3.1 Source of water for point of use treatment units

Point of use (POU) treatment units are devices that treat water at the place where it is used, such as a household kitchen, a school, a clinic or an office, rather than at a central treatment works. The source of water feeding the unit decides what contaminants must be removed and therefore which unit is suitable: rainwater, borehole water, river water and piped utility water each carry different problems. A POU unit chosen without knowing the source may fail, for example a carbon filter cannot remove the salts found in some borehole water. Example: a rural dispensary that collects water from a shallow well first has the water tested, finds bacteria and turbidity, and chooses a ceramic filter with a sediment pre-filter rather than a carbon filter.

Types of water sources for point of use treatment

Households and institutions in Kenya draw water from several sources, often using more than one through the year depending on the season and supply reliability. Each source has typical quality problems that guide the choice of POU treatment.

Piped utility water

Piped water from a water service provider has normally been treated and chlorinated, but it can be recontaminated by leaking pipes, intermittent supply that draws in dirty water, and storage in dirty tanks. POU units are used to remove residual chlorine taste, sediment from old pipes and any contamination picked up after treatment.

Rainwater

Rainwater harvested from roofs is soft and low in dissolved salts, but it picks up bird droppings, dust, leaves and sometimes metals from the roof and gutters. It often needs filtration and disinfection before drinking, and first-flush diversion reduces the dirt reaching the tank.

Boreholes and wells

Groundwater from boreholes and wells is usually clear and lower in bacteria than surface water, especially from deep boreholes. However, it may contain dissolved iron, manganese, hardness, salinity or fluoride, which are high in parts of the Rift Valley and arid areas. Shallow wells near latrines can also be contaminated with bacteria and nitrates.

Springs

Spring water emerges naturally from the ground and is often of good quality when the spring is properly protected. Unprotected springs are easily contaminated by surface runoff, animals and people collecting water.

Surface water

Rivers, streams, dams, water pans and lakes are the most contaminated sources. They carry high turbidity, bacteria, viruses and parasites from human and animal waste, and may contain agricultural chemicals and algae. They need settling, filtration and disinfection before drinking.

Vended water

Water bought from kiosks, handcart vendors and tankers may come from any of the sources above, and its quality is often unknown. It is commonly recontaminated in containers during transport and storage, so POU treatment is advisable.

Comparison of sources and suitable point of use treatment

Factors in assessing a water source for point of use treatment

• Microbiological quality: The presence of E. coli or coliform bacteria shows faecal contamination, which must be removed or killed before drinking.
• Turbidity: Cloudy water clogs filters quickly and shields microorganisms from disinfection, so very turbid water needs pre-treatment such as settling.
• Dissolved chemicals: Salts, hardness, fluoride, iron and nitrates are not removed by simple filters and need units such as ion exchange or reverse osmosis.
• Seasonal variation: Surface water quality worsens in the rainy season, while some groundwater becomes saltier in droughts, so units must cope with the worst season.
• Quantity and reliability: The amount of water needed daily and how reliably the source supplies it affects the size and storage of the unit.
• Pressure availability: Some units, such as reverse osmosis, need mains pressure, while gravity units suit sources without piped pressure.
• Cost and maintenance: The user's ability to pay for the unit, replacement cartridges and servicing determines what can be sustained.

Procedure for assessing a source before selecting a point of use unit

1. Identify all sources used: List every source the household or institution uses through the year, including dry and wet season sources.
2. Inspect the source: Carry out a sanitary inspection, checking for nearby latrines, animals, broken covers, dirty containers and runoff paths.
3. Collect water samples: Take samples in clean, sterile bottles from the point where water is actually collected or stored.
4. Test the water: Test for E. coli, turbidity, pH, conductivity or salinity, and, where relevant, fluoride, iron and nitrate, at an accredited laboratory or with field kits.
5. Compare with standards: Compare the results with the drinking water standards to identify which parameters exceed the limits.
6. Estimate water demand: Work out the daily volume of drinking and cooking water the unit must supply.
7. Select the treatment unit: Choose a unit, or a combination of units, that removes the contaminants found and suits the demand, pressure and budget.
8. Plan monitoring: Arrange periodic retesting and maintenance, especially after seasonal changes in the source.

Estimating the daily water demand for a point of use unit

POU units usually treat only drinking and cooking water, since this is the water swallowed. A common planning figure is about 7.5 litres per person per day for drinking and cooking.

Daily treated water needed = number of users × litres per person per day

Example: A household of 5 people uses a gravity filter that produces 2 litres per hour. Find the daily treated water needed and the hours of filtration required.

Daily treated water = 5 × 7.5 = 37.5 litres

Filtration time = 37.5 ÷ 2 = 18.75 hours

Answer: 37.5 litres per day, needing about 18.75 hours of filtration, so the household should refill the filter regularly through the day or choose a larger unit.