Limiting microplastic pollution from municipal wastewater treatment

DEGREE PROJECT IN ENVIRONMENTAL ENGINEERING, SECOND CYCLE, 30 CREDITS

STOCKHOLM, SWEDEN 2020

Limiting microplastic pollution

from municipal wastewater

treatment

A circular economic approach

Limiting microplastic pollution from

municipal wastewater treatment

A circular economic approach

Jordy van Osch

Supervisor

Andreas Feldmann

Examiner

Monika Olsson

Degree Project in Sustainable Technology KTH Royal Institute of Technology

School of Architecture and Built Environment

Department of Sustainable Development, Environmental Science and Engineering SE-100 44 Stockholm, Sweden

www.kth.se

Sammanfattning på svenska

Den ökande mängden mikroplast som finns i miljön har understrukit brådskan i att identifiera, utveckla och tillämpa strategier där kommunala avloppsreningsverk (MWWTP) begränsar utsläpp av urbana mikroplaster. Samtidigt har den globala trenden mot en cirkulär ekonomi definierat villkoren för dessa scenarier i förhållande till vatten-energi-näring-näxan.

Denna studie har tagit fram ett nytt ramverk mellan studier om reningsteknologier för avlägsnande av mikroplast i avloppsvattenströmmar och cirkulära ekonomiska mål från beslutsfattare med avseende på water-energy-nutrient nexus. Resultaten av denna studie bygger på befintliga bevis på att kommunala avloppsreningsverk släpper ut betydande mängder mikroplast i både mark- och vattenmiljöer. Denna studie har visat hur Multi-Criteria Analysis (MCA) kan användas för att analysera avloppsreningsscenarier utifrån deras förmåga att begränsa mikroplastföroreningar från reningsverk, samtidigt som man tar hänsyn till vatten-energi-näring-näxan. MCA har identifierat MBR-inci-eco som det bäst presterande cirkulära ekonomiska scenariot för att begränsa mikroplastföroreningar från nya verk. Detta scenario inkluderar en Membrane Bioreactor (MBR) med anaerobisk nebrytning, energiåtervinning genom förbränning och fosforåtervinning genom Ecophos. Om redan befintliga verk ska uppgradera sin anläggning för att begränsa mikroplastföroreningar, ses CASPACUF med Pyreg som energi-näringsåtervinning som det bästa scenariot. Det pulveraktiverade kolet med ultrafiltreringssystemet (PAC-UF) skulle sedan installeras som ett ytterligare poleringssteg till ett befintligt konventionellt system för aktiverat slam (CAS), vilket avsevärt minskar investeringskostnaderna.

Abstract

The increasing amount of microplastics found in the environment have underscored the urgency to identify, develop and deploy scenarios in which municipal wastewater treatment plants (MWWTPs) limit the release of urban microplastics into the environment. Simultaneously, the global trend towards a circular economy has defined the conditions for these scenarios in relation to the water-energy-nutrient nexus.

This study has created a novel framework between studies into treatment technologies for microplastics removal in wastewater streams andcircular economic objectives from policymakers with regard to the water-energy-nutrient nexus. The results of this study build on the existing evidence that MWWTPs release significant amounts of microplastics to both terrestrial and aquatic environments. This study has demonstrated how Multi-criteria Analysis (MCA) can be applied to analyse wastewater treatment scenarios for their ability to limit microplastic pollution from MWWTPs, whilst taking the water-energy-nutrient nexus into account. The MCA has identified MBR inci-eco as the best performing circular economic scenario for limiting microplastic pollution from MWWTPs in to be constructed plants. This scenario includes a Membrane Bioreactor (MBR) with Anaerobic Digestion, energy recovery through incineration and Phosphorus recovery through Ecophos. If already existing MWWTPs aim to upgrade their facility to limit microplastic pollution, CASPACUF with Pyreg as an energy-nutrient recovery is seen as the best performing scenario. The powder activated carbon with ultra filtration (PAC-UF) system would then be installed as an additional polishing step to an existing conventional activated sludge (CAS) system, significantly reducing upfront investment costs.

Academia can build upon these results to initiate additional research into novel microplastic filtration specific technologies, business model innovation for wastewater treatment and microplastic pollution prevention at the source and in stormwaters. National and international policymakers should ban the distribution and sale of biosolids for direct land application to limit the pollution of microplastics from bio-solids. Furthermore, efforts should be put in place to limit microplastic pollution at the source by stimulating policies for a ban on the use of microbeads, limit tyre wear and improving design for e.q. washing machines.

Keywords:

Acknowledgement

Firstly, I would like to thank my supervisor, Andreas Feldmann, for the many supervising meetings in which the progress of this study was discussed. His knowledge and past experience were a helpful guidance to my research. Secondly, I would like to thank my examiner, Monika Olsson, for taking her consideration and time to assess this report and attend the final seminar.

My gratitude also goes out to the survey and interview respondents, the results in this study would not have been achievable without their expert knowledge and experience; Waterschap Vechtstromen, Roslangsvatten AB, Hoogheemraadschap van Schieland en de Krimpenerwaard, Waterschap AA en Maas, Svenskt Vatten, the US EPA and STOWA. In particular I would like to thank the Development Engineer from Gryaab AB for taking the time to partake in a video-conference, for sharing important insights, comments and feedback.

Furthermore, I would like to thank my friends, family and classmates for their support, feedback and bright ideas during the completion of this research.

Table of Content

-1.1 Goal and research questions ... 9

-2. Literature review ... 10

-2.1 Municipal wastewater treatment plants and microplastic pollution ... 10

-2.1.1 Microplastic removal ... 10

-2.1.2 Microplastic leakage ... 12

-2.2 Existing and emerging technologies to remove microplastics from wastewater ... 13

-2.3 From wastewater treatment to resource recovery ... 16

-2.3.1 Water recovery ... 16 -2.3.2 Energy recovery ... 16 -2.3.3 Nutrient Recovery ... 18 -2.3.4 Other resources ... 20 -3. Methods ... 21 -3.1 Scenario creation ... 21 -3.2 Scenario analysis ... 21

-3.2.1 Decision context and system boundaries ... 21

-3.2.2 Criteria ... 21

-3.2.3 Weighting of criteria ... 23

-3.2.4 Grouping ... 23

-3.2.5 Scoring ... 23

-3.2.6 Normalization and sensitivity analysis ... 23

-3.3 Interviews ... 24

-3.4 Focus ... 24

-3.5 Ethical review ... 24

-4. Circular economic scenarios ... 25

-5. Results ... 26

-5.1 Analysis of scenarios to separate microplastics from wastewater ... 26

-5.2 Sensitivity analysis ... 28

-5.3 Validation of MCA results ... 29

-6. Discussion ... 31

-6.1 Interpretation of results ... 31

-6.2 Implications ... 32

-6.3 Limitations ... Fout! Bladwijzer niet gedefinieerd. 7. Conclusion and recommendations ... 35

References ... 37

Appendices ... 44

Appendix A: Selected Precovery technologies after incineration or pyrolysis ... 44

Appendix B: Criteria and grouping ranges ... 45

Appendix C: Stakeholders to interview ... 47

Appendix D: Scenario data ... 48

-List of Figures

Figure 1 Ratio of treated and untreated wastewaters around the world (Corcoran, et al, 2010) ... 10

-Figure 2 Typical MWWTP in Sweden, including primary, secondary, tertiary and sludge treatment (Persson,2011), adjusted to include estimated microplastic flow in MWWTP (Sun et al., 2019) ... 11

