Sanitary Engineering

• Sewer Design Criteria

  • Determination of sanitary sewage and storm sewage for suitable design period (25-30 years)
  • Select the type of sewerage system (separate or combined)
  • Select shape and type of sewer (closed for sanitary sewage and open drain for storm flow)
  • Sewer size should be at least 15 cm but recommended is 20 cm
  • Sewer gradient should be such that self-cleaning and non scouring velocity is maintained. (minimum gradient 1:100 and maximum is 1:20)
  • Sewers are designed to flow 1/2 to 2/3 full at peak or maximum discharge
• Design period


  Sewerage system is normally designed for a design period of 25-30 years but it depends on: Fund available, Rate of interest, Life of material, Anticipated expansion of town, Ease/difficulty in expansion
• Self cleaning velocity


  Minimum velocity at which solids do not get deposited in the invert of the sewer. May not have SCV whole day but should have at least once in a day. Design velocity of flow is normally 0.6-0.9 m/s
• Calculation of Self Cleaning Velocity


  Shields' Formula: Vs = k(8f(S-1)gd)^0.5 where, k = dimensionless constant, f = Darcy Weisbach friction factor, S = Specific gravity of sediments, g = gravity acceleration, d = diameter of grain
• Non-scouring/Limiting/Maximum Velocity


  Maximum velocity at which no scour or abrasion of sewer takes place. Depends on the materials of sewer.
• Calculation of Velocity


  Chezy's Formula: V = C√(RS)

  Hazen William's Formula: V = 0.85CR^0.63S^0.54

  Crimp and Bruge's Formula: V = 47.83R^0.5S^0.5

  Manning's Formula: V = (1/n)R^(2/3)S^(1/2)
• Shape of Sewers


  • Circular
  • Rectangular
  • Semi-circular
  • Parabolic
  • U-shape
  • Horse shoe shape
  • Basket handle
  • Egg shaped
• Circular Section


  • Least perimeter for the given area (maximum hydraulic mean depth hence hydraulically efficient section which gives higher discharge)
  • Less chance of deposition of organic matters
  • Easy to construct, transport and handle
  • Useful in separate systems
  • Self cleaning velocity can not be maintained at DWF conditions in combined system
• Egg-Shaped


  • Depth is 1.5 times of width
  • Hydraulically efficient section than circular section
  • Self cleaning velocity is maintained even in DWF
  • Difficult in construction and less stable
• Sewer Material


  • Brick
  • Cement concrete
  • Stone ware/ vitrified clay
  • Cast iron
  • Plastic pipes
• Requirements for sewer


  • Resistant to corrosion and abrasion
  • Strong and durable
  • Should be impervious
  • Should have minimum possible weight
  • Hydraulically efficient for given discharge
  • Economic in construction and material itself
• Vitrified clay/stone ware


  • Manufactured from clay and shales of special qualities and grades
  • Inner surface is made smooth
  • Cheap and resistant to corrosion and abrasion
  • Bulky and heavy in weight
  • Brittle in nature
  • Weak in tension
  • Highly impervious
• Cast Iron Pipes


  • Structurally stronger, long life but costlier
  • Heavy and brittle
  • Can withstand high internal and external pressure and vibration
  • Can bear tensile, bending and compressive strength
• PCC/RCC Pipes


  • Widely used in sewerage system
  • Non pressure type classified as NP2, NP3, NP4
  • Joined by bell and spigot joints and collar joints
  • PCC is used for upto 60 cm diameter
  • Grade M20 is normally used

  Merits: strong enough to withstand external and internal pressure, can be cast in site in required shape and size, Economical for medium and large size, Maintenance cost is low, Can be laid under water

  Demerits: heavy to handle and transport, May be affected by acids and alkali and salt water, Most liable to crown corrosion
• Crown Corrosion


  Sulphate present in sewage are converted into sulphide by bacteria and then to hydrogen sulphide gas. Further bacterial action forms sulphuric acid droplets at the crown. This makes crown uneven and its thickness reduces with respect to time which is called as crown corrosion.
• Method of Construction of Sewer


  1. Setting out centre line

  2. Alignment and gradient fixing

  3. Excavation, timbering and dewatering of trenches

  4. Laying and jointing of sewers

  5. Testing of sewer

  6. Backfilling
• Setting out centre line

  General practice to lay sewer line between manholes and setting out from tail end or outfall and proceeding upwards

  Method I: A offset line parallel to centre line is marked at a distance of (1/2 trench width + 0.6m). Pegs are driven along this line at an interval of 7.5-15m

  Method II: Two vertical posts are driven into the ground at a known distance from centre line peg and one horizontal rail known as sight rail is fixed between these posts. Cord is driven between sight rails of consecutive posts.
• Alignment and gradient fixing


  Sewers are laid to correct alignment and gradient with the help of boning rods or sight rails or sometimes with level. Modified levels of invert are first obtained by adding a suitable vertical length to the invert levels marked on L-section. These modified levels are marked on sight rails either by fixing nails on sight rails or by adjusting the top of sight rails to the modified invert levels of sewer lines. The boning rod or traveler of same height is travelled on the cord to obtain required gradient.
• Excavation, timbering and dewatering of trenches

  Minimum depth above sewer is 0.9m and width is 0.6m. When depth exceeds 1.5-2.0m, timbering (support the side by sheeting and bracing) of trench is necessary. If high water table condition found during excavation, dewatering should be adopted.
• Laying and Joining of Sewers


  Before placing the sewer pipe, the gradient of the bottom of the trench should be checked. Pipes are laid with their socket end faces at upgradient. Jointing are properly done and filled with cement mortar or bitumen. Normally collar or bell and spigot joints are used.
• Testing


  Water Test: Pipe lines are filled with water (plugging lower end and upper end with provision of air outlet in upper end and a provision of funnel in lower end). The water loss should not be greater than 2l/cm diameter of pipe per km length.

