|Operation and Maintenance of Water and Sewerage Systems (Ministry of Water - Tanzania - Rwegarulila Water Resources Institute, 1999, 90 p.)|
|C. General Guide Lines in Construction of Water Structure|
1.1 Outline of Intake Structures
Intake is a well type masonry or concrete structure, whose function is provide clam and still water, free from floating matter for water supply schemes. Its main purpose is to provide cals and still water conditions so that comparatively pure water may be conveniently collected from the source. While selecting site for locating the intake, the below mentioned points should be carefully attended to:-
(i) Intake work should provide purer water so that its treatment may become less exhaustive.
(ii) Heavy water currents should not strike the intake directly. This aspect can be achieved by suitably shifting the proposed intake.
(iii) Intake should be located at such a situation where sufficient quantity of water remains available under all the circumstances.
(iv) Site should be well connected by good type of roads.
(v) Site should be such that intake should be in a position to provide more water if required to do so.
(vi) It should not be located in navigation channels, because water of such channels is generally polluted.
(vii) During floods in rivers, flood waters should not be concentrated towards the intake.
(viii) It should not be located on the curve of the river. If there is no alternative then intake should be located on the outer bank and not on the inner bank.
(ix) Intake should be located on up stream side of the town. Water will not be contaminated on this side due to sewage disposal of the city.
In spite of all the effort and precautions, problems may still be there due to natural causes. Temperature, seasonal variations in quantity and quality, wind currents etc. may affect the stability and safety of the intake works.
1.2 Design of Intake
Intake should be designed on the basis of the following considerations:-
(i) Intake should be sufficiently heavy so that it may not start floating due to upthrust of water. Also a heavy intake will not be washed away by heavy water currents.
(ii) All the forces which are expected to work on intake should be carefully analysed and intake should be designed to withstand all these forces.
(iii) The foundation of the intake should be taken sufficiently deep. This will avoid overturning of the structure.
(iv) Intake should not be constructed in a navigation channel as possible. If it has to be constructed it should be protected by cluster of piles all round from forces caused by moving ships and steamers.
(v) Strainers in the form of wire mesh should be provided on all the intake inlets. This will avoid entry of large floating objects and fishes into the intake.
(vi) Intake should be of such size and so located that sufficient quantity of water can be obtained from the intake in all circumstances.
1.3 Types of Intake
(i) River intake
(ii) Canal intake
(iii) Reservoir intake
(iv) Lake intake
Intake work = inlet (+pump for suction and transport)
Requirements inlet constructions:
* protection inlet against damages (navigation, floods)
* raw water quality considerations
- avoid coarse floating materials and fish
- quality at various depth
(swing pipe, closeable openings etc.)
Some intake works
Intake from a river:
Unprotected river intake
Pumped river water intake
River intake station
Intake from a impounded reservoir or lake
Water intake from reservoirs through hinged tubes.
Dam and intake tower for an impounded surface-water supply.
Intake by means of river bed infiltration:
Erosion of the embankment damaging the suction pipes.
Bank river intake using infiltration drains.
Pumping station in embankment water abstraction in the river bed.
2.1 Lay-out of pumping stations
For the lay-out of pumping stations no overall criteria can be formulated, as local conditions will be of great influence. Generally the lay-out will be the result of a design process in which all the foregoing design factors will be taken into account.
In general it can be said that the lay-out of a pumping station is a logic fit of all functions of the stations, with sufficient room to move between machinery for erection and maintenance purposes, but without unnecessary empty spaces nor in a horizontal plane, nor in vertical direction.
In principles, flow lines should be as short as possible and no unnecessary bends should be present in the piping.
Spaces that may be required are:
- pump hall/engine room;
- transformer station including high voltage switch gear;
- low voltage switch gear, switch board, control desk, often combined and stores;
- workshop and stores;
- office, toilet (including water supply);
- (central) heating and/or ventilation
- generator room
All spaces should be well lighted. Also outside lighting may be required. Whether a pumping station will include all these facilities will clearly depend on the importance of the plant and on its location.
