Validated innovative approaches for energy-efficient resource recovery and re-use from municipal wastewater: From anaerobic treatment systems to a biorefinery concept

Abstract The development of innovative technologies in wastewater treatment create the concept of biorefinery in wastewater treatment plants (WWTPs), placing anaerobic processes in the highlight. Starting from the conventional anaerobic treatment processes to ‘closing the loop’ scheme, next generation WWTPs are ready to serve for water, energy and materials mining. While bioenergy is still dominating the resource recovery, recovery of value-added materials (i.e. struvite, biopolymers, cellulose) are receiving significant attention in recent years. So, what are the state-of-the-art approaches for energy-efficient resource recovery and re-use from municipal wastewater? This paper follows a critical review on the validated technologies in operational environment available and further suggests possible market routes for the recovered materials in WWTPs. Considering the development and verification of a novel technology together with the valorization of the obtained products, biorefinery and resource recovery approaches were gathered in this review paper from a circular economy point of view. General currently-faced barriers were briefly addressed to pave the way to create to-the-point establishments of resource recovery facilities in the future. Graphical Abstract


Introduction
During the last years, wastewater treatment plants (WWTPs) have moved from the concept of 'waste treatment', aimed at discharging treated wastewater into surface waters, to the concept of 'water resource recovery facility' (WRRF). This transformation from pollutants removal to valuable resources frames wastewater management in the broader context of the circular economy. The question that arises is which are the possible recovered and safely reusable resources to help closing the loop in WRRF?
First of all, the reclaimed water: Water reuse is particularly important because it is considered as an effective approach to address water shortage problems and water quality deterioration issues (Sun et al., 2016). Water reuse can be one of the methods of recycling treated wastewater for beneficial purposes, such as agricultural and landscape irrigation, industrial processes, non-potable domestic use (e.g. toilet flushing), and groundwater replenishing. At EU level the minimum quality standards for water reuse have been proposed in May 2018; this proposal for regulation lays down minimum requirements for water quality and monitoring as well as the obligation to carry out specified key risk management tasks. Classes of reclaimed water quality, minimum treatment requirements, allowed uses, irrigation methods and minimum requirements for water quality are set (http://www.europarl.europa.eu/RegData/etudes/BRIE/2018/625171/EPRS_ BRI(2018)625171_EN.pdf).
On site energy recovery in WWTPs, particularly as biogas production, is widely diffused as an alternative source of energy, for the recovery of thermal, electrical and mechanical energy, to be consumed either inside (also achieving energy self-sufficiency) or outside the plant. Nowadays energy recovery takes place mostly in the sludge line and actions in water line are much rarer but more and more of interest (Papa, Foladori, Guglielmi, & Bertanza, 2017). Biogas is the main resource of anaerobic treatment systems. In the last years; however, two-step bioconversion comes into prominence as more value is derived to volatile fatty acids (VFAs) production before ending up to other end-products. Moreover, anaerobic processes offer much more than conventional wastewater treatment, provide recovering sustainable energy and valuable biochemicals. This scenario helps to recognize conventional and innovative (i.e. anaerobic membrane bioreactor -AnMBR) anaerobic processes as the core of biorefinery general concept (Puyol et al., 2017, Krzeminski, Leverette, Malamis, & Katsou, 2017. Nutrient recovery and recycling take an important role in circular economy. Recovered nutrients from the wastewater can be utilized as soil amendments or fertilizers for beneficial uses in agriculture. In particular, NH 4 þ form is advantageous because it predominates in anaerobic reactor effluents and can be useful for fertigation purposes. Phosphorus recovery (i.e. in the form struvite or phosphorous salts) becomes essential for preventing eutrophication in the aquatic environment and for alleviating economic dependence on phosphate rocks. Addressing raw materials conservation, arising from phosphorus scarcity is described as one of the greatest global challenges of the 21st Century (Peng, Dai, Wu, Peng, & Lu, 2018). The resources mentioned above are those most commonly recovered in WWTPs; in addition to them there are more innovative ones that can be originated from cellulosic primary sludge (CPS) and polyhydroxyalkanoate (PHA) rich sludge. The cellulosic sludge can be separated by upstream dynamic sieving. The CPS can then be anaerobically digested to produce biogas, or, under optimal acidogenic fermenting conditions it produces VFAs. Here the propionate content can be more than 30% and can optimize the enhanced biological phosphorus removal (BPR) processes (Crutchik, Frison, Eusebi, & Fatone, 2018). Long-term operation indicated that anaerobic alkaline fermentation for VFA production from sewage sludge is both technically and economically feasible (Liu, Han, et al., 2018). Regarding PHA recovery, primary and secondary sewage sludges are potential feedstock (Kumar, Ghosh, Khosla, & Thakur, 2018). PHAs have comparable properties to petrochemical plastics and can also serve as biofuel or building blocks for the synthesis of various chemicals (Kleerebezem, Joosse, Rozendal, & Van Loosdrecht, 2015).
While some of the above-mentioned reuse and recovery approaches towards wastewater are already efficiently implemented, some of them still lack the convenient technology together with social-technological planning and design methodology to identify their potential end-use and market requirements (Van Der Hoek, De Fooij, & Struker, 2016).
At European level there are several EU projects founded by Horizon 2020 that aim at recovering materials from centralized and decentralized WWTPs. For example, SMART-Plant is an Innovation Action that aims at reducing the energy and environmental footprint and, contemporary, at recovering valuable materials (SMART-Products are water, cellulose, biopolymers, nutrients) that are valued in construction, chemical and agriculture supply chain (smart-plant.eu). POWERSTEP is another Innovation Action aims at energy positive WWTP, biogas production and carbon extraction (powerstep.eu). P-REX, similarly, aimed at sustainable sewage sludge management promoting phosphorus recycling and energy efficiency (p-rex.eu).
This review critically analyses innovative anaerobic processes to recover materials and energy from municipal wastewater, state of the art WWTPs and future aspects. The energy-efficient resource recovery is examined by the critical analysis only of the anaerobic processes. Moreover, the discussed technologies are selected based on the validation criteria of the demonstrative or full scales applications to ensure the robustness of the technologies supporting the characteristics, as quantity and quality, of the products to be marketed. Hence, this review paper aims to provide a comprehensive overview to the biorefinery concept, recent leading technologies and further sustainable scenarios to fulfill circular economy goals. Although great previous efforts have been done towards anaerobic processes and the concept of resource recovery, environmental technology verification (ETV) has further reviewed innovative technologies together with the possible valorization market alternatives, bottlenecks or barriers of the recovered products.
2. Brief evolution of anaerobic schemes as the core of the biorefinery approach Anaerobic treatment is one of the most promising treatment technologies for developing more sustainable sanitation; while it is considered to be the core technology for resource and energy recovery (Stazi & Tomei, 2018). Upflow anaerobic sludge blanket (UASB) was successfully implemented and established within a wide acceptance in municipal WWTPs, especially in tropical and subtropical regions where the temperature of the wastewater is usually above 20 C (Lohani et al., 2016). Expanded granular sludge bed (EGSB) was further developed to enhance substrate-biomass interaction within the treatment system by expanding the sludge bed and increasing hydraulic mixing compared to UASB (Niwa et al., 2016;Cuff et al., 2018). Although well-established UASB and/or EGSB configurations mostly meet the requirements necessary for anaerobic treatment, unfavorable conditions regarding the disintegration of granules led to the development of AnMBR by coupling membrane technology with anaerobic treatment. Meanwhile, combined heat and power (CHP) systems using the anaerobic digestion (AD) of sludge has become the most adopted technology in the existing energy self-sufficient WWTPs. Based on life cycle comparison, AnMBR technology was found to produce more net energy as biogas compared to conventional activated sludge coupled with AD (Gu et al., 2017). The main advantage of AnMBR as a mainline wastewater treatment process is the capacity to recover most of the energy potential in the wastewater rather than the fraction currently recovered by the aerobic-anaerobic treatment.
The recent results obtained from pilot-scale AnMBRs treating domestic wastewaters were reviewed and discussed in detail (Shin & Bae, 2018). Table 1 provides a schematic representation of different flow schemes to enhance the role of anaerobic processes as core of biorefinery approach. Table 2 refers to resource recovery associated with the schemes in Table 1; all of them recover methane, which is the main produced-resource in WWTPs where anaerobic processes are implemented. Moreover, some of them recover N, P and VFAs due to coupling with the membrane technology. The optimization of anaerobic processes brings the key towards energy self-sufficient WWTPs.

