Examensarbete TVVR 10/5020 Runoff water quality from a green roof and in an open storm water system NAEEM AHMED Division of Water Resources Engineering Department of Building and Environmental Technology Lund University 1 Advelningen för Teknisk Vattenresurslära TVVR-10/5020 ISSN-1101-9824 Runoff water quality from a green roof and in an open storm water system NAEEM AHMED Supervised By: Justyna Czemiel Berndtsson 2 Acknowledgment The author of the thesis, Naeem Ahmed, and the supervisor Dr Justyna Czemiel Berndtsson gratefully acknowledge the financial support from Åke and Greta Lished Stiftelse for the project that was used for covering the cost of analysis of the water samples. First of all I would like to pay thanks to my supervisor Dr. Justyna Czemiel Berndtsson who has helped me a lot in writing this thesis and without her strong support I would have not completed. I would like to pay thanks to my wife also as she helped me to give time to my thesis work. I can’t forget to acknowledge this work to my mother’s sole and my father. 3 Runoff water quality from a green roof and in an open storm water system Abstract: Augustenborg storm water system has become well known for its incorporation of urban green spaces. Open storm water system was developed in late 1990s by disconnecting impervious surfaces from combined sewers as a result of flooding problem in the basements after heavy rain events. The open storm water management system consists of open channels and ponds, a small wetland, green infiltration areas, green roofs, and permeable paving surfaces. The Augustenborg catchment area includes council offices, residential apartments separated by parking places, a school, courtyards and roads. This study consists of two parts. In the first one the storm water quality in Augustenborg storm water system is investigated and the degree of water pollution is assessed by comparing with other studies of storm water quality in urban areas. In the second part the changes in green roof influence on runoff water quality depending on roof age are assessed by comparing current results with the previous studies on the same green roof in Augustenborg. Water samples are taken from four different locations after the four different rain events; these are rain water sample, green roof runoff sample, runoff sample from the open channel and finally the sample from the last pond situated on downstream of the Augustenborg storm water system. All samples are analysed for nutrients and heavy metals. Analysed nutrients are potassium (K), nitrate nitrogen (NO3-N), total nitrogen (Tot-N), phosphate phosphorus (PO4-P), total phosphorus (Tot-P), dissolved organic carbon (DOC) and analyzed metals are; Cd, Cu, Zn, Pb, and Ni. The study revealed that the influence of green roof on runoff quality changes with the roof age. The major trend is that the phosphorus and potassium concentrations in roof runoff water are less the older the roof. The green roof keeps its ability to remove 60% of nitrogen from rain water so that the concentrations in green roof runoff are less than the concentrations in rain water. Regarding the open storm water system in Augustenborg the study found generally that the storm water quality was better comparing with other urban sites as found by other studies. 4 Table of Contents Abstract 1. Introduction.................................................................................................................7 2. Background................................................................................................................. 9 2.1 Storm water systems and their elements..................................................... 9 Urban drainage........................................................................................9 Swales/filter strips...................................................................................12 Infiltration systems..................................................................................13 Porous surfaces........................................................................................13 Gully pots.................................................................................................14 Above ground water storage facilities………………………………………………….14 Constructed wetlands..............................................................................14 Green roofs..............................................................................................15 2.2 Storm water pollutants..................................................................................17 Nutrients..................................................................................................18 Oil and grease..........................................................................................18 Sediments/solids..................................................................................... 19 Heavy metals........................................................................................... 19 Organic compounds................................................................................. 20 Microbial contaminants .......................................................................... 21 2.3 Legislative framework....................................................................................22 3. Methodology ..............................................................................................................23 3.1 Study site description.....................................................................................23 3.2 Methodology of sampling and analyses.........................................................26 3.3 Methodology of load calculations…………………………………………………………… …28 4. Results and discussion.................................................................................................31 5 4.1 Evaluation of green roof influence on runoff water quality with the passage of time. Load of contaminants in rain water and green roof runoff………………… ………………30 4.2 Comparisons of water quality between rain water, storm water quality in green roof runoff, open channel, and pond in Augustenborg open storm water system……….36 4.3 Comparison of the findings of this study with literature……………….. ………….42 5 Conclusions...................................................................................................................47 6 References................................................................................................................ ….49 6 1. Introduction Many catchments around the world are undergoing fast urbanization as a result of population increase. Increasing urbanization leads to an increase in runoff and this process concentrates fresh water flows to localized receiving waters including man-made systems such as retention ponds, detention basins and natural environments like lakes, estuaries and near shore coastal waters. The impact of urbanisation on terms of water resource management in combination with effect of climate change has been studied by Kleidorfer et al. 2009. They found out that 20% increase in rain intensity as a result of climate change has the same effect on average as a 40 % increase of the impervious area. They also concluded that