STOCKHOLM SWEDEN 2019,
Impact of Stormwater reuse
(Rainwater Harvesting) in areas with combined sewer network
CASE STUDY: KUNGSHOLMEN AREA - STOCKHOLM
ROAA HAMID
KTH ROYAL INSTITUTE OF TECHNOLOGY
SCHOOL OF ARCHITECTURE AND THE BUILT ENVIRONMENT
www.kth.se
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1.1 SUMMARY
Due to the combined effect of intense rainfall events together with the expected impact of climate change, this will put pressure on the existing and future infrastructure for stormwater management.
One of the challenges related to this is the combined sewer system which is still operating in large areas of many cities worldwide. In Stockholm, combined sewer represents around 50% of the total sewer pipe length. In a Combined sewer system, once the conveyed discharge exceeds the system capacity, the system overflows, which can result in a diverse range of health and environmental problems. The cause of overflow has been strongly linked to runoff from intense rainfall events.
Therefore, a key proposal to overcome this problem is to disconnect runoff from hard surfaces.
This research aims to investigate the impact of applying a rainwater harvesting (RWH) and reuse system to collect runoff water from roof surfaces in areas with combined sewer system. A simulation water balance model for a rooftop RWH system was developed and two reuse purposes were considered, which entails toilet flushing and garden irrigation within the property. The study area consists of one building block within Kungsholmen area in Stockholm.
The obtained results indicate that applying such systems can reduce runoff to the sewer system.
Toilet flushing reuse shows a higher reduction impact on sewer flow than the use for irrigation.
Toilet flushing reuse reduces annual runoff volumes to sewer in a range of 49.5% - 93.4% while irrigation provided reduction in a range of 11.6% - 26.3%. Regarding number of times that overflow from the combined sewer system occurs, toilet flushing reuse demonstrated reduction of 40% - 100% while 20% to 60% was reduced by irrigation reuse. For overflow volume, a reduction rate of 11% to 100% was reached through toilet flushing in contrast to 9% to 43% reduction from irrigation reuse. 19% to 37% of toilet flushing water demand was covered by the tank, while a range of 48% to 100% was covered for irrigation demand. All these parameters were found to be sensitive to change in tank size where increasing the size result in higher flow reduction rates.
When considering implementing a reuse system, it is important to consider the applicability of RWH and reuse within the specific property. In areas that are under development, either of the two reuses can be considered depending on local conditions. However, in already built up area it is difficult to introduce a system that requires significant adjustment to existing pipe networks, such as reuse systems for toilet flushing. Systems for outdoor irrigation are possible to implement in most situations. When it comes to tank size, the optimal size will depend on the intended reuse,
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the catchment area and the objective of the system. For example, if the main objective is to reduce potable water consumption, a smaller tank can be used compared to where the main objective is to reduce sewer overflow. Hence, when considering implementing a rainwater reuse systems, each project will need to consider the local conditions as well as the individual objectives when determining the optimal reuse purpose and tank size. A cost-benefit analysis should also be considered when determining the optimal tank size for the intended use.
Keywords: Rainwater harvesting, rooftop runoff, Combined Sewer Runoff- CSO, Toilet Flushing, Garden Irrigation
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1.2 ACKNOWLEDGEMENT
This thesis is written as a complementary research for the Environmental Engineering and Sustainable Infrastructure Master’s Program. By completion of this research I would like to extend my thanks to my Tyréns supervisor Olof Jonasson, for his continuous support and encouragement along the journey. I would like also to thank my examiner from KTH Anders Wörman for his helpful advice regarding the project. By completion of this master’s degree, I acknowledge the great support of Swedish Institute for my scholarship and their support all through my study time in Sweden. At last, I would like to thank my family and friends for their support and motivation, and finally to the one who was always with me despite the long distance, my beloved husband Abubaker.
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1.3 ABBREVIATIONS
CSO Combined Sewer Overflow 𝑅𝑟𝑜𝑜𝑓 Runoff from roof surface area 𝑅𝑟𝑜𝑎𝑑 Runoff from road surface area
𝑂𝑔𝑟𝑒𝑒𝑛 Overflow from green surfaces after infiltration RWH Rainwater Harvesting
WWTP Wastewater Treatment Plant TOC Time of Concentration
𝑁𝑒𝑥𝑐𝑒𝑒𝑑 Number of times that rainwater runoff to combined sewer exceeds threshold runoff 𝑉𝑒𝑥𝑐𝑒𝑒𝑑 Volume of runoff that exceeds threshold runoff
SMHI Swedish Meteorological and Hydrological Institute DHI Danish Hydraulic Institute
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C ONTENTS
1.1 SUMMARY I
1.2 ACKNOWLEDGEMENT III
1.3 ABBREVIATIONS IV
1 INTRODUCTION 1
1.1 BACKGROUND 1
1.2 AIM AND OBJECTIVES 1
1.3 METHODS 1
2 THEORETICAL BACKGROUND & LITERATURE REVIEW 3
2.1 CLIMATE CHANGE AND ITS IMPACTS ON WATER MANAGEMENT IN SWEDEN 3
2.2 URBAN STORMWATER MANAGEMENT STRATEGIES IN SWEDEN 6
2.3 COMBINED SEWER OVERFLOW 8
2.4 SUSTAINABLE MANAGEMENT OF URBAN STORMWATER 11
2.4.1 SUSTAINABLE STORMWATER TECHNIQUES FOR URBAN RUNOFF CONTROL 12
2.5 RWH AND REUSE 14
2.5.1 RWH AND REUSE FOR CSO REDUCTION 16
3 MATERIALS AND METHODS 19
3.1 DATA SOURCES AND CHARACTERISTICS 20
3.2 MODEL DESCRIPTION 21
3.2.1 MODEL PARAMETERS: 22
3.2.2 MODEL SCENARIOS AND REUSE PURPOSES 24
4 CASE STUDY 28
4.1 STUDY AREA DESCRIPTION 28
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5 RESULTS 32
5.1 TOILETFLUSHINGREUSE 32
5.2 IRRIGATION REUSE 38
6 DISCUSSION 44
7 CONCLUSION AND LIMITATIONS 45
8 REFERENCES 48
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1 Introduction
1.1 BACKGROUND
On average, there is 550 mm/year of precipitation in Stockholm. Of this amount 450-500 mm/year is drained from hard surfaces and needs to be managed. According to climate change forecasting models, this amount is expected to increase with time in addition to expected increase in the frequency of high intensity rainfall events. This change and its impact put a demand on climate adaptation not only in the new construction and development urban projects, but as well on the existing ones within the urban environment. One of the infrastructures vulnerable to climate change is stormwater drainage systems. In Stockholm there is a total of about 2000 km of pipeline, of which 50% is combined sewer system where wastewater and stormwater are conveyed in one pipe. The other 50% consists of duplicate systems in which stormwater is separately drained through a special management system (Stockholm Stad, 2013).
