2013

Gordon Hirst

18th June 2013

Issue 1

Initial study for rainwater

harvesting

Rain water harvesting for Nong het district, Xiangkhouang Province

12 target villages

Synopsys

Study into alleviation of dry season water shortage in 12 target villages in Nong het district by use of

rain water harvesting systems.

Scope

The scope of study is an initial look at the possibility and methodology of harvesting rain water in the

12 target villages to supplement their existing spring water & domestic rain water harvesting supply,

particularly for the ‘lean’ months in the dry season. The study was conducted over two days with

presentation of initial results at the CF Nong Het office.

General Notes

This report covers both generic and specific solutions for the proposal for rainwater harvesting

systems. The general notes also covers other issues which could be considered outside the scope of

this report. However not all problems and solutions can be considered in isolation, therefore they

have been included. The report concludes with notes on specific villages visited.

Loss of water from existing system

It has been recognised by CF and also observed during the study that the loss of water through:

1. Brocken pipes (often due to construction work)

2. Lost, stolen or broken taps

3. Taps being left on

4. Illicit ‘tapping’ of water pipes

Before any supplementary water system can be examined, attention must be made to these existing

water loss problems.

In the issue of taps, the taps supplied are inferior Chinese manufacture and are easily broken or

stolen or just left on.

The resolution of this issue can be approached in two different ways:

1. Education program on the need for conserving water.

A very easy statement to make but as CF will know a very very difficult concept to

implement. This would be a program that would take several years to implement but a long

persistent program will achieve results in the end.

2. Installation of ‘tamper proof’ tap timer switches in the village cisterns.

As this issue is not covered in the report no resources have been used in sourcing such a tap

(if one exists)

The layout is surmised in the fig 1.

Generic solution

The survey conducted looked at all possible points of capture and concluded that the roof areas of

the recently constructed school buildings offer the best solution. The buildings are new and well

constructed and cover a large area they are also elevated from the ground giving necessary fall for

the water.

Other sources of rain water collection such as collecting from the road gulley’s using a Ram pump

system were rejected as the heavy use of paraquat in the area would make the water unusable.

This is also true of all systems which rely on ground water runoff.

Supply and demand

The most critical factor in designing a proposed solution is to identify what the water demand will

be. In this, clarification must be made as to the actual water usage is stated in Litres per person per

day. (L/p/d). WHO guidelines state the bare minimum is 7.5 L/p/d for consumption, basic hygiene

and laundry. However the consumption in the villages when water is available is much higher with

the Poverty reduction fund stating a figure of 40 L/p/d. A general consensus of agreement is that this

figure is high and as a measured consumption rate must be barred in mind that there is a lot of lost

water in the system.

http://www.who.int/water_sanitation_health/emergencies/qa/emergencies_qa5/en/

Notes:

• The village cistern systems were supplemented by household rainwater collection

• A distinction must be made between water and drinking water. Drinking water is to a higher

standard of sanitation and will require post processing after delivery.

• Demand figures must also take into account the expanding population figure given at 2.9%

per year.

In our deliberations we have developed an algorithm for water demand which is best shown

graphically as in Figure 2.

The supply shortfall is represented by the shaded area on the graph

Notes:

1. An increased demand has been shown for the dry season although this is theoretical.

2. Supply and consumption from the gravity feed (natural springs) cisterns was measured

during the rainy season. It is assumed that there will still be a supply during the dry season

although this figure is not known.

3. Inclusion for supplementary water capture by households, no accurate figures are available

but needed to be included in the algorithm.

4. All villages were asked in which months do the experience water shortages. This did vary

slightly but usually between 3 and 4 months of the year.

As we are providing a supplementary system (not a replacement) we have used a value of 20 L/p/d

Demand calculation

Q (Ʃvolume of water: litre) = n (number of people) x d (demand (20)) x d (number of days shortage)

Example:

Q = 304 x 20 x 90 = 547,200 Liters

Supply calculation

Our preliminary proposal is the use of rain water capture systems, this principal driving force for this

being the construction of school buildings in the target villages, some of which are still under

construction. The large roof is of the school buildings offer a good opportunity for rain water

harvesting.

Calculation:

The average rainfall in Nong het district is between 1500mm - 2000mm per annum (1.5m - 2m)

This however should be verified and the setting up of a rain gauge would be a good idea, this will

need to be measured over a one year period.

Assuming 10% losses from wash over.

Example:

Therefore a roof measuring 30m x 10m will be expected: 30 x 10 x 2 x 0.9 = 540 m³ (540,000 litres) of

water.

From the data gathered on site of the size of the roofs, it than therefore be calculated the difference

in the supply and demand this is represented in table 1.

From the above table it is conceivable that in the majority of cases the supply is complimentary to

demand. With a few exceptions, however the issue becomes the practicality on the size and number

of the tanks. I.E. storage of 540,000 litres of water will require 18 off 30,000 litre storage tanks, a

clearly impractical proposition. Following this study it must be decided what is the practical number

of tanks would be required for the target installation. To complement the shortfall on practical rain

water storage tanks a system of domestic systems be introduced.

The generic designs are based on the installation of a number of tanks. Short squat tanks are

preferred as these will fit easily under the eaves and allow 5° fall on the guttering. 5° is recommend

as a minimum which would entail a drop of 1.75 m over a 20 metre stretch meaning the positioning

of the tanks is also critical.

Ideally the larger the tank the better limited only by the height of the tank and the logistics of

shipping. See figs 3, 4 & 5

Fig 3

Fig 4

Fig 5

It is proposed that at least initially the units to be ‘stand alone’ with the rainwater capture tanks

feeding a single tap. Discussions were made on the possibility of linking up with the existing village

cisterns in the future and therefore this must be carried over into the initial design.

