Moving bed biofilm reactor as an alternative wastewater treatment process for nutrient removal and recovery in the circular economy model

Journal Pre-proofs Review Moving bed biofilm reactor as an alternative wastewater treatment process for nutrient removal and recovery in the circular economy model J.C. Leyva-Díaz, A. Monteoliva, J. Martín-Pascual, M.M. Munio, J.J. GarcíaMesa, J.M. Poyatos PII: DOI: Reference:

S0960-8524(19)31861-9 https://doi.org/10.1016/j.biortech.2019.122631 BITE 122631

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Bioresource Technology

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27 October 2019 12 December 2019 15 December 2019

Please cite this article as: Leyva-Díaz, J.C., Monteoliva, A., Martín-Pascual, J., Munio, M.M., García-Mesa, J.J., Poyatos, J.M., Moving bed biofilm reactor as an alternative wastewater treatment process for nutrient removal and recovery in the circular economy model, Bioresource Technology (2019), doi: https://doi.org/10.1016/j.biortech. 2019.122631

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Moving bed biofilm reactor as an alternative wastewater treatment process for nutrient removal and recovery in the circular economy model J.C. Leyva-Díaz a*, A. Monteoliva b, J. Martín-Pascual b, M.M. Muñío c, J.J. García-Mesa c, J.M. Poyatos b a

Department of Chemical and Environmental Engineering, University of Oviedo, 33006 Oviedo, Spain b

c

Department of Civil Engineering, University of Granada, 18071 Granada, Spain

Department of Chemical Engineering, University of Granada, 18071 Granada, Spain

*corresponding author, email: [email protected], phone number: +34 985103509, Department of Chemical and Environmental Engineering, University of Oviedo, 33006 Oviedo, Spain

ABSTRACT Over the last years, an increasing concern has emerged regarding the eco-friendly management of wastewater. Apart from the role of wastewater treatment plants (WWTPs) for wastewater and sewage sludge treatment, the increasing need of the recovery of the resources contained in wastewater, such as nutrients and water, should be highlighted. This would allow for transforming a wastewater treatment plant (WWTP) into a sustainable technological system. The objective of this review is to propose a moving bed biofilm reactor (MBBR) as a novel technology that contributes to the circularity of the wastewater treatment sector according to the principles of circular economy. In this regard, this paper aims to consider the MBBR process as the initial step for water reuse, and nutrient removal and recovery, within the circular economy model. Keywords: Circular economy, Moving bed biofilm reactor, Nutrient, Resource recovery, Water reuse.

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1.

Introduction

Today, the European economy remains heavily dependent on primary resources, which are becoming increasingly scarce on the basis of the current linear economy model. According to the economic study carried out in several of the most important sectors in European Union, the way in which resources are used following the linear economy model entails direct costs of primary resources that reach 1.8 trillion euros per year in Europe, to which must be added indirect costs of 5.4 trillion euros per year, for a total of 7.2 trillion euros annually. Since only 40% of discarded products are reintroduced into the economy, the current economic model results in the loss of 95% of the value of the resources used. The replacement of the linear economic model by a circular economy model offers a great opportunity to increase the resource productivity, reducing waste and consequently dependence on primary resources. As a result, this change could generate a profit of up to 0.6 trillion euros per year associated with the direct use of primary resources and an indirect profit of up to 1.2 trillion euros, bringing the total annual profit to about 1.8 trillion euros by 2030, which represents a 25% decrease in costs compared to the linear economy model (Ellen MacArthur Foundation, 2015). In light of this, the wastewater treatment sector plays a key role in the shift towards a circular economy (IWA, 2016). Urban wastewater treatment has been considered as a step to reduce the emission of pollutants into the environment, avoiding pollution of the receiving water bodies. Nevertheless, urban wastewater contains large amounts of resources that could have a secondary use in the economy (Sfeza et al., 2019). Amongst them, is wastewater sludge, which could be a valuable source of nutrients (Verstraete and Vlaeminck, 2011), and treated wastewater, which could be reused after the

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corresponding treatment. Therefore, recovering and giving a second life to resources available in wastewater can contribute to this paradigm shift (Vanrolleghem and Vaneeckhaute, 2014; Molina-Moreno et al., 2017). Thus, it implies that wastewater streams are considered as a resource, and not as a waste, in order to comply with one of the three principles of a circular economy which states that no waste should be produced by circular systems, only by-products and resources to be used in further processes (Djuric Ilic et al., 2018; Molina-Sánchez et al., 2018). The concept of a circular economy has emerged as an alternative to the linear economy model that is based on the ‘take-make-use-dispose’ system, considering waste as the last step of the product life cycle (Ghisellini et al., 2016). However, the circular economy is restorative and regenerative by intention and design (Ellen MacArthur Foundation, 2015; Geissdoerfer et al., 2017), and aims to reduce the resources escaping from the process in order to get an optimal performance (Smol et al., 2015). Consequently, the circular economy model intends to convert a waste into a resource to be fed back into the process as a raw material, closing the resources loop and extending their lifespan as long as possible (Lieder and Rashid, 2016; Ghisellini et al., 2016; Iacovidou et al., 2017). This model allows for fostering a sustainable and resourceefficient management, so diminishing the consumption of primary resources (European Environment Agency, 2014; Heshmati, 2017; Voulvoulis, 2018). Based on the European Commission Report (European Commission, 2008), the increase of the amount of wastewater sludge represents a global concern for wastewater treatment plants (WWTPs) as more than ten million tons of sludge were produced in the European Union in 2008, with an estimation of growth up to thirteen million tons by 2020 (Kelessidis and Stasinakis, 2012). This causes negative externalities from an environmental and socio-economic perspective as sludge disposal processes entail 40% 3

