'01 ~ ~ ~ ~ ~ ~ ~ ~ ~ t-.4 =- c f~~~~~~~~~~~~~ 4%&rC- FE~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~c. t-l -O al 0 71<6~~~~~~~~~~~~~~~~~~. > > i loci. '.. Simplified Sewerage: Design Guidelines by Alexander Bakalian Albert Wright Richard Otis Jose de Azevedo Netto E UNDP-World Bank Water & Sanitation Program Copyright C 1994 International Bank for Reconstruction and Development/The World Bank 1818 H Street, NW Washington, DC 20433 USA First printing, May 1994. Cover photos by Alexander Bakalian and David Kinley. Other photos by Alexander Bakalian and Richard Otis. The UNDP-World Bank Water and Sanitation Program was organized as a joint endeavor of the United Nations Development Programme and the World Bank, and it has been one of the primary actors in worldwide efforts to meet the challenge of providing basic water supply and sanitation services to those most in need in the developing world. Partners in this venture are the developing countries themselves and the multilateral and bilateral agencies that fund the Program's activities. The Program is part of the Transportation, Water and Urban Development Department of the World Bank, and has regional offices in Abidjan, Jakarta, Nairobi, and New Delhi. The Program's publications include two series, a Water and Sanitation Program Report series and a Water and Sanitation Discussion Paper series. The Program Report series presents formal discussions of the Program's operations and research as well as examinations of relevant projects and trends within the water and sanitation sector. Program Reports are subjectto rigorous external review byindependentauthorities from appropriate fields. The Discussion Paper series is a less formal means of communicating timely and topical observations, findings, and opinions concerning Program activities and sector issues. This document has been prepared and published by the Program, and copies may be obtained from the Washington, DC office. Material may be quoted with proper attribution. Any maps that accompany the text have been prepared solely for the convenience of readers. The boundaries, denominations, and classifications of maps do not imply, on the part of the UNDP-World Bank Water and Sanitation Program, the United Nations Development Programme, the World Bank Group, or any affiliated organization, any judgment on the legal or other status of any territory, or endorsement or acceptance of any boundary. The findings, interpretations, and conclusions expressed in this paper are entirely those of the author(s) and should not be attributed in any manner to the UNDP-World Bank Water and Sanitation Program, the United Nations Development Programme, the World Bank Group, or any affiliated organizations. Contents Abstract v Acknowledgements v In Memoriam v 1. Introduction 1 1.1 Background 1 1.2 Unconventional Alternatives: Intermediate-Cost Sewerage 2 1.3 Objectives 3 2. System Description: Origin and Development 5 3. Design Criteria 7 3.1 Layout 7 3.2 Hydraulics 7 3.3 Service Connection 9 3.4 Depth of Sewers 9 3.5 Manholes and Other Appurtenances 12 3.6 Materials 15 4. Operational Experience 17 4.1 Operational Problems 17 4.2 Maintenance Requirements 17 4.3 Equipment 17 5. Costs 19 5.1 Capital Costs 19 5.2 Operation and Maintenance Costs 19 6. Discussion 21 6.1 Risk Estimation 21 6.2 Flexibility 21 6.3 Applicability 21 6.4 Requirements 22 6.5 Additional Work 22 7. Conclusions 23 Annex 1: Design Peak Factor 25 Annex 2: Tractive Force 27 Annex 3: Examples 29 Annex 4: Cost Comparison Between Conventional and Simplified Design 31 References 3 3 Figures Figure 3.1 Typical layout of simplified sewer system (SABESP) Figure 3.2a Connection-inspection box (Sao Paulo State, Brazil) Figure 3.2b Interior connection-inspection box (Sao Paulo State, Brazil) Figure 3.2c Interior of connection-inspection box (Sao Paulo State, Brazil) Figure 3.3 Inspection-cleanout (SABESP) Figure 3.4a Typical residential connection Figure 3.4b Typical baffled box (Sao Paulo State, Brazil) Figure 3.5 Minimum depths of sewers (SABESP) Figure 3.6 Schematic of cross-block connection Figure 3.7 Junction inspection-cleanout Figure 3.8 Terminal cleanout (SABESP) Figure 3.9a Comparison of conventional and simplified manholes Figure 3.9b Interior of simplified manhole (Sao Paulo State, Brazil) Figure 3.10a Buried box for change in direction (SABESP) Figure 3.10b Buried box for change in slope (SABESP) Figure Al Elements of circular conduit Tables Table 1 Some Simplified Alternatives to Conventional Manholes Table 2 Costs of Selected Simplified Sewerage Projects Table 3 Sensitivity Analysis of Costs of Individual Design Variations in Two Egyptian Towns Table Al Hydraulic Elements of Circular Pipes Table A2 Comparison of Design Results for the Two Methods iv Abstract Acknowledgements Heavy reliance on high-cost conventional sewers Thanks are due to the staff and management of has produced inadequate sanitation service SABESP and SANEPAR, the water companies coverage in many urban areas. In the recent past, of the states of Sao Paulo and Parana, respectively. low-cost, on-site systems have been gaining They have generously provided us with most of increased acceptance as alternatives; however, the data in this report. The authors are grateful in areas where housing densities and levels of for comments from Donald Gray, West Virginia waterconsumption are high, waterborne solutions University; James Kreissl, United States are required. In an effort to reduce the cost of Environmental Protection Agency; sewered systems, a critical review of the basis of John Kalbermatten, Kalbermatten Associates, the conventional design standards has been Inc.; and John Briscoe and Andrew Macoun, the carried out in Brazil. The result has been the World Bank. Any errors or omissions are, of development of a modified approach for sewer course, the responsibility of the authors. design based on hydraulic theory, satisfactory experience elsewhere, and redefinition of In M emoriam acceptable risk. Systems designed according to these new criteria are known as "simplified sewers." They operate as conventional sewers This work is dedicated to the memory of but with a number of modifications: the minimum Jose M. Azevedo Netto, who, throughout his diameter and the minimum cover are reduced, career, was a strong motivating force in bringing the slope is determined by using the tractive positive changes to the practice of our profession. force concept rather than the minimum velocity concept, sewers are installed below sidewalks where possible, and many costly manholes are eliminated or replaced with less-expensive cleanouts. Experience with these systems has shown that cost savings of 20 percent to 50 percent have been achieved. Operation and maintenance requirements have been similar to conventional sewers. 