Urban drainage full_2011

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Information about Urban drainage full_2011

Published on March 13, 2014

Author: DelftOpenEr

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sanitary engineering - ct3420 UrbanDrainage

2 sanitary engineering - ct3420urban drainage Table of contents 1. Introduction 3 1.1 Urban drainage and sewerage 3 1.2 History of urban drainage and sewerage 3 1.3 Purpose of urban drainage systems 5 2. Approaches to urban drainage 5 2.1 Piped or natural systems 5 2.2 Combined and separate systems 6 2.3 Improved combined and improved separate sewer systems 7 2.4 Pressurised and vacuum systems 8 2.5 Components of sewer systems 10 2.6 Concepts, definitions and abbreviations 11 3. Inflows into urban drainage systems 12 3.1 Amount of wastewater 12 3.1.1 Domestic wastewater 3.1.2 Industrial wastewater 3.1.3 Infiltration and exfiltration 3.1.4 Drainage water 3.1.5 Extraneous water 3.2 Amount of stormwater 15 3.2.1 Amount of precipitation and runoff behaviour 3.2.2 Rainfall intensity duration frequency and rainfall mass curves 3.2.3 Using rainfall mass curves 3.2.4 Run-off processes 4. Environmental requirements for sewer design 20 4.1 Development of environmental guidelines for sewer systems 20 4.2 “Basic effort/Basisinspanning” 21 4.3 European Framework Directive 22 4.4 Storage and stormwater pumping capacity 23 4.5 Hydrodynamic evaluation of environmental impacts of sewer systems 24 4.6 References 26 5. Hydraulics for sewer design 27 5.1 Uniform channel flow in partially filled sewer pipelines 27 5.2 Hydraulic resistance in components of sewer systems 31 5.2.1 Sewer overflow weirs 5.2.2 Inlets, outlets and manholes

3 sanitary engineering - ct3420 urban drainage 1. Introduction 1.1 Urban drainage and sewerage Drainage is defined in the Cambridge Online Dictionary as the process of water or waste liquids flowing away from somewhere into the ground or down pipes. In the case of urban drainage, “somewhere” is the urban environment, mainly constituted of roads, houses and green spaces. Urban drainage may be used to describe the process of collecting and transporting wastewater, rainwater/stormwater or a combination of both. Sewage is water-carried wastes, in either solution or suspension, that is intended to flow away from a community. “Sewage” and “Sewerage” may be used interchangeably in the USA but elsewhere they retain separate and different meanings - sewage being the liquid material and sewerage being the pipes, pumps and infrastructure through which sewage flows. Similar to urban drainage, sewerage may refer to systems collecting wastewater or rainwater or a combination of both. 1.2 History of urban drainage and sewerage Artificial drainage systems were developed as soon as humans attempted to control their environment. Archaeological evidence reveals that drainage was provided to the buildings of many ancient civilisation such as the Mespotamians, the Minoans (Crete) and the Greeks (Athens). Romans The Romans are well known for their public health engineering achievements, particularly the impressive aquaducts bringing water into the city. Equally vital were the artificial drains they built, of which the most famous is the cloaca maxima, built to drain the Forum Romanum (and still in use today) (Butler and Davies, 2004). The Romans were proud of their “rooms of easement” (i.e., latrines). Public baths included such rooms -- adjacent to gardens. There Roman officials would sometimes continue discussions with visiting dignitaries while sitting on the latrines. Elongated rectangular platforms with several adjacent seats were utilized (some with privacy partitions, but most without). These latrine rooms were often co-ed, as were the baths. Public latrines were used by many people, but for the most part, human wastes were thrown into the street. Rome had extensive street washing programs (with water supplied by aqueducts, the first being built in 312 BCE). Only a few homes had water piped directly from the aqueducts; the vast majority of the people came to fountains to gather their water. Even though not many homes were directly plumbed into the sewers, when the wastes were thrown into the street, the street washing resulted in most of the human wastes ending up in the sewers anyway. Direct connection of homes to the sewers was not mandated until nearly 100 CE (cost was a factor; also mandating such a connection was then considered an invasion of privacy). Sewage resulting from the public baths and the included latrines was discharged into sewers and eventually to the Tiber River. It is worth noting that the Romans recognized the value of their water (which had been transported to the city via aqueducts, often over a distance of 20-30 miles); as such, any wastewater from the public bath facilities was often re-used, frequently as the flushing water that flowed continuously through the public latrine facilities. From the latrines, it flowed to a point of discharge into the sewer system. Middle Ages to 20th century The Roman Empire fell in early CE along with the concepts of baths, basic sanitation, aqueducts, engineered water or sewage systems, etc. Sanitation reverted back to the basics: very primitive. During the so-called “Dark Ages,” there arose a brotherhood among men noted for skill in combat. There also evolved a creed that uncleanliness was next to godliness. As such, bathing/sanitation became quite uncommon; homes, towns, and streams became filthy. Diseases were commonplace; epidemics decimated towns and villages. Twenty-five percent (or more) of the ancient European population died of disease (cholera, plague, etc.). The major transmitter of the plague was rats (actually bacteria conveyed from rats to people via flea bites). The rat population thrived amongst the mess and stench commonplace in medieval times.