of the total greenhouse gas emissions from WWTPs (Brown et al., 2010; Gherghel et al., 2019). Thus, wastewater sludge can be considered as a source of resources, such as carbon and nutrients, among others. This could replace high quantities of raw materials so avoiding their production from non-renewable sources with a serious environmental impact (Fijalkowski et al., 2017). In this regard, the considerable nutrient content in sludge should be highlighted, ranging from 2.4% to 5.0% for nitrogen and varying from 0.5% to 0.7% for phosphorus, as a proteinaceous material. This could be extracted from sludge and used as a fertiliser (Tyagi and Lo, 2013). Consequently, this could reduce the demand of conventional fossil-based fertilisers and the subsequent ecological footprint associated with them (Zhang et al., 2017). It is specially important for the phosphorus recovery since phosphate rock is limited and recycling of phosphorus from wastewater may be viewed as an alternative source for industries that depend on this chemical element (Hermassi et al., 2017; Bacelo et al., 2020). However, other resources can also be extracted, although in a lesser extent, from the wastewater sludge. For example, bioplastic recovery from wastewater sludge could be an interesting alternative to contribute to replace synthetic fossil-fuel polymers with bio-based bio-degradable plastics in order to drastically reduce plastic pollution in the environment (Mannina et al., 2019a). Apart from the use of wastewater sludge, water can also be obtained from wastewater streams. In addition to the possibility of recovering nutrients from treated wastewater streams, as happens with sewage sludge, water reuse is necessary due to the huge demand on water resources derived from agricultural and industrial activities, urbanisation, climate change, population growth and unceasing consumption of water (Goswami et al., 2018). Treated wastewater constitutes an alternative source of water, which could reduce the demand from agricultural to industrial uses (European 4

Environmental Agency, 2012; U.S. Environmental Protection Agency, 2012; Lazarova et al., 2013; Becerra-Castro et al., 2015). In light of this, in order to overcome the currently existing barriers for the application of nutrients recovery and/or water reuse in the concept of circular economy, several measures should be carried out. This implies, among others, the elaboration of a regulation that facilitates the reuse of resources, a budget to address the necessary investments and the need for a cultural change in society. Although they are currently considered waste, properly treated wastewater and wastewater sludge have a high added value, as previously mentioned. For this reason, regulations should be drawn up in order to promote their reuse as resources or source of energy, therefore replacing the use of other non-renewable resources. In this sense, the European Commission has recently elaborated a proposal for a directive that establishes the minimum quality requirements for the reuse of treated urban wastewater in agriculture (European Commission, 2018a). However, regulation should be also developed to make use of the wastewater sludge, whose disposal treatment requires high amounts of energy, assuming approximately 50% of the total operation cost of WWTPs (Collivignarelli et al., 2019). It can be noted that regulations must be developed and aligned with other sectors, as there may be existing regulations in other sectors that do not allow the use of reclaimed wastewater or wastewater sludge as fertiliser. However, to be technically and economically feasible to comply with the regulations, their implementation must be gradual over time and have incentives and/or European funding. Otherwise, countries could focus their efforts and budgets on a limited number of WWTPs, achieving adverse results by leaving many other sources of pollution untreated.

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Finally, there must be an awareness that allows a change in the perception that population has of WWTPs. These should be understood as facilities that not only improve the water quality of the receiving bodies, but also generate an alternative water source, low-cost fertilisers and energy from biogas. Therefore, the conception of wastewater as a major social and environmental problem should be converted into an opportunity of achieving the 2030 Agenda for Sustainable Development within the circular economy paradigm (United Nations, 2017). The recovery of resources from wastewater entails environmental benefits and the transformation of the current linear economy model into a circular economy one. Thus, wastewater treatment and recycling, by suitable technologies, could limit its release into the environment. In this regard, the urban WWTPs emerge as an essential part of the circular economy because of the integration of resource recovery and clean water production (Mo and Zhang, 2013; Rashidi et al., 2015). As a consequence of the forthcoming implementation of the European Directive on the circular economy, it is necessary to have technologies that are reliable and readily transferable to the market. The importance of novel technologies to achieve the transition towards a circular economy is present in Europe. In light of this, the moving bed biofilm reactor (MBBR) could be used to foster the recovery of resources from wastewater, which constitutes the first step in the context of a circular economy. In light of this, the limitation of freshwater supply and the requirement of rising water demand for economic development confer the potential in minimising the gap between the availability and demand on MBBR technology. In an MBBR process, biomass mainly grows attached to plastic elements, called carriers, as biofilm (Ødegaard et al.,

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1994; Ødegaard, 2006). These elements keep moving within the bioreactor due to the agitation caused by aeration in an aerobic reactor, or by mechanical stirrers in anoxic/anaerobic reactors (Leyva-Díaz et al., 2013). MBBR technology shows important advantages in relation to conventional processes for wastewater treatment. In light of this, this process has less space requirement, a higher surface area for mass transfer, higher organic loading rates at lower hydraulic retention times (HRTs), and higher effluent quality regarding organic matter and nutrients (Leyva-Díaz et al., 2014). Currently, this technology faces some challenges such as high energy costs due to aeration for carriers mixing purpose, possible formation of stagnant zones within the reactor that hinder the complete mixing, and capital costs associated with reactor construction and carriers (di Biase et al., 2019). Therefore, this technology could be appropriate for water reuse and recycling. Fig. 1 shows a diagram about the implementation of the circular economy model in urban wastewater treatment for resource recovery (closing the loop), with potential applications in agricultural and/or industrial uses. Wastewater would be treated in an MBBR-based WWTP that produces discharged water and sewage sludge. The water stream could be processed for reuse, and the sludge stream is treated for recovery of products such as nutrients (nitrogen and phosphorus). This could reduce water and fertiliser demands. 2.