The layout should be made in such a way the free movement of travelling cranes and crabs is not hampered by the presence of delivery pipelines or exhaust pipes of diesel engines. In principle the crane should be able to reach every part of the equipment, both for installation and maintenance or overhauling. The equipment will have to be moved to the workshop floor and/or to the entrance doors of the building.
Proper railings are required along stairs or on platform (catwalks) overlooking the engine room.
Flooring of the station should be such that it will not become slippery by oil leaks from equipment or by/grease lost during maintenance work.
Drainage openings should be provided for scrubbing.
The site lay-out of the grounds around the station should take care of sufficient access to the station, both for the erection and possibly removal!) of the mechanical equipment. Supply of fuel should be possible. Proper fencing will be required, especially in the case of automated stations.
If appropriate, sufficient space should be available for future extension of the station.
2.2 Design procedure
In a normal design procedure several steps have to be taken in succession.
In the first step all relevant design criteria should be gathered. In the second step preliminary choices will have to be made as to the number and type of pumps to be used, the types of drives and transmissions and auxiliary equipment. Several of these possibilities will warrant further elaboration and sketches for a building will be made.
After a first evaluation of these first set-ups, one or more solutions will be worked out in more detail, resulting in one or more preliminary designs. These preliminary designs will have to be compared both on technical and on economical merits.
After a final choice has been made, in which also other factors may play a role, the definite design will be made.
On the basis of the definite design specifications can be written. Generally this will be done separately for the mechanical installations and the civil engineering constructions. In larger installations also the mechanical installations can be split up; e.g. separate specifications for electrical installations or specific items like mechanical trash-racks raking equipment, generator sets, etc.
On the basis of the specifications tenders can be invited from manufacturers and contractors. Especially for the mechanical installations specifications will have to be very clear in specifying required quality standards and performance requirements.
Of course, it will be possible to ask for offers on the basis of the design alone, without specifications. However, the great disadvantage of this procedure will be that offers will be received that cannot be compared with respect to quality and performance. This procedure should therefore be discouraged.
After all tenders have been compared, contracts can be made between manufacturers, contractors and the commissioner, purchaser or principal.
After agreement on the contract has been reached (and under the assumption that financing has been arranged) manufacture of pumps can start and construction on the site can be initiated.
During manufacture and construction sufficient supervision will be required on the part of the principal. This supervision will not only deal with the actual construction of the station and the installation of the equipment, but it will often be necessary also to check the production of the mechanical equipment in the various factories.
Detailed drawings of the equipment will also have to be checked on behalf of the principal, before the actual production has started.
Finally, when the station becomes operational tests will have to be performed in order to check the performance of pumps, etc.
In the meantime the organization for the future operation and maintenance of the station must be created, sometime with the aid of the pump manufacturer, and/or a consultant.
The fore-mentioned procedure may be simplified for small pumping stations and/or when standard pumps plus drives are used from a reliable manufacturer.
However, also in this case the contract will have to be specific about quality standards, required performances, etc.
During all these design stages a good co-ordination is required between all disciplines involved in the design process. In principle this should be done by - or on behalf of - the commissioning agency or principal for the project. The coordination involves the timely preparation of drawings, production and shipment of equipment, the construction of the station and the installation of equipment on the site. In this respect the preparation of a time schedule - to which all participants in the construction (including the principal!) - will have to be bound is part of the co-ordination task.
A pump is a mechanical device or machine and is used for lifting the water or any fluid to higher elevations or at higher pressure. The operation of lifting water or any fluid is called pumping. Pumping may be adopted for the following purposes in the water supply scheme.
1. To increase the water pressure at certain points in the distribution system
2. To lift treated water to elevated storage tanks so that it may flow automatically under gravity into distribution system.
3. To lift raw river or lake water to carry it to treatment plant
4. To lift well water to elevated storage tanks.
Classification of pumps
Based on Principle of operation
(a) Air lift pumps
(b) Centrifugal pumps
(c) Displacement pumps
(d) Miscellaneous pumps
Based on type of power required:
(a) Steam engine pumps
(b) Diesel engine pumps
(c) Electrically driven pumps
(d) Atomic power and other sources of power drive pump
Based on type of service or on function
(a) Low lift pumps
(b) High lift pumps
(c) Deep well pumps
(d) Booster pumps
(e) Stand - by pumps
The most importance classification is in which mechanical principles of their working are involved and it is this classification which gives different types of pumps.