4 sanitary engineering - ct3420urban drainage Living conditions aggravated most seriously in large cities like London and Paris. In the beginning, natural streams were used as sewers. As cities developed, these natural drains were structurally covered. Early on, these sewers were used primarily for storm waters. For instance in Paris, the Menilmontant sewer, first noted in the early 1400s, was initially an open wash and later a closed conduit. It intercepted surface flows from Paris’ north slope area (i.e., that area lying on the right bank of the Seine River). It was called the “Great Drain” (grand ègout or ègout de ceinture). Prior to the wide use of cesspools in Paris, cesspits (ones that percolate) were widely used. Their use in combination with the large growing population, however, resulted in the subsoil of Paris becoming putrid. Cesspools, instead, were then encouraged. However, they required periodic/routine cleaning, which the city couldn’t adequately provide. Another stinky mess arose. A “Nite Soil” program started to facilitate the collection and disposal (elsewhere) of the wastes (in community cesspools, rivers, vegetable gardens). The problem was that all of the people could not afford the service. In the 1830s a series of cholera epidemics started in Paris. To combat the epidemics, new and bigger sewers (called “Les egouts” ) began to be constructed in the 1840s-1890s. They became the pride of Paris. The design father of the complex system of sewers under Paris was Eugéne Belguard. The construction of this newer/larger system started in 1850, on borrowed money. By 1870, over 500 km of new sewers were either in service or under construction. By 1930, the entire system (a “combined” system) was finished: “One sewer for each street.” From these times, “Sewerman” became a profession. Tours of the sewers were given by the “sewermen” on weekends. Some of the sludge found in the sewers was removed through manholes. Most of it was moved downstream via boats (with “wings”) to the discharge point of the sewer into the river -- where the sludge was pushed onto barges, from whence it was transported to various places of reuse or disposal. London’s oldest “sewer,” known as the Ludgate Hill Sewer, was constructed in 1668. (Initially, it was an open channel fed by springs, big enough to be used by boats. It was covered in 1732.) Early sewers (initially, natural watercourses that had been covered) started in the London area in the 1730s -- primarily for storm water. Privies/ cesspools were used to collect home wastes; some of these facilities also “collected” the methane generated by the decaying waste. The result was often explosions/fires ... and death. 1858-59 were the years of the “Big Stink” in London. The Thames River received wastes of thousands of people who lived upstream of Parliament. Many of the sewers tributary to the Thames River could only physically drain during low tide. The problem was that at low tide, the river did not have enough flow to carry the waste downstream and out to sea. The incoming tide pushed the waste upstream. This cycle resulted in the river becoming virtually a wide-open-to- the-sunlight cesspool for the excrement of nearly three million people! Parliament had to shut down often in summer months. This situation created an even greater problem: the Thames was also the source of water for a large portion of London! During these years, various ways to minimize sewer odors were tried, including the addition to the sewers (especially in warm weather) of large quantities of lime or chloride of lime. Sir Joseph William Bazalgette, CB (28 March 1819 – 15 March 1891) was chief engineer of London’s Metropolitan Board of Works and his major achievement was the creation in of a sewer network for central London which was instrumental in relieving the city from the Great Stink. Bazalgette’s solution was to construct 1,100 miles (1,800 km) of underground brick main sewers to intercept sewage outflows, and 1,100 miles (1,800 km) of street sewers, to intercept the raw sewage which up until then flowed freely through the streets and thoroughfares of London. The outflows were diverted downstream where they were dumped, untreated, into the Thames. The scheme involved major pumping stations on the north and south sides of the Thames.

5 sanitary engineering - ct3420 urban drainage Bazalgette’s foresight may be seen in the diameter of the sewers. When planning the network he took the densest population, gave every person the most generous allowance of sewage production and came up with a diameter of pipe needed. He then said ‘Well, we’re only going to do this once and there’s always the unforeseen.’ and doubled the diameter to be used. As it is the sewers are still in use to this day. The unintended consequence of the new sewer system was to eliminate cholera not only in places that no longer stank, but wherever water supplies ceased to be contaminated by sewage. This result was unintentional, since disease was believed to be transferred by bad odours. It was not until 1854 that Dr. Snow made the connection between human wastes (from over-loaded privies) and water supplies (wells) within the “Broad Street Neighborhood”: he found that a well at 40 Broad Street was found to be contaminated with sewage from a nearby overloaded/flowing privy; the well was removed from service and the cholera outbreak ended. In the mid-1800s Louis Pasteur proved disease could be caused by germs. The link between bacteria and infectious diseases was beginning to be understood. 1.3 Purpose of urban drainage systems The four objectives of drain and sewer systems are (NEN-EN752:2008): • Public health and safety; • Environmental protection; • Sustainable development; • Occupational health and safety. Drain and Sewer systems are provided in order to prevent spread of disease by contact with faecal and other waterborne waste, to protect drinking water sources from contamination by waterborne waste and to carry runoff and surface water away while minimising hazards to the public. Additionally, the impact of drain and sewer systems on the receiving waters shall meet the requirements of any national or local regulations or the relevant authority. Finally, sewer systems should be designed, constructed, operated, maintained and rehabilitated at the best environmental, social and economical costs so that it uses materials that minimise the depletion of finite resources, can be operated with the minimum practicable use of energy and can be constructed, operated and, at the end of their life, decommissioned with the minimum practicable impact on the environment. To protect the health of sewer workers occupational health and safety risks likely to arise during installation, operation, maintenance, and rehabilitation should be minimised. 2. Approaches to urban drainage 2.1 Piped or natural systems Historical developments of urban drainage systems has been from natural towards piped systems: natural channels were used to collect rainwater and as cities developed these were structurally covered to create additional space for urban developments and to contain the bad smells arising from the waste collection channels. The urban drainage systems that were designed in the 19th century and onwards consisted mainly of underground pipes, because they had to be incorporated into existing, densely built cities. The recent trend, that started in the 1970’s, has been to move towards a more natural means of drainage, using infiltration and storage properties of semi-natural features such as constructed wetlands, ponds and permeable pavements. The movement towards increased use of natural drainage mechanisms has been termed differently in different countries. In the US, these techniques are usually called ‘best management practices’ (BMPs). In the UK they are called Sustainable Urban Drainage Solutions (SUDS). In Australia the term ‘water sensitive urban design’ is often used to refer to the incorporation of natural drainage mechanisms in urban areas. This series of lectures is mainly dedicated to piped systems that constitute the majority of urban drainage systems in existing urban areas in the Netherlands and many other western European countries.

6 sanitary engineering - ct3420urban drainage 2.2 Combined and separate systems Urban drainage systems collect and transport to types of flows: wastewater and stormwater. In combined systems these flows are drained through one and the same system (see figure 2.1), in separate systems wastewater and stormwater are drained through separate pipes. In western Europe, most older systems are combined systems. In the Netherlands about 70% of the population is connected to combined systems, while a little over 25% is connected to separate systems. The same percentages apply to the UK, France and Germany. Combined sewers systems During dry weather, combined systems carry only wastewater flow. During rainfall, the flow increases as a result of the inflow of stormwater. The combined flow of wastewater and stormwater is transported towards a wastewater treatment plant. The stormwater flow exceeds the wastewater flow even under light rainfall conditions. During heavy rainfall, stormwater flow exceeds wastewater flow by a factor 100 to 1000 or more. It is not economically feasible to provide capacity for the total flow under these conditions. Therefore, in combined systems so-called combined sewer overflows are installed to discharge excess water that cannot be transported towards the wastewater treatment plant, to surface water. The overflow water is a mixture of wastewater and stormwater and as a result the quality of the surface water, in most cases temporarily, is extremely reduced. The dying off of fish, foul odours and visual pollution is often the result. Separate systems do not have this drawback of mixing relatively clean stormwater with wastewater. A separate sewer system consists of two sewer pipelines, one for wastewater and one for the drainage of stormwater (See figure 2.2). Some cities, like the city of Amsterdam, started Figure 2.1 – Principle combined system Figure 2.2 – Principle separate system