Figure 3 Visualization of membrane bioreactor system (Pandey et al., 2014). ... 14

Figure 4 Visualization RO process (Pervov et al., 2013). ... 15

Figure 5 Visualization of PACUF system (Voigt et al., 2020). ... 15

Figure 6 Circular economic cycles in regards to MWWTPs ... 16

Figure 7 Hotspots for P recovery from the wastewater stream (GWRC, 2019) ... 18

Figure 8 Process flow of methodology ... 21

Figure 9 Visualisation of the system boundaries of this study ... 24

Figure 10 Visualisation of CE wastewater pathways for limiting microplastic pollution ... 25

Figure 11 MCA Results visualised ... 26

Figure 12 Sensitivity analyses of the MCA results ... 28

Figure 13 Schematic flow of the EcoPhos process (Kabbe, 2019). ... 44

Figure 14 Pyreg process illutrated (Pyreg GmbH, 2020). ... 44

-List of Tables

Table 2 P resource recovery technologies for sewage sludge ash ... 19

Table 3 P resource recovery technologies after or during pyrolysis ... 20

Table 4 Selected criteria for multicriteria analysis ... 22

Table 5 CE scenarios for limiting microplastic pollution from MWWTPs ... 25

Table 6 MCA Microplastic pathways in wastewater treatment ... 27

Table 7 Overview of criteria and grouping range ... 45

Table 8 List of stakeholders and experts for survey and interviews ... 47

Table 9 Data for seven different scenarios ... 48

-Abbreviation

AD Anaerobic digestion BAF Biologically Active Filters

CE Circular Economy

DAF Dissolved air floatation

DF Discfilter

EU European Union

GAC Granulated Activated Carbon GHG Greenhouse gas

IVL Swedish Environmental Research Institute MBR Membrane bioreactor

MWW Municipal Wastewater

MWWTP Municipal Wastewater Treatment Plant PAAS Product-as-a-Service

PAC Powdered Activated Carbon RSF Rapid sand filtration

SSA Sewage Sludge Ash

UF Ultrafiltration

1. Introduction

In August 2019, the World Health Organization (WHO) underscored the need for further research into the leakage of microplastics into the environment (WHO, 2019). Microplastics have been found almost everywhere on Earth, from Antarctic ice sheets to the stomachs of seabirds, to our own faeces (Waller et al., 2017; Carr et al., 2016; Schwabl et al., 2019). Microplastics are particles smaller than 5 mm, consisting of tiny plastic granulates or fibres (Sharma and Chatterjee, 2017). Due to the chemical composition of plastics, these particles are resistant to degradation. Their small size makes them easily accessible to a vast range of organisms and transferable along the food chain. This results in the accumulation of potentially hazardous effects on both organisms and humans alike, causing alternations in chromosomes leading to obesity, infertility and cancer (Sharma and Chatterjee, 2017).

Microplastics mainly originate from sources such as synthetic fibres, automobile tire wear, industrial processes, household dust, and the deterioration of plastic surfaces (Nizetto et al., 2016). In high-income countries, the runoff from these emissions is conveyed to municipal wastewater treatment plants (MWWTPs). MWWTPs are fairly good at retaining microplastics through conventional treatment, over 90 percent of microplastics are retained in sewage sludge, a by-product of the wastewater treatment process (Nizetto et al., 2016). Nonetheless, MWWTPs are found to be significant sources of microplastic leakage into the environment (Mason et al., 2016). Due to the sheer volume of continuous discharge into the aquatic environment, the final effluent acts as an exit route for microplastics (Talvitie et al., 2017). It is estimated that between 3 billion and 23 billion microplastics are being released from MWWTPs each day in the United States alone (Mason et al., 2016).

Whilst this pollutant release is significant, it dwarfs in comparison to the terrestrial microplastic emissions stemming from MWWTPs. In developed regions, about 50 percent of all sewage sludge is converted into agricultural fertilizer. This indicates that at least 50 percent of the retained microplastics will be released on farmlands (Nizetto et al., 2016). Neither European nor North American regulations mention microplastics in their restrictions on using sewage sludge containing harmful substances. It is therefore suggested that MWWTPs are a significant source of microplastic pollution to not only marine, but also to terrestrial, and freshwater environments (Bayo et al., 2016).

Concurrently, the EU commission, parliament and council reached an agreement to facilitate increased use of waste-based fertilizers in 2016 (European Commission, 2018). Following in line with policymakers around the world, who are pushing forward circular economic objectives in order to reduce socio-technical pressures on the environment (Geissdoerfer et al., 2017). In a circular economy (CE), products and materials should remain in the economy for as long as possible, retaining their value, whilst waste is treated as a secondary raw-material that can be re-used, re-purposed or re-cycled (Ghisellini et al., 2016). MWWTPs consume large amounts of materials and energy to comply with discharge regulations, while wastewater simultaneously contains large amounts of energy, nutrients and other resources (Mo and Zhang, 2013). For MWWTPs to comply with the global movement towards a circular economy, it is crucial that the water-energy-nutrient nexus is taken into account in decision making. By complying to the circular economic objectives of policy makers, MWWTPS can transition from waste handlers to resource recovery facilities (Rodriguez, 2018).

1.1 Goal and research questions

This study establishes and assesses several scenarios for limiting microplastic pollution from MWWTPs and making MWWTPs a sustainable actor in a global circular economic system. The following research questions are answered in this report:

1. To what extent do MWWTPs act as barriers and/or entry points for microplastic pollution into the environment?

2. Which full-scale wastewater treatment technologies are able to remove microplastics from wastewaters whilst taking the water-energy-nutrient nexus into account?

3. Which full-scale sludge handling technologies are able to destroy microplastics whilst taking the water-energy-nutrient nexus into account?

4. What are the relevant sustainability criteria for assessing wastewater handling scenarios that limit microplastic pollution from MWWTPs whilst taking the water-energy-nutrient nexus into account?

5. Which wastewater treatment scenario limits microplastic pollution from MWWTPs whilst taking the water-energy-nutrient nexus into account?

2. Literature review

In this literature review, the challenge of urban microplastic pollution through MWWTPs is evaluated in response to research question 1. Many recent studies have assessed how microplastics flow through MWWTPs and to what extent they are emitted in effluent streams. Furthermore, the research frontline in terms of microplastic removal and destruction technologies for MWWTPs is defined in response to research question 2 and 3.

2.1 Municipal wastewater treatment plants and microplastic pollution

A MWWTP is a facility in which several combined processes treat municipal wastewater (MWW) and remove a number of pollutants (Hreiz et al., 2015). MWW is comprised of blackwater, consisting of excreta, urine and faecal sludge – and greywater, from laundry, bathing, commercial and industrial effluent (Corcoran, et al., 2010). Wastewater treatment aims to create an effluent that can be discharged safely into natural water bodies, with minimal impacts on the environment. Globally, there are many differences in terms of treatment ratios. High-income countries treat around 70 percent of the generated municipal and industrial wastewaters, while middle-income countries only treat about 28 to 38 percent. In low-income nations, only 8 percent of wastewaters get treated. In total, over 80 percent of all global wastewaters are released into the environment untreated (WWAP, 2017). This is mainly due to technical and institutional capacities, lacking infrastructure and financing (WWAP, 2017).

Figure 1 shows the ratio of treated and untreated wastewater, flowing into natural water bodies, for ten global regions (Corcoran, et al., 2010). This thesis will focus on high-income regions in which the treatment of wastewater is dominant, namely the North Atlantic, Western Europe and the Baltic Sea. These treatment facilities have a similar mix of wastewater entering the plant, and therefore use similar treatment processes (Nizetto et al., 2016).

