Question

A small coastal town in Queensland is subject to an increasing permanent population and also a transient influx of tourists during the summer period. Council already receives frequent complaints of restricted water supply. The water supply system comprises a rising trunk main from the treatment plant A to the service reservoir B, the service reservoir, and service mains to the consumption areas. Two consumption areas C and D at different elevations are gravity supplied from the service reservoir. The service reservoir has a capacity (total storage) of 12.5ML and is fed through the rising trunk main from the water treatment plant located 5 km away. The static lift between the water treatment plant and reservoir is 40 m. Other relevant information is shown in the sketch below. You are provided with data on the population and consumption as follows: Population (persons) Area C Area D Present maximum population (Dec – Mar) 2000 4500 Design maximum population (Dec – Mar) 3000 6000 Design maximum population (Apr – Nov) 1500 3000 Consumption (litres/person.day) Average daily demand (Apr – Nov., ie.245 days) 240 Average daily demand (Dec – Mar., ie.120 days) 600 Peak daily demand (Dec- Mar) 1350 Peak hour demand (Dec – Mar) based on L/person.day 2700 A. The present rising main is unable to meet the water demand and frequently breaks down. Council proposes to quickly replace the rising main. Select a suitable pipe diameter that will meet present and the future maximum population demand. Assume MDMM=1.5 x AD. The pumps for the rising main are restricted to operate not more than 20 hours a day. Once designed, estimate the pump power required for the present and future population for both scenarios of flow rate. It is assumed that minor losses between the water treatment plant clear-water tank outlet and service reservoir are negligible and may be ignored. The design lifetime of the rising main is considered to be 20 years. Cost of pipeline per m/25 mm diameter $150 Cost of pump per kW installed power $2500 Pump/motor efficiency 0.65 Cost of power per kWh $2 Hours of pumping operation per day 20 Total installed power allowing 50% standby 150% Annual charges in capital 10% Power factor 0.85 B. The ground elevations at area C vary between 15 RL and 30 RL, and that in area D vary between 12 m RL and 20 m RL. The Queensland Guidelines for planning and Design of Urban Water Supply Schemes require the minimum allowable pressure in reticulation or service mains to be 22 m head. Other provisions of the Guidelines referred to in the study material are also to be considered. Minor loss in the service mains is estimated to be 1.5 times the velocity head. You are required to investigate and report to the Council what population is adequately served by the existing service trunk mains. Then calculate and comment on the immediate and future shortfall of population served (if any). C. Assume that the service reservoir operating storage is 30% of the peak day demand; the required fire storage is 20 L/s for 4 hours and the breakdown provision is 1 peak day demand. Find out what population can be adequately served by the existing service reservoir and comment on the immediate and future shortfall of population served (if any). Comment on the capacity (total storage) of the service reservoir to meet the existing and future demand & supply conditions. Assume that the service reservoir capacity must meet the following criteria:  capacity must meet 3(PD-MDMM) demand+ greater of ( breakdown storage or fire-fighting storage)) as per Queensland Guidelines  capacity must not empty at 3 days consecutive peak day demand as per local criteria  At average daily demand, the system should have a capacity to refill from empty to full within 5 days of consecutive operation at this demand as per local criteria D. Based on the results of your investigation suggest at least two effective immediate and long term measures for existing service reservoir and service mains (where appropriate) that may be considered to satisfy present demands and that at the end of the design period. Support your proposals with relevant calculations. Ductile iron pipes are used and have the following dimensions: Nominal size, mm Mean internal diameter, mm Pipe wall roughness, mm 100 107.5 0.06 150 161.7 0.06 200 215.4 0.06 225 241.7 0.06 250 268.0 0.06 300 326.4 0.06 You should summarise the results for each system components supported with detailed calculations in an appendix. Refer to the marking scheme for assignment sheet for the details. Marks will also be apportioned for documentation and clarity.

Answer 1