Wastewater treatment line
The biogas produced during anaerobic treatment of wastewater can be utilized as an energy source (Table 3). However, a significant amount of CH 4 cannot be recovered since a major proportion is dissolved in the effluent, even if the biogas exhibits high CH 4 content (Liu, Han, et al., 2018;Souza, Chernicharo, & Aquino, 2011). Anaerobic wastewater treatment processes such as UASB is therefore limited because of low liquid upflow velocity and inadequate mixing (Yeo & Lee, 2013). CH 4 losses recorded in different anaerobic reactors at a pilot-scale were listed and discussed and average CH 4 loss in the effluents were stated between 19% and 85% (Crone, Garland, Sorial, & Vane, 2017). In municipal wastewater treatment up to 30%-40% of the produced CH 4 was reported to be loss in the application Table 1. of AnMBR (Krzeminski et al., 2017). Although many efforts were directed towards recovering dissolved CH 4 from anaerobic effluents, such as hollow fiber membrane contactors (Cookney et al., 2016) or down-flow hanging sponge reactors (Hatamoto, Miyauchi, Kindaichi, Ozaki, & Ohashi, 2011), scaling up is still missing and validation is required.
In the concept of valorization of municipal wastewater, two-step bioconversion stands as an attractive alternative route  in the fermentation reactors (Table 3). Complex organic matter in wastewater is simply converted to VFAs before ending up as other valuable end-products (Zhou, Yan, Wong, & Zhang, 2018). This allows a separated optimization of bioconversion mechanisms into a more straight-forward bioproduction process (Pan et al., 2018). The concentration and speciation of VFAs during this process often determines the desired quality of the end-products (Reyhanitash, Kersten, & Schuur, 2017).
Although VFAs are so-called intermediate products, they have various potential applications within the WWTP. Acetate is the most-preferred VFA product for denitrification within WWTP followed by butyrate and propionate (Elefsiniotis & Wareham, 2007). Propionate can enhance BPR processes in biological nutrient removal systems (Chen, Randall, & Mccue, 2004). However, further application, either within WWTP or the product value, will determine the desired VFA concentration and speciation (Peces, Astals, Clarke, & Jensen, 2016) as discussed in the following section. Microbial fuel cells (MFCs) is also another option to produce electricity from VFAs by fermentative hydrogen production (Teng et al., 2010); however, this technology has not been validated yet. Low power density and high operating cost of MFCs limit their implementations on a large-scale (He et al., 2017).
AnMBRs have been successfully implemented to treat municipal wastewaters with high COD removal rates for the main water reuse purpose. However, the discharge of the treated effluents into the aquatic environment or water reuse is usually not possible without further nutrient removal (Ruiz-Martinez, Martin Garcia, Romero, Seco, & Ferrer, 2012;Batstone, H€ ulsen, Mehta, & Keller, 2015). In this regard, nutrient removal  (Draaijer, Maas, Schaapman, & Khan, 1992) from AnMBR effluent using microalgae was proposed (Viruela et al., 2016). In addition, within the context of EU LIFE Project MEMORY (life-memory.eu), submerged AnMBR was demonstrated to combine AD and membrane technology. Such innovative pilot-scale implementations suggest promising technologies for municipal wastewater treatment and resource recovery (https://ec.europa.eu/info/research-and-innovation/law-and-regulations/identifying-barriers-innovation_en).

