Heavy rain is a godsend for a city without water. In fact, if the collected rainfall is effectively treated and utilized, it is more economical and convenient than the treatment of light polluted domestic sewage.

How can we collect this rainwater well without letting it go for nothing? The main experience of urban rainwater utilization in developed countries abroad is that a series of laws and regulations on rainwater utilization have been formulated. A complete roof water storage system and surface irrigation system composed of infiltration ponds, Wells, grasslands and permeable ground were established. The rainwater collected is mainly used to flush toilets, wash cars, water yards, wash clothes and recharge groundwater.

The United States: mandatory local detention of flood waters in new development zones

Rainwater use in the United States is often aimed at improving natural infiltration capacity. For example, the "LeakyAreas" underground recharging system in Fresno, California, had a total of 133.8 million cubic meters of groundwater recharging over a 10-year period (1971-1980), accounting for 20 percent of the city's annual water use. An underground tunnel impoundment system was built in Chicago to solve the city's flood and rainwater problems. In many other cities, roof water storage and surface recharging systems consisting of infiltrating ponds, Wells, grasslands and permeable ground have also been established. The United States not only attaches importance to engineering measures, but also has laws and regulations to support the use of rainwater. For example, Colorado (1974), Florida (1974) and Pennsylvania (1978) respectively enacted the rainwater utilization regulations. These regulations stipulate that the rainstorm, flood and flood peak flow of the new development zone should not exceed the pre-development level. All new development areas (excluding single-family households) must be subject to mandatory "local demilitarized water storage".

The century dome of London: use rainwater to flush toilets

Taking London's century dome rainwater collection and utilization system as an example, the Thames water company designed the century dome demonstration project, a British exhibition building in 2000, in order to study different scale water circulation schemes. Within the building, 500 cubic metres of water are recycled daily to flush toilets in the building, of which 100 cubic metres are collected from roofs. This makes it the largest water circulation facility in a building in Europe. Rainwater collected from 100, 000 square metres of domes the size of 12 football fields is fed into surface-water drains through 24 specially designed sinks. The initial rainfall water contains pollutants washed down from the domes and is discharged directly into the Thames through surface-water drains. Due to the limited storage capacity, the surplus rainwater is discharged into the Thames. Rainwater collected is treated in reed beds, which can also increase the diversity of urban landscape.

Germany: using rainwater can save 2,430 cubic meters of drinking water every year

Rainwater can be collected by using public water pipes. After simple treatment, the water quality standard of mixed water can be reached, which can be used for toilet flushing and courtyard pouring in block apartments. For example, the HlankWitzBeless-luedeckeStrasse apartment in Berlin was built in the 1950s. After renovation and expansion, the number of residents increased rapidly and the roof area increased only slightly. By adopting new hygiene principles and effectively combining with rainwater collection, the maximum rainwater collection is realized. Rainwater collected from roofs, surrounding streets, parking lots and passageways is channelled through separate rainwater pipes into underground storage tanks. The storage tank has a capacity of 160 cubic meters, which is used for flushing toilets and pouring courtyards after simple treatment. Using rainwater can save 2430 cubic metres of drinking water per year.

In addition, Germany has formulated a series of laws and regulations on the use of rainwater. For example, before the construction of new residential areas in Germany, rain water utilization facilities should be designed in both industrial, commercial and residential areas. If there is no rainwater utilization measures, the government will collect the rainwater drainage facilities fees and rainwater discharge fees.

Rain water is a substitute for water to reduce groundwater consumption

More than 98 percent of Denmark's water supply is groundwater. However, because the current groundwater utilization rate is less than 1 except in the area around Copenhagen, the aquifer in some areas has been overexploited. As a result, alternative water sources have been sought in Denmark to reduce groundwater consumption. Rainwater is collected from roofs in urban areas, and the collected rainwater is stored in storage tanks through pre-filtration equipment at the bottom of the collection pipe. When used, the pump is filtered by a floating filter at the inlet, which is used for flushing toilets and washing clothes. During a seven-month rain period (from July to January), the amount of water collected from the roof is enough to flush the toilet. The amount of water needed to wash clothes is only four months. 6.45 million cubic metres of rain can be collected from the roofs of households each year. If it is used to flush toilets and wash clothes, it will account for 68 per cent of the actual water used by residents to flush toilets and wash clothes. This is equivalent to 22% of the total water consumption of residents and 7% of the total drinking water output of the municipal government.