the impact of increased urbanization is sometimes significantly higher than the one expected from the global warming. During urbanisation some catchments experience water quality degradation due to high pollution loads from different non-point sources. Storm water runoff has been recognised as one of the major driving force of non-point source discharge such as heavy metals, polycyclic aromatic hydrocarbons, nutrients and other toxic compounds of anthropogenic activities (US EPA, 1995; Characklis and Wiesner, 1997). Non-point source pollutants have recently become the great concern for the scientists, decision makers and for public, greater that the point source pollutants. The reason is that the point source has been identified and remedied but the non-point sources are complex to identify and manage. Storm water pollutants have different characterised depending on the land use. As storm water runoff pollutants are generated and transported in a diffuse manner and their sources are related to physiographic factors of individual catchment thus storm runoff pollution usually presents spatial variation with land use being one of the most important factors (Tong and Chen, 2002; Yusop et al., 2005). The storm runoff pollution has negative consequences on our aquatic resources (streams, lakes, estuaries, rivers, aquifers and other water bodies) which are most valuable assets of every nation. The resulting negative impact includes change in flow, increased sedimentation, higher water temperature, lower dissolved oxygen, degradation of aquatic habitat structure, loss of fish and other aquatic populations, and decreased water quality due to increased level of nutrients, metals, hydrocarbons, bacteria and other constituents (Karlsson, 2006). Generally, storm water is conveyed in two ways. One is conventional storm water systems which include combined and separated storm water sewers through pipe arrangements and the second approach is open storm water system which includes wetlands, ponds, infiltration systems, open channel etc. Objectives In this thesis an example of an open storm water system from Augustenborg, Malmö, Sweden is studied. The purpose of this study is to investigate the storm water quality in 7 Augustenborg open storm water system and to compare it with the urban storm water quality as found by other studies. In addition to this, performance of green roof in Augustenborg on runoff quality with the passage of time is also investigated. The study was done on small scale area where storm water system was only managed so sanitary waste water has not been discussed and only open storm water system was studied instead of conventional sewer system. 8 2. Background 2.1 Storm water systems and elements Urban drainage Human activity interacts the natural water cycle in two ways. One way is the abstraction of water for water supply and other way is by creating obstruction to the natural rainfall cycle through impervious surfaces. These two ways lead to the development of different drainage systems in urban areas, namely wastewater and storm water (Butler and Davis, 2004). Urban drainage systems handle these two types of water in order to minimize problems to human health and to the environment. When rain falls on a surface it is either evaporated or infiltrated while rest water becomes runoff. In urban areas infiltration is very little as a result of large extension of more impervious surfaces. Infrastructures in urban areas obstruct infiltration and concentrates runoff (Villarreal et. al, 2005). The principles of how urbanization influences the urban water cycle are shown in figure 1. The conversion of roof runoff into the waste water in terms of its entry into the sewer system means not taking advantage of this resource. The potential uses of storm water runoff from roofs may otherwise be irrigation, drinking water or different uses at household. Fig. 1. Effect of urbanization on runoff (taken from Butler and Davis, 2004). Drainage systems are categorized on base of their means of drainage. The means of drainage can be natural or conventional. Natural mean of drainage is the drainage through infiltration and storage properties of semi natural features. For example swales, ponds, 9 detention basins, ground depression, pervious pavements etc. These will be discussed in detail later. Conventional mean of drainage is pipe drainage. There are two types of piped storm water systems: Combined and separate. In the combined sewer system storm water and waste water are carried together in the same pipe. The pipe is normally loaded 10 % of the total volume and the rest is only filled in the wet weather. Thus most of the time the system is not used in its full capacity. This is the main drawback of combined sewer system. In addition, combined sewer overflow (CSO) is occuring in case of heavy storms to reduce load on the tretment plant. CSO diverts untreated waste water into natural water courses. It is diluted with storm water but still carrying gross pollutants into recipients. In cold climate during snow melt period CSO has been seen often (Thorolfsson, 2000). The separate system consists of two separate sewers, storm water sewer and waste water sewer, usually laid side by side (Davis and Butler, 2004). The advantage of separate system is that the storm water does not mix with waste water and thus it can be discharged to the water body at any suitable place in the vicinity of the sewer system. Nevertheless, the storm water is now recognised as polluted because it is washing off the pollutants from the urban impermeable surfaces. As two pipes are required the system is more expensive than the combined sewer system. The cost however does not need to be doubled as both pipes may be laid in the same excavation. During dry weather the storm sewer is to large extent empty. There might be some inflitration or inflow from storm sewer to waste water sewer due to some crackes at waste water sewer joints or along the length. On the other hand the inflow to the sewer can be sometimes a result of wrong connections between storm sewer and wastwater sewer; for example it is found that many households connections of garden drains are made through manhole for waste water. There may also be illiegal connections from some households conecting household wastewater to storm water drains. In many European countries like Germany, France, UK the majority of sewer systems is combined (Butler and Davies, 2004). In Sweden about 15 % of the sewer systems is combined and the rest is separate (Berggren, 2007). The separate storm water system can be conventional (piped), open, or a combination of those two. An open system may, among others, contain following elements: ponds, channels, wetlands, green roofs, green infiltration surfaces and porous paving. The common advantages of open systems are storm water treatment and slowing runoff thus minimising risk of flooding. The elements of open systems and other structural and non-structural measures aiming at improving water quality and minimising the environmental impact of the systems are called BMPs (best management practices). BMPs are considered as the sustainable urban drainage approaches in many countries (Scholes et. al, 2007). 10