STUDY OF TWO-GENERATION LIFE-CYCLE EVALUATION BY BOTH COST AND CO2 EMISSIONS FOR THE SELECTION OF SEWER SYSTEM ALTERNATIVES WITH CASES OF USING RECYCLED PRODUCTS

This paper proposes a selection procedure for sewer systems based on their alternatives for reducing overall CO2 emission and total cost, whilst considering recycled products made from wastes/discarded articles instead of new products as alternatives. The sewer systems evaluated are:a conventional gravity-type sewer system (hereinafter, conventional gravity system).a modified gravity-type sewer system with multiple pump settings in manhole (hereinafter, modified gravity system),a vacuum-type sewer system (hereinafter, vacuum system),The estimates of both total costs and overall CO2 emissions were carried out over a two-generation life-cycle of 100 years of use of sewer pipes in order to evaluate the impact of recycled products from raw materials regenerated from wastes/discarded articles. The two-generation life-cycle costs (LCC2) and CO2 emissions (LC-CO2 emission2) over 100 years were estimated by dividing these into the initial period, operation and maintenance(O&M), renewal and discarding periods, respectively. Furthermore the evaluations were conducted applying two integrated indicators for comparing each case with the alternatives.In the paper, case studies were carried out to verify and evaluate the studies in two actual districts, where District A has an area of 147 hectare (ha) consisting of three separate villages on mostly flat and with a high groundwater level, District B was an area of 30ha on sloping ground with a low groundwater level. Ten alternatives were prepared in total, half of which used products manufactured from raw materials either made from crude oil/ore (hereinafter, new products), with the other half using products regenerated from wastes/discarded articles after their effective working lives had expired during the O&M and renewal periods (hereinafter, recycled products).The annual present worth costs per capita and annual CO2 emissions per capita were estimated and evaluated for each case. There was a small difference in the cost per capita between three sewer systems, but a significant difference was found between Districts A and B. The annual CO2 emissions per capita show a fairly large difference between each system and between two districts. The two integrated indicators show clearly that applying the modified gravity system was a little advantageous, but applying the vacuum system was fairly greater advantageous than applying the conventional gravity system. Furthermore, two indicators (1) and (2) suggest a small advantage based on less LC-CO2 emission can be achieved by using recycled products instead of new products.