Rainwater infiltration facilities throughout the front and back of buildings

In 1963, Japan began to build flood storage ponds for flood detention and storage of rainwater, which is also used to spray road surfaces, irrigate green areas and other urban miscellaneous water. Most of these facilities are built underground to make full use of underground space. And those built on the ground can be used for as many purposes as possible, such as constructing a sports field in the flood control pond, storing up the flood in the rainy season and using it as a sports field in ordinary times. In recent years, various rainwater infiltration facilities have been rapidly developed in Japan, including seepage Wells, seepage trenches, seepage ponds, etc. These facilities cover a small area and can be built in accordance with local conditions in front and behind buildings. In 1992, Japan promulgated the "second-generation urban drainage master plan", which officially took rainwater seepage trenches, seepage ponds and permeable ground as an integral part of the overall urban planning, requiring large public buildings to be built and rebuilt with on-site rainwater infiltration facilities. According to the simulated test conducted by the association of rainfall storage and infiltration technology of Japan, in the areas where the combined rainwater pipe system is used, the density of various infiltration facilities should be reasonably configured to strengthen the infiltration of rainwater, so that rainfall can enter the ground at a rate of 5mm/h.

In conclusion, the main experience of urban rainwater utilization in developed countries in foreign countries is that a series of laws and regulations have been formulated. A complete roof water storage system and surface irrigation system composed of infiltration ponds, Wells, grasslands and permeable ground were established. The rainwater collected is mainly used to flush toilets, wash cars, water yards, wash clothes and recharge groundwater.

Beijing tuan cheng: a masterpiece of ancient rainwater utilization projects

The idea of rainwater utilization in urban areas in China has a long history, which is produced with the construction and development of the ancient capital. In the real sense, the research and application of urban urban rainwater utilization began in the 1980s and developed in the 1990s. But in general, the technology is still backward, lack of systematization, and lack of laws and regulations guarantee system. Since the 1990s, some buildings in China's megacities have built rainwater collection systems, but no treatment and reuse systems. If the collected rainfall can be effectively treated and utilized, it is more economical and easier to handle than light polluted domestic sewage.

Next, the ancient rainwater utilization project of group city of beihai park in Beijing is introduced in order to enlighten the current rainwater utilization.

The ground of tuan cheng rises 4.6 meters above the surrounding ground and is paved with green bricks. However, many ancient trees in the city have grown for hundreds of years. The oldest trees in the city are over 800 years old and still have luxuriant branches. The reason is that the city has a unique rainwater discharge and utilization system.

Tuan cheng is an isolated and closed unit, and the ground is 5.64 meters above the water surface of the north sea lake. Therefore, the water needed for ancient trees to grow is difficult to be replenished from the groundwater, only by natural precipitation.

With a total area of 5,760 square meters, the annual average annual precipitation in Beijing is 595 millimetres. In addition to the uncontrollable evaporation loss, the ancient people gave careful consideration to how to make use of the remaining rainfall water to meet the needs of the growth of ancient trees.

Unlike other ancient buildings in Beijing (such as the Palace Museum, zhengyang gate archery building, drum tower, Great Wall and beihai white tower, etc.), there are surface drainage ditch and drainage stone trough (spitting mouth) on the wall, indicating that the ancient people had intended to keep the precipitation in the city as far as possible.

The flooring of tuan cheng is different from that of other ancient buildings. Except buildings and ancient trees occupy part of the ground, the rest of the ground is paved with green bricks. According to the form and function, the ground of green brick paving is divided into two types: a small part of square brick and brick paving into yongdao, non-seepage water, for people to walk; The rest is made of trapezoidal green bricks with large and small sides facing up and down while paving. There are gaps between bricks and bricks.

There is a drainage corridor in the underground of the city, which is arranged around the city. It is built of green bricks. The drainage corridor is also built with green bricks. When the water is small, the water will gradually infiltrate into the surrounding soil through green bricks.

In a word, the ground of tuancheng adopts the practice of paving inverted trapezoidal green bricks and installing underground drainage corridor, making full use of the natural precipitation, and creating a suitable growth environment for ancient trees. This technology has a long history and exquisite ideas, and is a masterpiece of human engineering in the use of rain water.

As far as today's cities are concerned, it is suggested to combine the rainwater collection system with the treatment and reuse system. Only the rainwater collection system, without the rainwater treatment and reuse system, cannot realize the reasonable utilization of rainwater resources. The collected rainwater can be used for a variety of purposes. The collected rainwater can be used to flush toilets, wash cars, water yards, wash clothes, water greenbelts, fire fighting and recharge groundwater directly or after proper treatment.