In high-income countries, the most common form of MWW treatment is through centralized collection, whereby a sewer system collects MWW from businesses, homes and industries and transports it to a treatment plant. Today, the most common form is a sanitary sewer system. This system inhibits storm waters from entering the sewage system and transports the municipal wastewaters directly to the plants. This reduces the need for larger pipes and limits the overflow of untreated wastewater into the environment. (WWAP, 2017). As populations and industries have grown, so has the quantity of pollutants in wastewater, meaning the total volume and treatment needs are increasing (EPA, 2004). Conventional MWWTPs use physical, biological and chemical processes to remove pollutants from the effluent and typically consist of preliminary, primary and secondary treatment stages (Persson, 2011). Figure 2, on the next page shows a visual overview of a typical MWWTP in Sweden (Persson, 2011, p. 147). There are many variations to MWW treatment processes, however the depicted stages are generally accepted as standard practice in high-income regions (Persson, 2011). It should be noted that additional treatments can be undertaken depending on the wastewater flow rates and contaminant loads.

2.1.1 Microplastic removal

New risks concerning ‘emerging pollutants’ have been recognized from the early 2000s onwards (Bolong et al., 2009). Emerging pollutants are defined as synthetic or natural substances that are not commonly controlled or monitored but have the potential to harm ecosystems and human health (WWAP, 2017). One emerging pollutant that has been detected extensively in MWWTPs are microplastics (Hu et al., 2019). Microplastics consist of tiny plastic granulates or fibres, smaller than 5 mm (Sharma and Chatterjee, 2017). These particles are resistant to degradation, as they are primarily made of

polyethylene, polypropylene and other synthetic and long-lasting polymers. They are easily transferred along the food chain, as their small size makes them accessible to a vast range of organisms. This results in potentially hazardous effects on organisms and humans alike, with some evidence showing chromosomal alternations that lead to obesity, infertility and cancer (Sharma and Chatterjee, 2017). It is estimated that humans consume between 74.000 and 121.000 microplastics per person annually (Cox et al., 2019). It is therefore of increasing importance to biological and human health to limit the pervasion of microplastics in the natural world.

The main origins of microplastics include synthetic textiles, automobile tire wear, industrial processes, household dust, and the deterioration of plastic surfaces (Nizetto et al., 2016). Fibres are the most common type of microplastic in MWWTPs representing 52.7 percent of the total. As most of the runoff from urban emissions is conveyed to MWWTPs, treatment can act as either a barrier or an entry point of microplastics into the environment. Conventional MWWTPs with a primary and secondary treatment stage retain over 86 percent of microplastics in the sewage sludge, whilst MWWTPs with tertiary treatment retain over 98 percent (Sun et al., 2019). Sun et al., (2019) have estimated the flow of microplastics within a conventional MWWTP including preliminary, primary, secondary and tertiary treatment. These are visualized in figure 2 (Sun et al., 2019). The liquid phase microplastic flow has been estimated based on reported data (red), whilst the microplastic flow in sludge phase (orange) was estimated according to the particle balance.

Preliminary treatment

Preliminary treatment prepares the wastewater for the other treatment stages by separating out larger particles and sand-like solids to prevent damage to equipment or interference with the other stages. The preliminary treatment include screening through a coarse screen, and grit removal through sedimentation (Persson, 2011). Preliminary treatment restricts microplastic flow to 41 ~ 65 percent relative to the influent (Sun et al., 2019).

Primary treatment

Primary treatment typically consists of gravity sedimentation to remove settleable solids, about half of which are removed during this treatment stage. The residue from this treatment stage is called primary sludge, which is a concentration of suspended particles in water. Some pathogenic organisms, organic compounds and nutrients are also contained in the primary sludge. Primary treatment reduces flows of microplastics to 2 ~ 50 percent relative to the influent (Sun et al., 2019).

Therefore, preliminarily and primary treatment remove the majority of microplastics from the wastewater stream (Sun et al., 2019). This is mainly due to the skimming of light floating microplastics and the settling of heavier microplastics in the sedimentation processes. Fibres, the most common type, are retained through settling as they become entrapped in flocculating particles (Magnusson and Norén, 2014).

Secondary treatment

The secondary treatment stage often consists of a biological treatment process to remove biodegradable organic material. For this, an activated sludge process is most common, in this study referred to as the conventional activated sludge process (CAS) (Persson, 2011). Biological treatments use microorganisms to oxidize organic materials, which flocculate to form settleable particles. These particles are separated in sedimentation tanks, creating a concentrated suspension called biological sludge. Biological treatment processes limit emissions of oxygen-demanding organic materials (Persson, 2011). During this treatment process, the retained microplastics accumulate with the sludge flocculate or bacterial extracellular polymers in the aeration tank. These flocculates then settle in the settling basin. Secondary treatment processes further decrease wastewater microplastic flows to between 0.2 ~ 14 percent relative to the influent (Sun et al., 2019).

The contact time of microplastics with wastewater in the treatment cycle is an important factor in microplastic removal, as longer contact time increases surface biofilm coatings on microplastics. These coatings modify the relative density of the microplastics, making them neutrally buoyant, and thus more likely to surpass both skimming and settling processes (Rummel et al., 2017). The secondary treatment mainly removes larger particles, one reason is that only the fibres with neutral buoyancy remain, thus being resistant to further removal (Sun et al., 2019). Studies have shown that microplastics larger than 500 µm were nearly absent from secondary effluent (Mintenig et al., 2017; Ziajahromi et al., 2017).

Tertiary or advanced treatment

Tertiary or advanced treatment steps have been introduced to further improve effluent quality. The sediments of tertiary treatments are added to the primary and secondary sludges. Tertiary treatments often include chemical treatment stages to reduce phosphorus and nitrogen leaching, minimizing the nutrient overload of surface waters (Persson, 2011). Phosphorus is typically removed by chemical precipitation, while nitrogen is removed by nitrification and denitrification. The process step in which precipitation chemicals are added can vary in different MWWTPs (Persson, 2011). Microplastics interact with flocculants such as aluminium salt and iron salt, reducing their effectiveness in the coagulation process. To achieve the same removal results, additional chemicals are needed, increasing the cost of chemical treatment, though it is unclear to what extent chemical processes contribute to microplastic removal (Sun et al., 2019; Zhang and Chen, 2019).

Another common type of tertiary treatment is the removal of pathogenic microorganisms and viruses through filtration-based technologies. Filtration-based tertiary treatment can provide additional microplastic removal, restricting their concentration to about 0.2 ~ 2 percent relative to the influent (Sun et al., 2019).

