# Hydraulic Design Of Low Cost Combined Sewerage Biology Essay

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The design of low-cost combined sewerage is based on a minimum tractive tension also known as boundary shear stress which is achieved at peak flow. This is stress experienced by settleable solids within the sewer due to the flow of the sewage which if large enough, keeps the solids in suspension and so prevents any deposition that may lead to sewer blockage (Mara, 1996 and Mara and Guimarães, 1999). It is also possible based on the flow of sewage that achieves a minimum self cleansing velocity at peak flow. However the minimum tractive tension design approach is more economical than the minimum self-cleansing velocity design approach because it results in gradients which are significantly lower, hence reduced excavation and thus resultantly reduction in cost (Mara 1996 and Mara et al 2001).

The flow in low-cost combined sewers is open channel flow and Manning equation is normally used (Guimarães and de Souza, 2004).

Figure 3.1: Definition of parameters for open channel flow in circular sewer

From figure 3.1 geometric relationships can be derived for the area of flow , the wetted perimeter and the hydraulic radius

The angle of flow Î¸ (in radians) subtended at the centre of the sewer by the water surface (as shown on figure 3.1), is given by the following equation

Where y is the depth of flow in m,

D is pipe diameter in m. The ratio y/D is termed the proportional depth of flow.

From the same figure, the area of flow and the wetted perimeter are related to an angle of flow and the diameter of the pipe section by the following equation

and

The hydraulic radius r is the ratio of the area of flow to the wetted perimeter

From equation (3.2 and 3.3)

The following equations are also used

Where, from equations 3.2 and 3.4 coefficients and can be given by

and

The relationship of proportional of depth of flow (y/D) and factorsand are presented in appendix 1

Manning equation for the velocity of flow has been found to be sufficiently accurate and simpler for the design of low cost combined sewerage. The velocity of flow at is related to the slope and hydraulic radius by the following equation

Where n is Manning roughness coefficient (it is commonly taken as 0.013. This value is independent of sewer material because it depends on the roughness of the bacterial slime layer which grows on the sewer wall) (Mara and Brome, 2008)

But

Where =flow at , m3/s

is the area of flow in m2

Therefore from equation 3.9

By substituting equation 3.6 into equation 3.9 and rearranging,

Substituting equations 3.5, 3.9 and 3.11 in equation 3.10 and rearranging, The ratio of flow to the square root of gradient can be given by

## 3.1 Sewage flow and Sewer Gradient

Sewage flow consist of domestic also called sanitary waste water (this is waste water discharged from residences and similar facilities), infiltration that enters through leaking joints, cracks and breaks, and sometimes possible contribution of industrial waste water (Metcalf & Eddy, 2003).

For Low-Cost combined sewerage this flow is assessed for the two cases; at the beginning of the project period (the initial sewage flow) and at the end of the design period (final sewage flow) (Guimarães and de Souza, 2004). The difference in these flows would be due to either an increase in population or an increase in water consumption, or both (Mara, 1996 and Mara et al, 2001). In a fully developed area where there is no room for expansion as well as changes in water consumption the initial sewage flow is equal to the final sewage flow.

## 3.1.1 Water Consumption

It is very complicated to determine amount of waste water produced by community. The approach in low-cost combined sewerage is to relate it to amount of domestic water (which is water supply intended for toilet flushing, bathing dish washing and other less intensive or less frequent purposes such as cooking and drinking) (Metcalf & Eddy, 2003). In urban areas of developing countries water supplied for consumption commonly varies between to per day while in industrialised countries the supply is usually greater than per day (Trifunovic 2006).

## 3.1.2 Return coefficient

There is a difference between the amount of water used and the amount of sewage flow discharged into the sewer (Tucker, 2010). However, sewage flow is commonly presented as a proportion of water supplied (Mara, 1996). This proportion is what is called the return coefficient. The typical values used for design are any values between 0.8 and 0.9. A fixed value of 0.85 is recommended for the design of low-cost combined sewerage (Mara 1996, and Guimarães and de Souza, 2004).

