Sanitation and Public Health Engineering Basics

What to remember

• Public health engineering supplies safe water and removes waste. It covers water sources, treatment, distribution, sewerage, sewage treatment and solid waste management.
• Water is made safe in steps. Screening, sedimentation with coagulation, filtration and then disinfection (chlorination) is the standard sequence for surface water.
• BOD measures how polluted sewage is. BOD is the oxygen used by micro-organisms to break down organic matter in 5 days at 20 °C. Higher BOD means stronger sewage.

Water demand, sources and quality

Sources of water: surface sources (rivers, lakes, reservoirs) and underground sources (wells, tube wells, springs, infiltration galleries). Rainwater harvesting adds to local supply. Groundwater is usually clearer but may be hard or contain dissolved minerals such as fluoride or iron. Surface water is usually soft but carries suspended matter and germs.

Water demand is the quantity needed per person per day (litres per capita per day, lpcd). Types of demand are domestic, industrial, public use (parks, street cleaning), fire demand and losses and wastage. A figure of about 135 lpcd is commonly adopted for towns with piped sewerage, and a smaller value where there is no sewerage. Check the latest national norm before using a value.

Rates of use: the maximum daily demand is about 1.8 times the average daily demand. The maximum hourly demand is about 1.5 times the maximum daily demand, which is about 2.7 times the average. Pipes and pumps are designed for the higher figure.

Fire demand by Kuichling's formula is Q = 3182 √P litres per minute, where P is the population in thousands.

Population forecasting methods (design period is usually 20 to 30 years):

• Arithmetic increase: Pn = P + n × x̄, where x̄ is the average increase per decade.
• Geometric increase: Pn = P (1 + r ÷ 100)ⁿ, with a constant growth percentage.
• Incremental increase: the arithmetic mean plus the increase of the incremental increase.
• Other methods are graphical and the comparison method.

Worked example: A town had 50,000 people in one census and 60,000 ten years later. The average increase is 10,000 per decade, so after another decade the arithmetic forecast is 70,000. At 135 lpcd the average demand is 70,000 × 135 = 9,450,000 litres per day, or 9.45 million litres per day.

Water quality is judged by physical, chemical and biological tests. Physical: turbidity, colour, taste, odour and temperature. Chemical: pH, hardness, chlorides, fluoride, nitrate, iron and total dissolved solids (TDS). Biological: bacteriological tests, with the coliform group (indicator of faecal contamination) tested by the MPN or membrane filter method. The Indian drinking water standard is IS 10500, and the acceptable pH range is 6.5 to 8.5. Hardness is caused by calcium and magnesium salts. Temporary hardness is removed by boiling. Permanent hardness needs the lime-soda or zeolite process. Excess fluoride causes fluorosis and excess nitrate harms infants (methaemoglobinaemia).

Water treatment and distribution

Units in the usual order:

Coagulant: alum (aluminium sulphate) is the most common. It needs alkalinity in the water to work.

Sedimentation tank design: surface overflow rate = Q ÷ A (m³/m²/day). It is independent of tank depth. A tank is rectangular, circular or hopper shaped. Detention time = Volume ÷ Q.

Worked example: Q = 1,000 m³/day and tank area A = 50 m². Overflow rate = 1,000 ÷ 50 = 20 m³/m²/day.

Filters:

• Slow sand filter: fine sand, works at a low rate (about 100 to 200 litres per hour per square metre). The top layer of biological film (schmutzdecke) removes germs. It is cleaned by scraping the top layer. It needs a large area.
• Rapid sand filter: coarser sand, works at a high rate (about 3,000 to 6,000 litres per hour per square metre). It needs coagulation before it. It is cleaned by backwashing.
• Pressure filter is a closed rapid filter, used in small plants.

Disinfection: chlorination is the most common. Forms: bleaching powder, liquid chlorine and sodium hypochlorite. Chlorine demand = dose − residual. A residual of about 0.2 mg/L at the user end is kept as protection. Break-point chlorination is adding chlorine until the free residual starts to increase. Pre-chlorination is applied before treatment and post-chlorination after filtration. Other methods are ozone, ultraviolet light and boiling.

Worked example: If the chlorine demand is 0.5 mg/L and a residual of 0.2 mg/L is required, the dose = 0.5 + 0.2 = 0.7 mg/L.

