sustainable urban sewerage and drainage

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SUSTAINABLE URBAN SEWERAGE AND DRAINAGE

Viet-Anh Nguyen, Assoc. Prof. Dr.
Vice Director, Institute of Environmental Science and Engineering (IESE),
Hanoi University of Civil Engineering (HUCE), Vietnam.
Add. 55 Giai Phong Rd., Hanoi, Vietnam.
Tel. (+84-4) 3628 4509, MP: (+84) 91320.9689.
E-mail:


ABSTRACT: Urban sewerage and drainage is among hot issues in fast developing urban
areas in Vietnam, especially in nowadays where climate change process is creating more and
more impacts. Inadequate infrastructure engineering systems in Vietnamese urban areas
cannot provide sufficient service for the socio-economic development. At the same time,
Vietnamese urban areas have lots of opportunities to apply new approaches which are more
appropriate and sustainable in term of service efficiency, financial sustainability,
environment friendly, etc. The paper presents new approaches and technical options in urban
drainage and sewerage management. Decentralized wastewater management, with low-cost
treatment technologies has number of advantages compared with conventional approach.
Sustainable urban drainage solutions (SUDS) are also introduced whereas flow diversion and
retention techniques can be applied. Rainwater harvesting has shown a very promising
adaptive water supply solution. Some good models applied in Vietnam and other countries
are presented as examples. Sustainable, integrated urban water management approach is
recommended.

KEY WORDS: Decentralized wastewater management; rainwater harvesting; urban
sewerage and drainage, Vietnam

1. INTRODUCTION

Improvement of sanitation systems for urban, peri-urban and rural areas in Vietnam is very
challenging and becoming more and more crucial issue in this fast-developing country.
According to the Ministry of Construction (MOC), from 760 urban centers, in the large cities,
50-80% of households use septic tanks, treating mostly black wastewater, while 10-20% uses
simple pit latrines. In the other cities (class 3-5), 20-50% uses septic tanks and 30-50% uses
pit latrines or double vault latrines. The current system does not have the capacity to respond
to the demands created by wastewater and run-off from urban areas, industrial centers, and
agricultural land. Water bodies such as lakes, streams, and canals increasingly serve as sinks
for domestic sewage and industrial wastes. It is estimated that only 10% of urban wastewater
is treated (Nguyen V.A., 2009).

In number of new urban areas, though domestic wastewater is separated from surface runoff
from the buildings, but they are again mixed up during discharge to the city’s combined
sewerage and drainage network. This problem is wasting invested money and polluting the
environment. Besides, there is poor control on ground level of number of new development
areas, roads and surrounding areas, leading to conflicts and negative impacts on each others.
Environmental and wastewater fees are too low, not enough to recover the system operation
and maintenance expenses.

While large centralized sanitation projects are not affordable for most cases in urban and rural
areas, the only way to increase sanitation coverage, especially for the poor, is to implement
low-cost alternatives with decentralized sanitation management schemes where local
community, administrative authorities and private sectors are involved in the decision making

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as well as in the exploitation process (Nguyen V.A., 2004). This concept is being developed
in Vietnam, even though it still requires more efforts and co-operations before it could be
widely disseminated in the practice.

Climate change is showing more and more evidences and becoming more and more serious
challenge for urban planning and engineering infrastructure, especially in coastal cities.
According to the World Bank study (2008), up to the year 2050, the most accepted scenario
show the sea level in Vietnam will be raised up for 30 cm more. Climate change will also lead
to negative consequences such as increased rainfall, changes of unpredictable urban
hydrology patterns, etc. what impact on collection and conveyance of urban wastewaters.

2. APPROACHES AND MODELS FOR SEWERAGE AND DRAINAGE IN
VIETNAMESE URBAN AREAS

Sustainable Urban Drainage Solutions
There is a growing tendency to argue that decentralized solutions would be more effective
than a sewer-based centralized system for pollution control. Decentralization of wastewater
management systems relate to planning and decision-making, design of physical infrastructure
and management arrangements for operations and maintenance (Parkinson and Tayler, 2003).
The decentralized approach offers important benefits, namely by the possibility of dealing
with wastewater locally, and applying pollution control measures at the source. By tackling
pollution problems close to their source, the large capital investment of trunk sewers
associated with centralized systems can be reduced, thus increasing the affordability of
wastewater management systems. Furthermore, decentralized systems allow an incremental
approach to provision of facilities. In some cases, the investment may require little more than
improvements to existing sanitation infrastructure (Parkinson and Tayler, 2003). Furthermore,
in a agriculture-based country like Vietnam, where wastewater reuse and nutrient recycling
has a long tradition, decentralized management systems are likely to be compatible with local
demands for wastewater reuse in urban and peri-urban agriculture.

