| Abstract: | 1. This project is within the framework of American Red Cross’s Tsunami Recovery
Program in cooperation with the Government of the Republic of Maldives with an overall
objective of improving the sanitation system and health of the inhabitant’s tsunami
affected islands.
2. Cardno International Pty Ltd in association with Riyan / LaMer Pte have been
commissioned by the American Red Cross (ARC) to undertake survey, investigation,
design and documentation of new reticulated sewerage services in Villingili and
Dhaandhoo Islands in Gaaf Alifu (GA) Atoll and Gadhdhoo Island in Gaaf Dhaal (GD)
Atoll.
3. The proposed project involves construction of new wastewater collection and disposal
systems including replacement of existing septic tanks on the islands of GA. Villingili, GA.
Dhaandhoo and GD. Gadhdhoo. The key objective of the project is improvement of
sanitation and health of the inhabitants of the islands through construction of the new
reticulated sewerage systems.
4. The proposed wastewater collection systems for all 3 islands will comprise of: Individual
on-site septic tanks for each lot; A small bore gravity sewerage reticulation network;
Reticulation pumping stations and pressure main system; Outfall pumping station and
ocean outfall pipeline and disposal of septage via outfall pipeline
5. Wastewater from houses and non-residential connections will drain to a septic tank
where partial treatment of the wastewater stream will occur with up to 90% removal of
floating / settleable solids and reduction of BOD by about 50% through natural die-off.
The partially treated effluent from the septic tanks will drain to the small bore sewerage
network comprising of gravity sewer pipes laid on a constant downward grade toward a
reticulation pumping station. Effluent will then be discharged via a series of pressure
sewer mains to the Outfall Pumping Station.
6. A large accumulation tank will be located at the Outfall Pumping Station to receive the
effluent inflows. Effluent will be discharged from the accumulation tank to deep water at
the outer edge of the reef via the outfall pipeline. A t-head diffuser structure fitted at the
end of the outfall pipeline and located at a depth of at least 10m, to ensure rapid dilution
and dispersal of the effluent.
7. Environmental examination of major project impact areas have been carried out as part
of setting out baseline conditions of those environmental components. These include
examination of groundwater of each island (determination of existing status, likely
impacts from the project and benefits due to the implementation of the project,
Furthermore examination of the marine environment in the context identifying of suitable
outfall locations were carried out
8. The impact that the wastewater collection and disposal system on the water balance
(freshwater lens) are discussed in the context of the size of the freshwater lens in each
island, the rate of extraction in the groundwater analysis. Two scenarios are considered;
A) all grey and black-water with the exception of the miscellaneous grey-water is
connected to the wastewater collection system and is discharged via an ocean outfall; B)
only black-water (toilets/septic tanks) is connected to the wastewater collection system
and discharged via an ocean outfall.
9. If all greywater (with the exception of miscellaneous greywater), and blackwater are
collected and discharged via an ocean outfall there will be a groundwater deficient on all
islands, that is the sustainable yield of the freshwater lens will be exceeded and there is a
high risk that the freshwater lens will deplete in size and become more saline over time.
Scenario A is therefore considered not an option for the wastewater collection and
disposal system. With respect to Scenario B where all grey-water is discharged to land,
that is not collected by the proposed wastewater collection system, Villingili will maintain
a groundwater surplus, but Dhaandhoo and Gadhdhoo will have a groundwater deficient
where the freshwater lens will be impacted over time.
10. Investigations on groundwater status and usage indicated high usage of groundwater on
all the islands when comparing to usage adopted by other studies in the Maldives.
However whether usage that produces grey-water is high or low does not have a
significant impact as most is returned to the lens via land discharge. The real concern
therefore in the water balance and impact of the project, is the volume of water used for
the toilet and sanitary cleansing, that is blackwater production. The values obtained from
the community survey are consistent across the three islands at about 55L/c/d. They are
however twice as much as stated by Beswick (2000) of 28L/c/d and higher than 50L/c/d
used by Falkland (2001) for cistern flush toilets. The community survey indicated that
cistern flush toilets were only used by 27% of the households on Villingili, 47% on
Gadhdhoo and by none of the households on Dhaandhoo. Therefore it would appear that
water usage for toilets/sanitary cleansing obtained by the community survey is high and
that the groundwater surplus or deficits estimated are either under or over-estimated.
