Effect of design peak flow factor to the capital cost of sewerage reticulation in Malaysia

Abstract

The design peak flow factor (DPFF) formula prescribed in MS1228:1991 is commonly adopted in sewerage industry in Malaysia although the validity of this formula has not been verified since its publication 24 years ago. Local research findings and feedback from industry revealed that the DPFF has been over provided for all sewerage services which lead to over design and increase in capital cost as well as operation and maintenance (O&M) cost. The Government has planned to raise the sewerage tariff to cover high O&M cost in sewage treatment plants. In view of RM10.3 billion of sewerage projects had been approved under the Tenth Malaysia Plan, the research on design and cost optimization in sewerage services is timely and in line with country needs. The aim of this research is to prove via statistical analysis that the DPFF is higher than both the actual peak flow factor (APFF) in sewerage services and international peak flow factor (IPFF); to recommend a more optimised peak flow factor (PFF) formula in lieu of the current DPFF; and to evaluate its financial effect to the sewerage reticulation projects in Malaysia. Hourly flow data was collected via online electromagnetic and ultrasonic flow meter connected to the Supervisory, Control and Data Acquisition (SCADA) system at five centralised sewage treatment plants (CSTP) with capacity ranging from 73,000 to 394,000 population equivalent (PE). The analysis revealed that the DPFF for all five CSTPs were overprovided by 23% to 63% as compared to the APFF. A comparison between Malaysian DPFF with six IPFF formulas, namely Babbitt, Duncan, Gifft, Harmon, Johson and Utah showed that Malaysian DPFF is the highest among all IPFFs from 10,000 PE to 1 million PE. In addition, the APFF calculated for five CSTPs are close to Utah, Babbitt and Harmon PFF. Based on the highest APFF among five CSTPs, a more optimised PFF formula: 3.8/P0.11 (named as Lee’s PFF) is recommended in lieu of the original DPFF of 4.7/P0.11. Three IPFF i.e. Utah, Babbitt and Harmon, together with Lee’s PFF were selected for financial analysis which involve proper engineering design using Manning equation by Professional Engineers for five actual centralised sewerage reticulation projects of 72,000 PE to 554,000 PE. The financial analysis revealed that capital cost saving of up to 7.6% (RM35.5 million) and 5.1% (RM23.6 million) can be achieved from the total capital cost of RM464 million for five projects, using IPFF (Babbitt) and Lee’s PFF, respectively. The above findings warrant a full scale review of the current DPFF formula prescribed in MS1228:1991 for the interest of the country.