Based on the drainage & sewage systems as mentioned above, the advantages and disadvantaged of each system and application for each project area s are analyzed as follows:
Regarding the common system Advantages:
- The construction cost be the lowest in the three alternatives.
- The system shall be operated continuously in the year (in both dry and rainy seasons). - It is necessary to construct the system with a suitable scale for rainwater as the wastewater shall account for a very small flow in comparison with the rainfall.
- Easy management. Disadvantages:
- Wastewater is not collected and treated, causing environmental population seriously and germs for diseases and sickness. This is the reasons for environmental pollution, especially on soil, water and air ... affecting the lives and health of communities in the project area and surrounding areas.
- If wastewater is collected and treated at the WTP, the capacity of the WTP shall be very high, leading to the great costs for construction and management & operation of the project as well as reduction of efficiency and feasibility in terms of socio-economy.
- The working condition of the system shall be not stable. In the dry season, the wastewater flow shall be small in comparison with the pipeline scale. The small flow rate shall cause settling and clogging inside the sewer. Therefore dredging should be done regularly for smooth flow. The operation & management cost is huge.
Regarding the separate system. Advantages:
- The environmental sanitary shall be guaranteed best without environmental pollution for surrounding areas.
- The wastewater shall be collected and treated thoroughly before discharging to the natural environment, create a fresh environment for the project area and for the city as well.
- The collecting system shall be separated for wastewater and rainwater, which shall facilities the flood control and environmental protection, saving of water resources. In the long term, rainwater collection system can be used as a reserved source for watering in green areas and parks in the dry season.
- Be suitable for new urban areas and urban areas which are not invested with the drainage & sewage systems.
- The costs for operation and management is lower than the two other alternatives. Disadvantages:
- The initial investment for construction will be high as two new network for drainage & sewage shall be constructed.
- In the dry season, the sewer network for rainwater shall not operate so often, therefore the investment in the sewer network may be not effective. However, the sewer network could work as a rainwater storage area in the dry season for watering the green areas and park, & fire-fighting.
- The flow rate of the dirty water is small, diameter of the pipeline is small with high slope and depth. Therefore it is necessary to have transition pumping stations.
- The management shall be more complicated as the 02 systems shall be operated at the same time.
Regarding the Common system with Combined Sewer Overflow (CSO) and interceptors to collect wastewater
Advantages:
- The existing system could be utilized, therefore the existing condition of the area shall remain and the cost for investment shall be deducted.
- The quantity of excavating in the central area is limited. - No environmental pollution due to construction activities.
- In accordance with the needs of urban renovation and upgrade of sewerage network systems, especially suitable with not huge amount of initial investment capital but high requirements about environment.
Disadvantages:
- The interceptors collect dirty water from CSO to the wastewater treatment station which is located at the end of main sewer. Therefore it is also necessary to construct different transition pumping stations.
- The outlets must be constructed with the auto gate in order to prevent the water flowing to the sewer in the dry season without rain.
- The management & operation for the system is very complicated.
- The working condition of the system is not stable. In the dry season, the wastewater amount is not so much with small velocity, resulting in the clogging in sewers. It is necessary to dredge the sewers regularly and the operation and maintenance costs is very high.
Based on the analysis about the advantages and disadvantages of each systems as stated above, based on the existing condition of drainage & sewage system in the District 2, where the drainage & sewage systems are almost unavailable, based on the fund for the project investment and plan for socio-economic development of the city which has been approved by the Prime Minister and the detailed planning of District 2, which is approved by HCMC, the District 2 shall become a new urban area and economic & commercial center of the city. And in accordance with the planning about the central wastewater treatment plant of the city and take advantages of the HCMC Environmental Sanitary Project, Phase 1 (Nhieu Loc – Thi Nghe), we would like to propose to select a design options of 02 separate for rainwater and wastewater collection. In the chase the system design is as follows:
- The wastewater shall be collected to a separate system. The total amount of wastewater in the extended area shall be collected to the extended wastewater collection system, which is collected with the existing main transmission pipeline to the WTP for treatment before discharging to the environment.
- Regarding the extended wastewater treatment, the households, offices and public works shall be connected directly to the sewage system, and no longer connect to the local treatment and discharge system. The direct connection shall facilitate significantly for the customers. The drainage shall be collected to a separate drainage system and conveyed to the nearest stream.
The option of a separate drainage system shall not only facilitate for the design of the drainage network but also for design of sewage network.
3.3.3. Identification of drainage system scale.