However, it is considered that once the wastewater collection system is installed and
people become more affluent, there will be an increase in cistern flush toilets and
groundwater use and that therefore the water balance issues will apply in the longer
term.
11. The wastewater disposal system comprises of an Outfall Pumping Station and an Outfall
Pipeline that will discharge effluent to deep water on the outer edge of each island’s
house reef. The Ocean Outfall Pumping Station will include an above ground
accumulation tank constructed from either cast-in-situ concrete or prefabricated from
fibreglass etc, to receive incoming flows from the reticulation pump stations. The
accumulation tank acts as a temporary storage reservoir for the effluent prior to being
discharged to the ocean outfall by the pumps.
12. There are no statutory dilution requirements within the Republic of The Maldives and so
two international criteria have been adopted. Nevertheless, Cardno International have
been advised that discharge is to be in depths of 10m or more and into waters that are
not likely to transport diluted effluent towards fishery and recreational areas. The adopted
water quality/dilution criteria are:- Initial, average plume dilution greater than 100:1.
13. 13 To ensure that beyond a mixing zone of about 200m in any direction (always along
the coast because of the prevailing currents), from the end of the outfall, faecal coliform
concentrations at PDWF (Ultimate) are less than about 150cfu/100ml. This water quality
guideline is presented in ANZECC (2001) for Australian and New Zealand water quality
guidelines. This region, where specified water quality guidelines are not fulfilled, is called
the mixing zone. The intention is to limit the size of this zone.
14. Two diffuser configurations were investigated as the mean of dispersal of effluents– an
end of pipe discharge and a Tee-head discharge (two ports discharging in opposite
directions) with diameters reduced from the outfall pipeline diameters. Generally, the
Tee-head option provides about 50% more initial dilution, whilst remaining a relatively
simple structure, and hence is the recommended configuration.
15. Under PWWF (Ultimate) conditions, dilution of 100:1 is achieved at the GA. Dhaandhoo
site, with the other two sites being very close to the required criterion. While not strictly
achieved, initial dilution is considered to be adequate under what are the most severe
flow conditions – commonly these water quality criteria need be met only in Peak Dry
Weather Flows.
16. The need for proper sanitation system is one of the priority areas for the community of all
three islands. Hence the community is eager for a better sanitation system. Highlights the
communities view point In terms of determining the best option taking into consideration
each community sanitation requirements level of affordability and sustainable
management system
17. Environmental monitoring and mitigation measures have been considered during both
construction and operation phase of the project to minimise and monitor impact on the
two major environmental components that would be affected i.e. groundwater and marine
outfall at the outfall locations. Furthermore, improvements on environmental and human
health would also be monitored through hospital records and general perceptions of the
community.
18. A number of important conclusions and recommendations were made based on the
proposed sewerage system’s impacts on groundwater and marine environment. It is
apparent that the groundwater usage in each of the island is currently high and
considerations were given particularly in regard to maintaining the groundwater balance
on each island, and these are recapped below.
19. The Communities of GA. Villingili, GA. Dhaandhoo and GD, Gadhdhoo should be
encouraged to practice the following:
• Reuse of grey water as means of reducing net abstraction from the aquifer. For
example: using laundry washing water and other greywater collected in buckets
to flush toilets.
• Land surface application of collected lightly soiled greywater on gardens and
other grassed or vegetated areas to help maintain positive groundwater balance.
• Use of low phosphorous detergents where greywater is disposed of via land
surface application.
• Minimise blackwater production and discharge to septic tank / sewerage system.
This could be accomplished by limiting pour flushing to one (1) small 6 litre
bucket and / or installation of water saving dual flush toilet cisterns (6 / 3 Litre).
• Groundwater should only be for non-potable usage, i.e. not for drinking or
cooking.
20 A supplementary community education program as has already been implemented on
other ARC sewer project islands in collaboration with MWSA should also be implemented
in parallel with the new sewerage services to promote conservative water usage practices
and raise awareness of the risk to the groundwater quality (salt water intrusion) from
over-abstraction.
21 Health benefits associated with the project are direct and apparent due to the
construction and operation of the proposed sewage systems as it directly improves the
water and sanitation situation in all three islands. |
