Treatment of Water For Town Use

Why We Treat Water for Town Use

Water from natural sources contains many impurities that make it unsafe to drink. River water in Nigeria carries soil particles from erosion, dissolved minerals, bacteria from human and animal waste, and sometimes industrial chemicals.

Lagos State gets municipal water from Oyan River and other sources. Abuja draws water from Usuma, Lower Usuma, and Gurara dams. This raw water contains microorganisms like Vibrio cholerae (causes cholera), E. coli bacteria, parasites like guinea worm, and physical impurities like sand and leaves.

Drinking untreated water causes diseases. Cholera outbreaks in some Nigerian communities happen when people drink contaminated water. Typhoid fever, dysentery, and diarrhea also spread through dirty water. Children under five are most vulnerable to these waterborne diseases.

The Lagos State Water Corporation and Federal Capital Territory Water Board treat billions of liters daily. Treatment removes 99.9% of harmful organisms and makes water clear, colorless, and safe to drink.

Stages in Water Treatment

1. Sedimentation (Settling)

Raw water from rivers flows into large sedimentation tanks. The water stays still for several hours. Heavy particles like sand, gravel, and silt sink to the bottom by gravity. This layer of settled material is called sediment.

The Ijora Water Treatment Plant in Lagos has huge rectangular sedimentation basins. Water flows slowly through them, giving particles time to settle. Workers regularly remove accumulated sediment from the bottom.

After sedimentation, about 60-70% of suspended solids are removed. However, tiny particles remain floating. These include clay particles, bacteria, and algae that are too light to sink quickly.

2. Coagulation and Flocculation

Chemical coagulants like aluminum sulfate (alum) are added to the partially clear water. The chemical formula of alum is Al₂(SO₄)₃·18H₂O. When mixed with water, alum produces sticky aluminum hydroxide particles.

These sticky particles attach to tiny suspended materials, bacteria, and other impurities. The process is called coagulation. As the sticky particles collect more impurities, they form larger clumps called “flocs.”

The water is gently stirred in flocculation tanks. This slow mixing helps flocs grow bigger without breaking apart. After 30-45 minutes, the flocs become heavy enough to sink. This removes particles that didn’t settle during the first sedimentation.

Typical alum dosage is 10-30 mg per liter of water, depending on water turbidity (cloudiness). Too much alum makes water acidic and leaves a metallic taste. Treatment plants carefully control the amount.

3. Filtration

Water passes through filter beds containing layers of different materials. A typical filter bed has these layers from top to bottom:

• Fine sand layer (60 cm thick): Removes tiny particles and remaining flocs
• Coarse sand layer (30 cm): Catches larger particles
• Gravel layer (40 cm): Provides support and allows water flow

As water slowly trickles down through these layers, remaining suspended particles, bacteria, and floc remnants get trapped. The sand grains act like a sieve with very tiny holes.

Some plants use rapid sand filters where water flows at 5-15 meters per hour. Others use slow sand filters operating at 0.1-0.3 meters per hour. Slow filtration is more effective at removing bacteria because a biological layer forms on the sand surface.

After filtration, water is 99% clear. Turbidity drops from 10-20 NTU (Nephelometric Turbidity Units) in raw water to less than 1 NTU. However, microscopic organisms may still survive.

4. Disinfection (Sterilization)

Chlorine gas (Cl₂) or sodium hypochlorite (bleaching solution) is added to kill remaining bacteria, viruses, and other microorganisms. This is the most important step for preventing waterborne diseases.

Chlorine reacts with water to form hypochlorous acid (HOCl), which destroys microorganisms by damaging their cell walls. The chemical reaction is:

Cl₂ + H₂O → HOCl + HCl

The dosage is typically 0.5-2.0 mg of chlorine per liter of water. This leaves a small amount of “residual chlorine” (0.2-0.5 mg/L) in the water. The residual protects water from recontamination as it travels through pipes to homes.

You can smell this chlorine in tap water. The slight smell confirms proper disinfection. Water without any chlorine smell might have picked up bacteria in the pipes.

Some plants use alternative disinfectants:

• Ozone (O₃): Very effective but leaves no residual protection
• Ultraviolet (UV) light: Kills microorganisms without chemicals but also has no residual effect
• Chlorine dioxide (ClO₂): More effective than chlorine but more expensive

5. pH Adjustment and Additional Treatment

Alum makes water slightly acidic. Lime (calcium hydroxide) is added to raise pH to 7.0-8.5, making water neutral to slightly alkaline. This prevents pipe corrosion and improves taste.

Some treatment plants add fluoride to prevent tooth decay. The recommended level is 0.5-1.0 mg/L. However, not all Nigerian plants add fluoride due to cost.