1. Introduction z7 0 few systems that are built. Consequently, many 1.1 Background plans for needed facilities have been postponed The unprecedented population explosion in ur- indefinitely. ban centers during the past two decades has severely strained the ability of cities to meet the One of the main reasons for this typically acute needs for services such as water supply and situation is the use of conventional sewerage wastes disposal. As local governments have tried systems. They are expensive even for industrial- to cope with insufficient resources, their efforts ized countries.2 To ensure that raw sewage flows have achieved mixed success. In planning for freely, conventional sewers are designed with additional services, priority has generally been large-diameter pipes at slopes that often require given to the high-income areas where full or extensive excavation. Flat terrain, high ground- partial cost recovery was considered feasible, and water table, manholes, other appurtenances, and poorer sections were often left unserved or were pumping stations also increase construction costs. served by woefully inadequate facilities. It is clear that exclusive reliance on conventional Further, in low-income areas where some service sewerage cannot solve the current predicament has been extended, planners and users have of increasing needs and dwindling resources. always given a higher priority to water supply Recognizing the magnitude of the problem, sec- than to sanitation.' Uneven expansion of water tor institutions have begun investigating the use coverage without parallel improvement in sani- of alternative technologies. Much of this work tation has increased water pollution and caused has been directed at on-site systems, and the public health problems. In trying to correct the ventilated improved pit (VIP) and pour-flush imbalance between water supply and sanitation latrines have emerged as technologies of choice: coverage, cities face severe financial hardships in they provide good service at reasonable cost. both building new sewer systems and extending However, in many situations-for example, high existing ones. housing density, impermeable soil, or high water consumption-on-site systems are not appropri- In many cities, parts of sewerage systems built in ate. Under those circumstances, sewered alter- the past either remain incomplete because of natives to conventional sewers are needed. cost overruns or are underutilized because of the mismatch between supply and demand. This affects financial viability and sustainability of the 1. It was estimated that in 1985,66 percent of urban dwellers in developing countries had access to water, but only 35 percent were served by sanitary facilities (WHO 1988). 2. Even in prosperous nations, serious efforts are made to find alternative sewage conveyance systems in order to reduce the costs of sanitation systems (Kreissl 1987). 1.2 Unconventional self-cleansing velocities and permits flatter gra- dients and shallower depths; the attenuation of E Alternativ sflow reduces the peak flow factor and makes it o *~III .ves. possible to use small-diameter sewers laid atmild EL I ntermed i ate-Cost gradients that require less excavation. First used o in Zambia and Australia, this modification of U Sewerage conventional sewerage is known as solids-free sewers, common effluent sewerage, or small- Attempts at developing lower-cost alternatives diameter gravity sewer systems (USEPA 1986). usually focus on elements in sewerage systems that most influence costs. Among such key cost Another example is the septic tank effluent factors are the average diameter and average pump (STEP) sewerage system, similar to the depth of sewers; average slope relative to ground solids-free sewer system except that the settled topography; the number and depths of man- effluent is pumped out into the sewer network; holes; and otherfactors suchas totalsewerlength, this permits further reduction in pipe size and z population density, set-up costs, and excavation slope. Other examples are the grinder pump D inrock.Consequently,sewercost-reducingmea- sewerage where wastewater is ground and sures have invariably been directed at modifying pumped into the sewer line, and the vacuum one or more of these cost-setting factors. sewerage system. It should be noted that these various solutions are location-specific and de- The resulting range of technological options3 is pend heavily on population densities and avail- collectively known as intermediate-cost sewer- ability of strong resources for maintenance. Most age or intermediate-cost sanitation systems. of them are only suitable for populations up to There are two types of intermediate-cost sewer- 10,000. A thorough review of experience with age: those that arise from changes in technology these systems has been the subject of a recent and those based on changes in design standards publication from the United States Environ- and guidelines.4 mental Protection Agency (1991). Changes in technology: A number of innova- Changes in design standards and guidelines: tions have been made in the design of sewer Sincethemostcostlycomponentofconventional systems through special ancillary appurtenances sewerage is the collecting system-accounting that permit a reduction in the depths and diam- for80-90percentof thetotalcost(Kreissl 1987)- eters of sewers. An example is the addition of a design criteria and standards can be carefully solids interceptor tank between house sewers modified to achieve cost savings from the use of and laterals. The tank captures and stores incom- shallower depths, smaller pipe diameters, fewer ing solids, attenuates the flow, and allows the appurtenances,etc.Suchmodificationshavebeen settled sewage to flow out by gravity. The ab- introduced without jeopardizing the reliability sence of settleable solids eliminates the need for and safety of the system. 3. The technologies discussed in this section are all self-cleaning systems. An innovative approach advocated in the Netherlands, however, questions the self-cleansing velocity method. This new approach examines the trade-off between installation and maintenance costs, and compares life-cycle costs of sewers (combined construction and operation costs) for various slopes and sizes and selecting slopes corresponding to the lowest total (DHV 1990). 4. Another approach that would affect the size and cost of the sewer network is the use of water-saving devices. This approach could be adopted with any one of the intermediate technologies and is undoubedly beneficial: it reduces the ever-increasing cost of water production and saves on the construction of new extensions of sewerage systems. Forexample, it isestimated thatthe use of a low-volume flush toilet (4-5 liters perflush ratherthan 18-20 liters)would reduce the amount of water consumed-and the amount of water discharged into the sewer-by 20 percent. It is also worth noting that a reduction in water consumption can also be achieved by pricing and legislative mechanisms. The latter has been used in the United States where effective January 1994, low volume flush toilets (not exceeding 2 1.6 US gallons per flush) are mandatory in new installations. Changes in design standards to produce lower- 1 .3 Objectives cost sewerage have been based on hydraulic theory, advances in technology, satisfactory This report presents design guidelines used for experience, and acceptable risk. One example is simplified sewers. It is based on information "flat-grade sewerage" in use for some 80 years in collected from a number of projects in Brazil and Nebraska (Gidley 1987). Based on changes in through discussions held with the staff of the oD design standards affecting only the minimum state water companies of Sao Paulo (SABESP) P diameters and minimum slopes, its use in and Parana (SANEPAR). Additional data from -' Nebraska's flat terrain and high groundwater the literature and other areas are also presented. table produces significant cost savings during construction (saving the cost of deeper sewers, This report is not intended to serve as a design deeper manholes, dewatering during sewer manual. It describes changes in design criteria laying, and pumping stations), and further savings that have been introduced in Brazil and found to during operation (savings in pumping costs and pose no significant threat to the operational in- in the maintenance of pumping stations). tegrity of sewer systems. In addition to pro- viding an insight into the development of this A similar system, known as "modified conven- new design approach, the report: tional gravity system," has been introduced in Australia (AWRC 1988) and includes modifica- reviews the modifications introduced to conven- tions such as reduction of minimum cover re- tional design standards and presents the argu- quirements, use of PVC pipes, use of 100-mm ments and rationale for the modifications; diameter sewers, revision of trench dimensions at shallow depths, increased separation between evaluates operational experience from selected manholes, and increased use of inspection shafts. projects; and The most extensive changes in design standards, however, have been carried out in Brazil and evaluates the cost-saving potential of the modi- have resulted in a system called "simplified fied system. sewerage," the subject of this paper. 