FIRST FLUSH CONTROL FOR URBAN RAINWATER HARVEST SYSTEMS

Transcription

1 FIRST FLUSH CONTROL FOR URBAN RAINWATER HARVEST SYSTEMS Che wu, Li junqi, Liu Yan and Wang wenhai (Beijing Institute of Civil Engineering and Architecture, Beijing, China) Liu hong, Meng guanghui, Wang hongling (Beijing Urban Water Conservation Office, Beijing, China) Abstract: First flush control is a very important measure to control water quality of urban rainwater harvest system due to the pollution of rainwater in urban area. This paper analyzes the regularity of runoff flush to pollutants on roof and road surfaces, introduces an effective facility of first flush control, which was designed based on a lot of measurements and the analysis, and it s applications in several rainwater utilization projects in Beijing. Key words : urban rainwater, harvest system, first flush, water quality control

2 1 INTRODUCTION Beijing faces critical challenge of water resource scarcity and water ecological problem. Rainwater harvest is a potential alternative to mitigate the water crisis and receives recently more and more attention. However in urban area, rainwater, especially, first flush of runoff may be polluted. So water quality is an important consideration in designing a harvest system. First flush control is a very important and efficient measure to control water quality of urban rainwater harvest system. This paper analyzes the regularity of runoff flush to pollutants on roof and road surfaces, introduces an effective facility of first flush control, which was designed based on a lot of measurements and the analysis, and it s applications in several rainwater utilization projects in Beijing city. 2 BRIEF DESCRIPTIONS ON URBAN RAINWATER QUALITY Urban rainwater quality is quite different in different cities or countries, which is related to local natural and environmental conditions. Generally, in regions where rainfall is relative small and environment pollution is more severe the quality is worse. Rainfall quality in some cities of developing countries could be bad. That should be carefully taken into account by decision-making and choosing of harvest alternatives. Table 1 and Table 2 show quality of urban runoff in Beijing. Some pollutants are high. 3 FIRST FLUSH CONTROL OF RAINWATER HARVEST SYSTEM 3.1 First flush of urban runoff Based on measurements of ten rainfall events during , Figure 1-3 show some typical curves of main pollutant concentration. A summary of the storm water quality are listed in table 1and table 2.

3 According to the statistical analysis of runoff, flush process to pollutants is principally as: Table 1: pollutant concentration of stormwater runoff from different catchments in Beijing city Catchments Stormwater from atmosphere Stormwater from roof Stormwater from road Pollutants Variation Variation (mg/l) Pitch Tile coefficient coefficient COD SS < Pb < Zn TP TN Table 2: Pollutants of first flush from different catchments in Beijing city Stormwater Catchments from atmosphere Stormwater from roof Stormwater from road Pollutants Variation Variation (mg/l) Pitch Tile coefficient coefficient COD SS < CAS Pb < Zn Phenol Oil and grease TP TN following: (1) Where:k is flush coefficient,co is pollutant concentration at the beginning of first flush,

4 h(t) is rainfall at duration t. The data for a storm event of June 9, 2002 is taken as an example (Figure 1 and Figure 2). Results show reasonable regression equations and degree of fit # $$ # $$ ,/ 8:7 1,.0 0 # ,/ 8:7 1,.0 0 # , # /:7, Figure 1: COD and SS from urban runoff in different catchments (9 June 2002) pollutants 2 % # %! # %174274,/ 0 # %! ,/ 0 # % ,/ 0 # %! ,/ 0 # /:7, Figure 2: TN and TP from urban runoff in different catchments (9 June, 2002) 3.2 Split volume of first flush According to the flush law, controlling a certain quantity of first flush can obviously cut down runoff pollutants load into a harvest system. Based on the formula (1) and actual flush curves, a critical split quantity of first flush H 0 could properly be determined, at which pollutants concentration falls to relatively low and stable levels.

