Proposal for the assessment and improvement of the
existing rainwater catchment system for the Dixcove
hospital
The goal of this proposal is to make an assessment and suggest improvement for the
existing rainwater catchment system of the Nana Hima Dekyi District Hospital located in
Dixcove, Ahanta West, Western Region, Ghana. It is part of the alternative water system
construction and training project directed by the NGO African Health Net and funded by
the European Union.
1 Assessment of the existing infrastructures
1.1 Rainwater catchment infrastructures
- Rainwater storage tank (16 000 liters):
Figure 1: Existing rainwater storage tank of 16 000 liters
This tank collects water form the concrete slab roofs. It is currently in use and water
is collected from a tap as shown in the picture above. There is an existing overflow
pipe of 1 inch diameter. The water inside the tank is full of suspended solids. There is
no existing filter and the water quality in the tank seems to be very poor. The tank
seems to be reusable and no obvious leaks were seen during the inspection. The
absence of a lid on the opening at the top of the tank was noted. The tank will have to
be thoroughly cleaned out.
- Underground storage tank (49 000 liters):
Figure 2: Existing underground storage tank of 49 000 liters
This tank is currently not in use. It seems to be in good condition but the fact that the lids
have not been in place has given access to animals. It will have to be thoroughly cleaned
out and the lids should be secured. The drainage system and valve seem to be in good
condition although it should be checked for any possible leaks.
There is evidence of an existing uncompleted rainwater catchment system for this tank
as shown in the pictures below:
Figure 3: Evidence of rainwater catchment system for underground tank, not completed
The gutters in place can be reused although their slope should be verified. The inlet
from the down pipes to the tank can also be reused.
1.2 Other infrastructures
- Pumping system:
Figure 4: Pumping room
The existing pumps are no more in place as it is shown in the above picture. The piping
seems to be in acceptable condition and can be reused. Pressure tests should be
carried out to check for any potential leaks in the piping and it will have to be
disinfected. The pump intake inside the tank seems to be in bad shape though and
should be replaced.
-Water storage tower (47 000 liters):
Figure 5: Water storage tower
This reservoir seems to be in good condition and can be rehabilitated. The inside of the
tank will have to be thoroughly cleaned out. The tubing to and from the reservoir and
the drainage system and valve seem to also be reusable. Pressure tests should be carried
out to check for any potential leaks in the piping and it will have to be disinfected.
-Poly-tanks:
Figure 6: Poly-tanks of 7 000 liters
The hospital has 4 poly-tanks of 7 000 liters. Three of them are currently on the concrete
slab roofs and one is at ground level outside the pharmacy. PVC plumbing to the tanks
on the roofs is already existent although it will have to be checked thoroughly for any
potential leaks by carrying out pressure tests. The tanks are all equipped with float based
top up system and can be filled up by the community water system.
One tank has caused cracks in the concrete structure of the roof as shown in the
second picture above and will have to be moved to another location on the concrete slabs,
preferably where there are pillars or a load-bearing wall. The poly-tanks will have to be
carefully positioned by making sure their weight will not affect the structures of the
building. Although, the locations of the two other tanks seem to be safe, the help of a
civil engineer may be sought so that no risks of affecting the structures are involved.
1.3 Available water storage
The total available storage of water for the hospital is:
-
Underground storage tank: 49 000 l
Water tower:
47 000 l
Rainwater storage tank:
16 000 l
4 Poly-tanks of 7 000 l:
28 000 l
Total:
140 000 l
Considering 100 people in the hospital each day using 200 liters of water, the need would
be of 140 000 liters per week. Thus the available storage of the hospital is enough to
provide a week use of water during a potential shortage from the pressurized
community water system.
2 Design for improvement of the rainwater harvesting system
2.1 Catchment surfaces
To fill the 49 000 liter underground tank, the most appropriate and most practical
catchment available is the roof top of the main ward shown in the pictures below:
Roof valley
Figure 7: View of the roof top of the main ward.
The surface available is estimated to be more than 700 m2. The yearly rainfall being 1700
mm and assuming a pessimistic runoff coefficient of 60%, the water available would be
around 700 000 l /year.
To fill the 16 000 liter rainwater tank, sections of the roof tops of the lab and consulting
rooms/pharmacy can be used. These sections of roofs are shown in the following
pictures:
Figure 8: View of the roof tops of the consulting rooms/pharmacy and lab.
The surface available is estimated to be more than 200 m2. Using the same calculation as
previously, the water available would be around 200 000 l /year.
The total amount of water available from rainfall will be about 900 000 liters a year.