2. System Description: Origin and Development rD The principal reason for the development of survivedintactorhadbecomemoreconservative simplified sewerage was the realization that the in Brazil and elsewhere, with very few excep- reasonforthehighcostofconventionalsewerage tions-the flush tanks and the open-grate man- was high design standards, and that these stan- holes disappeared long ago. The idea of self- dards were hindering the expansion of service cleansing sewers had become the central design coverage to middle- and lower-income urban criterion, and a minimum velocity of 0.6m/s was communities; this led to a review of design crite- set as the design parameter. The cost of sewer ria used in Brazil for conventional sewerage systems based on century-old criteria was too (Azevedo-Netto 1975, 1984; Diniz 1983). high for many cities, and engineers in Brazil questioned the appropriateness of such systems The review showed that the prevailing design in their cities. criteria were very similar to (and in some cases even more stringent than) those used by George The ensuing critical review led to sweeping WaringJr. in his design of the first separate sewer changes in conventional sewer design standards. system in the United States in 1880. The changes were based on findings of recent research in hydraulics, satisfactory experience, The 1880 sewer system was designed to carry and redundancy. The use of these new standards peak flows at the minimum velocity of 0.60 m/s. has produced a lower-cost system that uses Waring had argued that if that velocity were smaller, flatter, and shallower sewers with fewer, reached at least once a day, the system would simpler manholes. perform without problems. To ensure complete removal of deposits, flush tanks were installed at The following section discusses the distinctive the head of each sewer line. Ventilation was features of simplified sewerage and the support- provided through open grates on manholes spaced ing rationale behind changes in design standards. at least 300 m (1,000 ft) apart. Waring's system Thissystemofsewerdesignhasbeenadoptedby did not work very well: frequent obstructions in a number of state water and sewerage companies the 100-mm and 150-mm pipes were reported in Brazil and has been incorporated into the (Metcalf and Eddy 1928). The review also noted Brazilian Sewer Code (ABNT 1988). It has also thatmostof these criteria and appurtenances had been used in Bolivia, Colombia, and Paraguay. 5 3. Design Criteria 0 opportunity cost of capital, uncertainties in pre- 3.1 Layout dicting future land-use patterns or directions of To avoid deep excavations, long trunk pipes to growth in developing-country cities, and the high interceptors, and large pumping stations, serious cost of maintaining large sewers with low flows. consideration is given to splitting the network into two or more separate smaller systems; al- The use of shorter design periods avoids such though network layout is also an importantpartof problems and reduces the large capital require- conventional design, the optimization of pipe ments in sewerage systems, facilitates financing, lengths and network subdivisions takes on even and enhances prospects ofachievinggreatercov- greater importance in the simplified system. erage with a given investment. With shorter design periods and construction byphases, start- Where feasible, a project area is defined by ing from upstream ends, the effects of errors in individual drainage basins, each with its own forecasting population growth and their water collectors and treatment plant. As needs and consumption can be minimized and corrected. resources increase, mini-networks can be con- For these reasons, simplified sewerage employs nected to a common interceptor for conveyance design periods of 20 years or less. In this regard, to a regional plant or local treatment system.5 itis noteworthy that the USEPAlimits the design period to 10-15 years (ASCE 1982). Furthermore, to minimize excavation and the cost of pavement restoration, sewers are, to the 3.2.2 Design flow extent possible, located away from traffic loads, 3 D generally under the sidewalks (on both sides of Wastewaterflow quantities are necessarily lower the street, if necessary) rather than down the than the quantity of water supplied because center of the street. To save pipe and excavation water is lost through leakage, garden watering, costs, sewers extend only to the last upstream house cleaning, etc. To determine the expected connection rather than to the end of the block amount of wastewater, it is important to keep (Figure 3.1). records of pumpage for each day and fluctuations during the day. 3.2 Hydraulics Reliance on estimates of water use from industri- alized countries or cities of similar characteristics 3.2.1 Design period can lead to erroneous design flows. Information In conventional design, it is common to design should be obtained from the area under consid- trunk sewers and interceptors for the projected eration. In arid areas of the United States, for peak flow expected during a 25 to 50-year pe- example, the return coefficient is as little as 0.4; riod or for the saturation population of the area. in Sao Paulo, this coefficient is 0.8. The design Such long design periods make it possible to flowisbasedonthisreturnedquantitymultiplied capture economies of scale in sewerage systems. by a peak factor, which is inversely related to However, these have to be balanced against the population size. 5. To treat its waste water, the city of Juiz de Fora (population 400,000) in the state of Minas Gerais plans to build 57 communal septic tanks with anaerobic filters and 17upflow anaerobic sludge blanket systems at a total estimated cost of $18 million. The cost of a central conventional treatment plant and the necessary interceptors was estimated at $75 million. 