A study on the effects on a wastewater treatment plant when recovering the heat from wastewater

(1)

A study on the effects on a

wastewater treatment plant when

recovering the heat from wastewater

Susanne Bergstrand

(2)

- ii

Master of Science Thesis TRITA-ITM-EX 2020:556

A study on the effects on a wastewater treatment plant when recovering the heat

from the wastewater

Susanne Bergstrand

Approved

Date

Examiner

Joachim Claesson

Supervisor

Jörgen Wallin

Commissioner

SEQWENS

Contact person

Davis Nilsson

Abstract

Wastewater flowing in the sewers underground in urban areas to the wastewater treatment plant is an unused source of thermal energy. Wastewater as a source of energy has gotten more attention in recent times, as the Sustainable Development Goals in combination with local challenges encourage new technical solutions that can replace old and outdated technology. By installing a heat exchanger to recover the heat, it can be used locally to heat buildings as well as urban areas. The technology and the opportunity to recover and use the energy is a way to operate in a circular way. Still, the infrastructure systems of water, wastewater, and thermal energy are intertwined, creating a complex and balanced environment where several actors representing their own interest and service to a community and urban area. Some actors may be the municipality handling the wastewater, real estate, and owner of buildings or the district heating company.

This complexity in the urban area where several actors are involved with their own goal and all using the infrastructure surrounding water and wastewater in different ways, creates challenges. The users want to get rid of the wastewater and possibly recover heat from the wastewater, while the producer is responsible for the treatment plant and treatment, and is dependent on the wastewater reaching the plant with the highest temperature possible, and thereby not disrupting the process by a decrease in temperature created by the upstream heat recovery systems.

The purpose of this study is to examine how the wastewater treatment plant react and is affected if the influent wastewater temperature decreases as a consequence of heat recovery from the wastewater, as the removal of nutrients in the treatment plant are chemical and biological processes which are depending on the temperature. By examining operational data from Henriksdals wastewater treatment plant and a model simulating the removal of nutrients in activated sludge, some light can be shed on how the processes of the treatment plant react to colder incoming wastewater. The legal framework and regulations around the wastewater treatment will also be examined due to the complexity of these systems, which is often overlooked.

(3)

- iii

Sammanfattning

Under markytan i urbana miljöer flödar avloppsvatten i avloppsnätet från byggnader till avloppsreningsverk.

Avloppsvattnet innehåller stora mängder termisk energi och är därmed en outnyttjad energikälla i form av värme. Denna energikälla och konceptet värmeåtervinning ur avloppsvatten har fått mer och mer uppmärksamhet från både den akademiska världen och från olika fastighetsägare under senaste tiden, då man sett potentialen med ett utvinna värme ur avloppsvatten. Att installera en värmeväxlare för ett sådant ändamål kan antingen göras lokalt i anslutningen till en byggnad eller i större skala i en stadsmiljö. Tekniken och möjligheten att återvinna värmen från avloppet är ett sätt att identifiera var resurser kan användas cirkulärt. Dock är infrastrukturen kring vatten och avlopp och fjärrvärme sammanläkande, vilket skapar komplexa relationer mellan de olika aktörerna inom energibolag, fastighetsägare samt kommunerna som renar avloppsvattnet.

I en stadsmiljö uppstår en komplexitet när flera olika aktörer använder infrastrukturen kring vatten och avlopp. Användarna använder infrastrukturen för att göra sig av med avloppsvatten och eventuellt utvinna värme ur det, medan producenterna är ansvariga för avloppsreningsverket och reningsprocessen.

Producenterna är beroende av att avloppsvattnet kommer fram till avloppsreningsverket med högsta möjliga temperatur eftersom processerna på reningsverket kan påverkas av en lägre inkommande temperatur av avloppsvattnet.

Syftet med denna studie är att undersöka hur avloppsreningsverket påverkas om det inkommande avloppsvattnets temperatur sänks som följd av användandet av värmeutvinning ur avloppsvattnet, givet att de kemiska och biologiska reningsprocesserna av näringsämnena kväve och fosfor är temperaturberoende.

Med driftdata från Henriksdals avloppsreningsverk i Stockholm som grund, kombinerat med modellering av reningen av fosfor och kväve i simulatorn JASS, Java Activated Sludge Simulator, kan denna uppsats bidra till en bättre förståelse för hur avloppsreningsverket påverkas av kallare inkommande avloppsvatten.

Den lagstiftning och riktlinjer som styr avloppsreningsverket som bidrar till systemens komplexitet och som ofta förbises har även undersökts.

Driftdata och modellering i denna studie indikerar särskilt en påverkan av kvävereningen vid lägre temperatur av det inkommande avloppsvattnet. Detta beror sannolikt på att kvävereningen sker huvudsakligen genom biologiska processer med bakterier och mikroorganismer. Detta resulterat i att en minimal mängd värme kan återvinnas under vintern och kallare perioder under året för att inte påverka reningen av avloppsvattnet. Samtidigt är den befintliga infrastrukturen kring vatten och avlopp byggda för årtionden sen och är i stort behov av renovering. Renovering av befintliga rör skulle kunna minska mängden tillskottsvatten reningsverket hanterar idag. De huvudsakliga resultaten och slutsatsen av detta examensarbete är att systemen och infrastrukturen kring energi och värme samt vatten och avlopp hänger ihop och påverkar varandra i en stadsmiljö där fjärrvärme används. Detta blir särskilt tydligt i ett system där värme utvinns uppströms från reningsverket. Baserat på analys av driftdata från Henriksdals reningsverk i Stockholm samt modellering av den biologiska reningen finns tydliga tecken på att den biologiska reningen av kväve och fosfor skulle påverkas om den inkommande temperaturen skulle minska till följd av värmeutvinning ur avloppsvattnet. Baserat på denna studie med driftdata från Henriksdals reningsverk, avtar den biologiska reningen av kväve nästan linjärt med avtagande temperatur på avloppsvattnet, då tillväxten av mikroorganismer i slammet minskar och avstannar med kallare temperatur. Mikroorganismernas roll i slammet är viktig då det är de som utför omvandlingen av ammoniumkväve till nitratkväve och sedan nitratkväve till kvävgas. Reningsgraden av mikroorganismerna noteras vara nästan linjär med varmare temperatur, men endast till ca 30°C då reningen blir mer konstant.

(4)

- iv

Denna studie återkommer vid flera tillfällen till behovet av en övergripande och ingående systemanalys som även inkluderar lagar, riktlinjer och aktörerna kring avloppssystemet. Idag finns det väldigt lite skrivet om hur en installation för värmeutvinning ur avloppsvattnet hanteras mellan huvudmannen för VA-systemet (kommunen) och användaren. Det som har skrivits återfinns ofta i kommunernas ABVA (Allmänna Bestämmelser för VA-anläggning), där punkterna ofta är vagt formulerade. Avsaknaden av ett regelverk gör användandet av tekniken otydlig då de olika aktörernas intressen skiljer sig; huvudmannen måste oavsett vad upprätthålla reningen vid reningsverket medan en fastighetsägares intresse kan ligga i att återvinna värmen nära till fastigheten. Dessa olika intressen kan skapa en obalans i det stora tekniska systemet som infrastrukturen kring vatten och avlopp är en viktig del av.