7 sanitary engineering - ct3420 urban drainage constructing separate systems early on, yet in most cities in the Netherlands separate systems started to be constructed from the 1970’s onwards, as the drawbacks of combined systems become more and more apparent. In theory, in separate systems stormwater does not become mixed with wastewater. It happens from time to time, that domestic sewer connections accidentally become connected to stormwater pipes. This puts a constant strain on the quality of the receiving surface water. Also the opposite can happen; the stormwater pipes become connected to the wastewater system. This can result in overloading of the wastewater system. In both cases one speaks of faulty or illegal connections. Initially, the assumption was that stormwater that was drained through stormwater systems was uncontaminated, since during drainage it did not get mixed or come in contact with domestic wastewater or other wastewater. However, research has shown that this assumption does not hold true. During runoff over urban surfaces rainwater takes up all sorts of contaminated substances. These substances settle on the surface as a result of traffic, human activities and direct deposition from the atmosphere. Stormwater systems drain towards surface water more than 50 times per year on average, during each precipitation of importance, which implies that polluted stormwater is discharged to surface water frequently. On the other combined systems overflow less than 10 times annually, on average. The result is that the annual pollution load to surface water from separate systems and combined systems can be equally large. Separatesystemshavetheadditionaldisadvantage that construction and maintenance costs of separate sewer systems are in most cases higher than in combined systems. Only in in cases where a large amount of surface water is present to locate stormwater oulets, so transport distances to surface water are short, separate systems can be cheaper than a combined system. 2.3 Improved combined and improved separate sewer systems As the drawbacks of combined and separate systems became apparent, attempts have been made to overcome the disadvantages to both combined systems and separate systems. Improved combined systems include so- called storage -and-set tlement basins (bergbezinkbassins, in Dutch) that are installed at sewer overflows to reduce the amount of water that is spilled through the combined sewer overflows (See figure 2.3). During heavy precipitation the overflowing water is retained in the basin. The amount of overflowing water decreases as a result of this and with that the contamination of the surface water is decreased as well. Once the basin is full the on-coming water is discharged to the surface water. Before the water reaches the (external) combined sewer overflow, it is partially stripped of pollutants, thanks to sedimentation in the basin. A storage and settlement basin, therefore, reduces the amount of combined sewer overflow water and in addition the overflowing water is less contaminated. However, one condition to this is of course that the design of the basin must be such that it allows sedimentation to occur. Combined sewer systems can also be improved through installing vortex overflows or by adjusting the combined sewer overflow itself (‘improved overflow drain’). The idea behind these special WWTP receiving water storage and settlement tank supply pipe 1 = internal combined sewer overflow 2 = extreme combined sewer overflow 1 2 Figure 2.3 – Improved combined system

8 sanitary engineering - ct3420urban drainage structures is that they remove pollutants from the overflow water by containing settleable material that many of the pollutants adhere to. Improved separate systems are devised to overcome the effect of faulty connections in separate sewer systems. This done by connecting the stormwater system with a wastewater sewer in a suitable place (See figure 2.4). The wastewater flow that undesirably goes through the storm water sewer is in this way led to the wastewater system and eventually to the wastewater treatment plant. A weir is installed at the outlet of the stormwater system to contain stormwater inside the system during small precipitation and transport it towards the wastewater system. The installation of a weir makes sure that a part of the contaminated rainwater (because of faulty connections) also undergoes treatment in the wastewater treatment plant. This solution does not entirely remove the harmful effects caused by connecting rainwater pipes to the wastewater system. However, due to the installation of extra pumping capacity for the transport of stormwater from the stormwater system, this capacity is also available to transport stormwater that was directly (and erroneously) connected to the wastewater system. This is indeed taken into account in the design of the wastewater treatment plant that should allow the treatment of an limited amount of rainwater from the improved separate system. The amount of stormwater that ends up in the wastewater treatment plant this way depends on the distribution of rainfall over small and large rainfall events: stormwater from most of the small events (up to about 5mm) are transported towards the wastewater treatment plant and only a part of the stormwater from larger events (> ±5mm) is discharged to surface water. One other possibility to reduce the adverse effects of sewer overflows is the enlargement of the capacity of sewer pumping stations that pump sewer water towards the wastewater treatment plant. The disadvantage of this is, that a larger amount of relatively clean water must be treated in the wastewater treatment plant and that the hydraulic capacity of the wastewater treatment plant must be enlarged to cope with the extra water (unless the existing capacity is sufficient). The efficiency of wastewater treatment plants is lower for less concentrated influent waters, so by transporting more stormwater to the treatment plant, its efficiency is reduced and fewer pollutants are removed during treatment. In the end, it comes down to finding a balance between the prevention of combined sewer overflows, reduction of polluted stormwater flows and maintenance of treatment efficiency at the wastewater treatment plant. This balance is studied in optimization studies for wastewater systems (in Dutch: Optimalisatie AfvalwaterSysteem, OAS). Furthermore, the amount of overflow water can be reduced by minimising the amount of runoff water that is connected to the urban drainage system. The reduction of the amount of connected runoff-surface is called disconnection of impermeable surfaces (in Dutch: afkoppelen). Of course disconnection can only take place when the stormwater can be transported to alternative facilities such as infiltration systems or open drainage channels and ponds. 2.4 Pressurised and vacuum systems In conventional sewerage methods wastewater is collected through a gravity-flow sewer, using gravity to transport wastewater. If the sewage cannot be transported under gravity, because ground level variations are small and transport distance are long, pressurized or vacuum WWTP rain water pump receiving water combined sewer overflow storm water sewer wastewater sewer Figure 2.4 – Improved separate system

9 sanitary engineering - ct3420 urban drainage systems can be applied. These systems are general installed in areas where a gravity flow sewer system is too expensive, such as detached buildings that are spread far apart. In general pressurized and vacuum system only transport wastewater and no stormwater, because the latter would necessitate a larger pipe diameter and pumping capacity and therefore reduce the financial advantages of a pressurized systems compared to gravity systems. Pressurised sewer systems consist of a few (small) pumping stations and pressurised mains. Wastewater from a few detached houses is led by a gravity-flow line to a pump chamber. From there the water is pumped into the pressurized main to the next pump chamber. One or more houses can be connected to these pump chambers. This method allows the wastewater from a few detached houses to be collected. Finally the wastewater is led to the gravity sewer systems that eventually transports it to the wastewater treatment plant (See figure 2.5). Many pressurized sewers in the Netherlands were constructed with state subsidy. The intention behind distributing the subsidy was to get as many buildings as possible connected to a sewer system. Currently 3.6% of the population in the Netherlands is connected through pressurized sewers. There are disadvantages to pressurised sewer systems. The most obvious ones are, in comparison to the gravity-flow sewer system, high maintenance and replacement costs. Foul odours occur as wastewater is transported over large distances as a result of anaerobic degradation of the transported wastewater. This results in the production of sulphuric gases (H2S) that lead to severe corrosion of concrete sewers where pressurised sewers are connected to gravity sewers. In vacuum sewers residential wastewater is led through vacuum pipelines to a pump chamber. From this pump chamber the following transport of the wastewater takes place under pressure. (See figure 2.6) Vacuum sewer systems have been put to little use in the Netherlands. Although the operational safety is comparable to pressurised sewer systems, its higher installation costs prevent its widespread usage. One disadvantage of vacuum sewer systems in relation to pressurised sewer Figure 2.5 – Example of a pressurized sewer system Figure 2.6 – Principle diagram vacuum sewerage system for a house boat