7 In industrialized countries, the peak factor is conservatively es- E timated to be between 2.0 and 3.3. In Brazil and Colombia, a 0 peak factor of 1.8 has been used - - l in simplified sewerage projects6 - (see Annex 1). Where water use l information is not available, the simplified sewerage system is - designed for a minimum flow of 1.5 l/s; infiltration is assumed to be 0.05-1.0 1/s/km of pipe. - - -------- ~~~~~~~~~~~~~~~~~0~~~~~~~~~~~~~~~ 3.2.3 Minimum diameter z A minimum diameter for sani- _ tary sewers is usually specified I in order to avoid clogging by _ large objects. In conventional P systems in the United States, the house connections are usu- ally 150 mm in diameter, but smaller sizes have been used. 0 a~ Therefore, forconventionalssew- O erage, the minimum diameter commonly specified for street sewers in many countries is Figure 3.1. Typical layout of simplified sewer system (SABESP) 200 mm. In the simplified sys- tem, smaller sizes are recom- mended because, in the upper reaches of a system where flow is low, the use of 3.2.4 Ensuring self-cleansing smaller-diameter sewers results in greater depths offlowandhighervelocities,andimprovescleans- Instead of the minimum velocity criterion of ing. Experience in Latin America and elsewhere 0.6 m/s as in conventional sewer design, simpli- (e.g., Nebraska) shows that 150-mm diameter fied sewer design is based on maintaining a street sewers do not present additional mainte- boundary shear stress of 0.1 kg/ml, which is nance problems compared to conventional sew- sufficient to resuspend a 1-mm particle of sand. erage. In Brazil, 100-mm diameter laterals or Many authors (Machado 1985; Paintal 1977; Yao branch sewers are being used in residential areas 1974, 1976) have proposed the use of critical for a maximum length of 400 m. The 100-mm shear stress for determining the minimum slope diameter pipes are usually specified for unpaved of sewers as a economical alternative to the streets of periurban communities.7 minimum velocity approach. For a minimum shear stress of 0.1 kg/m2, pipes smaller than 1,050 mm can be made flatter than when de- 6. This factor is the product of two ratios: (a) KV, the ratio of the maximum day flow over the average day flow (equal to 1.2), and (b) K2, the maximum hour flow over the average hour flow (equal to 1.5). In other words, the maximum sewage flow will be the hourly maximum, or the peak rate of the maximum day (plus the maximum infiltration). 8 signed according to the minimum-velocity ap- in the public right of way (Figures 3.2a, 3.2b, proach,andpipeslargerthanl,050mmshouldbe 3.2c). A simpler cleanout could be substituted made steeper to maintain self cleansing. In Bra- (Figure 3.3). zil, for design of simplified sewers, the following equation is used: In certain areas of Sao Paulo, where the risk of 3 obstruction is believed to be high (e.g., in com- -c I. = 0.0055 Q.04mercial establishments), baffled boxes have been q added downstream of each building sewer (Fig- where I, is the minimum slope of the sewer and ures 3.4a, 3.4b), in addition to the connection or Q. is the initial flow in I/s (current flow). For inspection box. Baffled boxes are usually 60 cm derivation and use of this equation, refer to x 60 cm x 80 cm concrete boxes with underflow Annex 2; to compare the advantages of this baffle located approximately 60 cm from the method over the conventional minimum veloc- inlet. Their purpose is to prevent trash and other ity method, see Annex 3. large settleable solids from entering the sewer. Their maintenance is usually the responsibility 3.3 Service Connection of the homeowner. In the simplified design, a 60-cm square or circu- 34 of lar connection (or inspection) box is placed be- Depth Sewers tween the building and the service line. All At the starting point of laterals the minimum sewers or drains from the house or building enter depth at which pipes are laid should suffice to the box. It is usually located under the sidewalk (a) make house connections and (b) have a layer - Figure 3.2a. Connection-inspection box (Sao Paulo State, Brazil) 7. Although the most commonly recommended minimum diameter for conventional systems in the United.States is 200 mm, a number of states and other countries such as France and the United Kingdom have adopted the minimum size of 150 mm. However, if there is evidence that the smaller pipes would add to operational problems, it would be more economical to install the next-larger size and avoid repetitive servicing and user dissatisfaction. 9 2 i; ll~~~~~~K CL 7 Figure 3.2b. Interior of connection-inspection box (Sao Paulo State, Brazil) 10 Figure 3.2c. Interior of connection-inspection box (Sao Paulo State, Brazil) 00 0 7~~~~~~~~~~~~~~~~~~ Figure 3.3. Inspection-cleanout (SABESP) 4-Baftled Do. Collector Main nection Box Sewer Utility NEProperty Owner Figure 3.4a. Typical residential connection X1 E Figure 3.4b. Typical baffled box (Sao Paulo State, Brazil) of soil over the crown to protect the pipe against mains are too low for connections by gravity, it is structural damage from external loads and frost. the responsibility of the property owner to find In conventional design, there is no one method other means of making a connection. In cases for determining the minimum depth of sewer as where topography permits, it may be necessary long it satisfies the above criteria. However, to use a longer building sewer to be able to some rules of thumb suggest that (1) the top of connect to aservice line,,provided easements can the sanitary sewer should not be less than I m be obtained from the neighboring owners (Fig- below the basement and (2) where there is no ure 3.6). In this context, it is noteworthy that the basement, the invert of the sanitary sewer should Brazilian Code disallows direct connection of not be less than 1.8 m below the top of the fixtures installed below the street level. foundation. In the simplified system, typical minimurn sewer 3 .5 Ma nh o les an d depths are much shallower: 0.65 m below side- Othe r walks, 0.95-1.50 m below residential streets (de- pending on the distance from the street centerline Appitn n e and the amount of traffic), and 2.5 m below State,eBrazil heavily traveled streetso (Figure 3.5). Building Manholes are an expensive component. They elevationsarenotconsideredinsettingtheinvert are now among the most familiar features of a elevation of the sewers. If buildings along the sewer system, but they were not widely used in 8. The determination of the minimum depth should still be made with regard to live and impact loads, pipe material (3-edge bearing strength), and bedding class (ASCE/vECF 1982). The minimum deprh of sewers proposed for the modified system (0.9 m cover) would be ample for a 600-mm extra-strength claf pipe (ASTM C 700) wirh 4,400 Ib/ ft three-edge bearing strength under the 10.000-lb weight of a truck wheel and with saturated topsoil backfill and class-C bedding (compacted granular bedding). Therefore, dhe depth of the sewer would not be dictated by a predetermined criterion belwy the pipe, bedding, and backfill types. Project designers could also look into the cost effectiveness of deeper excavations compared to 3he use of higher-strength materials in shallower trencaes. 1 2 early sewers. They came into wide use with combined systems where } n Sldewalk they facilitated removal of grit. The criteria for manhole use have gradu- ally become more conservative and have contributed significantly to the n high cost of sewerage. The cost of manholes is a function of depth, spac- Mal., Ad.d. ing, and strength of design. The use Figure 3.5. Minimum depths of sewers (SABESP) of shallower depths is one way to reduce these costs. Early sewer systems used simple ARE appurtenances such as lampholes. Some variations of these earlier ER LN systems are being reinrroduced STREET in Brazil, for example, the inspec- SEWER LINE . UILDING SEWER tion tube (Figure 3.7) and the terminal cleanout (Figure 3.8). Figure 3.6. Schematic of cross-block connection The former is similar to the old lamphole, and the latter replaces manholes at the upstream ends ofsewer lines. The present requirement of placing manholes about 100 m apart was intro- duced when sewers were cleaned with CO>NR\T rods and canes. The use of modern = cleaningequipmentcalls for a reviewof - manhole location and spacing guide- Q 7 A I/SAN lines. ----- - BEDBING In conventional systems, manholes are generally located at (i) the upper ends of all laterals, (ii) changes in direction and slope, (iii) pipe junctions, except building connections, and (iv) at inter- vals not greater than lOO m for pipes up to 600 mm diameter, and at less than 120 m for sewers between 700 mm and 1,200 mm diameter. In the United Kingdom the dis- tance between manholes has been changed from 110 m to 180 m (Escritt and Haworth Y /B -' :-'X;/ 1:.7F- K\ 1984);however,fortheCairosewerageproject N in the late 1970s as little as 35 m between - ltl-Xi _ -____ manholes was proposed for sewers less than . . . . 