Nutrient removal in moving bed biofilm reactors in comparison with CAS and MBR technologies

Nowadays, the increasing pressure to meet more stringent discharge standards, especially in terms of nutrient release, has led to the new implementation of advanced biological treatment processes (Güneş et al., 2019). Nutrients in wastewater have a potential effect in environmental contamination (Wang et al., 2019). For this reason, the removal of these components from wastewater is essential in such a way to obtain 7

treated water with no negative impact in the environment. A wide range of wastewater treatments have been developed attending to the different necessities and singularities of each effluent, with Biological Nutrient Removal (BNR) being the preferred group of methods to obtain the desired reduction (Shore et al., 2012). BNR proceeds slowly because the microorganisms responsible for the removal reactions grow slowly (Chaali et al., 2018). Conventional Activated Sludge (CAS) is one of the most extended treatment systems with BNR, although some authors have questioned its effectiveness mainly due to energy efficiency and operational problems. Additionally, it requires high volume reactors to achieve a good performance level in the nitrification process (Güneş et al., 2019). For that reason, MBBR could be an efficient system for the incorporation of biomass as biofilm instead of flocs. Güneş et al. (2019) compared a pilot MBBR plant vs a CAS wastewater plant, tested for nitrogen removal efficiency. It was concluded that MBBR reduced plant footprint by 50% vs a CAS, leading to a capital investment and operational cost reduction that impacted in a lower cost for treated wastewater. Simulations done for that research showed that with such a reduced footprint for MBBR, nitrogen levels were slightly higher in MBBR that for CAS, at 9 mg/L vs 6 mg/L ,while orthophosphate was significantly lower at 0.7 mg/L vs 4.2 mg/L (Günes et al., 2019). Aziz et al. (2019) carried out a review study on different biological treatment technologies for slaughterhouse and meat industry wastewater. Slaughterhouse wastewater has a high content of nutrients, being a challenging media to test the effectiveness of different technologies. In that research, findings in lab tests for CAS showed that the maximum COD, ammonia, and phosphorous removal efficiencies were

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98%, 99%, and 65%, respectively, while for MBBR technology, 81% of nitrogen and 96% of phosphorous removal were achieved. In the same line, Iannacone et al. (2019) concluded that for low dissolved oxygen (DO) levels the advantages of the use of MBBR results in lower reactor volumes and sludge recycle elimination, typical of CAS systems, resulting in a further decrease of investment and operational costs. Additionally, they evaluated the C/N ratio effect in MBBR, giving 2.7 as the value where lower nitrogen levels are obtained, and also achieving significant phosphorous removal. Chaali et al. (2018) reviewed nitrogen removal for different biofilm reactors in wastewater treatments. Regarding their findings, MBBR technology has a high, or very high, rate for nitrogen removal while MBBR varies between high for synthetic wastewater, to medium-high for real wastewater. MBBR can achieve full nitrification when used for carbon and ammonia oxidation in both configurations, alone in a pure system or following a CAS. Being focused on the nitrification process, 75% of DO is needed for ammonia oxidation while the other 25% is used to oxidise nitrite to nitrate (di Biase et al., 2019). Lima et al. (2016) observed that the role of the suspended biomass in the overall nitrification and denitrification can be very significant in highly loaded MBBRs and should not be neglected, even at low HRTs. Moreover, to run a MBBR with low SRT values, as well as low temperatures, is possible to obtain high ammonium removal efficiency, since a large fraction of nitrification activity will take place in the biofilm (Di Trapani et al. 2013). A simultaneous nitrification-denitrification membrane bioreactor (SND-MBR) was able to remove 80% of the total nitrogen (TN) with 48% of influent TN being removed by SND and 31% of TN being removed by a conventional membrane bioreactor (C-MBR).

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Characterisation of the microbial populations within each MBR revealed that a C-MBR favoured the development of Nitrospira and Nitrobacter while the SND-MBR favoured Paracoccus and Nitrosospira species (Paetkau and Cicek, 2011). Later research has shown that the performance level of C-MBR, and MBBR alone, can be improved by a hybrid system of both called Integrated Fixed Film Activated Sludge Membrane Bioreactor (IFAS-MBR) (Mannina et al., 2017) Experimental results obtained from pilot plants working with real urban wastewater indicate that the nitrification and denitrification processes in the moving bed biofilm reactors were slightly more effective than in the membrane bioreactor, 67.34 ± 11.22% vs 65.17 ± 7.41%. The tendency for phosphorus removal was similar to that for the nitrogen one, 50.65% ± 11.13% vs 48.31% ± 10.77% (Leyva-Díaz et al., 2013) Using tannery wastewater, Shodi et al. (2018) compared CAS technology with MANODOX, which is a CAS upgraded system that includes a MBBR process. Results for nitrates, phosphates and sulphates removal showed a much higher performance for the process considering MBBR, especially for shorter digestion times. Other authors such as Ashrafi et al. (2019) have studied the most sophisticated configuration for MBBR, the hybrid 5-stage Bardenpho. This was tested exhibiting a maximum performance for both organics and nutrients removal under a total HRT of 15.74 h, corresponding to an HRT in the anaerobic compartment of 2 h and a nitrate recycle ratio of 2 h. Satisfactory COD, NH4+-N, TN and TP removal efficiencies were obtained of about 98.20%, 96.50%, 92.54% and 94.70%, respectively. Two quadratic response surface models for TN and TP removal displayed a satisfactory, and statistically significant, fitting between the experimental and the predicted values (Ashrafi et al., 2019).

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The cost effectiveness of phosphorus removal processes in municipal wastewater treatment for MBR and MBBR, as part of an IFAS, was studied by Bashar et al. (2018). This research considered energy consumption, chemicals, maintenance costs, and sludge disposal. This study concluded that MBR is less cost effective, with a cost of more than $60/lb-P removed compared to IFAS at $42.22/lb-P removed, mainly driven by chemical and sludge treatment. Independently of the removal efficiency, the influence of MBR and CAS on direct and indirect greenhouse gas (GHG) emissions and operation costs was studied by Mannina et al. (2019b). These authors obtained higher indirect GHG emissions for MBR than CAS, whose result is related to the higher energy consumption in MBR. Multiple BNR systems are being used and studied with CAS, MBR and MBBR being some of the most representative. MBBR improves CAS performance, requiring a significantly smaller footprint and operational cost, and, in fact, it is an option to support CAS plant upgrades to reduce the nutrient levels after wastewater treatment. On the other hand, comparing MBBR with MBR for nutrient removals, there is no clear performance differentiation between them, although MBBR is supposed to provide lower operational costs. In addition, the operational control of the aerobic and anaerobic conditions needed for nitrification and denitrification are very important for the efficiency of the process and can be difficult. 3.