Air lift Pumps
In this type of pumps compressed air is used to lift water. These pumps are used principally in well pumping, but they are also used for handling thin sludges in sewage treatment processes.
If a vessel having liquid in it, is rotated about a point centrifugal force will cause the liquid level to rise to a point by . The open impeller type is better, suited to pumping liquid which carry solids such as sewage or muddy water, but the enclosed impeller is generally more efficient in operations and is therefore, more after used in water supply.
(a) Reciprocating pumps
Because the discharge through the pump is not continuous reciprocating pumps may be used only where heads are very high and where the capacity required is great enough to justify the more expansive types.
(b) Rotary pumps
In this type of pump there are two cams or gears which mesh together and rotate in opposite direction. Its capacity depends upon the size and shape of the cams or gears.
(i) Hydraulic ram
(ii) Jet pump
This pump is a centrifugal pump with the driving motor in the well which is fitted just below the impellars.
These are the pumps which can be installed to lift water from shallow wells in small quantities required only for household uses.
Fig. 21a - Selfpriming centrifugal pump
Fig. 22 - Propellor pump
Fig. 23 - Mixed-flow pump
Fig. 24 - Centrifugal pump with double inflow
Fig. 16 - Hydraulic ram
Fig. 17 - Air lift
Fig. 18 - Injection pump (Jet pump)
Fig. 107 - Lay-out of water distribution station
Fig. 108 - Electrically driven distribution station including emergency/peakload generators
General guide lines
The rising main should be as far as possible be laid with an even gradient with a minimum of sharp bends and curves. It should be laid in a trench with a minimum of 0.6m of earth cover, and if brought above ground for any reason should beheld firmly by clamps to concrete blocks. It may be necessary, where pressures are high to anchor the pipe at bends even if under the ground, and again this is usually done by means of concrete blocks.
In the trench the pipe should be supported along its barrel and not on its joints. This means deeper excavation at every joint, sufficient only to have the bottom of the joint not resting on the trench bottom. In rocky trench it is advisable to lay the pipe on prepared bed of sand.
A wash-out valve is inserted in the rising main immediately outside the pump house with a stop valve or non-return valve immediately downstream of it, to prevent the wash water flowing back to the pump. A non-return valve or reflux valve is better than a stop valve as it cannot be inadvertently left closed when the pump starts up again. Unless the rising main is a long one, it will seldom be necessary to incorporate an air-valve and only in exceptional conditions should this be done for a short rising main.
It is also bad practice to take off connections direct from the rising main, and generally the rising main should convey the water direct from the pump to the high-level storage tank without being tapped or having water drawn off in any way. It may be necessary in certain circumstances e.g. where the rising main passes close to an isolated house which is a long distance from the storage tank and which would require a every expensive gravity pipe back from the tank to supply it. In these cases care should be taken that the connection feeds a storage tank with a ball valve and doesnt discharge freely at the end.
The diameter of the rising main should be such as to keep the velocity of the water fairly low, and consequently the head lost in friction. A speed of up to 1.5 m/sec. should not be exceeded, and if it is, a thought should be given to installing a larger diameter pipe, balancing this against lower friction head and small pumping units.
General guide lines
Water treatment involves physical, chemical, bacteriological and biological transformation of raw water into potable quality. The quality of water for use is controlled by world Health Organisations standards or Tanzania temporary standards.
Design of water treatment plant requires the information given below:
- the water demand: the treatment works should be designed for the peak day demand.
- The raw water quality: It is required to decide the type of treatment.
The following methods of water treatment are considered to be suitable for Tanzania.
- Screening or straining
- Plain sedimentation
- Chemical coagulation, flocculation and settling
- Control of algae
- Taste and odour control
- Removal of iron and manganese
- Defluoridation of water
Based on the life span of the different structures the following recommendations are made:-
- Pumps, steel tanks, mechanical and electrical equipment and internal piping have to be designed for the future demand of 15 years.