250 mm (Taylor & Sons, Binnie and Partners CR 1977). CONCRETE SAND BEDDING In light of experience in Brazil, the simplified system is designed with these guidelines: Figure 3.8. Terminal cleanout (SABESP) 13 Et Where possible, conventional manholes arereplacedwith "simplified" manholes, _ _ _ . sm E cleanouts, or buried boxes, and man- '0 Simplified manholes (Figures 3.9a, 3.9b) o are similar to conventional manholes, _ A- but they are reduced in size from 1.5 m to 0.6-0.9 m diameter. The need for maintenance personnel to enter the manholes is eliminated by the shallower depths and the availability of modern 1.5 hydraulic cleaning equipment; for small CONVENTIONAL SIMPLIFIED -r sewers, and where infiltration is not a major concern, manholes can be built Figure 3.9a. Comparison of conventional and simplified major concern, manholes can be built manholes with precast concrete pipes or concrete z rings with precast slabs and bottoms. cially since up to 90 percent of manholes are Et Manholes at changes of direction or slope are never opened. In 1881 Waring wrote, "It seems replaced by simple underground boxes or cham- to me decidedly advantageous to use inspection bers (Figures 3.1Oa, 3.1Ob); pipes, or even lampholes on 6-inch and 8-inch sewers, rather than build manholes and inspec- House connections are adjusted to serve as in- tion chambers" (USEPA 1986). spection devices aswell; a small box is builtunder the walkway and connected to the sewer with a There are situations, however, where manholes curve of 45 degrees and a "Y" (the cleaning rod should not be eliminated: (i) very deep sewers is introduced through this box). (more than 3 m), (ii) slopes smaller than required, (iii) sewers with drops, and (iv) points of con- These guidelines for the design of manholes nections from certain commercial and industrial considerably lower the costs of the system, espe- establishments, i.e., points of sampling and flow 14 Figure 3. 9b. Interior of simplified manhole (Sao Paulo State, Brazil) measurements. Guidelinesformanholereplace- durability and resistance to corrosion. These ment are summarized in Table 1. pipes are, however, especially suitable when the water table is low. On the other hand, PVC pipes offer the advantage of longer sizes, fewer joints 3.6 MA ateri a s (i.e., less infiltration), light weight, watertight- The types of pipe materials used in simplified ness, and uniformity. o 0 sewerage are similar to those used in conven- tional sewers. Those most frequently used in Mortar is commonly used for vitrified clay pipe Brazil for simplified sewerage are vitrified clay, joints. However, SANEPAR also uses okum and asbestos cement, and polyvinyl chloride (PVC) asphalt. Rubber gasketjoints arecommonlyused pipes. The vitrified clay pipes, generally 90 cm with plastic and fiber concrete pipe. long, are considered ideal for sewers due to their Plan View Section A-A 0 A s B 0 00-) (m) | n() X A () | IS0 O.f0 0.23 0.53 0.18 200 0.00 030 0.00 023 230 0.75 0.38 0.60 0.30 300 0.90 0.45 0.7S 0.36 375 1.10 0.56 0.0S 0.43 400 [ 130 0.69 0.0 0.31 Figure 3. 1 Oa. Buried box for change in direction (SABESP) 15 .:.~~~~~~~~.t k: a; - :: - :- Co. E0 Concrete Blocks CZ PlnConcete Slab vB Section t-B B v~~~~~~~~~~~~~~~~~~~~uv z Prohnle A-A A A Plan Figure 3.1 Oh. Buried box for change in slope (SABESP) \ - | ~Table 1: Some Simplified Alternatives to Conventional Manholes Situation Starting point of a sewer Solution Inspection & cleaning terminal Long straight sewer Intermediate inspection tube Horizontal curve of 90 Two separate 45-degree degrees curves Service connection Y branch and one 45-degree curve Change of diameter Underground concrete box | ~~~Change of slope Underground concrete box 16 4. Operational experience 0 4 1 0 e rati o n a I past history oftrouble areas, which gives mainte- 4.1* OPeationa nance crews some guidance where and how often ProblemI preventive maintenance should be performed and the type of maintenance that would be Simplified sewerage systems were first adopted effective. Therequirements forpreventivemain- in Brazil in early 1980s (Sao Paulo9 and Parana) tenance are similar to those of a conventional and have subsequently been applied in other system. Minimum maintenance includes clean- parts of Latin America. Although specific data on ing, flushing, repairs, and supervision of connec- operational problems are not readily available, it tions and disconnections (WPCF 1985). To be is known that no significant problems have been effective, the program should, at the very least, reported. In Sao Paulo, it has been estimated that include: there are about 75 obstructions per 1,000 km of sewers each month. (Further data collection in determination of the types of problems and this area is under consideration.) The infrequent trouble areas with a closed circuit camera, by occurrence of obstruction supports the strategy visual surface, or by direct inspection, and by of minimizing the number of manholes. Engi- keeping an information data base; neers in SABESP reckon that it would be eco- nomical to install only a few manholes initially prompt removal of any accumulation of foreign and install additional ones as needed (i.e., at material; and points of frequent obstructions). ' occasional flushing of the sewer lines. Similarly, field surveys have reported no prob- lems related to excess hydrogen sulfide genera- 4.3 E u i me nt tion. Although no measurement or monitoring * Equp has been indicated, the designer should calculate There are different types of cleaning equipment the potential for sulfide concentration in a new and methods an agency could select depending system. on budget, facilities, and the experience of the staff. A survey of simplified systems in Brazil 4.2 Mai ntenance shows that the cleaning devices most commonly used are rodding machines and flushing equip- Requ i rements ment; use of the latter is increasing rapidly. Preventive maintenance consists ofinspection of the system and analysis ofexisting data regarding 9. As of 1988, in Sao Paulo State alone this technology has been adopted in 26 cities. Plans to adopt it are being made for at least 36 other cities. 1 7 5. Costs 0 Table 2 summarizes cost information on some of 5.1 Capital Costs the systems reviewed for this paper. The cost per Simplified sewers have been shown to cost sig- person varies between $51 and $151. nificantlyless than conventional systems. In many places, cost savings of from 20 percent to 50 The total savings that these modifications gener- percent have been reported. In the State of Sao ate will depend on the number of modifications Paulo, Brazil, the first projects have shown a that are deemed feasible in a particular project, construction cost reduction of 30 percent; but given factors such as population density, topog- after about 8 years of experience, the cost reduc- raphy, soil and water conditions, etc. For ex- tions are estimated to more closely approximate ample, in a sensitivity analysis of costs of differ- 40percent.Thecostreductioninsewagecollect- ent design choices carried out in Egypt, savings ing systems in the city of Sao Paulo is reported to of up to 23 percent were shown to be achievable be 35 percent. (Table 3). In another project in Bogota, Colom- bia, itwasestimated thatthe costsavingwould be SABESP estimates the following average con- about 50 percent. Annex 4 shows a breakdown of struction costs (1988 prices) for small towns (not