Water

Lessons

Senior 1 · Chemistry

Water

5 lessons

Lesson 4 of 5

Treating and Purifying Water: at the Works and at Home

25 min

From a brown river to a clear tap

In the rainy season the Nyabarongo runs the colour of milky tea, thick with soil washed off the hills. Yet much of the water that comes out of Kigali's taps started in this river. How is brown river water made safe to drink, and what can a family do when there is no tap? In Unit 4, Lesson 5 you met the separation methods used at a treatment works. This lesson follows water through a real Rwandan works and looks at the chemistry that happens at each stage.

Definition

Turbidity: the cloudiness of water caused by tiny particles of soil, clay or other matter suspended in it. Example: the Nyabarongo after heavy rain has a high turbidity; distilled water has almost none.

Case study: the Nzove treatment works

The Water and Sanitation Corporation (WASAC) runs the Nzove treatment plants, which take water from the Nyabarongo River north-west of central Kigali. What follows is only what has been published; the figures are design capacities at the dates shown, and the plants often produce less, for example when the river is very muddy.

Table 4.1: Nzove in published figures

Fact	Source and date
The upper Nyabarongo catchment is "Kigali's main source of water".	InfoNile, August 2020
The river water's main problems are a high content of iron and manganese, high turbidity and suspended solids.	Culligan, the plant's equipment supplier (page archived 2024)
The intake on the river has a sand trap so that large sediment does not enter the plant.	Tetraturk, designer of Nzove II
Nzove I: $40{,}000\,\mathrm{m^3}$ per day, expandable to $65{,}000\,\mathrm{m^3}$ . Nzove II: upgraded from $25,000$ to $40{,}000\,\mathrm{m^3}$ per day.	Ministry of Infrastructure, 6 May 2019
Planned total for the Nzove catchment after all upgrades: $145{,}000\,\mathrm{m^3}$ per day.	Ministry of Infrastructure, 6 May 2019
Heavy flooding and erosion made the river so muddy that production at Nzove and Kimisagara fell by nearly half.	WASAC, quoted by InfoNile, August 2020

Figure 4.1: The main stages at a river-water treatment works such as Nzove. WASAC has not published the exact chemicals it uses, so the coagulant and disinfectant shown are the ones most works use.

The chemistry of the stages

Coagulation. The finest clay particles are too small to settle or to be caught by sand. A coagulant such as aluminium sulfate (alum) is added. It reacts with hydrogencarbonates dissolved in the water to form a sticky, jelly-like precipitate of aluminium hydroxide, which traps the particles into large clumps called flocs:

$\mathrm{Al_2(SO_4)_3(aq) + 3Ca(HCO_3)_2(aq) \rightarrow 2Al(OH)_3(s) + 3CaSO_4(aq) + 6CO_2(g)}$

Removing iron and manganese. Dissolved iron and manganese give water a metallic taste and stain clothes and sinks brown or black. Air or chlorine converts them into insoluble compounds, which the filters then catch:

dissolved iron + oxygen → insoluble iron(III) hydroxide (brown solid)

Disinfection. Chlorine reacts with water to form hypochlorous acid, $\mathrm{HOCl}$ , which kills bacteria:

$\mathrm{Cl_2(g) + H_2O(l) \rightarrow HCl(aq) + HOCl(aq)}$

A little more chlorine is added than is needed at once, so that some is left in the water to keep killing any germs that get into the pipes on the way to your tap.

Definition

Residual chlorine: the small amount of chlorine deliberately left in treated water after disinfection, so that the water stays protected in reservoirs and pipes. It is what gives tap water its faint "swimming-pool" smell.

Worked example

Problem. In December 2019 Kigali needed at least $120{,}000\,\mathrm{m^3}$ of water a day, but could produce about $90{,}000\,\mathrm{m^3}$ a day (figures quoted by InfoNile, 2020). (a) What was the daily shortfall, in litres? (b) What percentage of the need was met? ( $1\,\mathrm{m^3} = 1000\,\mathrm{L}$ .)

Solution.

(a) Shortfall $= 120{,}000 - 90{,}000 = 30{,}000\,\mathrm{m^3}$ , which is $30{,}000 \times 1000 = 30{,}000{,}000\,\mathrm{L}$ : thirty million litres a day.
(b) $\frac{90{,}000}{120{,}000} \times 100 = 75\%$

Answer: (a) 30 million litres a day; (b) 75%. This is why the Nzove plants were upgraded.

Worked example

Problem. Nzove II was upgraded from $25,000$ to $40{,}000\,\mathrm{m^3}$ per day. By what percentage did its capacity increase?

Solution.

Increase $= 40{,}000 - 25{,}000 = 15{,}000\,\mathrm{m^3}$ per day.
$\text{percentage increase} = \frac{15{,}000}{25{,}000} \times 100 = 60\%$

Answer: 60%. Divide by the original value, not the new one.

Making water safe at home

Families who collect water from a spring, a well, a river or a roof tank must treat it themselves. Whatever the method, remove the particles first: let muddy water stand so that the soil settles, then pour it off or strain it through a clean folded cloth. Particles shield germs from heat, chlorine and sunlight.

Boiling. Bring the water to a rolling boil for at least one minute (three minutes above about 2,000 m, according to the US Centers for Disease Control). Boiling kills germs but does not remove mud or dissolved chemicals, and it uses fuel.
Solar disinfection (SODIS). Fill clear PET plastic bottles of 2 litres or less with clear water and lay them in full sun (Figure 4.2). Ultraviolet light and heat kill the germs.
Filters. Ceramic pot filters and sand filters remove particles and many germs. A home-made bottle filter like the one you built in Unit 4 clears water but does not make it safe.
Chlorine. A measured dose of the chlorine solution sold in pharmacies, followed by 30 minutes' wait, kills germs cheaply (see the dose calculation in Unit 4, Lesson 5).