Sludge treatment line
Sewage sludge management constitutes a major part of the operating expenses of municipal WWTPs. In full-scale WWTPs sewage sludge usually undergoes AD to recover energy (CH 4 -rich biogas), and thus produce heat and electricity within the concept of CHP plants. There is also a growing trend to use sludge as feedstock in other value-added processes together with bioenergy (Zacharof & Lovitt, 2013) as shown in Table 4.

Recovered materials: VFA
More attention has been paid in the recent years to recover VFAs through the acidogenic fermentation of sewage sludge on down-stream processes Longo et al., 2015). In a wider biorefinery concept, carbon upgrading to VFAs seems an energy-efficient and cost-effective strategy. However, utmost importance lies here when considering the WWTP as an integrated process, since extracting VFA will reduce the amount of organic matter fed to AD, which will eventually decrease the energy recovery (Peces et al., 2016). In this regard, the benefits of VFA production and extraction from sewage sludge should be well-designed and optimized in order not to outshadow methane recovery. The optimization should focus on two main criteria: (i) the cost (capital investment and operating expenses) of the fermentation and extraction process and further earnings from VFA use or sale; and (ii) the impact on CH 4 generation (Peces et al., 2016).
Depending on the selective production of VFAs from the acidogenic fermentation of sewage sludge, VFAs also have high economic values such as materials used in the production of bioplastics and biotextiles (Zacharof & Lovitt, 2013;Lin, Li, & Li, 2018). For instance, acetate and butyrate are preferred for polyhydroxybutyrate (PHB) production, while propionate is required when producing polyhydroxyvalerate (PHV) (Shen et al., 2014;Peces et al., 2016). In addition, some important characteristics such as higher flexibility, low stiffness and brittleness as well as higher tensile strength and toughness are highlighted to promote the production of co-polymers using VFAs with higher   propionate/acetate ratio (Frison, Katsou, Malamis, Oehmen, & Fatone, 2015;Crutchik et al., 2018). However, establishing consistence VFA concentration and proportion remains a significant challenge.

Recovered materials: nutrients
As mentioned earlier, there is large interest in decreasing costs and elevating sustainability by energy-efficient resource recovery in the concept of biorefinery (Raheem et al., 2018). So far, validated biorefinery products from WWTPs include nutrients (i.e. N, P), biopolymers (i.e. PHA) and cellulose (Zijp et al., 2017;Raheem et al., 2018). The recovered nutrients from WWTPs can be utilized for struvite and/or Ca-P precipitation (Cie slik & Konieczka, 2017;Melia, Cundy, Sohi, Hooda, & Busquets, 2017) or biochar adsorption (Huggins, Haeger, Biffinger, & Ren, 2016). Struvite (MgNH 4 PO 4 ) crystallization has been successfully used for simultaneous recovery of nutrients from wastewater (Hermassi et al., 2018) together with calcium phosphate (Ca 3 (PO 4 ) 2 ) (Le Corre, Valsami-Jones, Hobbs, & Parsons, 2009). Struvite is more preferred in agricultural use due to the fact that Mg, N and P are released simultaneously (1:1:1 M ratio), and that the rate of nutrient release is slow compared to other fertilizers (Puchongkawarin, Gomez-Mont, Stuckey, & Chachuat, 2015). Deficient concentrations of phosphorus, on the other hand, limit the struvite precipitation. Even the presence of toxic compounds and/or micropollutants in wastewater restrain its purity and further agricultural application. Hence, alternative nutrient recovery technologies (i.e. membrane, electrodialysis) should be considered to improve the quality of the recovered nutrients (Xie, Shon, Gray, & Elimelech, 2016). Several other benefits are also associated with P recovery by crystallization. For instance, the volume of the sludge produced together with other undesired precipitates diminishes, which eventually decreases the cost of sludge disposal (Hermassi et al., 2018).
Integration of the Short Cut Enhanced Nutrient Abatement (SCENA) system into the Carbonera WWTP, Italy, was previously evaluated (Longo, Frison, Renzi, Fatone, & Hospido, 2017) and the results motivated the Horizon 2020 'SMART-Plant' action which is currently running and investigates the optimization of the best scenario for SCENA within SMARTech4a and SMARTech4b. Briefly, the SCENA system integrates the following processes: optional upstream concentration of cellulosic sludge, fermentation of dynamic thickened sewage sludge to produce VFAs as carbon source, and nitrogen and phosphorus removal (by P-bioaccumulation) via nitrite from sludge reject water using an SBR. In this configuration, nitrogen is removed through the bioprocesses of nitritation/denitritation, and Enhanced BPR (EBPR) via nitrite using the VFAs from sludge fermentation liquid as carbon source. SMARTech4b is another validated SCENA pilot-scale system at the WWTP of Psyttalia, Greece. It enables the integration of the enhanced biogas recovery (by thermal pressure hydrolysis) of sewage sludge with side stream energy-efficient and compact nitrogen removal and phosphorus recovery. The CAMBI TM thermal hydrolysis process has been installed to treat 50% of the produced sludge before it is sent for AD. The integration of CAMBI TM with AD produces, after dewatering, a reject water stream that has a very high ammonium nitrogen concentration (>1.2 gN/L) to be removed in the SCENA unit.
Furthermore, many technologies are applied in full size for specific objective of phosphorous salts recovery. In fact, CalPrex TM reactor (Predigestion P-recovery) placed between the acid phase and gas phase digesters enables dissolved phosphorus in dewatered centrate precipitates and is recovered as a brushite crystal. Similarly, the AirPrex TM reactor (Sludge optimization and P-recovery) placed between the anaerobic digester and the dewatering equipment converts the orthophosphate into struvite crystals, which are harvested from the bottom of the reactor. Both the WASSTRIP TM and Ostara Pearl TM processes are already in operation in a number of municipal installations and achieve efficient P recovery (Point, Kemp, & Marten, 2017). Examples of commercial processes for P recovery and the different final P products derived were thoroughly listed and discussed (Melia et al., 2017). Potassium recovery from wastewater has not been substantially considered and is an emerging issue (Batstone et al., 2015). There are also quite number of other promising nutrient recovery technologies that are yet invalidated, such as microbial recovery cell-anaerobic osmotic membrane bioreactor ((MRC)-AnOMBR system (Hou et al., 2017), reactive sorbents (Hermassi et al., 2018) and microalgae (Viruela et al., 2016).

