Nitty-Gritty of Sewage System Design

nirmal-acharya

Globally 50% of the world’s population lives in urban areas or cities. Clean city is the ultimate dream of every citizen.  In general, main factors of cleanliness of cities have been regarded as better solid waste management and sewage system design. Both of these factors are related to transformation of healthy environment of cities and good health of people.

Every city is an ecosystem, and maintaining its good status is crucial for the health and happiness of city residents. This implies granting a good balance between green and built areas, sustaining biodiversity, restoring natural habitats and providing ecological corridors for wild species. Municipal waste accounts for only about 10% of the total waste generated. However, it has a very high political profile because of its complex character, due to its composition, its distribution among many sources of waste, and its link to consumption patterns. The first objective of any waste policy should be to minimize the negative effects of the generation and management of waste on human health and the environment. Waste policy should also aim at reducing the use of resources, and favor the practical application of the waste hierarchy (i.e. prevention, preparing for re-use, recycling, other recovery, e.g. energy recovery, and disposal).

Growing urbanization makes it necessary for city managers to have a precise overview of land uses. A smart sustainable city is indeed one in which residential areas, green spaces and services are equally distributed, and a good proportion exists between the green volume and the built “grey” volume, which is a necessary condition not only to protect cities’ ecosystems and biodiversity, but also to grant a healthy life to their inhabitants. Making a city clean requires constant efforts not just from the government but also from the citizens. Merely passing laws and issuing circulars do not ensure a clean city. Conscious sustained efforts from citizens, sometimes at a minor personal inconvenience are required to back the efforts of a willing government structure.

In this write up, I would like to discuss about some nitty gritty about sewage system design. As mentioned above, a good sewage system in a city exhibits its healthy environment.  Although, we can find numerous books and literatures about sewer system design; however practical aspects are always missing there, so I would like to inject some of my design experiences so far. History of centralized or water carried sewer system in Nepali cities is not that old and not much engineering involved as well. So, it is hoped that practicing City Engineers will be benefited from this design input.

In Nepali urban areas, it has been observed that more focus has been given for solid waste management. As private properties in big cities do not have large open lots, people tend to dispose their garbage in public spaces. Public open spaces are the property of local government, so they are compelled to protect and clean their properties. But in case of sewage management, this seems to be private responsibility. Except few cities, inhabitants of the urban areas are mostly relied on on-site systems like using open defecation, latrine pits and septic tanks. Efforts from the government part have not been observed towards to developing a proper water carriage sewage management scheme. Most of the developed sewage piping systems has been coming through pressure of communities and done in so haphazardly that the results are in front of us, the polluted rivers, lakes and other water bodies. With no scientific system for disposing of sewage and garbage, human and animal waste has polluted the water bodies. The water bodies are polluted in such extreme that we cannot touch the water; even worse to that we cannot go near to them. This means, we are not learning from history from either bigger or smaller cities like New York, London, Vienna, Surat; which had been suffered (Cholera, Typhoid, Yellow Fever etc.) a lot from their polluted river and land.

According to the World Health Organization, pollution represents today a greater threat than epidemics like Ebola and HIV. So, what are the solutions to free our Nepali waterbodies from being polluted and deteriorating aesthetics every day? The only answer to the above question is a best management practice of waste water or sewage. Water is an elemental part of the fabric of urban lives, providing sustenance and sanitation, commerce and connectivity. Indeed, the modern field of public health owes a strong debt to the Sanitary Engineers who attempted to provide potable water and safe disposal of human wastes in mushrooming cities.

Sewage Management Phenomenon

In practice, every day, wastewater goes down toilets and drains in homes, schools, businesses and factories and then flows into City’s sewer system. Runoff from rain and melting snow, street and sidewalks, and other outdoor activities flows into catch basins in the streets and from there into the sewers. Here, waste water management includes safe passage of all the sanitary and industrial wastewater including infiltration with some solids (containing poops, kitchen wastes, greases, metal dusts, papers etc. which constitutes less than 1%) coming out from the residential houses, commercial and industrial facilities, health facilities, schools etc. and storm water from roofs and premises of above mentioned structures through a underground piping system, treating properly in Sewage Treatment Plants (STP) and disposing in nearby receiving waterbodies as depicted in Fig 1. Here, sanitary wastewater includes liquid waste originating solely from humans and human activities, such as wastes collected from toilets, showers, wash basins, sinks, washing machines etc. In this process, the wastewater also undergoes treatment process, so that waterbodies be out of harmful pollutants. This is called the water carries sewer system.