10 sanitary engineering - ct3420urban drainage systems is also that vacuum sewer systems can barely overcome slight vertical obstacles. This is because the negative pressure (vacuum) in the system itself is limited. 2.5 Components of sewer systems A sewer system consists of a network of sewer pipelines that are connected through manholes. The sewer pipelines are made up of pipes, which can be circular shaped, rectangular or oval. In old sewers large rectangular pipes are sometimes covered with a barrel vaulting. The pipes (Dutch: rioolleidingen) are made of concrete, cast iron, PVC, HDPE, PE, glazed stoneware (in Dutch: gres) or brickwork. Most sewer pipes in the Netherlands (72% of the total sewer length (RIONED, 2010) are made of concrete. Manholes (Dutch: rioolputten) are constructed where more than two sewer connect and at regular distances to allow inspection of the sewer system. Manholes need to be accessible so that it is possible enter materials for inspection and maintenance of sewer systems. This means that the maximum distance between manholes is of the order of 40 to 60 m. Manholes in systems of concrete sewers are usually made of concrete, sometimes of brickwork. Manholes in systems of PVC sewers are made of PVC. The manholes are covered with a manhole cover. These are most often made of cast iron, as is the frame in which the cover is encased. Sealed covers (Dutch: geknevelde putdeksels) can be applied at locations where excess pressure sometimes causes water to push off the manhole cover as water flows out of the manhole, creating a dangerous traffic situation. Internal weirs (Dutch: interne stuwen) are installed in combined sewer systems with the aim to optimise in-sewer storage, especially when different part of the sewer systems have different bottom levels. As a result of this more water is stored inside the sewer system before combined sewer overflows take place, thus combined sewer overflows occur less frequently and surface water is less polluted. Combined sewer overflows (Dutch: gemengde overstorten) are implemented in combined sewer systems to relieve the pressure on the system during precipitation. Outlets (Dutch: regenwateruitlaten) are implemented in separate stormwater systems to allow outflow of stormwater towards surface water. Outlets in separate stormwater systems have no weirs; there is a constant open connection between surface water and the stormwater system. Emergency outlets (Dutch: nooduitlaten) are installed in the pumping stations’ collection chambers. Emergency outlets are supposed to work exclusively in case the pumping station is broken down for a long period. Combined sewer overflows, outfalls and emergency outfalls are usually made of concrete, and occasionally of brickwork. Gully pots or sewer inlets (Dutch: kolken) collect water from roads and transport it to the sewer system. House connections (Dutch: huisaansluitingen) collect water from households and transport it to the sewer system. Combined house connections collect wastewater and stormwater and transport it to a combined sewer system; separate house connections collect wastewater or stormwater and transport it respectively to a separate wastewater or stormwater system. House connections are predominantly made of PVC. Old connections are usually made of glazened stoneware. Pumping stations (Dutch: pompstations) are indispensable in flat areas like the western and northern parts of the Netherlands. Sewer pumping stations have capacities anywhere between a few m3/h to a few thousand m3/h, depending on the size of the system that drains to the pumping

11 sanitary engineering - ct3420 urban drainage station. The design, building and installation of bigger sewage pumping stations, is a matter for specialists. Shutting down pumps creates shockwaves in the pressurised mains. Pumping stations with a large capacity that are connected to pressurised mains that are a few kilometres long may need to buffer the shockwaves by mounting so-called water hammer provisions. These are usually surge towers or pressure vessels. Even the dimensioning of water shock provisions is a matter for specialists. Pressurised mains (Dutch: drukleidingen) are components of pressurised sewer systems. Small small pressurised systems (Dutch: drukriolering) connect detached houses at long distances from a gravity system to the gravity system. Large pressurised systems (Dutch: persleidingen) consist of pressurised pipelines of large capacity that transport water from a gravity system to a wastewater treatment plant or between subcatchments of a gravity system. The pressurised mains are predominantly made of HDPE or PE. Storage and Settling Basins or Tanks (Dutch: bergbezinkbassins) are placed behind combined sewer overflows to provide extra storage capacity, contain pollutants and limit the frequency and pollution content of combined sewer overflows. With careful design the tanks are able to contribute nicely towards improving the surface quality. Storage and Settling Basins are usually designed as closed basins, especially if they are installed close to buildings. The basins are usually made of concrete. 2.6 C onc e pt s , d e f i nit ions an d abbreviations In this paragraph a list is given with the most important concepts and abbreviations in the sewerage field. The terms in bold are defined just as in the NEN 3300: ‘Buitenriolering; termen en definities’ (Drains and sewers outside buildings; terms and definitions’). In addition, a list of these with the technical terms found in English, French and German literature is provided. Abbreviations WWTP Wastewater Treatment Plant BOD Biochemical Oxygen Demand COD Chemical Oxygen Demand DWF Dry Weather Flow TKN Total Kjeldahl Nitrogen OF Overflow Frequency POC Pump Over-Capacity WWF Wet Weather Flow Nederlands Engels Duits Frans afvalwater wastewater Schmutzwasser eaux usées afvloeiingscoëfficiënt runoff coefficient Abflußbeiwert coefficient de ruisselement afvoerend oppervlak catchment area Einzugsgebiet bassin versant droogweerafvoer dry weather flow Trockenwetterabfluß débit de te,ps sec gemengd rioolstelsel combined system Mischsystem réseau unitaire gescheiden rioolstelsel separate system Trennsystem réseau (de type) séparatif grondwater groundwater Grundwasser eaux souterrain huisaansluiting house sewer system Grundstück entwässerung drainage domestique huishoudelijk afvalwater domestic sewage Häusliches Schmutzwasser eaux usées domestiques industrieel afvalwater trade effluent Betriebliches Schmutzwasser effluent industriel lekwater infiltration/exfiltration Infiltration/Exfiltration infiltration afstromend hemelwater stormwater Regenwasser eaux de ruissement regenwaterafvoer rain discharge Regenabfluß écoulement de pluie riolering sewer system Kanalisation réseau d’assainissement riool sewer Abwasserkanal émissionaire rioolwater sewage Abwasser effluent