When the discharge in combined sewer pipes exceeds the system’s capacity, excess overflow water causes what is known as combined sewer overflow (CSO). This problem is associated with several environmental and health problems (Ekelund, 2007). The percentage of domestic wastewater was found to be constant during overflow events for each individual municipality for different years (Wennberg et al., 2017), only 10% of the overflow volume is wastewater, which indicates the correlation between and heavy rainfall events (Stockholm Vatten och Avfall, 2019). To overcome the CSO problem, one key proposal, is to reduce the amount of runoff from hard surfaces that goes into combined sewer system (Svenskt Vatten AB, 2016).
1.2 AIM AND OBJECTIVES
This research project aims to study the impact of rooftop rainwater harvesting system (RWH) on reducing sewer runoff flow that goes into a combined sewer system. In reducing sewer runoff flow this should in turn lead to the reduction of combined sewer overflow CSO.
1.3 METHODS
To achieve the aims of this research, a review of literature related to rainwater management methods and sustainability has been conducted. Following this, a case study for a small urban building block in Stockholm was carried out.
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In the literature review, information about rainfall in Stockholm and expected future impact of climate change has been collected and studied. Current and historical systems of Stormwater management in Sweden have been studied and related challenges to these systems have been identified with focus on the combined sewer overflow problem. Furthermore, selected studies and theories regarding sustainable stormwater management were reviewed together with the international and national (Swedish) best practice on RWH and reuse. A number of previous relevant studies about RWH and reuse systems have been critically reviewed and analysed to identify knowledge gaps that could be achieved though this study.
The Case study had been selected to be within a residential area in Kungsholmen, Stockholm.
The study considered a sub-catchment (part of a bigger sewer shed) in the area to assess the effect of applying a RWH and reuse system on the sewer runoff. For assessment, an excel model had been built and implemented.
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2 Theoretical Background & Literature Review
This chapter describes the theoretical background related to urban RWH and reuse. It comprises a brief about climate change in Stockholm-Sweden and its impact on stormwater discharge systems currently and in the future. One section describes local systems of stormwater management in Sweden and how it has changed over time, as well as their pros, cons and related challenges. One of these problems is the combined sewer overflow which is connected to the combined sewer system. The chapter briefly describes the CSO problem and reviews several previous scientific studies about it. Moreover, sustainable management approaches for stormwater are described from scientific literature and previous studies. Finally, the literature regarding rainwater reuse is reviewed and special focus is put on reducing sewer runoff and CSO, as well as how this problem has been studied and what solutions were proposed before.
The main source of literature was assembled using the KTH library search engine with focus on recent studies within the last two decades.
2.1 CLIMATE CHANGE AND ITS IMPACTS ON WATER MANAGEMENT IN SWEDEN
Climate change has become a real challenge in today’s life. It has a clear impact on rainfall patterns and distribution, which as a result affects the stormwater drainage system. In Svenskt Vatten’s Report, (Svenskt Vatten AB, 2007), the study discusses two different climate change factors and how they would impact the sewage system in Sweden. In addition to this, it also discusses a change in rainfall and water level rise in recipient water bodies. The increased rainfall would produce more runoff which increases the risk of urban flooding and overflow from sewer systems. Furthermore, treatment plants have limited treatment capacity regarding water volume as well as limited pollutant load. So, when they are faced with extra loads, this would lead to release of untreated waters into the receiving water bodies. This eventually would reduce water quality within water bodies and drinking water (Hellström et al., 2014).
In Sweden, the predicted changes in rainfall include a risk of having more intense short-term rain with changed patterns and distribution than what has been in the past; e.g. long-duration rainfall events. This can have consequences and impacts that include; increased risk of flooding especially during short intense storms during cold months, increased runoff volumes that need to be managed in addition to the expected longer rainy seasons with less evaporation during the
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colder months, leading to and saturated soil. This decreases the infiltration rate and increases the runoff volumes . (Svenskt Vatten AB, 2007).
As stated in the Journal of Hydrology (Madsen et al., 2014), a review study for trend analysis of extreme precipitation and hydrological floods based on historical observations and future climate projections in Europe has been conducted. General findings from this study indicate an increase of extreme precipitation events in the future. This increase has been found to be particularly in winter precipitation while summer rainfall events are observed to decrease in different locations within western and central Europe (Zolina, 2012) (Madsen et al., 2014). For Sweden, historical studies for 90 years in southern part of the country have found that there was no trend in maximum daily precipitation. However, climate forecasts anticipate an increase in short term extreme precipitation (30 min to 24 hours) particularly in winter (Madsen et al., 2014, Olsson, 2009).
Over the next 100 years, the predicted changes in precipitation is strongly connected to climate change (Svenskt Vatten AB, 2007). According to the Swedish Meteorological and Hydrological Institute’s (SMHI) climate forecast models, the precipitation over the next 100 years will increase. For instance; in the Stockholm area a gradual increase will happen in the annual precipitation. That change is shown in Figure 1 & Figure 2, in which the black line represents s the mean change while the expected range of change is shown within the grey area of the graph.
However, another results from SMHI indicate that in Sweden, the change in precipitation varies among seasons; during summer months (June, July and August), the rainfall will decrease in most of the country, while the rainfall and temperature will increase during winter months (Svenskt Vatten AB, 2007). Particularly in winter, an increase of 5-10% is forecasted for 2011- 2040 and 25% is forecasted for the period 2071-2100 with base line period 1961-1990 (Ekelund, 2007). This redistribution of rainfall during autumn, winter and spring with low evapotranspiration would lead to an increase in the drained volumes during this time of the year which imply higher loads in wastewater treatment plants (WWTP) (Svenskt Vatten AB, 2007).
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From SMHI climate models results, the future prediction for change in separate rainfall events is shown through calculated change in maximum daily precipitation (%) during years of 1961- 2100 using historical rainfall data from 1960 to 2005 (Figure 1). From graph in Figure 1, the general trend of change indicates a mild increase in the future maximum daily precipitation.
However, in Figure 3, the change in annual number of days with heavy precipitation is showing a clear forcasted incresed change during the coming years.
Figure 3:calculated change in annual number of days with heavy precipitation (days) in Stockholm area during the years 1961- 2100 compared with normal (mean for 1961-1990) (SMHI, 2014)
These changes in climate would have several impacts on the environment. They are expected to happen slowly over time which gives an opportunity for preparing and applying the needed adaptation measures. Slow change towards higher rainfall intensities eventually leads to more frequent flooding in the rainfall drainage system, which in turn needs to be considered within the continuous improvement measures for drainage systems (Svenskt Vatten AB, 2007). On a
Figure 2: annual Precipitation change (%) over time from 1960 to 2100, with historical data from 1960 to 2005 (SMHI, 2015)
Figure 1:Calculated change in annual maximum daily precipitation (%) for the period 2071-2100 (SMHI, 2014)
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regular basis, the drainage network is reviewed and adjusted to the buildings and new changes within the service area (drainage catchment). However, risk of flooding differs from one area to another according to the normal variation of rainfall distribution and the current status of drainage network. As a result, application of the adaptation measures will vary accordingly taking into account, how critical the flooding risk is, in each area of the drainage catchment (Svenskt Vatten AB, 2007).
