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Newsletter and Technical Publications
Freshwater Management Series No. 10

Managing Urban Sewage
An Introductory Guide for Decision-makers


VIII. Optimizing Resource Efficiency

A. Utilization of Reclaimed Water

One of the key policies now being considered by many cities is to optimize the use of abundant, high-quality effluent from treatment plants, recognizing that this is an important water resource. For example, in 1984, a water recycling centre was established in Tokyo to provide water recycling services using rapid sand filtration and chlorination to produce a recycled effluent for toilet flushing in 25 high-rise buildings. The recycled water in this application is charged at a rate lower than that for conventional city water. Area-wide recycling using advanced wastewater treatment is an option that can be considered in areas where large-scale urban development projects are planned.

B. Utilization of Sludge

There are a number of useful by-products that can be derived from municipal sewage sludge, including:

  • Concrete products made from sludge ash, such as reinforced-concrete pipes
  • Lightweight aggregate made from sludge ash to replace natural aggregate used in water-permeable bricks and building construction materials
  • Compressed baked blocks, based on press and burn technology whereby the pressed molded sludge ash is burned at a temperature of about 1050 degrees Celsius to manufacture durable interlocking bricks that it can be used as for pavement and park landscaping.
  • Sludge melted slag where the sludge is heated up to 1500 C, the organic matter is decomposed and burnt, and the remaining inorganic matter is melted to a liquid state. When cooled and solidified, this melted inorganic matter, referred to as sludge melted slag, has half the volume of sludge ash and is highly stable with no dissolution of heavy metals contained in the slag. Several uses for sludge melted slag have been investigated, including its use in road bed or construction materials.

All of these applications make use of a valuable resource that would otherwise be wasted.

C. Utilization of Wastewater Heat

In most cities, the temperature wastewater is lower in summer and higher in winter than the ambient atmospheric temperature. This characteristic of wastewater can be used as a heat source for cooling and heating which can contribute to energy savings and associated reductions in air pollution. Since the use of wastewater for heating and cooling does not involve the construction of a cooling tower, it is also serves as a way of avoiding unnecessary noise.

D. Pretreatment Requirements for Industrial Wastewater

To preserve both the function and physical structure of the public sewer system, as well as to ensure the quality of the water that runs through public water bodies, factories and other establishments should not permitted to directly pump wastewater containing toxic substances into the public sewer system. Before discharging wastewater into the system, these stakeholders should be required to treat the wastewater so that it conforms with predetermined water quality standards (sewage effluent standards).

In some cities, sewer use bylaws and ordinances have been developed prohibiting factories and businesses using the public sewer system from discharging wastewater that does not satisfy acceptable pretreatment standards. In most of these cities, the local sewage department provides guidance for appropriate wastewater quality control, monitors the wastewater pretreatment systems of factories and businesses, by conducting regular performance inspections.

It should also be noted that the pre-treatment of some industrial effluents can provide significant economic benefits as a result of resource recovery and reuse.

E. Finance

The ongoing financing of sewage infrastructure is a significant challenge. In most cities, a local public utility is responsible for the establishment of a financial accounting system. By separating capital accounting from revenue accounting, the local utility is able to analyze the effectiveness of management and promote incremental improvement in the operation of the system.

In most cities, the sources of revenue for the construction of sewage facilities are often national subsidies, bonds, and funds appropriated from general tax revenues. In some cases, national subsidies are provided for the construction of pumping stations, trunk sewers, and treatment plants. In addition to national subsidies, a major portion of the capital expenditure is paid by bonds and other long term borrowing of funds, with the remainder of the funds coming from revenues derived as income obtained from user fees. The main items of revenue expenditure are maintenance and management, depreciation and interest payments. Although expenditures on wastewater treatment are usually paid by users in the form of service charges, expenditures on stormwater runoff are usually supported by general tax revenues.

 

Annex I

How Sewage Treatment Works (PDF)

 

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