5 From a lot of monitoring data, generally, at the same storm event, the initial pollutant concentration from roof is usually lower than from road and k of road is smaller than that of roof. That is to say, H 0 and the stable value of C h from roof are smaller than from road surface. The comparisons between typical flushing curve of runoff from roof and road showed in figure3. C h from different road surface is higher than from roof and k is smaller than from roof. The rainfall at which concentration from roof fall to low levels is about 3mm and from road is 12-14mm. H 0 for road runoff is about 3-4 times higher than that for roof. Figure 3: Comparison of typical flushes from roof and road catchments A large quantity of monitoring data and theoretical analysis indicate that C h from roof tends to keep a lower stable value at 1-3mm rainfall (pitch roof at top limit, tile roof at bottom limit), C h from road surface tends to keep a lower value at about 6-8mm rainfall at most conditions. And it could cut down about 60% or more pollution load entering into a harvest system from a rainfall event if the stated volume of first flush is split away. 3.3 Spliter of first flush Figure 4 shows an effective and practical facility of first-flush controlling designed to

6 split first flush from main flow into harvest system, which could automatically control about 60%-80% or more pollutant load for each rainfall event depending on rainfall and H 0 chosen. In order to make volume of the first flush tank small when catchment area is relative big a new mode (patent) has been developed, by which the volume could be only one several tenth smaller than normal type and with comparable convenience and efficiency. Figure 4: A type of first flush spliter 4 APPLICATIONS project 1: This is a residential area with a total 29-hectare area, a small lake of 3.8 ha surface and 15.3 ha greenbelt, located in outskirt of Beijing city. In order to save water resource and prevent the lake from pollution an ecological water system (see Figure 5) has been designed. amount of 87 thousand m 3 rainfall harvest per year could be utilized.

7 Figure 5: An ecological water system with rainfall harvesting project 2: An Elementary School is located at western district of Beijing. Total plot area is 5,800 m 2, including floor area 1,420 m 2, playground & road area 4,100 m 2 and green area of 280 m 2. It is required that rainwater from building roof and partial playground rainwater would be collected and utilized for a ship-playing tank, a waterscape tank and greenbelt demand after purification. Purification requirements for this project follows water quality standard (CJIT ) of the regenerated water for landscaping water: Additionally, due to the uneven distribution of rainfall in Beijing area, to maintain the continuous operation of the system, washing wastewater from kitchen is also collected. The process flow sheet is shown in Figure 6.

8 Figure 6: Flow Sheet of the Primary School Rainwater Harvest & Utilization project 3: Figure 7 is a flow sheet of harvest system in the courtyard of Haidian District Government Office Area. The catchment area in the courtyard is about 3 ha and only half of it is used for harvest. First flush spliters are also adopted in this system as a very important operation. Figure 7:Flow sheet of a rainwater harvest system Note: 1-road runoff, 1 -roof runoff, 2-spliter facilities, 3-stormwater manhole, 4-downstream manhole, 5-municipal sewer, 6-storage tank, 7-purified rainwater tank, a-splited first flush, b-overflow REFERENCES [1] Che wu, Liu yan and Li junqi. Flush Model of Runoff on Urban Non-Point Source Pollutants and Analysis. (adopted by IWA, Water in China-Water and Environmental Management Series) [2] Che Wu, Wang huizhen, Li junqi etc. The Quality and Major Influencing Factors of Runoff in Beijing s Urban Area. 10 th international Conference on Rainwater Catchment Systems. Fakt and IRCSA/Europe. Mannheim (Germany), 2001:13-16 [3] Che Wu, Liu Hong, Wang huizhen etc. Research on Utilization and Pollution of Roof Runoff in Beijing[J]. China Water and Wastewater, 2001,17(6):57-61

9 [4] Che Wu, Liu Yan and Li Junqi. Evaluation and Control of Pollutant Input from Urban Runoff in Beijing. (adopted by 3 RD SOUTH PACIFIC CONFERENCE ON STORMWATER MANAGEMENT, AUCKLAND, NEW ZEALAND 14-16, MAY, 2003)