2.2 URBAN STORMWATER MANAGEMENT STRATEGIES IN SWEDEN
The Swedish urban stormwater management could be considered as an international model for cities within the same climatic zone. Cities like Malmö and Växjö are perceived as pioneers in integrating stormwater issues within their urban planning. This includes collaboration between relevant institutions which resulted in multifunctional blue and green solutions (Hellström et al., 2014).
There are three alternative types of municipal drainage systems that have been used and developed over time in Sweden. They comprise; combined sewer, duplicate system and separate system. In Stockholm, stormwater and wastewater had been drained together in combined system to a nearby recipient since 1860. In the 1930s wastewater treatment plants started to be built and combined water was treated there before final disposal. Combined systems were the dominating system until the 1950s, where thereafter duplicate systems were introduced by 1963 to reduce the load from WWTPs (Stockholm Stad, 2013).
In general, a combined sewer system is known as the system in which wastewater and stormwater are collected in one pipe and conveyed to the wastewater treatment plant before final disposal in water ways as shown in Figure 4 (Svenskt Vatten, 2007).
Figure 4: Sketch for combined sewer system components. from roofs, roads and household wastewater (Svenskt Vatten, 2007).
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In the duplicate system represented by Figure 5, the wastewater and stormwater are handled in different sewage systems; the lowest pipe system is designed for taking wastewater while the upper ones are devoted for stormwater. In Stockholm, the use of duplicate system diverted pollutant load of stormwater to the city’s recipient water bodies (Stockholm Stad, 2013).
Figure 5: Sketch for duplicate sewer system where stormwater is collected by upper pipe and household wastewater is collected separately by the lower pipe (Svenskt Vatten, 2007)
Finally, in a separate sewer system, wastewater is handled separately by its own sewage system while stormwater is diverted into another drainage facility, which can be a ditch system or a pipeline. This system has started to be applied within residential areas since the early 1900s, the main reason was to reduce the construction cost in residential areas, where only wastewater and sometimes roof water are handled through a piping system (Svenskt Vatten AB, 2007).
Figure 6: Sketch for Separate sewer system where stormwater is handled separately and household wastewater is collected by wastewater pipes (Svenskt Vatten, 2007)
As discussed before, climate change with anticipated wetter seasons imposes new challenges with the current physical infrastructure (City of Stockholm, 2018). Furthermore, the rapid growth of urban areas in Stockholm city increases the total area of hard surfaces; buildings, roofs, roads etc. Furthermore, this would change the natural pattern of stormwater discharge through infiltration and delayed discharge. In areas with combined sewer system, at times of
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high flows, problems with sewer overflow are experienced in some places of Stockholm (Stockholm Stad, 2013).
2.3 COMBINED SEWER OVERFLOW
As described before, different types of sewer systems are used to manage runoff. In case of combined sewer systems (Figure 4), runoff water is drained together with domestic wastewater in one pipe system. When total combined runoff flow exceed the capacity of sewer systems or the capacity of treatment plants, the excess untreated water is often discharged directly into surface water. This outcome is known as Combined Sewer Overflow (CSO) (Alyaseri and Zhou, 2016).
Intense rainfall events produce large amounts of runoff which can be a direct cause for overflow in sewer systems and wastewater treatment plants. The discharge of untreated combined runoff results in contamination of water bodies with sewage that leads to degradation of surface water quality (Ekelund, 2007). Stormwater itself washes away any pollutants in the flow path, including bacteria, viruses, pet waste, road runoff and debris. When collecting this water in combined sewers, it mixes with untreated domestic wastewater which itself contains pathogens, organic and chemical pollutants. This results in a toxic cocktail within a combined sewer system. In case of CSO, part of this water is directly discharged into water bodies which represents an obvious risk to the environment and public health (Stoner, 2007).
In many places worldwide, CSO is the reason behind closing beaches, drinking water contamination and spread of water borne diseases. Furthermore, it could pose an economic risk due to health care expenses, loss of productivity especially for vulnerable community members of children and elderly people (Stoner, 2007).
In 2011, over 18.7 billion gallons of combined sewage was dumped in the Great Lakes area in the USA. This CSO caused wide range of environmental problems as well as economic ones.
Number of beaches were closed which resulted in lost revenues with value range of $20-36 per person per day. In addition, the area experienced water degradation and environmental consequences (Lyandres and Welch, 2012).
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In Sweden, as shown in chapter 2.1, intense rainfall events are one of the critical impacts of climate change. Moreover, expansion in urban development and impervious surface areas increase the runoff volumes to be managed through sewer system, instead of this water being absorbed into soil and naturally circulated through the hydrological cycle. As a result, the risk of CSO consequently increases. For the Swedish Water and Wastewater Association Development SVU; The Danish Hydraulic Institute (DHI) has conducted a study for CSO in a selection of Swedish municipalities using hydrological and hydraulic models for the combined system in each area (Wennberg et al., 2017). The study results showed that CSO volume varies from 0.7-4.1% of the total runoff flow that goes to a sewage treatment plant. For most municipalities, the overflow occurs in one or a few points throughout the sewer network. This would help in deciding upon action and give priority to these points. The percentage of domestic wastewater in overflow water is found to be constant during overflow events for each individual municipality for different years (Wennberg et al., 2017). The study concluded that CSO have marginal environmental impacts. However, for some recipients, it could be responsible for significant amount of total phosphorus emissions from sewage. In addition, CSO can have negative consequences on revenues from bathing water and drinking water (Wennberg et al., 2017).
In Stockholm city, the sewer network is expanding with both combined and duplicate sewer systems. In older areas of Stockholm where a combined sewer system is used, incidents of sewer overflow are experienced. In Figure 7 the measured amount of CSO from year 2000 to 2017, is shown (Stockholm Vatten och Avfall, 2019). The average annual overflow value calculated for 10 years (2008 to 2017) that is discharged into water bodies in Stockholm was found to be 453,000 m3. This value didn’t meet the target guideline value for the sewer network condition which is 325,000 m3. Of this volume only 10% is wastewater and overflow mainly occurs with intensive rainfall events when sewer networks became unable to handle the excessive runoff amounts (Stockholm Vatten och Avfall, 2019). However, Stockholm city was able to reduce its CSO by half during the last 25 years (Wennberg et al., 2017). In a 2015 study (Stockholm Vatten, 2015), the future changes of CSO in Stockholm as a result of climate change and population growth was investigated. The study found that the total overflow volume is expected to increase by 5-10% within the near future and around 20-40% overflow increase is forecasted
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by the end of the century. Nevertheless, population growth is found to have smaller significance in the total overflow increase. Only 5% increase in overflow volume is corresponding to 25%
population growth (Stockholm Vatten, 2015).
Figure 7: Measured volumes of Combined sewer overflow in Stockholm through years of 2000 to 2017. (Stockholm Vatten och Avfall, 2019)
Figure 8: Chart shows the percentage of combined sewer in a selection of Swedish municipalities (x-axis) against the percentage of overflow volume from total runoff (Wennberg et al., 2017)
One way to overcome CSO problem, is to delay runoff volume that goes to combined sewers.