- All other structures have to be designed for the future demand of 20 years ahead.
Water Quality Standards used in Tanzania
Domestic water supplied to the community should be free from particles and pathogens hazardous to human being and livestock lives, taste, colour and odour should be kept at low limits; to attain the quality; two notable set of standards are used in Tanzania.
The standards are mostly applied to Urban Water Supplies and large rural water supplies. The temporary water quality standards for domestic water supply in Tanzania are applied to small rural water supply.
Name of Constituent
International Standards WHO 1963
Tanzania Standards for Rural W/S-1974
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6.5 - 9.2
6.5 - 9.2
The International Standards of Quality of Domestic water has to types of criteria, these being acceptable and allowable. In the table, only allowable values are shown. The international standards apply to water distributed through water sources or systems serving cities, municipalities and townships - water supplies. Furthermore, they apply to those water systems serving rural population of more than 5000 people - large scale rural water supplies; and lastly all those water systems which have treatment system more complex than simple sedimentation and or rapid filtration appliances.
A gravity main is of course the most preferable in respect of economy in construction, operation and maintenance. The main shall always be of such a size that the total quantity required for the future projected peak day demand is able to flow through the pipe in 24 hours.
The gravity main should be as far as possible on a constant falling gradient, avoiding high points and low valleys. Where the static pressure exceeds the allowable pipe pressures a break pressure tank with ball valve should be installed. Excessive high points should be avoided and at no account should the pipeline be laid higher than the hydraulic gradient (negative pressure).
Where it is unavoidable, for the pipeline to be laid above ground the pipe need to be fixed freely on concrete supports and held in place by metal ring brackets which are set into the concrete support. The pipe need to have free movement within the ring bracket. Pipes above ground can only be of galvanized steel or ductile iron. The pillars should be placed at length of the pipe.
Concrete anchors should be constructed at every 200m on all gradients and at much lesser distances in steep gradients and still lesser distances in steeper gradients. Also anchors should be provided at all horizontal and vertical changes of direction, at all equal tees and at all valves. The concrete should be formed so that it follows the curvature of the pipe. Suitable air valves should be located at all high points fitted with stop valves. Even in flat areas an air valve at every 1000m. is preferable. At all low points wash out arrangements should be positioned. These did not to be the same diameter as the main.
Non return or reflux valves can be located at distance of 3 to 5km. to facilitate maintenance and repair and in addition will help in reducing water hammer. For the purpose of inspection, maintenance and replacements unions or flanged joints should be provided every 350 to 500m in all pipelines.
Pipe should be prepared in well prepared trenches, 0.6m wide by 0.75m. deep, for pipes up to 100mm; and 0.8m wide by 1.0m deep for bigger pipes. The pipe should be laid on a prepared level bed cushion of sand or soil, free of stones under the pipe. Also all the back fill material should be free of stones around the pipe.
Special case should be taken to where mains are crossing roads and tracks, where a minimum of 1m. cover should be provided. It is an advantage when the pipes can be laid inside a bigger diameter steel, D1 or C1 pipe. These covers and protections should be extend 3m. beyond the width of the road at either sides.
The gravity mains are so designed that the available pressure head is just lost in overcoming the frictional resistance to the flow of water. The velocity to be generated are therefore so maintained that they are, neither too small to require a large size diameter pipe; nor too high to cause excessive loss of pressure and head.
Water tanks are provided for the purpose of balancing the constant supply rate from the water source or treatment plant with the fluctuating water demand in the distribution area. The storage volume should be large enough to accommodate the cummulative differences between water supply and demand and in case of breakdowns between source and tanks. If there would be no storage of water in the distribution areas, the source of supply and the treatment plant would have to be able to follow all fluctuations of the water demand of the community served. This is generally not economical and sometimes not even technically feasible. For that purpose, a water tank is provided. It also helps to maintain adequate pressure in the distribution.