12 sanitary engineering - ct3420urban drainage 3. Inflows into urban drainage systems Sewer water consists of wastewater or rainwater or a mix of both. Wastewater is broken down into domestic wastewater and industrial wastewater, infiltration and drainage water and extraneous water. The wastewater flow is also known as dry weather flow; wet weather flow refers to the flow under rainfall conditions. 3.1 Amount of wastewater 3.1.1 Domestic wastewater The amount of daily domestic wastewater depends on the drinking-water consumption per person, loss and the number of individuals consuming drinking water. When designing wastewater systems future developments must be taken into account, such as an expected population growth or changes in the drinking-water consumption per person. Additionally the amount of wastewater fluctuations throughout the day must also be taken into account. Drinking water consumption per person Average drinking-water consumption depends on both prosperity and climate conditions. In many poor areas, toilet flushing does not occur. In prosperous areas in the tropics, subtropics and arid areas, a lot of drinking water is consumed to water gardens to fill pools and more water is used to bathe compared to colder climates. In the Netherlands the average drinking water consumption is 125 to 135 liters per person per day. The drinking water consumption per person can be subdivided into various domestic activities, such as washing clothes, cooking bathing, toilet flushing and washing dishes. Less than circa 40 % of the supplied drinking water is used for personal hygiene, drinking and cooking. The other 60% of the drinking water is used for purposes that do not require drinking water quality. In recent years drinking water companies have investigated possibilities to produce second, lower quality water for such purposes. Application of second quality water in households requires a second distribution network for this water, which in most cases does not make such solutions cost-efficient. For industrial purposes, requiring large volumes of second quality water, the production of separate lowe quality water can be beneficial. Losses A part of the supplied drinking water is lost during the consumption process and does not reach the sewer system. This refers for instance to water used for watering gardens and water that evaporates, for example from laundry. In the Netherlands these losses amount to nearly 10% of the drinking water consumption, so that the amount of wastewater is around 115 to 120 liters per person. Where the drinking-water supply is less than 20 liters per inhabitant per day (only one faucet in the house) nothing gets drained to the sewer. With a drinking-water supply of circa 80 liters per inhabitant per day virtually all the supplied drinking water is drained through the sewer. Above that amount, increasing degrees of water usage does not reach the sewer. See figure 3.1. Leakage losses that occur in the distribution network also play a large part in explaining the difference between drinking water consumption and wastewater production. These losses can make up 10 to 50% of drinking-water production in some countries. In the Netherlands leakage Figure 3.1 – Relationship between drinking water supply and wastewater drainage

13 sanitary engineering - ct3420 urban drainage losses amount to less than 5% of production. In table 3.1 drinking-water supply (read: drinking water production) and the amount of wastewater for a few places is shown. In many areas the amount of wastewater is substantially smaller than the amount of supplied drinking water. In Amsterdam and the Grand Rapids wastewater flow is however larger than drinking water supply. In Amsterdam this can be attributed to the infiltration of groundwater into the sewer network. Presumably something similar is going on in Grand Rapids. The presented figures demonstrate that when dimensioning a wastewater system based on the amount of drinking-water production it can lead to either over or under designing of wastewater system capacity. Distribution over the day Wastewater is not produced evenly throughout the day; at night there is barely any wastewater flow. The largest amount of wastewater is produced during about 10 hours of the day. Wastewater production in most systems shows a peak in the morning and evening. See figure 3.2. In the Netherlands it is common practice to take into account a domestic wastewater flow equal to 12 l/(inhabitant.h) in the design of sewer systems. Hereby is assumed that the total amount of domestic wastewater of 120 l/(inh. day) is discharged in 10 hours. The peak factor employed in the Netherlands for wastewater flow then becomes: • peak drainage is: 120/10 = 12 l/(inh.h) • average drainage is: 120/24 = 5 l/(inh.h) • peak factor is: 12/5 = 2,4 In France the following peak factor is applied: P = peak factor (1,5 < P < 3) - qm = drinking water supply l/s In the United States the following various formulas are used: For Desmoines, for example, the following formula applies: where I is the population times thousand. In table 3.2 the peak factor is given for varying populations. The previous information shows that the magnitude of the applied peak factors from country to country differs. This is explained by the empirical character of these peak factors. city supply (lpppd) drainage (lpppd) Las Vegas (US) 1.560 760 Little Rock (US) 190 190 Wyoming (US) 570 300 Boston (US) 550 530 Caïro (Egypt) 800 150 Amsterdam (part of the city) 130 209 Grand Rapids (US) 670 720 Table 3.1 – Drinking water supply and wastewater drainage Figure 3.2 – Daily dry weather flow fluctuations Number of inhabitants France USA The Netherlands 100 3,0 4,2 2,4 1.000 3,0 3,8 2,4 100.000 1,7 2,0 2,4 Table 3.2 – Peak factors. Peak factors calculated for France are limited between 1.5 and 3. P qm = +1 5 2 5 . . (3.1) P l l = + + 18 4 (3.2)