## 3.1.3 Peak waste water flow factor

Sewage flow is expected to vary with time of the day. That is to say, the peak waste water flow is expected to occur at least once in a day during the peak hour which might occur, for example, in the morningÂ when a lot of people are taking showers and preparing food. A peak hour design of 1.5 is recommended in the design of sewerage (Guimarães and de Souza, 2004). Similarly, waste water flow also varies season. That it is to say the sewage flow is higher in summer (when water usage tends to be higher) than in winter. A season factor of 1.2 is recommended (Guimarães and de Souza, 2004). By incorporating peak hour and seasonal factors, the suitable design peak waste water flow factor used is in the design of low-cost combined sewerage 1.8 (thus,)

## 3.1.4 Peak Sewage flow

To estimate peak sewage flow generated by a community it is important to consider the above factors as well as the population concerned. The peak sewage flow can then be estimated by;

Where

is the peak waste water flow factor

is the return coefficient

is water consumed in litres per capita per day

is the upstream population of the sewer section

86400 is a conversion factor of day to seconds

By substituting of and , equation 3.13 can be written as

Alternatively, peak sewage flow can also be estimated from the number of households discharging into that section. That is to say, the Equation 3.14 can be modified as

Where is the average household size.

is the Number of households

For long section of sewers, infiltration that enters through leaking joints, cracks and breaks because significant and should taken into account. The value of Infiltration varies from 0.5 to 1l/s per km of pipe length depending on the infiltration capacity of the soil (Tucker, 2010 and Guimarães and de Souza, 2004).

## 3.1.5.1Minimum sewage flow

To maintain the self cleansing velocity of flow a minimum peak flow requirement it is necessary to set the minimum peak flow the system can accept. The standard minimum peak flow used in practice is . When calculated has a value which is less than below , this value is adopted. It is an estimate of peak discharge from a flush toilet. However this flow may be attenuated in the house connections. Current Brazil practice assumes a minimum peak flow of (instead of ) (Guimarães and de Souza, 2004).

## 3.1.6 Sewer Gradient

Sewer gradient is supposed to be determined when wastewater flow is at minimum because that is the critical flow to maintain self cleansing velocity. For low-cost combined sewerage this flow is equivalent to the sewage flow which occurs at the beginning of the design period (Guimarães and de Souza, 2004).

Figure 3.2: Parameters for tractive tension in Circular sewers

The approach used to determine the gradient is tractive tension method. Tractive tension is the tangential force exerted by the flow of sewage per unit of wetted boundary area. It is denoted by the symbol and has units of N/m2 (pascals).

From figure 3.2, considering a mass of sewage of length , in m and cross-section area a in m2, which has a wetted perimeter of , the tractive tension can be given by the component of the weight ( in Newtons) of this mass sewage in the direction of flow divided by its corresponding wetted boundary area

But weight W is given by

Where

is the density of sewage in kg/m3

is acceleration due to gravity in m/s2

But the hydraulic radius r is given by . Substituting r and W in the equation

For small values of , .

But is the sewer gradient, therefore equation 3.19 can also be written as

Substituting equation 3.20 in 3.6 (with the pivot) and rearranging

Substituting equation 3.21 in 3.12 and rearranging

Rearranging equation 3.22 for gradient

In low-cost combined sewerage the design limits of y/D are 0.2<y/D<0.8 (Guimarães and de Souza, 2004 and Mara 1996). The lower limit ensures that there is sufficient velocity to flow to prevent solids deposition in the initial part of the design period, and the upper limit provides for sufficient ventilation at the end of the design period (Mara 1996).

The minimum gradient, is given substituting this equation with at y/D=0.2, ( and from Appendix 1). With Ï=1000kg/m3, g=9.81m/s2 and n=0.013 equation 3.23 can be written as

In low cost combined sewerage a satisfactory of is 1Pa. Substituting this value in equation 3.24.

Changing units of from to gives the design equation

In developed countries where water consumption is usually greater than per capita per day a minimum tractive tension of 1Pa may not be considered sufficient to secure self- cleansing in low-cost combined sewers. However, minimum tractive tension of 2.5 Pa at full-bore flow (but also at half bore flow since in both cases the hydraulic radius is D/4) is recommended to achieve self cleansing (Mara and Guimaraes, 1999). Therefore the equation 3.24 can be recalculated by substituting 1Pa with 2.5Pa and for y/D=0.5 to become.

