**1. Introduction**

Climate change, together with the growing population expected over the coming years, makes food production a crucial issue. In this context, prevention and minimization of food waste are recognized as key actions [1]. In addition, food waste is highly polluting as it leads to the misuse of resources and significant greenhouse gas emission levels. To address these issues, the adoption of the biorefinery concept (circular economy approach), particularly strengthening the agri-food waste biorefinery, is a strategic point not only to make the cost of the process economical but also to reduce the pressure on natural resources [2,3]. There is no explicit mention or definition of the valorization of food supply chain waste in the Waste Framework Directive. However, the objective of using waste for value-added production comes within the spirit of the directive [4,5].

A circular economy provides a different flow model, where no resources are wasted; on contrary, they are considered as feedstocks. In particular, open-loop material flow patterns bring new supplies of secondary materials into the raw material pool that can be reclaimed by other industries [6,7]. Rice hulls are the largest by-product of rice milling in producing countries and most of them are thrown away as a waste byproduct, which will undoubtedly have too many negative influences on the global environment. Worldwide production amounts to approximately 100 million tonnes per year [8]. Rice hulls, the lignocellulosic outer coats of rice, have been only considered as combustible to recover energy or animal bedding, because of their low nutritive value as animal feed [9]. Although rice hulls have long been identified as a source for energy production, experiences from large-scale rice husk firing are quite limited, because of the high quantity of ash (about 20%) [10].

**Citation:** Costa, S.; Summa, D.; Zappaterra, F.; Blo, R.; Tamburini, E. *Aspergillus oryzae* Grown on Rice Hulls Used as an Additive for Pretreatment of Starch-Containing Wastewater from the Pulp and Paper Industry. *Fermentation* **2021**, *7*, 317. https://doi.org/10.3390/ fermentation7040317

Academic Editor: Giuseppa Di Bella

Received: 15 November 2021 Accepted: 13 December 2021 Published: 16 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Many efforts have been made to use rice hulls as feedstock to produce fermentable sugars, followed by ethanol fermentation, but the application on large scale is still affected by the out-of-market costs of pretreatments and saccharification [9]. The pretreatment process is considered the most expensive step in the valorization of lignocellulosic byproducts, where it can contribute to about 30% of the total cost. In fact, even those experiences have demonstrated that rice hulls can be more conveniently re-used as untreated material [11]. For example, promising research has been carried out on the use of rice hulls, as well as other agricultural lignocellulosic waste, as inexpensive and efficient biosorbent for heavy metals removal from contaminated wastewater [12]. From this perspective, rice hulls have also been used as solid support for the production of various fermented products and enzymes by solid-state fermentation (SSF) [13]. SSF is defined as the cultivation of microorganisms on inert carriers or on insoluble substrates that can, in addition, be used as carbon and energy source. The fermentation takes place in the absence or near absence of free water, thus being close to the natural environment to which microorganisms, especially fungi, are adapted [14].

SSF aims to bring the cultivated fungi or bacteria into tight contact with the insoluble substrate and thus to achieve the highest substrate concentrations for fermentation [15]. This technology results, although so far only on a small scale, in several processing advantages of significant potential economic and ecological importance as compared with submerged fermentation. SSF holds tremendous potential to produce enzymes, as amylases, proteases, or extracellular lipases by fungal strains belonging to the genus *Aspergillus*. Several experiences have been reported on the production of amylase and glucosidase by *Aspergillus niger* from sugarcane bagasse, corn cobs and rice hulls using SSF [16], production of amylase by *Aspergillus oryzae* on spent brewing grain as solid substrate in SSF [17].

In this work, rice hulls have been used as support for SSF of the amylase-producing *Aspergillus oryzae* to verify the feasibility to add an amount of dried powder of *A. oryzae* adherent on rice hulls, as an additive in starch-containing wastewater treatments from pulp-and-paper mill [18]. Starch is presently the third most prevalent component by weight in papermaking, only surpassed by cellulose fiber and mineral pigments. It is used as a flocculant and retention aid, as a bonding agent, as a surface size, as a binder for coatings, and as an adhesive in corrugated board, laminated grades, writing paper, and other products. The starch-containing effluents generated by the papermaking industry are usually destined for the anaerobic digestion process or degraded by aerobes microorganisms in fluidized bed bioreactors. In this context, simple and low-cost strategies for accelerating starch digestion are desirable before both anaerobic and aerobic effluent treatments.

In a wider perspective, this application could represent a promising example of industrial symbiosis, where agri-food waste valorization can become the point of connection between two different supply chains, in which one company's waste is used as raw material by another company.