Storage tanks may be classified in the following way:-
(i) classification based on the position of the tank. Under this category of classification the tanks may be;
(a) surface storage tanks
(b) elevated storage tanks
(ii) Classification based upon the material of construction.
(a) R.C.C. tanks
(b) Masonry tanks
(c) Concrete tanks
(d) Steel tanks
(iii) Classification based on the shape of the tank.
(a) Circular tanks
(b) Rectangular tanks
(c) Intze tanks
Calculating capacity of the reservoir or tanks.
The total capacity of the reservoir can be divided into three categories. The total of all the three categories determines the storage capacity of the reservoir.
(a) Balancing reserve
Demand of water always keeps on varying hour to hour but, treated water always comes out of treatment plants at a constant rate. Balancing reserve is that quantity of water required to be stored for balancing the variable demand in the distribution system. It is mostly calculated by means of mass curve or hydrographs.
(b) Break-down reserve.
This storage is estimated to be 1½ to 2 hours of average daily supply.
(c) Fire reserve = (F C) T
F = Fire demand
C = Reserve for pumping capacity
T = fire duration (10 hours) and (2-5) hour
After having treated the raw water in water treatment plants, to the required standards, the last and final stage of water supply schemes comprises distribution of water to the consumers. The main purpose of the distribution systems is to develop adequate water pressure at various points i.e. depends upon the topography of the area of distribution and its elevation with respect to the location of the water treatment plants. The distribution system may be classified into three categories.
(i) Gravity system
(ii) Pumping system without storage
(iii) Dual system with storage
Water from storage tanks can be distributed to consumers by arranging pipelines in two ways.
(i) Dead-end System (Branches system)
In this system water is fed into secondary pipes from on side
(a) It is possible to calculate accurately the discharge and pressure at any point in the distribution system. Calculations are simple and easy to do.
(b) This method requires comparatively less number of cut off valves, hence cheap.
(c) Pipe lines can be laid in the street of any pattern which may not be standardised.
(d) The diameter of mains are to be designed for the population they have to serve. This fact may make the system cheap and economical.
(a) During break downs and repairs large areas which are served by this pipe, go without water and thus cause great inconvenience to the public.
(b) In this system there are large number of dead ends, where water does not circulate but remains static, which may get contaminated due to stagnation.
(c) Water available for fire fighting will be limited is being supplied by only one water main.
(ii) Grid-iron System (looped system)
In this system, water is fed into secondary pipes from two sides.
(a) At the time of repair or breakdown only small portion of the distribution layout is affected.
(b) As there are no dead ends and water circulation is free throughout, it is not liable to contamination.
(c) Water reaches all the points with minimum loss of head.
(d) At the time of fires, by manipulating the cut off valve, plenty of water supply may be diverted and concentrated for fire fighting.
(a) Cost of pipe laying is more because relatively more length of pipe is required.
(b) It is difficult to calculate pressures and discharges at various points of distribution systems
(c) More number of valves are required.
8.1 Energy concept
Before starting the design, consideration should be given to the energy concept of a sewerage. The aim of our system is to transport sewage from any part of the town, or area to be sewered, to a predetermined point where it is going to be treated or discharged into the surface water. To transport water from one point to another, energy is needed. This energy is available in the form of potential energy, provided the point the water enters the system is higher in lever than the point where it is discharged. The lost potential energy is equal to the wall friction in the sewer, multiplied by the distance the sewage has traveled. Knowing that the friction is proportional to the square of the velocity, we see that in flat areas we have to be thrifty with energy and design our sewers to operate with the minimum allowable velocity. Nevertheless, if insufficient energy is available, an additional supply could be provided by means of pumping. When more than the minimum requirement of energy is available, we can allow a higher flow velocity. If it occurs that the maximum allowable flow velocity does not utilize all of the available energy, then the surplus has to be dissipated, for instance, by adopting drop manholes. According to these considerations, it can be concluded that, especially in flat areas, the utmost care must be taken not to lose energy unnecessarily. This means that turbulence and considerable fluctuations of flow velocities in the sewer system have to be avoided, manhoes and bends have to be streamlined, no application of drop manholes, overflows have to be designed with a minimum of energy loss for DWF and transitions have to be designed properly.
