1998, Yang et al. 2011, Zgheib et al. 2011)
Depending on the wastewater composition and the coating on the ENPs, previous studies have shown that a substantial amount of ENPs may remain in the treated wastewater effluent. (Jarvie et al. 2009, Limbach et al. 2008) Previous studies have also shown that organic compounds, such as natural and dissolved organic matter (DOM), are able to stabilize a range of different nanoparticles, both NONPs and ENPs, in laboratory setting or environmental conditions. (Aiken, et al. 2011, Domingos et al. 2009, Mosley and Hunter 2003, Navarro et al. 2009) Therefore, the stabilization of discharged nanoparticles in the environment could aid their transport in the receiving water body upon their release. With rising concerns about contaminant transport on nanoparticles and the impact of nanoparticles on wastewater treatment processes and on the ecosystem, understanding the production and removal of particles of different sizes in water resource recovery facilities (WRRFs, formerly wastewater treatment plants) will thus improve our current knowledge of nanoparticles behaviour and contaminant transport.
Due to the dearth of literature on the production and behaviour of particles of different sizes in WRRFs, this study aims to understand the prevalence and size of particles along the wastewater treatment process and comparatively analyse them with the water quality parameters and activated sludge EPS.