(5)

- v

Nomenclature

ABVA: In Swedish, Allmänna Bestämmelser för VA-anläggning. Costumer agreement on water and wastewater services.

Combined sewage systems: System designed to collect stormwater and sewage in the same pipe.

Population equivalent (pe): One pe corresponds to the amount of degradable organic material with

a biochemical oxygen consumption and is often describe the treatment capacity of a wastewater treatment plant.

Recipient: is a river, lake or sea that receives untreated or treated wastewater or stormwater.

Separate sewage systems: Systems where sewage and stormwater are collected in separate pipes. In this case, stormwater does not enter the wastewater treatment plant.

Stormwater: is temporary flows of rainwater, meltwater and flushing water, as well as emerging groundwater.

Wastewater: Domestic wastewater comes from households and consists of water used to flush toilets and water from baths, washing-up and laundry facilities. Industrial wastewater is the wastewater discharged

from areas used for commercial or industrial activities.

Wastewater treatment plant: Plant designed to treat wastewater before it is released to a recipient.

(6)

- vi

Preface and Acknowledgement

Today, every action and decision we make have an impact on the environment. In order to ensure we have a positive impact on the planet, knowledge of what has a positive or negative impact on the climate is key.

During the years of studies in the field of Energy and Environmental Engineering at Kungliga Tekniska Högskolan, KTH, in Stockholm I’ve gained a better understanding of what is needed to contribute to a better world.

This thesis has primarily been written between March and September in 2020 and includes traces of an interdisciplinary fusion of courses from the bachelor level and also from the master Environmental Engineering and Sustainable Infrastructure. Courses over the years at KTH include studies in energy- systems, infrastructure, water and wastewater treatment and some introduction to environmental law among courses in sustainable development. From the start, reading and writing about wastewater heat recovery and wastewater treatment on a deeper level, have given me the opportunity to use my collected knowledge from several courses and has painted a picture of an even more complex system than I imagined when I started.

In this section I would like to thank my supervisor, Jörgen Wallin at the department of Energy Technology, for his guidance and support during this process. I also want to thank all the people involved in the SEQWENS-project at KTH for their inputs and wise comments made at the seminars during the writing process.

In this thesis, operating data is used from Henriksdals wastewater treatment plant. This data and information about Henriksdal wastewater treatment plant was delivered by Sofia Andersson working at the treatment plant. In the beginning of this project, Sofia, together with her colleague, Johanna Blomberg were kind enough to make time and meet for discussing and bouncing ideas of some possible challenges in wastewater system and wastewater heat recovery. I am thankful for the information they have provided me with and the time given. This would not have been possible without you.

Equally as important, I need to thank to my family, who has always been here to support and motivate me.

Their advice and comments have been helpful during the process of writing this thesis.

(7)

- vii

Table of Content

1 Introduction – From sustainable development goals to local water systems ... 1

1.1 Background – Wastewater system in urban areas ... 2

1.2 Aim and Objectives ... 3

1.3 Scope and Limitations ... 3

1.4 Previous Studies... 4

1.5 Outline of the Study... 5

2 Methodology ... 6

2.1 Research Methodology and Choice of Method ... 6

2.2 Operational Data ... 7

2.3 Java Activated Sludge Simulator, JASS ... 8

3 Literature Review ... 9

3.1 Research and examples on Wastewater Heat Recovery ... 9

3.2 Research on Effects on Wastewater Treatment Plant... 11

4 Theoretical Background ... 13

4.1 Large Urban Infrastructure Systems ... 13

4.2 Wastewater Heat Recovery ... 14

4.3 Nitrogen and Phosphorus... 14

4.3.1 Usage ... 14

4.3.2 Eutrophication and nutrients in the sea ... 15

5 Henriksdals Wastewater Treatment plant ... 16

5.1 Mechanical treatment... 17

5.2 Chemical treatment ... 18

5.3 Biological treatment ... 18

5.3.1 The Nitrogen cycle and removal from wastewater ... 18

5.3.2 Phosphorus removal from wastewater ... 19

5.4 Challenges in the treatment process ... 19

6 Legal limitations of wastewater heat recovery ... 21

7 Results ... 23

7.1 Results from Henriksdals wastewater treatment plant ... 23

7.1.1 Temperature over a year... 23

7.1.2 Removal of Nitrogen compounds ... 25

7.1.3 Removal of P-tot ... 35

7.2 Results from Modeling in JASS ... 38

7.2.1 Removal of Nitrogen Compounds ... 38

7.2.2 Removal of Phosphorus and Nitrogen Compounds ... 40

7.3 Aggregated Results ... 41

(8)

- viii

8 Uncertainty Analysis ... 43

8.1 Validation of the model ... 43

9 Discussion ... 48

10 Conclusion ... 51

11 Future Research ... 53

Bibliography ... 54

(9)

- ix

List of Figures

Figure 1: Illustration on the phases and process of this master thesis. ... 6

Figure 2: How the influent temperature varies during the year in 2017. ... 24

Figure 5: Concentration of NH4-N in the effluent water over the temperature from cold to warm of the influent wastewater in 2017. ... 26

Figure 6: Percentage removed NH4-N, meaning the effluent concentration over the influent concentration, the verses the influent temperature of the wastewater in 2017. ... 26

Figure 11: Influent and effluent concentration of the nitrogen in NO3 -N in 2017 verses the influent temperature of the wastewater. ... 30

Figure 12: Influent and effluent concentration of the nitrogen in NO3-N in 2018 verses the influent temperature of the wastewater. ... 30

Figure 13: Influent and effluent concentration of the nitrogen in NO3- N in 2019 verses the influent temperature of the wastewater. ... 31

Figure 14: The concentration of nitrogen in several molecules in the effluent water in 2017 verses the influent temperature of the wastewater. ... 31

Figure 15: Percentage removed Tot-N, meaning the effluent concentration over the influent concentration, the verses the influent temperature of the wastewater in 2017. ... 32

Figure 20: The concentration of phosphorus in several molecules in the effluent water in 2017. ... 36

Figure 23: Simulation of the concentration of NH4-N and Tot-N over temperature in the biological treatment simulator. The initial concentration of NH4-N for this simulation was 12 mg/l. ... 38

Figure 24: Simulation of the concentration of NH4-N and Tot-N over temperature in the biological treatment simulator. The initial concentration of NH4-N for this simulation was 33 mg/l. ... 39

Figure 25: Simulation of the concentration of NH4-N and Tot-N over temperature in the biological treatment simulator. The initial concentration of NH4-N for this simulation was 45 mg/l. ... 39 Figure 26: Results of the concentration of NO3-N from modelling when the model is set to match the effluent concentration of the NO3-N from Henriksdals wastewater treatment plant. The input is based on