cost savings. including the per capita costs of treatment and house connection, which are approximately $40 5.2 O peration and and $50, respectively): Conventional systems $150-$300/capita NMaintenance Costs Simplified systems $80-$150/capita No cost data on operation and maintenance have been made available from this survey. It may be Table 2: Costs of Selected Simplified Sewerage Projects ------ Sao Paulo state ------ Parana state Sao Paulo Cardosa Coraodos Toledo Total cost of $1,897,000 $48,125 $68,194 $3,762,066 collection system Population served 13,200 950 780 65,500 Average cost per $76 $13 $8 $21 meter of sewer Average cost per $151 $51 $87 $59 person __i 1 9 difficult to separate operating costs for the sim- pendently operated and maintained by the mu- plified system from the overall operation and nicipalities under the umbrella of SABESP. It is E maintenancecostofalargeutilitycompanysuch important to obtain this information to make as SABESP, but it should be possible to obtain meaningful cost comparisons between simpli- &L this information from the systems that are inde- fied sewerage and other alternatives. C c ~~~~Table 3. Sensitivity Analysis of Costs of Individual Design Variations in Two Egyptian Towns 0 (Figures are percentages of the total cost of alternative A.) z z Beni Suef Kafr el Shokr A Conventional standard 100.0 100.0 B Houses connected to sewer lines (instead of 92.4 90.3 manholes) C Manhole spacing 50% greaterer than conventional 97.8 98.0 D Lighter manhole covers (80 kg and 175 kg instead of 96.1 96.1 285 kg) E No manhole at upstream end of branch NA NA B + C 86.9 83.0 C+D 93.9 94.6 B+C+D 83.2 80.1 B+C+D+E 77.3 76.3 Source: Gakenheimer and Brando 1984 20 6. Discussion 0 Ln 6.1 Risk Estimation 6.2 Flexibility The present conventional engineering practice Despitethefactthatmost(if notall)ofthecriteria in sewer design was introduced more than a discussed in this report have been integrated in century ago and has changed little since. More the Braziliancode, flexibilityin theuseofcriteria thana decadeago, engineersin Braziltookaclose is unavoidable. In fact, the basic pillar of the look at the rationale for design criteria and found philosophy behind the revision of the conven- ample room for change and simplification with- tional standards has been the strategy of select- out jeopardizing the operational integrity and ing standards to fit existing conditions. Since the safety of the system. resources needed to provide sanitation services are huge, even a small percentage reduction in Itis common knowledge thatengineering design the total cost translates into large savings. More- is not conceived exclusively on the basis of rigid, over, the use of the simplified sewerage design exact scientific facts; it is rather heavily based on approach does not necessarily require the use of empirical data supplemented with probability all the modified criteria: the designer is called to and risk criteria. The safety coefficients embed- make professional judgements about specific ded in many design criteria (design flow, mini- standardsthatcouldbeusedundergivencircum- mum diameter, depth of sewers, etc.) should not stances. be uniformly applied in all situations. For ex- ample, there is no valid reason to apply the same 6 ApplIcabIlt conservative standards in business districts, where 6. App I i cab i I ity breakdowns and repairs could cause heavy eco- Thesimplifiedseweragesystemswerefirstimple- nomic loss and great inconvenience, as in the mented in Brazil (Sao Paulo state"0 and Parana outskirts, where the impact of a malfunction is State), and later applied in Bolivia (Cochabamba more limited. and Oruro), and Colombia (Bogota and Cartegena). New projects using this approach in In addition to economic aspects, the probability design are being considered for the towns of of breakdowns should be a prime consideration Chilayo, Peru, San Bernandino, Paraguay, and in design of a sewerage system. While Kumasi, Ghana. Gakenheimer and Brando (1983) suggest addi- tional research on uncertainty as it relates to The simplified sewerage system differs from a infrastructure standards, they argue that there is conventional system only in the standards ap- enough evidence to move away from the strin- plied in the design. Since most ofthe major cities gent standards prevalent in industrialized coun- in Brazil already had conventional sewerage in tries. They contend that "when resource-limited central districts prior to the introduction of sim- countries are using conservative standards, risk is plified sewerage, the current experience with lowered in one locality at the cost of fully expos- this new system has been mainly in the periurban ing another." areas and secondary towns. Since most of the modified standards are based on sound analysis, it would be safe to assume that they could be 10. As of 1988, in Sao Paulo State simplified sewerage systems were implemented in about 30 locations and were being planned for at least 36 more. 2 1 applied in any design, recognizing their limita- A parallel field evaluation of a simplified sewer tions which are generally obvious. system and a comparable conventional system. E The purpose of this study would be to monitor, o 6.4L Requirements evaluate, and compare directly the operational CL problems of both systems, initially for a six- o Since simplified sewerage is not fundamentally month observation period with possibility of differentfrom the conventionalsewerage except continuing over a longer period. in the levels or values of design criteria, the institutional requirements for their use are also 'r The long-term problems of the systems, for generally similar. In the case of the two water example, corrosion, generation of sulfides and companies using this system that were visited in methane, etc. Since some of these systems have Brazil during this work, no changes had been been in place for periods rangingfrom under one -C adopted in their procedures service provision. year to eight years, it is recommended that stud- O ies be carried out to identify levels of sulfides at c.. However, even though the anecdotal evidence selected locations in the system. z does not indicate any increase in operational problems in the areas covered in this report, one More complete information on cost variation cannot discount an intuitive tendency among between different types of simplified systems, sector professionals to anticipate additional op- and costeomparisons with conventional systems. erational requirements for simplified sewerage. In particular, difference in operation and mainte- Where this is the case, a strengthening of the nance costs are needed for complete cost com- operation and maintenance capability of the water parisons. company should be given upfront consideration. Survey of how current design criteria vary among 6.5 Additional W ork industrialized countries (and within certain coun- tries, forexample, the various states ofthe United This new approach for designing sewer systems States); a similar survey (in the United States was introduced in the early 1980s and is consid- only) conducted in 1942 showed a large variation ered an "infant" technology. This review, which in the design criteria used in different parts of the was based on a small number of projects, is one of country (Boston SocietyofCivil Engineers 1942). the first attempts to document and disseminate the experience. To increase confidence in the Field measurementofflowvariations indifferent technology, additionalresearch iswarranted. The parts of a city. Most designs have relied on peak following are suggestions for further work: factors determinedindevelopedcountries. These factors may be excessive and could be modified for use in different developing countries. 