4.3.1 Recycling of water resources in the United States and its effects
4.3.1.1 Reclamation and reuse of wastewater in the United States
Reclamation and reuse of urban wastewater in the United States started earlier. At present, the amount of urban wastewater reused in the United States is 9.37×18m3/d, including ① reuse irrigation 5.81×18m3/d (accounting for 62%), including agricultural irrigation 2.75×18m3/d, landscape irrigation .46×18m3/d and others 2.6× 18m3/d; (2) Industrial recycling is 2.86×18m3/d (accounting for 31.6%), including .91×18m3/d for process water, 1.96×18m3/d for cooling water and .9× 18m3/d for boiler make-up water; ③ Recharge groundwater .47× 18m3/d; ④ Other recycling (entertainment, fish culture, wildlife habitat, etc.) is .13× 18m3/d..
There are 536 reclaimed water reuse points in the United States, including 238 in California. Here are some examples of wastewater regeneration and reuse in the United States.
① Reclaimed water from 21st century waterworks in Orange County, California is reinjected underground. Due to the over-exploitation of groundwater in Juzi County, the groundwater level is lower than the sea level, which promotes the continuous flow of seawater inland, resulting in the degradation of underground fresh water and unfit for drinking. In order to prevent seawater intrusion caused by the drop of groundwater level, as early as 1965, Juzi County began to study the reinjection of tertiary treated water into the ground to prevent seawater intrusion. Juzi County has built the 21st Century Water Plant with a design capacity of 5678m3/d.. The raw water is the effluent of secondary treatment of municipal sewage, which is further treated by precipitation, filtration and activated carbon and then reinjected into groundwater. Because the limit of total dissolved solids in groundwater recharge is 5mg/L, a part of reclaimed water is desalted by reverse osmosis before recharging groundwater. In the 21st century, the purified water from waterworks is injected into four aquifers through 23 multi-point injection tube wells, and mixed with deep aquifer well water at the ratio of 2∶1 to prevent seawater intrusion. The project shows that it is feasible to control seawater intrusion manually. After advanced treatment, municipal wastewater can meet the drinking water quality standard; The project has been proved to be stable and reliable by long-term operation.
② wastewater regeneration and reuse in St. Petersburg, Florida. St Petersburg is one of the pioneers of urban wastewater reuse. In 1978, a dual water distribution system was implemented to provide users with two kinds of water (drinking water and non-drinking water), and reclaimed water began to be used for non-drinking purposes. In 1991, the city provided 8×14m3/d reclaimed water to more than 7, families and office buildings, and used it as irrigation water for parks, playgrounds and golf courses, as well as cooling water for air conditioning systems and fire fighting water. There are four wastewater treatment plants in this city, with a total treatment capacity of 27× 13m3/d; Activated sludge biological treatment process is adopted, and aluminum salt coagulation, filtration and disinfection treatment are added. The pipeline of double-pipe water delivery system is 42km long. Excess reclaimed water is injected into saline aquifer through 1 deep wells, and about 6% reclaimed water is injected into deep wells on average in a year. Because the use of reclaimed water saves high-quality water, although the population of this city has increased by 1%, drinking water can still meet the supply.
(3) Recycle reclaimed water as cooling water in Palo Verde nuclear power plant in Arizona. Palo Verde nuclear power plant is the largest nuclear power plant in the United States. Three reactors in the third phase were put into operation in 1982, 1984 and 1986 respectively, each with a power generation capacity of 127MW. In addition, it is planned to build two more reactors. The nuclear power plant is located in the desert, which is severely dry, so reclaimed water is used as cooling water. Reclaimed water comes from the secondary biological treatment effluent of wastewater treatment in two cities, and is transported to nuclear power plants for supplementary treatment to make it reach the required water quality. The nuclear power plant adopts cooling water system, and the make-up water is about 2× 14m3/d.
4.3.1.2 Effect of recycling water resources in the United States
In the past 5 years, the water consumption in the United States has reflected the water consumption change process of a country that has completed its industrialization task and entered post-industrialization in different periods (as shown in Figure 4.1). In 195, the total water consumption of American national economy was only about 25 billion m3, among which agriculture was the largest water user. Since then, with the development of American economy, the water consumption has continued to grow, reaching a peak of about 61 billion m3 in 198. After 198, the water consumption dropped significantly, and basically stabilized at around 55 billion m3. By 2, industrial water consumption had decreased, and the total water consumption had dropped to about 48 billion m3.
Figure 4.1 Changes of water consumption in the United States from 195 to 2
During the 3 years from 195 to 198, the national economic water consumption in the United States increased rapidly, during which the American economy developed rapidly, and heavy industries dominated by metallurgy and chemical industry developed rapidly. With the development of these high water-consuming industries, industrial water consumption increased rapidly, from 16.3 billion m3 in 195 to 35 billion m3 in 198. Although agricultural water consumption is also growing rapidly, the growth rate is less than that of industry, and industry has become the largest water user. Since 198, emerging industries and service industries, mainly electronic industries, have become the leading industries to stimulate economic growth. The proportion of service industry in GDP has been rising. At the same time, technological progress has greatly improved water use efficiency, and industrial and agricultural water consumption has been declining, making the total water consumption basically stable and slightly declining. Although the domestic water consumption has changed, it has little overall impact on the change of water demand because of its small proportion.
