Centralized and decentralized mode of wastewater treatment

As an important part of environmental engineering, wastewater treatment takes an important position in sustainable development. However, wastewater treatment is generally divided into centralized and decentralized. The choice of these two modes will become a very debatable topic regarding wastewater treatment in the future.

Centralized wastewater treatment

Traditional wastewater treatment plants are centralized wastewater treatment systems and are suitable for densely populated urban areas. As a more widely used technology, it has the following advantages:

  • Reliable and effective management of water plants or wastewater treatment plants.
  • When providing the services to areas with large populations and developed wastewater pipe networks, large wastewater treatment plants are cheaper than smaller wastewater treatment in terms of unit water investment (excluding pipe network investment) and operating costs (excluding pump station operating costs).
centralized wastewater treatment

However, this mode also contains the following disadvantages:

  • Large engineering costs are needed to build a complicated drainage network. In areas with low population density, this investment will be much higher but has a lower efficiency. Especially in rural areas, where residential groups are scattered.
  • From a long-term and overall perspective, centralized treatment will inevitably cause energy and material loss. Even the most effective wastewater treatment plant has more than 20 % Nitrogen, 5% Phosphorus and more than 90% Potassium is lost;
  • The sludge produced is very polluted, and mostly mixed with harmful substances such as pathogenic organisms, household chemicals, drugs and heavy metals, which are difficult to transform into useful products.

Generally, for the same drainage type, the smaller the water volume, the larger the cost per unit of water volume will be. Therefore, large-scale or regional centralized processing should be considered in the planning stage. However, centralized wastewater treatment requires a considerable number of pipelines and pumping stations, and its construction period is long and the cost is high.

For small and medium-sized cities (towns), large rural areas and remote areas with relatively scattered residences, due to geographical conditions and economic factors, it may not be suitable for centralized treatment of domestic wastewater. At this time, choose and develop decentralized domestic wastewater treatment based on locations and needs. In-situ processing technology.

Distributed wastewater treatment

Distributed wastewater treatment is a new, economical and environmentally friendly wastewater treatment mode.

Distributed wastewater treatment

Its sewage treatment characteristics are very different from the centralized type in many aspects:

Investment cost: The construction investment of a decentralized wastewater treatment system is significantly lower than that of a centralized system. The centralized system requires the construction of a large-scale wastewater collection pipe network, and its investment cost accounts for 80% of the total cost. The average design service life of the pipe network is 50 years. The daily maintenance and replacement of centralized systems require a lot of investment.

Operating cost: Although the centralized wastewater treatment system has advantages in terms of unit wastewater treatment costs in densely populated areas with developed municipal wastewater pipe networks, the peak flow rate considered in its design will increase the system capacity and reduce the treatment efficiency. Besides, long-distance water transmission requires the construction of pumping stations with higher energy consumption. These factors directly lead to an increase in operating costs.

Water quality safety: The centralized wastewater treatment system discharges a large amount of water, which may cause eutrophication of the water accepting area. The decentralized system will reduce the probability of eutrophication due to its small displacement.

Monitoring and management: With the advancement of remote monitoring technology, the use of remote-control equipment can already easily achieve operation and maintenance, greatly reducing the number of personnel on site. In terms of management, the decentralized wastewater treatment system has more “visibility” than the centralized system so that local residents can better participate and also promote the public’s water conservation and environmental protection awareness.

Development perspective: Decentralized wastewater treatment infrastructure investment can be made gradually, without being in place at one time. Compared with centralized systems, it is more suitable for rural and townships where infrastructure is still relatively backward in my country.

Source separation: It is difficult for centralized wastewater treatment to achieve wastewater source separation, but decentralized systems are easier to implement.

However, compared to centralized processing, it also has the following disadvantages:

1. The number of sewage plants is larger, and the total operating cost is higher, which makes it difficult to form scale benefits;

2. The total investment of all sewage plants is relatively large;

3. Some sewage plants are located near residential areas, which may result in poor sanitary conditions;

4. Each of the scattered wastewater treatment plants is based on its own operation, which is easy to cause environmental disputes.

Distributed wastewater treatment technology is sustainable due to its in-situ treatment and reasonable resource recycling. This feature will be more and more developed in the future. Although the centralized wastewater treatment system can currently meet the needs of densely populated areas, it is not generally seen from the development trend. The new concept of “future cities” is likely to focus on decentralization.

From the perspective of technology research and development, decentralized wastewater treatment systems have been fully recognized, but the transfer of technology to engineering applications is still slightly weak. The “conservative mentality” of managers has become one of the main obstacles to the promotion and application of new technologies.

On the contrary, for areas where wastewater collection facilities are in urgent need of replacement or reconstruction, replacing the decentralized wastewater treatment system is a good opportunity to change the traditional wastewater treatment mode.

However, this does not mean that the centralized wastewater treatment system is completely useless. High-density and dense areas have been served by centralized wastewater treatment plants, and the pipeline network system has been continuously expanded with the development of urbanization.

In these cases, the decentralized system is not a suitable and feasible alternative in short term. The compromise method is to concentrate, decentralize and coexist separately. For residential buildings, commercial complexes and hospitals, the decentralized system can be applied, which is easier to handle.

Reference

Russell, R. (2014). Waste not, want not? Evaluating the urban sustainability implications of centralized versus decentralized wastewater treatment in Tijuana, Mexico. Urban Geography, 35(6), 805-821. https://doi.org/10.1080/02723638.2014.917909

BOGUNIEWICZ-ZABŁOCKA, J., & CAPODAGLIO, A. (2017). Sustainable Wastewater Treatment Solutions for Rural Communities’: Public (Centralized) or Individual (On-Site) – Case Study. Economic And Environmental Studies, 17(44), 1103-1119. https://doi.org/10.25167/ees.2017.44.29

Seymour, Z. (2007). Life Cycle Assessment of Decentralized Wastewater Systems and its Comparison to Centralized Wastewater Systems. Proceedings Of The Water Environment Federation, 2007(18), 1084-1107. https://doi.org/10.2175/193864707787452787

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