Harvesting Hong Kong’s Rain
By Angel Wong 9 May, 2012
Angel Wong of AECOM explains how harvesting HK's rain can turn cities into catchment areas
In Hong Kong, water is taken for granted because it’s readily available, of good quality, and very cheap. We turn on the tap and it’s always there. In reality, Hong Kong is in a vulnerable position as it relies on mainland China for 70-80% of its water and Hong Kong’s water is only guaranteed to 2014. In 2011, Hong Kong renewed its water supply contract with Guangdong for another three years. Hong Kong’s water from China comes from the Dongjiang River, a major tributary to the Pearl River, 83 km north of Hong Kong. The Dongjiang also serves as a major supplier of fresh water to seven other cities including the heavy industrial and commercial centers of Guangzhou, Shenzhen, and Dongguan. Given that Guangdong province is water stressed, plans to harvest Hong Kong’s rainwater may have to be one of the solutions for sustainable water use in the future. Angel Wong from AECOM explains how the city can be used as catchment below…
While population increase raises the demand for water, climate change makes water supplies from natural sources more unpredictable. Rainwater harvesting, an ancient method, now poses a modern solution in urban development. Harvesting rainwater on site provides a sustainable source of water, reduces a development’s dependence on the municipal water supply and alleviates the burden of stormwater drainage.
AECOM has been working with the Hong Kong Housing Authority to incorporate rainwater harvesting into a local housing development plan, an application of water sensitive urban design (WSUD) techniques pioneered mostly in Australia to a new metropolitan context. It’s an example of a global best practice that AECOM experts have promulgated across the U.S. and Europe and are now applying in China.
WSUD integrates stormwater treatment measures into landscape to achieve both water sustainability and aesthetic amenity. WSUD can be applied to harvest rainwater for reuse in urban environments as well as to reduce flooding risks and improve the quality of runoff entering the ecosystem. WSUD measures provide a higher degree of treatment from multiple pollutant removal mechanisms, including filtration, adsorption to soil particles and plant roots and biological degradation by plants.
WSUD often incorporates vegetation elements, with common features including bioretention ponds (also called rain gardens) and bioswales (a similar measure in linear shape). Selecting the appropriate design application depends on the overall project objectives and specific project and site constraints.
Bioretention can provide physical (sedimentation and filtration) and biological (pollutant degradation) treatment, while being largely self-sufficient and requiring low maintenance. When rainwater enters the bioretention basin, it gradually percolates down through the different layers. As the inflow exceeds the infiltration rate, or when the basin begins to fill up, the detention space above the filter begins to be filled. Once the detention space is full, the high-flow bypass will begin to work. Treated water is drained to the storage tank via the perforated pipe. During heavy rainstorms, when rainfall exceeds the treatment capacity, excess water overflows to the manhole, which acts as a high-flow bypass without any treatment and is carried away to the stormwater drainage network.
In the Hong Kong Housing Authority project, AECOM conducted a feasibility study and detailed design of the rainwater harvesting system. The rainwater is collected from rooftops of buildings, green roofs, roofs of covered walkway and planted slopes. The water treated by the bioretention will first be stored in the storage tank. Ultraviolet disinfection is selected as the post-treatment before usage, mostly for irrigation, to safeguard the bacteria level in the rainwater discharged from the water tanks.
Before commencing the detailed design, AECOM applied geographic information system (GIS) analysis and model for urban stormwater improvement conceptualization (MUSIC) for evaluating the feasibility and the performance of the system respectively. Site analysis is one of the key aspects of feasibility study. The physical constraints of the site (slope, soils, groundwater, land use) were analyzed using GIS prior to the conceptual plan and proceeding into the detailed design of the rainwater harvesting system.
The topography study helps identify the flow path of the water and ideal locations of the treatment and storage units. Topography can also be a constraint to the location of WSUD measures: in this instance the steeply sloping terrain of the development site was a major issue. Many WSUD treatment devices will not work effectively on steep slopes, and are best located in the flatter areas of the landscape. Bioretention systems can be located in steeper sites, but a local flat area should be formed for the bioretention basin. The best strategy for low carbon and economical design is to collect rainwater for reuse from the highest elevation, treat and store rainwater at lower elevation and reuse at the lowest elevation.
The analysis of rock level is primarily for constructability considerations. If bioretention or a water tank is to be located at ground level, it can be installed at the sand layer so as to reduce the cost of excavating rock. By integrating the analysis of finished ground level, groundwater level and rock level with the masterplan, the optimal locations for constructing bioretention were proposed in the case of the Hong Kong project.
Developed by Australia Cooperative Research Centre, MUSIC is gaining popularity among government, academy and engineering consultancies for conceptual planning of stormwater management. Melbourne Water uses MUSIC for urban planning and stormwater treatment measure design for instance. AECOM applied MUSIC in this study to evaluate the performance of the bioretention both with respect to the quantity of water provided and the quality that could be expected.
MUSIC can simulate the removal of some key pollutants, namely total suspended solids (TSS), total nitrogen (TN) and total phosphorus (TP). The model results indicated that annual TSS removal could reach 70 percent and 40 to 60 percent respectively for TP and TN annual removal.
Chlorination is applied to minimize the health impact brought by pathogens and provide residue disinfection effect along the distribution pipes. Since MUSIC is only used for conceptual planning, in order to safeguard the rainwater quality, water samples must be collected on-site during testing and commissioning stage and on a regular basis during the maintenance period.
Besides the evaluation of water quality, MUSIC can also estimate the amount of water that will be available in the water storage tanks. The size of water tanks is adjusted based on the modeling results to optimize the storage of water while balancing the site conditions and the available area for catchment, bioretention and tanks. The model results showed these water tanks would be 50 percent full for over 100 days in a year. Supplementary water is required during the dry days.
With the incorporation of civil, plumbing, geotechnical, electrical and mechanical engineering design as well as the landscape and architectural design, the detailed design of the bioretention and the whole rainwater harvesting system were developed.
The project incorporates educational elements to introduce this natural treatment system to the general public and promote sustainable urban ecosystem development and water conservation. This project has been showcased at industry conferences and has led to ongoing cooperation between AECOM and the government of Hong Kong.
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