E. Earth and Space Systems · Water Systems

From Lake to Tap and Back: How Municipalities Process and Manage Water

E2.7 A2.1 A3.1 A1.5About 55 minutes

Learning goals

I can describe the steps a municipality uses to treat drinking water.
I can describe how wastewater is treated before it returns to a lake or river, and how stormwater is different.
I can explain ways municipalities manage water use, such as meters, pricing, bylaws and leak detection.
I can read and write simple code that automates part of a water system.

Words to know

municipality
A city, town or township with its own local government.

raw water
Untreated water taken from a lake, river or well.

coagulation
Adding a chemical (a coagulant, such as alum) that makes tiny particles stick together into clumps called floc.

sedimentation
Letting heavy clumps settle to the bottom of a tank.

filtration
Passing water through layers such as sand and anthracite coal, or through membranes, to remove remaining particles.

disinfection
Killing or inactivating germs, using chlorine, ultraviolet (UV) light or ozone.

wastewater
Used water from sinks, toilets, showers, laundry and industry, carried by sanitary sewers.

stormwater
Rain and snowmelt that runs off roofs, roads and lawns into storm drains.

SCADA
Supervisory Control and Data Acquisition: computer systems with sensors and code that monitor and control water plants and pipes automatically.

Remember from Grade 7

In Grade 7 you learned how mixtures are separated (screening, settling, filtering) and built a model water filter. A water treatment plant uses the same ideas, at a huge scale, plus disinfection to kill germs.

Where does your tap water come from?

Turn on the tap and clean, safe water comes out, day and night. Behind it is a whole system: intake pipes in a lake, a treatment plant, pumps, reservoirs, water towers and hundreds of kilometres of pipes, run by certified operators and watched by sensors and computers. Then there's a second system that takes the used water away and cleans it again.

Sources: many Ontario cities, including Toronto, Hamilton and Kingston, take water from Lake Ontario; Ottawa uses the Ottawa River. Other communities, such as Guelph, rely mostly on groundwater from wells.

Aerial view of a water treatment plant beside a large lake, with rows of rectangular concrete tanks and buildings. — Treatment plants use settling and filtering tanks at a huge scale, the same ideas as the model water filter you built in Grade 7. AI-generated image

Step	What happens	What it removes
1. Intake and screening	Raw water enters through pipes far out in the lake; screens block fish, weeds and debris	Large objects
2. Coagulation and flocculation	A coagulant (such as alum) is mixed in; tiny particles stick together into larger clumps (floc)	Prepares fine particles for removal
3. Sedimentation	Water sits in big tanks; heavy floc settles to the bottom	Most particles
4. Filtration	Water passes through sand and anthracite (or membranes)	Remaining particles and many microorganisms
5. Disinfection	Chlorine, UV light or ozone kills or inactivates germs; a small amount of chlorine is left in the water to protect it in the pipes	Bacteria, viruses, parasites
6. Adjust and test	pH may be adjusted to protect pipes; some cities add fluoride; water is tested constantly	Checks safety
7. Storage and distribution	Water is pumped to reservoirs and water towers; gravity from towers keeps pressure in the pipes	Delivers water to homes

Typical drinking water treatment steps for surface water. Groundwater often needs fewer steps, but is always disinfected in municipal systems.

Typical drinking water treatment for lake water, in order. Each step removes what the step before missed, and disinfection is the safety barrier against germs.

After the drain: wastewater treatment

Used water flows through sanitary sewers to a wastewater treatment plant:

Preliminary: screens remove rags, wipes and plastics; grit tanks settle sand and gravel.
Primary: in settling tanks, solids sink (sludge) and grease floats.
Secondary: air is bubbled in so bacteria can eat the dissolved organic matter (this is biology doing the cleaning!).
Extra steps: many plants remove phosphorus and disinfect the water (often with UV) before releasing it.
The cleaned water (effluent) returns to a river or lake. Treated solids (biosolids) may be used as fertilizer, landfilled or burned.

Stormwater is different. In most neighbourhoods, storm drains in the street carry rain straight to a creek or lake without treatment. That's why only rain should go down the storm drain. Some newer areas send stormwater to stormwater ponds first, where sediment settles. In parts of some older cities, sewage and stormwater share the same pipes (combined sewers), which can overflow into lakes during heavy rain.

Two separate pipe systems (not to scale). Water from sinks and toilets goes through the sanitary sewer to a treatment plant; rain in a street storm drain usually goes straight to a creek or lake without treatment.

Managing how much water people use

Municipalities also manage demand, because building bigger plants is expensive and taking too much water harms lakes, rivers and aquifers.

Water meters and pricing: homes pay for each cubic metre (1 m³ = 1 000 L) they use, so saving water saves money.
Outdoor watering bylaws: for example, lawn watering only on certain days or hours in summer, when demand peaks.
Rebates for low-flow toilets, efficient washing machines or rain barrels.
Leak detection: finding and fixing leaky water mains (a lot of treated water can be lost through leaks), sometimes using acoustic sensors that "listen" for leaks.
Smart meters that show hourly use and warn of leaks in homes.
Education campaigns.
Permits: in Ontario, large water takings (most over 50 000 L per day) need a provincial Permit to Take Water.

Ontario passed the Safe Drinking Water Act (2002) after the Walkerton tragedy (more in Lesson 11). It requires certified operators, regular testing, and reporting of any bad test results.

Just for fun

In 2017, sewer workers in London, England, found a fatberg (a giant lump of wipes and grease) about 250 m long: as long as about four hockey rinks end to end. Moral of the story: only toilet paper gets flushed!

