Preliminary Treatment of Wastewater
Wastewater contains varying quantities of floating and also suspended solids, some of considerable size. Preliminary Treatment processes remove floating materials from wastewater like dead bodies of animals, pieces of wood, papers, rags, metal containers, plastic or rubber containers, grease etc. These are also very heavy settle-able inorganic solids like grit, fragments of masonry etc.
Various processing units therefore involved in Wastewater Preliminary Treatment are –
- Grit Chamber or Detritus Tank
- Flotation units and Skimming Tanks
- Flow measuring units
1. Screens and Racks
Screening is the first physical unit operation in wastewater preliminary treatment plants. Thus the screening elements consists of bars, rods, gratings or wire mesh or perforated plates. Generally these bars are of circular or rectangular in shape. These bars therefore run vertically or at a slope varying from 30° to 80° with horizontal.
Depending upon the size of screens, however these are of three types –
- Coarse screens (bar size of 40 mm placed at spacing of 75 – 150 mm).
- Medium screens (bar size of 10 mm placed at spacing of 20 – 50 mm).
- Fine screens (perforated plates having perforation of 1.5 – 3 mm in size or woven cloth / mesh / bar size of 6 – 9 mm with clear openings of less than 20 mm).
|Dimensions of bar (mm)||Clear spacing (mm)|
The material obtained from screenings are disposed off by the following methods –
(i) Burial (ii) Incineration (iii) Digestion (iv) Grinding
2. Comminutor and Barminutor
A comminutor consists of a vertical revolving drum, therefore screen with 6 – 10 mm slots. The coarse material is then cut by cutting teeth and the solids are carried away by stationary comb. Then passed through the slots of the drum out and of a bottom opening through an inverted siphon.
Another form of comminutor consists of stationary semi circular screen and a rotating circular cutting disk. It is also termed as stationary screen type comminutor.
Barminutor is another comminuting unit which is generally used for flows exceeding 50 mLd. Thus it is a combined screening and cutting machine consisting of a bar screen of special design with small openings, along with rotating cutters comprising a comminuting unit which travels up and down the screen cutting the retained solids.
3. Grit Chambers
Grit is thus heavy material found in raw sewage. It contains therefore sand, gravel, silt, cinders, broken glass, small fragments of metal, inorganic solids. It also includes eggshells, bone chips, coffee grounds, seeds, food wastes. Grit is generally nonputrescible. However it has specific gravity ranging from 2.4 to 2.65.
Thus Grit Chamber reduce the formation of heavy deposits in pipe lines, channels and conduits. It also reduce the frequency of digester cleaning that may be required as a result of excessive accumulation of grits in such units.
There are two types of grit chambers – Horizontal and Aerated
(i) Horizontal flow grit chambers –
Horizontal flow grit chambers therefore are designed to maintain a velocity as close to 0.3 m/s as practical. The most important consideration in the design of grit chambers is that the flow velocity should neither be low as to cause settling of lighter organic matter, nor should it be so high as not to cause the settlement of the entire silt and grit present in the sewage. In order to control the velocity within recommended range, a control section is placed at the end of the channel. Such control sections include –
- Proportional flow weir
- Sutro weir
- Parshall flumes
- Palmer-Bowlus flumes
A Proportional flow weir is a combination of a weir and also an orifice. It thus consists of rectangular plate. Each grit chamber, therefore should be provided with a separate control weirs.
A Sutro weir is; a half proportional flow weir cut symmetrically and also centrally along the vertical axis. The orifice thus has a straight horizontal bottom forming the weir.
A Parshall flume is; an open constricted channel which thus placed at downstream end of the parabolic section grit chamber which can control the velocity. This can also be used as a measuring device. the flume is self-cleansing and there is no problem of clogging. It is therefore a rectangular control section.
(ii) Aerated grit chambers –
An aerated grit chamber is a special form of grit chamber. Thus it is consisting of a standard spiral flow aeration tank. It has provided with air diffusion tubes, thus placed on one side of the tank, 0.6 – 1 m from the bottom. Therefore these control the separation of inorganic and organic solids by producing a rolling flow pattern. As heavier settle on the floor and lighter ones carried with the roll of spiral motion and eventually out of the tank. The velocity of flow however should not exceed critical velocity of scour of grit particles. Head loss is also negligible through it.
Dimensions and other parameters of aerated grit chamber is such that –
- Depth = 2 – 5 m
- Length = 7.5 – 20 m
- Width = 2.5 – 7 m
- Width to depth ratio = 1:1 – 5:1 (generally we take 2:1)
- Detention time at peak flow = 2 – 5 minutes (generally we take 3 min.)
- Air supply = 0.15 – 0.45 cubic meter per minute per in of length (generally we take 0.3)
4. Detritus Tank
A detritus tank is a grit removal unit which also removes silt as well as some organic matter along with grit. This is because the flow through velocity is less an detention time is more in a detritus tank. Therefore it is install to remove the finer particles forwarded by grit chambers. It is a continuous tank which is generally of rectangular or square shape. The sides are vertical above and also tapered but at the bottom.
Depth and other parameters of detritus tank is such that –
- Depth = 2.5 – 3.5 m (generally we take 3)
- Detention time at peak flow = 3 – 4 minutes
- Velocity of flow = 0.2 – 0.3 m per sec.
5. Skimming Tanks
Skimming Tanks are installed; just ahead of sedimentation tanks so as to remove the floating substances like grease, oil, fats, waxes, soap, free fatty acids. It also includes pieces of cork or wood, fruit skins, vegetables debris etc. Much of these oily, greasy substances however enter the sewers from kitchens of restaurant / houses, motor garages, oil refineries, soap and candle factories etc. If these are not remove, then they seriously affect the working of various treatment units.
Skimming Tanks are arrange so that the floating matter rises and remains on the surface of wastewater until removed, while the liquid flows through out continuously. These are generally rectangular or circular in shape, having detention time of 3 to 5 minutes.
For efficient working of skimming tank, however air diffusers are provide at the bottom of tank. Thus sewage is aerated.
Advantages of aeration of sewage in skimming tank
- Raise oil and grease above its surface.
- Freshens the sewage through supplying DO.
- Objectionable gas such as H2S; are driven out.
- Flocculation of colloidal matter.
The efficiency of skimming tank increased 3 to 4 times by passing chlorine gas.
The waste removed from skimming tank is disposed off by –
(i) Burial in low lying areas (ii) Burning (iii) Digestion
6. Vacuum Flotation
Grease, oil etc. can also removed by subjecting the aerated sewage to vacuum pressure of about 0-25 cm of mercury for 10 to 15 minutes in a vacuater. This process is termed as Vacuum Flotation. Hence in the vacuum flotation method, auxiliary equipment include an aeration tank, vacuum pumps, vacuum, sludge pumps.