2.1.2 Microplastic leakage

Despite high retention ratios, MWWTPs leak significant amounts of microplastics into the environment, with most MMWTPs processing millions of litres of wastewater each day (Mason et al., 2016; Sun et al., 2019). The smallest sizes of microplastics, between 20–100 µm and 100–190 µm are most abundant in MWWTPs effluent (Ziajahromi et al., 2017). Relatively speaking, more fibres are found in the effluent, as they can pass membranes or filters longitudinally (Sun et al., 2019). Mason et al. (2016), found that from an analysis of 17 plants in the United States, MWWTPs release over 4 million microplastics per plant per day through effluent water. It is estimated that between 3 billion and 23 billion microplastics are released from MWWTPs each day in the United States alone (Mason et al., 2016), amounting to between 5 600 and 43 000 tons of microplastic annually. Similarly, Sun et al. (2019), estimate that MWWTPs release 2 billion microplastics per plant per day on average. In the European Union there are 18 000 MWWTPs in operation, of these 68.4 percent treat at tertiary level, whilst 28.5 percent and 3.1 percent treat at secondary level and primary level respectively (WaterWorld, 2017; EurEau, 2019; EurEau, 2017). MWWTPs with tertiary treatment processes have an estimated 0 ~ 51 microplastics / L in their effluent, whereas effluent from primary or secondary treatment contains between 0,0009 ~ 447 microplastics / L (Sun et al., 2019). This means that, conservatively, European MWWTPs could be releasing 36 billion microplastics per day, or 67 000 tons annually, through their effluent. These high discharge levels suggest that microplastic targeting treatment technologies are necessary to avoid further emissions into aquatic environments.

phasing out in most countries (PURE, 2012). Alternatives for ocean dumping or landfilling are energy recovery through incineration, or nutrient recovery through agricultural use (Usman et al., 2012). In the United States, approximately 55 percent of all produced biosolids are applied to land (WEF, 2010). Within the 26 EU Member States, about 49.2 percent of the produced biosolids is applied to agricultural lands (EurEau, 2017). The percentage of nutrient recycling varies greatly between the member states, in Ireland up to 80 percent of produced biosolids are applied in agriculture, whereas Greece and the Netherlands have no application of biosolids on land at all due to safety concerns of possible hazardous content (Mahon et al., 2017; Milieu Ltd et al., 2010). Nonetheless, the total amount of nutrient recycling through biosolid application is increasing within the EU (Milieu Ltd et al., 2010). The high usage of biosolids for agricultural applications, indicates that a significant amount the retained microplastics will be released on farmlands (Nizetto et al., 2016). Concentrations of microplastics can reach 1500 - 17 000 microplastics/Kg in dried treated sludge (Sun et al., 2019).

Nizetto et al., (2016) estimate that “a total yearly input of 63 000 - 430 000 and 44 000 - 300 000 tons microplastics is emitted to European and North American farmlands respectively” through the application of biosolids. Neither European nor North American regulations mention microplastics in their restrictions on using sewage sludge containing harmful substances (Bayo et al., 2016). Underscoring the need to identify, develop and deploy wastewater treatment and sludge handling methods that safeguard policymakers’ circular economic objectives whilst limiting microplastic pollution.

Whilst the above-mentioned estimations have underscored the significance of microplastic pollution from MWWTPs, it is important to note that only 10 – 15 percent of all microplastic pollution arrives in the influent of MWWTPs. The other 85 – 90 percent enters the environment through e.g. storm water and surface deterioration of larger plastic pieces in the environment (EUreau, 2019).

2.2 Existing and emerging technologies to remove microplastics from wastewater

Several studies suggest that advanced final-stage treatment technologies can further improve the microplastic removal ratios (Carr et al., 2016; Mintenig et al., 2017; Ziajahromi et al., 2017). Due to the sheer volume of effluent flow, only a microplastic removal ratio of 100 percent is considered satisfactory purification (Baresel et al., 2017). Tertiary filtration-based technologies retain the highest quantity of microplastics.

To achieve a 100 percent removal of microplastics, the Swedish Environmental Institute has proposed a combination of several different tertiary or advanced treatment technologies: Ultrafiltration (UF) is a complementary technology to Powdered Activated Carbon (PAC-UF) or Biologically Active Filters (UF-BAF). However, in a study by Talvitie et al., (2017), BAF does not have a positive effect on microplastic removal rates, and thus will not be considered in this report. The complementary UF is proposed to be operating with a membrane of a nominal pore size between 0.01-0.1µm and pressure differences between 0.5 - 10 bar. Besides UF methods, reverse osmosis (RO) also provides complete removal of microplastics, with the correct pore size (Baresel et al., 2017; Mason et al., 2016; Mintenig et al., 2017; Talvitie et al., 2016).

The removal efficiency of microplastics between rapid sand filtration (RSF) and dissolved air flotation (DAF) in secondary effluent treatment, and membrane bioreactor (MBR) in primary effluent treatment was compared by Talvitie et al. (2017). The highest removal efficiency was achieved with MBR (99.9 percent), followed by RSF (97 percent) and DAF (95 percent). Further research shows that when a pore size of 0,04 µm is used during the MBR process, all microplastics between 1 µm and 5 mm are removed from the wastewater stream (Baresel et al., 2017). The feasibility of microplastic removal through a dynamic membrane (DM) was evaluated by Li et al., (2018), and suggests that DM’s are an energy efficient way to filter microplastics out from wastewater, though further research is needed. None of the aforementioned technologies are specially designed for the removal of microplastics from wastewater. Treatment technologies that specifically target microplastics are still undergoing preliminary research, and thus have not been included in this study (Sun et al., 2019)

Table 1 Overview of existing technologies for microplastic removal

No. Removal

technology Description Targets Estimated microplastic removal ratio

Remarks Source

1 MBR (UF) Membrane based on ultra-filtration followed by anaerobic digestion Microplastics and other contaminants 99.9 % - 100 % Can be implemented as additional treatment to existing MWWTP or replace CAS systems.

(Talvitie et al., 2017; Baresel et al., 2017)

2 RO Reverse osmosis Microplastics, Antibiotics and other contaminants 99 – 100 % Can be implemented as additional treatment to existing MWWTP. (Baresel et al., 2017; Sun et al., 2019)

3 PAC-UF Powdered activated carbon and an ultrafiltration Microplastics, Antibiotics and other contaminants 99 – 100 % Can be integrated or supplementary purification step in existing MWWTP. (Baresel et al., 2017) 4 DM Dynamic membrane of

non-woven fabric, woven filter and stainless-steel mesh.

Microplastics and other small particles 99.5 % Could be energy efficient, can be implemented as additional treatment to existing MWWTP. (Li et al., 2018; Zhang and Chen, 2019)

5 RSF Sand filtration with flow

through pump. Microplastics and other contaminants

97 % Can be implemented as add on to MWWTP, often polishing stage.

(Talvitie et al., 2017)

6 DAF Physical separation of microparticles with dissolved air flotation

Microplastics and other small particles 95 % Already implemented at many MWWTP as primary treatment. (Talvitie et al., 2017)

7 UF-BAF Granulated Activated Carbon, provide surface for microorganisms to attach and consume organic material.

Microplastics, Antibiotics and other contaminants

Inconclusive Can be implemented as additional treatment to existing MWWTP. Mixed results from several studies.

(Baresel et al., 2017; Talvitie et al., 2017)

Selected treatment technologies

This report only includes treatment technologies that can achieve 100 percent microplastic removal ratios in full scale operations, as a 100 percent removal ratio is the only satisfactory purification result. Therefore, MBR, PAC-UF and RO/NF have been selected to be further analysed in this report in response to research question 2.

• Membrane Bioreactor

A Membrane Bioreactor or MBR, is an activated sludge system that uses microporous membranes for solid and liquid separation, as shown in figure 3. The basic principle of membrane filtration is a pressure difference that draws raw wastewater through a microfiltration membrane surface, removing suspended material in the process. The solid / liquid separation process is enabled through a vacuum state downstream of the membrane, and air is introduced into the system to drive the biological treatment. An MBR system requires relatively high amounts of power for the pressure pump to prevent the membrane from fouling and force the water through the membrane. The production of reusable water, ease of operation and compactness make MBR ideal for municipal wastewater treatment, of which there were over 1500 in operation around the world in 2014. (Pandey et al., 2014).