The first step is to design the layout. It is not possible to follow a fixed procedure. In general, an attempt should be made to follow the natural drainage pattern. It is advisable to work from the overall plan to the details. First, the border lines of the area to be sewered (catchment area), the watersheds and the main valleys have to be marked on a map. Afterwards the sewer lines have to be arranged tentatively to fit the most economical flow pattern and the main sewer line in the area has to be determined. It has to be kept in mind that the shortest way always has to be chosen. Figures 6 give some examples of flow patterns.
Some general rules to follow during the design of the layout are:
(a) No sewer should pass underneath a building
(b) Avoid crossings with railroads, canals, rivers etc.
(c) Manholes have to be plotted at all junctions and changes of slope or direction. The maximum manhole distance should be 35 to 50 meters. Between manholes a sewer has to be straight. From these manholes, the sewer can be inspected to locate obstructions and cleansing device with buckets and steel cable can be passed through them. This is not possible in the case of curved sewers. These have to be used sometimes in winding and narrow streets.
(d) Big accessible sewers may be curved and have manholes at 100 to 200 meter intervals.
(e) The locations of the sewer depends on the type of sewer and the width of the street.
Now, on the layout, the sites for the pumping stations, the storm water overflows and the treatment plants can be indicated.
By having this complete layout, and by knowing the population at the end of the design period, the sewage production and storm water flow and by the whole system can be calculated. Normally this is done with the aid of a computation list. The procedure is as follows:
(a) Mark the border lines of the community or the catchment area of the main sewer in that area, including future extension areas to be connected;
(b) Mark the internal border lines of the tributary areas for every computation section of the sewer lines. This includes the partial tributary areas along the main sewer as well as the tributary areas which are connected through secondary sewers to the main sewer. These areas form the flow pattern for the domestic and industrial waste water as well as the storm water.
Find the surface of each area by planimetry and multiply this value with the runoff coefficient of that particular area (average reduction factor at the end of the design period) to find the surface of the reduced tributary area;
(c) Compute the tributary population by multiplying the surface of the area with the population density;
(d) Find the total average and total maximum dry weather flow (DWF) - domestic flow, industrial flow, including infiltration water;
(e) Estimate the concentration time at the section under consideration, and compute the total storm flow (RWF);
(f) Find the maximum combined flow in case of a combined sewerage system;
(g) Determine the elevation and slope of the invert; choose diameter of sewer and find capacity and flow velocity when flowing full. Minimum sewer sizes are: 0.15m for house connection; 0.20m. for sanitary sewers, 0.30m for storm sewers.
(h) Compute flow data for actual sewage flow and storm flow or sewage flow and combined flow (combined system). Check estimated flow time with actual flow time; correct if necessary.
(i) Design and calculate structures such as overflows, pumping stations, retention basins etc.
In case of a sanitary sewer or a storm sewer has to be designed the same computation list can be employed, by merely deleting the redundant columns. As an example a typical layout schemes of a waste water collection system has been attached below
Fig. 1. Typical lay-out scheme of a waste water collection system
Fig. 2. Combined system
Fig. 3. Separate system
Fig. 4. Pseudo separate or partially separate system
Fig 5. Layout
Fig.6. Patterns of sewerage systems
Perpendicular pattern for storm sewer combined system
Perpendicular pattern with interceptor for combined system
Pattern with transversal main for sanitary system
Pattern for different levels for combined system
Parallel pattern for combined system, possible to flush the system with river water
Fan pattern for sanitary system
Multiple fan pattern (sanitary)
Radial system (sanitary or combined)
8.4 Responsibility for Operation and maintenance
RESPONSIBILITY FOR OPERATION and maintenance is usually divided between the household and one or more institutions. The most important point is that all concerned should know unambiguously their responsibilities and duties as early in the project as possible. It is better for responsibilities to be spelled out in formal documents, signed by all interested parties, than to rely on the spoken work.
Under normal circumstances, the householder is responsible for the operation and maintenance of all infrastructure within the plot boundary and possibly up to the junction of the house sewer and collector sewer.