There are two distinct benefits of water carriage sewer system. Firstly, it is cost effective than individual pit latrine/Septic tank system. Cost of a water-carriage sewer system divided over a period of thirty years would be less than the cost of keeping privy vaults and cesspools clean. Secondly, Water carriage sewer systems eliminated most maintenance work by the homeowner. Thus, at least in Nepali context, it would release the burden of house owners to finding cleaning workers and their unusual wages bargaining. However, it is not like that centralized sewer system does not have any disadvantages. Main disadvantages of sewer system are human waste that might be used for fertilizer would now be lost, there would be an increased danger of contamination of the subsoil by leakage, pollution of the waterways with threats to drinking water supplies and shellfish, and the generation of disease-bearing sewer gas, and a heavy capital cost in the beginning. But with the establishment of improved STPs, all the disadvantages of central system have been overcome. A powerful STP can remove pollutants very quickly, produce bio-solids, which has high nutrient and organic contents so can be used as fertilizers or soil conditioners, and a proper investment plan yielding higher socio-economic return.

 Separate Sewer System Vs Combined Sewer System

City’s sewer system can be designed in two ways. If the neighborhoods runoff from the streets as well as households, institutional and industrial premises is carried by separate storm sewers directly to local streams, rivers and bays, then it is called as Separate Sewer System (SSS). This system has been depicted in Fig 2. Whereas, if the City’s sanitary and industrial wastewater, rainwater and street runoff are collected in the same sewers (pipe or drain) and then conveyed together to the City’s treatment plants, this will be known as a Combined Sewer System (CSS). Fig. 3 shows schematic sketch of Combined Sewer System (CSS). Sometimes, during heavy rains or snow, combined sewers fill to capacity and are unable to carry the combined sanitary and storm sewage to the waste treatment plants. When this occurs; the mix of excess storm water and untreated sewage flows directly into the City’s waterways. This is called a Combined Sewer Overflow (CSO). If storm runoff of some areas of neighborhoods mixed with Sanitary Sewer system, it is called as Partially Combined Sewer System.  We have also a system called Partial Separate Sewer System. In this system the sewage and storm water of buildings are carried by one set of sewers while the storm water from roads, streets, pavements etc. are carried by other system of sewers usually open drains.

Many old US and European cities like New York, Boston, London etc. have adopted CSS over SSS. A main reason to adopt CSS was cost aspect. As SSS required two conduits to remove storm water and sanitary wastewater; so it was costlier than CSS. Before invention of Vitrified Clay Pipe (VCP); brick or stone conduits were used which required much time to construct; so, became expensive. Wooden conduits also have been found in use in some locations of New York City.

SSSs later became economically attractive after introduction of VCP and RCC Hume Pipes. Clay pipes could be constructed with smaller diameters and in different shapes than traditional wood, brick, or stone sewers. Clay pipes had economic advantages over traditional brick pipes because their smaller size reduced material costs and their ability to be delivered precast reduced labor costs. Clay pipes also had sanitary and performance advantages over the larger combined-sewer conduits.

Complication of Sewer System

In separate system, number of things need to be taken in consideration. Main point is vertical level difference between sanitary pipe and storm water pipe. To facilitate the household connections, sanitary pipe is usually laid at least 2.55 m (8.5 ft) below the road surface as shown in Fig 1 whereas, storm water pipe is laid at least 1.2 m (4 ft) below the road surface. If due to any specific circumstances, pipe cannot be laid below 4 ft, then Ductile Iron Pipe (DIP) shall be used instead of Vitrified clay pipe or RCC Pipes. Minimum cover is required to withstand load and avoid freezing conditions. Theoretically there is only 4.5 ft level difference between invert level of sanitary pipe and top level of storm water pipe. So, in case of larger pipe diameter this 4.5 ft difference would not be enough and the condition when storm water and sanitary pipes need to be crossed, there will be problem. In this case, we need to go sanitary pipe down. As we have already laid storm water at minimum depth so we cannot reduce the depth of pipe, hence we do not have other option than going sanitary pipe down. Here now, we also can’t go extra deep to accommodate the pipe crossing. From constructability point of view, the maximum depth of pipe could be 5.4 m (18 ft). Let maintain 1.2 m (4 ft) horizontal space between two pipes and 0.60 m (2 ft) vertical space between invert of storm water pipe and top invert of sanitary pipe. By maintaining this difference; houses at both sides of road can be connected easily to the sewer system.