(10)

- x

the influent conditions at Henriksdals waste water treatment plant. The three lines indicate where they match and differs 15 %. ... 45 Figure 27: Results of the concentration of NH4-N from modelling when the results of the model is set to match the effluent concentration of the NO3-N from Henriksdals wastewater treatment. The input is based on the influent conditions at Henriksdals waste water treatment plant. The three lines indicate where they match, half and double concentration. ... 45 Figure 28: Results of the concentration of NH4-N from modelling when the model is set to match the effluent concentration of the NH4-N from Henriksdals wastewater treatment plant. The input is based on the influent conditions at Henriksdals waste water treatment plant. The three lines indicate where they match and differs 15 %. ... 46 Figure 29: Results of the concentration of NO3-N from modelling when the results of the model is set to match the effluent concentration of the NH4-N from Henriksdals wastewater treatment. The input is based on the influent conditions at Henriksdals waste water treatment plant. The three lines indicate where they match, half and double concentration. ... 46

(11)

- xi

List of Tables

Table 1: Description of where in the wastewater treatment plant each parameter is measured ... 8

Table 2: Removal requirement for nitrogen and phosphorus. ... 17

Table 3: Allowed concentration in the effluent water and results from the years 2017-2019. ... 35

Table 4: Allowed concentration of the total phosphorus in the effluent water and results from the years 2017-2019. ... 37

Table 5: Results from ASM2d simulation for different input concentrations on NH4 and PO4. ... 40

Table 6: Aggregated result from modeling in JASS. ... 41

Table 7: Aggregated results from Henriksdals wastewater treatment plant... 42

(12)

1

1 Introduction – From sustainable development goals to

local water systems

In today’s society, the importance of sustainability is evident, and the concept of sustainability is increasingly being integrated in local, regional, national and international contexts. In 2015, the Member States of the United Nations unanimously agreed on the 2030 Agenda outlining 17 Sustainable Development Goals, designed to be the guiding "blueprint to achieve a better and more sustainable future for all" (Sustainable Development Goals, 2020). Apart from the unprecedented ambition of the goals, this decision also had a unique feature compared to the predecessor of the Millennium Development Goals: universality. While the previous Millennium Development Goals were aimed towards the least developed countries, these new goals are pertinent for all UN Member States and their respective local adaptations, regardless of national income or level of development. According to the overall aim of the 2030 Agenda, sustainability is to be mainstreamed throughout the goals, and specific goals put sustainability as a goal in itself. For example: goal number 6 is “Clean Water and Sanitation”, goal number 7 is “Affordable and Clean Energy”, goal number 9 is “Industry, Innovation and Infrastructure”, and goal number 11 is “Sustainable Cities and Communities”.

As such, Sweden has a clear commitment to work towards to above-mentioned goals nationally, regionally and locally (Regeringskansliet, 2015).

To reach the Sustainable Development Goals (SDG) and continue and maintain a certain standard of living, technical solutions are needed. Global sustainability often requires local solutions and therefore also indirectly studies on how local sustainable systems are best developed. An important part of sustainable systems is water infrastructure as well as energy systems, i.e. how can we adapt local infrastructure system providing us necessities like water and heat in a sustainable and resilient way? Worth remembering, water is a limited resource, essential to life as we know it. In parts of the world, pollutions are contaminating the water and decreasing the access to fresh water, putting pressure on treatment and finding ways to reduce water usage and reuse water when and where it is possible (Olsson, 2012). As such, research on how to better understand water and wastewater treatment infrastructure is needed to answer questions like this. As the world becomes more and more urbanized, understanding and developing sustainable urban infrastructure for water and energy is necessary. Energy systems, water systems and wastewater systems are connected and linked; energy is used to collect and treat water and wastewater, having an effect on the quality while the energy sector might be one of the largest consumers of water (Olsson, 2012). Given the global and national priority of sustainable water- and energy systems, this master thesis is inspired by the Sustainable Development Goals dealing with sustainable cities and communities and industry, innovation and infrastructure, by shedding light on the effects on a wastewater treatment plant when recovering the heat from the wastewater. While scholarly attention has increasingly been given to understanding wastewater and heat recovery, less attention has however been given to downstream consequences. As, this thesis is looking more into the effects of the wastewater treatment as wastewater heat recovery is used, SDG goal number 6 is of particular importance as the municipality in charge of the treatment of wastewater has a responsibility not to release untreated water to the Baltic Sea. Another SDG, number 7, about affordable and clean energy comes into play as the thermal energy which is flushed away from an urban area, in this case Stockholm, can be recycled back and used in the district heating system of the city. SDG number 11 is important as wastewater heat recovery could be one step towards becoming more sustainable as the thermal energy is not flushed out from the city, it is used to heat the city by the district heating system instead of some other source with a potential harmful effect on the environment.

With a holistic point of view and an interdisciplinary communication between engineers and policy makers as well as between producers and consumers is important. Therefore, this master thesis will look at this

(13)

-2-

specific aspect of the possible effects on wastewater treatment plant if heat recovery from wastewater is used, using Henriksdals wastewater treatment plant as a case study.

1.1 Background – Wastewater system in urban areas

In general, urban areas, heat from wastewater has long been an unused source of energy and thus presents opportunities to be utilized in urban areas. It could even be said that energy is literally flushed down the toilet (and of course drains) (Olsson, 2012). Arnell et. al (2017), estimates the average temperature of wastewater in Sweden has a temperature of about 20˚C as it leaves a residential building. Needless to say, the inlet temperature differs during the year due to seasonal changes, but also due to more external shocks and weather phenomena such as heavy rainfall and snow melting. During the year of 2019, the highest influent temperature of wastewater treatment plant Henriksdal in Stockholm was 24,5 °C the coldest was 9,9 °C and the average temperature over the year was 16,2 °C. The corresponding temperatures of the year 2018 were 26,9 °C as the warmest, 8,3 °C as the coldest and 15,3 °C was the average temperature. One way of recovering this heat is by installing a heat exchanger (which will be further explained in the following section), where the heat from the wastewater is recovered and used to heat a city or urban area by district heating (Arnell, et al., 2017). Using water as a source of energy in an urban area for heating buildings is favorable due to its high heat capacity. The amount of energy in the wastewater system may indicate how water and energy is linked and connected and therefore it could be argued that they should be discussed and planned together (Olsson, 2012).

However, when the temperature of wastewater is decreased, it affects and often disturbs or disrupts the treatment process and removal of nutrients (like nitrogen or phosphorus) from the wastewater. This could be a cause for concern, since an increased concentration of nutrients from Henriksdals wastewater treatment plant and other treatment plants along the eastern coast of Sweden released to the recipient, here the Baltic Sea, may contribute to the process of eutrophication. It could also be noted that eutrophication could lead to other concerns like: dead zones in seas, algae blooms and oxygen depleted seabed. Eutrophication has long been a priority in Sweden’s environmental policy: in the year 2000, Sweden adopted the environmental objectives system, a policy to safeguard the environment of Sweden. One of the goals of the environmental objectives is no eutrophication, meaning decreasing the load of nitrogen compounds and phosphorus compounds from human activity, which is contributing to the eutrophication process. (Naturvårdsverket , 2012).