Case studies in hybrid technology for treated wastewater reuse: Regulations and quality parameters

Water scarcity is a growing problem worldwide, exacerbated by the mismanagement of water resources, increasingly constrained by overexploitation, contamination and climate change (Connor et al., 2017). In recent years this has caused the requirement for

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new sustainable water resource management strategies. Amongst them, it is unthinkable not to take advantage of the opportunities derived from the reuse of wastewater (United Nations, 2017). This practice not only prevents pollution of freshwater resources by incorporating wastewater into the value chain but may also significantly reduce demands on water resources, in line with circular economy policies (European Commission, 2015). For this reason, many international authorities are currently updating, or developing, treated wastewater reuse programs and the regulations in order to promote the regeneration of wastewater for reuse instead of discharge. For example, while many states in the United States have had specific regulations governing this practice for many years, the Environmental Protection Agency of the United States has recently advised state governments to update, or otherwise establish, water reuse regulations to encourage the adoption of this practice (Environmental Protection Agency, 2019). On the other hand, in the specific case of Europe, the European Commission has recently drawn up the first proposal of a directive for wastewater reuse, which aims to give adequate treatment to wastewater for use to help alleviate the demand for water in agriculture (European Commission, 2018a). In this proposal, reclaimed wastewater must meet stringent quality requirements for organic matter (up to BOD5 < 10 mg/L), suspended particles (up to SS < 10 mg/L), turbidity (up to < 5 NTU), microorganisms (up to E. Coli < 10 UFC/100 mL), and even the possibility of including contaminants of emerging concern (CECs) removal requirements is contemplated (European Commission, 2018b). As a consequence, water reuse is driving new advances in water treatment technologies that allow for the production of high-quality reclaimed wastewater that can reliably meet water reuse standards. Among them, hybrid MBBR technology, based on attached 12

and suspended biomass combined growth by adding plastic carriers into the aeration tanks, has become a suitable alternative in order to meet more stringent effluent parameters (Mannina and Viviani, 2009). Recently, several studies have been performed on hybrid MBBR systems, producing interesting results in the removal of many pollutants and showing the efficiency of these systems for the production of high-quality reclaimed wastewater (Leyva-Díaz et al., 2017). They are listed in Table 1. Shreve and Brennan (2019) conducted a comprehensive study of the performance of hybrid MBBR systems at full-scale. They assessed six full-scale hybrid MBBR based urban WWTPs in the Eastern United States working with different carriers (AnoxKaldnes K1, AnoxKaldnes K3, AnoxKaldnes K5, BiofilmChip P and ActiveCell 450) and filling ratios (from 0.35 to 0.67). Their study showed excellent effluent quality in terms of TSS (< 4.8 mg/L), COD (< 29.5 mgO2/L), and ammonia (< 0.4 mgN/L), and TN and TP removal yields ranging from 48.7 to 94.4 %, and from 30 to 96 %, respectively (Shreve and Brennan, 2019). Yadav et al. (2019) evaluated the performance of four full-scale urban WWTPs operating with different secondary treatment technologies in Australia, among which was a hybrid MBBR-based WWTP. They also observed that the hybrid MBBR based WWTP was able to produce a high quality effluent (Yadav et al., 2019). BOD5, TSS, COD and NH4 of the effluent were 11.0, 6.2, 79.0 and 1.0 mg/L, respectively, and TN and TP removal yields were 95.80 and 41.86 %, respectively. All of these are in accordance with that obtained by Najibul et al. (2019) in their study. They carried out a similar study in which they evaluated organics and nutrients removal in seven full-scale WWTPs from North India, based on different processes, for two years (Najibul et al., 2019). In general terms, they observed that highest quality effluent was produced in the hybrid MBBR-based WWTP. With this 13

technology, an effluent with an organic matter content of 12 mgO2/L as BOD5 and < 50 mgO2/L as COD was obtained. TSS removal yield was higher than 90%, whereas nutrient removal yields were 93 and 60 % for NH4-N and PO4-P, respectively. Due to its high efficiency and affordable cost, hybrid MBBR systems are also one of the most promising treatment technologies for the removal of CECs from urban wastewater. The ability of full-scale hybrid MBBR-based WWTPs to remove CECs was also evaluated by Shreve and Brennan (2019), and Yadav et al. (2019), in their respective works. Shreve and Brennan (2019) analysed the removal of 98 CECs in several fullscale hybrid MBBR-based WWTPs. They stated that hybrid MBBR technology is capable enough in completely degrading up to 17 CECs from urban wastewater such as 1,7-dimethylxanthine, caffeine, estriol and methylparaben. Furthermore, up to 23 CECs presented removal yields above 90 %. These findings demonstrated potentially higher hybrid MBBR removal efficiencies for a number of CECs in comparison with CAS technology reported in the literature. On the other hand, Yadav et al. (2019) evaluated the removal of CECs in several full-scale WWTPs with different treatment technologies. Hybrid MBBR-based WWTP resulted in the most effective technology for removing CECs. They also obtained removal yields around 90% for targeted CECs. Both researches concluded that hybrid MBBR is very effective in CEC removal as a consequence of a higher biodegradation due to the higher presence of MLSS and the combined presence of both quick and slow-growth microorganisms. Furthermore, in industry it has proven to be a reliable technology to achieve a high quality effluent for potential reused. For example, Baddour et al. (2016) assessed the performance of this system in the treatment of poultry slaughterhouse wastewater. The system was able to achieve removal yields up to 94.1% COD, 95.49 % BOD5, 96.61 % TSS, 50.8% NO3-, and 33.7% orthophosphate (PO43-). Pinto and Souza (2018) also 14

