Review of Water Conservation Technologies and practices

Review of Water Conservation Technologies and practices

Qiu Shenchu
(National Engineering Research Center for Urban Water & Wastewater)

(Abstract of Comprehensive Report)

1. The term of water conservation

The term of water conservation means the preservation, control and development of water resources (both surface and ground water) to ensure that the largest possible amount of water is made available for all purposes in the most suitable and economical way. So water conservation is an inseparable part of water management.

2. Main Situation of Water Conservation

It has been estimated that by the 2025 one third of the world‘s population will be living in a water-scarce environment. Today high population areas such as China, the Indian sub-continent, northern Africa and the Middle East are already suffering the consequences of over-using their groundwater and are anxiously looking for alternative ways to increase their water supplies. Some experts say the situation is beyond remedy and bound to increase global tensions, eventually leading to wars over water.

There is a shortage of water resources in China. The total amount of fresh water resources in China is approximately 2.62 trillion cubic meters annually, the average per capita figure being 2392m3/year, which is about one fourth of the average of the entire world and stands 110th on the global list. Along with the rapid national economy growth and increasing activities of cities, the shortage and contamination of water resources have occurred in many places, especially in the north region and some coastal cities in China. Severe draughts in cities, such as Taiyuan, Qingdao and Dalian, have given rise to a strong incentive for practicing wastewater reclamation and reuse.

In recent years, the government has paid close attention to water shortage and pollution problems. The construction of water and wastewater works has been promoted. Up to the end of 1997, the urban water supply capacity of China reached 190 million m3/d, but many cities in China still lack water.

At present, there are 668 cities in China. Among them, about 330 cities are short of water. As estimated according to current rate of economic growth, water shortage in cities by the year 2000 will exceed 2.86 billion m3 annually. The total amount of urban wastewater discharge is about 21.2 billion cubic meters per year, but the amount of wastewater treated is approximately 13.7% of the total. Hence, most of the wastewater is discharged to the water body without being treated. It was observed that surface water was heavily polluted. In addition, about two thirds of China‘s cities depend on groundwater as the main water source. At present, there are many problems existing with groundwater development and protection, among them are inadequate management, over exploitation, continuous lowing of the groundwater table, and the general pollution of urban subterranean water. These problems directly affect the sustainable utilization of groundwater resources. For this reason, many cities have no choice but to take their water from distant sources, hence the number of long distant water conveyance projects increase with time. Under the above-mentioned circumstances, shortage and contamination of water resources have become a factor, which is causing the restriction of national economic growth in China. At present, sustainable growth is the leading policy in further development aiming at economic growth without negative impact on the environment. Therefore, adopting effective countermeasure of water conservation, which tallies with China‘s national condition, is of vital importance.

3. Water Tariff

The water tariff is an important issue to water conservation and it should be reasonably fixed. Rational water tariff can promote the social and economic development, reduce the water demand and improve the water environment. As commonly known, water is a kind of commodity, but the current water tariff is made much lower than its market value. Low water tariff is unfavorable to encourage consumers to take effective water-saving measures and also disadvantageous to sustainable development of water industry. In short, the tap-based unified water fee collection system should be applied as soon as possible. Once the concept of water as an economic good has been accepted, it is necessary to move on to calculate not only the costs involved in the provision of water, but also the economic and environmental externalities connected with its use. The context within which included water prices, effluent charges and incentives for pollution control must be fixed.

In most cities of China, each household is provided with a meter. The clean waterside knows the consumption of water for each household, so they can calculate the water charge, which may include capital, maintenance and operation cost for each household based upon that data. In the case of dirty waterside, they should also utilize that data from the same water meter and they can estimate the sanitary sewage portion. Sewer user charge can be calculated in the same way as water charge by using water consumption data based upon water meter separately and might become much higher than water charge. In addition, an incremental charge system for water and wastewater needs to be introduced for fairer allocation of expenses. In this system, the unit charge for 1 m3 increases as the total volume increases. The water quality-based user charge system for all kinds of industries should be also introduced in the near future. The establishment of water and wastewater charge system will certainly promote the progress of water conservation in China.