The legal framework of Sweden regulating the wastewater system and the responsibility of heat recovery from wastewater is shared among several actors in the system, ranging from municipalities to the owner of a building, making it challenging for actors to even get approval for a project on heat recovery from wastewater. One could therefore argue that clear responsibility as well as a system analysis is essential in order to further develop infrastructure on wastewater and heat recovery, which few studies on the subject of wastewater heat recovery have taken into account. Although there are several previous studies, such as Arnell et. al in 2017, Oskar Wanner and Davin Dürrenmatt in 2014, that have studied wastewater heat recovery on how the temperature differs between the residential buildings and the inlet of the wastewater treatment plant, there has been little scholarly attention regarding if, and more importantly how the treatment processes are affected (Arnell, et al., 2017).

(14)

-3-

1.2 Aim and Objectives

The aim of this master thesis is to analyze how the processes in a wastewater treatment plant is affected by heat recovery from wastewater, before the wastewater arrives at the treatment plant, in particular the removal of nutrients (compounds consisting of nitrogen and phosphorus). To better understand these issues, and in order to reach the aim, this master thesis will collect and analyze data from Henriksdals wastewater treatment plant in Stockholm and produce simulation and data modeling. In addition, since operational systems of wastewater treatment plants are subject to rules and legislation, and in order to achieve a more comprehensive understanding of what measures are both technically as well as legally possible, this thesis will also partly examine the relevant legal framework.

The objectives of this master thesis are:

• Analyze and examine how building installed wastewater heat recovery affects the removals of nutrients in the treatment plant.

• Find suggestions on how the processes of the wastewater treatment plant and the wastewater system can be modified to functioning well if the inlet temperature is decreased due to heat recovery from the wastewater.

In order to meet the aim and objectives, the following research questions will be the focus of the discussions for this master thesis:

• How do the processes of the wastewater treatment plant work?

• What issues does a wastewater treatment plant experience today?

• With the case of Henriksdal, how will the treatment process react to temperature change if there were heat recovery of wastewater?

• What legal framework is controlling the process of heat removal from the wastewater?

• How could the treatment plant or the wastewater system be modified for a better process of removal of nutrients if there were heat recovery?

Given that these are extensive issues, this master thesis does not claim to find the complete or definitive answer to these questions, but rather shedding a light on these issues while examining the wastewater treatment plant of Henriksdal, and thus contributing to the broader research on wastewater and heat recovery.

1.3 Scope and Limitations

Water treatment plant systems are complex and there are several aspects and dimensions to the processes.

Accordingly, there are several interesting dimensions to study on this topic, but in order to have a clear focus area and to be able to shed light on one specific dimension, this thesis will mainly focus on the wastewater treatment plant in the case of usage of wastewater heat recovery upstream. The basis for this master thesis is the advantage of having available operating data from the wastewater treatment plant.

Although it would have been desirable to be able to repeat modeling and observations several times and accordingly increase reliability, the time limits of this thesis make repetitive modeling difficult to undertake.

Additionally, the limitations make it difficult to control for omitted factors. Within the subject of heat recovery from wastewater, there are many components in the whole system from when the water is leaving a building until it reaches the treatment plant. This project will focus on the Henriksdals wastewater treatment plant in Stockholm, from where operating data will be collected in order to analyze it and hopefully gain a better understanding for the wastewater system. This project will mostly be theoretical, as

(15)

-4-

it will be based on modeling and literature, since it has not been possible within the scope of this thesis to install a heat exchanger and observe how the system change with variations in temperature.

Every moment at the wastewater treatment plant has a distinctive inflow depending on the daily and seasonal variations of the area from where the wastewater originates. It is therefore important to note that these variations may present a skewed result. Moreover, given the case study nature of this thesis, conclusions regarding overall wastewater treatment operations in general cannot be made since the observations made in the Stockholm area and Henriksdals may not apply to another city in Sweden or anywhere else. However, the advantage of applying a case study approach is the possibility of being able to examine specific factors and see specific correlations.

This research field covering wastewater heat recovery in a large technical system is relatively new. The issue on how different actors, technologies and regulations interact with each other on a fully operational level has not fully been studied. Since these aspects is a relatively new field of research, there have not been as much research done yet, which makes the theoretical framework and the number of empirical studies less developed, which also presents certain limitations.

The main focus of this thesis is to apply a more system-based research approach as previous studies have mainly examined specific factors. Given the time restrictions of this thesis, a comprehensive and exhaustive analysis taking into account the system in its entirety is beyond the scope of this thesis. Nevertheless, hopefully this thesis will have the added value of an attempt towards a system-based approach.

1.4 Previous Studies

As mentioned earlier in the introduction and background section, wastewater heat recovery has been given more and more research attention. Previous studies of wastewater heat recovery have examined how the temperature differs between the residential buildings and the inlet of the wastewater treatment plant (Olsson, 2012). The effects, more specifically, if the treatment and how processes are affected and possibly disturbed have not yet been thoroughly studied (Arnell, et al., 2017). The European Union has funded several projects, for example Celsius and ReUseHeat, in order to identifying potential sources of energy in cities and urban areas. Within these projects several examples existing wastewater heat recovery installations are listed, but only how much heat they deliver and as far as known, they have not considered the problems of compounds discussed in this master thesis.

Although studies have been made looking at specific dimensions of heat recovery and wastewater heat recovery at a buildings level, less scholarly attention has been given to examining heat recovery at a system level. Still, it can be argued that a system-based study is needed due to the complexity of wastewater heat recovery. One example of this could be The International Water Association’s report in 2012, “Water and Energy: Threats and Opportunities” which explores the connections and links between energy and water, including an overall study on wastewater heat recovery. Nevertheless, system-based studies remain limited, which indicates that this field needs further research. A system-based approach is also needed in order to take into account the number of different actors, since there are possibilities of conflicting interests between actors, which adds to the complex nature of wastewater heat recovery and frameworks surrounding it.

Therefore, this thesis will aim to add to this research gap by assuming a more system-based perspective.

(16)

-5-

Still, it is worth noting that the limited number of similar studies does mean that there are fewer empirical examples to fall back on. Additionally, this means that, although this master thesis aims at answering new research questions set out in this thesis, it is likely to also raise several questions for future research.