The outlet tap must be lower than the lowest point of the tank, this is not always possible with

exiting site configurations, in some cases a pump and a header tank will be needed.

Notes on tank installations:

1. Use of a ‘first flush’ diverter should be used, these are simple devices which use the first few

minutes of rainfall to ‘clean’ the roof surfaces, gutters and feeder pipes. They operate

automatically requiring o input aside from the flow of water. They come in a number of sizes

and designs the following is a typical unit. See fig 6

Fig 6

2. Location of tanks

In addition of the critical consideration of the vertical location, the lateral location could also

be an issue. With the fall requirement of 5° the tank has to be located close to the gutter

drop to take into account for the drop requirement and the diverter. It is suggested that a

row of tanks on the ‘back side’ of the school buildings but leaving enough room to allow

natural light into the class rooms. Stacking the tanks at the ends of the building is also an

option however this becomes an issue with the fall. This can be circumvented by dropping

the gutter water into underground ‘carrier’ pipes. See Fig 7

Fig 7

3. Gutter maintenance

The most imperative issue with the installation of rain water harvesting systems is keeping

the gutters and drop pipes clean. This will mean the instigation of maintenance routines, a

thorough clean and check over prior to the rainy season should suffice in normal

circumstances however if the roof has a tendency to be covered in vegetation more regular

cleans will be necessary.

4. Guttering and carrier pipes

As part of the installation is a critical factor that the guttering, drop pipes and carriers should

be installed to the highest standards possible. This covers the standard of the guttering and

pipes and hangers. The new school buildings generally are recent constructions and in good

order. Also the wooden roof struts are made from local hardwood and make excellent fixing

points. The guttering supplies can also be extremely useful for the supplementary domestic

rain water capture. Careful attention must be made to taking the ‘tap point’ from the roof at

the ‘lowest’ point.

5. Tank sanitation and keeping water fresh

A priority consideration for water storage is the need to keep the water fresh, free from

algae growths, pathogens and stagnation. In general rain water which storage tanks and is

capture from buildings keep fresh for long periods. This is due to the water being captured

from ‘inert’ surfaces (particularly combined with a ‘first flush’ diverter, removing a good deal

of the organic matter). Secondly a ‘closed’ tank will stop a good deal of ingress of organic

matter, animals and insects (particularly mosquitoes). The tank lining material is also inert

and does not promote growth of pathogens.

This issue was a point of some discussion and it was concluded that some form of ‘agitator’

be included in the installation of the tanks. The purpose of this agitator will be to regularly

stir the water, averting the onset of stagnation. The agitator could be a stirrer powered from

a small motor or a small submersible pump. Either could be either mains or solar powered.

6. Water filtration

As part of a more comprehensive water supply program the twelve target villages are

undertaking a water filtration program with Abundant Water. This is mentioned on this

report as consideration should be taken at the time of installation feed to the water

filtration system and the possible requirement for a pump and header tank

7. Shipping to site

The proposal is recommending the installation of large plastic rain water tank. There

advantage over construction concrete tanks on site is they arrive as a working unit. They can

be shipped to site on a the back of a truck and are relatively easy to dismount move and put

into position (three people can easily maneuver a 30,000 liter tank)

Houaydeua is the only village where shipping to site might be an issue

Fig 8

8. Installation base and coupling

Prior to the arrival of the tanks the site must be prepared. This essentially flattening the

ground that the tanks will sit on, a shallow sand base is also recommended, this will take up

the contours on the base so that no stress points are introduced. It is recommended that all

the tanks be linked together with a 4” pipe at the base. This will make the egress of water

much easier.

9. Linking with gravity feed spring water tanks

Although the agreement on site was the proposed systems be ‘stand alone’ it would be wise

to consider integration into the existing water system in a future phase of the project. For

this it will be a general requirement for the ‘lift’ of the water to be up to 200m, requiring as

single phase motor to produce a 20bar pressure. These are available and a power

consumption between 5 KW and 10 KW per pump be designated. Also the pipe and fittings

at the discharge end of the pump must be designed to withstand 20 bar pressure.

Housing clusters and domestic rain water capture systems

From the calculations we see that in many cases the demand outstrips supply and there is the issue

mentioned the actual number of tanks required will be impractical. At present crude domestic rain

water capture systems are installed in a number of households. The effectiveness of this can be

greatly increased by the use of effective guttering which will be supplied similar as that for the rain

water tanks. The scope and size of the domestic rain water harvesting again will be limited on

practical sizes. In deed there is scope for a number of houses to ‘cluster’ and use a central larger

storage tank.

Grey water

As part of the overall concept of water usage, re usage and water conservation the concept of grey

water should be mentioned. Grey water is the capture and use of ‘secondary water’ from non

polluting sources (showers and laundry: not latrines this is considered black water).

During the study it was noticed a large quantity of water escaping from the cisterns, especially

where the taps have been removed.

This water could be re used see Fig 9

Fig 9

Notes on Grey water:

1. By definition grey water is collected at the lowest point. Therefore it will always require a

pumping system.

2. Care must be taken as to collect directly from water source using piping or gulley’s

otherwise an issue with paraquat contamination might occur.

3. Its usage lifetime is very short, normally if not used within 24 hours it should be considered

as black water. If being used for irrigation an instantaneous pumping system should be

considered.

Ram Pumps

Ram water pumps have been around for decades and are essentially a simple gravity powered

pumping system. The pump relies of a flow of water (such as a river or stream), the momentum of

the water operates a combination of two valves and a pressure chamber which pulses as the system

equalizes and unequalizes in cycles. Note: there is always waste water flow. The amount of water

which can be pumped is calculated from how much flow water there is and how high the water

needs to be pumped.

http://www.youtube.com/watch?v=qWqDurunnK8

Fig 10 & 11