Water Matters

Why Rain Water Harvesting?

Previously in rural Cambodia the emphasis has been on accessing drinking water through ground water sources such as tube wells and surface water sources such as rivers and shallow ponds. It is now evident that not all these sources meet drinking water standards and not all areas have enough water to meet even basic needs. Problems range from arsenic and iron contamination in groundwater through to fecal contamination and pesticide residues collecting and concentrating in open ponds. During the height of the dry season some villagers must travel for up to five hours followed by long waits in queues to access the nearest water source, which may be of extremely poor quality.

Due to these issues considerable effort has been directed into treatment methods that provide “point of use” treatment for drinking water. In particular household filters such as bio-sand filtration or ceramic filters have been developed along with approaches such as solar disinfection (SODIS), chlorination and boiling. Piped water from small scale water treatment plants are now being developed for bigger villages – typically 300 plus families. These methods have proven effective in dealing with fecal contamination and some other pathogens. Unfortunately they cannot cost effectively address heavy metal contamination or remove agricultural waste products, they are also of limited benefit where insufficient water exists in the first place.

Many people throughout Cambodia already practice rain water harvesting on an informal basis collecting run off in large jars, typically of about 500 litres in capacity. Each household will have more or less of these jars depending on income level but very few have enough to reach the national standard of 3,000 litres of storage capacity per household. The jars are usually left uncovered and this can lead to the breeding of mosquitos and the growth of algae and pathogens in the jar. The large opening also encourages the use of dippers to access the stored water, a further source of contamination in villages where open defecation is considered normal.

These issues are not insurmountable however and RainWater Cambodia has been working hard over the last eight years to establish a fomalised method of rainwater harvesting providing high quality, disease and heavy metal free rain water to thousands of households, community centres and schools. We have also trained local entreprenuers to follow our risk managment approach to rainwater harvesting and they are replicating this successful programme in their own businesses.

Formalised Water Collection

RainWater Cambodia has identified measures to combat the risks in traditional rainwater harvesting and has developed a formalised, risk management approach to rain water harvesting. A typical design of our rainwater harvesting systems considers the following elements:

1. Monthly rainfall data
2. Available harvesting area
3. Harvesting area runoff coefficient – this takes into account losses from the roof/catchment system and gives an estimate of the actual amonut of water that can be expected to be collected from the roof
4. Usage/demand per user

The systems can be sized based on supply, demand and budget.

Typically a graph plotting cumulative rainfall against cumulative demand will determine if the supply will meet the demand. From this type of graph the maximum storage that would be required can also be determined. This is particularly significant in areas where there is a monsoonal or single wet season, such is the case in Cambodia.

Harvesting area:

1. The bigger the harvesting area, the more water can be collected during any rainfall period.
2. The smoother the harvesting area, the greater the % of water that can be collected.
3. Metal roofs may provide some additional benefit if subject to the hot sun as this may contribute to bacterial die-off.
   

Conveyance system – guttering and pipes:

1. This part of the system helps to maximise the quantity of water that flows to the storage tank.
2. The conveyance system can be enhanced to maximise the quality by adding leaf guards and first flush diverters.
   

Storage tank:

1. Most emphasis is usually placed on this component as it is generally the most expensive part.
2. The storage capacity needs to be adequate enough to enable water to be stored to last through the dry period.
   

Risk management components:

1. See detailed comments in the next section.

Risk Management Approach

Our risk management approach is based on a system design which removes the risks associated with traditional rainwater harvesting and can be seen in the diagram below.

Risk management features :

1. Leaf gaurd – keeps out larger objects which may contribute to bacterial loading or may provide nutrients for microorganisms.
2. First flush diverter – diverts the first flow of water from a roof area away from the tank. This flush of water contains the highest levels of roof contaminants and these are effectively excluded from entering the tank.
3. Enclosed tank – prevents human access and excludes light preventing algal growth which provides nutrients for microorganisms.
4. Insect and animal screening – prevents animals entering the tank and dying which can be the most significant contributor of contamination to the storage tank.
5. Graded base sloped towards a cleaning outlet – enables easy cleaning of the sediment layer which forms in the bottom of the tank. This layer contains microorganisms which if left to build up may contaminate the water. Yearly cleaning of the tank is recommended to prevente this build-up contaminating the water.

As a means to try to ensure consistent high quality water for drinking purposes, the WHO have introduced a risk management approach to drinking water supplies based on Hazard Analysis of Critical Control Points (HACCP).

This is documented as a water safety plan which “provides for an organised and structured system to minimise the chance of failure through oversight or lapse of management and for contingency plans to respond to system failures of unforeseen events”.

RainWater Cambodia has developed a water safety plan for rainwater harvesting, based on WHO recommendations. We are happy to share this with interested parties as we understand it is the only Water Safety Plan document specific to rain water harvesting systems.

Arsenic in Groundwater

Until recent decades, arsenic poisoning was regarded as an obscure or local phenomenom , however, it is now recognised to be a global problem. Natural arsenic pollution of groundwater and, to a lesser extent, surface water, is known to have exposed more than 140 million people to arsenic concentrations above the WHO guideline of 10 micrograms per litre in at least 70 countries. Symptoms of chronic arsenic poisoning include hyperpigmentation, dipygmentation, keratosis, skin cnacer, internal cancer and even death.

Recent data from UNICEF indicates that up to 2.4 million Cambodians are at risk from arsenic poisoning, a figure that represents approximately 27% of the countries population, due to groundwater being a huge source of drinking water for rural Cambodians with thousands of wells in use around the country. Not all are contaminated with arsenic however and RDIC is currently undertaking a huge water survey of all the provinves in Cambodia to identify and map which wells and areas are contaminated and to determine water use patterns. Other significant threats to health have been identified in groundwater including Manganese and high levels of Nitrates. The scale of the drinking water situation in Cambodia is dramatic to say the least, however many organisations are working in conjuntion with the Ministry of Rural Development to achieve the Governments Millenium Development Goals whereby 2025 one hundred percent of the population will have access to safe drinking water.

Impact of Air Pollution

Cambodia is a relatively undeveloped country with little heavy industry and few cars outside of the three main regional centres of Phnom Penh, Siem Reap and Battambang. This is great news for people interested in rainwater harvesting as it means that a clean source of drinking water is available – literally on the doorstep, free from contaminants which often occur in rainwater in industrialised countries. This is particularly good news when surface and ground water are often not safe to drink due to high levels of contamination and involve long hours spent collecting, especially during the four month long dry season.