• Reverse Osmosis

Reverse Osmosis or RO has the finest membrane filters of all membrane filtration systems, with a pore size ranging down to 0.1 nm, as shown in figure 4. RO closely resembles other forms of membrane filtration, as it is driven by high operating pressures, however it can remove much smaller molecules, including chemicals and pesticides (Ochando-Pulido et al., 2019). Pre-treatment is necessary, as the membrane is highly susceptible to fouling. The utilization of RO systems has increased in the last decade as system costs have decreased and water quality regulations have increased (Ramaswami et al., 2018).

• Powdered Activated Carbon with Ultrafiltration

Powder activated carbon ultrafiltration processes, or PAC-UF processes, add PAC to a recirculation loop of the membrane system. The contaminants in the wastewater are absorbed by the activated carbon particles, whilst the ultrafiltration membrane separates the particles from the water. Figure 5 shows a visual overview of the PAC-UF system (Voigt et al., 2020). PAC-UF systems are used to treat wastewater contaminated with organic matter and micro-pollutants (Wintgens et al., 2005). The combination of PAC and UF is particularly interesting as UF membranes are capable of retaining all PAC particles. As wastewater regulations are tightening and membrane costs are decreasing, PAC-UF has become increasingly interesting for wastewater treatment applications (Löwenberg and Wintgens, 2017).

Whilst the above-mentioned research and technologies help reduce the amount of microplastics in wastewater effluent, they do not solve the issue of microplastics in sludge. Indeed, a more efficient microplastic filtration technology only means that the microplastic content in sludge is increasing. The Swedish IVL institute has stated that there is currently no method to separate microplastics from sewage sludge in a cost-effective way, therefore they see thermal treatment, such as mono-incineration, co-incineration and pyrolysis, as the only methods to destroy microplastics. A thermal sludge treatment process thus makes MWWTPs a sink for urban microplastic emissions (Baresel et al., 2017). In addition to removing microplastics from MWWTPs, future efforts should be directed towards the prevention of microplastics entering MWWTPs as will be further elaborated on in the discussion section of this report.

Figure 5 Visualization of PAC-UF system (Voigt et al., 2020).

2.3 From wastewater treatment to resource recovery

Society’s current linear economic system is material-, resource- and waste-intensive. Our planet’s resources are finite, and a linear system is therefore not feasible for long-term human development. The Circular Economy (CE) offers an alternative approach, in which materials and products are transformed into resources for other processes at the end of life through using, cycling, pairing and re-manufacturing. This minimizes waste by closing the loops in urban systems (Stahel, W, 2016). A shift towards a CE would reduce humanity’s pressure on the planet significantly by reducing greenhouse-gas (GHG) emissions by up to 70 percent (Stahel, W, 2016).

However, recovering resources in a closed loop system requires clean waste-streams, and is often energy intensive and expensive due to technological complexities (Graedel and Allenby, 2010, p. 36). There is thus need for additional research into finding new and better pathways for the recovery of materials in waste, including wastewater. In 2015, the European Commission launched the ‘Circular Economy Action Plan’ to contribute to “closing the loops” and improving recycling and re-using of resources (European Commission, 2018). MWWTPs can play an important part in a circular system, as energy production and resource recovery are both achievable during the production of clean water (Neczaj and Grosser, 2018). The main drivers for creating a wastewater recovery industry are global demand for nutrients, recovery of water and energy production, or the water-energy-nutrient nexus (Mo and Zhang, 2013). Figure 6 shows the proposed cycles of wastewater handling in a circular economic model. The paragraphs below describe the different ways of maximizing water, energy and nutrient recovery in wastewater treatment, whilst taking microplastic filtration and destruction into account.

2.3.1 Water recovery

CE strategies can function as a first line of defence against water scarcity. In an anticipated CE system for MWW treatment, water is cascading in accordance to different water quality standards for each use purpose (IWA, 2016). Wastewater reuse can be direct (planned) or indirect (unplanned). Indirect use originates from discharged wastewater into groundwater or surface water resources and will eventually end up as drinking water supply. Direct use of correctly treated wastewater includes e.g. agriculture and land irrigation, industrial purposes and groundwater replenishing. MWWTP effluent has been used for agricultural irrigation for many years, and the nutrients in wastewater reduces the need for synthetic fertilizers. Challenges associated with wastewater reuse are human health risk and high costs for reclaimed water delivery systems. Improvement of MWWTP effluent is crucial for sustainable water recovery strategies, as it would simultaneously increase the quality of natural waterbodies. MBR, RO and PAC-UF are suitable treatment processes for water recovery purposes, as microplastics, antibiotics, pathogens and other contaminants can be removed effectively. (Neczaj and Grosser, 2018).

2.3.2 Energy recovery

MWWTPs consume a significant amount of energy (Maktabifard et al., 2018). Currently, wastewater treatment consumes 21 terawatt-hours annually in the United States alone, about 4 percent of the total electrical power produced in the United States (Maktabifard et al., 2018; Xu et al., 2017). In the EU, wastewater treatment consumes about 1 percent of the total electricity supply (Haslinger et al., 2016). Electricity demand for wastewater in high-income nations is expected to increase by 20 percent over the next decade, underscoring the need for further development of energy recovery (Yan et al., 2017).

MWWTPs have the potential to recover energy through sewage sludge treatment. Nonetheless, the potential energy extracted from sewage sludge is typically not sufficient to create an energy neutral plant; therefore, adding renewable energy sources will increase the sustainability performance of MWWTPs in terms of GHG emissions. In addition, energy recovery would help significantly reduce MWWTP operation costs, as energy is the second-biggest operational cost after labour (Maktabifard et al., 2018). This report will consider those sludge treatment energy recovery practices that reduce microplastic pollution, including anaerobic digestion with biogas utilization, and thermal treatments with heat or power generation (Stillwell et al., 2010; Cowger et al., 2019).

Anaerobic digestion with biogas utilization

Creating biosolids from sewage sludge is often done through digestion. A common form of digestion is anaerobic digestion (AD), which produces biogas and biosolids. Biogas contains 60 – 70 percent methane (CH4) and 30 – 40 percent carbon dioxide (CO2) and other trace gasses, making it a useful fuel for heat, power or combined heat and power (CHP) systems (Ma et al., 2015). A MWWTP with an AD consumes about 40 percent less net energy as opposed to one without an aerobic digestor, significantly reducing its energy footprint (Neczaj and Grosser, 2018). AD has been used for MWWTPs with a flow of less than 4.000 m3 _{per day to flows of up to 757.000 m}3 _{per day (Ghazy et al. 2011).} Nonetheless, a capacity of at least 19.000 m3 _{of wastewater per day is necessary in order} to produce electricity in a cost-effective way (Stillwell et al., 2010).

As AD reduces the organic matter in sewage sludge by up to 50 percent, it is seen an essential step prior to drying and incineration. The reduction in organic matter optimizes the post-treatment process, while reducing costs. Today, AD is the dominant sludge stabilization technology in North-America, with 48 percent of all created sludge being digested (Schafer et al., 2002; Hale, 2018). In Sweden, approximately 70 percent of sewage sludge is treated in digesters to produce biogas (Bhasin, 2017). For MWWTPs to become resource recovery facilities with an energy neutral or energy positive footprint, AD can be applied as an essential component in the plant’s treatment processes (Edwards et al., 2015). This study therefore considers anaerobic digestion as standard practice for sludge handling systems, and the process will thus be included in this study. Few studies have researched the effects of AD on microplastic abundance, however Mohan et al., (2017) suggest that AD has the potential to destroy microplastics contained in the digestate, though not considerably. Therefore, the biogas digestate, which remains after the digestion process, should be thermally treated or destroyed to prevent microplastic leakage from the digestate. It is recorded that the presence of microplastics in sewage sludge can reduce the quantity of biogas produced during AD processes by up to 27.5 percent, underscoring the importance of preventing microplastics from entering MWWTPs (Zhang and Chen, 2019; Jin et al., 2019).

