Cellulosic materials recovery from municipal wastewater: from treatment plants to the market

IntroductionCellulose is the most abundant available biopolymer (up to 1011 t/annum production) with a high molecular weight (Kumar et al., 2020 ). Cellulose is a long linear polysaccharide polymer consisting of β-1,4-linked glucose units (C5H8O4)m, and exhibits in structural cell wall tissues of all higher plans and some algae, while some bacteria also excrete cellulose. Considering the increasing demand for lignocellulosic raw materials in a huge number of industries, undervalued side streams with a high potential should be well utilized in order to embrace a more circular economy approach that reduces waste and allows for the continual use and re-use of resources (Keijsers et al., 2013; Puyol et al., 2017). For thousands of years, mankind process cellulose derived from plants in the production of pulp, paper, and derivatives. With the adoption of toilet paper in modern societies, cellulose fibers became a primary insoluble substrate entering wastewater treatment plants (WWTPs). In the last decade, cellulose recovery from wastewater has become a topic of interest which has attracted a remarkable attention leading to considerable investments (Palmieri et al., 2019; Ruiken et al., 2013). By introducing a microsieving (mesh size <350 μm) downstream of the coarse sieving, it is possible to recover the cellulose from cellulosic primary sludge (CPS; Gherghel et al., 2019). In a wastewater treatment scheme, sieving is a mechanical method often used to remove solid particles. A significant part of the sieved material includes cellulosic fibers (approx. 30–50%), which mainly originates from toilet papers (Espíndola et al., 2021) together with a minor contribution of clothes, fruits, and vegetables. This is a very significant amount since approximately 10 kg toilet paper is used on average per person each year. This number varies greatly, and it is location dependent. For instance, the average toilet paper use in the United States is 22.68 kg/capita year, which translates into more than 2 × 104 ton of toilet paper entering WWTPs daily, assuming that at least 90% of toilet paper is flushed after use. Meanwhile, although the annual consumption of toilet paper per capita is considerably lower in China (i.e., 2.9 kg/capita year), the total amount of toilet paper entering WWTPs is also significant because of its vast number of consumers (Li et al., 2020). Recovering and further adding value to these cellulosic fibers can easily contribute to the sustainability of wastewater treatment processes with the possibility of generating new eco-efficient products while requiring less energy and cost for water reclamation (Mussatto and Loosdrecht, 2016). Then, recovered cellulose can be processed into various value added molecules, building bioblocks, bioplastics, and flocculants (Glińska et al., 2020).