4.3 Biological pretreatment of raw water

Technical description

This technology is being used in China to pre treat slightly polluted water sources which have to be used as raw water sources for water treatment works. Generally, such sources are polluted by wastewater, and the primary concerns relate to the presence of carbonaceous and nitrogenous compounds in the raw water which may not satisfy water source standards. Most water treatment works commonly use a coagulation-sedimentation-filtration-disinfection process. However, when polluted water is used as a source water to produce water for public consumption, reactions between carbonaceous compounds in the raw water and Cl₂ used in the disinfection stage, and between Cl₂ and nitrogeneous compounds in the raw water, results in formation of compounds such as trihalomethanes which are harmful to human health. These compounds also consume greater volumes of Cl₂, beyond those necessary for disinfection, resulting in a waste of Cl₂. In such cases, bio pretreatment is one options for improving the safety and quality of the water produced by the waterworks. Various kinds of commercial plastic products may be used as growth substrates to promote biological pretreatment of the raw water. Alternatively, sometimes local materials such as the shells of shellfish can be used for this purpose in coastal cities. The treatment process is shown diagrammatically in Figure 24.

Extent of Use

This technology has been used by small municipalities and industries.

Operation and Maintenance

Biological pretreatment of raw water feeds to waterworks requires good operational and maintenance supervision by qualified staff. Constant monitoring of the water quality parameters is also necessary. Such monitoring requires a fully equipped laboratory and trained staff to operate the laboratory equipment.

Level of Involvement

This technology is typically implemented at the municipal level or at the level of individual large industries.

Costs

Table 17 summarises the costs associated with the use of this technology.

TABLE 17. Cost of the Biotreatment Process in China

Figure 24. A Process Diagram for Biological Pretreatment

Effectiveness of the Technology

This is an effective technology for ensuring the quality of raw water for municipal and industrial purposes. Tables 18 and 19 present an example of the effectiveness of this technology from a glass factory in Zhejiang Province, China. Table 18 shows the water quality parameters of the water source used by the glass factory, and the industrial water standards. Table 19 shows the water quality of the product water at various points in the treatment process (refer to Figure 24).

TABLE 18. Source Water Quality and Industrial Water Standards for Industrial Use.

Advantages

This technology is effective in reducing the concentrations of pollutants in natural waters, thereby improving water quality for a variety of water uses after application of conventional water treatment techniques.

Disadvantages

The primary disadvantage of this technology is that it requires trained staff to implement. The biological pretreatment technology needs to be carefully controlled and closely monitored to ensure consistent quality product water is produced.

Cultural Acceptability

This technology is acceptable, with no known cultural problems.

TABLE 19. Effluent Water Quality Using Biological Pretreatment Processes in China.

Further Development of the Technology

This technology is considered to be fully developed. Pilot projects are needed to popularize the use of this technology.

Information Sources

Professor Chi Bute, Tingji University, 1239 Shiping Road, Shanghai 200009, China, Fax: 86 2165028965.