To design based on self cleansing velocity, the Macedo-Manning equation is used and it is essentially a modified Manning equation (Mara 1996). It is given by

Where

For any values of y/D between 0.14 and 0.92, M is essentially constant and has a value equal to 0.61. Substituting this value in equation 3.28 (n is equal to 0.013)

Assuming (the minimum self cleansing velocity which has shown to be applicable in Brazil), and changing q from m3/s to l/s. Equation 3.30 can be rearranged as

Where

is the minimum gradient in m/m

## 3.2 Storm flow and Pipe diameter

In low-cost combined sewerage the determination of diameter of a sewer is based on the maximum peak flow expected in the design period. This maximum is estimated a summation of storm water flow for a specified return period and peak sewage flow expected at the end of the design period. Storm water flow is the runoff resulting from rainfall (Eddy & Metcalf, 2003) and for small catchment areas, it is usually estimated from the rational method developed by Lloyd and Davies (Shaw, 1994)

## 3.2.1 Rational method

The Rational Method is an intensity based runoff prediction with the following assumptions (Meadows and Walski, 1999).

The rate of rainfall is constant throughout the storm and uniform over the whole catchment area.

Catchment imperviousness is constant throughout the storm

Contributing impervious area is uniform over the whole catchment.

The catchment area is small

Considering all these assumption, the rate of runoff at a design point is a direct function of the catchment characteristics and the average intensity of rainfall excess up to the time of concentration (Crobeddu et al, 2007, Guo, 2001 and Shaw, 1994). This relationship is expressed by equation 3.32.

Where is peak runoff flow in m3/s

is the Coefficient of runoff (dependent on catchment Characteristics)

is the intensity of rainfall (in mm/h) in time of concentration Tc

is the area of Catchment in km2

## 3.2.1.1 Rational Coefficient of runoff

The rational coefficient of runoff,C, is the parameter which is open to engineering judgement which relates the rate of rainfall over a catchment to the rate of discharge from the same catchment (Guo 2001). Its value varies from 0 to 1. It is highly dependent on the land use and slope such that the more covered and impervious soil is the value of C approaches 1.

## 3.2.1.2 Intensity and Time of Concentration

The intensity of the storm is directly related to its duration and its return period (frequency). Historical storm data for an area under consideration is compiled and analysed to predict the storm characteristics, and is presented in various forms of equation. One example of such equations is presented as.

Where is the intensity of rainfall in mm/hr

are constants depending on local precipitation characteristics and frequency

is the time of concentration (duration)

The time of concentration ) refers to the time at which the entire catchment begins to contribute to runoff. Any constant rainfall intensity is at peak value when the duration of rainfall and time of concentration are equal (Butler and Davies, 2000). it is required to determine the time of concentration for each design point within the drainage basin. Its value is calculated as the time taken for runoff to flow from the most hydraulically remote point of the catchment area to the point under investigation (Crobeddu et al, 2007 and Butler and Davies, 2000) . The time of concentration is therefore viewed as having two components

Where

is the time of entry (time of flow over the ground surface) in minutes, which vary with catchment characteristics such as surface roughness, slope and length of the overland flow. Typical values of are in the range of 8 and 12minutes (Department of Environment, 1981). In Brazil values between 5 to15 minutes have been used for considerable size of catchment (Guimarães and de Souza, 2004).

is the time of flow in minutes through the pipe system to the point under consideration, based on the pipe-full velocity (Department of Environment, 1981). Its value can be calculated from the hydraulic properties of pipes. With known sewer length and flow characteristics, time of flow can be determined. Values between 10 to 15 minutes are recommended (Guimarães and de Souza, 2004)

Equations are typically used only in regions where the rainfall data has already been analysed and an appropriate equation has been fit to the data. However, choosing equation coefficients to be used in such equations is much more substantial task.

An alternative and most common way of determining rainfall intensity is to use Intensity-Duration-Frequency (IDF) curves for the area under concern. These curves present the rainfall characteristics of a catchment in terms the relationship between the intensity and duration for a specified return period. An example of such curves is shown in figure 3.3. They are usually available from a local regulatory agency or meteorological offices/bureaus

Figure 3.3: Intensity-Duration-Frequency Curve

## 3.2.1.3 Return Period/ Storm Frequency

The return period or storm frequency of an annual maximum flooding event can be described as the long-term average of the time interval between specific occurrences. The choice of design storm frequency, therefore determines the degree of protection from storm water flooding by the system (Butler and Davies, 2000). The protection should be related to the cost of any damage or disruption that might be caused by flooding. In practice, cost-benefit studies are rarely conducted for ordinary urban drainage projects; a decision on design storm return period is made simply on the basis of judgement and precedence. It is very difficult to determine the return period of flooding because of absence of comprehensive storm runoff data. However it is possible to assess and specify design rainfall return period, and a reasonable approach is to assume that the frequency of rainfall is equivalent to the frequency of runoff (Butler and Davies, 2000). The standard practice for the design of low cost combined sewerage is to use the return period of 10 years (Guimarães and de Souza, 2004). Note the duration should be equal to the time of concentration (equation 3.34) to achieve peak runoff.