14 sanitary engineering - ct3420urban drainage When it comes to appropriate design of wastewater systems, the prevention of rainwater connections to a separate wastewater system is of much greater importance than the value of the applied peak factor. This is illustrated by the following: The peak wastewater production per inhabitant is 12 l/h in the Netherlands. The amount of impervious surface connected to a sewer system is about 60 m2 per person. This amounts to a wastewater flow of 0.2 mm/hour (12l/60m2 ). Wastewater sewer systems are designed for a filling rate of 50%. The filling rate is equal to the water depth in the sewer divided by the diameter. The full capacity of the sewers is for a filling rate of 100%, double the design inflow. This margin is kept to allow for future changes, expected or unexpected, such as the connection of new urban developments to the system and especially faulty connections of stormwater to the wastewater system. When the design capacity of a wastewater system equals 0.2 mm/h, faulty connections of stormwater from 1/100th of the total impervious surface fill up the wastewater system to full capacity for a rainfall intensity of 20 mm/h. Larger rainfall intensities and larger amounts of impervious surfaces connected through faulty connections lead to overloading of the wastewater system. This may result in wastewater flowing back up into the houses through the house connections. 3.1.2 Industrial wastewater The amount of industrial drinking-water consumed varies per type of company. When a sewer system needs to be designed for a new neighbourhood an estimation of the industrial wastewater production is made based on average drinking water consumption figures for the expected type of industries. If it is not known which industry will establish itself an assumption of a load of 2 l/ (s.ha of industrial area) is often made. This load is derived from the gross impervious surface, since the net impervious surface is not yet known at the moment the design is made. If the plan is for an existing neighbourhood then the amount of wastewater can be calculated based on data from the drinking water company and an estimation of losses during the industrial process. It must be realized however that some industries acting in their own interest supply their own water by abstracting ground water. In that case a permit for groundwater abstraction must be obtained from the local authorities (the province). The water consumption from such industry can be derived from the text on the permit. Sometimes certain industries (the soft drink industries, breweries) do not drain their water into the sewerage system. This must be taken into account in setting up the design for the sewer system. 3.1.3 Infiltration and exfiltration Sewer systems are constructed to be watertight. However, in older systems, particularly in areas with poor soil conditions, this is often not the case. Joints between sewer pipes become leaky as a result of degradation of the rubber sealing rings or because pipes move due to ground settlement. Therefore, in sewer system design a leak flow rate of 0.2 m3 /(km/h) sewer per hour is often taken into account for sewers below the groundwater table (in the United States up to 3 m3 /(km/h)). When sewers are constructed above the groundwater table, leakage of wastewater out of the system can give rise to groundwater contamination. 3.1.4 Drainage water Drains are often laid in areas with high groundwater tables to keep the groundwater table at a minimum depth below roads and building foundations. When the drainage water cannot be discharges to surface water because there no water courses nearby, the drainage water is often discharged by connecting the drains to the sewer system. Water quality managers are generally not in favour of transporting comparatively clean drainage water to sewers and to a wastewater treatment plant. Therefore drainage water, if connected to sewers if preferably connected to separate stormwater sewers.

15 sanitary engineering - ct3420 urban drainage If the expected amount of drainage water is known, this can be taken into account in the design of the sewer system. If not, the drainage water is assumed to be incorporated in the leakage water. 3.1.5 Extraneous water Brooks or covered waterways can also make up part of sewer systems, particularly in the eastern and southern parts of the Netherlands. The term extraneous water applies to the flow that is transported from a brook or waterway into the sewer system. In the dimensioning of a sewer system or in control calculations for existing systems this flow must be properly taken into account. 3.2 Amount of stormwater One of the design requirements for urban drainage systems is that houses and buildings may not become flooded during precipitation of any intensity and volume. Damages of flood events caused by heavy rainfall are reported in the press at regular intervals. To prevent such damages it is of great importance to have good insight into the possible expectations of the amounts of precipitation and the runoff behavior over the lifetime of a system. 3.2.1 Amount of precipitation and runoff behaviour In the Hydrology course fundamental concepts of rainfall events were explained, which are briefly repeated here: Every rainfall event or storm event can be described by its temporal and spatial characteristics: • Rainfall volume, mostly expressed in terms of rainfall depth (d) on the surface [mm] • Duration (t) of the rainfall event [hours] • Rainfall intensity (P or I): amount of rainfall per unit of time [mm/h] • Frequency: frequency of occurrence, usually expressed in terms of a frequency scale T (i.e. the parameter under consideration occurs once every T years) • Area: geographical scale of rainfall [m2 or km2 ] Rainfall depth, duration and intensity in one station (point measurement) have the following relationship: And the average rainfall intensity is: A mass curve presents the accumulation of rainfall with time. A hyetograph presents the variation of rainfall intensity with time. Total rainfall depth (d), rainfall duration (t) and frequency scale (T) are also called the external statistics of a storm event and can be described by probabilistic distributions. The internal statistics of a storm event refer to the distribution of rainfall intensity with time, during the storm event. This distribution is often depicted in the form of a histogram. The distribution of rainfall intensity with time is important for urban drainage, because run-off processes in urban areas are fast and peak rainfall intensities within a storm event determine whether system capacity gets overloaded and flooding occurs. 3.2.2 Rainfall intensity duration frequency and rainfall mass curves Use of intensity duration frequency curves (IDF- curves) and mass curves is widespread in the design and analysis of urban drainage systems, sewer systems as well as open channels systems, retention basins and storage and settlement tanks. IDF-curves and mass curves provide information on the occurrence of rainfall intensities over a given period of time for certain return periods. Mass curves and IDF-curves, based on available precipitation information, can be compiled in two ways: • partial series • extreme values series With partial series all the peaks that are above a certain threshold are taken into account in d Pdt t = ∫0 (3.3)[L] P d t = (3.4)[LT-1] P d t =

16 sanitary engineering - ct3420urban drainage statistical analysis. In extreme value series only the maximum occurrences that take place within a certain period (a year for example) are taken into account. In the latter case high precipitation that is nevertheless lower than the annual maximum precipitation is not included in statistical analysis. This precipitation can however be higher than the maximum that occurs in one of the other years in consideration. As a result not all the information is processed and some information gets lost. An example of a mass curve that has been frequently used in urban drainage design in the Netherlands are Braak’s curves, dating from 1933. In recent year, these curves have been updated by KNMI and Meteoconsult (Wijngaard et al., 2004; Malda et al., 2006). Braak’s curves are compiled from 37 years of analysed rainfall values. Recent curves by KNMI are based on hourly rainfall data for one location, the Bilt, for a period of 97 years (1906-2003). Meteoconsult has extended this analysis by including rainfall data from 23 rainfall stations in the Netherlands, including the Bilt, for 10 years of rainfall data per station. The analysis for the Bilt has been extended by Buishand and Wijngaarden (2007) to include short duration rainfall down to a 5 minute time-step. It is clear that the differences in the length of the examined periods and the data time step will result in variations between rainfall duration curves. The reliability of the various rainfall duration curves depends on the period examined and the applied method. Figure 3.3 is extracted from the publication by Buishand and Wijngaarden, KNMI technical report TR-295. The return period and rainfall duration that is to be used in analysis and design depends on the type of system. Urban drainage systems typically require rainfall data of the order of several minutes up to hours. In rural areas like polders, run-off processes are slower and rainfall over a period of several days is critical to determine the required water system capacities. The return period that is used in design depends mainly on economic factors such as expected damage to buildings or agricultural products. Influence of area size In the previous text rainfall was described as a point measurement. In reality, storms have spatial dimensions and rainfall intensities usually decrease with distance from the center of the storm. This implies that average rainfall over a certain area decreases with the size of the area (see figure 3.4). Figure 3.3 - Return periods for exceedance frequencies of once per 1, 10, 100, 250 and 1000 years voor rainfall duration between 5 and 120 minutes (Buishand and Wijngaarden, 2007). Figure 3.4 - Influence of area size on average rainfall depth over area