"Sewer user charge system in Osaka municipal government in Japan" and "fee collection system for water supply and wastewater treatment in Florida, USA", and "urban sewer user charge system in China" were introduced in this report.

4. Technical Measures on Urban Water Conservation

In order to relieve the crisis in water shortage regions, water conservation must be taken as an important policy. Water conservation should be promoted by following economic, technical and engineering measures:

It is important to improve leak-detecting technique in an effort to reduce water leakage in pipe network. In some cities, sensitive leak detectors are employed which can find accurately out the leakage section. A widespread adoption of new pipe and ring joint has obtained some results in the decrease of leakage loss.

A great deal of water can be saved by taking discharging water from water treatment plant for its own use. Recently in the design of water treatment plant, the backwash water in filter and the discharging water in sedimentation tank are to be thickened and the supernatant will be reclaimed.

The application and dissemination of perfect water-saving facilities will have significant effect on the domestic and public water saving. Especially, adoption of water saving flesh toilet will not only lead to the economic use of great amount of water, but will also reduce the water charge, and so, it should be popularized.

5. Technical Measures on Industrial Water Conservation

Industrial water conservation is comprised of the recirculation use of cooling water, reutilization of process water among various processes and plants. The recirculating use of cooling water is the main water conservation measure of industry. It is mainly dependent on the efficiency of cooling tower, water quality stabilization technique, and type and category of media used in cooling tower. Innovation and popularization of water saving production process is of vital importance. It is possible to have the plant achieving zero discharge by upgrading wastewater treatment technology and using the treated water for production and form a closed recirculation system in the plant. Economizing water use in industry can not only save running cost but also can control water pollution. Low or zero water-consuming industrial processes should be advocated for adoption in order to raise the efficiency of water conservation.

6. New Development and Practice of Wastewater Treatment

A so-called "Advanced Primary Treatment (APT)", an alternative for Biological Secondary Treatment (BST), is introduced to expand the treatment capacity of existing treatment plant without spending more money. It is possible by APT to obtain a clear effluent substantially free from matter in suspension or in the colloidal state. From 80 to 90 percent of the total suspended solid, 50 to 70 percent of BOD, 40 to 60 percent of COD, 80 to 90 percent of the bacteria, and 70 to 90 percent of phosphorus can be removed by APT. In comparison, when conventional sedimentation is used, only 50 to 60 percent of total suspended solid and 25 to 40 percent of BOD settles out. Therefore, special concern for APT is necessary in the process evaluation and selection.

6.1 Description of major Biological Secondary Treatment (BST) processes

The existing requirement on the efficiency of wastewater treatment plant for nutrient removal has been increased considerably with regard to the pollution load of water bodies. For this reason, much attention has been focused on the removal of phosphorous and nitrogen in order to meet the requirement of wastewater discharging standard and to prevent water bodies from eutrophication and reuse wastewater in the region where lack water. New wastewater treatment plants have been constructed and existing plants have been modified for nutrient removal.

Conventional biological secondary wastewater treatments (BST) allow efficient treatment of organic substances, greatly reduced the BOD of the effluent. However, the removal efficiency of phosphorous and nitrogen is low. Under this circumstance, wastewater treatment plants discharging the effluent into closed and semi-closed water bodies are required to promote nutrient removal for eutrophication prevention. Among phosphorous and nitrogen removal processes, two major biological nutrient removal processes, which have been developed and put to practical application in China, are briefly described below:

6.1.1 Cyclic Activated Sludge Technology (CAST)

According to the information report and a survey of some SBR systems to be undertaken in United States by the author, numerous variations of SBR are currently in use. Among them the CAST process developed by Goronszy typically treats municipal wastewater with a 4-hour cycle (2 hours of aeration and 2hours of non-aeration), and storm-water flows with a 2-hour cycle. Influent enters the reactor at aeration phase and there is no inflow for non-aeration phase, so that normal system operation consists of the following phases: fill-aeration, non-fill-settle, non-fill-decant and fill-idle. This system is preceded by an anaerobic captive selector with sludge recycling from main aeration zone to the selector. Many Chinese experts pay close attention to CAST process. In addition to industrial wastewater application, now CAST process has already been applied for municipal wastewater treatment in China.