When full, the available water storage will allow the hospital to have running water
during one week considering a community water shortage. The water shortages do not
usually last more than a few days, thus the amount of stored water will be enough to
cover the hospital needs during a community water deficiency.
2.2 Gutters and downspouts
The gutters will consist of 200 mm diameter half-round PVC pipes. They should be
fixed securely to the wooden planks at the end of the roof tops. The fixing method used
for the remaining gutters from the uncompleted catchment system seems to be efficient
and should be reused. It is shown in the picture below:
Figure 9: Fixing the gutters for the collection of rainwater
Blocks of wood are nailed into the wooden planks at the end of the roof and metal hooks
are fixed on them to support the gutter. The hooks will have to be solidly and accurately
fixed with screws to firmly support the gutter and allow an ideal slope of 5 mm per
meter.
On the roof of the main ward, 2 roof valleys have been identified. These are where the
two sections of roof come together and the flow of water during a rainfall in these valleys
can be very strong. This strong flow was confirmed by the hospital administrator.
Rainwater diverters should be placed in these valleys to spread the accumulated flow of
water so that it doesn’t pour too hard into the gutters. These can be made by cutting a
piece of aluminium (about 40 x 10 cm) from a roof panel. This piece will be smoothed
out and bent at a right angle so the water will flow on either side. Two diverters will be
placed in each roof valleys, one in the middle and one at the end. They will be fixed onto
the roof with small screws. The diverters will be placed so that the holes made by the
screws will be on an overlapping part of roof panel to make sure there will be no leaks
created in the process.
The gutters will have end caps where rainwater begins to flow in direction of the tank.
The downspouts will be 200 mm diameter round PVC pipes. These will bring the
water from the gutters vertically down. They should be clamped firmly to the wall using
metal hooks and long screws.
2.3 Roof washers
Roof washers allow the first flush of rainwater to be diverted from the collection tanks.
This first flush of rainwater can contain potential roof contamination accumulated
between rainfalls. These contaminants can be dust, insect bodies, bird and rodent feces,
other airborne residues etc, which can contribute to bacterial growth in the tanks.
The principle of the roof washer and rainwater catchment system that will be used for this
project is described in the figure below:
Figure 10: Rainwater catchment principle and components
The Texas Manual on Rainwater Harvesting suggests one gallon or 4.546 liters of washer
capacity for each 100 square feet or 9.29 m2 of roof.
-
Volume of roof washer for the 49 000 liter underground tank: 350 liters
-
Volume of roof washer for the 16 000 liters rainwater tank: 100 liters
The roof washers will be made out of plastic drums available locally. The volumes will
have to be the closest as possible to the ones calculated above within the range of drum
sizes available. A piece of fine mosquito mesh will act as the filter and will prevent
mosquitoes entering the tank. It will be fastened over the top of the drum. It will prevent
any type of coarse organic matter to enter the tank. Two PVC pipes will have to be fitted
and sealed into the drums:
- At the top of the drum, a PVC pipe will be fitted and connected to the tank.
- At the bottom of the drum, another PVC pipe will act as a trickle drain and will be of
very small diameter (0.5 cm or smaller). It will unable the first flush to run out.
The description for this type of roof washer is shown in figure 11 below:
Figure 11: Roof washer
For both tanks, the roof washers will be situated against the building walls directly under
the downspouts. A concrete platform of 1 m2 x 10 cm thick will be laid to place the
drums on. For the 16 000 liter rainwater tank the drum may have to be slightly higher
than the ground level to insure gravity feed from to the tank. Thus the concrete platform
may have to be slightly elevated.
The downspouts carrying water into the roof washers should have two 90° angles at their
ends so that the water flowing down will be slowed and won’t put too much strain on the
mosquito nets. The mesh should be checked regularly and if torn or open should be
repaired or replaced immediately.
2.4 Rehabilitation of storage tanks
For the underground and rainwater tanks 160 mm diameter PVC overflow pipes will
have to be put in at the top of the tanks. In case of heavy rain, the overflow pipes will
allow the excess rain water to be safely disposed of without causing any flooding. The
existing one on the rainwater tank is of too small diameter and should be replaced. They
will have mosquito meshes at the bottom to prevent animals or insects getting in. The
meshes should be checked regularly and if torn or open should be repaired or replaced
immediately. It should also be ensured that the overflow water is drained away
effectively and not allowed to cause flooding.
An example of an overflow pipe on a rainwater tank is shown in the picture below:
Figure 12: Example of overflow pipe on rainwater tank
For the 16 000 liter rainwater tank an inlet identical to the existing one on the 49 000 liter
underground tank will also have to be created.