17 sanitary engineering - ct3420 urban drainage Urban drainage systems usually drain small areas compared to surface water systems in rural areas. This means that high intensity rainfall over short periods of time is of importance for the design of system capacity. It also means that system transport capacity must be large compared to that of rural water systems. This would lead to very large pipe dimensions if the same return periods were applied to rural and urban systems. In the Netherlands, relatively low return periods are applied for design calculations, mostly T=2 years. Urban drainage system are designed to get overloaded at a relatively high frequency, in which case streets take over a part of the transport and storage capacity of underground systems. This is not a problem as long as buildings are located sufficiently high above street level. While in some locations flooding occurs more frequently as a result of increase in impervious areas that were connected to urban drainage systems over the past decades, the use of street storage to compensate for a lack of transport capacity in sewer systems is a debated topic. In the end, economic considerations are likely to end the discussion in favour of street storage instead of having to increase pipe capacities in existing systems. 3.2.3 Using rainfall mass curves The recorded values in the rainfall duration curves refer to the amount of precipitation that falls within a certain period of time, or the external storm event statistics. The internal storm event statistics for a rainfall depth of for instance 20 mm over a duration of 60 minutes can be composed in many different ways, for example: 1. 5 mm of steady rainfall in the first 50 minutes, followed by the remaining 15 mm in the next 10 minutes; 2. 15 mm steady rainfall in the first 10 minutes, followed by the remaining amount in the next 50 minutes; 3. steady precipitation over the whole period of 60 minutes. Suppose that a storage settlement tank must be dimensioned for precipitation that falls in 60 minutes and for a return period of 5 years. From figure 3.3 it can be read that the designer for T=5 must take a rainfall volume of about 20 mm into account. The capacity of the pumping station that empties the tank is 0.2 mm per minute (12 mm/h). The required tank volume is determined by the greatest difference between the supply (rainfall) and drainage (pumping) that can occur at any moment. Figure 3.5 shows that for the first and second storm characteristics the required storage capacity must be 13 mm, while a straightforward use of the rainfall mass curves yields a required storage capacity of 8 mm. This example clearly shows that using rainfall mass curves can cause under-designing in sewerage systems or its components. The vast majority of rain showers do not fall linearly. This means that the amount of combined Figure 3.5 – Relationship of distribution precipitation – necessary storage

18 sanitary engineering - ct3420urban drainage storm water overflows and storm water overflow frequencies will be greater than what is derived by the calculation based on rainfall duration curves. It is also important to consider that a previous storm can lead to filling of (a part of) available system storage, thus influencing starting conditions for the following storm. This means that under-designing of retention basins can occur. Other important factors can conversely lead to over-designing. The most important are: • Not taking rainfall losses into account (evaporation, surface wetting, retention on the surface, interception by vegetation etc.); • Overestimation of the runoff coefficient (see following paraghraph; runoff coefficients in the Dutch Guidelines for Sewer Design (Leidraad Riolering) are on the high side, so the calculated runoff load to the stormwater system is higher than what occurs in practice.); • Not taking storage in drains and house connections into account; • Not taking runoff delay into account (in general this has little influence on the design of sewer systems, since runoff processes are fast). 3.2.4 Run-off processes Part of the precipitation does not run off to the sewer system, thus does not add to inflow into the system. Careful estimation of the amount of precipitation that actually does run off to the sewer system is important to prevent under-design or over-design of the sewer system. Runoff behaviour depends on the following processes: • interception • evapotranspiration • infiltration • retention through ponding Interception is the process in which part of the precipitation on the surface is absorbed and does not run off. Evapotranspiration is constituted of rainwater that directly evaporates from the ground, plants and building as well as water that evaporates indirectly through plants. The term infiltration refers to water that sinks into the soil through pervious surfaces. Retention through ponding occurs on uneven surfaces, where water does not flow towards a sewer inlet. Runoff behaviour is taken into account with the help of a runoff coefficient in the design of sewer systems. In table 3.3 a few typical values are shown. In general in the Netherlands values used in sewer design as a first approximation are: runoff coefficient C=1 for impervious areas (roofs and roads) and C=0 for pervious areas (parks and gardens). The runoff coefficient shows which part of the precipitation runs off the surface and comes into the sewer system. The record shows that the runoff coefficient amounts to 48% of the total built up area. This is 2% less than the percentage of impervious surfaces that is present in the built up area. This is nearly the case in each built up area. This is why when examining the working of sewer systems the drainage is assumed exclusively for Surface of the earth runoff coefficient slate roofs 0,95 tiled roofs 0,90 flat roofs 0,50 - 0,70 asphalt roads 0,85 - 0,90 tile paths 0,75 - 0,85 cobblestone pavement 0,25 - 0,60 gravel roads 0,15 - 0,30 bare surfaces 0,10 - 0,20 parks, strips of land 0,05 - 0,10 Table 3.3 – Runoff coefficient Proportion Runoff coefficient Relative proportion Asphalt roads 22,5% 0,90 0,20 Cobblestone roads 7,5% 0,50 0,08 Pitched roofs 10,0% 0,90 0,09 Flat roofs 10,0% 0,70 0,07 Unpaved/pervious runoff 25,0% 0,15 0,04 Impervious runoff 25,0% 0,00 0,00 Total 100 0,48 Table 3.4 – Estimation compounded runoff coefficient

19 sanitary engineering - ct3420 urban drainage impervious surfaces. The runoff coefficient, which in that case is applied, naturally has the value 1! Traditionally, based on years of experience, Dutch sewer systems were designed to be able to process a constant rainfall intensity of 60 l/s/ha. For hilly areas sometimes 90 l/s/ha was chosen in order to promote better flood prevention. The hectares included relate to the total impervious surface. Most older sewer systems, up to the 1970’s were designed this way. In the 1980’s computers started to be used for sewer design and more advanced calculations could be made, taking rainfall intensity variations into account. More details on rainfall characteristics and sewer design are explained in the Fundamentals of Urban Drainage course (CT4491). Question: 1. Given a residential urban area of 1 ha, 100 houses and 250 inhabitants. a. Calculate the average annual stormwater flow and the average annual wastewater flow for this area. b. Calculate the peak stormwater flow and the peak wastewater flow for this area Answer: a. Annual stormwater flow: assume annual rainfall: 800 mm; annual stormwater flow: annual rainfall*area: 0.8x104 = 8000 m3 Annual wastewater flow: assume daily wastewater production: 120 l/p/day; annual wastewater flow: daily ww prod*nr of inhab*days/year: 250x120x365 = 10,950 m3 b. • Assume peak stormwater flow: 60 l/s/ha (or 90 or 120 l/s/ha); peak stormwater flow: peak flow*area: 60x104 /1000 = 600 l/s • Assume peak wastewater flow: 1.5x120 l/p/ day; peak wastewater flow: peak flow*nr of inhab*conversion days to seconds: 1.5*120*250 / 24/3600 = 0.5 l/s

