HDT 55: Low Cost Technologies For Sewer Systems
Eng. Roberto Mejia; Antioquia University, Colombia. September 1993.
Spanish version: HDT 55: Tecnologías de bajo costo para sistemas de alcantarillado
One of the major causes of morbidity and mortality in Latin America is the low coverage of waste water disposal and heces services; only 49% of the population has sewer service, 38% disposes their heces through latrines and 13% (60 million Latin Americans) practice outdoor defecation.
This has led to various investigations that have tried to look for simple low-cost solutions that involve the community in planning, designing, construction and operation of the sewer system.
These new options are:
- Sewerage networks without entrained solids.
- Simplified sewerage networks.
- Sewerage system in condominiums.
Sewerage Networks Without Entrained Solids
Sewers without entrained solids is also known as small-diameter sewer or decanted sewage networks. The system was originally conceived in the United States Department of Agriculture, in the early seventies to troubleshoot infiltration problems in areas with little capacity to absorb effluent from septic tanks; this technology has spread worldwide and its first application in Latin America was in 1979 in Brazil.
In the decanted network system for waste water, these are decanted or settle before being taken out networks in order to retain the solid portion; Part flowing liquid collectors.
The process of solids sedimentation is done in septic tanks or interceptor tanks with a single camera and can receive waste water from one or several houses.
- Advantages and disadvantages of the system.
The main advantage is the cost reduction:
- Excavations, due to the absence of solids in the system, it is not necessary to ensure the flow with minimal self-cleansing velocities, which decreases the slope of the collectors.
- Pipes, because small diameter collectors are employed.
- Auxiliary works due to manholes are replaced by simpler structures such as visit boxes and cleaning records and inspection.
- Waste water treatment, because the primary treatment is performed in septic tanks, and it is no longer necessary to project this process in the processing units.
The main system disadvantage is the maintenance of the septic tank which requires periodic evacuation and the disposal of accumulated solids. For this reason, the decanted sewer should only be built when there is an organization that ensures the maintenance.
This organization must perform strict control to prevent illegal connections that may lack of an interceptor tank or have bad connections that allow the entrance of solid or rainwater that could cause serious problems for operation and maintenance.
- System components, Decanted sewer networks consist of the following parts:
- Domestic connections.
- Interceptor tank.
- Records of Inspection and cleaning and visit boxes.
a. Domestic connections This connection is placed at the entrance of the interceptor tank. Through it enters to the system all household waste; rainwater and solid waste, must be excluded. The diameter of these collectors are 75-100 mm.
b. Interceptor tank is septic tank and is an essential component of the system. This tank should be adapted and be constructed easily; must remove floating and settleable solids. It has inlet and outlet pipes, these last are connected to the system through a T and an elbow and may have less diameter than the inlet pipe.
c. Collecting tubes, collectors are small diameter (minimum 50 mm) polyvinyl chloride (PVC) pipes that are buried deep enough to collect the sedimented water. Unlike conventional collectors, they are not necessarily placed on a uniform gradient with a straight alignment between the cleaning and inspection records.
Because not carry solids, this allows the pipe sections work properly even under pressure, with positive or negative slops, as long as the line pressure does not cause backflow into septic tanks connected to the section.
It is not necessary to consider the slope and the minimum and maximum speeds because the liquid is free of solids. Therefore, the tubes can follow the topography, making the maximum energy from the difference in height between upstream and downstream.
The collectors may be placed in green or pedestrian areas, to reduce the risks of live loads due to traffic, thereby decreasing the excavations.
d. Cleaning and inspection records and visit boxes, the cleaning and inspection records and visit boxes allow access to collectors for inspection and maintenance. In many circumstances cleaning records are preferred before the visit boxes because they cost less and can be hermetically sealed; most of the infiltration and sand that commonly enters through the walls and lids of the visit boxes are avoided.
The visit boxes are recommended in the collectors main point, drastic direction changes or in places where it is difficult to build a record, because of a very deep pipeline. See Figures 1 and 2 (Ref 4)
Records of inspection and cleaning must be arranged in the headers of the network, at the intersection of two or more collectors, in very drastic changes of direction, at high points to prevent the accumulation of gases and straight sections every 200 m. See Figures 3 and 4 (Refs. 4 and 1).
- Conditions for installation The decanted sewerage network is a system that is better suited to small communities, peripheral areas of low population density, high water table level, coastal villages, to isolated groups of houses and rural settlements.
- Construction costs represent about a fifth of a conventional sewerage; the system also provides the primary treatment of sewage. In relatively large towns with economic resources the acquisition of mechanical vacuum is possible for cleaning the interceptor tanks. For small communities, cleaning should be performed by users, under the supervision of an experienced entity in this field.
Simplified Sewerage Networks
Simplified sewerage networks consist of a set of pipes and accessories which have the purpose of collect and transport drains for disposal. These differ from conventional sewers on simplification and minimization of materials and in construction criteria.
The main differences with conventional sewers are:
- They are designed from household connections.
- Their excavation depth is reduced. For this reason, the pipes are projected through green or pedestrian areas to avoid vehicular areas that require protection against mechanical impacts . In some cases double networks are projected.
- Its design period is shorter and can be builted in stages.
- Are sized according to per capita consumption and the socio economic conditions of the population.
- Sedimentation in the pipes is controlled, with the concept of drag force, which is more practical than control sedimentation through the the criteria of a nominal minimum speed.
- Requires less manholes and the cost of construction of these structures is reduced.