Recovered materials: PHA
Biopolymers are a group of polymers with similar properties to petroleumbased plastics, produced from renewable sources also by different types of bacteria using carbon as substrate (Raheem et al., 2018). The main advantages of PHAs are the possibility of being completely biodegradable and nontoxic. PHA-storing bacteria are well-known to grow in activated sludge processes of WWTPs that store these polymers as carbon source and energy reserve . The series of operations needed for microbial production of PHAs are substantially described in the literature (Tamis, Marang, Jiang, van Loosdrecht, & Kleerebezem, 2014;Anjum et al., 2016). On-going pilot-scale demonstrations in recent years offer fundamental experience to produce PHA from waste materials in enough quantities to inspire value chains and investment within first bio-based value chains.
To launch the private and public relationships that will drive the economic and regulatory framework, it is crucial to verify and explore technology process basis, to validate recovered material flows to marketable renewable resources (Valentino et al., 2017). SMARTech2b stands as the key to enable secondary mainstream energy-efficient resource recovery in Manresa WRRF, Spain. It applies the mainstream SCEPPHAR (Short-cut Enhanced Phosphorus and PHA Recovery) and consists of two sequencing batch reactors (SBRs); one for heterotrophic bacterial growth operated under anaerobic/anoxic/aerobic sequence (HET-SBR), and another SBR for autotrophic nitrifiers growth (AUT-SBR), an interchange vessel and a chemical system for P-recovery as struvite. The integrated system accomplishes enhanced N-removal and P-recovery in municipal WWTP. PHA is recovered from the anaerobic purge of the SBR. SMARTech5 also applies the SCEPPHAR concept in Carbonera WWTP, Italy, which was conceived as a modified version of SCENA where PHA recovery is an economically sustainable option. It accounts of the following subprocesses: (i) cellulosic primary sludge fermentation to enhance the production of VFAs and release nitrogen and phosphorus in soluble forms (i.e. ammonia and phosphate); (ii) solid and liquid separation of the fermentation products and recovery of struvite form the sewage sludge fermentation liquid by the addition of Mg(OH) 2 to favor the precipitation; (iii) ammonium conversion to nitrite accomplished in a SBR; (iv) selection of PHA storing biomass in a SBR by the alternation of aerobic feast conditions and followed by anoxic famine conditions for denitritation driven by internally stored PHA as carbon source; (v) PHA accumulation using a fed-batch reactor to maximize the cellular PHA content of the biomass harvested from the selection stage. Within the context of the INCOVER Project, pilot-scale mainstream phototrophic PHA recovery is conducted in Viladecans, Spain. PHA is produced through photo-bioreactors in which microalgae and cyanobacteria communities grow in a symbiotic relationship, removing pollutants from urban and agricultural wastewaters and accumulating PHA. Produced biomass is then fed into AD with sewage sludge or other biomass sources as co-substrate for biogas production (incover-project.eu). An innovative biogas upgrading technology is also implemented, based on the symbiosis between microalgae and bacteria and the photosynthetic fixation of CO, which removes CO and HS to produce biomethane of 92%. In El Torno WWTP, Spain, PHA production is produced through two-stage anaerobic-photosynthetic high rate algae pond systems that are consisting of pulse feeding of municipal wastewater pretreated in an UASB reactor with molasses as COD source. Similarly, after PHA production, the remaining biomass is converted into biogas using thermal pretreatment and an anaerobic codigestion process followed by biogas upgrading.

