Gravity-Fed Water Filtration System
An easy to maintain system to convert untreated water with a turbidity of more than 50 nephelometric turbidity units (NTU) into safe, potable water (below five NTU) using locally available resources. The large version of the system can produce 5,500 litres of potable water in eight hours and is easy to maintain.
Dera Murad Jamali project, Balochistan, Pakistan
- Raw water storage tanks (capacity of 89,000 litres each for the full system)
- Sandbags (jute or canvas)
- Activated charcoal (if colour and odour are a concern)
- 8” and 6” diameter PVC pipes, perforated with 0.75” diameter holes
- PVC woven, nylon or fabric covering with 0.5–1 mm pores
- Water pumps
- Pipes (preferably polypropylene) 1.5” and 0.75”
- Chlorine 50 HTH (powdered)
- Aluminium chloride (powdered)
- Pool tester
- Turbidity tube
- Water-powered, non-electric chemical injector (Dosatron recommend)
- Water storage tanks: overhead tank, chlorine contact time tank, and dispensing tank or point-of-use tank
- Protective gear: respirators, apron and goggles
- Two dosing buckets: for aluminium chloride solution & chlorine solution
- Engineer method
Balochistan is facing a major water crisis. Poor communities cannot afford bottled water and have to rely on unsafe canal or creek water. At the Divisional Headquarters Hospital in Dera Murad Jamali water was pumped in from a local canal and supplied through a municipal pipeline. It arrived with a high level of turbidity (cloudiness caused by particles in the water), and contained pathogens and small maggots. It was not fit for any purpose in the hospital so an alternative had to be found. Could we create a system to supply safe water sustainably, using only locally available materials?
Part 1: My observations
When I visited a number of MSF inpatient therapeutic feeding centres (ITFCs) in my region, I found that sourcing safe water was their greatest challenge. Poor communities couldn’t afford bottled water and for many the only available source was canal or creek water. I thought it might be possible to create a water filtration system, using local materials and powered by gravity.
Start small, test and develop
Part 2: Creating a smaller pilot system
I first prepared a small, low-cost model to see if my idea for a gravity-fed water system could work. I dug a six foot deep trench in a creek near to the Monjo Shori ITFC. I laid a 6” diameter pipe, which ran the length of the trench. The pipe was perforated with 1” holes and wrapped in a PVC woven covering.
I buried the submerged pipe in gravel, leaving only 1” of piping exposed. I attached a pump to the end of the exposed pipe and added a layer of sandbags on top of the gravel to fill in the trench within the creek. Water passed through the layers of gravel and sand, which filtered out many of the impurities. The pump then drew up the filtered water. I checked the turbidity of the water and the results were very encouraging.
Getting buy in
Part 3: Designing the full gravity-fed system
Encouraged by the success of the pilot system, I scaled-up the design to create a full system. I conducted a feasibility study and sent this and the new design to the HQ Water and Sanitation Advisor. After some consultation, the design was approved. I prepared detailed drawings with design specifications and a list of materials, with estimated costs for the whole system.
The estimated budget was approved and allocated for construction of the system, which I named the Gravity-fed Water Treatment System.
Part 4: Setting up the full system
For the full system, we set up a large concrete filtration tank able to hold around 100,000 litres near the Dera Murad Jamali hospital. The tank was connected to the existing pipeline, which brought water up to the hospital from the river. Within the tank, we dug a trench, six feet wide, nine feet long and five feet deep, which we named the filtration gallery. At the base of the trench we laid two 8” perforated pipes wrapped in the PVC covering.
The horizontal pipes were buried in a deep layer of gravel, on top of which we added a layer of activated charcoal to help remove chemicals and odours from the water. This was followed by a separator layer of fine PVC mesh, and a layer of bricks to help capture very fine particles. We topped this off with another layer of the PVC mesh. Finally, we placed sandbags on top to add one final layer of filtration and to hold it all in place. The sandbags need to be replaced every six months, but older ones can be washed and reused. All of these materials were locally available.
River water is propelled down through the layers by gravity, losing turbidity with each one. The pipe we laid at the base of the trench is connected to a pump, which draws the filtered water out into the flocculation tank. By passing through the filtration gallery, the water turbidity dropped to below 5 NTU. This already a significant improvement, before flocculation even begins.
Part 5: Flocculating the water
In the flocculation tank we were able to further purify the water. Flocculation is the process by which particles in water are joined together using a flocculant, so that they become heavy enough to sink to the bottom of the tank, making it easier to remove them from the water. We used aluminium chloride as our flocculant because very effective, cheap, easily available and can be stored for years with no fear of it expiring.
The tank was fitted with jets so that, when the water was pumped in, it swirled round, which further aided the flocculation. We tested the water as it arrived to monitor how effective the filtration tank had been; we were consistently impressed by the drop in turbidity. After the flocculation process, water turbidity was at 0 NTU.
Part 6: The final testing and purifying
Once we judged that the water had reached 0 NTU, we moved it to the clear water tank. The flocculation tank was fitted with another 8” PVC pipe, which we had covered with mesh to prevent any remaining flocs flowing back into the system.
As a final step, the water in the third tank was disinfected with chlorine solution using a Dosatron water-powered injector. Thirty minutes after dosing, a pool tester was used to check the free residual chlorine levels. Once the level was within the required range (0.2 to 0.5mg/l) we were able to transport it to the hospital’s storage tanks for use in sterile cleaning and oral consumption.
Part 7: Conclusion
The system we set up at Dera Murad Jamali hospital shows that it is feasible to provide safe drinking and cleaning water through a gravity-fed filtration system made using only locally available materials. All the people at this busy hospital are now able to enjoy clean, safe water, roughly 10,000 litres every seven hours.
The cost of installing the full system was £19,675 (including installation of eight 2,000 litre storage tanks) and ongoing maintenance and running costs are expected to be no more than £70 per month. As far as I know, the hospital has not had to spend anything on maintenance or repairs so far. Not only is this an economical and efficient design, which could be adapted to other contexts with similar water quality issues, it could also be used in domestic settings with minimal maintenance and running costs.
Are you in a location where source water quality has high turbidity or cloudiness?
Is the current treatment to remove turbidity working well?
Can a gravity-fed filtration system improve access to clean water?
Implementing this design requires review and coordination with HQ. Note that, at the time of going to print, OCA (Biserka Pop-Stefanija, WatSan Advisor) has validated the contents of this feature. If you have any technical questions, please get in touch with your WatSan Advisor.