Discussion on Technical Development in Water Supply Enterprise

Fan Jie
(National Engineering Research Center for Urban Water & Wastewater)

Abstract: Brief description on the some technological development trends in water supply enterprise in this paper, including water quality goals and treatment process, process monitoring and control, and process residuals treatment and disposal. The author perceives that it is an important role and significance to study and master above fundamental issues for promoting the technical progressive and increasing the management competence in water supply enterprise.
Key words: Water quality Treatment process Process control Residuals disposal

1. Induction

Over a billion people worldwide lack access to adequate water. In China, the total average annual water resources is about 2800 billion m3 and ranked sixth in the world. But the average annual water resources per capita is only 2730m3, less than one fourth of the world average. 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. At present, more than half of 668 large and medium sized cities face shortages and 108 were severely deprived. On the other hand, the total amount of urban wastewater discharge is about 20 billion m3 per year, but the amount of wastewater treated is about 10% of the total. 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. So, increasing the competence of the research and application of water supply technology is of vital importance.

In understanding and resolving the issues related to water, it is important to consider the following trends:
- Continuing urbanization
- Increasing domestic and industrial water consumption
- Continued pollution of surface and groundwater from human activity and chemical process
- Rising costs of construction and operation
- Rapid Development of water supply technology
- New requirement in environmental protection

The key considerations when addressing water supply technology issues include water quality goals of drinking water and relative treatment processes options, process monitoring and control, and residuals treatment and disposal that has been mentioned and is gradually recognized in recent years.

2 Water Quality Goals and Treatment Processes Options

When water supply treatment engineering first evolved in the early part of the twentieth century, its main goal was to ensure that infectious organisms in drinking water supplies were removed or inactivated. Filtration and chlorination practices were applied with tremendous success. As a result, in the 1960s, 1970s, and even 1980s, The microbiological contamination in surface waters could be eliminated by providing filtration, with suitable pretreatment.

The principal challenge to water supply treatment engineers in 1970s and 1980s was engineering cost-effectiveness: how to accomplish these simple treatment goals at the lowest total cost. Thus, in these decades many new techniques and processes were developed to clarify surface water economically. These developments included improvements to sedimentation basin designs: high-rate clarification processes such as tube settlers, plate settles, and floatation tank; high-rate filtration processes; and proprietary package equipment integrating coagulation, settling, and filtration processes.

However, In the 1980s and 1990s, a new drinking water concern arose: the potential long-term health risks posed by trace amounts of organic compounds present in drinking water. So certain new water pollution index and regulations were laid down, for example total trihalomethanes(TTHMs), volatile organic chemicals(VOCs) and maximum contaminant levels(MCLs). In response to this concern and resulting treatment needs, water supply treatment engineers have successfully devised new methods of water treatment to remove organic compounds. These methods, such as oxidation, activated carbon adsorption, and enhanced coagulation, have been the primary focus of water supply treatment engineering over the last 10 years.

As illustrated in Table 1, many water supply treatment processes options are available to the designer to achieve the desired water quality results.

Table 1 shows that rapid sand filtration with coagulation in the most common type of water treatment in use. However, the addition of preoxidation, adsorption and combination of them is major method of resolving the polluted water resource at present, and the biological pretreatment process has been also used in China. The membrane filtration is mentioned will greatly develop in the future.

There are many treatment processes options and combination of them available to the designer. But, it is essential that all viable options be investigated and analyzed for each treatment application. Some issues may be considered as follows:
- Construction cost
- Annual operation costs
- Site area required
- Complexity and risks of operation
- Environmental impact
- Flexibility of plant arrangement for future changes

Water quality parameter

Process components Turbidity Rapid sand filter(conventional): Coagulation, Sedimentation, Filtration Rapid sand filter(direct mode): Coagulation, Filtration Membrane filtration Color Coagulation/Rapid sand filtration Adsorption: Granular(Powdered) activated carbon, ion exchange resins Oxidation: Ozone, Chlorine, Potassium permanganate, Chlorine dioxide Taste and odor Oxidation: Ozone, Chlorine, Potassium permanganate, Chlorine dioxide Biologically activated carbon Volatile organic chemicals Air stripping Granular activated carbon Combination of above THM and HA Precursor reduction
Enhanced coagulation; Granular activated carbon; Biologically activated carbon By-product removal
Granular activated carbon; Air stripping Organic chemicals Ion exchange Biologically activated carbon Membrane filtration Bacteria and viruses Partial reduction - filtration Inactivation - disinfection: Chlorine, Chloramine, Chlorine dioxide, Ozone

In sum, The water treatment plant design is continually changing as new technologies emerge, offering new unite processes for water supply treatment and making currently used processes more efficient or economical.

3 Process Monitoring and Control

In the past, many water treatment plants have been manually operated. But, there is need to add a lot of electronic instrumentation, automatic controllers, and computer system now. Some of the major reasons are:
- More stringent water standards require continuous control
- Plant managers require fast response to changing raw water quality and emergency situations
- Economics dictates more efficient use of chemicals and energy

Optimal control requires accurate signals from process-variable transmitters. Accurate signals require high-quality electronic instrumentation instilled with attention to detail. Accurate reporting of process parameters in orderly and timely requires a kind of computer system. Optimal water treatment can be made possible by a coordinated application of hydraulic, process, and equipment monitoring, coupled with hydraulic and process controls, as illustrated in Figure 1.