tested this technology for the treatment of pesticide industry wastewater for water reuse. The treatment of the industrial wastewater with this technology proved to be an efficient solution regarding the discharge limits requirements (COD and NH4). COD concentration in the effluent ranged between 75 and 89 mgO2/L, resulting in a removal yield of 77%. On the other hand, NH4 was almost completely removed (95% of removal yield), the NH4 content in the effluent being between 0.7 and 3.1 mgN/L. TSS and turbidity also were measured in the effluent, presenting average values of 28 mg/L (80.68%) and 6.1 NTU (84.36%) respectively. Hybrid MBBR technology has also been tested for the treatment and reuse of industrial laundry wastewater. Mozia et al. (2016) used a hybrid MBBR pilot-scale WWTP consisting of a 1 m3 aerobic bioreactor with Picobells® carriers inside, and a secondary settling tank. Results were promising, achieving average values of 36 and 22 mgO2/L of COD (95 %) and BOD5 (92 %), respectively, in the effluent. However, nutrient removal yields were not excessively high (16 % and 29 % for NT and PT, respectively). When very stringent effluent requirements are difficult to meet, membrane bioreactors (MBRs) are being favoured. MBR technology is based on the activated sludge process with biomass separation by MF/UF membranes. However, combining MBR with moving bed results in an interesting alternative called hybrid MBBR-MBR (Ødegaard, 2017). To the best of our knowledge, hybrid MBBR-MBR has not yet been used in full-scale plants (Ødegaard, 2017). However, several authors have demonstrated the feasibility of combining MBR with MBBR for the removal of organic matter, nutrients and CECs at a pilot-scale. The work of Mannina et al. (2018) evaluated and compared the different removal yields for organic matter and nutrients obtained in an MBR with respect to those obtained in a 15

MBBR-MBR. They used an MBR system that worked with Amitech® carriers when operated as MBBR-MBR, with filling ratios of 40% in the aerobic zone and 15 % in the anoxic zone. They observed that at a C/N ratio of 5 in the influent, the MBBR-MBR system was able to achieve a higher removal efficiency (80%) in comparison to that observed in the MBR system (74%). Regarding nutrients removal, they also noticed that nitrogen removal was enhanced in the MBBR-MBR system as a consequence of the combined presence of suspended and attached biomass. Nitrogen removal increased from the 39 % achieved with the MBR system, to the 53 % obtained in the MBBRMBR system. Likewise, the phosphorus removal was higher in the MBBR-MBR system compared to the MBR. At the same inlet C/N = 5 ratio, phosphorus removal in the MBBR-MBR was above 60 % while no removal was observed in the MBR system. A similar study was carried out by Rodríguez-Sánchez et al. (2018) in which they examined the same comparison, but for the treatment of variable salinity wastewater, in order to simulate coastal-located WWTPs. They used three bioreactors operating in parallel, each one consisting of three aerated tanks, one anoxic tank, and one membrane tank. The MBR configuration did not work with carriers in the tanks, while for the MBBR-MBR configurations, one reactor operated with carriers only in the aerated tanks, and the other with carriers in both the anoxic and aerated tanks at a filling ratio of 35 %. They observed that in general terms, COD removal was similar in the three systems, but it was slightly higher in the MBR configuration. Instead, results suggested that hybrid MBBR-MBR technology was the best one for nitrogen removal, with removal yields not exceeding 45 %. They justified the low nitrogen removal yields obtained in this study because salinity inhibits the nitrogen removal process. The performance of an Electro MBBR-MBR in terms of nutrients removal was also assessed by Borea et al. (2017). The peculiarity of this system is that an intermittent 16

voltage gradient of 3 V/cm is applied inside the bioreactor in order to palliate fouling formation in the membranes due to the extra EPS generation as a consequence of the biofilm (Borea et al., 2017). The system was filled with BIOMASTER BCN 012 KLS Amitec® carriers in a 30 % ratio. They concluded that the synergy produced by the combined treatment achieved NH4-N and PO4-P removal yields of 55.0 % and 98.7 %, respectively, values higher than those obtained in the MBBR-MBR when no voltage gradient was applied in the reactor (49.8 % and 76.7 %, respectively). Regarding CEC removal in MBBR-MBR, there is a work carried out by De la Torre et al. (2015) in which removal yields of 39 different CECs in MBR, pure MBBR-MBR, and hybrid MBBR-MBR are compared. This study concluded that the hybrid MBBRMBR system is the most efficient in CEC removal. The biofilm presence yielded to an enhancement of CEC removal efficiency due to the contribution of the biofilm (63.8, 50.9, and 71.8 %, for MBR, pure MBBR-MBR, and hybrid MBBR-MBR, respectively). On the one hand, it leads to different aerobic/anoxic/anaerobic conditions, which increases the degradation possibilities, while on the other hand, the higher sludge age of the biofilm allows a better acclimation of the microorganisms to the contaminants. Recently, Monteoliva-García et al. (2019) have performed a comprehensive study about the effects of carrier addition on water quality and pharmaceutical removal capacity of a membrane bioreactor – advanced oxidation process combined treatment. In that study, they analyzed BOD5, SS, turbidity, E. coli and CEC removal (European Commission, 2018b) in order to produce reclaimed water from urban wastewater in a membrane bioreactor – advanced oxidation process (MBR-AOP) and in a moving bed biofilm reactor – membrane bioreactor – advanced oxidation process (MBBR-MBR-AOP). Concerning exclusively the biological treatment, both MBR and MBBR-MBR treatments showed similar and great potentials to produce high quality reclaimed 17

wastewater (BOD5 < 7 mgO2/L, SS < 1 mg/L, turbidity < 1 NTU and no presence of E. coli). However, the addition of carriers (MBBR-MBR) did improve the pharmaceutical biodegradation in comparison with the MBR treatment, which had as a consequence the MBBR-MBR-AOP showing a complete degradation of pharmaceuticals after 5 min AOP treatment. 4.