In combined system, there will be only one pipe in use, so design of the same is easier than separate system. There will be no any vertical difference consideration needed to be taken in account. Initial pipe laying cost will be less and easy to handle during construction time. Minimum velocity of 0.90 m (3 ft) per sec. shall be maintained for self-flushing of pipe and should not be more than 4.5 m (15 ft) per sec to protect from abrasion of pipe wall, water hammering, emission of Hydrogen Sulphide gas (H2S) etc. When flows drop below 3 feet per second (0.9 meter per second); pipes can be clogged due to siltation. Flows can be designed to as low as 2.5 feet per second with justification in the hydraulic report. In no case should the slope exceed 10%. In the case of very steep slopes, drop manholes should be provided so that maximum velocity is within allowable limit.

In traditional practice, invert elevations of sewers are used to match at all junctions and manholes. However, pipe crowns (top point) of differing diameter, branch or trunk lines should match the same elevation when entering and exiting junctions. Matching the crown elevation of the pipes, will prevent backflow in the smaller pipe. In general, the sewer should slope in the same direction as the street. A larger pipe must never connect into a smaller pipe. This will clogged the downstream small pipe in no time.

Interference with Other Amenities

There should be no any interference of sewer line with other amenities like water supply, gas line, electric/telephone lines etc. Especial care shall be taken so that there will be no any physical connections between a public or private potable water supply system and a sewer, or appurtenance thereto which may permit the passage of any sewage or polluted water into the potable supply. This fact is related to health of citizens, which remains in a mass. No potable water pipe shall pass through or come into contact with any part of a sewer manhole. Sewers shall always be laid below than potable water pipe line. Potable water main supply line shall be in a separate trench or on an undisturbed earth shelf located on one side of the sewer. Sewers shall be laid at least ten (10) feet horizontally from any existing or proposed potable water main. Sewers crossing potable water mains shall be laid to provide a minimum vertical separation of eighteen (18) inches between the outside of the potable water main and the outside of the sewer. Where a new sewer line crosses a new potable water main, a full length of pipe shall be used for both the sewer line and potable water main and the crossing shall be arranged so that the joints of each line shall be as far as possible from the point of crossing and each other.

Sewer System in Flat Areas

In flat terrain areas the natural drainage system for surface water and sewage is often poor. It is very difficult to maintain gravity sewer system in flat terrain (Terai/Madhesh) cities like Biratnagar, Janakpur, Birgunj, Bhairahawa, Nepalganj etc. So, sewage of these flat terrain cities needs to be pumped in certain locations to maintain the required velocity. The gravity sewer system is always cheaper than pumping system. As of now our Terai cities have not employed any centralized sewer system, so they do not have any experience of pumping system. During rainy seasons, flooding of Terai cities is the result of lacking application of storm water pumping system. Now, it has become high time that all Metropolitan, Sub-metropolitan and Municipal Cities in Terai region to plan a sustainable sewage system to make itself a smart and flourished city. There is no any option than applying a pumping sewage system in Terai cities. Thus, Cities have to manage enough budget to bear extra pumping cost.

Pump stations shall be designed to be fully operational during flooding to the twenty five (25) year flood elevation. Pump station structures and equipment shall be protected from physical damage by flooding to the one hundred (100) year flood elevation. These cities also try to build structures to infiltrate more storm water into the ground below hence reducing the quantity of surface runoff. Infiltration wells or infiltration ditches will be suitable when the soil is with high permeability. For low permeability soils, ponds or artificial lakes shall be constructed to store the storm water in rainy season. This will also increase aesthetics of the cities. Several historical ponds remained in Janakpur City must have been helping to recharge the underground water level. As shown in Fig 4 many cities (for example New York) in developed countries have constructed Bioswales or road side gardens to lessen the surface runoff and increase the aesthetics of city. Terai Cities can construct Bioswales/Road side Garden along wide streets and Chowks. Citizens having larger plots in the city also can be encouraged to construct infiltration wells in their premises to capture the storm water. By doing this, they will send less quantity of storm water in city’s drainage system and also increase the underground water table.

Here through this article, I want to send a message to City/Municipality Engineers/Planners that sewage management is a complicated process to correct if not planned properly in time. Though, Combined System is cheaper initially but in long term it will have many consequences. There are flooding in street due to backflow in heavy rainfall time, extra load in treatment plants and polluted receiving water bodies due to raw sewage outflows. Newly formed Nepali Metropolitan cities, Sub-metropolitan cities and Municipalities have to start immediately for sustainable sewage management planning. All Cities in Terai Region also need to execute best management practice of Infiltration well, Ponds and Roadside Gardens regarding storm water management to reduce flooding in streets and neighborhoods.

(PhD in Civil Engineering, Works in New York City, Dept. of Environmental Protection)