1.5 Outline of the Study

After this introduction, the methodology section will describe the methodological approach of this thesis, including what type of data was collected from Henriksdals wastewater treatment plant, how the modeling was carried out, which will serve as the basis for the discussion in a later section. Then, a literature review is presented, looking at the various previous on this topic as well on projects and examples of wastewater heat recovery. Next, this paper continues in giving the reader an understanding of what is known in the field of wastewater heat recovery today and how a large technical system works based on Hughes theories on the development of large technical systems in a society (Summerton, 1998). The following section will describe how nitrogen and phosphorus affects the recipient, here the Baltic Sea, and how the processes at the Henriksdals wastewater treatment plant works. This paper will then examine what laws and regulations surround wastewater heat recovery and the wastewater treatment plant. Then, this paper will present the results for each modeling and simulation, showing the specific results for the three observed years. A discussion on the results will follow, along with conclusions that can be made from the results. Lastly, this master thesis will conclude with some reflections regarding future studies on this topic.

(17)

-6-

2 Methodology

The methodology that has been used for this master thesis, including reflections on the choice of method as well as the choice of Henriksdals wastewater treatment plant for this case study.

This master thesis is primarily focusing on an examination of operating data, meaning that the research strategy departs from a quantitative approach. Given the ambition of this thesis to also apply a more system- based understanding of the issues, a literature review and an overview of the legal framework of wastewater treatment have also been included, the latter naturally having a more qualitative approach.

This study was divided into several phases, as described in Figure 1 below. First, the research area was identified, along with the key focus area of wastewater heat recovery. After having discovered the limited amount of research on the whole system and the effects on the wastewater treatment plant, the aims and objective were outlined. Second, a review of the literature on wastewater heat recovery and projects was made. The legal framework was also further examined. Third, data was collected and processed in the different simulations in JASS. Fourth, the simulations were analyzed, including compared with data in regards to temperature and removal of nitrogen and phosphorus. Fifth, the results were interpreted and conclusions discussed. Finally, a concluding reflection on future work was made.

Figure 1: Illustration on the phases and process of this master thesis.

2.1 Research Methodology and Choice of Method

As described in the first section, the aim and objectives of this project are to clarify the possibility to recover heat from wastewater from a legal perspective and to identify how the different processes of a wastewater treatment plant are affected. A part of the aim and objectives can be achieved by reading literature and examining previous studies. By collecting operating data from Henriksdals wastewater treatment plant and evaluate the data, together with modeling the processes as a function of the inlet temperature, each step of the treatment process can be theoretically evaluated.

(18)

-7-

The choice of Henriksdal treatment plant for this case study was done for several reasons. First, there is available operating data that was both possible to share with the public, and possible to use in modeling.

Second, given that Henriksdal is a large wastewater treatment plant, located in the south of Stockholm, providing wastewater services and treatment to the people in Stockholm and surrounding municipalities, it is a good example the complexity wastewater treatment plants are. Third, the type of treatment includes mechanical, chemical and biological treatment, where the biological treatment is of the type activated sludge and the main phase of nutrient removal. Given the aim of this thesis, the main focus will be on the biological treatment phase, due to this phase being the most sensitive to external shocks.

Modeling is a practical tool for isolating factors and its effects (Epstein, 2008). Treatment of wastewater is a dynamic process where many factors determine the results in the removal of organic matter and nutrients.

Modeling the nutrient removal can be a strong tool for understanding a complex process and how different parameters affect each other. Still the results from using a model needs to be handled and understood with caution due to the nature of the model being a simplified representation of the true world (Plaza, 2017c).

In this project, modeling will be done in a simulator called JASS, Java Activated Sludge Simulator. The simulator was made and updated by several master thesis work by the Department of System and Control at Uppsala University, and is accessible online. The purposes of the simulator are design, forecasting, research and educating the user how activated sludge works as a biological treatment in a wastewater treatment plant. The model itself is based on empirical studies and operating data from a wastewater treatment plant located in Uppsala, a city located about 70 km north of Stockholm. There are two different models within the simulator, ASM1 and ASM2d. In the ASM1 model, the processes of removal of organic matter, nitrification and denitrification is modeled, while the ASM2d model the process the removal of phosphorus (Carlsson et. al, 2001).

2.2 Operational Data

Data in the form of the official operating data from Henriksdal wastewater treatment plan, has been collected. Given the first-hand nature of the data, it should be considered highly reliable as a source.

Nevertheless, the accuracy and the preciseness are subject to caution, given that data can be affected by several factors that are beyond the scope of this thesis. Further detailed examination on uncertainty factors will be discussed in section 10 and 11.

The data consists of influent flow and temperature and concentrations of nitrogen and phosphorus on a weekly basis starting from May 2016 to May 2020. The concentrations are measured at three different points in the treatment plant; at the inlet (before any treatment), outlet (after treatment) and a point called FV (försedimenterat vatten in Swedish). The FV-point is located in the plant after the chemical treatment, but before the biological treatment. In table 1, a list of what parameter is measured where in the treatment plant and the measured unit can been seen.

(19)

-8-

Table 1: Description of where in the wastewater treatment plant each parameter is measured

Parameter Measured Inlet FV Outlet

Temperature (°C) x

Flow (m³/s) x

Tot-N (mg/l) x x x

NH4 (mg/l) x x x

NO3 (mg/l) x x

Tot-P (mg/l) x x x

PO4 (mg/l) x x x

Summary of the operation data has also been collected from the yearly environmental report over Henriksdals treatment plant. The report also provided further information on the performance of treatment and from where the wastewater originates. Further, by comparing the concentration values in the effluent and the concentration in the tank of pre-sedimented water and simply calculating the difference of these two values, the performance of the biological treatment can be determined. This has been plotted for the different years of 2017, 2018 and 2019. The most recent data was chosen (2019) and the years before to make comparisons possible.

2.3 Java Activated Sludge Simulator, JASS

The background of the model is mentioned in section 2.1, while how the model is used in this master is presented here.

The setup of the simulator is simple with the different tanks and arrows in between where flow and parameters can be changed. The removal of nitrogen and phosphorus is displayed in a graph as well as in a table, making it easy to follow the process in each step.

In the ASM1 simulator, the focus is on removing nitrogen. The simulator consists of 10 tanks, where five are aerated and five are anoxic. In this simulation, the temperature was varied for different influent concentrations of ammonia (NH4) and total nitrogen, (Tot-N). The mean, maximum and minimum concentrations of ammonia (NH4) and total nitrogen (Tot-N) from Henriksdals wastewater treatment plant were chosen as input and starting point for the simulator. The temperature variable was varied from 6 °C to 26 °C on a two-step scale.

The ASM2d simulator focuses on removing phosphorus in the biological treatment tank. The inputs for this simulation were the mean, minimum and maximum concentration of phosphate (PO4) and ammonia (NH4). The different results from the modeling are presented in graphs and tables in section 7.2.

(20)

-9-

3 Literature Review

The following section examines the existing literature on wastewater treatment and relevant projects examining effects of wastewater heat recovery. More specifically, this literary review gives particular attention to studies that have examined different possibilities of recovering heat in an urban area, and that have examined existing heat recovery installations from wastewater, where existing installations have been located after the treatment. Additionally, this section also examines some aspects that previous studies have not taken into account, which this thesis aims at shedding further light on.