6.1.2 Oxidation Ditch Process

Oxidation Ditch Process is proved to have important advantages in comparision with conventional systems:

- Pre-sedimentation was not needed;
- The process was stable and reliable;
- The process control was simple;
- The sludge was relatively stabilized and could be dewatered and dried directly without digestion.

The system performance showed reduction rates of 95-98% for BOD and 80-90% for COD respectively together with a high ammonium removal degree. The purification results were better than with conventional system. However, the traditional oxidation ditch has some disadvantages, eg. the land area required was large because of limitations to applicable channel depth.

Three types of oxidation ditch process are introduced in this report, ie, Carrousel System, Alternating System, and Orbal System. Each type of the oxidation ditch system has its advantage and disadvantage and their mutual feature is the increased process adaptability for nitrogen and phosphorus removal. By using purely biological nitrogen/phosphorus removal means, level of 6-10 mg/L of TN and 1-2 mg/L of TP in the effluent can be reached. Now the application of oxidation ditch process for municipal wastewater treatment has been popularized in China.

6.2 Description of Advanced Primary Treatment (APT)

Advanced primary treatment (APT) of phosphorus, suspended solids, and organics has been applied for a long time in the wastewater treatment in several countries, particularly in Scandinavia, Switzerland, and the province of Ontario in Canada. Many plants are today also in operation in the US and Germany. In conventional wastewater treatment, chemical precipitation has thus mostly been used as a complementary addition to a biological process in order to achieve a high degree of phosphorus reduction. In addition to phosphorus removal, a portion of the organic matter of the raw wastewater will be removed. Bacteria, virus, and eggs of intestinal parasites are also effectively removed from the wastewater as well as heavy metals. In Scandinavia, the weather is very cold and the price of chemical coagulant is rather low, so APT achieves a good purification of wastewater at low capital and operating costs. The situation in China is different from Scandinavia, the process selection should be considered in a way, which is compatible with China‘s national condition. For effluent total phosphorus concentrations greater than 0.5 mg/L, metal salt dosage will typically vary from 1 to 2 moles of metal salt added per mole of phosphorus removed. In order to comply with the stringent legal limit for phosphorus, the operatingl costs of APT will be much higher than those of BST. Operating and maintenance costs of APT system are typically the largest component of the total present worth for the system. Operating costs include chemicals added to the wastewater, power, labor, maintenance, and costs of increased sludge handling. The additional generation of chemical sludge from APT, which in turn must be disposed of, is a very complicated problem to deal with. To a large treatment plant in an urban community, the metal salt dosage and the additional volume of sludge for disposal can be very costly, while the impact at a small rural facility can be minor. At present, the APT system could be used for wastewater treatment in small-sized cities and regions of severe cold in China.

7. Wastewater reclamation and reuse

It is said that the municipal wastewater will be one of the major water resources in the urban areas in China and more studies on requirements and health considerations associated with non-potable reuse will be needed. The economic development of the country requires additional water resources, and efforts to develop non-conventional sources are continuing. Treated wastewater already comprises an unplanned, but significant component of our nation‘s fresh water supplies through discharge to streams, lakes, and groundwater basins used to supply domestic, industrial, and agricultural water demands. Wastewater must be treated by appropriate technologies to assure that potentially harmful contaminants are reduced to levels, which will not impair the use of the receiving water. Reclamation and reuse of municipal wastewater has become a vital issue in the development and preservation of the national water resources. Since 1980s, China has made efforts in development of wastewater reuse projects. Appropriately treated wastewater can be used directly for industrial purposes as well as irrigation including food crops, golf courses, parks and playgrounds and to supply recreational water bodies used for boating, fishing etc. Treated wastewater is also used to provide recharge to groundwater basins. The government strongly supports the use of reclaimed wastewater for non-potable purpose as a means of conserving limited quantities of high-quality potable water supplies.