Institutions are usually responsible for the entire communal network. Where more than one institution is involved (for example where one is responsible for sewage collection and the other for sewage treatment) the most important need is to demarcate clearly areas of responsibility.
There is usually very little choice in the selection of which institutions will be involved, but it is necessary to review them to ensure that they have sufficient skills, equipment and funds to carry out their duties correctly. There may be need for institutional strengthening prior to or during the project. Institutional responsibilities need not be carried out by the institution itself; they can be subcontracted to private companies. Such an approach has the advantage of reducing the size of institution and reducing the cost of maintenance. This approach has worked successfully on large schemes such as Orangi in Pakistan, where the demand for routine maintenance is sufficient to support a number of private companies who compete with each other for business. On schemes where the volume of work is small, privatizing maintenance is unlikely to produce significant benefits.
There may be circumstances where institutions should be responsible for maintenance of components on private property. An obvious example would be the emptying of interceptor tanks.
There are a number of examples of communities being responsible for the maintenance of collector sewers, particularly where they run across household plots. Such responsibilities can be household basis where the plot owner is responsible for the length of sewer crossing or adjacent to his/her plot. Alternatively, a group of plot-owners may work together to maintain the collector sewer that they jointly use. It appears that joint responsibility is more appropriate to systems where the collector sewer runs under public land and individual responsibility works better where the sewer crosses the users plots.
Attempts to make communities responsible for trunk sewers have met with little success. Most communities still feel that the removal of waste from large groups of people should remain the responsibility of institutions.
The stability of the community has a bearing on the level of responsibility it can support. It is more difficult to enforce responsibility for maintenance in a changing population or where the sewers are installed before properties are occupied. In such areas it is more appropriate to keep consumer responsibility to within the plot boundary.
The private sector
The use of the private sector for operation and, particularly, maintenance is often overlooked but has a number of advantages. Competition for work between companies tends to reduce unit costs. The drive for larger profits speeds up work, getting jobs completed faster. The implementing agency may be able to reduce an over-inflated and under-utilized labour force resulting in savings in salaries and management. Contractors are usually less affected by political interference and union demarcations, producing a flexible and swift response of problems.
However, the use of contractors has its draw backs as the drive for profits can lead to the use of untrained staff and the taking of short cuts, both of which can produce unsatisfactory standards of work. Contractors can use their wealth to corrupt the selection process. Contributions to politicians re-election funds and under-paid government officials are not unknown.
Operation and maintenance using contractors must therefore be properly managed. Minimum standards of performance must be laid down and monitored. Contractor selection processes must be open to public scrutiny and seen to be fair and reasonable.
8.5 Supervision Operation and maintenance
All operation and maintenance activities must be supervised to ensure that they are carried out safely, to an approved standard, at a fair cost and within an acceptable time. Good supervision is based on knowledge of what work must be done, why it is necessary, who is responsible for doing it and what measurable standards must be achieved. This requires the supervisors to be well-educated and trained.
Responsibility for supervision usually lies with the implementing agency, which is often a government agency. It is common for day activities to be contracted out to private companies. Whilst this can be much cheaper than using in-house staff, it is important that the responsible agency retains overall management control. It should set and monitor standards and take responsibility for introducing new working practices when required.
Supervision of sewerage that has been implemented, community groups often disband and leave day-to-day maintenance to individual residents. This may be satisfactory for minor problems such as local blockages, but dealing with major problems is difficult. Problems affecting a large number of properties require residents to regroup, collect funds and implement repairs. This all takes time and can create considerable social tensions. Localized maintenance by residents with no overall supervision can produce variable standards of workmanship. Residents with minimal funds available for emergencies will spend as little as possible on sewerage repair. This can lead to the long-term deterioration of the network.
In general, supervision by staff with local knowledge is an advantage. It allows for continuous contact between supervisors, the network and its users. Any problems that occur can be dealt with quickly and without the need for extensive bureaucracy. However, supervision of on-plot construction is one area where the use of local staff can be disadvantage. Personal knowledge of the families constructing the sewers, and community pressure, may cause the supervisor to accept a standard of construction lower than that, which would normally be acceptable.
