Using rainwater tanks as stormwater control measures to improve runoff and water quality management in urban areas : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Water Resource Management at Lincoln University

Abstract

Urbanisation and associated human activities impact rivers and streams that flow through urban areas. Impacts include receiving large volumes of stormwater runoff loaded with high concentrations of contaminants during rainfall events. Different decentralised stormwater control measures such as raingardens and rainwater harvesting tanks have been used to mitigate stormwater runoff in urban areas, and different natural materials such as mussel shell waste have been incorporated in these measures to remove contaminants in runoff near pollution sources. Rainwater tanks have been recently recognised as stormwater control measures based on the tank’s ability to detain roof runoff during rainfall events. Despite the increased attention on using rainwater tanks to mitigate stormwater runoff in urban areas, there is still a lack of knowledge regarding their effectiveness to mitigate stormwater runoff at different scales of urban areas such as industrial and residential lands. Furthermore, the use of filtration units in the rainwater tanks to improve the mitigation performance and to remove common contaminants in roof runoff such as zinc have not been investigated yet. The mussel shell wastes have been used to remove dissolved zinc from stormwater runoff in stormwater control measures. However, the removal efficiency of zinc during different filtration conditions such as varied flow rates and short contact times with water have not been fully investigated yet. Therefore, this thesis investigated the use of mussel shell waste as filtration media in rainwater tanks to remove zinc from roof runoff, and evaluated the use of rainwater tanks to mitigate stormwater runoff at residential and industrial scales in Christchurch, New Zealand. The effectiveness of mussel shell to remove zinc was investigated for untreated mussel shell (UTMS), and heat-treated mussel shell (TMS). Two types of filtration units were designed to investigate the use of UTMS and TMS as filtration media in the rainwater tank. The first filtration units included 1.0 m depths of the filtration media connected to a gravity-driven outlet, and the second filtration units included 0.8 m depths of the filtration media connected to a siphonic-driven outlet. An actual roof runoff was collected from galvanised roofing, and the removal performance of zinc was estimated during controlled saturated flow rates of 1, 3, 5, 10 L/min. The collected roof runoff showed high concentrations of dissolved zinc with an average zinc concentration of 3347.2 μg/L, which is ca. 200 times higher than the recommended concentration of zinc in urban streams (i.e. 15 μg/L) to protect 90% of the freshwater organisms according to the ANZECC’s guidelines (ANZECC, 2000). Both the TMS and UTMS demonstrated high removal efficiencies of dissolved zinc. The heat treatment of the mussel shell generally improved the removal performance of dissolved zinc. The TMS media showed significantly higher (p ≤0.05) removal performance of zinc compared to the UTMS media for the tested flow rates during 0.8 m depths of filtration media, while the TMS showed higher average removal efficiencies but with there was no significant difference (p > 0.05) for the tested flow rates during the 1.0 depths. For all flow rates, the overall average removal efficiencies of the TMS were estimated at 94% and 82% for 1.0 m and 0.8 m depths of filtration media respectively, while the overall average removal efficiencies of the UTMS were estimated at 92% and 72% for 1.0 m and 0.8 m depths of filtration media respectively. The removal performance of zinc decreased as water flow rates through the TMS and UTMS increased. The EPA's Storm Water Management Model (SWMM) was used to evaluate the mitigation performance of rainwater tanks for two urban blocks that represented the residential and industrial land use in Christchurch, New Zealand. Three management scenarios were simulated using 5-min time steps throughout a 12-year period starting from 4 November 2007 until 4 November 2019. The first management scenario represented the Business As Usual ( BAU) conditions, and used as the reference of actual stormwater peaks and total volumes of outflow in the selected blocks. The second management scenario represents using Rainwater Harvesting for Toilet Flushing (RWH-TF) uses which simulated the use of collected rainwater to supply water for toilet flushing uses in the existing buildings of the selected blocks. The third management scenario represents using Rainwater Harvesting for Stormwater Treatment (RWH-ST) uses which simulated the use of rainwater tanks and filtration units as stormwater detention units to collect, and slowly discharge treated roof runoff into the stormwater network. The simulation results showed effective mitigation performance in both the residential and industrial blocks. The average reductions in peak runoff during the RWH-ST scenarios were estimated at 52.9% and 45% in the residential and industrial blocks respectively, while the average reductions in runoff volumes were estimated for 37.3% and 19.5%. The RWH-TF scenarios showed lower peak reductions with averages estimated at 36.3% and 23.9% in the residential and industrial blocks respectively, while the average volume reductions estimated at 42.9% and 27.5% in the residential and industrial blocks respectively. During RWH-ST scenario, the integrated rainwater tanks with filtration units showed effective treatment performance throughout the simulation period with more than 99% and 58% treatment ratio of roof runoff in the simulated residential and industrial blocks respectively. The results of this thesis exposed potential benefits for using integrated rainwater harvesting tanks (i.e. with filtration units) as stormwater control solutions to improve runoff and water quality management in urban areas. In particular, the use of mussel shell waste as filtration media in rainwater tanks provided cost-effective solutions to remove metals from runoff in order to protect the ecological conditions in urban waterways, and the use of rainwater tanks reduced stormwater runoff volumes and peak flows during rainfall events at both residential and industrial scales.