Distribution systems: by gravity, by pumping, or both (dual). Layouts of pipes: dead-end (tree system, simple but water may stagnate), grid iron (interconnected, good circulation), ring (around the area) and radial (from a central reservoir). A service reservoir stores treated water and balances the difference between supply and demand. Valves include sluice valves, air valves, reflux (non-return) valves, and fire hydrants are provided at intervals. Pipe materials: cast iron, ductile iron, steel, PVC, HDPE and asbestos-free cement pipes.

Sewerage and sewage treatment

Sewage is waste water from homes, industry and other sources. Sewerage is the system of collecting and carrying it. Dry weather flow (DWF) is the flow with no storm water.

Systems:

• Separate system: one set of sewers for sewage and another for storm water.
• Combined system: one sewer for both. It is cheaper to lay but costly to treat in rains.
• Partially separate system: part of the storm water enters the sanitary sewers.

Sewer shapes are circular (common), egg-shaped and rectangular. A sewer must run at a self-cleansing velocity, which is about 0.6 to 0.9 m/s at least, to avoid the settling of solids. A maximum velocity is also kept to avoid scouring. Manholes give access for inspection and cleaning. They are fixed at bends, junctions and changes of gradient, and at intervals on straight stretches. A drop manhole takes a branch sewer at a higher level down to the main sewer.

Sewage treatment stages:

• Preliminary: screens and grit chambers remove coarse material and grit.
• Primary: sedimentation tank removes settleable solids, giving sludge.
• Secondary (biological): the trickling filter and the activated sludge process use bacteria to break down dissolved organic matter. Oxidation ponds and aerated lagoons are also used.
• Tertiary: removal of nutrients, and polishing and disinfection.

Sludge from treatment is thickened, digested in anaerobic digesters (giving biogas, which is mostly methane) and dried on sand drying beds.

Terms of pollution: Dissolved oxygen (DO) is the oxygen dissolved in water. Healthy water has DO of about 4 to 6 mg/L or more. BOD is the oxygen used by micro-organisms in decomposing organic matter. BOD5 is measured at 20 °C over 5 days. COD is the oxygen needed to oxidise matter by a strong chemical oxidant, and it is always higher than or equal to BOD.

On-site systems: a septic tank receives toilet waste, lets solids settle and digest in anaerobic conditions and passes the effluent to a soak pit or a dispersion trench. An Imhoff tank combines sedimentation and digestion. A soak pit lets the effluent seep into the ground.

House drainage, plumbing and solid waste

Traps hold a water seal that stops sewer gas from entering a building. Common traps are the P-trap, Q-trap and S-trap (named for the shape) and the gully trap. Systems of plumbing: the single-stack system has one pipe for soil and waste. In the two-pipe system soil pipes and waste pipes are separate. The one-pipe system carries all through a soil pipe with ventilation. Vent pipes release gases and prevent the water seal from breaking. House sewers are connected to the street sewer through an inspection chamber.

Solid waste management: the steps are generation, storage, collection, transport, processing and disposal. Segregation at source into wet (biodegradable), dry (recyclable) and hazardous waste is the first step.

Leachate is the liquid draining out of a landfill, and it needs a liner and collection. Waste-to-energy and recycling reduce disposal.

Programmes: Swachh Bharat Mission (sanitation and solid waste, for urban and rural areas), AMRUT (urban water supply and sewerage) and Jal Jeevan Mission (tap water in rural homes). Andhra Pradesh runs a Clean Andhra Pradesh programme for door-to-door waste collection and cleanliness. Check the latest official release for current details. Ward-level sanitation staff in village and ward secretariats look after cleanliness, drainage, street lights, water supply and waste collection in their wards.

Exam traps

• BOD uses 5 days at 20 °C. Do not mix with COD.
• Surface overflow rate depends on the plan area, not the depth.
• Slow sand filters need no coagulant. Rapid sand filters need one.
• Slow sand filters are cleaned by scraping. Rapid sand filters are cleaned by backwashing.
• Chlorine demand = dose − residual.
• Dead-end layout may have stagnation. Grid iron gives circulation.
• Separate system: two sets of sewers. Combined system: one set of sewers.
• A trap prevents sewer gas from entering. A vent pipe allows gas to escape.