22 7. Conclusions 3 7 CD A concerted effort to review, adopt, and dissemi- the new design approach does not create a sub- nate these modified criteria would be a timely standard level of service; it rationalizes design initiative, given the tremendous needs of devel- standards without sacrificing quality or lowering oping countries for sanitation services and the the level of service; potential for large savings. simplified sewerage systems cost a fraction of This paper has presented information on simpli- what conventional systems cost and therefore fied sewerage, a design strategy based primarily make funds available to extend service coverage on experience from Brazil that offers a new cost- to larger populations; and saving approach to the design of sewer systems. The review has shown that the system is the the cost of simplified sewerage can be reduced equal of conventional sewerage in effectiveness. further through use of community-participation Itisbasedmainlyoncost-savingrationalchanges methods of service provision as applied in in long-standing traditional sewer design stan- condominial sewerage in Brazil or as used in the dards. The review shows that: Orangi project in Pakistan. simplified sewerage technology is being applied Little is known about the system outside Brazil. successfully, and it is a viable, lower-cost alterna- Engineers in other parts of the world will become tive to conventional systems; more familiar with simplified sewerage as expe- rience is accumulated and reported. A growing design modifications introduced in simplified number of cities are discovering that the simpli- sewerage systems are based on sound engineer- fied system is attractive, and they are achieving ing principles; considerable cost savings by making use of it. 23 Annex 1: Design Peak Factor O CD In conventional design, the peak factor is deter- Although SANEPARrecommends usingthefac- mined from curves developed from data gath- tors mentioned in the main text of this report, ered in industrialized countries. Although it is Freitas (1989) has used SANEPAR data to pro- usually recommended to generate local data to pose a set of equations (derived from fitting estimate this factor, these curves are commonly curves to a set of six data points) for the calcula- used. In simplified sewer design, emphasis is put tions of these factors: on estimating the peak factor for the city under considerationfromflowmeasurementrecords.If K2 = -10.848 + 19.656 K1 - 7.801 K,2 records are not available, efforts should be spent togenerate them quicklyto avoid overdesigning K, = [-19.651 + (47.653 -31.205 K2)1/2)/ -15.603 the system. The peak factor will depend on a number of elements such as the contribution of According to this study, which also draws on the commercial, industrial, and institutional otherreports,insmallcommunities(underlO,000) wastewater, and the social and economic make- with commercial and institutional users, K, is up of the area under design. between 1.0 and 1.1. This gives K, values be- tween 1.0 and 1.3. 25 Annex 2: Tractive Force Thetractiveforcemethodisadesignprocessthat Q - 7.68710-8 - (1/n) * (T8/3) * `136 (6) 3 is widely used in the design of open channels. r 0 Like the minimum velocity design methodol- Assuming n = 0.013 and X 0.1 kg/m2 and ogy, it is based on the concept of "threshold of expressing Q in lI/s, equation (6) can be solved for rD movement" and makes use of the minimum the minimum slope CD force required to move a certain size of settled particle. The resistance equation is given by I= 0.0054 Q-0.462 (7) F = FRI (1) (The equation proposed in Machado [19851 is I = 0.0055 Q047; the observed differences are where X is the boundary shear stress, r is the probably due to rounding off.) specificweightofwater, R is the hydraulic radius, and I is the slope of the conduit. The minimum To complete the design, the following procedure designslopeisderivedbyincorporatingequation is suggested, which is similar to the one pre- (1) into Manning's equation: sented by Yao (1974): Q = (1/n) * A * R2/3 * I'll (2) 1. Solve equation (7) forI.,, using the initial flow, Q; 2. Compute Qi/ I' where Qf is the flow at the end and solving for the minimum slope with the of the design period, m3/s. assumption that the depth of the minimum flow 3. Find the value of Qf/I.11 in Table A.1 where d/D is two tenths of the diameter; the hydraulic is closest to and preferably less than 0.75 (d/D is elements for this condition are derived from the ratio of depth of flow to the pipe diameter). geometric relationships in Figure A. 1 Select the corresponding pipe diameter D as the minimum size of the sewer pipe. cos 012 = 1 - 2d/D 4. Compute the final flow velocity, Vf, from the corresponding value of V/I1`5 given in the table. therefore, for d/D = 0.2, 0 = 106.26. The corre- Check if Vf is less than 5 m/s. sponding cross-sectional flow area is 5. Compute the critical velocity V = 6 (gR)°.s where g is the acceleration of gravity and R is the A - D2/4(7iO/360 - sinO/2) hydraulic radius; to ensure adequate ventilation, check if Vf is less than V.; if not go back to step 3 A = 0.1118 DI (3) and select a new diameter which corresponds to a value of d/D closest to 0.5 instead of 0.75. and the hydraulic radius R = (D/4)(1-360sinO/itO) R=0.1206D (4) Inserting equation (4) in equation (1), the diam- o/ eter D can also be expressed as _- _ D =/0. 1206FI (5) Inserting (3) and (4) into (2), and then replacing D by its equivalent given in (5), with r = 1,000 kg/m3, Manning's equation becomes Figure Al. Elements of Circular Conduit 27 co UNDP-World Bank Water & Sanitation Program DIAMETER (m) d/D 0.100 0.150 0.200 0.250 0.300 0.375 0.400 0.450 0.500 Wll ^ 0.5 Q/l -'0.5 Wll-^0.5 Q/l-^0.5 Wll ^ 0.5 Qll ^-0.! W/l-0^.5 Qll ^-0.' Wll ^^.5 Qll ^-0.! Vll ^-0.5 Q/l - 0.!W Vll ^ ^0.Q/1 ^^.! Wll^-0.56 Qll^^O.S Vll^-0.5 Qll -^0.5_ 0.025 1.0733 0.0001 1.4064 0.0002 1.7037 0.0004 1.9770 0.0006 2.2325 0.0011 2.5905 0.0019 2.7044 0.0023 2.9253 0.0031 3.1381 0.0041 -q 0.050 1.6902 0.0002 2.2147 0.0007 2.6829 0.0016 3.1132 0.0029 3.5155 0.0046 4.0793 0.0084 4.2586 0.0100 4.6064 0.0137 4.9416 0.0181 0.075 2.1968 0.0006 2.8785 0.0017 3.4870 0.0037 4.0463 0.0068 4.5892 0.0110 5.3020 0.0200 5.5350 0.0237 5.9871 0.0324 6.4227 0.0430 CD 0.100 2.6392 0.0011 3.4583 0.0032 4.1893 0.0068 4.8612 0.0124 5.4894 0.0202 6.3698 0.0366 6.6498 0.0435 7.1930 0.0595 7.7163 0.0789 > 0.125 3.0368 0.0017 3.9792 0.0051 4.8204 0.0109 5.5935 0.0198 6.3163 0.0322 7.3293 0.0584 7.6515 0.0694 8.2765 0.0950 8.8787 0.1258 0.150 3.3999 0.0025 4.4550 0.0074 5.3988 0.0159 6.2623 0.0289 7.0716 0.0470 8.2057 0.0852 8.5684 0.1013 9.2661 0.1386 9.9403 0.18368 0.175 3.7350 0.0034 4.8941 0.0102 5.9286 0.0219 6.8795 0.0397 7.7685 0.0645 9.0144 0.1170 9.4107 0.1390 10.1793 0.1903 10.9199 0.2520 0.200 4.0463 0.0045 5.3021 0.0133 6.4229 0.0287 7.4530 0.0521 8.4161 0.0847 9.7659 0.1536 10.1952 0.1824 11.0279 0.2497 11.8303 0.3307 0.225 4.3371 0.0057 5.6830 0.0169 6.8844 0.0364 7.9885 0.0660 9.0208 0.1074 10.4676 0.1947 10.9277 0.2313 11.8203 0.3166 12.6803 0.4193 0.250 4.6095 0.0071 6.0400 0.0209 7.3168 0.0449 8.4902 0.0815 9.5874 0.1325 11.1250 0.2402 11.6141 0.2853 12.5827 0.3906 13.4768 0.5173 _ 0.275 4.8653 0.0085 6.3752 0.0252 7.7228 0.0542 8.9814 0.0983 10.1195 0.1599 11.7425 0.2899 12.2587 0.3443 13.2599 0.4714 14.2247 0.6243 rm 0.300 5.1059 0.0101 6.6904 0.0298 8.1047 0.0842 9.4046 0.1165 10.6199 0.1894 12.3231 0.3434 12.8649 0.4079 13.9156 0.5584 14.9281 0.7396 , D 0.325 5.3324 0.0118 6.9872 0.0348 8.4842 0.0749 9.8217 0.1359 11.0909 0.2209 12.8697 0.4006 13.4355 0.4758 14.5328 0.6514 15.5902 0.8627 9 0.350 5.5456 0.0136 7.2665 0.0401 8.8026 0.0863 10.2144 0.1564 11.5344 0.2543 13.3843 0.4611 13.9727 0.5477 15.1139 0.7498 16.2136 0.9930 _ . D 0.375 5.7462 0.0155 7.5295 0.0458 9.1212 0.0981 10.5840 0.1780 11.9518 0.2894 13.8686 0.5246 14.4783 0.6232 15.6808 0.8531 16.8003 1.1299 9 u 0.400 5.9349 0.0174 7.7768 0.0513 9.4207 0.1105 10.9316 0.2004 12.3443 0.3259 14.3240 0.5909 14.9537 0.7019 16.1751 0.9609 17.3520 1.2726 . O 0.425 6.1122 0.0194 8.0090 0.0573 9.7020 0.1234 11.2580 0.2237 12.7129 0.3638 14.7518 0.6596 15.4003 0.7835 16.8582 1.0726 17.8702 1.4206 un' ( 0.450 6.2783 0.0215 8.2267 0.0634 9.9657 0.1366 11.5640 0.2477 13.0584 0.4029 15.1528 0.7304 15.8189 0.8676 17.1109 1.1877 18.3559 1.5730 _. 