After all treatment, water is stored in clean reservoirs before distribution. The storage tanks are covered to prevent recontamination from birds, insects, or dust.

Water Treatment Process Flow

Quality Standards for Treated Water

NAFDAC and the Standards Organization of Nigeria (SON) set water quality standards. Treated municipal water must meet these requirements:

• Physical: Colorless, odorless, tasteless; turbidity less than 5 NTU
• Chemical: pH 6.5-8.5; residual chlorine 0.2-0.5 mg/L; no toxic metals
• Biological: Zero coliform bacteria per 100 mL sample
• Fluoride: 0.5-1.5 mg/L (if added)
• Total dissolved solids (TDS): Less than 500 mg/L for good quality

Water boards test samples daily at different distribution points. If coliform bacteria appear in tests, that section of the network is flushed and re-chlorinated.

Challenges in Nigerian Water Treatment

Many Nigerian cities struggle with water treatment. Common problems include:

Insufficient treatment capacity: Lagos needs over 700 million liters daily but produces only about 210 million liters. Many areas rely on boreholes and water vendors.

Old infrastructure: Pipes in some areas are over 40 years old. They leak treated water and allow dirt to enter, recontaminating clean water.

Power supply issues: Treatment plants need constant electricity for pumps and equipment. Generator costs increase water prices.

High pollution in source water: Industrial waste, sewage, and agricultural runoff make raw water heavily polluted. This requires more chemicals and treatment time.

Funding constraints: Treatment chemicals, equipment maintenance, and skilled staff require significant investment.

Common Exam Mistakes

WAEC examiners report these frequent errors:

• Wrong sequence of treatment stages: Always follow the correct order: sedimentation → coagulation → filtration → disinfection. Don’t write filtration before sedimentation
• Confusing alum’s role: Alum is used in coagulation, not disinfection. Chlorine kills germs, alum makes particles stick together
• Writing “disinfection” as “desinfection”: The correct spelling has an “i” not “e”
• Incomplete explanations: When asked to “explain” a stage, don’t just name it. Describe what happens and why
• Not mentioning specific chemicals: If the question asks about chemicals used, mention alum for coagulation, chlorine for disinfection, and lime for pH adjustment
• Confusing sedimentation with filtration: Sedimentation uses gravity in tanks; filtration uses sand/gravel beds

Practice Questions

Multiple Choice Questions

1. What is the main purpose of adding alum during water treatment?
   • a) To kill bacteria
   • b) To make particles stick together ✓
   • c) To improve water taste
   • d) To add minerals
2. Which chemical is commonly used for disinfection in water treatment?
   • a) Alum
   • b) Lime
   • c) Chlorine ✓
   • d) Fluoride
3. In which stage of water treatment do heavy particles like sand settle by gravity?
   • a) Filtration
   • b) Sedimentation ✓
   • c) Coagulation
   • d) Disinfection
4. What is the typical pH range for treated drinking water?
   • a) 4.0-5.5
   • b) 6.5-8.5 ✓
   • c) 9.0-11.0
   • d) 12.0-14.0

Essay Questions

1. Describe the four main stages involved in treating water for town use. (10 marks)
   
   Examiner tip: Use separate paragraphs for each stage. Explain what happens, mention any chemicals used, and state what impurities are removed. “Describe” needs more detail than “state.”
2. Explain why chlorine is added during water treatment and state one disadvantage of using chlorine. (5 marks)
   
   Examiner tip: First explain chlorine’s role in killing microorganisms (3 marks), then mention a disadvantage like unpleasant taste/smell or formation of harmful byproducts (2 marks).
3. Distinguish between sedimentation and filtration in water treatment. (6 marks)
   
   Examiner tip: “Distinguish” means show clear differences. Define each process, explain how they work differently, and mention what type of impurities each removes.
4. State three reasons why water from rivers must be treated before use in towns. (3 marks)
   
   Examiner tip: One sentence per reason is enough. Focus on health dangers like disease-causing organisms, physical impurities, and chemical pollutants.

Memory Aids

Treatment stages (SCFD):

• Sedimentation
• Coagulation
• Filtration
• Disinfection

Chemicals used (ACL):

• Alum for coagulation
• Chlorine for disinfection
• Lime for pH adjustment

What each stage removes:

• Sedimentation = Sand and Silt
• Coagulation = Clay and Colloids
• Filtration = Fine particles
• Disinfection = Disease germs

Related Topics

• Water Definition and Properties
• Hard and Soft Water Treatment
• Water Pollution Sources and Control
• Purification of Water in the Laboratory
• Waterborne Diseases and Prevention