## 3.2.1.4 Catchment Area

The catchment area is one of the most important parameter for good prediction of storm water. This is the entire geographical area drained by a sewer section. The boundaries of the catchment to be drained can be defined with reasonable precision either by field survey or use of contour maps (Butler and Davies, 2000). They should be positioned such that any rain that falls within them will be directed under Gravity to a point of discharge or outfall (Butler and Davies, 2000). Rational method of estimating storm water runoff can only be accurate if used in the design runoff from small catchment. For low Low-cost combined sewerage design the maximum threshold for catchment area is 12km2 (Guimarães, and de Souza 2004).

## 3.2.2 Wallingford Modified Rational method

Increased understanding of the rainfall-runoff process has led to further development of the Rational Method to improve its accuracy. The Modified Rational Method which is recommended in the Wallingford Procedure has shown to be more accurate for small catchment and it is the recommended approach in the design of low-cost combined sewerage (Department of Environment, 1981).

In this procedure the runoff coefficient is considered to consist of two components

Where is the volumetric runoff coefficient.

is the dimensionless routing coefficient

The dimensionless routing coefficient,varies between 1 and 2. It accounts for the effect of rainfall characteristics (such as peakedness), catchment shape and the magnitude. A fixed value of 1.30 is recommended for design.

Incorporating this value in the Rational formula (equation 3.32)

is the intensity of rainfall (in mm/h) in time of concentration Tc

is the area of Catchment in km2

Volumetric runoff coefficient,, is the proportion of rainfall falling on the catchment that appears as surface runoff in the drainage system. Its value depends on whether the whole drainage basin is under consideration or only the impervious areas alone. If only the impervious areas are considered is given by;

The percentage impermeable area of the catchment (PIMP) is the degree of urban development of the catchment. It is estimated by the equation

is impervious (roofs and paved areas) area (in km2)

is total Catchment Area (in km2)

Alternatively, the percentage impermeable area (PIMP) can also be related approximately to the density of housing development using the following relationship

Where J is the housing density (dwelling/ha). The value of PIMP varies between 25 and 100 (Butler and Davies 2000).

Percentage Runoff (PR) on the other hand is the dimensionless runoff coefficient. It is measured by defining the impervious surfaces such as roads, roofs and other paved surfaces.

Values of range from 0.6 to 0.9 (Department of Environment, 1981). The lower values indicate a rapidly draining area while higher values relate to heavy clay soils. By assuming to be equal to 0.75 (the average value), equation 3.36 can be written as.

Changing the value offrom m3/s to l/s

With a maximum threshold of area as 12km2, the value of in l/s should not exceed

## 3.2.5 Sewer Diameter

The sewer pipe is designed to carry the maximum possible flow which is composed of the summation of storm water flow and sewage flow at the end of the design period. Rearranging equation 3.12

Where is the diameter

is the final sewage flow at the end of the design period in m3/s(from equation 3.13 converted to m3/s)

is the peak runoff in m3/s (from equation 3.39)

and are geometric parameters at y/D equal to 0.8 (From appendix 1 =0.6736 and =0.3042)

The following sequence of calculations is recommended to determine diameter:

Calculate using equation 3.12 the final sewage flows (, l/s), which is the flow occurring at the end of the design period.

Calculate peak runoff using equation 3.39

Calculate the diameter D using equation. 3.40

The minimum and maximum diameters recommended for low-cost combined sewerage are 400mm and 1500mm respectively (Guimarães and de Souza, 2004).

Alternatively and the simplest way of determining diameter used in practice is the design Chart in Appendix 2. The chart can be used by the following procedure;

Calculate with in m3/s and find this value in the chart where y/D is close to but less than 0.8. The sewer diameter is given at the top of the column in which is found.

Read the corresponding values of from the chart and calculate . This is based on arrangement that incorporates

Calculate with in m3/s and find this value in the same column as in the step above and read the corresponding value of y/D and calculate from the corresponding value of .

Using the chart therefore permits determination of the velocities and proportional depths at the beginning and the end of the design period

## 3.3 Design Summary

The hydraulic design of low-cost combined sewerage aims at determining the minimum recommended sewer gradient and the required diameter for the sewer pipes. The following steps summarize the design process.