Figure 4.2: Solar water disinfection (SODIS).

Clear plastic bottles of water laid on a metal sheet in the sun, the SODIS method, in Indonesia. Photo: SODIS Eawag, CC BY 3.0, via Wikimedia Commons

Table 4.2: Home methods compared

Method	Kills germs?	Removes mud?	What it needs	Main drawback
Settling and cloth straining	no	yes, the larger particles	time and a clean cloth	only a first step
Boiling	yes	no	fuel or electricity	costs fuel; charcoal smoke indoors
SODIS	yes	no	clear bottles and sunshine	slow; fails if the water is cloudy or the day is rainy
Ceramic or sand filter	most of them	yes	a filter, kept clean	must be cleaned and eventually replaced
Chlorine solution	yes	no	the solution and a measure	taste; wrong dose is useless or harmful

Worked example

Problem. A family near a river has no fuel to spare and no chlorine, but it is the dry season and very sunny. Their water is brown with mud. Plan how they can make safe drinking water, and justify each step.

Solution.

Let the water stand in a bucket for several hours so that the soil settles, then pour off the clearer water through a folded clean cloth. This lowers the turbidity, because SODIS does not work in cloudy water.
Fill clear PET bottles of 2 litres or less, close them and lay them flat on a sunny metal roof for at least 6 hours. The ultraviolet light and heat kill the germs.
Drink straight from the bottles, or pour into clean covered containers, so the water is not contaminated again.

Answer: settle and strain, then SODIS. Boiling would also work but uses fuel they do not have.

Warning

Never drink untreated water, however clear it looks. Water from a clean-looking spring, a well or a roof tank can carry the germs of cholera, typhoid and diarrhoea. Treat it, store it in a clean covered container, and pour it out rather than dipping cups into it.

Tip

"Clear first, then kill." Every treatment, at Nzove or at home, removes particles before it disinfects.

FuseSchool: How does water treatment work? (4 min). Compare the stages in the video with Figure 4.1.

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Practice

What is turbidity, and why is it high in the Nyabarongo in the rainy season?
Put these stages in order: filtration, coagulation, disinfection, screening, sedimentation.
What is the job of the coagulant? Name the sticky solid that alum forms.
Write the equation for the reaction of chlorine with water, and name the product that kills bacteria.
Why is a little chlorine left in the water that leaves the works?
Nzove I can be expanded from $40,000$ to $65{,}000\,\mathrm{m^3}$ per day. Calculate the percentage increase.
Give one advantage and one disadvantage of SODIS compared with boiling.
A neighbour says: "Our spring water is crystal clear, so we don't need to treat it." Evaluate this claim.

Show answersHide answers

The cloudiness caused by suspended particles; heavy rain washes soil off the hills into the river.
Screening, coagulation, sedimentation, filtration, disinfection.
To make the fine particles clump together into flocs that can settle or be filtered; aluminium hydroxide, $\mathrm{Al(OH)_3}$ .
$\mathrm{Cl_2(g) + H_2O(l) \rightarrow HCl(aq) + HOCl(aq)}$ ; hypochlorous acid, HOCl.
So that it goes on killing any germs that enter the water in the reservoirs and pipes (residual chlorine).
$\frac{25{,}000}{40{,}000} \times 100 = 62.5\%$ .
Advantage: it needs no fuel and costs almost nothing. Disadvantage: it is slow (6 hours to 2 days), needs sunshine, and does not work with cloudy water.
The claim is unsafe: clearness shows that there are few particles, not that there are no germs. Spring water can carry the germs of diarrhoea, typhoid and cholera, so it should be boiled, chlorinated or treated by SODIS before drinking.

Sources and credits

Water purification (coagulation with aluminium sulfate, flocculation, sedimentation, filtration and disinfection with chlorine), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Solar water disinfection (the SODIS method: clear PET bottles of 2 litres or less, low turbidity, and exposure to full sun), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Facts on Nzove from: "The Prime Minister inaugurates the new Nzove I and the upgrade of Nzove II Water Treatment Plants", Ministry of Infrastructure, Rwanda, 6 May 2019; "Amidst urban water shortages, Rwanda government commits to expanding Kigali's water supply", Clementine Uwimana, InfoNile, 24 August 2020; "Nzove 1 2 Municipal Water Treatment Plants", Culligan Africa (archived 7 September 2024); "Water supply design project of Nzove II", Tetraturk. Facts only; the text was written by Ganzaa.
Boiling and SODIS times from "Water Disinfection for Travelers", US Centers for Disease Control and Prevention (CDC Yellow Book). Facts only.
Image: Nyabarongo River Scene with Bugesera Marshes - Outside Kigali - Rwanda - 01 (7679548058).jpg by Adam Jones, via Wikimedia Commons, licensed under CC BY-SA 2.0.
Image: Water treatment plant in Ramadi.jpg by Jeremy M. Giacomino, U.S. Marine Corps, via Wikimedia Commons, in the public domain.
Image: Indonesia-sodis-compressed.jpg by SODIS Eawag, via Wikimedia Commons, licensed under CC BY 3.0.
Video: How Does Water Treatment Work | Environmental Chemistry | Chemistry, FuseSchool - Global Education, embedded from YouTube.
The diagrams, worked examples, Practice items and exercises were written and drawn by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1From a brown river to a clear tap

2Case study: the Nzove treatment works

3The chemistry of the stages

4Making water safe at home

From a brown river to a clear tap

Case study: the Nzove treatment works

The chemistry of the stages

Making water safe at home