One-liners

• 1. Alum is the most common coagulant.
• 2. The acceptable pH range for drinking water is 6.5 to 8.5.
• 3. Coliform bacteria indicate faecal contamination.
• 4. Chlorination is the most common disinfection method.
• 5. Rapid sand filters are cleaned by backwashing.
• 6. The surface overflow rate = Q ÷ A.
• 7. BOD is measured at 20 °C over 5 days.
• 8. Manholes give access to sewers.
• 9. A septic tank provides on-site sewage treatment.
• 10. Anaerobic digestion gives biogas.
• 11. Kuichling's formula gives fire demand.
• 12. Segregation at source separates wet and dry waste.

Practice questions

1. A town had 50,000 people in one census and 60,000 ten years later. The arithmetic increase method gives a population after another ten years of

   1. 80,000
   2. 70,000
   3. 72,000
   4. 65,000
   Answer

   B. 70,000

   Increase per decade = 10,000, so 60,000 + 10,000 = 70,000.

2. For a population of 70,000 at 135 lpcd, the average daily demand is

   1. 0.945 million litres per day
   2. 9.45 million litres per day
   3. 945,000 litres per day
   4. 94.5 million litres per day
   Answer

   B. 9.45 million litres per day

   70,000 × 135 = 9,450,000 litres per day.

3. A sedimentation tank with Q = 1,000 m³/day and plan area 50 m² has a surface overflow rate of

   1. 50 m³/m²/day
   2. 200 m³/m²/day
   3. 0.05 m³/m²/day
   4. 20 m³/m²/day
   Answer

   D. 20 m³/m²/day

   Overflow rate = Q ÷ A = 1,000 ÷ 50 = 20 m³/m²/day.

4. The chlorine demand is 0.5 mg/L and a residual of 0.2 mg/L is required. The chlorine dose is

   1. 0.3 mg/L
   2. 0.7 mg/L
   3. 0.1 mg/L
   4. 1.0 mg/L
   Answer

   B. 0.7 mg/L

   Dose = demand + residual = 0.5 + 0.2 = 0.7 mg/L.

5. By Kuichling's formula Q = 3182 √P (P in thousands), the fire demand for a population of 4,000 is

   1. 12,728 litres/min
   2. 1,591 litres/min
   3. 3,182 litres/min
   4. 6,364 litres/min
   Answer

   D. 6,364 litres/min

   √4 = 2, so Q = 3182 × 2 = 6,364 litres per minute.

6. If the average daily demand is 10 million litres and the maximum daily demand is 1.8 times the average, the maximum daily demand is

   1. 27 million litres
   2. 15 million litres
   3. 10 million litres
   4. 18 million litres
   Answer

   D. 18 million litres

   10 × 1.8 = 18 million litres per day.

7. If the maximum hourly demand is 2.7 times the average, an average demand of 5 million litres per day gives a peak rate equivalent to

   1. 27 million litres per day
   2. 13.5 million litres per day
   3. 9.0 million litres per day
   4. 7.5 million litres per day
   Answer

   B. 13.5 million litres per day

   5 × 2.7 = 13.5 million litres per day.

8. A town of 10,000 grows by 10% per decade (geometric increase). After two decades the population is

   1. 12,500
   2. 12,000
   3. 12,100
   4. 11,000
   Answer

   C. 12,100

   10,000 × 1.1 × 1.1 = 12,100.

9. A tank of volume 250 m³ receives 1,000 m³/day. The detention time is

   1. 6 hours
   2. 12 hours
   3. 4 hours
   4. 24 hours
   Answer

   A. 6 hours

   250 ÷ 1,000 = 0.25 day = 6 hours.

10. A rapid sand filter works at 5 m³/h per m² and must treat 300 m³/h. The filter area required is

    1. 15 m²
    2. 1,500 m²
    3. 600 m²
    4. 60 m²
    Answer

    D. 60 m²

    Area = 300 ÷ 5 = 60 m².

11. A slow sand filter works at 2.4 m³ per m² per day and must treat 240 m³/day. The area required is

    1. 10 m²
    2. 240 m²
    3. 100 m²
    4. 576 m²
    Answer

    C. 100 m²

    Area = 240 ÷ 2.4 = 100 m².