Three categories of reclaimed wastewater applications are mentioned which are important in the implementation of wastewater reuse in China- farmland irrigation, groundwater recharge, and industrial cooling.

7.1 Farmland Irrigation

The farmers in China have begun to make use of municipal wastewater for farmland irrigation since 1950s. In addition to providing a source of additional water for irrigation in water shortage areas and nutrients for growing valuable crops, returning the wastewater to the land often prevent pollution of rivers, lakes and oceans and aids in protecting the quality and safety of drinking water sources. In the past, because most wastewater were untreated and used for irrigation directly, they made soil, crops and groundwater polluted to a certain degree. From detailed analysis of epidemiological studies on health effects of wastewater reuse on agriculture, we conclude that there is evidence of transmission of diseases, all associated with the use of raw or only very poorly treated wastewater. However, due to the shortage of water resources in China and agriculture water use can‘t be guaranteed, the municipal wastewater has to be used continuously. From general point of view, it is imperative that the reclaimed wastewater will be mainly used for farmland irrigation. The key issue is that municipal wastewater treatment should be strengthened, and the quality of irrigation water should be improved constantly.

7.2 Groundwater recharge

Groundwater recharge with surplus surface water and reclaimed wastewater is an approach to water reuse that results in the augmentation of groundwater supplies. The purpose of recharge of groundwater have been to reduce, stop, or even reverse declines of groundwater levels, and to store surface water, including flood and other surplus water such as treated effluent, for future use. Groundwater recharge is carried out by intermittent flood of spreading basins according to a cycle that varies with climate and hydrological conditions.

The infiltration and percolation of reclaimed wastewater through the soil and the groundwater zone may render additional treatment reliability to the overall wastewater treatment system, resulting in loss of identity between reclaimed water and groundwater. The treatment capacity is particularly high in the unsaturated zone of aquifer where suspended solids are removed by filtration, dissolved organic material is removed by biodegradation and micro-organisms are removed by filtration and adsorption. The separation in space and time in groundwater recharge is an important public health consideration where potable water supply aquifers may be affected by reclaimed water. Hence, the appropriate pretreatment is crucial for groundwater recharge and its requirements vary considerably, depending on sources of wastewater and location.

7.3 Industrial cooling

There are significant variations among large industrial cooling systems. The range includes once-through non-contact cooling such as a large power station or refineries near the ocean, direct contact cooling of relatively inert material as the primary metals industries, and non-contact recirculating cooling at large inland industries with limited water resources.

Industrial cooling tower operations face four water quality problems: scaling, corrosion, biological growth and fouling in heat exchangers and condensers. Both freshwater and reclaimed municipal wastewater contains contaminants, which can cause these problems, but their concentrations in reclaimed water are generally higher. When reclaimed water is used for cooling, the assurance of adequate disinfection is a primary concern to protect the worker‘s health. The most restrictive requirement, limiting total coliform concentration to 2.2 MPN per 1000 ml, would be applied, if there were a potential for workers to be exposed to spray. In many cases, the water quality requirements for the use of reclaimed municipal wastewater are the same as those for freshwater. The water quality requirement could be achieved with advanced water treatment designed to reduce concentration of phosphorus (control scaling), suspended solids (control fouling), the microorganisms (control biological growth and pathogens). Additionally, internal chemical treatment such as zinc/chromate treatment would be required to minimize corrosion caused by other contaminants. These include total dissolved solids and minerals, which are expensive to remove in conventional unit processes and operations.

