Rainwater harvesting - An investigation into the potential for rainwater harvesting in Bradford

摘要

This report provides a brief review of rainwater harvesting and rainwater harvesting tools, which are then used in case study examples for domestic, office block and warehouse rain water harvesting scenarios. Rainwater harvesting is placed in an historical context as a source of water supply and in a modern context as being complementary to centralised water distribution networks with benefits for wider water management including flood risk treatment as well as providing environmental and economic benefits. A range of readily available rainwater harvesting tools are presented and compared using data supplied by the City of Bradford Metropolitan District Council (CBMDC). Rainwater harvesting is discussed in the context of applicable British Standards, from which formula for assessing rainwater harvesting potential and requirements, such as maximum storage tank sizes, are given. Using data from 2008 to 2010 and provided by CBMDC, case study examples are given for rainwater harvesting potential and reductions in rainfall run-off volumes. These include city centre office blocks, domestic properties and estates, and large commercial warehouses: Offices in Bradford are assessed for water demand and rainwater harvesting potential using the ‘RainCycle’ rainwater harvesting tool. The report identifies that whilst reducing demand at source the most effective way of reducing water use, rainwater harvesting can also contribute to reductions in water demand and rainfall run-off even from office buildings with low roof area to occupancy ratios. The report considers rainwater harvesting from individual domestic properties and from a small urban estate using rainfall data, roof area data and alternative methods of determining dwelling occupancy numbers (occupants verses roof area and statistical occupant numbers). Using roof yield coefficients identified in literature and acknowledging the difficulties in determining occupancy numbers (and therefore water demand), the report notes that individual domestic properties can contribute to surface water management though reducing rainfall run-off via rainwater harvesting. This potential is increased as greater numbers of domestic dwellings use rainwater harvesting. Using the same simplistic method of determining rainwater harvesting potential, the report considers rainwater harvesting from large, warehouse-type buildings, firstly by only considering available rainfall collected from a large warehouse, and secondly in considering a large warehouse with comparatively low occupancy rates but high water demand (through vehicle maintenance). Both example detail volumes of rainfall that could be captured, thus reducing run-off, whilst the second example also details potential reductions in mains supplied water through comparing mains water supply volumes with potential volumes of harvested rainwater. As an illustrative example, the report considers the potential for reducing rainfall run-off from a city centre area, and therefore contributing to flood management, should all roof areas be connected to a rainwater harvesting system and disconnected from urban drainage systems. This illustration details the potential volumes of rainfall that could be collected from roofs within a two kilometre square city centre area, detailing an approximate reduction in run-off of 23%. This compensates for the anticipated increase in runoff that will be generated by climate change by the end of the 21st Century. The report briefly discusses and gives examples of the potential benefits associated with rainwater harvesting (reduced energy costs associated with unnecessary cleaning and transporting of potable water supplies, increased capacity in drainage networks, less water abstraction, and potentially reduced demand for reservoir capacity) and uses for harvested rainwater (vehicle and road gully cleaning, urban irrigation schemes, toilet flushing and industrial cleaning operations). Additional benefits include urban flood management and less tangible benefits to the wider environment such as groundwater recharge and reduced water abstraction from rivers. Constraints on rainwater harvesting such as cost factors and uncertainty of economic gains are noted along with uptake by businesses and the public. The report concludes that rainwater harvesting has potential for non-potable water use and wider water management through reductions in run-off, particularly so if implemented within urban areas and on a larger, city wide scale, but that this is unlikely to happen unless an appropriate degree of leadership and coordination is provided.