Recovered materials: cellulose
Municipal wastewater contains high amounts of cellulose fiber (30%-50% of the total suspended solids) that is mainly originated from toilet papers (Behera, Santoro, Gernaey, & Sin, 2018). These cellulose fibers easily enter biological treatment systems of WWTPs if they are not separated during the primary treatment; while the biodegradation of cellulose is comparatively difficult and depends on many factors (Ruiken, Breuer, Klaversma, Santiago, & van Loosdrecht, 2013;Crutchik et al., 2018). On the other hand, cellulose fibers hold a great potential as a resource which can be recovered from wastewater by sieving (Ruiken et al., 2013). The benefits of cellulose dewatering sludge are: minimization of chemical consumption, lower electricity consumption for aeration, less chance of phosphate release and much lower sludge volume to discharge that reduces sludge handling and management cost. Cellulose harvesting is expected to have added benefits to the WWTP's downstream biological process and provided outside the WWTP for the downstream blending with PHA and processing for final biocomposite production. SMARTech1 comprises an innovative integration of dynamic fine-sieving together with in-situ post-processing that is currently validated in the municipal WWTP of Geestmerambacht, Netherlands. CirTec has developed flow scheme with filter for primary treatment (Salsnes Filter TM ) and separating cellulosic fibers to produce a highly-concentrated sludge. The produced cake layer or fine sieved fraction (FSF) harvested from Salsnes Filter TM has a very heterogeneous composition containing mainly cellulosic fibers originating from toilet paper. The result is a market-ready cellulose that has been cleaned, dried and disinfected. Examples of the recovered materials from WWTPs in SMART-Plant are shown in Figure 1.

Technologies to the market: focus on the environmental technology verification and other performance certifications
At EU level, innovative environmental technologies are validated by ETV Program to prove the reliability of the developed claims and help technology purchasers identify innovations that suit their needs. Hence, the best long-term technical, environmental and economic performances are validated by ETV protocol. ETV ensures that the performance claims are as structured and completed in order to present a clear assessment of the technology's potential and value. However, it does not cover the evaluation of the technology's performance against standard or pre-defined criteria. More information can be found at ETV's official website (https://ec.europa. eu/environment/ecoap/etv_en).
In addition to validation of a technology, the functional properties of recovered materials should be determined using specific functional tests to compare recovered products with industrial products. The use of phosphate salts, biochar and pyrolysis materials, is more controlled and regulated compared to other recovered materials. The European Sustainable Phosphorus Platform (ESPP) are implementing many activities for the sustainable management of phosphorus and other nutrients. STRUBIAS -EU Fertilizers Regulationsets criteria for nutrient recovery rules within EU Fertilizing Products Regulation. At national level, authorisations of struvite/ recovered phosphates as fertilizers together with phosphorus recycling legislations are in force in some EU countries. The main challenge of these organic-based fertilizers is to ensure that their application is not resulting in an accumulation of different organic non-biogenic and inorganic compounds (e.g. toxic metals and non-metals) (Hermassi et al., 2018). Strong debates continue regarding the social awareness of consumers and framework regularities about food security (3rd European Nutrient Event, 2018).
Inconsistency and high-variability of available sources for recovered PHA quality in routine production is still a hidden gem (Valentino et al., 2017). For instance, the extracted PHA from municipal secondary wastewater was examined using 13 C NMR spectroscopy (Kumar et al., 2018). Size exclusion chromatography (SEC) and differential scanning calorimetry (SDC) were also used for the characterization of the recovered PHA (molecular number, molecular weight, glass transition temperature etc.) (Frison et al., 2015). However, consistent quality of recovered PHA derived from wastewater as feedstock has not been proven or refuted presenting a big challenge in scaled-up implementations (Valentino et al., 2017).

Valorization of recovered materials to consumer/industrial products
Sustainability assessment of the recovered materials from wastewater was conducted (Zijp et al., 2017) with respect to 6 different categories as follows: economic welfare, resource depletion, environmental and biological quality, technical welfare, human health and social welfare. It was concluded that PHA and struvite seemed economically feasible in terms of production costs and market values. However, PHA needs urgent and further investigations as it exhibits some critical barriers, which have to do with the possible emissions of toxic compounds during the production stage and concerns regarding the perception of the market on the food security. Similarly, struvite utilization also depends on location-specific aspects and legislations and needs further assessment. Cellulose recovery and application, on the other hand, seems less feasible due to the costs of extra hygiene step when used in the paper and carton industry. This step is not required in construction applications which makes cellulose recovery more beneficial for all resource themes. In a recent study, better value was derived from valorizing CPS to VFAs and struvite from the fermentation liquid, then CH 4 was further recovered after AD of remaining fermentation solids (Crutchik et al., 2018). The authors made a simple comparison by assuming CH 4 market price of 0.11 e/m 3 , the best valorization of CH 4 from CPS could be up to 0.46 e/capitaÁyear. Acetate and propionate price could be as high as 0.45 and 1.01 e/kg, respectively, meanwhile struvite could be sold up to 0.76 e/kg. Therefore, the VFAs and struvite route before biomethanization have the potential to increase the market value potential of CPS up to 1.55-1.95 e/capitaÁyear (Crutchik et al., 2018). Overall, potential end-use of the recovered materials with respect to market requirements highly influence its role within circular economy. Some of the potential end-uses of the recovered materials from WWTPs are discussed with existing market values and possible valorization alternatives.
The different market possibilities and the discussion of the advantages and disadvantages for the recovered materials commercialization opportunities are summarized in Table 5.