8. Case Analysis of the Feasibility Study on Wastewater Treatment of Shijiazhuang City

Shijiazhuang, the capital city of Hebei Province, is the demonstrative area of UNDP Project - "21st Century Urban Water Management in China" and a pilot project will be carried out in this city. The World Bank is preparing a loan to Hebei Province for water supply and sanitation in selected cities. It is proposed to provide financing for water supply and wastewater treatment in Shijiazhuang and other cities. According to the Project Document of "21st Century Urban Water Management in China", high complementarily between the two programs will be achieved with UNDP providing upstream technical assistance for institutional reform and capacity building, followed by World Bank financing for infrastructure upgrading and expansion. A "Feasibility Study on Qiaodong Wastewater Treatment of Shijiazhuang City" has been accomplished and three processes for treating the wastewater flow of 500,000m3/d were worked out in the study. They are:

1. A2/O Process
2. Oxidation Ditch Process (Carrousel-2000)
3. Traditional Activated Sludge Process

The economical/technical comparisons among these three processes listed in table 11~13 are summarized below:

In traditional activated sludge system, the sludge loading in the aeration tank is high, with corresponding short hydraulic retention time, in order to reduce the volume of aeration tank. The excess sludge is usually stabilized in anaerobic sludge digesters. The capital and operating costs of traditional activated sludge system are also low. The main weakness of this process is low efficiency for phosphorus and nitrogen removal.

A2/O Process has proved to have advantages of high phosphorus and nitrogen removal efficiency, and operating reliability. The major disadvantages of this process are its larger land requirement and higher capital cost.

Oxidation Ditch Process has the advantages of higher ability of resistance against shock-loading, and better operating stability/simplicity as compared with A2/O Process and Traditional Activated Sludge Process. The removal efficiency for phosphorus and nitrogen of this process is much higher than that of traditional activated sludge Process.

Comparing the advantages and disadvantages among the above-mentioned processes, it can be seen that the Oxidation Ditch Process is the optimum process for the construction of Qiaodong wastewater treatment Plant in Shijiazhuang City.

9. Land Disposal and Agricultural Use of Municipal Wastewater Sludge

Along with the development of wastewater treatment in China, the generation of sludge that in turn must be used or disposal of has increased correspondingly. Most of the municipal wastewater sludge contain approx. 50~60% VS, 3.0% TN, and 2.0% P205. Due to its relatively high nutrient, agricultural use of sludge has been widely used for a long history in China. Agricultural use is often extremely economical. Farmers may save money by reducing their dependence on expensive fertilizers. In some regions of China, the water added to the soil during application is also a valuable source. Sludge with 5~20% dry solid are usually given without charge to the farmers who take and carry the sludge as soon as they are running out from the digester or the dewatering equipment.

Most of the municipal wastewater sludge in China contains a large proportion of industrial discharges. Where such sludge are used in agriculture, the metals may contaminate land to such an extent that the quality and yield is affected. In order to avoid these problems, the industrial pretreatment and source control for sludge application should be emphasized. Besides, a significant proportion of bacteria, virus and parasite ascaris in wastewater becomes concentrated in sludge during wastewater treatment. A small percentage of these organisms may be pathogenic. Pathogen reduction can be substantially reduced by anaerobic digestion.

Both liquid and dewatered sludge can be applied to land. Decrease in moisture content may be beneficial, since it reduces the costs for transportation. Generally, the reduction in transportation is more important for large municipalities. Some China small-sized cities will not need to invest dewatering equipment. It is hoped to achieve a combination of disposal and utilization, which is sustainable in the long-term.

10. Utilization of seawater in coastal cities

In the coastal cities, it is a beneficial measure in saving fresh water to make full use of seawater as cooling water or process water. Of course, some protections against corrosion shall be taken for the pipeline and equipment. In some Soda Plant, seawater has been used as cooling water and then used for dissolving salt. It has the advantages of saving salt dissolving fresh water and energy consumption and utilizing salt in seawater as well. In Tianjin, Dalian, Qingdao, Ningbo, etc., many power plants in seashore have gained rich experience in the utilization of seawater as cooling water. A great amount of water has been saved every year.

There has been some development in seawater desalination technology, but seawater cannot be a major water source in a short time due to its high cost and energy consumption of desalination. However along with the passage of time, the cost of desalination will decrease and the shortage of fresh water might be more acute. Under this situation, the utilization of seawater will be an important trend in the release of water crisis.