Environmental and financial performance evaluation of rainwater harvesting (RWH) systems to identify optimized options and comparison with two popular water-sensitive urban design (WSUD) components

Since late 1990, there have been many initiatives to manage the urban water cycle more sustainably. An increasingly popular approach in implementing total water cycle management philosophy is to decrease water demand, reduce stormwater runoff and improve pollutant wash-offs from urban catchments by adopting sustainable design practices. Water-sensitive urban design (WSUD) is one of the most effective and environmentally friendly measures to manage the urban water cycle. Stormwater treatment trains are the most common WSUDs though total water cycle management is the prime focus of WSUDs. Rainwater harvesting (RWH), one of the WSUD elements, reduces potable water demand, runoff, and pollutant loads to natural water bodies. RWH reduces water demand, runoff, and pollutant loads toward natural water bodies, so its performance must be compared with other WSUD components. These comparisons would help in selecting competing options for sustainable urban development. There is a widespread perception that RWH is an environmentally sustainable water supply option. This perception is not always precise for all RWH components and systems arrangements. All potential features of RWH need to be thoroughly studied to substantiate its environmental impacts. For having the RWH system as a part of the total water cycle management practice, it is required to consider aspects such as energy consumption, life cycle assessment (LCA), and financial feasibility to determine the combined impacts of the RWH systems. Broadly, two perspectives of LCA are available on a household-scale RWH in the literature, which can be identified as the selection of tank material on the one hand and the saving of potable water on the other. Most of these studies used High-Density Polyethylene (HDPE) as tank material, where its fabrication, transportation, installation, and disposal were included. However, those studies did not include runoff and pollutant load reductions and the recycling phase at the end of the life cycle in net environmental impact assessments. In this study, the LCA approach was applied to quantify the catchment-scale net environmental impacts of RWH systems made of different tank materials (e.g. HDPE, LDPE, ferrocement, and steel), plumbing components (with pump, no pump, pipes and accessories, and raised tank bases), tank sizes (2000, 3000, 4500, 6500 and 9000 L), roof sizes (150, 200 and 300 m2), varying annual rainfall patterns (driest, average and wettest rainfall year) for varying RWH supply reliabilities. The life cycle cost analysis (LCCA) was included in this study to identify financial feasibility and environmentally feasible scenarios. It also included comparing environmental impacts between the RWH systems and other WSUD elements, such as the rain garden (RG) and the wetland, for generating an equivalent runoff quantity in the given catchment. Results show that the HDPE tank has a lesser impact in most of the impact categories than other material tanks. RWH systems with tank sizes of 2000 and 3000 L without pumps and tank sizes of 2000 L with a pump under a 150 m2 roof are the most feasible scenarios in terms of environmental and economic perspectives. The water-saving and pollutant load reductions cannot compensate for the pump operation's impacts, except for very few cases, like the 2000 L tank during the driest rain year at lower reliabilities. A tank size larger than 2000 L would not be effective. On average, 70% of the total net life cycle impacts were developed from the operational phase of the RWH system (with a pump). The pumping operation provided about 6.5 times higher CO2-eq emission than the water-main supply. The outlined strategy would help select tank material, size, and accessories based on annual rainfall and roof size available to introduce definite engineering standards, government rebates, and guidelines for RWH system design. This approach of quantifying the environmental impacts will explain further insights into the RWH system design and its components. A comparison was conducted between the scenarios with and without RG with the most feasible (environmentally) RWH system. Three RG sizes, 3, 4, and 6 m2, are considered in driest, average, and wettest annual rainfall conditions. The catchment-scale results showed that the runoff generation impacts of the RGs' operation phase were about (24-54%), (21-49%), (21-47%), and (14-45%) of the system without RG on eutrophication, human toxicity-carcinogenic, ecotoxicity-freshwater, and ecotoxicity-marine, respectively. However, once the installation & fabrication (I&F) phase was added, RG had much higher net impacts than without RG, except for eutrophication and ecotoxicity-freshwater. Hence, the net ecotoxicity-freshwater impact was lower for all scenarios except 4 and 6 m2 RG sizes during driest rainfall. The most feasible RWH scenarios (e.g. 2000 and 3000 L tanks) had net impacts of 3–81% of the RG systems on global warming, human toxicity-carcinogenic, and ecotoxicity-terrestrial categories. On the other hand, RWH had net impacts of 105-200% on ozone depletion and eutrophication and 51-119% on the ecotoxicity-freshwater and ecotoxicity-marine of the RG systems. By comparing RG with the RWH system, the study indicates that RWH is preferable over RG if the prevention of emission to the air is emphasized. RG is preferable over RWH if prevention of emission to water is emphasized. The study shows that the runoff generation impacts of the catchment without wetland were about (2.7-3.5), (3.2- 4.7), 4, and (3.5- 4.5) times of catchment with wetland on eutrophication, human toxicity-carcinogenic, ecotoxicity-freshwater, and ecotoxicity-marine impact categories respectively. In contrast, its I&F phase was about 6%, 19%, 33-78%, 4%, 18-26%, 5%, and 9% of the net impacts of the water-main system (without wetland) for global warming, ozone depletion, eutrophication, human toxicity-carcinogenic, ecotoxicity-freshwater, ecotoxicity-marine, and ecotoxicity-terrestrial impact categories respectively. The runoff generation catchment impacts of the most feasible RWH system scenarios (tank only (TO) 2000 and 3000 L) were about (1.5-2.7), (1.4-3.8), (1.8-3.2) and (2-3.3) times the wetland runoff catchment impacts on eutrophication, human toxicity-carcinogenic, ecotoxicity-freshwater, and ecotoxicity-marine impact categories, respectively. The net catchment impacts of the most feasible RWH scenarios (2000 and 3000 L tank) had about (41-98%), (32-90%) and (7-63%) of the system with wetland (wetland + water-main) net catchment impacts on global warming, human toxicity-carcinogenic and ecotoxicity-terrestrial, respectively. The net catchment impacts of the same most feasible RWH scenarios had about (450-500%), (146-190%), (110-145%) and (70-136%) of the system with wetland (wetland + water-main) net catchment impacts on ozone depletion, eutrophication, ecotoxicity-freshwater, and ecotoxicity-marine respectively. By comparing the wetland with the RWH system, the study indicates that RWH is similarly preferable over the wetland if the prevention of emission to air is emphasized. A wetland is preferable over RWH if the prevention of emissions to water is emphasized.