Inadequate operation and maintenance is the commonest reason for sewerage schemes to fail, and persistent attention is needed to achieve long-term success. Whoever is responsible for carrying out the operation and maintenance of a scheme must be managed by an organization with the skills and legal duty to ensure that it is carried out effectively.
ACHIEVING SUSTAINABLE MAINTENANCE
Photograph 9: Cast iron squatting plate with integral drop pipe, fitted in a toilet block with interceptor tank below. The squatting plate is removed to empty the tank
Photograph 10: A well-stocked plumbing and drainage store in Pakistan, promoting private sector maintenance of the sewer network.
Photograph 11: A low-income housing area in Pakistan with a community constructed collection sewer. The concrete access point covers are poorly fitted, allowing silt and garbage to enter the sewer to cause maintenance problems.
Photograph 12: Specially shaped concrete blocks have been manufactured in Brazil to allow the construction of small diameter access chambers.
8.6 Glossary of terms
Points of entry into the sewer network for observation and maintenance. They are usually large enough to allow the entry of a person or persons (figure A 1.1). They are located at sewer intersections, changes in sewer direction, gradient or size and at spacing dictated by the methods of sewer cleaning being used.
Figure A 1.1 Detail of a typical access chamber for a sewer
1.35 - 3.0m deep
Source: Adapted from WAA (1989)
Access point on the highway boundary
The same as any other property access point located on the property and adjacent to the highway boundary. It is used for observation and maintenance of the sewer and also marks the change of responsibility. Upstream it is the responsibility of the property-owner whilst downstream it is the responsibility of the authority in charge of maintenance of the communal sewerage network.
Pipes connected to the waste outlet to water-using appliances such as WCs and wash-basins after the water-trap. As such they usually run above ground.
Collects effluent from properties and other waste-producing sources (such as industries) and carries them by gravity and in increasing volume to the discharge point. They are commonly constructed under public highways so as to be accessible to properties on both sides of the road and easy to reach for maintenance.
A small underground tank attached to a house sewer. It is designed to collect excess grease and/or sand and prevents them precipitating in the nearby sewer network and causing blockages. (Sand is used for washing pots pans.)
Gully traps are installed where branch drains enter house sewers. They allow the entry of sullage into the house sewers whilst preventing the escape of foul sewer gases.
House sewer (for house drain)
Collect wastes discharged from individual or groups of sanitary fittings via the branch drains inside a building. The house sewer is nearly always below ground.
Underground storage tanks set in the line of the house sewer, usually near the highway boundary, designed to reduce the amount of solids in the sewer network. Small tanks have a liquid volume of around 250 litres and provide 4 to 6 hours retention, while large tanks provide approximately 24 hours storage.
Property access point
A hole over the house sewer which gives access for observation and maintenance. Access points are commonly provided wherever the house sewer changes directions, slope or size and at sewer intersections.
A sewer receiving effluent from a number of collector sewers and carrying sewage in bulk from one point to another. It usually receives very little effluent directly from individual properties.
A vertical pipe extending from the house sewer to a point above the highest opening in the property. The pipe is open at the top and allows the escape of the foul gases produced in the sewer system to be discharged into the atmosphere without causing annoyance to residents.
The ventilation pipe also conveys wastes from upper floors to the house sewer.
Liquid waste of community - waste water.
A pipe or conduit that carries waste water or rainwater.
Process of removing sewage.
The entire waste water collection system (pipes appurtenances, pumping stations, etc.
Sanitary sewage, or house sewage or foul waste water.
Industrial waste water
Trade waste water
Storm water, rainwater or drainage water.
Sewer for domestic sewage.
Rainwater sewer or sewer for storm water
Sewer for domestic sewage and rainwater
Dry Weather Flow-Flow in combined sewer during dry weather periods. (no rain) - domestic industrial waste water + infiltration water (leak).
Rain Water Flow - Flow in storm sewer during rainstorms
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Combined flow =
D.W.F. + R. W. F.