Thermal treatments with heat or power generation

As mentioned, biosolids still contain microplastics after digestion and must be disposed of. Although most legislators encourage the reuse of biosolids nutrients in agricultural application, thermal treatments are the only full-scale pathways for microplastic destruction (Baresel et al., 2017; Cowger et al., 2019). If combustion temperatures are high (400 – 550 °C) microplastics and other organic components can be destroyed (Cowger et al., 2019). Thermal treatments include mono-incineration, co-incineration, pyrolysis and gasification.

• Mono- and co-incineration

Sludge incineration is done through one of two commercially available technologies: fluidized bed furnaces or multiple hearth furnaces. Multiple hearth furnaces burn the biosolids in steps using hot air recycling and can be operated continuously or intermittently. Fluidized bed furnaces are newer, more efficient, easier to operate and stable, however only continuous operation is possible. Both incineration processes create heat, powering a steam turbine to generate electricity, and are suitable for medium to large MWWTPs. Advantages of incineration is the significant potential through energy recovery, the thermal destruction of microplastics and other contaminants retained in biosolids, and the possibility to recover phosphorus from the incinerated ashes (Stillwell et al., 2010; Baresel et al., 2017). Disadvantages are high investment and operational costs. Nonetheless, biosolid incineration is a feasible energy management strategy for MWWTPs, as up to 40 percent of the treatment plants energy consumption can be generated through biosolid incineration (Stillwell et al., 2010).

organic materials (EPA, 2017). An alternative to mono-incineration, to avoid high dewatering costs, is co-incineration, in which 5 – 15 percent of sludge is added to the incineration process of household waste (PURE, 2012). However, the potential for nutrient recovery is greatly reduced when sludge is co-incinerated and will therefore not be included in this study (Bhasin, 2017). In Europe, only about 15 percent of all created sewage sludge is incinerated; however tighter regulations, including limits to landfilling and agricultural use, are leading to an increase in sludge incineration. The destruction of harmful substances such as microplastics, combined with the potential of energy and nutrient recovery make incineration of sewage sludge a well-equipped circular economic solution (Bhasin, 2017).

• Pyrolysis

Pyrolysis is the process of thermally decomposing organic substances in an anoxic environment in temperatures ranging from 300 – 900 °C. Pyrolysis techniques, including catalytic pyrolysis, thermal pyrolysis and microwave-assisted pyrolysis, have been used to treat plastic waste in the past (Sun et al., 2019). This method decomposes the long chain polymers into oligomers. Recent studies have shown how co-pyrolysis with biomass could be a treatment method to turn microplastic containing sewage sludge into valuable gas, liquid and solid products, without producing toxic emissions (Burra and Gupta, 2018; Jin et al., 2019; Fytili et al., 2008). The pyrolysis gas and char can be used as fuels, whilst pyrolysis oils can be used in the chemical industry as raw materials (Fytili et al., 2008). Karaca et al., (2018) indicate that high temperature pyrolysis is the most efficient process for energy recovery from sewage sludge. High temperature pyrolysis operates at 850 °C and is sufficient to destroy harmful substances such as microplastics. Furthermore, it is possible to recover nutrients from the by-product sewage sludge char, making pyrolysis a circular economic solution for sludge handling. A pyrolysis system which includes AD has a better energy balance and a higher reduction in total greenhouse gas emission, as opposed to a pyrolysis system without AD as pre-treatment (Cao and Pawlowski, 2013). Therefore, a combination of AD with pyrolysis will be included in this study.

• Gasification

Another novel thermal sludge treatment technology is gasification; however, it lacks widespread application due to relatively high investment costs and a lack of a developed by-product market (Samolada and Zabaniotou, 2014). As such, gasification is not considered in this study.

2.3.3 Nutrient Recovery

The application of synthetic fertilizer has greatly increased the demand for extractable nitrogen and phosphates. As these nutrients are not recycled or brought back to their source, phosphate mineral resources are expected to become scarce or exhausted within 50 to 100 years (Van Vuuren et al., 2010). Effectively, the European Commission listed phosphate rock as a critical raw material in May 2014 (Neczaj and Grosser, 2018). It is therefore of increasing importance that nutrients in MWW and run-off water are recovered and reused. The biosolids created in MWWTPs can fulfil an important role in the nutrient recovery cycle. Nutrient recycling reduces the demand for fossil-based synthetic fertilizers and reduces the consumption of energy, resources and water (Neczaj and Grosser, 2018). A circular system for nutrient recycling can contribute to sustainable agricultural practices (Usman et al., 2012).

As shown in figure 7, there are currently three main commercially-scaled pathways for the recycling of phosphorus from sewage sludge, being 1) direct land application of biosolids; 2) P recovery from aqueous phase, and 3) P recovery from sewage sludge ash or char (Egle et al., 2016; GWRC, 2019).

1) Direct application of biosolids

The phosphorus recycling rate for direct biosolids application is 90 percent as compared to MWWTP influent (Egle et al., 2016), and it is by far the most common type of sludge disposal in high-income nations (WEF, 2010; EurEau, 2017). However, there are currently no viable solutions to separate microplastics from the biosolids (Baresel et al., 2017; Cowger et al., 2019), and biosolids can contain 1500-17 000 microplastics/Kg (Sun et al., 2019). Thus, in the context of making MWWTPs microplastic sinks instead of sources, direct land application of treated biosolids is not seen as a viable solution and will not be considered in this study.

2) P recovery from aqueous phase

Phosphorus recycling from wastewaters can also be achieved through technical recycling of wastewater and sewage sludge. Nonetheless, microplastic particles are not destroyed during aqueous phase P-recovery processes, and the potential for leakage exists. Thus, P P-recovery from the aqueous phase is not seen as a viable solution in the context of microplastic removal and will not be considered in this study (Stowa, 2019).

3) P recovery after thermal treatment of sludge

There are opportunities for combined nutrient and energy recovery, however widespread application is hindered by limited market opportunities and economies of scale. P containing products can be produced out of ashes from incinerated sludge, or the pyrolysis process. The retrieval rate of P in sewage sludge ash (SSA) varies from 80 – 100 percent with respect to the wastewater input (Egle et al., 2016). One of the main advantages of P recovery after thermal treatment is the fact that microplastics and other contaminants have been destroyed during the treatment. Table 2 describes the different available P recovery technologies applied after incineration, whilst table 3 on the next page describes P recovery technologies after or during pyrolysis. Selection criteria were based on a P recovery rate of more than 80 percent (Phosphorus Platform EU, 2020). The technologies described all have a minimum of "TRL 7”, meaning that there is a system prototype in the operational environment (De Rose et al., 2017). As the P recovery market is rather novel, some new technologies might be missing from the selection. Based upon the technologies described in table 2 and 3, this study will take EcoPhos for P recovery after incineration and Pyreg for P recovery after pyrolysis into consideration. Both these technologies show the most promising advantages in terms of P recovery rates, integration and costs. Appendix A gives a further description of both selected P recovery technologies.