Market possibilities for recovered materials: nutrients
Adding nutrient-loaded sorbents enhances the soil quality in terms of agricultural yield and nutritional quality. However, socioeconomic conditions  (Keijsers et al. 2013) highly influence whether such materials can be applicable in commercial agriculture. The key factors that influence the application of post-sorbent fertilizers are the availability of feedstock, the technology to manufacture fertilizers, and the investment costs and capacity. (Hermassi et al., 2018). In addition, soil measurements together with plant bioavailability indices can help to determine fertilizer performance (Peng et al., 2018). Effects of struvite as fertilizer on various plants can be found (Kataki, West, Clarke, & Baruah, 2016). The market value of struvite varies from 188 e/t to 763 e/t struvite in the recent years (Molinos-Senante, Hern andez-Sancho, Sala-Garrido, & Garrido-Baserba, 2011;Desmidt et al., 2015). Although economic feasibility of struvite recovery is limited by high operational costs, it was also determined that when the struvite sale price is assumed as 560 e/ton, the net profit of 445.62 e/day was obtained for a full-scale fertilizer production industry with a 500 m 3 /day capacity. (Yetilmezsoy, Ilhan, Kocak, & Akbin, 2017). The European Commission's draft 'market study' assesses the possible sources of raw materials for nutrient recycling, STRUBIAS technologies and economic aspects. High quality of these struvite-based products enables them to be used as effective slow-release fertilizers for agriculture practices. Furthermore, P recovery also aids to cease eutrophication in aquatic environments. In this regard, if economic aspects for P recovery are not satisfactory, environmental benefits and government regulations could be the driving force (Peng et al., 2018). During the market development strategy for struvite, the focus should be based on a holistic approach considering pricing, demand, purity, size, storage, transportation and distribution with respect to the existing regulatory framework of contaminants and eco-toxicity (Desmidt et al., 2015;Kataki et al., 2016).

Market possibilities for recovered materials: biopolymers
Biopolymers must compete with petroleum-based polymers, which are available in high amounts at relatively low prices. Biogas could be also considered the main competitor for biopolymer production in WWTPs since organic carbon from waste material will not be diverted for the production of biopolymers when the production of biogas is more convenient (Kleerebezem et al., 2015). Thus, the market potential of bioplastics seems limited so far (Van Der Hoek et al., 2016). However, (EEA report No 8/ 2018) reported that the global production of plastics is estimated to account for about 7% of the world's fossil fuel consumption. The proportion of bioplastics is still low, currently below 1%. However, the worldwide biopolymer production capacity is forecast to increase from 6.6 million tonnes in 2016 to 8.5 million tonnes in 2021.
Production of biopolymers from waste feedstock seems advantageous and economical depending on the market requirements. It has multiple applications especially in material and packaging industries and the utilization of waste feedstock as substrate makes a great contribution to waste management and reduces environmental pollution. For instance, these waste materials are proved to be efficient substrates producing significant amounts of PHA or extracellular polymeric substances (EPS) that can help to reduce the production cost by eliminating the usage of pure carbon sources. Research is still on-going for the lower-cost production of PHAs by utilization of such low cost wastes and using wild and mutant strains of microorganisms (Anjum et al., 2016). Optimization of the processing techniques can pave the way to take PHA formation from waste materials to industrial level and then into the market (Pakalapati, Chang, Show, Arumugasamy, & Lan, 2018). At the moment, the bottleneck of the process seems to be the extraction of PHA from the biomass which requires thermal and/or chemical processes which are usually expensive.