Recovery of nutrients from waste sludge in the circular economy model

In this section a comparison of various techniques applied to the recovery of nutrients from various effluents of wastewater treatment plants is made. Since phosphorus cannot be replaced by any other element in biochemical processes, it becomes an essential element for all living organisms. It is currently obtained from phosphate rock, but this resource is not renewable and is running out. Recovery, and recycling, of the phosphorus plays a very important role in the closing of the phosphorus cycle and in the implementation of the circular economy (Ohtake and Tsuneda, 2019). The reduction of nutrients, nitrogen, and phosphorus, from the wastewaters has appeared as an alternative for their recovery, both from the sludge line and from the water line, in which 70% of them are still discharged (Guaya et al., 2018). Table 2 shows the main techniques used for the recovery of nutrients from wastewater treatment plants effluents. If we focus on the chemical recovery of phosphorus from the liquid phase of wastewater treatment plants, several commercial techniques can be found such as AirPrex®, Crystalactor®, NuReSys® or PHOSPAQ® (Li et al., 2017; Ye et al ., 2017; Zhou et al., 2019; Gherghel et al., 2019). These processes are based on chemical precipitation in which mostly calcium and magnesium are used as reagents to form hydroxyapatite (Ca5(OH)(PO4)3) and struvite (MgNH4PO4·6H2O), respectively (Desmidt et al., 2015). In all of them the phosphorus is recovered in the form of struvite except for the Crystalactor® technique in which phosphorus is recovered as hydroxyapatite (HAP). 18

The recovery rates of these techniques range between 70 and 95% depending on the case, and in almost all cases, the process is carried out in continuous stirred tank reactors except in the case of Crystalactor® in which a fluidised bed is used (Ye et al., 2017). Both the pH and the ratio between the reagents are aspects that must be controlled, mainly due to their influence on the available amount of phosphates and ammonium in the case of pH (Bi et al., 2014). In addition, higher doses of magnesium are usually used than the stoichiometric ratio necessary (Tong and Chen, 2007). In some countries the AirPrex® process in which MgCl2 and flocculants are used as reagents, has already been applied, and phosphorus recoveries that reach 90% have been obtained when the process was applied to digested sludge (Desmidt et al., 2015). The Crystalactor® technique can reach 80% phosphorus recovery yields (Cornel and Schaum, 2009) through the use of Ca(OH)2, NaOH, H2SO4 and sand. In the NuReSys® process (Moerman, 2012), both MgCl2 and NaOH are used as reagents for the obtention of struvite precipitates and values greater than 85% of phosphate recovery can be achieved. Finally, the PHOSPAQ® technique, which uses only MgO as a reagent, was applied to a mixture of effluents from wastewater treatment plants and 1.2 tons per day of struvite was obtained (Abma et al ., 2010). In the liquid phase, phosphorus recovery can also be carried out by adsorption techniques. This technique applied to the recovery of phosphorus is not expensive and it is effective when phosphorus is dissolved in the treated liquid. In addition, some of the adsorbents used can be used as fertilisers in agriculture as after the process they are enriched in phosphorus (Wendling et al., 2013). Different options for phosphorus adsorbents with different retention capacities can be found in the literature. In this kind of technique, the pH is also a variable to be controlled in the case of metal adsorbents, since it can change the surface charges of them (Xie and Zhao, 2016) and the phosphate 19

species (Dai et al., 2014). Phosphorus recovery of more than 99% can be achieved by increasing the amount of adsorbent (Dai et al., 2014). In the case of not directly using the adsorbent for a specific purpose, it is necessary to include a desorption stage. In this step, salts, alkalis and acids are usually used, although the subsequent use of the recovered phosphorus will limit the use of each of them (Delaney et al., 2011). The phosphorus-rich liquid obtained after desorption should be passed through a chemical precipitation process to form phosphorus precipitates if necessary (Loganathan et al., 2014). Natural phyllosilicate minerals usually retain a few amounts of phosphate when working at neutral pH, however, increasing the pH also increases the adsorbent capacity (Edzwald et al., 1976). On the other hand, the oxides or hydroxides of iron, aluminium or zirconium are potent phosphate adsorbents, and amongst them, the ones with the highest selectivity are natural or synthetic iron oxides (Ryden et al., 1987). Later, it was found that phosphate adsorption can be maximised by using a binary oxide of Fe and Mn (Zhang et al., 2009). The zeolites can be used both naturally and modified, and in the latter way, phosphate adsorption almost five times greater than natural can be achieved (Wendling et al., 2013). Porous nano-structured Ca-silicate (NCS), mesoporous silica nanoparticles Mobil Crystalline Material (MCM-41), or mesoporous structures based on titanium are some of the porous nanosilicates used as phosphorus adsorbents in aqueous solutions (Southam et al., 2008; Zhang et al., 2011, Choi et al., 2011). Natural zeolite of the clinoptilolite type in its potassium form can be impregnated with hydrated metal oxides to prepare a wet reactive adsorbent that is capable of retaining both ammonium and phosphate from the effluent of treated urban wastewater (Guaya et al., 2018). An important fact is that it has been proven that this zeolite could be used in 20

agriculture by reducing the impact of nitrogen on the hydrological system. In addition, other types of materials, such as dry or chemically modified materials (Rockfos® and lanthanum-modified bentonite), can be used for the same purpose, such as phosphorus adsorbents with adsorption capacities of up to 45.6 mg/g (Kasprzyk and Gajewska, 2019) In the case of the recovery of the phosphorus from sludge, other options should be used since both the sewage sludge and the sewage sludge ash have the phosphorous fixed. For the phosphorus recovery of these two phases, a wet chemical process is usually used as well as a thermochemical process (Ye et al., 2017). In the first case, the chemical process, acids and bases are usually used to recover phosphorus. The strong acids usually used are HCl, H2SO4, or even H3PO4 in the case of treating sewage sludge ash. With these treatments, more than 99% of the phosphorus can be dissolved (Petzet et al., 2012). The base that is most used is NaOH since it dissolves more phosphorus that other species (Torres and LLoréns, 2008) and up to 70% phosphorus recovery can also be achieved from sewage sludge ash (Ye et al., 2017). In the thermochemical treatment high temperatures are used and the presence of chlorinated additives, mixed with the phase to be processed, is necessary. In this way a double effect is achieved, the formation of compounds such as NaCaPO4 that capture phosphorus, and the elimination of heavy metals present in the media that are transformed into volatile compounds included in the flue gasses, which can be treated (Herzel et al., 2016; Vogel et al., 2016). As with the liquid phase, when sludge line is used as a source for nutrient recovery, a couple of commercial processes that have been applied on a real scale, such as Seaborne® and AshDec®, can be found. Both processes require specific equipment that increases the costs and makes the process more complicated (Ye et al., 2017). In the 21