The conventional practice is to construct large deep tunneling storage systems with large volume capacity to hold the excess combined sewer flow until it is possible to be treated later in wastewater treatment plants. These systems are proved to be effective in reducing CSO surges if well sized and constructed. In Stockholm, adapting measures to CSO are planned to be included with the new tunneling project that conveys wastewater to the new Henriksdal WWTP after closure of Bromma WWTP. This new installation is expected to significantly reduce
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overflow and cut the volume in half (Stockholm Vatten och Avfall, 2019). Yet, such projects take several years to be constructed in addition to their high cost (Stoner, 2007). In the case of Stockholm, the city accepts proposals from business regarding local measures to overcome CSO (Stockholm Vatten och Avfall, 2019). Therefore, it is important to consider more sustainable solutions when designing climate change adaptation measures.
2.4 SUSTAINABLE MANAGEMENT OF URBAN STORMWATER
Conventional stormwater drainage systems are designed to discharge runoff from impervious surfaces as discussed earlier in this chapter. However, volumes of runoff are increasing with time as a result of current expansions in impervious surface areas (such as roads, roofs, parking lots, etc.). Moreover, climate change impacts of higher rainfall intensities and consequent flooding add another factor for increased runoff volumes to be managed. The traditional measure to adapt with increased runoff is to gradually renew and expand the existing drainage networks over time (Svenskt Vatten AB, 2007). However, such measures have been proven to lack sustainability, and are hard to apply on a long term basis especially in densely urban areas in addition to their high cost (Qin et al., 2013).
The term “sustainable stormwater management” is referred to the management that fulfills today’s requirements for stormwater disposal as well as considering the future challenges (Stockholm Stad, 2013). Different terms have been assigned to these techniques in different regions; in the US low impact development LID, Water sensitive Urban Design in Australia and Sustainable Drainage Systems in UK.
In Sweden as discussed before in chapter 2.1, climate change adaptation measures are needed to deal with the impacts of changed rainfall patterns and increased frequency of intense rainfall events. Regarding this, sustainable measures for stormwater management would play an important role to minimize the generation of stormwater runoff and mimic natural drainage patterns. To this, the Stockholm strategy for sustainable management of stormwater included four main goals (Stockholm Stad, 2013);
1. Improved water quality in the city’s water bodies 2. Robust and climate-adapted stormwater management
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3. Resource and value creation for the city
4. Environmentally and cost-effective implementation.
2.4.1 Sustainable Stormwater Techniques for Urban Runoff Control
Several techniques have been recently developed as sustainable alternative solutions for increased urban runoff problem. Green roofs, rain gardens and retention ponds are some examples. Such techniques manage stormwater through retaining, infiltrating, reduction of imperviousness and stormwater reusing locally within the development site (Qin et al., 2013).
(Qin et al., 2013) studied the effect of three LID techniques on urban flooding (swales, permeable pavements and green roofs). By comparing the reduction in total flood volume between these three techniques and conventional drainage system during a storm event, the results indicated that LID systems were more effective in reducing flood volumes than the conventional system. In a study conducted for St. Louis city using Green infrastructure (permeable pavement), as one onsite methods for rainwater management that aims for reducing rainfall runoff, it has been found that the stormwater runoff was reduced by 36%, 13% and 46%
as a result of using permeable concrete, permeable asphalt and permeable pavers respectively (Alyaseri and Zhou, 2016). Furthermore, green infrastructures have been studies by (Stoner, 2007) for purpose of controlling CSO. Through their function of capturing stormwater runoff before it goes to combined sewer system, green infrastructure reduces total runoff volume into sewer system. In the city of Seattle, a multifaceted green infrastructure was implemented for stormwater control. As a result, they succeeded to reduce the discharge into sewers. Since 2002 there has been no runoff from stormwater recorded at any of the project sites (Stoner, 2007). In Lyandres and Welch, (2012), municipalities of Great Lakes areas in the USA have decided to use different green infrastructure methods to solve CSO in lakes including green roofs, vegetated curb extension and permeable pavements.
Based on their functions, sustainable stormwater management techniques can be categorized as follows (Embertsén, 2012);
1- Reduction of Peak flows (e.g. Retention ponds and structures, Open flow canals)
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2- Runoff volume reduction (e.g. Green roofs, Permeable surface layers, Bioswales, Trenches and vegetated infiltration zones, Infiltration zones, Skeleton soil structure, Storage tanks) (Figure 9).
3- Treatment (e.g. Filter, Floating Biomass, Bioretention, Biotope) (Figure 10).
4- Reuse (e.g. Rooftop RWH for non-potable water use)
Figure 10: Ditch and infiltrating green areas (Stockholm Stad, 2013).
In Sweden, during the last 20 years a paradigm shift has been made towards more sustainable stormwater management. The efforts are made towards reducing the amount of stormwater that is diverted to a piping system. For Sweden, the international market trend and water management strategy put water reuse high on agenda, which emphasize the importance of developing management methods that make reuse possible in order for Sweden to stay as an important player in the global water market (Hellström et al., 2014).
Stockholm city has formulated a new strategy for stormwater management in 2013. This strategy describes the city’s approach towards more sustainable management of storm water. It
crusher ditch Green roof Plant bed
Figure 9: Examples for runoff volume reduction measures from left to right crusher ditch on parking area, Green roofs on garage buildings Plant bed for parking surface water (Sweco) (Stockholm Stad, 2013)
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puts focus on preserving water quality as well as stresses on concepts of stormwater utilisation to a larger extent. The overall policy in this strategy called local handling “Lokalt omhändertagande (LOD)”, it states that “stormwater should primarily be handled by infiltration and delay at source before diverting to its final pool” with a main aim of water balance and reduction of the load on mains and wastewater treatment plants (Stockholm Stad, 2013) According to the Stockholm city plan; to achieve the plan’s objective effectively, regional cooperation is needed for large scale technical facilities including water and sewage works.
Though, this cooperation is also required for small scale solutions (City of Stockholm, 2018).
The third goal in the Stockholm city strategy for sustainable stormwater management aims for
“resource and value creation for the city”, to handle the increased urban runoff volumes in a sustainable way; stormwater should be utilized as a resource within urban communities. This requires selecting solutions for urban stormwater management from hard surfaces and roofs to aim for utilising rainwater as far as possible (Stockholm Stad, 2013). In this research, the focus would be particularly on RWH and reuse.
2.5 RWH AND REUSE
RWH and reuse systems are known as “the practice of collecting rainwater or stormwater from impervious surfaces such as roofs or ground surfaces, treating and storing it for future uses”
(Begum, 2008, Ding, 2017). Rainwater can be directly used for domestic purposes with less- water quality demand such as toilet flushing, cleaning, washing cars and irrigation of house gardens (Campisano et al., 2013). Rainwater reuse provides several advantages when applied in a proper manner such as:
▪ Reducing potable water demand from mains
▪ Reduction in stormwater flows which can provide flood risk reduction in places with flood problems and sewer system overflow
▪ Reducing contamination loads into treatment plants and downstream waterways (Campisano et al., 2013)
These benefits are limited by a number of factors within the reuse scheme such as; local climate conditions especially rainfall, local land use in the catchment which affects the quality and quantity of runoff, local condition of drainage system in terms of sewage type and overflow
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condition, reuse demand and its variation over time and finally; the specific design for reuse system and how water is delivered for reuse. (Begum, 2008).