Table 2 P resource recovery technologies for sewage sludge ash

P recovery

technology Resource recovered Process Results

TRL _{Advantages Disadvantages Source}

1 EcoPhos (applied after incineration) Fertiliser / technical / feed grade phosphoric acid or DCP Phosphates present in ash are dissolved using phosphoric acid. Insoluble containing metals go to a waste stream. 93 - 98% P recovered. 9 Low energy consumption; Low investment costs; High value co-products; easy integration in incineration facility.

Limit number of full-scale operations in use. (Phospho rus Platform EU, 2020; EcoPhos, 2020) 2 ICL – Fertilizer industry (applied after incineration) Standard mineral fertilizers Recovered materials are mixed into the phosphate rock or phosphoric acid-based fertilizer production process 100 % P

recovered 9 No new process needed; Infrastructure of fertilizer industry can be used.

Contaminants in ash are diluted in final product; (Phospho rus Platform EU, 2020) 4 Kubota (applied after incineration) P-containing slag. Thermal treatment with temperature 1300°C. Part of the heavy metals, copper and zinc are volatilized.

90 % of P

3 Ash2Phos (applied after incineration) Di-calcium phosphate (DCP), - mono-ammonium phosphate (MAP). Sewage sludge ash is dissolved in hydrochloric acid + lime (ambient temperature, no pressure). 90% of phosphorus , 10-20% of ion and 60% of aluminum recovered

7 Low labor intensity; Low investment costs; Aluminum hydroxide as a raw material for coagulants created;

Residue can be used in cement or concrete industry; only two full-scale operations. (Phospho rus Platform EU, 2020; Easymini ng, 2020)

Table 3 P resource recovery technologies after or during pyrolysis

P recovery

technology Resource recovered Process Results

TRL _{Advantages Disadvantages Source}

1 Pyreg (applied after AD) P from pyrolyzed dry sludge Carbonization reactor operated at 500 – 800 °C. This temperature results in a biochar with labile organic carbon content < 1%. 90 % recovery rate. 9 Decentralized system, so integration at MWWTP possible; Operating in almost 30 full scale units in Europe today; Biochar not included in EU fertilizing products regulation. (Phospho rus Platform EU, 2020) 2 Enersludge (applied after AD) P from pyrolyzed dry sludge Pyrolysis of dry sludge at 450°C and a pressure 1–5 kPa in the absence of oxygen 100 % P recovered and char created. 7 Several types of fuels created; volume of solid residue low; low gas emissions.

Only 1 full scale

plant in operation. (Szaja., 2013)

2.3.4 Other resources

3. Methods

Firstly, the literature study defined current MWWTP processes, their capabilities of retaining microplastics and novel technologies to enable MWWTPs to become microplastic sinks. Secondly, a multi-criteria analysis (MCA) method was conducted, by which several different scenarios for transforming MWWTPs from microplastic sources into sinks have been assessed. Thirdly, interviews were held to identify gaps between the selected scenarios and their implementation. Figure 8 shows a visual overview of the methods used in different phases of this research.

Figure 8 Process flow of methodology

3.1 Scenario creation

Qualitative research was conducted through a literature review of peer-reviewed studies, to obtain a clear image of the knowledge state on the subjects of MWWTPs, microplastic pollution and CE objectives for wastewater treatment. The literature review allowed for the identification of relevant theories, and gaps in existing academic work. The review demonstrates the understanding of relevant theories and concepts and has formed the foundation for the assessment of existing and emerging microplastic filtration technologies for MWWTPs.

The literature study has been conducted through the collection, evaluation and analysis of a variety of publications in scientific databases. The research material consists of peer-reviewed papers, published reports and official websites of the EU and US EPA. Only literature sources released in the last 20 years (2000-2020) have been reviewed to ensure the relevance of the data. Data was held to high-quality standards, nonetheless certain limitations exist, such as reduced accuracy of data due to the use of multiple data sources.

3.2 Scenario analysis

The identified scenarios were compared with each other through a matrix. This matrix was based on the literature study. Multi-criteria analysis (MCA) is a decision-making tool, predominantly used to assess decisions with environmental implication. An MCA-matrix compares impacts by giving a weight and score, to assess the impact of each criteria (Dodgson et al., 2009).

3.2.1 Decision context and system boundaries

This MCA’s decision context defines which wastewater treatment scenario will convert MWWTPs from microplastic sources into sinks, whilst taking the water-energy-nutrient nexus into account. The different scenarios were assessed and compared with one another using several different criteria. All scenarios were based on the same functional unit of one cubic metre (m3_{) of influent wastewater. This} functional unit was based on a collection of LCA studies performed on wastewater treatment processes (Larsen, 2018). The temporal boundary of the study is set to be 30 years, being the average lifespan of MWWTPs (Risch et al., 2015). None of the upstream processes related to construction materials and end of life stages of the treatment facilities were considered.

3.2.2 Criteria

The chosen evaluation criteria were based on the literature review, CE objectives in regard to the water-energy-nutrient nexus and the three pillars of sustainability in response to research question 4 (Hansmann et al., 2012). The criteria were chosen based on measurability, non-redundancy and relevance. As such, the criteria in table 4 on the next page were selected. A further description of each criteria is given in Appendix B.

Table 4 Selected criteria for multi-criteria analysis

Pillar Objective Criteria Type Indicator

Environmental

Minimize microplastic leakage in

waterbodies Removal of microplastics from wastewater Quantitative % of microplastics removed from Effluent. Minimize microplastic leakage in

environment Destruction of microplastics during sludge treatment Quantitative % of retained microplastics destroyed. Minimize GHG emissions into atmosphere Global Warming Potential of scenario Quantitative kg CO2- eq./m3 effluent

Maximize effluent water quality Water recovery potential Qualitative Low / Moderate / High

Maximize utilization of phosphorus Nutrient recovery potential. Quantitative % of phosphorus extraction ratio Economic

Minimization of costs Operational costs Quantitative USD / m3 effluent

Investment costs Quantitative USD / m3 effluent

Maximization MWWTP income Additional revenue streams Quantitative Low / Moderate / High Social

Maximization of public acceptance Consensus Qualitative Low / Moderate / High

Minimization of the technological

3.2.3 Weighting of criteria

The weight of each criteria is determined according to the weighted sum model. The weighted sum model has been used, as the use of weighted averages underscores the independence between the judged strength of one criterion in relation to the judged strength of the other criteria’s (Dodgson et al., 2009). The weighted sum model is the most frequently utilized method for single dimension decisions (Pohekar and Ramachandran, 2004).

Weights are based on the relative importance to the decision and founded upon a structured survey send out to a panel of experts. The weights for each criterion are given in Appendix B. The full lists of experts that weighted the criteria is given in Appendix C. The relative weight of each criterion was created by averaging the survey results. In the survey, each criterion has been weighted on a scale from 1 – 5, taking their relative importance into account.

1: Very low importance 2: Low importance 3: Medium importance 4: High importance 5: Very high importance

3.2.4 Grouping

Each criterion has been assigned an indicator and these have been evaluated on a grouping scale from 0 to 5. The grouping was created in order to aggregate the criteria into quantifiable values, that can be used in the evaluation (Tsoutsos et al., 2009). A 5 represents a criterion with the greatest favourable performance, whereas 0 stands for a criterion with the least favourable performance. Groupings are both quantitative scales, as well as qualitative scales, depending on the availability of reliable data and the nature of the criteria. The qualitative scores are determined on their favourability, and thus can vary between several different qualitative criteria. The qualitative data has been evaluated on the same binary scale to create compatible results for both qualitative and quantitative criteria

The grouping generated a range of values against which the criteria score can be judged. As scales are dependent on the nature of the criterion, not all scales are linear, and some have shortened intervals on one end to emphasize the relative importance of incremental differences.