The three tanks should be totally swept and cleaned out of any loose debris. The surfaces
should then be cleaned with a steel brush or pressure washer. A cement based water
proof coating (Maxseal or similar) should be applied to the entire inside surfaces of the
tanks to prevent any leaks and to make sure that the water that will enter will be in
contact with clean surfaces. The sealant can be applied by trowel or brush on about 1/4 of
an inch thick.
Lids with handles and efficient locking systems should be used for each tank to insure
nothing can get in.
2.5 Description of global rainwater feeding system
The global rainwater feeding system will consist of 3 different systems that can be used
independently:
1. Water tower feeding system:
This will be the main rainwater feeding system for the hospital. The over head tower
will allow water to flow into the plumbing system of the hospital by gravity. A valve will
be operated manually by hospital staff when water shortages from the community system
occur. This system will be the most significant in terms of water quantity and cost
effectiveness.
2. Rainwater tank feeding system:
This system can be used as a back up to the water tower. A booster pump will suck
water from the 16 000 liters rainwater tank into the plumbing system maintaining a
minimum pressure. It will be switched on manually by hospital staff.
3. Poly-tanks feeding system:
The poly-tanks can be used as a last back up to the two previous systems. These tanks
will have been filled up previously by the community water system and will feed into the
plumbing system by gravity. It will also mean that when a community shortage occurs,
the hospital will always have water even without switching to one of the two systems
above although the pressure will be much lower.
A schematic description of the 3 systems is described in the following figure 13:
3. Poly-tanks feeding system
Water tower
(47 000 liters)
4 Poly-tanks on roof,
extra storage when no
more rainwater
Rainwater
from roof tops
Hospital
Pump
Underground tank
(49 000 liters)
Rainwater from
roof tops
Manual valve to
open when
community water
shortage occurs
Rainwater tank
Booster pump to start (16 000 liters)
when shortage from
water tower system
1. Water tower feeding system
Figure 13: Schematic description of rainwater feeding systems
2. Rainwater tank
feeding system
Important note: Check valves will have to be installed on the main pipes feeding into
the hospital plumbing for the community water system, the water tower system and the
rainwater tank system.
2.6 Pumps and pumping room
- Pump replacement for transfer of water from underground tank to water tower:
Suggested pump model:
DAB K 14/400 M single impeller centrifugal pump
Technical data :
(See appendix 1)
Provenance:
AQUATEC PLACE, 222 ASHIAKLE AVENUE, ABELENKPE, ACCRA
TEL: 03 02 76 87 58/9
For the head needed which is about 13 m the discharge would be of 30 m3/h.
This type of pump is available in Ghana through aquatec services limited in Accra for
about 700 US dollars.
Since the rainwater harvesting system will be a back up to the community water system, a
second back up pump is not compulsory.
Two float based systems will have to be installed in the water tower and the underground
tank. The water tower will have a minimum level that will switch the pump on and a
maximum level that will switch the pump off. The underground tank will have a
minimum level that will not enable the pump to turn on.
The pumping room will have to be rehabilitated and the door should be replaced and a
functioning lock will be put into place.
- Booster pumps for rainwater tank:
Suggested model:
DAB 1 KVCX 50 vertical multistage centrifugal pump
Technical data :
(See appendix 2)
Provenance:
AQUATEC PLACE, 222 ASHIAKLE AVENUE, ABELENKPE, ACCRA
TEL: 03 02 76 87 58/9
This type of booster pump is available in Ghana through aquatec services limited in
Accra for about 500 US dollars.
A small pump room of 1 m3 can be built out of concrete adjacent to the rainwater tank. A
door with a lock will have to be installed on it.
This system will be turned on manually by the hospital staff when a second water
shortage occurs from the rainwater underground tank and tower and will maintain a
constant pressure in the hospital network that can be set manually from 2.5 to 3.5 bars.
2.7 Fencing of site
The underground storage tank, the pumping room and the water tower will have to be
fenced off to avoid any intrusions that may result in degradations or theft of equipment. It
will also secure the tank from any potential contaminations.
A green PVC coated chain link fence can be used for this purpose. The length of fence is
approximately 100 m. Concrete pillars to hold up the fence are still in place and can be
reused.
References:
1 Government of Karnataka, Manual on rooftop rainwater harvesting systems in schools
2 Gretchen Rupp, PE, Extension Engineer/Specialist, Rainwater harvesting systems for Montana
3 Practical Action, Rainwater harvesting practical brief
4 Texas water development board, The Texas manual on rainwater harvesting
APPENDIX 1: Pump technical data
APPENDIX 2: Booster pump technical data