Figure 1 Monitoring and Control for Water Treatment

The major types of instrumentation for water treatment processes include flowmeters, level switches, transmitters, pressure switches and transmitters, temperature switches and transmitters, analytical instrumentation (chlorine analyzers and detectors, turbidity detectors, and other analyzers and detectors)

The types of controls in water treatment processes include pressure controls, flow controls, filter backwash controls, control of pH, coagulation controls, chlorination controls, etc.

In addition, rapid development in dynamic simulation technology has become a gradually powerful tool for the water treatment process in recent years. It can be used across a range of functions, including optimization of design, commissioning, operational decision support and training. The kind of approach includes expert systems, neural networks and dynamic process simulation. Each of these techniques has its particular role, and increasingly attention is being given to combining them in an integrated manner. Process simulation is probably the most powerful tool available at present for application across the entire processes of design and operation.

In a word, the potential of monitoring and control of treatment processes has been demonstrated. The means that it is necessary to alter operating practices which ensure stability of output at minimum cost.

4 Residuals Treatment and Disposal

In the past, the primary focus of the regulatory agency and water supply enterprise was providing high-quality water to meet state drinking water quality standards. Little attention was paid to the treatment and final disposal of water supply plant residuals, including sludge and nonsolid wastewater that must be treated or controlled, which were typically discharge directly into a nearby water body or to a local wastewater treatment plant. Both of these practices, however, are receiving increased attention from the environmental communities.

In recent years, the requirement in environmental protection and the decrease in land have caused essentially all water treatment plant residuals to be recycled or treated in some manner before disposal. Methods for processing and disposing of residuals should be investigated early in the planning phase for new building water treatment plant. Process residuals treatment and disposal may represent a substantial portion of the investment and operating costs of providing treated water and may influence the source water selection and the method of treatment.

The general steps in designing residuals handling, recovery and disposal are:
- Determining the types, characteristic, quality, and quantity of residuals
- Evaluating treatment and disposal options that are available
- Reviewing the regulations and restrictions affecting each disposal method
- Comparing feasibility of the economics, advantage and disadvantages of all alternatives
- Selecting the best treatment or disposal alternatives

The various types of wastewater resulting from water treatment processes include filter backwash water, supernatant from sludge treatment, and water from washing equipment and pipelines. The most amount of them above is the filter backwash water, and it typically represents 2% to 5% of total water processes. Filter backwash water can be troublesome to handle, and it has low solids content even after thickening. It usually dewaters poorly, and most of the solids are difficulty to gravity separate without using coagulant aids. Filter backwash water deserves careful consideration not only with regard to solids disposal, but become of potential recovery of a significant volume of water. It has generally been returned to an early segment of the treatment processes as a means of water conservation and the easiest method of treatment. This can not only economize on chemicals and energy, but also improve the effect of sedimentation.

Typical sources of sludge discharges from water treatment plants include sedimentation basin sludge and solids in filter backwash water, and the amount of the sedimentation basin sludge holds a major position. Solids in sedimentation basin sludge generally fall into two categories: suspended solids removed from raw water and chemical precipitates created by the treatment process. The three primary categories of sludge treatment processes as follows:

- Adjustment Process: Filter backwash water and sedimentation basin sludge are discharged intermittently, and the quality and quantity of them don‘t constant. Adjustment installation has been set up to ensure the well-distributed of thickening basin load.

- Thickening Process: Thickening is typically towards reducing the quantity of water treatment residuals. The products of the process are a low solids return flow and a thickened solids flow. Return flow is generally returned to the wastewater treatment processes, and the thickened solids flow is further disposal. There are three types of thickening: gravity, flotation and mechanical, and gravity thickening is the most predominantly used technique at water treatment plants.

- Dewatering Process: The function of dewatering is to further remove the moisture and the volume of thickened sludge flow, and to product sludge cakes. The methods of dewatering are natural dewatering and mechanical dewatering, and mechanical equipment used for dewatering water treatment sludge includes belt filter presses, belt presses, centrifuges and vacuum fitration.

Three methods of final disposal are typically considered for the sludge from the treatment processes: resource recovery of sludge cake, agriculture application, and burial in a landfill.

The alternative treatment and disposal methods of residuals from water treatment plant are now being increasing considered. Although there are many considerations in determining the most appropriate method of residuals treatment and disposal, environmental issues are gaining more significance. It appears likely that direct discharge to receiving streams and wastewater treatment plants will become increasingly disfavored, and the beneficial disposal alternatives will gain greater acceptance.

5 Conclusion

As mentioned above, I think that the water treatment processes in water supply enterprise cannot still remain conventional treatment processes, but the appropriate measures should be taken in accordance with the variations of water resource quality. Meanwhile, the monitoring and control of water treatment processes may be enhanced, and the management competence should be increased in water supply enterprise. These can ensure to provide high-quality drinking water reliably at lower cost. In addition, the process residuals must be treated and disposed from now. Thus, the wastewater and sludge are discharged cannot made the water body polluted. This is the responsibility must be borne by water supply enterprise.

Reference
[1] American Water Works Association. Water Treatment Plant Design. 3rd ed. New York, 1998
[2] John Neate. Water and Technology - Trends and Challenges. Water Quality International, (1/2):9,1999
[3] F.Porta. Management Systems of Drinking Water Production and Distribution Services in the EU Member States. AQUA, 47(8):176,1998
[4] Richard Head. Dynamic Simulation of Water Treatment. Water Quality International, (5/6):23,1998
[5] Cui Yuchuan, Fu Tao. Current Status and Features of Municipal Water Supply in China. China Water and Wastewater. 15(2):52, 1999
[6] Le Linsheng et al. Study on Sludge Treatment of Water Works. China Water and Wastewater. 15(6):14, 1999