Coding a large system (A2.1)

A city water system has hundreds of pumps, valves and sensors. Operators use SCADA systems: sensors send readings to computers, and code makes many routine decisions automatically, alerting a person when something is wrong. Here is a simplified example for a water tower and its pump:

SET LOW_LEVEL to 40 (percent full)

SET HIGH_LEVEL to 90

SET MIN_CHLORINE to the minimum the operator sets (mg/L)

REPEAT every minute

level ← read tower level sensor

IF level < LOW_LEVEL THEN turn pump ON

IF level > HIGH_LEVEL THEN turn pump OFF

chlorine ← read chlorine sensor

IF chlorine < MIN_CHLORINE THEN

increase chlorine dose a little

send ALERT to operator on duty

record level, pump state and chlorine in the log

END REPEAT

Pseudocode for automating a water tower pump and checking chlorine. Real systems also have backups, alarms and human checks.

Worked example

Let's trace the code

At 6:00 a.m., the tower is 38% full and the pump is OFF. People wake up and use lots of water, but the pump fills faster than they use it: the level rises 2 percentage points per minute. What does the code do at 6:00, and at about what time does the pump turn OFF?

At 6:00, level = 38%. Is 38 < 40? Yes, so the code turns the pump ON.
The level now rises 2 points per minute: 40, 42, 44, …
The pump turns off when level > 90. From 38 to above 90 is more than 52 points. 52 ÷ 2 = 26 minutes gets to exactly 90, which is not greater than 90.
One more minute: 27 minutes → 92% > 90. The pump turns OFF.
6:00 + 27 minutes = 6:27 a.m.

The pump turns ON at 6:00 and OFF at about 6:27 a.m., when the level reaches 92%.

Hands-on activity

Water detective: your home connection (A1.5)

You need: a notebook, food colouring, a clock, and (if your home has one) access to the water meter with a grown-up.

Trace your water: find out where your water comes from (lake, river or well) and which treatment plant serves you. Your municipality's website or water bill usually says. If you have a private well, find out how it is tested.
Toilet leak test: with a grown-up's help, lift the heavy toilet tank lid, set it on a towel, and add 5 to 10 drops of food colouring to the tank (not the bowl). Don't flush for 15 minutes. If colour appears in the bowl, the toilet is leaking. Flush afterwards.
Meter test (if you have a meter): with a grown-up, read the meter, then make sure no one uses any water for 1 hour. Read it again. If it moved, there's a hidden leak.
Storm drain check: find the nearest storm drain. Where does it lead? (Many municipalities post maps; some drains are painted with a fish symbol and "Drains to river/lake".)
Communicate: make a labelled flow diagram: source → treatment plant → your tap → sanitary sewer → wastewater plant → back to the water body. Add the storm drain as a separate path.

What to look for: a correct, labelled diagram that shows two separate paths for wastewater and stormwater.

Safety: a grown-up handles the water meter (it may be in a basement or meter pit). Don't put anything except food colouring in the toilet tank.

Careers connection (A3.1)

Water and wastewater treatment operators (certified in Ontario), plumbers and steamfitters/pipefitters (skilled trades), industrial electricians, instrumentation and control technicians (who maintain SCADA sensors), civil engineers and public health inspectors all keep municipal water safe.

Your turn

List the main steps of drinking water treatment in order.

What does a coagulant such as alum do?

Name two ways drinking water is disinfected.

Why is a small amount of chlorine left in the water after it leaves the plant?

What happens in secondary wastewater treatment?

What is the difference between a sanitary sewer and a storm drain?

Name three ways a municipality can reduce water use.

More practice: warm-up

How many litres are in 1 cubic metre?

What is floc?

What keeps water pressure in the pipes in many towns?

What does SCADA do?

What should go down a storm drain?

More practice: on your own

A family uses 18 m³ of water in a month. If the price were $2.00 per m³ (an example rate), what would the water charge be? How many litres is that?

Why should wipes never be flushed, even "flushable" ones?

Explain why combined sewers can harm water quality during heavy rain.

Why do some cities limit lawn watering in summer?

In the pump code, what would happen if the HIGH_LEVEL check were missing?

Challenge

Modify the pseudocode so that, if the level is ever below 15%, it turns on a second pump and sends an emergency alert. Write the new lines.

Why do groundwater systems in Ontario still have to be disinfected, even though water is filtered as it soaks through the ground?

Automation can make water systems more efficient, but what is one risk, and how can it be reduced?

Check your understanding

Quiz

Which step makes tiny particles clump together?

What is the main job of disinfection?

In most neighbourhoods, water in a street storm drain goes...

In secondary wastewater treatment, what does most of the cleaning?

Which is a way municipalities manage water USE?

In the pump code, the tower is at 95% and the pump is ON. What happens?

1 m³ of water is...

For parents and teachers

Big idea (E2.7): municipalities take raw water from lakes, rivers or wells; treat it by screening, coagulation, sedimentation, filtration and disinfection; test and distribute it; collect and treat wastewater; keep stormwater separate; and manage demand with meters, pricing, bylaws, rebates and leak detection. The pseudocode connects to A2.1 (automating large systems); careers to A3.1; the flow diagram to A1.5.

How to help: look at your water bill or municipal water report together (most municipalities publish an annual drinking water quality report). Do the toilet leak test. Many treatment plants offer public tours or virtual tours.

Common difficulties:

Thinking storm drains lead to treatment. Usually they don't.
Mixing up sedimentation and filtration. Settling vs. passing through layers.
Code logic: "greater than 90" is not true at exactly 90.

Safety: adults handle water meters; nothing but food colouring in toilet tanks.

Sources: Ontario Safe Drinking Water Act, 2002; Ontario Permit to Take Water rules (over 50 000 L/day); municipal water websites (Toronto, Ottawa, Guelph). The $2.00 per m³ rate is an example only; real rates vary by municipality.