0.475 6.4337 0.0237 8.4302 0.0697 10.2123 0.1502 11.8502 0.2723 13.3816 0.4428 15.5277 0,8029 16.2103 0.9537 17.5343 1.3056 18.8101 1.7292 0.500 6.5784 0.0258 8.6200 0.0762 10.4422 0.1640 12.1169 0.2974 13.6827 0.4836 15.8772 0.8768 16.5751 1.0414 17.9290 1.4257 19.2334 1.8882 3 FT 0.525 6.7129 0.0280 8.7961 0.0827 10.6555 0.1780 12.3845 0.3228 13.9823 0.5249 16.2016 0.9516 16.9138 1.1304 18.2953 1.5475 19.6264 2.0494 9 - __ ~~___ _- _.c_ 0.550 6.8370 0.0303 8.9588 0.0892 10.8526 0.1921 12.5932 0.3484 14.2208 0.5865 18.5013 1.0271 17.2267 1.2200 18.6337 1.6701 19.9895 2.2119 , D 0.575 6.9510 0.0325 9.1082 0.0958 11.0338 0.2063 12.8031 0.3740 14.4577 0.6082 16.7784 1.1027 17.5139 1.3098 18.9444 1.7931 20.3227 2.3748 _. 0.600 7.0548 0.0347 9.2442 0.1023 11.1983 0.2204 12.9943 0.3996 14.6735 0.6498 17.0269 1.1781 17.7754 1.3994 19.2273 1.9157 20.6262 2.5372 co 0.625 7.1484 0.0369 9.3668 0.1088 11.3468 0.2344 13.1667 0.4249 14.8682 0.6910 17.2527 1.2528 18.0112 1.4881 19.4823 2.0372 20.8998 2.6981 0.650 7.2316 0.0391 9.4759 0.1152 11.4790 0.2481 13.3200 0.4499 15.0413 0.7316 17.4536 1.3264 18.2209 1.5755 19.7091 2.1569 21.1431 2.8565 Qn eQ 0.675 7.3043 0.0412 9.5711 0.1215 11.5944 0.2616 13.4539 0.4743 15.1925 0.7713 17.6291 1.3984 18.4041 1.6610 19.9073 2.2738 21.3557 3.0115 O 0.700 7.3663 0.0433 9.6523 0.1275 11.6927 0.2747 13.5680 0.4980 15.3214 0.8097 17.7786 1.4681 18.5602 1.7439 20.0762 2.3873 21.5369 3.1618 0.725 7.4171 0.0452 9.7189 0.1334 11.7734 0.2872 13.6616 0.5207 15.4271 0.8467 17.9013 1.5352 18.6883 1.8235 20.2147 2.4964 21.6855 3.3063 0.750 7.4564 0.0471 9.7704 0.1389 11.8357 0.2991 13.7340 0.5424 15.5088 0.8819 17.9961 1.5990 18.7872 1.8993 20.3217 2.6002 21.8003 3.4436 0.775 7.4835 0.0489 9.8059 0.1441 11.8788 0.3103 13.7839 0.5627 15.5652 0.9149 18.0615 1.6589 18.8555 1.9704 20.3956 2.6975 21.8795 3.5725 0.800 7.4976 0.0505 9.8244 0.1489 11.9012 0.3207 13.8099 0.5814 15.5946 0.9454 18.0956 1.7140 18.8911 2.0359 20.4341 2.7872 21.9209 3.6913 Cg 0.825 7.4978 0.0520 9.8246 0.1532 11.9014 0.3299 13.8101 0.5982 15.5948 0.9728 18.0959 1.7637 18.8914 2.0949 20.4344 2.8680 21.9212 3.7983 0.850 7.4824 0.0532 9.8045 0.1570 11.8770 0.3380 13.7819 0.6129 15.5629 0.9966 18.0589 1.8069 18.8527 2.1463 20.3926 2.9382 21.8763 3.8914 m 0.875 7.4496 0.0543 9.7614 0.1601 11.8249 0.3447 13.7214 0.6250 15.4946 1.0162 17.9796 1.8425 18.7700 2.1885 20.3031 2.9961 21.7803 3.9680 0.900 7.3961 0.0551 9.6914 0.1623 11.7401 0.3496 13.6230 0.6339 15.3835 1.0308 17.8507 1.8689 18.6354 2.2199 20.1575 3.0391 21.6241 4.0249 0.925 7.3171 0.0555 9.5879 0.1637 11.6147 0.3525 13.4775 0.6390 15.2192 1.0391 17.6600 1.8840 18.4363 2.2378 19.9422 3.0636 21.3932 4.0574 0.950 7.2032 0.0555 9.4386 0.1637 11.4339 0.3525 13.2676 0.6391 14.9822 1.0392 17.3851 1.8842 18.1493 2.2381 19.6317 3.0639 21.0601 4.0578 0.975 7.0314 0.0549 9.2134 0.1617 11.1611 0.3483 12.9511 0.6315 14.6247 1.0269 16.9703 1.8618 17.7163 2.2115 19.1633 3.0275 20.5576 4.0096 1.000 6.5784 0.0517 8.6200 0.1523 10.4422 0.3280 12.1169 0.5948 13.6827 0.9672 15.8772 1.7536 16.5751 2.0829 17.9290 2.8515 19.2334 3.7765 Annex 3: Examples 0 Comparison of the Minimum Velocity (Conventional) m and the Tractive Force Design Approaches (Simplified) 0 Problem: Design an interceptor sewer line to pipe.ThecorrespondingslopeisO.0016;thetotal carry away the sewage of a town with a current capacityofthispipeis67.41/s;foradischargeratio population of 10,800 people which is expected to of 60/67.4 = 0.89, the corresponding d/D is 0.73 grow to 14,400 in about 20 years. and the velocity is 0.68 m/s (for n constant with depth). At the initial phase, for a discharge ratio Other data: of45/67.4 = 0.67 the d/D ratio will be 0.60 and the corresponding velocity will be 0.65 m/s. - no significant industries or commercial activi- ties The tractive force method: the Brazilian equation. For an initial flow of 45 1/s the mini- - water consumption: 250 Vcap/d mum slope would be Im 0.00093 (using equa- tion 7, Annex 2); Qf/Il in"5 = 1.967; from Table - return coefficient: 80 percent A. 1, select a diameter of 400 mm at y/d of 0.775; the corresponding V/I"' is 18.85 which translates - peak factor: 1.8 into Vf = 0.57 m/s; the initial Qj/I05 = 1.5 which is equivalent to about y/d = 0.63 and the initial - topography: flat., no risk of infiltration velocity would be 0.55 m/s. Initial Flow: 250 * 10,800 * 0.8 * 1.8 = 3,388,000 Table 5 summarizes the results of the various l/d or 45 I/s approaches which suggest that the use oftractive force has reduced the required slopes byabout50 Design Flow: 250 * 14,400 * 0.8 * 1.8 = 5,184,000 percent compared to conventional design, even l/d or 60 1/s though the required pipe diameter has increased from 375 mm to 400 mm; the cost savings from Conventional method: the minimum velocity reduced excavation alone would offset the small approach. From a prepared nomograph (Metcalf cost differential between the two pipe sizes. and Eddy 1981) for the design flow of 60 Vs and Assuming that the line is 1,000 m long and the a minimum velocity of 0.6 m/s, choose a 375 mm trench is 1 m wide, the reduction in excavation volume would be 350 m3. Table A2. Comparison of Design Results for the Two Methods Velocity--- Pipe diameter Slope Initial Final (mm) (m/m) (m/s) (mWs) Conventional 375 0.0016 0.65 0.68 oBrazaiade 400 0.00093 0.55 0.57 29 Annex 4: Cost Comparison Between Conventional and Simplified Design 3 (All costs in Colombian pesos; US$1 Col$335 [Nov 1988]) Conventional --S---- ------iimplifled Unit Quantity Cost % Quantity Cost °/3 Excavation m3 2,038 2,411,449 14.5 721 587,382 3.5 Pipes m 1,530 5,870,110 35.4 1,510 3,471,726 20.9 Manholes ea. 27 2,128,380 12.8 18 1,035,755 6.2 Appurtenances ea. 46 442,698 2.7 Connections ea. 258 6,091,638 36.7 258 2,380,338 14.4 Other 82,656 0.5 74,068 0.4 Total 16,584,233 100.0 7,991,967 48.1 a. Percent of the conventional system total. Source: Angulo (1988) 31 de Baixo Custo: Redes Coletoras de Esgotos." 2 References Instituto de Saneamento Ambiental, Pontificia Universidade Catolica do Parana. American Society of Civil Engineers and Water Gakenheimer, R. and C. H. J. Brando. 1984. 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M. 1974 Sewer line design based on critical o'i shear stress. Journal of the Environmental Engi- a- neeringDivision, American Society of CivilEngineers z (April). Yao, K. M. 1976. Functional design of sanitary sewers,.J. WaterPollution ControlFederation (July). 34 UNDP-World Bank Water and Sanitation Program UNDP-World Bank Water and Sanitation Program The World Bank 1818 H Street. NW Washington. DC 20433 USA United Nations Development Programme One United Nations Plaza New York, NY 10017 USA Regional Water and Sanitation Groups Eastern and Southern Africa c/o The World Bank P.O.Box 30577 Nairobi, Kenya West Africa c/o The World Bank B.P. 1850 Abidjan 01, Cote d'lvoire East Asia and the Pacific c/o The World Bank P.O.Box 324/JKT Jakarta, Indonesia South Asia c/o The World Bank P.O.Box 416 New Delhi 110003, India