Determine the expected peak sewage flow from upstream of the section for the beginning and the end of the design period using either equation 3.14 if the population is the information available or equation 3.15 if the number of household is the information available. The difference in these flows would be a result of either an increase in population or an increase in water consumption, or both. Both final and initial sewage flow values should not be less than (adopt this value if less).

Calculate the storm water flow from upstream of the section using equation 3.39 where the intensity is determined by a 10 year return period of storm and a duration equivalent to the time of concentration. The catchment area should also not exceed 12km2

Using the peak sewage flow at the beginning of the design period (calculated in 1 above) determine the minimum sewer gradient using either equation 3.26 when designing in developing countries where water supply is less than or 3.27 when designing in industrialised countries where water consumption is more than .

By summing up the storm water flow and the peak sewage flow at the end of the design period, determine the required pipe diameter for the section by either using equation 3.42 or the design chart in appendix 2. The minimum and maximum diameters recommended are 400mm and 1500mm respectively. Please note that if the calculated diameter is not available on the market the practice is to choose the nest higher available size. That is to say, if the calculated diameter is 470mm and the available diameters are 400mm and 500mm then 500mm section would be the right option.

## 3.4 Storm Water Inlet Design

The key objective when designing inlets is to minimize the spread of water across the sewer and in the gutter (in storm drainage, the gutter is the channel in which runoff is conveyed to storm sewer inlets) (Meadows and Walski 1999). These inlet structures are located in box drains which are sited on either side of a pavement gutter.

## 3.4.1Types of inlets

Two types of inlets are commonly used for storm water. They include grate inlets and curb inlets.

Access to the storm drain system through a grate inlet is excellent because it is removable. However, it is difficult to maintain grate inlets and they also have a higher probability to collect debris which obstruct the flow of surface water into the inlet (Meadows and Walski, 1999). Figure 3.4 gives typical grate inlets used.

Figure 3.4: Grate inlet in Gutter and Some Typical Grate Types (source: Meadows and Walski, 1999)

Curb inlets (figure 3.5) are openings within the curb and are used in areas where grate inlet are prone to clogging. The efficiency of curb inlet is based on the ratio of the actual inlet length to the inlet length necessary to capture 100% of the total runoff.

Figure 3.5: Curb inlet

Combination inlets such as curb and grate can also be useful in some configurations. They offer overflow drain if part of the inlet becomes completely or severely clogged by debris (Guo 2000). Maintenance of combination inlets can also be simplified by the fact that the grate is removable, providing easy access to the inlet and associated storm drain system.

## 3.4.2 Location of inlets

Drainage inlets are either located on a grade to intercept portion of runoff that flows past or on a sag where all the runoff may collect. There is a difference in equation used for designing inlets for these locations.

Inlets located in sag are assumed to capture 100% of flow because once collected the runoff in the sag has no other place to go. As opposed to inlets located on a grade, the size and type of inlet directly affects the spread. The computations for calculating the amount of flow intercepted by inlets in sag are based on the principle of weir flow and orifice flow.

For an inlet which is not submerged, operates as a weir, the flow capacity is calculated as

Where is flow intercepted by the inlet operating as weir (m3/s)

is the weir coefficient

is the perimeter of the inlet (m)

is the flow depth at the curb (m)

If the gutter is depressed, the perimeter of the grate P is calculated as

Where is grate length (m)

is grate width (m)

Otherwise

The depth, d, for both types of inlets is measured from the projected normal cross-slope. For a curb inlet, the perimeter is equivalent to the length of the inlet.

If the inlet is submerged and is operating as an orifice, its capacity becomes:

Where is the flow intercepted by the inlet operating as an orifice (m3/s)

is the Orifice coefficient

is the area of the opening (m2). A multiplier of about 0.5 is recommended to be applied to the measured area as a factor of safety.

is the acceleration due to gravity and is equal to 9.81m/s2

is the effective head at the orifice (m)

Note

For a grate inlet the effective head, d, is simply the water depth along the curb.

Figure 3.6: Different curb inlet throat types

For curb inlet, the effective head (shown in figure 3.6) is expressed as;

Where is depth at lip of curb in m

is the curb throat opening height in m

is the inclination of the curb throat measured from the vertical direction

The following procedure should be used for determining a grate inlet capacity on a sag:

Choose a grate of standard dimensions to use as a basis for calculations.

Determine an allowable head () for the inlet location. This should be the lower of the curb height and the depth associated with the allowable ponded width.