This thesis is part of SEQWENS, Ensuring sustainability and equality of water and energy systems during actor-driven disruptive innovation, an interdisciplinary research project at the Royal Institute of Technology in Stockholm, Sweden. One of the aims SEQWENS is to deepen the knowledge about wastewater heat recovery in a city- wide system. In this project, participants from real estate, private and municipal actors in water, energy contribute with their specific knowledge as well as three other master theses.

As discussed, in section 1.2 Previous studies, wastewater heat recovery has been getting more attention and funding for research for performance and integrating it into existing buildings. However, not much attention has yet been given to the downstream consequences of wastewater heat recovery, including overviews of the whole system. This section will look further into existing literature examining this specific aspect.

3.1 Research and examples on Wastewater Heat Recovery

There is increasing literature and studies on wastewater heat recovery, in particular case studies examining the potential of heat that can be recovered in an urban area and locating the most optimal position for a heat exchanger in order to obtain as much heat as possible.

One of the very few examples that exists of large-scale wastewater heat recovery installations and one that is up and running is found in Canada. Around the year 2010, when Vancouver in western Canada hosted the Winter Olympics, an old industrial area was rebuilt to the housing area for the athletes and later on sold to the public. Today, the area is a modern district and it also reflects in the technology in houses. When the district was rebuilt, the False Creek Neighborhood Energy Utility was created, and in order to run and maintain a system of wastewater heat recovery was installed. A heat pump is extracting heat from the wastewater and heats up water to a temperature of 80°C, and then used in the district heating system. The False Creek Neighborhood Energy Utility had a vision to recycle heat and energy back to the community, and therefore reducing pollution associated with heating buildings and water (City of Vancouver, 2020). The wastewater from this area is led to two different treatment plants in the region that at the moment only have the primary treatment step. Currently, the metropolitan area of Vancouver is planning to update their wastewater treatment plant to a more advanced treatment. While this is an example of a large system, no conclusion can be made on secondary treatment and the biological removal nitrogen and phosphorus removal is affected by wastewater heat recovery (Metro Vancouver, 2015).

In 2013, an interdisciplinary study on wastewater heat recovery in cold climates was conducted by different departments the University of Oulu by Lauri Mikkonen, Jaakko Rämö, Riitta L. Keiski and Eva Pongrácz.

The university is located in the northern town Oulu in Finland. Mikkonen et. al have made an important contribution to the field of wastewater heat recovery by doing an assessment of the potential of heat recovery from wastewater in northern areas and have been looking at wastewater heat recovery in Lapua in Finland and Sandvika, outside of Oslo, in Norway. Both towns have installed heat pumps and recover heat

(21)

-10-

after the treatment and lowers the temperature to 3° C. In these cities, the wastewater heat recovery therefore has no effect on the treatment due to the location of the heat pumps (Mikkonen et.al., 2013).

The European Union has funded a project called Celsius, which is supported by Swedish Energy Agency, as a way to support the Sustainable Development Goals for a more sustainable future. The purpose of the Celsius project is to “Accelerate the energy transition through the deployment of smart and sustainable heating and cooling solutions in cities” by assisting cities in city planning and district heating and cooling systems” (Celsius, 2020). Several cities and their heating system have been examined in order to inspire other cities in doing the same. Among these examples is Sandvika, a town outside of Oslo in Norway. The same system of wastewater heat recover was set up and has been up and running since 1989. The system was originally installed due to an urban expansion in the area and a way to deliver both heating during the cold part of the year as well as cooling in the warm parts of the year. (Celcius, 2020). The Sandvika system has an installed heating capacity of 23 MW in Sandvika (Oslofjord varme, 2020), and the heat recovery is located after the treatment plant where the temperature of the treated water is lowered to 3°C.

Another project funded by the European Union is ReUseHeat, which focuses on “technical and non- technical barriers to unlocking urban waste heat recovery projects and investments across Europe”. The project started in 2017 and is planned to be running until 2021. The project has produced a handbook, listing 25 cases of urban heat recovery in Europe, both heating and cooling. Out of the 25 systems, some are up and running while others are in the planning stage. As the focus of the project indicates, the project identifies where heat can be recovered in the whole society, where a few of the cases are from wastewater.

(Petersen, 2017). In the handbook, Gothenburg is listed as an example. Gothenburg is located on the west coast of Sweden, where the city’s treated wastewater flows through heat pumps (Göteborg Energi, 2015), where the temperature is lowered to 3 °C. The installed heating capacity is 160 Mw. (Petersen, 2017)

The city of Turku in Finland has invested in renewable energy for the past 20 years. As part of this investment, recovering heat from wastewater at the Kakola wastewater treatment plant has been identified as a source of energy and heat and therefore has constructed a facility nearby for recovering this after the wastewater is treated (Lundqvist, 2015). The recovered heat is distributing to the district heat system in Turku, and is estimated to be of an effect of 42 MW (City of Turku, 2020). The installation of wastewater heat recovery and connection to the district has increased the amount of renewable energy used in the city and is one of many projects in Finland where a local decision can have effects on a global scale by reducing its emissions of CO2 (Lundqvist, 2015).

Several studies presented above are examples of large installations. However, a bachelor’s thesis written by Haci Candemir and Philip Model at KTH has examined the conditions of buildings that have considered installing wastewater heat recovery. They concluded that each building has its own set of conditions determining if wastewater heat recovery is possible and profitable or not. The conditions of a building are a consequence of when it was built since the city of Stockholm has been built over a long period of time with different styles and building techniques trending (Candemir and Model, 2015).

(22)

-11-

3.2 Research on Effects on Wastewater Treatment Plant

There is also increasing literature and studies on the specific effects of the removal processes on wastewater treatment plants. More specifically, much of this research has been led by the organization Swedish Water.

Swedish Water (Svenskt vatten) is an organization founded in 1962 by the municipalities in Sweden. The purpose for the organization was to assist with technical and administrative issues. Another reason was to get a unified voice for the municipalities when discussing regulation. Throughout the years they have published several reports on water and wastewater and also setting a standard of components in the system.

As part of the organization, a project called HÅVA, in Swedish “Hållbarhetsanalys av värmeåtervinning ur avloppsvatten”, (in English, Sustainability Analysis for wastewater heat recovery) is currently being conducted, with the purpose of conducting an analysis of sustainability, examining the effects on the wastewater treatment and creating a simulator over the wastewater system when using wastewater heat recovery. This project is carried out together with Lund University (Svenskt Vatten, 2020).

