Part 5 – Wastewater Drainage
5.3 Calculations of Flows
A statement is to be submitted with each plan to show that the design of the wastewater network has been calculated to meet the projected flows of the area under consideration. The designer shall consider the appropriate allowances for growth and clearly define any assumptions or basis for design inputs.
The pipe system shall be designed to ensure that the system is self cleaning and that the pipe gradients are such that the velocity at peak daily flow meets this requirement.
Design shall be in accordance with the information given in Figure 5.1. (See also Section 5.5 Pipeline Minimum Grade Guideline.)
Figure 5.1 Design Data |
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| Calculation of Flows | |||
| 2.1 | Domestic wastewater flows are a function of water consumption, ground water infiltration and surface water ingress and shall be calculated as follows: | ||
2.2 |
The wastewater flow is calculated as the product of the water consumption, the peaking factor, and the population equivalent of the area being served. The water consumption is 200 litres per person per day. The variable peaking factor dependent on population density for residential areas is determined from Figure 5.2. The following equivalent population densities per hectare should be adopted in the absence of specific supportable design data: |
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| Urban Commercial |
= 45 persons per hectare = 30 persons per hectare (except central business area) |
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| Other establishments should be treated as follows: | |||
| Primary Schools Secondary Schools Hospitals Boarding houses/motels |
= equivalent to 45 persons = equivalent to 150 persons = equivalent to 3.5 persons/bed = equivalent to 0.6 persons/bed |
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2.3 |
The infiltration allowance is 2250 litres per hectare per day. The surface water ingress allowance is 16,500 litres per hectare per day. The Average Daily Flow is calculated as the sum of the infiltration allowance and the daily wastewater flow (product of water consumption and the population equivalent). The Peak Daily Flow is calculated as the sum of 2.2 and 2.3. The Peak Wet Weather Flow is calculated as the sum of 2.2, 2.3 and 2.4. |
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Industrial Domestic Flow and Trade Waste |
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2.8 2.9 2.10 |
Where the industrial domestic waste and trade waste flows from a particular industry are known, these shall be used as the basis for the wastewater design. When this information is not available, then flows shall be calculated as above, except that the industrial peaking factor shall be used as shown on Figure 5.2 and the equivalent population density shall be 45 persons per hectare. |
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The Hydraulic Design of Pipelines |
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| 2.11 2.12 |
All wastewater pipelines shall be designed such that they have sufficient capacity to cater for the design wet weather flow from the area they serve without surcharge and that on at least one occasion every day a minimum velocity for solids re-suspension (self cleaning) is achieved. The minimum velocity for self cleaning at peak daily flow will be deemed to be 0.6 m/s. The capacity and velocity of flow in wastewater pipelines shall be determined by using the Colebrook White formula as shown in Figure 4.5 in Part 4 – Stormwater Design with a roughness coefficient (k) of 1.5 mm. Figure 5.3 can be used as a check of the design so that if the operational point falls within the appropriate envelope, then the pipeline will have adequate capacity for peak wet weather flows and achieve a self cleaning velocity at least once every day. |
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Figure 5.2 Wastewater Peaking Factors
| Population Equivalent for Catchment or Sub-Catchment Area | Wastewater Peaking Factor | |
| Residential | Commercial | |
| 15 20 25 30 35 40 45 50 55 60 65 70 75 80 90 100 125 150 175 200 250 300 350 400 450 500 600 700 800 900 1000 1500 2000 2500 3000 3500 |
14 12 10 9.1 8.5 8.0 7.5 7.0 6.8 6.7 6.3 6.2 6.0 5.9 5.8 5.5 5.3 5.0 4.8 4.4 4.1 4.0 3.8 3.7 3.5 3.4 3.3 3.2 3.2 3.1 3.0 3.0 2.9 2.8 2.8 2.7 2.6 |
13 11 9.5 8.5 8.0 7.5 7.2 6.9 6.3 6.0 5.7 5.5 5.4 5.3 5.1 5.0 4.8 4.2 4.0 3.8 3.7 3.5 3.3 3.1 3.0 2.9 2.8 2.7 2.6 2.55 2.5 2.4 2.2 2.1 2.0 1.9 1.85 |
Figure 5.3 - Residential Sections Connected