Determine the capacity of a grate inlet operating as a weir from equation 3.44

Determine the capacity of a grate inlet operating under orifice flow from equation 3.46

Compare the two calculated capacities and choose the lower value as the design capacity. The design capacity of a grated inlet in a sag is based on the minimum flow calculated from weir and orifice conditions.

Likewise when designing on a curb inlet the procedure would be

Determine the required flow to be intercepted

Determine an allowable head ) for the inlet location from equation 3.47

Determine the length of the curb of the inlet when operating under weir conditions from equations 3.44 and 3.45,

determine the inlet length of the curb opening from the intercepted flow operating as an orifice from equation 3.46 where

choose the larger of the two computed lengths as being the required length.

Select a standard inlet length that is greater than the required length

Proper attention should be given to the conveyance of storm water through sag inlets, because they frequently encounter water ponding which can increase water spread over the surface (Brown et al, 2001). Grate inlets designed as a solo inlet for installation in sags are not recommended because they have a propensity to clog and exacerbate ponding during severe weather (Almedeij et al, 2006 and Meadows and Walski, 1999). A combination of grate and curb inlets may be a better alternative. At low flow depths the capacity of a combination inlet where the grate inlet length equals the curb inlet length is equivalent to the capacity of the grate inlet alone. At higher flow depths, for the same type of inlet, both the curb inlet and grate inlet act as orifices working in conjunction (Meadows and Walski, 1999). The total intercepted flow is then calculated as the sum of the flows intercepted by the grate and the curb opening.

Designs of inlets on grade are based on how much flow will be intercepted for a given total flow (gutter discharge) to the inlet. Their design are based on efficiency.

Where is inlet efficiency

is the intercepted flow (m3/s)

is the total gutter flow (m3/s)

Flow that is not intercepted by drainage inlet on grade is bypassed and carried-over to another inlet downstream, or is lost to a stream. When designing inlets on grade, grate inlets have shown to be more efficient efficient than curb inlets (Meadows and Walski, 1999)

## 3.5 Sewer Gradient and Ground Slope

It is vital to consider the relations between the calculated sewer gradient and the slope of the ground (Mara 1996). The slope of the ground surface (S, m/m) may be (a) less than, (b) equal to, (c) greater than, or much greater than, the minimum sewer gradient calculated from equation.

Figure 3.7: The minimum depth to which the sewer is laid is the sum of the minimum depth cover C and the sewer diameter D (source mara 1996)

Furthermore, the depth to the invert of the upstream end of the length of sewer under consideration may be (a), equal to, or (b) greater than the minimum depth permitted , which is given by

Where

is minimum required cover (its value varies from 0.20m for in-block sewers to 0.40m for street sewers) (shown in figure 3.7).

is sewer diameter, m

There are six cases likely to be encountered in practice

Case 1. and the invert depth of the upstream end of the sewer ; choose and calculate the invert of the downstream end of the sewer as:

Where is the length of the sewer and consideration

Case 2. and ; choose and

Case 3. and ;choose and

Case 4. and ; choose and calculate the sewer gradient from

Subject to

Case 5. and , as in Case 4, but an alternative solution is to choose and calculate from equation in Case 1. The choice between these alternative solutions is made on the basis of minimum excavation.

Case 6. and ; here, it usually sensible to devide L into two or more substretches with and (but obviously ) in order to minimize excavation.

## 3.6 Selecting Sewer Pipe Materials

In general, selection of a sewer pipe material is dependent on the physical characteristics which include; durability, abrasion-resistance, corrosion-resistance, imperviousness and strength (Butler and Davies 2000).

The types of pipe materials used in low-cost combined sewerage are similar to those used in conventional sewers.

Vitrified clay is a commonly-used material for small- to medium-sized pipes. Its major advantages are its strength, durability and resistance to corrosion and are considered ideal for low cost combined sewers especially when the water table is low (Butler and Davies 2000 and Bakalian et al, 1994). However, clay is both heavy and brittle and therefore, susceptible to damage during handling. Mortar is commonly used for vitrified clay pipe joints. Rubber gasket joints are commonly used with plastic and fiber concrete pipe

Plain, reinforced and prestressed concrete pipe is generally used for medium to large-sized pipes. It is particularly suited to use in sewers because of its size, abrasion resistance, strength and cost (Butler and Davies 2000).

PVC pipes offer the advantage of longer sizes, fewer joints (i.e., less infiltration), light weight, water tightness, and uniformity (Bakalian et al, 1994).