first case, work is carried out in continuous stirred tank reactors and recoveries of more than 90% of the phosphorus are obtained, while in the latter technique, it is necessary to use an oven. About the generation of sludge, it should be mentioned that there is a wide variety of processes for water treatment depending on whether carriers for biofilm formation are used or if microorganisms are simply kept in suspension. In addition, the type of solidliquid separation also influences decisively, such as in activated sludge technology, or through membrane technology. The separation of the sludge in the MBBR processes can be carried out by using membrane technology or settling tank, in both cases giving rise to a mixed sludge between the sludge in suspension and the biofilm detached from the support (Leyva-Díaz et al., 2017). In general, the characteristics of the sludge obtained will depend on the operating conditions, mainly the sludge retention time (SRT) and the method of separation of the biomass used, settling tank or membranes (Massé et al., 2006). The most recent research is now directed at hydrothermal carbonisation (HTC), which can be a promising alternative for the treatment of sewage sludge and also employs acids to improve recovery yields. In fact, the simple combustion of the sludge generated in the treatment plants can result in waste in terms of nutrients such as phosphorus and nitrogen. The HTC process consists in the development of a wet conversion in which the water present in the sludge is used both as a solvent and as a reagent. The flocculated structure of the sludge is broken and the hydrolysis of the organic matter is produced, giving rise to the hydrochar that, being hydrophobic, can be easily separated. The nature of the products will vary depending on the temperature and residence time used in the treatment (Libra et al., 2011). By using this technique, the precipitation of phosphate and nitrogen in the form of struvite can be achieved. Becker et al. (2019) 22

have been able to treat anaerobically digested sludge, containing high loads of aluminium and iron salts, by hydrothermal carbonisation. In this case, phosphorus release was not possible with a single HTC process due to the initial characteristics of the sludge where the phosphates were strongly bound to iron and aluminium. So that an acid treatment can be used to remove the phosphorus from the hydrochar and transfer it to the aqueous phase, in percentages that reached 94.8%, in addition the liquid part obtained from HTC proved to be a source of ammonium containing up to 291 mmol/L of ammonium. So, after a pH adjustment and the addition of a source of magnesium, high purity struvite can be precipitated. On the other hand, the addition of nitric acid can improve carbonisation in such a way that phosphorus recovery is improved, reaching phosphorus recovery values of 82.5% by weight, of the sludge. Other authors used the same technique applied to dehydrated sludge from a treatment plant with anaerobic/anoxic/aerobic treatment (Shi et al., 2019). In this case, an acidification by HCl was also used to transfer the phosphorus obtained to the aqueous phase, in percentages greater than 80%, so that the bioavailability increased considerably and phosphorus recovery values greater than 98% were reached in the struvite form with a purity of 90.41%. Techniques such as the combination of ozonation and microwaves also prove to be effective for the concentration and recovery of phosphorus in sludge from the aerated biological reactor and from the reuse line of a wastewater station (Cosgun and Semerci, 2019). This study compared acidic and basic pre-treatments followed by microwave treatments and with ozonation/microwave combinations. Basic pre-treatments greatly increased solubilisation and release of phosphorus, but decreased ammonium, magnesium, and calcium, that could suggest a loss of phosphorus due to the precipitation of struvite or apatite. The best combination of processes turned out to be 23

the acid/ozonation/microwave pre-treatment, however, it was not far from the values obtained with the same process but without ozonation, which due to the need to reduce costs proved to be the most economically effective and viable alternative. Once an increase in phosphorus solubility of 479% was achieved, its precipitation in the form of struvite can be improved by adding it in the form of fine seeds. Membrane technology also has a place in the recovery of wastewater phosphorus. The combination of an iron-doped membrane bioreactor and an acidogenic fermentation, focused on the elimination and recovery of phosphorus in wastewater treatments, can lead to phosphorus recovery efficiencies of 62% of the influent of residual water (Li et al., 2018 a). In ferric iron-dosing MBR, a greater phosphorus removal is achieved (98.1%), while in the fermenter and subsequent denitrification, some of the phosphorus not eliminated (53.4%) is recovered and a great elimination of nitrogen (91%) is achieved (Li et al., 2018 b). The acid recovery of the phosphorus that takes place in the fermentation is carried out in the form of a precipitate such as vivianite, although with other impurities such as iron hydroxides and iron carbonates. However, these substances will not affect the use of vivianite as a fertiliser since they are naturally present in the environment (Li et al., 2019). Membrane technology can also be used for the fractionation of nutrients from the effluent obtained from the anaerobic digester (digestate). After pre-treatment, diafiltration is carried out with ultrafiltration followed by nanofiltation. Through this system phosphorus retention of 6.78 mmol/L was achieved during UF, and 31.8 mmol L in NF. As for ammonium, 13.4 mmol/L was retained (Zacharof et al., 2019). WWTPs in general are recipients of microplastics which mostly remain in the sewage sludge and may have a risk in the use of this sludge for agriculture (Mahon et al., 2016). In fact, after different analyses, it has been shown that the amount of microplastics 24