3.2.5 Scoring

Scoring results have been analysed according to a linear additive model, with the assumption that uncertainty is not built into the model and criteria are independent from one another. The linear additive model is commonly used by decision makers, for its robustness and effectiveness (Dodgson et al., 2009). The model multiplies the value score of each criterion by its assigned weight, after which the weighted scores are added together. A higher weighted score represents a more favourable outcome.

The weighted score (WSx) of each criterion was calculated, by multiplying the scores (Sx) and weights

(Wx) of the criterion (x) (1). As both weighting and scores were judged on a scale from 0 to 5, a maximum

score of 25 and a minimum score of 0 can be obtained.

!" ! = %!× '! (1)

3.2.6 Normalization and sensitivity analysis

After weighting and scoring results for each scenario, the total scores for each pillar of sustainability were normalized in order to attain a final score between 0 and 5. Normalization ensures a balance between the three pillars of sustainability, despite the pillars not having an equal number of indicators. The overall normalized total (Nt) was generated to rank the scenarios against one another. The normalized weighted score for each pillar (WSp) was determined, by dividing the sum of all weighted scores (Ux) from one scenario by the sum of the weight for that pillar (y) (2)

!"( = ∑!" #! ∑!

" $ (2)

The normalized total (Nt) was obtained by summing the weighted score for each pillar (WSp) and divide these normalized scores by three, the total number of pillars (3).

)* =∑#" &'(

Great effort was put in ensuring the use of high-quality data, however uncertainties and limitations in data quality are unavoidable. Hence, a sensitivity analysis was conducted to determine the results dependence on particular preferences or weights. The weighting for each sustainability pillar was modified to reflect potential changes in preference, as weights are subjective figures, and may be vulnerable to bias.

3.3 Interviews

The structured interviews were formulated through an online survey. The semi-structured interviews were conducted through a video conference. Several major stakeholders have been interviewed. These stakeholders include, and are not limited to the management of MWWTPs, national and regional waterboards and academic experts. The interviews aimed to define the weighing’s for each criterion in the MCA and to identify the bottlenecks between suitable scenarios and their deployment in MWWTP processes. Finally, the compiled data was assessed to discuss how the identified bottlenecks can be overcome. The key stakeholders selected include the following:

1) Three professionals employed at MWWTPs operations, engineer and management.

2) Three professionals employed at national and regional waterboards, with a portfolio on wastewater treatment.

3) Two academic experts employed at an environmental protection agency and a research institute. Table 8 in appendix C shows a full list of the stakeholders and experts that have been interviewed.

3.4 Focus

This research aimed to aid in the implementation of wastewater handling scenarios in the context of municipal wastewater treatment. This research focussed on wastewater treatment plants that are operating for municipalities within high-income nations. These facilities likely have a similar mix of wastewater entering the plant due to similar consumption patterns amongst nations (Nizetto et al., 2016). Furthermore, these MWWTPs generally have similar characteristics in terms of primary and secondary treatment processes. The system boundaries of this study are visualised in figure 9, included in the system were municipal wastewater influent, the municipal wastewater treatment plant and sludge treatment methods. The sources of the municipal wastewater influent, the stormwater runoff and the destination of municipal wastewater effluent, nutrient application and heat and power recovery were excluded from the system boundaries.

Figure 9 Visualisation of the system boundaries of this study

3.5 Ethical review

4. Circular economic scenarios

Considering the literature review findings, several circular economic scenarios appear for limiting MWWTPs related microplastic pollution. The selected technologies for each circular economic objective are visualized in figure 10.

Figure 10 Visualisation of CE wastewater pathways for limiting microplastic pollution

The only known full-scale treatment technologies with the potential to achieve 100 percent removal of microplastics are MBR, RO and PAC-UF, and will thus be considered in the scenarios. In addition, these treatment technologies create sufficiently filtered effluent water for water recovery. Furthermore, CAS has been selected as a baseline wastewater treatment process

In order to safeguard circular economic objectives, anaerobic digestion for energy recovery is considered an essential part of each scenario. Furthermore, energy recovery through thermal treatment, being incineration or pyrolysis, is deemed essential, as thermal treatment is the only available method to completely destroy microplastics. Nutrient recovery, in the form of P recovery is deemed an essential aspect of the circular economic scenarios as P is a critical raw material (Neczaj and Grosser, 2018). Several applicable methods for nutrient recovery have been identified in response to the advantages and disadvantages of these technologies, among them EcoPhos and Pyreg are selected for this study. In addition, land application of biosolids has been selected as a baseline P recovery technology.

The technology selection in figure 10, have let to the creation of the scenarios in table 5, these scenarios will be further analysed in this report. In terms of energy recovery, anaerobic digestion is a sludge pre-treatment process that is applicable to all scenarios. Conversely, incineration and pyrolysis are two different thermal sludge treatment pathways. These applications result in three separate nutrient recovery scenarios.

Table 5 CE scenarios for limiting microplastic pollution from MWWTPs

No. Scenario name Treatment for

microplastic removal / water recovery Energy recovery (2) / microplastic destruction Nutrient recovery

1 CAS CAS - Land application of biosolids

2 MBR inci-eco MBR Incineration P recovery from sewage sludge ash with EcoPhos

3 CASRO inci-eco RO Incineration P recovery from sewage sludge ash with EcoPhos

4 CASPACUF inci-eco PAC-UF Incineration P recovery from sewage sludge ash with EcoPhos

5 MBR Pyreg MBR Pyrolysis with Pyreg and P recovery from sewage sludge char ash

6 CASRO Pyreg RO Pyrolysis with Pyreg and P recovery from sewage sludge char ash

5. Results

This section outlines the results of the MCA comparison between the seven identified scenarios, in response to research question 5. The robustness of results is debated in a sensitivity analysis.

5.1 Analysis of scenarios to separate microplastics from wastewater

The MCA results are visualised in figure 11 below. The sources and calculation of the data can be found in Appendix D. The highest scoring circular economic scenario for limiting microplastic pollution from MWWTPs, is MBR inci-eco with a normalized overall score of 3,11. This scenario includes MBR with Anaerobic Digestion, energy recovery through incineration and P recovery through Ecophos.

The second-highest scoring scenario is MBR Pyreg, with a normalized overall score of 2,67. This scenario includes MBR with Anaerobic Digestion and energy-nutrient recovery through Pyreg. The high performing scenarios which include MBR are mainly useful in the decision process of yet to be constructed MWWTPs. Since the MBR process can make the CAS process obsolete.

For already existing MWWTPs aiming to upgrade their facility in order to limit microplastic pollution from their plant, CASPACUF with Pyreg as an energy-nutrient recovery is seen as the highest scoring scenario. The PAC-UF system can be installed as an additional polishing step to an existing CAS system, significantly reducing upfront investment costs. The PAC-UF system scores a 2,65 as a normalized overall score.

The baseline CASLand has a normalized overall score of 2,63. Three of the six analysed circular economic scenarios performed below the baseline. These scenarios are CASPACUF inci-eco with a score of 2,62, and the two circular economic scenarios with the lowest normalized overall scores being; the CASRO inci-eco scenario with a score of 2,33 and CASRO Pyreg scenario with a score of 1,94. Table 6 on the next page shows the full results of the MCA.

Figure 11 MCA Results visualised