20 sanitary engineering - ct3420urban drainage 4. Environmental requirements for sewer design 4.1 Development of environmental guidelines for sewer systems In the beginning of the 20th century, surface waters in many urban areas became heavily polluted as a result of increased, untreated overflows from sewer systems and industries. Wastewater treatment plants began to be built near large cities to collect and treat polluted water and control the pollution of urban surface waters. In the Netherlands, in 1970, the Surface Water Pollution Act (Wet Verontreiniging Opper- vlaktewateren, WVO) was adopted, aiming at the reduction and prevention of polluted overflows to surface water. It enforced treatment of polluted overflows to surface waters based on a system of pollution permits. This Act has been replaced by the Dutch Water Law (Waterwet) in December 2009. In the Netherlands, in 1970, the Surface Water Pollution Act (Wet Verontreiniging Opper- vlaktewateren, WVO) was adopted, aiming at the reduction and prevention of polluted overflows to surface water. It enforced treatment of polluted overflows to surface waters based on a system of pollution permits. This Act has been replaced by the Dutch Water Law (Waterwet) in December 2009. In 1986 a fire at the Santoz chemical plant in Switzerland had disastrous consequences for the Rhine. Thousands of gallons of toxic chemicals were washed into the river and millions of fish and other wildlife were killed. There was a public outcry and politicians from all the Rhine countries agreed that action had to be taken. The result was the Rhine Action Programme of 1987. The Rhine Action Programme stated that by 1995: • Discharge of the most important noxious substances should be cut by 50% compared with 1985. • Safety norms in industrial plants should be tightened. • Weirs must be fitted with fish passages to allow the fish to travel upstream and spawning grounds must be restored in the upper tributaries. • The riverside environment should be restored to allow the return of plants and animals typical of the Rhine. The first point has been an important trigger for reduction of (combined) overflows from urban drainage systems. Still there are approximately 13,000 overflow structures of (improved) combined sewer systems in the Netherlands. During heavy rainfall, sewage water diluted with stormwater is discharged, together with disturbed sewage sludge, from the combined Figure 4.1 - Polluted spills to surface waters

21 sanitary engineering - ct3420 urban drainage sewer systems via the overflow structures into the surface water. In separate systems, collected stormwater is discharged to surface water together with pollutions washed down from impervious surfaces. Many overflow structures are situated on small(semi-)stagnant watercourses. As a result, the influence of discharges from overflow structures on surface water quality, sediments and the aquatic ecosystem is considerable. Initially, requirements for granting discharge permits based on the Surface Water Pollution Act, mainly concerned the average frequency at which overflows from combined sewer systems can occur. No permit was necessary for stormwater outlets of separate systems. Meanwhile research has shown that the overflow frequency is representative to a limited extent only for the pollution load and the effects on surface water. From 1983 to 1990 the National Sewerage and Water Quality Working Group (Nationale Werkgroep Riolering en Waterkwaliteit, NWRW) conducted thorough research on combined sewer overflows from sewer systems and its effects on surface water quality. Intermezzo: Conclusions of the National Sewerage and Water Quality Working Group (Nationale Werkgroep Riolering en Waterkwaliteit, NWRW) The most important conclusions are: • The annual pollution loads from combined and from separate sewer systems is of the same order of magnitude; • Improved separate system produce the lowest pollution loads, compared to (improved) combined systems and (non-improved) separate systems; • Combined sewer overflows and stormwater outlets should preferably be located at large, non-stagnant waters. Discharge to small brooks, canals and isolated ponds should be avoided. • Only stormwater that runs off from roads with low traffic intensities can be directly drained to surface water; • The most efficient method to reduce overflow loads from combined sewer systems is to add a storage and settling tank between the combined sewer overflow and surface water (this implies installing an improved combined system). 4.2 “Basic effort/Basisinspanning” Based on the results of NWRW research, the CUWVO (Committee for execution of the Surface Water Pollution Act) recommended a basic effort (in Dutch: Basisinspanning) to reduce the adverse effects on surface water quality caused by overflows from sewer systems in their report (CUWVO, 1992). This basic effort in principle applies to all sewer systems. The recommended basic effort is based on the principle of best practicable means and has been defined so as to act as a reference effort corresponding with a certain expected type and amount of pollution emission. Depending on local circumstances, a solution may be selected based on lowest costs , provided that the expected pollution load corresponds with that of the defined basic effort. The basic effort has been defined in the form of reference systems for three different circumstances: • for sewer systems to be newly built: the preferred system is an improved separate sewer system; • for existing combined sewer systems: systems should have an in-sewer storage capacity equivalent to 7 mm of rainfall, a stormwater pumping capacity (Dutch: pompovercapaciteit) of 0.7 mm/h and additional storage capacity equivalent to 2 mm in storage and settling tanks; • for existing separate sewer systems: systems should be converted to improved separate sewer systems. Treatment of stormwater at stormwater outlets should only be implemented in cases of highly polluted impervious surfaces connected to the storm water system and large expected adverse effects on the receiving water. The recommended storage capacities and stormwater pumping capacity for combined

22 sanitary engineering - ct3420urban drainage sewer systems were based on the NWRW results for annual pollution loads and an analysis of combined sewer overflow frequencies. The defined capacities would correspond with an overflow frequency of about 10 times per year and an annual pollution load (according to NWRW results) of 350 kgCOD/ha/yr. Itisfurthermoreofimportancethatthemaintenance condition and lay-out of sewer systems are such that sedimentation is kept to a minimum in order to prevent pollutions adhering to sediments from being discharged to surface water. Discharges to sensitive surface waters should be especially prevented. In the CUWVO 1992 report, the year 1998 was in principle taken as a final date for realisation of the recommended basic effort. In the execution of improvement measures, priority should be given to situations where adverse effects on surface water quality are most severe. In 2001, the Committee for Integrated Watermanagement (CIW, successor of CUWVO), issued a new report (CIW, 2001) that provided a further elaboration of the “basic effort” definition, which upon practical application had proved to be open for different interpretations. This report states that “The reference system defined by CUWVO is taken as the starting point for establishing the criterion for average annual emissions of pollutants, which is expressed in kilograms of organic material (COD) per hectare of relevant impervious surface per year.” The report establishes the following criterion: the criterion for maximum annual pollution is a maximum load of 50 kg of organic material (COD) per hectare per year, totaled over all combined systems within a municipality. The criterion applies to the total area of impervious surface connected to the sewerage system within the drainage area. The evaluation is based on the following principles: • The average annual emission of pollutants from existing combined systems must not exceed the criterion. • The average annual emission of po

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