12. To disinfect 10 million litres per day with a chlorine dose of 2 mg/L, the chlorine needed per day is

    1. 200 kg
    2. 20 kg
    3. 0.2 kg
    4. 2 kg
    Answer

    B. 20 kg

    2 mg/L = 2 g/m³; 10,000 m³ × 2 g = 20,000 g = 20 kg.

13. A water sample has DO of 8 mg/L at the start and 3 mg/L after 5 days at 20 °C (no dilution). Its BOD5 is

    1. 8 mg/L
    2. 5 mg/L
    3. 11 mg/L
    4. 3 mg/L
    Answer

    B. 5 mg/L

    BOD = initial DO − final DO = 8 − 3 = 5 mg/L.

14. A town of 20,000 people at 135 lpcd needs an average of

    1. 2.7 million litres per day
    2. 0.27 million litres per day
    3. 2.7 litres per day
    4. 27 million litres per day
    Answer

    A. 2.7 million litres per day

    20,000 × 135 = 2,700,000 litres per day.

15. The most commonly used method of disinfecting municipal water is

    1. screening
    2. plain sedimentation
    3. chlorination
    4. aeration
    Answer

    C. chlorination

    Chlorine is cheap, effective and leaves a residual.

16. The most commonly used coagulant in water treatment is

    1. alum
    2. common salt
    3. lime only
    4. sodium chloride
    Answer

    A. alum

    Aluminium sulphate gathers fine particles into flocs.

17. The acceptable pH range for drinking water in the Indian standard is

    1. 4.0 to 6.0
    2. 1.0 to 14.0
    3. 9.0 to 11.0
    4. 6.5 to 8.5
    Answer

    D. 6.5 to 8.5

    IS 10500 acceptable pH range is 6.5 to 8.5.

18. Presence of coliform bacteria in water indicates

    1. high hardness
    2. excess fluoride
    3. faecal contamination
    4. high turbidity only
    Answer

    C. faecal contamination

    Coliforms are indicators of contamination by human or animal waste.

19. Temporary hardness of water can be removed by

    1. chlorination
    2. screening
    3. sedimentation
    4. boiling
    Answer

    D. boiling

    Boiling precipitates carbonates of calcium and magnesium.

20. A rapid sand filter is cleaned by

    1. scraping the top layer
    2. backwashing
    3. burning
    4. adding chlorine
    Answer

    B. backwashing

    A slow sand filter is cleaned by scraping.

21. The schmutzdecke in a slow sand filter is

    1. a pump
    2. a biological film that removes germs
    3. a pipe valve
    4. a type of coagulant
    Answer

    B. a biological film that removes germs

    The film on the top layer of sand traps and feeds on organisms.

22. A drawback of the dead-end system of distribution is

    1. high pumping cost only
    2. need for many valves
    3. stagnation of water in dead ends
    4. excess pressure everywhere
    Answer

    C. stagnation of water in dead ends

    The tree layout has no circulation at the ends of branches.

23. In the separate system of sewerage

    1. both are carried together
    2. only storm water is carried
    3. there are no manholes
    4. sewage and storm water are carried by different sewers
    Answer

    D. sewage and storm water are carried by different sewers

    Combined systems carry both in one sewer.

24. Sewers are designed to have a self-cleansing velocity so that

    1. solids do not settle in the sewer
    2. flow is always zero
    3. scouring is maximum
    4. the sewer does not need manholes
    Answer

    A. solids do not settle in the sewer

    Minimum velocity prevents deposition of solids.

25. Manholes are provided in sewers mainly for

    1. inspection and cleaning
    2. pumping
    3. treatment of sewage
    4. storage of sewage
    Answer

    A. inspection and cleaning

    They give access at bends, junctions and along straight lines.

26. The trickling filter belongs to which stage of sewage treatment?

    1. Disinfection
    2. Secondary (biological) treatment
    3. Preliminary treatment
    4. Sludge drying
    Answer

    B. Secondary (biological) treatment

    Bacteria on the filter media break down dissolved organic matter.

27. BOD5 is the oxygen consumed by micro-organisms in

    1. 5 hours at 20 °C
    2. 20 days at 5 °C
    3. 5 days at 20 °C
    4. 1 day at 5 °C
    Answer

    C. 5 days at 20 °C

    This is the standard definition of BOD.