Another advantage of decentralized systems in developing countries is that the systems are
mostly developed and run with more community-based approach, where users are involved
from the early stages of infrastructure system planning. Experience from number of
infrastructure development projects in this region have shown risk of failures of centralized

waste management systems due to there are no ‘’willingness-to-connect’’ and ‘’willingness-
to-pay’’ from the local users. Local resource contribution in decentralized wastewater
management systems make the system financial requirements affordable and feasible, the user
participation and the decision-making process more committed. Besides availability of
appropriate technologies, those conditions are necessary for sustainability of infrastructure
system.

The Decentralized Sanitation (DESA) study team at the Institute of Environmental Science
and Engineering (IESE), Hanoi University of Civil Engineering has been developing number
of decentralized sanitation solutions applicable for Vietnamese conditions. The team has
developed the package wastewater treatment plant BASTAFAT, a combination of anaerobic
treatment step in a baffled septic tank with anaerobic filter, and aerobic treatment unit with
attached growth treatment process. The tanks are made from a durable material of fibro-
reinforced plastics (FRP). The system BASTAFAT can achieve effluent standard QCVN
14/2008-BTNMT, Class A, appropriate for reuse or discharge to the environment (Figure 1).


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Figure 1. On-site, prefabricated package wastewater treatment system BASTAFAT

Sustainable urban drainage solutions – SUDS
Urbanization process has been creating negative impacts on the natural drainage, such as
change of natural flow patterns, loss of natural retention capacity of the green spaces and soil,
which have been replaced by impermeable surfaces of roofs, concrete and asphalt. The latest
covers significantly increase surface runoff flows (Figure 2.a) which again result in increase
of soil erosion and mud sedimentation. The above mentioned factors impact on environment,
flooding and aquatic life.

Figure 2. Principle of sustainable urban drainage

(a) Change of flow concentration due to changed urban surface;
(b) Return to original natural flow patterns thanks to surface flow retention solutions;
(c) Reduction of flow to be drained off thanks to infiltration and flow retention solutions.

Conventional drainage system is often designed in order to convey surface water away as
soon as possible. Costs for investment, operation and maintenance of such systems are often
very high, while their capacity and upgrading possibility are often limited. The consequences
are high probability of flood, erosion and pollution in the downstream areas, as well as loss of
chance for the on-site recharge of the valuable groundwater sources.

In recent years, new engineering alternative solutions of been developed in order to overcome
the mentioned drawbacks. The solutions are based on the following principles: retaining of
natural patterns of intensity, volume and quality of the flows, maximum control of the flows
at source, minimization of direct drain off areas, enabling of on-site flow retention and
infiltration associated with pollution control. Those are principles of SUDS.

The SUDS flow retention approach is to avoid flow concentration in a short period of time
(Figure 2), as limited cross section diameter of the collector, though expensive, cannot be
sufficient in case of heavy rainfall, what may cause overflows and flooding. Combination of
flow retention solutions often work best, using regulation ponds on the catchment and
conveyance area, using of surface of the city itself for infiltration through green spaces. Those
BASTAFAT

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solutions also bring to added values of recreational landscaping and micro-climate
conditioning (Figure 2.b, c).

In case of limited on-site flow control, SUDS can be applied by diversion of flows from sub-
basins, conveyance of flows by open, shallow canals, retention of flow in some ponds, and
infiltration through soil, swales, constructed wetlands, etc.

Recommendations for sewerage and drainage systems in Vietnamese urban areas
A scheme of urban sewerage and drainage should be based on the local specifics. The
following general principles of urban sewerage and drainage planning are recommended:

- For the existing urban areas: in coming decade, the existing combined sewerage and
drainage is still to be utilized, whereas ring or by-pass sewers are to be built for collection of
wastewater and first flush storm water and to convey them to the wastewater treatment plants,
while preventing direct discharge of those flows into the rivers and lakes. In order to avoid
overloads of wastewater treatment plants combined sewer overflows chambers are built on
those collectors in order to discharge high flows of storm water and diluted wastewater over
weirs to the receiving water bodies. The selection of centralized or decentralized wastewater
management systems, or combination of them, with low-cost of high-tech technologies, is
based on local natural conditions, population density, land availability, wastewater reuse
needs, etc.