At Lund University, a literature review called “Sustainability analysis of wastewater heat recovery” has been conducted by Magnus Arnell, Emma Lundin and Ulf Jeppson. The study was presented in 2017 and is focusing on discussing the positioning of the heat exchanger in the system, by using different models that predict the potential heat that can be recovered into the system again, either close to the building or to the district heating system. Legal framework and economic feasibility are also discussed in the study. The review highlighted the complexity of the wastewater heat recovery and showed that the area is still not fully investigated. (Arnell, et al., 2017). Within the project at Lund University, a master thesis was written in 2019 by Diego Reyes. The thesis’, “Modelling Heat Recovery from Urban Wastewater Systems - Case study from Malmö”, main focus was to determine locations for the heat exchangers and also estimate the amount of heat that can be recovered in the city of Malmö, based on modeling and empirical data. Reyes estimates that about 13% of what the district heating system in Malmö deliver today could be recovered from wastewater, corresponding to 36 MW from the city of Malmö. However, that would potentially affect the wastewater treatment plant. A better location would be in the existing pumping stations as a high volume of water flows through there, and would have less effect on the treatment plant. (Reyes, 2019)

In 2005 a study in Switzerland by Oskar Wanner, Vassileios Panagiotidis, Peter Clavadetscher, Hansruedi Siegrist, explored the effects of heat recovery on the nitrogen removal process. The study was based on recorded data from a wastewater treatment plant in Zurich in 2003, trying to establish a relation between a decrease of the influent wastewater temperature and the performance of the nitrification process, and the process of converting ammonium (NH4) to nitrate (NO3). The study focused on the nitrification process, without further discussing the process of denitrification, converting nitrate (NO3) into nitrogen gas (N2), which is released into the atmosphere. In the study the conclusion was made that a decrease of the influent temperature reduces capacity to remove ammonium, giving us the valuable observation for other studies on the removal of nitrogen. However, the study is made in Zurich with different basis than other places in the world making the results and specific conclusions difficult to adapt or transfer to another city or wastewater treatment plants (Clavadetscher et. al, 2005).

Oskar Wanner, together with Davin Dürrenmatt, presented another study on the subject of wastewater heat recovery in 2014. This time they presented a simulator, TEMPSET, created by the writers. TEMPSET is built with the purpose to “predict the effect of heat recovery on the wastewater temperature in sewers”, and be a useful tool when determine the best location for a heat exchanger in the wastewater system. The writers estimate that 1,16 kWh of heat energy can be gained from a 1 m³ of wastewater if the temperature is decreased by 1 °C. In Switzerland and Zurich, where the authors are based and collected their observations,

(23)

-12-

a local law requires that the temperature of incoming wastewater to the treatment plant cannot be colder than 10 °C. This temperature level is set in consideration of the microbes in the activated sludge, as they might be flushed out from the sludge if the surrounding environment of the microbes is to cold. The specific conditions of the wastewater system, seasonal variations and the legal framework are important factors to analyze when considering installing the technology of wastewater heat recovery (Dürrenmatt & Wanner, 2014).

Several studies are mentioned above, and many studies have investigated the possibility of installing a wastewater heat recovery system and some cities have already installed it on a larger scale. It is also known how the processes of a wastewater treatment plant works. However, these two fields, both located in the wastewater system have not fully been investigated together on a large scale or in a system overview.

From this literature review, several conclusions on this research field can be drawn. Firstly, the work on wastewater has shown some indication that lower temperature of the influent wastewater affects their performance of the microbes in the biological treatment of nitrogen, although final and definitive conclusion should be made with caution on the exact impact. Secondly, it can be concluded that much research has solely focused on either the technology of heat recovery or on the wastewater treatment plant. This is much due to limitation on the number of large facilities and projects dealing with wastewater heat recovery and in combination with the fact that they have not been operation long enough to make long-term conclusions.

There are many examples of cities and treatment plants where the heat recovery is located by the effluent water, and not too many studies have been conducted upstream from the plant with a system-based approach. Lastly, it is also interesting to note that several of the most significant examples of projects that used types of wastewater heat recovery technology have been largely driven by societal factors, such as urban expansion or other. It could therefore be argued that studies examining projects on wastewater heat recovery needs to take societal factors into consideration to fully understand the driven force behind the project to begin with.

(24)

-13-

4 Theoretical Background

In this section, the theoretical framework that this thesis draws upon will be presented. Key concepts relevant for this thesis will be defined, as well as description of the main features of water systems processes relevant for this study.

4.1 Large Urban Infrastructure Systems

The urban water cycle and the wastewater treatment system can be seen as a large technical system. The main theory behind a large technical system was introduced by Thomas P. Hughes in “Dynamics of Technological Change”. According to Hughes, a large technical system undergoes three phases before it is established: innovation, expansion and system growth. In his description of the effects of different components in a large technical system he uses terms from the military to mark their position to the set and an unbalanced system; salient and reverse salient. If a component is marked as salient, it is well developed and is in front of the general mass of components, while components marked as reverse salient are behind the general mass of components and holding back the development of the whole system (Summerton, 1998).

A large technical system is not always what it seems. Looking at the wastewater and sewage system, the consumer use what is in their house; a shower and toilet for example, while the producer, in this case the treatment plant, is responsible for the pipes and connection between the house and the treatment plant and treatment itself. A large technical system also includes legal framework, organizations and actors. A complex frame is set for each technical system and for it to work all the parts need to be harmonized. Building a large technical system and establishing it takes time. However, once a system is established and is up and running, changes within the system are slow and hard to make since each modification could cause more changes. In order for a large technical system to change, there is a need for a driving force of some kind (Summerton, 1998).

In Sweden before the 1930’s, the driving force for the society to build and extend the water and wastewater systems was population growth and urbanization. As more people moved to urban areas, the demand for clean water and sanitation increased to prevent spreading of diseases (Levlin, 2018a). This can be seen as the expansion and growing phase of a system based on Hughes theoretical framework. Once the system was established, with residential buildings where each apartment having their own tap with cold and hot water, an increase standard of living occurred and the usage of domestic water increased as people where better off economically than before. This, of course led to more wastewater for the system to handle. During this development, an increase of energy was also noted (Olsson, 2012).

Today, the system has reached a phase of saturation, meaning a state where the users are connected to the system and a so-called top-down structure. In such a system, the consumer and producer can be seen as upstream actors and activity and downstream actors and activities, where the system is strongly controlled by the producer, leaving the consumer and users without much or any influence (Blomkvist & Nilsson, 2017). The wastewater system in Stockholm is an example of this; the Stockholm municipality supply the residents of the city with water and treat the wastewater from users, excluding the population for involvement in the water and wastewater system. Blomkvist et. al (2020) introduce the concept of Vertical integration, a concept where one actor or producer control the whole supply chain. In Stockholm, and Sweden, the vertical integration can be considered strong, since the wastewater system is controlled by the municipality of Stockholm. In a well-established system with clear structure, new ideas and technologies find a place to flourish in the so-called critical interface, a level and place considered outside the current

(25)

-14-

system, breaking the logic of the system, which in turn may cause disturbance in the existing system (Blomkvist & Nilsson, 2017).