Stormwater reuse system can have number of drawbacks in its different scales such as environmental impacts of the storage, potential health risk and relative high cost of treated stormwater. However, these possible limitations of reuse systems are determined by the nature of scheme and local environment. Therefore, reuse of stormwater could be suitable more for non-potable purposes such as residential non-potable uses, irrigation, recreational water features, industrial and commercial uses (Begum, 2008).
Rainwater and stormwater harvesting techniques and design methodologies vary according to purpose of water reuse and catchment area. Some of these methods are used worldwide such as;
▪ Aquifer storage and recovery for ground water recharge,
▪ Urban lakes for recreational purposes,
▪ Constructed wetlands for enhancing pollutants removal,
▪ Rainwater tanks for rainwater storage and reuse (Begum, 2008).
Considering rainwater reuse on small scale; our study further investigates RWH tank system.
Storage tank systems have been adopted by many modern cities despite the existence of water supply system. Many reasons lay behind implementing such system based on level of water scarcity and development of the area. At first, in areas with water scarcity challenges and low development level, rainwater tanks provide an alternative or secondary water source. In areas with no water supply connections, they can provide better water quality compared to other water sources. (Bocanegra-Martínez et al., 2014) applied a model for optimal design of RWH system to satisfy domestic use in city of Morelia in Mexico. A reduction range of 80-87.6% in freshwater consumption was achieved. Secondly, in water rich areas and developed countries, rainwater tanks have been adopted for addressing urban sustainability issues such as reducing environmental impacts of stormwater flooding and drainage such as CSO. Moreover, it can be used as an addition water supply for non-potable demands in places where high quality of drinking water is available (Sharma and Begbie, 2015).
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2.5.1 RWH and reuse for CSO reduction
As mentioned earlier in this chapter, areas with combined sewer system are prone to experience problems of overflow (CSO) as result of heavy rains. To overcome such challenge, number of studies have proposed use of stormwater management measures that adopt the concept of control runoff from source. These measures reduce and/or delay the volume of runoff in the upstream part of the drainage catchment (Makropoulos et al., 2001). Different stormwater management measures have different impact on runoff. The impact vary between annual basis and precipitation event basis. As well, some measures have multiple impacts on sewer runoff as shown in Figure 11 and Figure 12. For instance, CSO-tank delays around 20% of runoff, while soil filter for CSO has an evapotranspiration effect on less than 10% of runoff, and delays about 70% of runoff based on storm event level. A key concept in runoff flow reduction is to disconnect hard surfaces from combined sewer network (Vaes and Berlamont, 2001). The use of storage facilities for RWH and reuse upstream sewer system can largely influence the amount of runoff to the sewer system, especially reducing peak flow (Vaes and Berlamont, 2001).
Figure 11 and Figure 12 demonstrate that almost 90% of annual runoff can be utilized through domestic reuse system and 65% from one design storm (Sieker, 2001).
Figure 11: Mean annual water balance for different stormwater management measures (Sieker, 2001)
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Figure 12:Water balance of different stormwater management measures for a typical design storm (Sieker, 2001)
Number of other authors studied the impact of RWH and reuse system on runoff reduction in various contexts. In their paper, Campisano et al., (2013), studied the potential of domestic RWH and reuse tank system in reducing storm sewer discharge to increase urban resilience for climate change impact in city of Fredrik stad in Norway. The reuse purpose in their study was limited to toilet flushing in houses, as this use is of constant daily rate with no large daily variation and no high quality for water is required in this purpose. The study results revealed that RWH tanks can significantly reduce water inflow into stormwater sewer in addition to reducing damages that result from extreme rain events. In another study, Zhang et al., (2012) investigated the potential of runoff reduction impact on reducing urban waterlogging problem in Nanjing in China. The study found that waterlogging problem can be reduced through RWH by range of 13.9%, 30.2% and 57.7% when the system was applied in three case study areas.
Regarding sewer runoff, Vaes and Berlamont, (2001) assessed the impact of rainfall runoff source control measures on the design storm for combined sewer systems. The study found that a well-designed tank system can significantly reduce runoff to sewer system.
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The reliability of the system to satisfy water demand and reduce runoff was investigated in several studies. It has been found that, system reliability is sensitive to tank size, catchment area and reuse demand. In this sense, Oliveira et al. applied a simulator to evaluate the benefits from RWH in three case studies. The results showed that higher benefits of RWH are correspondent to higher non-potable water consumption and larger collection areas. Petrucci et al., (2012) through an experimental case study, investigated the effect of applying RWH tank system on preventing sewer overflow. The results recognized that urban evolution of the catchment, particularly small-scale land cover modifications, have affected the hydrological behavior in the catchment. However, tanks alone were not able to prevent sewer overflow during heavy rains. The tank system inefficiency was found to be correlated to tank size and not to the harvesting area, where installing larger tanks was found to have higher ability in preventing overflows (Petrucci et al., 2012). In the same regard, Tsai and Chiu, (2012) evaluated the performance of multi-purpose RWH systems in Taipei City in Taiwan. The study found that using RWH storage system with capacity ranging from 55 m3 to 183 m3, reduced runoff volume in a range between 26.5% to 100% and reduced peak flow by 15% to 100% respectively. These findings agree as well with results of Sample and Liu, (2014).
However, little quantitative studies were found for the city of Stockholm. Furthermore, further studies for investigating the impact of RWH that use sub-daily time scale for rainfall records are recommended especially for studies related to CSO and urban flooding adaptation (Campisano et al., 2013). Therefore, this research investigates the effect of RWH system in combined sewer runoff considering two rainwater reuses within properties; toilet flushing and garden irrigation. The study assesses RWH impact on two levels; local sewer network level and the impact on main sewer network within main drainage catchment. Methodological details will be presented in the next chapter (Materials and Methods).
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3 Materials and Methods
To support the decision making process of applying RWH systems, the need would evolve for realistic data analysis and quantitative studies that use mathematical models and simulate rainwater systems. Such studies support decision making not only with regard to meeting potable and non-potable water demands, but as well, reducing stormwater discharge and its associated environmental problems. Through such quantitative studies, the selection of modelling approach can influence the expected outcomes from the model. Furthermore, different choices in the model parameters would significantly influence the results, for instance, time-step, model algorithm and simulation length (Sharma and Begbie, 2015).
Modelling tools vary in their complexity; some are very simple, they only consider annual variation in rainfall to determine the tank sizes. Whereas other approaches use analytical formulas to estimate tank sizes considering the expected water use rate, local climate data in addition to the required level of the tank reliability in meeting water demand over time. It has been proposed in number of studies such as; Bocanegra-Martínez et al., (2014), Sample and Liu, (2014). Such methods are very location specific; they use local rainfall records as input for probabilistic relationships. Models from these approaches can provide planners and engineers with a robust local template for quick decision making about rainwater system (Sharma and Begbie, 2015).