Market possibilities for recovered materials: PHA
PHA has gained greater attraction in the recent years due to their many advantages such as biodegradability, biocompatibility, controllable thermal and mechanical properties as well as molecular weight diversity, which allow them to be used as bioimplant materials for medical and therapeutic applications (Zhang, Shishatskaya, Volova, da Silva, & Chen, 2018). Although the utilization of waste materials for the synthesis of high-class materials such as PHAs has led to cost reduction as previously mentioned, the final products cannot be used in medical applications where high purity products with nontoxic nature are of utmost considerations (Raza, Abid, & Banat, 2018). PHAs recovered from waste materials can contain viral, plasmid, bacterial or genetic contaminations that hinder their potential usage for medical applications. Impurities in PHA regarding proteins, lipids, endotoxins, antifoam agents, DNA and hypochlorite have been previously reported (Koller, Niebelsch€ utz, & Braunegg, 2013;Koller, Sandholzer, Salerno, Braunegg, & Narodoslawsky, 2013). Such impurities require specific post recovery washing procedures that eventually cause a major increase in product cost (Raza et al., 2018).
In this regard, the majority of recovered PHA applications take place in a wide range of products including paper coatings, bags, containers, food packaging materials, bottles, cups etc. (Muhammadi, Afzal, & Hameed, 2015). For instance, water-resistant layer for paper, film or cardboard can be produced out of the latex of PHAs (Anderson & Dawes, 1990;Bourbonnais & Marchessault, 2010). PHAs can also be used to replace petrochemical polymers in toner and developer compositions as well as ion-conducting polymers (Muhammadi et al., 2015).
Industrial PHAs and their applications were discussed by Anjum and colleagues (Anjum et al., 2016). Among these materials and their applications, recovered PHA can find its own place in such practices: Biopol (copolymer of poly (3-hydroxybutyrate-co-3-hydroxyvalerate)), currently produced by Metabolix (Cambridge, MA, USA), can be used in packaging materials, shampoo bottles, disposable razors, disposable cups, disposable knives and forks. Nodax (PHA copolymer family consisting of 3-hydroxybutyrate) (P&G Chemicals, USA/Japan) is available as foams, fibers or nonwovens, films and latex among others. Biogreen (Mitsubishi Gas Chemicals) developed the production of P (3HB) from methanol, and markets it under the trade name Biogreen. Furthermore, PHAs can be used to make foils and diaphragms, pressure sensors for keyboards, stretch and acceleration measuring instruments, material testing, shockwave sensors, lighters, gas lighters; acoustics, and for ultrasonic therapy and atomization of liquids (Anjum et al., 2016). A high performance of PHA biopolymer, Minerv-PHA (Minerv, Italy), takes the place of highly pollutant materials such as PET, PP, PE, HDPE and LDPE. The most known commerciallyavailable PHA products can be found elsewhere (Bugnicourt, Cinelli, Lazzeri, & Alvarez, 2014).
Other than being used mainly as environmentally friendly plastics for packaging purposes, PHAs are considered as a source for the synthesis of chiral compounds which highlight them as raw materials for the production of paints (Reddy, Ghai, Rashmi, & Kalia, 2003;Muhammadi et al., 2015). Furthermore, PHA can be hydrolyzed chemically, and the monomers can then be converted into molecules such as 2-alkenoic acids, ß-hydroxy acids, ß-hydroxyalkanols, ß-hydroxyacid esters, ß-acyllactones, ß-amino acids, which hold great potential as biodegradable solvents (Madison & Huisman, 1999;Muhammadi et al., 2015). Other important, industrial applications of PHAs are printing and photography, art-smart gels, heatsensitive-adhesives and also fishing equipment (Pakalapati et al., 2018).
Blending PHAs, in particular PHB, with other polymers, or with plasticizers, creates opportunities to enhance their properties by decreasing the processing temperature and lowering the brittleness of PHAs based plastics. So far many blends containing PHB/PHAs have been investigated and several types of plasticizers have been proposed (Bugnicourt et al., 2014).
In addition, nanocomposites of PHA were also reported (Anjum et al., 2016). For example, the preparation of biodegradable nanocomposites using nanofibrillated cellulose (NFC) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate, PHBV) as the polymer matrix has been investigated (Srithep et al., 2013). This can be a good alternative to valorize two types of recovered materials from WWTPs in one application.
PHA yield on carbon source, its productivity and downstream costs determine their introduction into the global market (Mo_ zejko-Ciesielska & Kiewisz, 2016). The selected end-use of PHA often determines the market specifications and requirements.
The current cost of PHAs production and recovery with aqueous twophase extraction method is stated as 5.77 USD/kg. However, utilizing a cheaper carbon source such as sludge has the potential to reduce the final PHA production price significantly (Leong et al., 2017). Hence, the theoretical price of PHAs produced in fed-batch mode using waste materials could reach up to 3.51 e/kg. Yet, they are still not cost-efficient when compared to their synthetic alternatives such as polypropylene and polyethylene, which cost 1.47 e/kg and 1.15 e/kg, respectively (Mo_ zejko-Ciesielska & Kiewisz, 2016). The current PHA price ranges between 2.2 e/kg and 5.0 e/kg, which depends on monomer composition and is usually higher for the copolymers. In spite of having several environmental advantages, the PHA prices are still not commercially-competitive with conventional petroleum-based polymers, which typically cost less than 1.0 e/kg (Valentino et al., 2017).

Market possibilities for recovered materials: EPS
EPS are biopolymers that are considered eco-friendly, cost effective and sustainable alternatives to substitute the existing chemical flocculants. Potential environmental applications of EPS can be listed as follows as summarized from (More, Yadav, Yan, Tyagi, & Surampalli, 2014): Water treatment (Wang, Hessler, Xue, & Seo, 2012), wastewater treatment (Li et al., 2013), color removal from wastewater (Liu, Yuan, Yang, & Li, 2009), sludge dewatering (Yang et al., 2012), metal removal or recovery (Mikutta, Baumg€ artner, Schippers, Haumaier, & Guggenberger, 2012), removal of toxic organic compounds (Zhang et al., 2011), landfill leachate treatment (Zouboulis, Chai, & Katsoyiannis, 2004) and soil remediation and reclamation (Chandran & Das, 2011). Existing literature is limited to lab-scale applications and further research is still needed to be scaled up to field applications since EPS can be used as a cost-effective treatment alternative. The cost of the EPS extraction and purification can be the limiting factor in the field application considered of various sectors with chemistry, structure and properties of interest (More et al., 2014).