It could be argued that an important driving force of today is urbanization and the need for new technology in order to reach the Sustainable Development Goals, including the need to examine systems on a deeper level. An important key in reaching a sustainable world and develop the water and wastewater infrastructure in such a way, overcoming and starting communication and education over previous separated levels and actors is needed. In educating the consumers and the users of the water and wastewater services that there is a large infrastructure underground and the challenges the wastewater treatment plants are facing, maybe some user will reflect on this, and in turn a change in behavior may appear (Olsson, 2012).

4.2 Wastewater Heat Recovery

The technology of heat recovery is well known and the idea to recover heat from wastewater is also been used for some time, especially when considered the amount of energy that flows underground in the sewers.

In 2012, the Swedish Energy Agency studied the domestic hot water usage in about 40 houses, and also compiled different template values used and estimated by several actors in the field of energy. The study concluded that about 20% of the energy used in a household is used to heat domestic hot water. This corresponds to an average person in Sweden using about to 1- 5 MWh to heat up water in a year. The interval is explained to be based on the different houses used in the study, ranging from large residential buildings to single houses or townhouses (Swedish Energy Agency, 2012).

The ability water has to carry heat makes wastewater heat recovery favorable, however, no technology can transfer heat without any losses. However, an increase of the energy prices and goal to find new solutions for using renewable resources, make heat recycling an opportunity (Olsson, 2012). The basic function of heat recovery is based on the concept of a heat exchanger, transferring heat from a warm to a colder medium. A common used medium for transferring heat is water, based on its property of carrying heat.

Wastewater heat recovery is using the same principle and depending on the position of a heat exchanger in the system. The heat can either be used and circled back into the building or connected to a high temperature heat load with the use of a heat pump.

4.3 Nitrogen and Phosphorus

Biological removal of phosphorus from wastewater started already in the 1970’s as the Swedish government invested heavily into developing and building better wastewater treatment plants and also invested in treatment at several point sources such as industries and mining sites, decreasing the amount of nutrients reaching the recipients (Naturvårdsverket, 2017)

In the 1990’s, people and the society have become more aware of the environment and how it is affected by anthropogenic actions, putting pressure on the system to become more sustainable and removing nitrogen (Levlin, 2018a).

4.3.1 Usage

Nitrogen and phosphorus are elements important to farming and agriculture as they are nutrient and used as fertilizer. Phosphorus is also considered to be limited resource (Renman, 2017). A key factor in reaching

(26)

-15-

the Sustainable Development Goals is to identify where recycling of resources can be used. In wastewater treatment plant, the nutrients are removed by being absorbed by the activated sludge. If the sludge has low concentrations of heavy metals and other trace elements, it can be reused on farmland (Naturvårdsverket, 2017).

4.3.2 Eutrophication and nutrients in the sea

Nitrogen and phosphorus are two elements that appear naturally in the water and sea but have a role in eutrophication process as more and more is released as a result of human activities. The compounds come from either an internal release within the system or from an external source. An external source can be discharge from a point source and human activity, such as a wastewater treatment plant or agricultural activities, while an internal source of the elements is released during the process of decomposing organic matter. As the concentration of nutrients increases in the water, more plants will grow and the bacteria population increases. During the process of decomposing, nitrogen and phosphorus appear in different species and consume oxygen. Even though the release of nutrients has decreased somewhat from external sources, the internal release continues to drive eutrophication (Havs och Vatten Myndigheten , 2019). Of the anthropogenic sources, the emissions of nutrients into the sea from wastewater were identified as the second largest contributor, after industrial emissions in 2014 (Naturvårdsverket, 2017).

An increase of nitrogen concentration in the Baltic Sea also means that organisms can accumulate more nitrogen species. This has been noticed in fish, and could therefore be a health risk. The process of converting NO3 to NO2, require oxygen and reduce the blood’s capacity of transport oxygen around in the body (Plaza, 2018a).

(27)

-16-

5 Henriksdals Wastewater Treatment plant

This section will look closer at the Henriksdals wastewater treatment plant, and the different processes there to treat the incoming wastewater from all the residents and industries connected to the treatment plant.

The influent wastewater to a treatment plant consists mostly of water but also organic matter and nutrients.

The mixture of several compounds, each with specific properties and is removed in different settings, making the treatment complex and is designed after how many residents the treatment plant serves. The Henriksdals wastewater treatment plant treats wastewater from about 860 000 residents in Stockholm but also a few surrounding municipalities such as Huddinge, Haninge, Nacka, Tyresö and an area in Solna (Stockholm Vatten och Avfall , 2020). The conditions for the wastewater treatment plant are changing as more and more housing and industrial areas are being connected (Naturvårdsverket, 2017). In 2018, 848 000 residents were connected to Henriksdal. However, a larger wastewater treatment plant has a higher potential of removing nitrogen than a smaller treatment plant due to the spatial capacity to have larger tanks and increasing retention time (Levlin, 2017b).

Between the beginning June 2016 and May 2020, Henriksdals wastewater treatment plant received 3,25 m³/s, minimum flow 1,97 m³/s and maximum flow 6,52 m³/s. The requirement of treatment and the allowed concentration in the effluent waters of species of nitrogen and phosphorus can be seen in the table 2 below (Stockholm Vatten och Avfall , 2020). Before the treated water flows out into the Baltic Sea, the temperature is lowered by seven heat exchangers at Hammarbyverket, recovering heat used to heat up the facilities at the Henriksdals wastewater treatment plant and heat delivered into the district heating system in Stockholm. Each year the heat exchangers deliver about 1 200 GWh worth of energy. According to Stockholm Exergi, this amount is enough to heat about 95 000 two-bedroom apartments a normal winter in the Stockholm area (Stockholm Exergi, 2020).

In Sweden, the regulation NFS 2016:6, Treatment and control of emissions of urban wastewater (in Swedish;

Rening och kontroll av utsläpp av avloppsvatten från tätbebyggelse), set the allowed concentration in the effluent water from a wastewater treatment plant if the wastewater is collected from an urban area with more than 2000 p.e.. If the wastewater treatment plant collects wastewater from more than 100 000 p.e. and an effluent point somewhere along the Swedish coastal line from Norrtälje kommun in the east to the boarder to Norway in the west, the NFS 2016:6 regulation also sets a limit on the allowed concentration of the total nitrogen to 15 mg/l as the mean value over a year for the nitrogen total (Naturvårdsverket, 2017).

Tot- N is the sum of the nitrogen consisting in the several nitrogen species such as ammonia, nitrate, nitrite and organic nitrogen, illustrated in the equation 1 below (Plaza, 2018b).

𝑇𝑜𝑡𝑁 = 𝑂𝑟𝑔 𝑁 + (𝑁𝐻_-− 𝑁) + (𝑁𝑂₀− 𝑁) + (𝑁𝑂₁− 𝑁) (1)

However, with backing of the Swedish Environmental Code, it is always possible to set stricter requirements on the effluent concentration based in local conditions. As the Henriksdal wastewater treatment plant release their treated wastewater into the Baltic Sea of the, stricter regulations are set.