More specifically, the impact of rain water harvesting on sewer flow was studied using different methodologies. Most of these methods were based on modelling water balance for the rainwater collection and reuse system such as in; Campisano et al., (2013) and Vaes and Berlamont, (2001) studies. Campisano et al., (2013) used daily water balance model with input of historical rainfall record as well as investigating the effect under future climate change scenarios.
According to Vaes and Berlamont, (2001), it is important to incorporate the real variability of rainfall for accurate estimate of RWH system impact on runoff reduction.
The continuous water balance modelling approach calculates rainwater in the tank as function of time by using historical rainfall records for inflow and assumed water demand for outflow
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(Basinger et al., 2010). Similar approach has been used in Sample and Liu, (2014) to evaluate decentralized RWH systems over different land uses and locations in Vergina, USA.
For studying the impacts of source control measures, simple reservoir models that use continuous simulation for rainfall runoff can be used. These models can also be used to determine the optimal design parameters for rainwater tanks. Furthermore, investigating the effect of upstream retention structures on CSO can be studied through these simple models for upstream retention measures and sewer system. It can help reducing calculation time with acceptable analysis results (Vaes and Berlamont, 2001). In their study, Vaes and Berlamont, (2001) assessed the impact of rainwater tank on the amount of the inflow going to the combined sewer. The study used historical rainfall series in a simple reservoir model with constant outflow representing the household consumption of harvested water. Overflow from the tank goes to the combined sewer with runoff from other hard surfaces.
In our research, a water balance method is used to assess the impact of rainwater tank system on the rainfall runoff flow to combined sewer. For this, a simulation model is built for rainwater collection and reuse over time. It is run for different tank sizes and their impact on flow reduction is assessed. The model is built using spreadsheet in Microsoft excel program.
3.1 DATA SOURCES AND CHARACTERISTICS
For modelling the system, different data sets were used;
1- Catchment area boundaries: the boundaries were determined with a Shapefile that contains data for two sub-catchments as part of a bigger sewer shed (Tyréns AB, 2019).
2- Population data: it includes an updated number of people living within buildings in the study area. It has been used to calculate toilet flushing water consumption within these buildings. The total number of people living in the study area buildings was calculated using data from (Eniro, 2019) and number of children was calculated based on the fact that an average Swedish family will have 1.75 children, and assuming that 50% of people live in families (Sweden.se, 2019, Statista, 2018).
3- Toilet Flushing water demand data was based on (Petersens, 2001)
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4- Water consumption for house gardens is calculated based on the assumption that the amount of water that can be disposed of through evapotranspiration and infiltration is equivalent to two times average evapotranspiration. Irrigated areas were determined through digitization of green areas within the study blocks using google earth.
5- Historical rainfall data for years 2015 and 2016 (selected as representative years based on annual precipitation). This data was collected in 15 minutes i (SMHI, 2019).
3.2 MODEL DESCRIPTION
To measure the impact of RWH on the total runoff that goes to combined sewer system; a water balance simulation model was built. It simulates sewer runoff volume with two scenarios; one scenario for sewer runoff considering the suggested RWH and reuse system, and the other one calculates the resultant combined sewer runoff without existence of any RWH system (Figure 13 and Figure 14).
The model simulates the impact of RWH system over a specified time period using historical rainfall data as input for runoff generation. Runoff is calculated for the different land uses; roofs, roads and the overflow from green areas, using rational method which considers the variation in runoff coefficients with the different land uses. The generated runoff (output) is then collected differently in each scenario; in scenario-1, the runoff from roofs is captured in RWH tank and reused within
Combined Sewer Network
Overflow from Green areas Roads Runoff (Rr)
Overflow from Storage
Tank
Combined Sewer Network
Overflow from Green
areas Roads Runoff
Roof Runoff
Figure 13: Model Scenario-1 with RWH tank
Figure 14: Model Scenario-2 without RWH system
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the building blocks, while runoff from roads and overflow from green areas is diverted to the sewer network (Figure 13). In the other scenario, scenario-2 rainfall runoff from different surfaces is diverted to combined sewer system (Figure 14).
For impact assessment, a threshold runoff is selected to be correspondent to the produced runoff from one rainfall with return period equal to the frequency of reoccurrence of CSO. In case of Stockholm, it is found to often be one month (Olsson and Al-shididi, 2014)
Generally, as shown in Figure 15 below, the suggested rainwater system consists of; collection system (roof, gutter, downpipe), storage system (tank), reuse installations (pump, toilet flush storage, irrigation outlet, etc.). The system could have a first flush device to discharge the first contaminated water from roof, but this was not modelled.
For rainwater reuse; household utilization of runoff and irrigation reuses can have a similar effect on runoff to sewer as shown in previous section (Figure 12). About 65% of rainwater runoff is utilized from one typical design storm. However, for annual effect, household utilization has greater effect as it reduce almost 90% of the annual runoff while irrigation reduce around 30% (Figure 11). In this study, two different purposes are selected to be individually assessed. Firstly, rainwater is used for toilet flushing within the domestic context and other reuse is irrigation of house gardens during summer.
3.2.1 Model parameters:
This section presents the model structure for assessing the impact of RWH system on combined sewer flow. The model structure consists of simulation of the following processes; rainfall runoff generation, rooftop RWH, combined sewer runoff generation and impact assessment through comparison between results of simulating the two different scenarios 1&2 (Figure 13
& Figure 14). In Figure 15, the considered RWH system is described which comprise a rainwater storage tank and reuse installations. In addition, runoff compartments from different surfaces are shown. They include; runoff from roof area (Rroof,), runoff from road surfaces (Rroad) and overflow from green area (Ogreen).
For runoff calculation from different surfaces (roofs, roads, green areas); the rational method is used in which;
23 Where:
𝑅𝑗 𝑖 ≡ 𝐺𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 𝑅𝑢𝑛𝑜𝑓𝑓 𝑓𝑟𝑜𝑚 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 (𝑗) 𝑖𝑛 𝑡𝑖𝑚𝑒 𝑠𝑡𝑒𝑝 (𝑖) (𝑚3) 𝑃 ≡ 𝑃𝑟𝑒𝑐𝑖𝑝𝑖𝑡𝑎𝑡𝑖𝑜𝑛 𝑑𝑒𝑝𝑡ℎ (𝑚)
𝐴𝑅≡ 𝑅𝑒𝑑𝑢𝑐𝑒𝑑 𝑎𝑟𝑒𝑎 (𝑚2) = 𝐴 ∗ 𝐶 𝐴 ≡ 𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 (𝑚2)
𝐶 ≡ 𝑅𝑢𝑛𝑜𝑓𝑓 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡
Figure 15: RWH and reuse system with components of combined sewer system flow (Vaes and Berlamont, 2001)
For green areas, runoff is used to calculate the overflow through excluding the infiltrated water volume. Direct infiltrated volume is calculated considering hydraulic conductivity of the soil as follows;
𝑉𝑑𝑖𝑟𝑒𝑐𝑡 𝑔𝑟𝑒𝑒𝑛 𝑖 = max (𝑅𝑔𝑟𝑒𝑒𝑛 𝑖, (𝐴𝑔𝑟𝑒𝑒𝑛∗ 𝐻𝑦𝑑𝑟𝑎𝑢𝑙𝑖𝑐 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑖𝑣𝑖𝑡𝑦)) (2) (P)
(Rroof)
(OGreen)+ (Rroad )
𝑅𝑗 𝑖 = 𝑃 ∗ 𝐴𝑅 (1)
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This direct infiltrated volume in each time step (𝑉𝑑𝑖𝑟𝑒𝑐𝑡 𝑔𝑟𝑒𝑒𝑛 𝑖) is then used to calculate water volume that overflows from green surfaces considering the change in infiltration capacity at each time step (i). As a result, the overflow from green surface (𝑂𝑔𝑟𝑒𝑒𝑛 𝑖) is calculated for each time step (i) which is supposed to be discharged into combined sewer system (Figure 15 and Figure 16).