  Air Test: Plug the sewer at both ends and from one end, air pressure equivalent to 100mm of water is given. If the pressure is maintained at 75 mm of water, the joint is assumed to be water tight.
• Backfilling


  Immediately done after successful testing. Backfilling soil should be free from pebbles, stones etc. Done in every 15 cm thickness, watered and evenly rammed. Should be about 60-90 cm above the crown and remain left for one week. After one week, backfilling is completed and filled 15 cm extra above the ground surface.
• Solid Waste


  Dry state waste matters that may be organic or inorganic, combustible or non-combustible
• Types of solid waste

  • Garbage
  • Rubbish

  
• Garbage


  Organic waste from kitchen in the form of waste food, vegetable and fruit peelings, grass, leaves, animal and bird excreta
• Rubbish


  Non-putrescible waste excluding ashes, including combustible and non-combustible wastes such as paper, broken furniture, glass, plastic bottles, dismantled building materials

  
• In an average 0.5 to 0.8 kg/capita/day of solid waste are produced in which 45% are inorganic and 55% are organic (for developing countries)
• Collection and Disposal


  1. Solid wastes are collected in individual houses in a small bin where it is removed or collected by scavengers

  2. Public dust bins may also be provided by the municipality at suitable location

  3. Hand carts may also be used to collect the solid waste

  4. Transport of waste by manually or by vehicles

  
• Methods of Solid Disposal

  • Dumping
  • Sanitary Landfill
  • Incineration
  • Composting

  
• Dumping


  Simply throw solid waste in the low lying areas, if possible waste without garbage is thrown, common in developing countries, cheaper but unhygienic

  
• Sanitary Landfill


  Improvement on dumping, solid wastes are dumped into low lying areas and covered by soil at least 20 cm thickness for each layer of 1-2 m (prevent from nuisance of flies and mosquitoes), the next layer is added only after 1 week, all the layers are compacted by moving vehicle or rollers

  
• Sanitary Landfill


  • Costly plants and equipments are not required, residue are not remained for further disposal, pits of low lying areas are reclaimed
  • Requires more land, create foul gases and nuisance near the site, covering soil may not be available, may pollute ground water and surface water, insectisides are required to prevent fly nuisance

  
• Incineration


  Solid wastes are separated into combustible and non-combustible, combustible wastes are burnt into properly constructed furnace or incinerator, hospital wastes are incinerated (in Nepal)
• Incineration


  • Hygienic method, no odor and dust nuisance, heat produced in incineration may be used for other heating purpose such as steam power, the produced clinker may be used in road construction, space requirement is less
  • High chimneys are required (to control air pollution), large initial cost, residue needs further disposal
• Composting


  The putrescible organic matters present in solid waste are decomposed aerobically or anaerobically and convert into humus and stable mineral compounds, hygienic method of converting the refuse into manure through the bacterial agencies, compost is widely used as a manure (rich in nitrogen content)
• Methods of Composting


  • Composting by trenching
  • Open windrow composting
  • Mechanical composting

  
• Composting by trenching


  Trenches of 3-12 m length, 2-3 m wide and 1-2 m deep with a clear spacing of 2 m, organic solid waste is filled in layers of 15 cm, each layer is filled with 5 cm thick layer of night soil or animal excreta on semi liquid form for adding bacteria, top layer is covered by about 10 cm of good earth, biological action starts in 2-3 days and temperature may rises to 750C, waste gets stabilized and changed into brown coloured odorless powder (humus) after 4-6 months

  
• Open Windrow Composting


  Organic and putrescible matters are separated and dumped on the ground in the form of 0.6-1.0 m high, 6 m long and 1-2 m wide piles at about 60% moisture content, covered with animal dung, cattle urine, night soil etc, aerobic reaction needed and the temperature rises to 700C, pH should be adjusted to 7.2-7.4, after the temperature reaches 700C, pile is overturned and the moisture content and pH is maintained, temperature do not increase after certain time which indicates completion of bacterial activity i.e composting is completed, takes about 7-10 weeks and compost thus produced can be sold as a good fertilizer
• Mechanical composting


  Requires small area compare to trenching and open windrow, composting carried out in the closed room, solid waste stabilizes within 3-7 days, sprayers are used to adjust moisture content and the coils are used to make temperature constant at 700C, costly and done in large scale, operation involved: reception of refuse, segregation, shredding, stabilization, marketing the humus