increases in the areas where sludge has been applied, which demonstrates that the sludge is a vehicle for soil contamination with microplastics (Corradini et al., 2019). No evidence has been found yet to determine the presence of microplastics in the phosphorus recovered from the sludge from sewage treatment plants. It would be interesting to analyse the effect of their presence in the different recovery treatments as well as their presence or absence in the phosphorus products obtained. Currently various processes, both commercial and developing, for the recovery of nutrients from the effluents of wastewater treatment plants, can be found. The precipitation of phosphorus in the form of struvite, apatite, or vivianite, is one of them, as well as adsorption. In addition, in the case of nitrogen it is also possible to recover it by forming struvite. Novel techniques such as ozonation, microwaves or membrane technology are giving way to the most implanted alternatives. Once different phosphorus recovery techniques have been analysed, if we transfer it in terms of operating costs, it is based on the fact that today the cost of phosphorus recovery is higher than that of phosphate rock for use as fertilisers. This is due to the difference between the market price of struvite versus that of superphosphate and raw phosphate rock (Desmidt et al., 2015). Taking into account that phosphorus recovery yields in the water and sludge line can reach up to 50% and up to 90%, respectively, an additional specific cost of a water treatment with recovery has been generally estimated of integrated recovery of phosphorus that ranges between € 2 and € 6 per capita and year (Cornel et al., 2009). Likewise, these costs in terms of phosphorus recovery can range from € 2.2 to 8.8 /kg P (Ye et al., 2017). It is clear that the overall cost of phosphorus recovery has a great dependence on the technology used. In specific phosphorus recovery processes from digested sewage sludge, with struvite recovery yields greater than 65%, an additional operating cost that 25

would increase wastewater fee by approximately € 0.15/m3 has been estimated (Mayer et al., 2019). After analyzing the necessary costs for the recovery of phosphorus, for example as precipitated struvite, it is concluded that it generates profits not only for the wastewater treatment plants but also for crop producers (Maaß et al., 2014). In addition, a 6-year return on investment (ROI) has been estimated with AirPrex ® commercial technology already operational in some countries of the European Union (Zhou et al., 2019). 5.

Conclusions

The use of advanced wastewater treatments is necessary not only to remove pollutants from water, but also to provide them with a high quality for reuse. Several studies have shown that MBBR and MBBR-MBR systems are suitable technologies for the production of high quality reclaimed wastewater. This could alleviate the increasing demand for water and, therefore, water scarcity. Additionally, numerous methods for the recovery of nutrients from WWTP’s effluents have been developed and many investigations are focused on it. Both nutrient recovery and treated wastewater reuse could contribute to the transition to a circular economy model by reducing the consumption of virgin resources. Acknowledgement The research was supported by the Spanish Ministry of Science, Innovation and Universities in the framework of the research project RTI2018-101270-B-I00. References 1.

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generations of the AirPrex® systems. Int. J. Environ. Sci. Technol. 16(5), 24772440.

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Figure captions Figure 1. Implementation of a circular economy model in urban wastewater treatment for resource recovery through moving bed biofilm reactor (MBBR) technology.

Wastewater sludge Urban wastewater

MBBR TECHNOLOGY (urban wastewater treatment)

Treated wastewater

SLUDGE PROCESSING

CIRCULAR ECONOMY WATER PROCESSING

Nutrients (N, P)

FARMING/INDUSTRIAL USES

Water and nutrients (N, P)

Figure 1

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Tables

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Table 1. Case studies of MBBR-based wastewater treatment for wastewater reclamation reported in literature. System

Scale

Carriers tested

Filling ratio

Hybrid MBBR

Full-scale urban WWTP

AnoxKaldnes K1, K3 and K5, BiofilmChip P and ActiveCell 450

35-67%

Hybrid MBBR

Full-scale urban WWTP

-

-

Hybrid MBBR

Full-scale urban WWTP

-

-

Hybrid MBBR

Poultry slaughterhouse Industrial WWTP

Hybrid MBBR

Pesticide Industrial WWTP

Hybrid MBBR-MBR Hybrid MBBR-MBR Electro MBBR-MBR Hybrid MBBR-MBR

Laundry industrial WWTP Pilot-scale urban WWTP Pilot-scale urban WWTP Lab-scale WWTP Semi-real-scale urban WWTP

Hybrid MBBR-MBR

Pilot-scale urban WWTP

Hybrid MBBR

5 mm polyethylene granules

AnoxKaldnes K1

Picobells®

-

50% -

Amitech®

40% aerobic 15% anoxic

AnoxKaldnes K1

35%

Biomaster BCN 012 KLS Amitec®

30%

Polypropylene plastic carriers (Christian Stöhr®)

50%

AnoxKaldnes K1

35%

Pollutant removal efficiency and effluent quality 4.8 mg/L TSS 48.7-94.4% TN 29.5 mgO2/L COD 30-96% TP 0.4 mg/L NH4+ > 90% for 40 CECs 11.0 mg/L BOD5 95.80% TN 6.2 mg/L TSS 41.86% TP 79.0 mg/L COD > 90% CECs + 1.0 mg/L NH4 12 mgO2/L BOD5 > 93% NH4 50 mgO2/L COD > 60% PO4-P > 90 % TSS 94.1% COD 95.49% BOD5 96.61% TSS

50.8% NO333.7% PO4-P

75-89 mgO2/L COD 28 mg/L TSS (80.68%) (70%) 6.1 NTU Turbidity 0.7-3.1 mg/L NH4 (84.36%) (95%) 36-22 mgO2/L COD 16% NT (95 %) 29% PT 92 % BOD5 80% COD 60% PT 53% NT > 90% COD 45% NT 55.0% NH4 98.7% PO4-P 71.8% CECs < 7 mgO2/L BOD5 < 1 mg/L SS < 1 NTU Turbidity

≈ 0 UFC/100mL E. Coli > 90% CECs

Reference Shreve and Brennan, 2019 Yadav et al., 2019

Najibul et al., 2019 Baddour et al., 2016 Pinto and Souza, 2018 Mozia et al., 2016 Mannina et al., 2018 Rodríguez-Sánchez et al., 2018 Borea et al., 2017 De la Torre et al., 2015 Monteoliva-García et al., 2019

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Table 2. Main techniques employed for the recovery of nutrients from wastewater treatment plants effluents. Wastewater Treatment Plants Treated wastewater Chemical precipitation

Adsorption technique

Wastewater sludge Hydrothermal Wet chemical Thermo-chemical carbonization process process (HTC) Membrane Technology

Ozonation/ microwave combination

Highlights > Resource recovery from urban wastewater to promote shift towards a circular economy > Pilot and full-scale studies show MBBR as suitable system for treated wastewater reuse > Numerous methods developed for nutrient recovery from WWTP’s effluents

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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