|
 |
|
 |
|
|
|
 |
|
|
|
There are two types of sewage (rainwater and
|
|
wastewater) collection systems, namely, separate sewer |
|
system and combined sewer systems. Separate sewer
|
|
systems consist of two pipes. One pipe conveys stormwater |
|
runoff from storm drains to public water bodies, and the |
|
other pipe conveys sanitary sewage to a local wastewater
|
|
treatment plant. Combined sewer systems (CSS) have only |
|
one pipe which conveys both wastewater and stormwater |
|
to a local wastewater treatment plant. |
|
|
|
|
|
 |
|
|
|
In combined sewer systems, flood control and water
|
|
quality preservation in public water bodies can be |
|
expected by construction of one pipe. Also, system |
|
configuration is rather simple and construction is |
|
relatively easy as only one pipe is constructed |
|
under the road while two pipes is needed in case of |
|
separate systems. Therefore, combined sewer systems |
|
widely adopted in large cities such as Tokyo, and |
|
Osaka where sewage works were promoted in early stage. |
|
In these areas, flood control and wastewater drainage |
|
had to be accomplished at the same time. |
|
|
|
However, during heavy rain event, overflows occur from |
|
these combined sewers which results in stromwater |
|
runoff and sewage being discharged into water bodies. |
|
As design capacity of sewer systems is limited, the |
|
exceed sewage including rainwater and wastewater is |
|
discharged to public water bodies from outlet installed |
|
in sewer systems. Deposited sludge in sewer pipe is |
|
also washed out to water bodies at the first stage of |
|
rain event. |
|
|
|
 |
|
Storm Overflow Chamber (Dry-weather) |
|
|
|
 |
|
|
|
|
|
 |
|
Combined Sewer Overflow in Rain Event |
|
|
|
 |
|
|
|
Therefore, in the case of CSS, Combined Sewer Overflow
|
|
(CSO) should be appropriately controlled to preserve |
|
the water quality in water bodies. |
|
|
|
In Japan, basically separate systems are adopted for
|
|
newly constructed sewerage systems, as only wastewater |
|
is transported to the treatment plant and there is no |
|
overflow during rain event. |
|
|
|
To solve the problems regarding CSO, in 2001, Ministry
|
|
of Land Infrastructure and Transport established "CSO |
|
Control Studying Committee", and finalize the report |
|
on the basic concept for rational implementation of |
|
CSO control. Also, in 2002, based on the study of the |
|
committee, "Guideline and Explanation for CSO Control" |
|
was published. |
|
|
|
To promote rational CSO Control, research and technology
|
|
development is needed, and in 2003, MLIT adopt the |
|
research topic, "Technology Development on CSO Control" |
|
as the first topic of SPRIT 21, Sewage Project, |
|
Integrated and Revolutionary Technology for the 21st |
|
Century which is managed on the initiative of MLIT and |
|
focuses on important problems in the field of sewage |
|
works where technology development is particularly |
|
needed. |
|
|
|
In this project, 24 technologies classified into five
|
|
categories, Debris Removal, High Rate Filtration, |
|
Coagulation Separation, On-site Disinfections, |
|
Measurements/Control were evaluated and specified |
|
performance in each technology was certified by |
|
technical committee established in JIWET, Japan |
|
Institute of Wastewater Engineering Technology. |
|
The project has been coordinated and managed by |
|
JIWET under the cooperation of municipalities, |
|
private sector and academic institution. |
|
|
|
|
|
http://www.jiwet-spirit21.jp/confluence/eng_02.html
|
|
http://www.jiwet-spirit21.jp/confluence/eng_03.html |
|
|
|
 |
|
Fig. One of developed technology by SPRIT 21 |
|
"Brush Screen System" |
|
|
|
In 2003, partial amendment of enforcement order of the
|
|
Sewerage Law was made, and technical standards for |
|
final effluent water quality were set up for CSO as |
|
well as the effluent water quality from Wastewater |
|
Treatment Plant. This regulation will be applied after |
|
10 years from the enforcement, and each municipality, |
|
which adopted CSS should submit the Comprehensive CSO |
|
Control Plan to MLIT within 10 years. |
|
|
|
Please refer to |
|
http://www.sbmc.or.jp/english/200407/Partial_amendment
of Enforcement_Order_of_the_Sewerage_Law.htm |
|
|
|
There are following conventional counter measures for
|
|
CSO in addition to the newly developed technology by
|
|
SPRIT 21. |
|
|
|
- |
Stormwater Storage Tank (Storage in rain event and |
|
|
treatment of stored sewage after rain event) |
|
- |
Permeable Pavement (Reduction of Rainwater) |
|
|
Cleaning of Sewer |
|
- |
Conversion to separate sewer system from CSS, etc. |
|
|
|
|
|
|
|
|
|