The calculated runoff compartments (𝑅𝑟𝑜𝑜𝑓 𝑖, 𝑅𝑟𝑜𝑎𝑑 𝑖 & 𝑂𝑔𝑟𝑒𝑒𝑛 𝑖) are then used as an input for the two model scenarios (Figure 13 & Figure 14) and the excess volume is the total runoff that goes to the combined sewer.
Figure 16: Green surface description
3.2.2 Model Scenarios and Reuse Purposes
Scenario 1: Combined sewer Runoff with implemented RWH system
In this scenario, the runoff that goes to combined sewer is impacted by the presence of a rooftop RWH system as described before in section (3.2) and Figure 13 .
➢ Water Balance in the Storage Tank;
𝑆𝑖 = 𝑆𝑖−1+ 𝑅𝑟𝑜𝑜𝑓 𝑖− 𝐷𝑖, (0 < 𝑆𝑖 < 𝑉𝑡𝑎𝑛𝑘) (3) Where
𝑆𝑖 ≡ 𝑊𝑎𝑡𝑒𝑟 𝑠𝑡𝑜𝑟𝑎𝑔𝑒 𝑖𝑛 𝑡ℎ𝑒 𝑡𝑎𝑛𝑘 𝑎𝑡 𝑡𝑖𝑚𝑒 𝑠𝑡𝑒𝑝 (𝑖) 𝑖 ≡ 𝑚𝑜𝑑𝑒𝑙 𝑡𝑖𝑚𝑒 𝑠𝑡𝑒𝑝,
𝐷𝑖 ≡ 𝐻𝑎𝑟𝑣𝑒𝑠𝑡𝑒𝑑 𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑚𝑎𝑛𝑑 𝑎 𝑡𝑖𝑚𝑒 𝑠𝑡𝑒𝑝 𝑖
➢ Overflow from storage tank (𝑂𝑡𝑎𝑛𝑘 𝑖) was calculated through
Overflow form green area
(𝑂𝑔𝑟𝑒𝑒𝑛 𝑖)
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𝑂𝑡𝑎𝑛𝑘 𝑖 = (𝑆𝑖−1+ 𝑅𝑟𝑜𝑜𝑓 𝑖− 𝐷𝑖) − 𝑉𝑡𝑎𝑛𝑘, 𝑖𝑓(𝑆𝑖−1+ 𝑅𝑟𝑜𝑜𝑓 𝑖− 𝐷𝑖) > 𝑉𝑡𝑎𝑛𝑘 (4)
➢ The runoff to combined sewer in this scenario (𝑅𝑠1) is found through;
𝑅𝑠1 𝑖 = 𝑂𝑡𝑎𝑛𝑘 𝑖+ 𝑅𝑟𝑜𝑎𝑑 𝑖 + 𝑂𝑔𝑟𝑒𝑒𝑛 𝑖 (5)
Rainwater reuse purposes; Toilet Flushing and House Gardens Irrigation
For toilet flushing reuse, water demand is assumed to be the same throughout the year. So, in the model it has not been adjusted for monthly variation. However, water demand for house gardens irrigation vary among the year’s seasons and months. It is high during warm months (May, June, July, August and September) and low or no demand during cold months. Therefore, in the model it has been adjusted for such variation.
➢ Water demand supplied by mains is calculated by:
𝑊𝑠 𝑖 = 𝐷𝑖 − 𝑆𝑖, 𝑖𝑓 (𝐷𝑖 > 𝑆𝑖) (6)
➢ the demand coverage is calculated through:
𝐷𝑐𝑜𝑣𝑒𝑟 𝑖(%) = (1 − 𝑊𝑠 𝑖 𝐷𝑖
⁄ ) ∗ 100 (7)
Scenario 2: Combined sewer Runoff without RWH system
In this scenario, no RWH is considered and runoff volume from different surfaces (𝑅𝑟𝑜𝑜𝑓, 𝑅𝑟𝑜𝑎𝑑 & 𝑂𝑔𝑟𝑒𝑒𝑛) goes directly to the combined sewer (Figure 15)
➢ The runoff to combined sewer in this scenario (𝑅𝑠2) is found through;
𝑅𝑠2 𝑖 = 𝑅𝑟𝑜𝑜𝑓 𝑖+ 𝑅𝑟𝑜𝑎𝑑 𝑖+ 𝑂𝑔𝑟𝑒𝑒𝑛 𝑖 (8)
Impact Assessment:
To quantify the impact of applying RWH system on sewer runoff reduction; a threshold runoff flow to sewer was determined (𝑅𝑡ℎ). This flow was determined to be generated from rainfall with a one month return period (based on previous modelling that indicates that systems in the
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area of the case study overflow in such small events, (Olsson and Al-shididi, 2014)) and with a duration similar to the expected time of concentration.
➢ The exceeding runoff flow over the threshold runoff (𝑂𝑠𝑒𝑤𝑒𝑟 𝑗 𝑖) is calculated as follows for each scenario and every time step (i);
𝑂𝑠𝑒𝑤𝑒𝑟 𝑗 𝑖 = 𝑅𝑠 𝑗 𝑖− 𝑅𝑡ℎ , 𝐼𝑓 (𝑅𝑠 𝑗 𝑖 > 𝑅𝑡ℎ) (9)
Where (j) indicate the scenario number.
➢ The model then calculates number of times (𝑁𝑒𝑥𝑐𝑒𝑒𝑑−𝑗) in which sewer runoff (𝑅𝑠 𝑗) exceeds the threshold flow (𝑅𝑡ℎ) over the study period, for both model scenarios 1 and 2.
➢ The total exceeding runoff volume ( 𝑉𝑒𝑥𝑐𝑒𝑒𝑑−𝑗= ∑ 𝑂𝑖 𝑠𝑒𝑤𝑒𝑟 𝑗 𝑖) is calculated for each scenario as well.
➢ These two values (𝑁𝑒𝑥𝑐𝑒𝑒𝑑 & 𝑉𝑒𝑥𝑐𝑒𝑒𝑑) are then compared between the two model scenarios 1 & 2, to assess the impact of RWH through calculating percentage of reduction in both values.