- Pipes with elastic junctions are used to reduce the infiltration.
- The relative strut (h / d) must be less or equal than 0.8 in order of not increasing the diameter of the pipe and allow the free movement of gases.
- Accepts a minimum diameter of 100 mm.
- Design innovations
a. Drag tension
Drag tension (O) is the unitary tangential force exercised by the liquid on the collector and on the material in the deposited. It is also considered as the drag force divided by the area over which it operates.
The drag force is the tangential component of the weight of the liquid that moves in an inclined plane. To consider the portion of the liquid contained in a section of length L, the following formula applies:
Formula To Determine The Drag Tension.
|F = y * A * L||
|Ft = Y*A*L*sine o||
|o = Y*A*L*sine o/P*L||
|o = Y*Rh*sin o
||Y specific weight, kg/m3
|o = 1000 * Rh*I||I collector slope, m/m|
It is important to note that this hydraulic radius must be the real and not hydraulic radius corresponding to the nominal flow or full pipe.
For avoid sedimentation in the pipes the value of the traction force is 0.15 kg / m2 for the removal of particles up to 2.0 mm diameter.
When designing with the drag force criterion is not desired, sedimentation can be prevented by controlling the actual flow velocity and the nominal speed or not filled tube; This speed can be set to a value greater than 0.3 m/s, since, according to studies conducted in Brazil, with values of 0.3 m/s collectors did not suffered any damage.
b. Inspection and cleaning devices
In conventional sewers is necessary to project inspection pits in starts, slope change, changes in diameter or direction, at the intersection of two or more collectors and straight sections with lengths greater than 100 m. These pit models are similar for each of these conditions, while the simplified sewerage use different models for each.
The proposed solutions are:
Dispositivos de inspección y limpieza
|Conventional sewer||Alternative solution|
|Initial section or start.||Terminal de inspección.|
|Slope change or diameter.||Visit box.|
|S Long straight collector.||Vertical inspection pipe.|
|Collectors intersections.||Visit box.|
The structures of the proposed alternative are simpler and cheaper; with the new mechanical sewer cleaning equipment is not necessary for a worker go down a manhole for inspection. It is necessary that all sewerage projects includes a description of the equipment with its technical specifications in order to perform maintenance.
Construction costs of simplified sewerage networks are 30% or 40% less than the cost of a conventional sewer without including cost savings in pumping installations and waste water treatment.
Sewerage System In Condominiums
This system comprises sewerage networks in condos within a block; In other words, networks in condominiums are projected by the courtyards of the houses, in order to reduce the maximum length of internal networks (inside the house) and external.
The basic collection system is designed as a simplified sewerage network. The derivation in condominiums within each block consist of a surperficial pipe í 100 mm and usually with a minimum slope greater than 1%.
As the sewage in condominiums within a block is built over successive private properties must first obtain the consent of the owners. It is therefore essential to carry out health education programs, personal hygiene and community participation in order to promote the project, explain the system, convince stakeholders and ensure community participation in the construction, maintenance and operation of the system.
- Easy construction and lower cost of household connections.
- Minor extension of the main collectors.
- Greater community involvement.
- Low cost of construction and operation of the entire system.
- Installation depends on the architectural layout of the houses; health services should be located on the back of these and must have free zones to extend networks.
- There may be legal issues as the managing entity must have legal authorization to inspect and repair the system; also owners will not be allowed to build upon the pipes.
- In other cases, may be prohibited to the drain of a property being discharged in lands of a neighbor.
- Some users may make improper use of the connection, by downloading solid waste or rainwater that would cause serious damage to the system.
These new sewer technologies are practical, simple and inexpensive. Should be implemented to increase the coverage of sewerage and thus reduce the morbidity in water borne diseases.
This new technology should be promoted together with the use of low discharge toilets (4 or 6 liters), showers and mixers with low consumption, which is considerably reduced per capita consumption of water.
(Documents available at the CEPIS libary)
- Otis, Richard J.: Mara, Duncan D. Diseño de alcantarillado de pequeño diámetro. Washington, D.C., World Bank, 1985. (TAG Nota Técnica, 14).
- Rizo Pombo, J.E. Asas; una nueva solución de saneamiento. Acodal, 28 (124):441-68, Mayo -Ago. 1985.
- Guimaraes, Augusto Sergio Pinto. Alternativas tecnologicas de baixo custo. Trabajos presentados al Seminario Regional de Investigación sobre Alternativas de Tecnología de Saneamiento de Bajo Costo para Zonas Urbano Marginadas. Lima, 1985.
- .Brasil. Ministerio de Desenvolvimento Urbano e Meio Ambiente. Redes de alcantarillado simplificado (RAS). Lima, CEPIS, 1987. Manual Técnico, 1.
- Vines, Marus; Reed, Bob. Low-cost unconventional sewerage. Waterlines: Journal of Appropriate Water Supply and Sanitation Technologies; 9(1):26-29, jul. 1990.
- Azevedo Netto, Jose Martiniano de. Esgotamento sanitario; soluçoes apropriadas. Sao Paulo, s.n., 1986.
Note: – This Technical Disclosure Sheet has been prepared by Eng. Roberto Mejia, from the Antioquia University (Colombia), during his internship at CEPIS as a participant in the Continuing Education Program for Latin American University Teachers GTZ / OPS / CEPIS Convention. Technical review of Eng. Ricardo Torres, from CORPOSANA, Asunción, Paraguay is appreciated.