4.3.2 water resources recycling in Japan and its effects
4.3.2.1 wastewater recycling and reuse in Japan
in recent 2 years, Japan has carried out a lot of research and development and engineering construction in wastewater recycling and utilization. In 1986, the reuse amount of urban wastewater reached 63×14m3/d, accounting for .8% of the total urban wastewater treatment. Reclaimed water is mainly used for middle waterways, industrial water, farmland irrigation, river water supply, etc. Various uses and their proportions are: 4% for middle waterway system, 29% for industrial water, 15% for agricultural water, and 16% for landscape and snow removal. The middle waterway system is a typical representative of sewage reuse in Japan. In 1988, there were 844 sets of middle waterways in Japan, among which office buildings and schools were large. Schools account for 18.1%, office buildings account for 17.3%, public buildings account for 9.2% and factories account for 8.4%. Reclaimed water from Middle Waterway is mainly used for flushing toilets (37%), roads (16%), urban green spaces (15%), cooling water (9%), cars (7%) and others (landscape, fire fighting, etc.) for 16%.
4.3.2.2 Effect of water resources recycling in Japan
According to the statistical survey data of the Ministry of International Trade and Industry and the National Land Agency of Japan, the industrial and domestic water consumption in Japan has increased rapidly since 1965, with the industrial water consumption increasing by 1.5 times and the domestic water consumption increasing by 1.3 times in the ten years from 1965 to 1975, which is the fastest period of water consumption growth in Japan. With the acceleration of industrialization and urbanization, The reuse rate of industrial water in Japan was 36% in 1965, increased to 67% in 1975 and reached 78% in 2. Urban water supply system can prevent water leakage by replacing aging tap water pipes in time, improve the popularity of water-saving appliances, and actively encourage the use of non-traditional water sources such as reclaimed water and rainwater. In agriculture, encourage the construction of wastewater treatment facilities, irrigate farmland with purified wastewater, change traditional irrigation methods and popularize water-saving irrigation technology. Since 197s, Japan's water consumption has basically stabilized at around 9 billion m3 (as shown in Figure 4.2), while agricultural water tends to be stable, industrial water slowly decreases and domestic water steadily increases. Due to the lack of resources, Japan's energy and raw materials industries, which consume a large amount of water, account for a small proportion in the national economy, and its high-tech processing and manufacturing industries are developed. As in the United States, industrial water use has not experienced a ups and downs due to the adjustment of industrial structure.
fig. 4.2 changes of water consumption in Japan from 195 to 2
4.3.3 Recycling of water resources in other countries and its effects
The world's first recycling plant directly using reclaimed urban wastewater as drinking water source is located in Windhoek, the capital of Namibia. The recovery plant was put into operation in 1968. The water yield in the first stage was 23m3/d, and the normal treatment capacity could reach 45m3/d, and then increased to 62m3/d.. The raw water is the secondary biological treatment effluent from the municipal wastewater plant, and the treatment process is shown in Figure 4.3.
figure 4.3 secondary biological treatment process of urban wastewater plant
the water quality of advanced treatment water has been strictly monitored and proved to meet the standards issued by the World Health Organization (WHO) and the US Environmental Protection Agency.
Israel is a semi-arid country, and reclaimed water has become one of the important water resources in the country. 1% of domestic wastewater and 72% of urban wastewater have been reused. According to the data in 1987, the national wastewater is 2.5×18m3, with a treatment capacity of 2.18×18m3, and the treatment rate is close to 9%. Reclaimed water is used for irrigation up to 1.46×18m3 (accounting for 42%), reinjected to underground water is .7×18m3 (accounting for about 29%), and the discharged seawater is .7×18m3 (accounting for about 29%). The wastewater is treated and stored in waste reservoirs. 127 abandoned reservoirs have been built in China, including 123 on the ground and 4 underground. Wastewater is generally treated by stabilizing pond system before agricultural irrigation. In some cities, the effluent from secondary biological treatment in cities is reused as industrial cooling water after physical and chemical treatment. In addition, after advanced treatment, the wastewater is reinjected into the groundwater, and then pumped to the pipe network system, or incorporated into the national water resources allocation system, and transported to the southern region, or used in the general water supply system, and even used as a drinking water source in the southernmost region. As a result of the above-mentioned wastewater reuse measures, Israel has greatly improved the effective utilization of water resources, thus alleviating the restrictive effect of water shortage on social and economic development.
Kuwait uses municipal wastewater after tertiary treatment for agricultural irrigation. By 1985, at least 2 farms in India had been irrigated with municipal wastewater, covering an area of 23hm2. Saudi Arabia used 9,m3/d of reclaimed water in 1975, and the planned water consumption in 2 is 19×14m3/d, 1% of which will come from reclaimed water of municipal wastewater after secondary treatment or even tertiary treatment.