1. 𝑹𝒆𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝒊𝒏 𝒕𝒊𝒎𝒆𝒔 𝒐𝒇 𝒇𝒍𝒐𝒘 𝒆𝒙𝒄𝒆𝒆𝒅𝒂𝒏𝒄𝒆 % =(𝑵𝒆𝒙𝒄𝒆𝒆𝒅−𝟐 − 𝑵𝒆𝒙𝒄𝒆𝒆𝒅−𝟏)
𝑵𝒆𝒙𝒄𝒆𝒆𝒅−𝟐 (10)
2. 𝑹𝒆𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝒊𝒏 𝒆𝒙𝒄𝒆𝒆𝒅𝒊𝒏𝒈 𝒇𝒍𝒐𝒘 𝒗𝒐𝒍𝒖𝒎𝒆 % = (𝑽𝒆𝒙𝒄𝒆𝒆𝒅−𝟐 − 𝑽𝒆𝒙𝒄𝒆𝒆𝒅−𝟏)
𝑽𝒆𝒙𝒄𝒆𝒆𝒅−𝟐 (11)
➢ The total water demand covered by collected rainwater from the tank, is calculated over the study period
3. 𝒘𝒂𝒕𝒆𝒓 𝒅𝒆𝒎𝒂𝒏𝒅 𝒄𝒐𝒏𝒗𝒆𝒓𝒆𝒅 𝒃𝒚 𝒕𝒉𝒆 𝒕𝒂𝒏𝒌 = ( ∑ 𝑫𝒊 𝒊∑ 𝑫− ∑ 𝑾𝒊 𝒔 𝒊)
𝒊
𝒊 (12)
➢ Finally, the model calculates duration of roof runoff to combined sewer system in both scenarios 1 and 2, then compares between them using the following relation;
4. 𝒓𝒆𝒅𝒖𝒄𝒕𝒊𝒐𝒏 𝒊𝒏 𝒓𝒐𝒐𝒇 𝒓𝒖𝒏𝒐𝒇𝒇 𝒅𝒖𝒓𝒂𝒕𝒊𝒐𝒏 % = 𝒕𝒐𝒕𝒂𝒍 𝒓𝒐𝒐𝒇 𝒓𝒖𝒏𝒐𝒇𝒇 𝒕𝒊𝒎𝒆 𝒊𝒏 𝑺𝒄−𝟐
𝒕𝒐𝒕𝒂𝒍 𝒕𝒂𝒏𝒌 𝒐𝒗𝒆𝒓𝒇𝒍𝒐𝒘 𝒕𝒊𝒎𝒆 𝒊𝒏 𝑺𝒄−𝟏 (13)
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The model was run for a series of different volume of the RWH tank while keeping the other parameters constant. This was applied for testing the model’s sensitivity of changing tank size.
In this sense, Oliveira et al. found that, assessment of potential benefits for various storage capacities helps in making decision process in each case. In addition, it allows for studying different system alternatives. The other model parameters are dependent on the case study area characteristics and the desired reuse purpose for harvested water.
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4 Case Study
To assess the impact of applying a RWH system on flow in combined sewers; the model was applied onto a case study area with combined sewer system.
4.1 STUDY AREA DESCRIPTION
The study area is a part of the Kungsholmen district within Stockholm city, Sweden (Figure 17). The area comprises a dense urban grid layout of streets and uniform height buildings. Each block contains small scale properties of a mixed-use nature between residential and business use (City of Stockholm, 2018). The drainage system in the study area is mainly combined sewer (Figure 18), in which the domestic wastewater and rainfall runoff from different surfaces are collected in one sewer system and conveyed to the WWTP (Stockholm Stad, 2013). When the total runoff in the combined sewer exceeds the system’s capacity, the drainage system in the area overflow which result in CSO as discussed before in Chapter 2.3. Furthermore, the city of Stockholm mapped future climate change impacts for flood risk based on intense rainfall with100 years return period rainfall; the results are shown in Figure 19 which highlight some flooding risk within study area.
Figure 17: Case Study Site Map; it shows the selected sub-catchments A & B in Kungsholmen district within Stockholm city.
This map is produced by QGIS using open street map. 15.10.2019, Stockholm districts (contributors, 2015).
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In this study, two building blocks with a total area of 8,240 m2 were selected. They represent sub-catchments (catchments A&B Figure 17) from the main combined sewer shed (Figure 18).
Each one of these sub catchments drains its runoff water into one collection point within the main combined sewer shed. Within buildings, the roof runoff is drained through gutters and downpipes directly to the sewer or to the street, and then together with road surface stormwater they are collected by combined sewer pipelines (Figure 20).
Figure 18: map shows the distribution of sewer system types over Stockholm county. The brown shaded areas are managed by combined sewer system while green shaded areas are managed by duplicate sewer system (Stockholm Stad, 2013)
Sub-catchment A is selected for modelling purposes in this study. It has a total area of 3,573 m2. Of this area, 2,929 m2is classified as roof surfaces which represents around 82% of its total area. Around 436 m2comprises green spaces and gardens which represents 12% of the total area. The rest of the area is classified as road surfaces.
In the model, two rainfall intervals were considered. Firstly, a 15 minute rainfall duration was considered which corresponds to the measurement interval of the acquired rainfall data, as well as representing the approximate time of concentration (TOC) for the local catchment which is used to assess the impact of the system on the local sewer network. The other time step was correspondent to TOC for the main sewer-shed, which is calculated to 79 minutes (Figure 18).
For consistency and simplification, a time interval of 75 minutes was used to assess the impact Main sewer shed
Kungsholmen
Case Study area
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of the RWH system on the main sewer line over the main sewer-shed. In correspondence with these rainfall duration time steps (15 & 75 min), the resultant runoff from rainfall event with one month return period was calculated for 𝑅𝑡ℎ values as it represents the frequency of CSO reoccurrence in the area (Olsson and Al-shididi, 2014).
Figure 19: map shows areas with possible flood risk in Stockholm as a result of intense rainfall with 100-year return period.
the study area is highlighted by the black rectangle where it appears to have risk of flooding under the three scenarios (Stockholm Stad, 2018)
Figure 20:rainfall drainage installations inside buildings. From left, a Pipe collects rainwater from roof area, then roof water collecting pipes and the conveying ditch to the main street and finally at the right, stormwater drains in the street.
(Photos taken by author) (Photos taken by author)
Kungsholmen
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In Sweden, water demand for toilet flushing counts for 25% of domestic water use (Petersens, 2001) which is a considerable amount of water. This demand is assumed in the model to be at a constant rate over time with no seasonal or daily variations. On the other hand, the other considered reuse of rainwater is irrigating for house gardens during the warmer months of summer (June, July, August and September) with variation in demand between these months as shown in Table 1 below, expressed as a percentage of the maximum irrigation demand (which occurs during the month of June) . In this study, RWH model was applied for the two reuse purposes (toilet flushing and irrigation) with different tank sizes considering the local parameters.
Table 1: Irrigation Water Use Ratios for Stockholm, expressed as % of the maximum irrigation demand.
Month January February March April May June July August September October November December
Water Use Ratio 0% 0% 0% 0% 72% 100% 88% 66% 35% 0% 0% 0%