28. Compared with BOD, the COD of a waste is

    1. unrelated
    2. always zero
    3. always lower
    4. equal or higher
    Answer

    D. equal or higher

    The strong chemical oxidant also oxidises matter that bacteria do not.

29. A septic tank

    1. is used for filtering drinking water
    2. is used to store rain water
    3. provides on-site anaerobic treatment of toilet waste
    4. is a type of sewer
    Answer

    C. provides on-site anaerobic treatment of toilet waste

    Solids settle and digest in the tank and the effluent goes to a soak pit.

30. The water seal in a trap prevents

    1. flow of water
    2. entry of sewer gas into the building
    3. entry of rain
    4. blocking of the pipes
    Answer

    B. entry of sewer gas into the building

    The standing water blocks foul gas.

31. The Jal Jeevan Mission aims to

    1. clear urban slums
    2. build roads
    3. supply electricity
    4. provide tap water connections to rural households
    Answer

    D. provide tap water connections to rural households

    It is the national programme for tap water in rural homes.

32. Segregation of solid waste at source means separating

    1. wet, dry and hazardous waste
    2. water and gas
    3. rain and sewage
    4. cement and sand
    Answer

    A. wet, dry and hazardous waste

    It makes composting and recycling easier.

33. Which of the following statements is/are correct? 1. Slow sand filters need no coagulant. 2. Rapid sand filters need coagulation before filtration.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Rapid filters work at high rates and need coagulated, settled water.

34. Which of the following statements is/are correct? 1. BOD5 is measured at 20 °C over five days. 2. COD is always lower than BOD.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    COD is equal to or higher than BOD.

35. Which of the following statements is/are correct? 1. Chlorine demand = dose + residual. 2. Break-point chlorination ends when the free residual starts to increase.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Demand = dose − residual.

36. Which of the following statements is/are correct? 1. A combined sewer carries both sewage and storm water. 2. In a separate system the same sewer carries both.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    A separate system uses two sets of sewers.

37. Which of the following statements is/are correct? 1. A septic tank gives anaerobic digestion of solids. 2. An Imhoff tank combines sedimentation and digestion.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are standard.

38. Which of the following statements is/are correct? 1. The surface overflow rate of a sedimentation tank depends on its depth. 2. Detention time = volume ÷ flow.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Overflow rate = Q ÷ plan area and is independent of depth.

39. Which of the following statements is/are correct? 1. A trap holds a water seal. 2. A vent pipe helps to keep the water seal from breaking.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are plumbing rules.

40. Which of the following statements is/are correct? 1. Leachate is the gas released from a landfill. 2. In a sanitary landfill the waste is spread, compacted and covered.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Leachate is the liquid draining from a landfill.

41. Which of the following statements is/are correct? 1. A grid iron layout has dead ends. 2. A ring layout is a single straight pipe.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    Grid iron is interconnected; a ring system encircles the area.

42. Which of the following statements is/are correct? 1. Flocculation is gentle mixing to grow flocs. 2. Screening removes dissolved salts.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Screening removes floating and large matter only.

43. Which of the following statements is/are correct? 1. Kuichling's formula gives the fire demand. 2. In the formula, population is taken in thousands.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Q = 3182 √P litres per minute with P in thousands.

44. Match the treatment unit with its purpose: (a) Screening (b) Coagulation (c) Disinfection. Purposes: (1) kills disease-causing organisms (2) removes floating objects (3) gathers colloids into flocs

    1. a-2, b-1, c-3
    2. a-1, b-2, c-3
    3. a-3, b-2, c-1
    4. a-2, b-3, c-1
    Answer

    D. a-2, b-3, c-1

    Screening removes floating objects, coagulation forms flocs, disinfection kills germs.

45. Match the unit with its role: (a) Trickling filter (b) Grit chamber (c) Septic tank. Roles: (1) preliminary treatment (2) secondary biological treatment (3) on-site anaerobic treatment

    1. a-3, b-1, c-2
    2. a-2, b-3, c-1
    3. a-1, b-2, c-3
    4. a-2, b-1, c-3
    Answer

    D. a-2, b-1, c-3

    Grit chamber is preliminary, trickling filter is secondary, septic tank is on-site.