- For the new development areas, separate sewerage system is to be built whereas wastewater
should be treated at adequate level before its discharge to the environment.

- In coming decades, where the separate sewerage and wastewater treatment is still not
affordable for most of cities in Vietnam, the role of septic tank for treatment of black
wastewater, or combination of black and grey wastewater from households, offices and
commercial points, is still highlighted. There should be adequate design, construction and
management of the septic tanks. Figure 3 introduces pre-fabricated reinforced concrete septic
tank designed and made by IESE in cooperation with the Vinaconex Xuan Mai Concrete
Company. The tank is now under commercializing and mass production.


Figure 3. Pre-fabricated reinforced concrete septic tank

- For the mountainous urban areas, with sufficient slope along the roads, the separate
sewerage system is recommended. The low-cost simplified sewerage, with small bore sewers,
lay along pavements, plus cross quarter (back-yard) connection scheme and maximum
utilization of existing channels for the surface runoff collection and conveyance is
recommended.

- For the urban areas in flat, plateau region, where the sewer slop is limited, it is
recommended to use increased sewer diameter, maximum utilize urban water bodies for the
flow retention and conveyance as well as for reduction of sewer depth.

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- For the urban areas along the coast, where flat topography is associated with limited number
of urban water bodies due to sandy loam prevalence in the soil structure, the tidal gate system
is recommended. Such manual or automatic functioning devices can be used for daily drain
off and flush of the sewerage network thanks to natural tidal run.

- SUDS principle should be applied as soon as possible by integration into urban planning. In
urban infrastructure and land use management, ground level should be strictly controlled.
Integration and harmonization of urban sewerage and drainage among urban hydrology as a
component of the whole basin whereas water resources, irrigation, flood prevention and
hydropower activities in up- and down-streams are being considered.

In the urban area, basic formula for determination of storm water flow to be drained is as
follows: Q = ψ.q.F (l/s), where ψ is flow coefficient, depending on the surface type; q is rain
intensity (l/s/ha) and F is drainage surface area (ha). Controllable factor, which may lead to
reduction of flow concentration to the collector, is a flow coefficient ψ. The ψ values of the
roof, asphalt, concrete surface is 0,9 - 0,95; of the earthen road: 0,4 - 0,5; of the park, green
space: 0,1 - 0,3. By reduction of ψ from 0.4 to 0.8, the flow will be 2 times less. Storm water
regulation ponds and open canals also play important role in SUDS application. Assuming the

urban drainage basin area is 100 ha; with average flow coefficient ψ is 0.6. In case of the 1-
year return period, 1-day rainfall of 120 mm, we have: Total volume of storm water to be
drained off is W
1
= 72,000 m
3
. If there are 7.0 ha of lakes in the basin, with storm water
regulating height before and after the rain is 0.5 m, the total regulating volume is W
2
= 35,000
m
3
. Then, the total volume of storm water to be drained off is only W
3
= 37,000 m
3
.
Therefore, if the water surface in the city is 7% of the total surface area of the basin, the total
water volume to be directly drained off remains half. Besides, there are other added values of
the lakes such as micro-improvement of the climate conditions, landscaping, ecology and
biodiversity, etc.

Large public spaces like squares, parking, pavements, and even roads, should be built from
permeable materials whereas surface runoff can be penetrated through the surface to the
underground infiltration trenches with the gravel bed. Along the sides of the highways the
filter strips should be designed for flow retention and removal of pollutants. From economics
point of view, damages caused by the flooding during the rains can be much more than
investment and operation and maintenance costs for such SUDS facilities. The damage caused
by the historical storm in Hanoi in October 2008 has been estimated as VND 8,000 billions,
which is equal to total investment cost of the phase 1 and phase 2 of the Hanoi sewerage and

drainage improvement project (1996 – 2015).


Figure 4. Filter strip for flow retention and pollutants control in public places in the UK

Rainwater harvesting
One of interesting counter measures of flooding remediation is a public participation in
rainwater harvesting. If each household in Hanoi has a 6 m3 rainwater tank, the water volume
is sufficient for toilet flushing around the year. Rainwater tanks in households and public