Market possibilities for recovered materials: cellulose
Classification of cellulosic materials as well as current and emerging markets for cellulose-based products were thoroughly discussed (Keijsers, Yilmaz, & Van Dam, 2013). Accordingly, cellulose markets were classified into 9 categories: Textile, non-woven, wood and timber, pulp/paper and board, cellulose dissolving pulp, cellulosic films, building materials, cellulosic fiber composites and green chemicals. The selected end-use of a certain lignocellulosic raw material often specifies the market requirements. Hence, the end-use of cellulose determines the market prices and volumes, which are directly linked to the cellulose quality and defined over physical properties, chemical composition, unwanted components, prior treatments of raw material, and physical, chemical, biological stability of the cellulose. For instance, end-uses for cellulose such as pulp, paper and board are pointed on brightness, tensile and tear, freeness, write-ability; the price range is between 450 and 650 raw material price e/ton. Polymeric cellulose will eventually have higher chemical purity requirements. Meanwhile, market requirement of building materials for extracted cellulose is based on its strength, moisture absorbency and fire retardancy. Details of market volume and market price of purified celluloses can be found in the literature (Keijsers et al., 2013).
Considering its nature and high energy content, FSF has started to gain attraction in countries such as the Netherlands (Ghasimi, Zandvoort, Adriaanse, van Lier, & de Kreuk, 2016). The recovered cellulose can be used either as raw material to make paper products, adhesion binders for asphalts (Crutchik et al., 2018) or as the fibrous reinforcement material in bricks (Kim, Lee, & Choi, 2017) when properly separated and refined. For instance, cellulosic material recovered from screenings were used as an ingredient in the production of asphalt to create a bike path near Beemster WWTP, the Netherlands (Selster A.S., 2018). Similarly, Makron (Finland) uses recycled cellulose fiber additives for asphalt.
The use of natural fibers as the adsorbent is another emerging trend in environmental engineering applications including environmental remediation and water filtration membranes as the fibers are abundant, readily available and are more environmentally friendly compared to carbon based materials (Carpenter, De Lannoy, & Wiesner, 2015). They are used as adsorbents in wastewater treatment (i.e. in the form of membrane) and for the removal of adsorbates such as oil, dyes, heavy metals and ionic compounds (Rahman, Yhaya, Azahari, & Ismail, 2018). As the production of effective adsorbents at low cost and low energy consumption is placed at the center of many researches, the properties and possible modifications of recovered cellulose need to be thoroughly investigated and understood. Furthermore, market projections of cellulose nanomaterial-enabled products are estimated (Cowie, Bilek, Wegner, & Shatkin, 2014) and recent developments in production of NFC were discussed (Nechyporchuk, Belgacem, & Bras, 2016). Fluorescent cellulose bio-based plastics were successfully fabricated based on the strong hydrogen bonding interaction between cellulose chains and conjugated dye molecules, and further suggested as a good candidate for making anti-counterfeiting banknotes (Wang, Cai, Chen, Liu, & Zhang, 2016). In another study, the transparent and flexible cellulose-based nanocomposite papers were fabricated to be used as solar cell substrates (Cheng et al., 2018). However, smooth surface was proposed to be maintained on the cellulosic material to avoid problems during the coating process. The use of cellulose nanocrystals for thermal insulation can be another option to value recovered cellulose from WWTPs (Septevani, Evans, Annamalai, & Martin, 2017). NFC can be combined with clay for the preparation of nanopaper to obtain unique brick-and mortar structure (Carosio, Cuttica, Medina, & Berglund, 2016).
The possibility to use the separated materials to safely produce toilet paper was also suggested as a real cradle to-cradle application, but difficulties in relation to social acceptance were also highlighted (Ruiken et al., 2013). In all conditions and possible applications, extraction and purification methods of cellulose must be thoroughly studied to assess its feasibility to meet the criteria of end-use markets.

Barriers to resource recovery and reuse and solutions to overcome
Actually, notwithstanding the important research activities and the developed technologies for resource recovery from real WWTPs, many bottlenecks for the market uptake and for their application could be identified.
In fact, the law for water reuse, actually, regulates only the irrigation purpose. None specific indications and legislations were clearly promoted for fertigation objective as highlighted even by EU Innovation Deal on sustainable wastewater treatment combining anaerobic membrane technology and water reuse. Many lacks have been identified both in the legal definition of the term discharge and for quality standards provisions adopted for wastewater effluents to be used for agriculture. Moreover, recognition of the economic and environmental benefits of water reuse within reclaimed water pricing have to be implemented.
For phosphorous and ammonia salts, more detailed studies and programs have been developed at European level to overcome regulatory barriers. Not similar evidences have been identified for PHA and cellulose potential recovery.
Moreover, the quality, the purity and the characteristics of the recovered resources change on the basis of the implemented process in the WWTPs. On the other hand, the different market sectors request inlet materials with diverse standards on the basis of the final productive application. For this reason the certification of the technologies, which also has to include the main properties of the recovered products, seems necessary to couple the recovery processes to the industrial sectors.
In this direction, the European criteria of 'End-of-waste' could be identified as possible legislative solution to support the resources recovery application in the WWTPs. In fact, this approach (Waste Framework Directive 2008/98/EC) specifies when certain waste ceases to be waste and obtains a status of a product or a secondary raw material. The obtainment of the end of waste status has to be supported by several conditions: (i) the substance or object is commonly used for specific purposes; (ii) there is an existing market or demand for the substance or object; (iii) the use is lawful (substance or object fulfills the technical requirements for the specific purposes and meets the existing legislation and standards applicable to products); (iv) the use will not lead to overall adverse environmental or human health impacts. Starting from this point, specific regulations, centered on the end of waste concept, could be implemented to support the regulatory framework of the resources recovery. This approach can justify and encourage the technological investments in the WWTPs to economically address and support the resource recovery and to promote the circular economy in the water sector (Guest, et al., 2009).
Finally, public perception and social acceptance are insufficiently developed for all the described materials. Therefore, specific formative and public dissemination activities have to be strongly supported.

Conclusions
This paper has provided a commentary on recent advances in energy-efficient resource recovery approaches in WWTPs. Anaerobic processes stand as the 'gold mine' in wastewater mining strategy while AnMBRs are the 'gold diggers'. While the biorefinery concept has been widely recognized, lab-scale studies are gradually evolving into validated pilot/full scale implementations. Onsite energy recovery is still getting the most attention.
However, recently-developed and validated processes, such as SCENA and SCEPPHAR, derive more value to VFAs, while achieving satisfactory nutrients and PHA recovery, respectively. Together with nutrients and PHA, cellulose is another value-added material to be recovered in WWTPs. Among all the energy and material recovery methods, some are consistently considered to be beneficial to improve sustainability, and some of them still need further research to achieve desired feasibility. Struvite has a comparatively large market, also brings strong debates on food security that needs to be addressed in the near future. Valorization of PHA and cellulose, by the way, should not be overlooked since there are huge market alternatives. Therefore, there is a need to develop the regulatory framework for resource recovery and carry out socioeconomic assessments considering the market potential and specific requirements.