preliminary treatment.pptx

Waste water treatment

1. MODULE 4 WASTE WATER TREATMENT

2. COMMINUTORS A device used to reduce the size of the solid materials in wastewater by shredding ( comminution ). The shredding action is like many scissors cutting to shreds all the large solids in the wastewater . The rotating-screen-type  comminutor  consists of a motor-driven, revolving, vertical drum-screen with 6mm – 10mm slots almost completely submerged in the wastewater flow. Material that is too large to pass through the slots is cut into pieces by the cutting members acting like shears . The sheared particles then pass through the slots of the drum out and of a bottom opening through an inverted siphon

3. COMMINUTOR

4. GRIT CHAMBERS Grit chamber is a long narrow or circular tank in the primary sewage treatment plant Designed to reduce the velocity of the flow of sewage to eliminate the girt materials (specific gravity 2.4-2.65) such as sand, ash and clinkers, eggshells, bone chips and many inert materials inorganic in nature . Grit settles more rapidly than organic materials They are provided to protect moving mechanical equipments from abrasion They reduce formation of heavy deposits in pipe lines, channels and conduits They also reduce the frequency of digester cleaning

5. types Horizontal flow grit chambers Aerated grit chambers 1. Horizontal flow- They are 10 to 18 m long narrow open channel with a depth of liquid 1 to 1.3 m are designed to maintain a velocity 0.3m/s Such a velocity will carry most organic particles through the chamber and permit the heavier grit to settle out The flow velocity should neither be low as to cause settling of lighter organic matter, nor should be so high as not to cause the settlement of entire silt and grit To maintain a constant velocity a control section is used such as proportional flow weirs, parshall flumes etc

6. Cleaning of grit chamber Mechanically cleaned Mechanically cleaned grit chambers come with mechanical equipment that accumulates and washes grit chambers, serving on either intermittent or continuous basis.    Manually Cleaned Manually run grit chambers are equipped with adequate capacity to hold the grits between the time of cleaning.  Disposal of grit is by dumping or burying or by sanitary land fill

7. 2. Aerated grit chambers A special form of grit chamber consisting of a standard spiral flow aeration tank with air diffusion tubes placed on one side of the tank 0.6 to 1m from bottom Aerated type control the separation of organic and inorganic solids by producing a rolling flow pattern The heavier particles drop down to the floor whereas lighter particles are carried with the roll of spiral motion and eventually out of the tank

8. DETRITUS TANK A grit removal unit which also removes silt as well as some organic matter Because flow through velocity is less and detention time is more Idea is to remove finer particles than those removed by a grit chamber It is continuous flow settling tank of rectangular or square shape The sides are vertical and tapered towards the bottom to form a trough for collection of detritus The overall depth of detritus tanks varies from 2.5m to 3.5 m Detention time – 3 to 4 mts Velocity of flow- 0.2 to 0.3 m/s The light organic matter can be washed out by passing compressed air, by washing etc

9. Skimming tank Installed just ahead of sedimentation tank To remove the floating matter like oil, fat, grease , soap, waxes, fatty acids, fruit skins etc., Major sources are kitchen, motor garrages , oil refineries, soap factories etc If these are not removed they seriously affect the working of various treatment units It’s a chamber so arranged that the floating matter like oil, fat, grease etc., rise and remain on the surface of the waste water until removed, while the liquid flows out continuously under partitions or baffles . Most of them are circular or rectangular, with a detention time of 3 to 5 mts The chamber is a long trough shaped structure divided up into two or three lateral compartments by vertical baffle walls having slots for a short distance below the sewage surface and permitting oil and grease to escape into stilling compartments.

10. For efficient working air diffusers are provided at the bottom The rising air bubbles congeal the greasy and oily material and push it to the side compartment The floating matters can be removed either manually or with mechanical equipment The skimmings obtained should be disposed off by burial or by burning or by digestion It may also be converted into soap lubricants, candles and other non edible products The digestion of skimmings produces of gases of high fuel value

11. Equilization tank When the characteristics of the waste water varies during the day and also when the discharge is either not uniform or else is discontinuous, equilization is necessary The process consists of holding the ww for some predetermined time, in continuously mixed holding tanks so as to get ww of uniform character and at uniform rate Effects of flow equalization 10-20% of BOD entering is stabilized in the equalization basin 23-47% of SS is further removed in the primary clarifier reduce shock load on biological process