INTRODUCTIONSewage Treatment Involves various physical unit operations sequentially with biological unit processes. Physical unit operations include screens, grit chambers, skimming tanks and grease traps and primary clarifiers. Biological processes(both aerobic and anaerobic) may be – attached growth processes with units like trickling filters or suspended growth processes with units like activated sludge process, aeration tanks, aerated lagoons.
PRIMARY & SECONDARY SEWAGE TREATMENTS Primary Treatment – removal of heavy suspended matter like faecal solids, kitchen refuse, cloth, waste paper, etc. It’s advantage is that it prevents damage or clog of the pumps and skimmers of primary treatment clarifiers (trash, tree limbs, leaves, etc.). Primary treatment involves sequential unit processes like screening, removal of grit &other floating matter. The remainder sewage flows to primary sedimentation tank or clarifier(PST).
Secondary Treatment – biological process for treatment of very fine suspended matter, colloids & dissolved solids in sewage from PST. The unit processes are biological oxidation and activated sludge process (converting sewage to heavier & bulkier form and then it gets settled in Secondary Settling Tank) This process stabilizes & makes the sewage completely harmless. The separated sewer-sludge is decomposed anaerobically in Sludge Digestion Tanks & the digested sludge is disposed of separately in Sludge Drying Beds.
RETURNED ACTIVATED SLUDGE TREATE RAW GRIT AERATION D SCREEN SEWAGWATER CHAMBER TANK E P.S.T. S.S.T PRIMAR SECONDA Y RY SLUDGE SLUDGE SLUDGE SLUDGE DIGESTION DRYING BEDS TANK
METHOD FOR NORMAL WATER TREATMENT P.S.T COAGULLATION FLOCULATION SETTLING FILTRATION DININFECTION STORAGE DISTRIBUTION
GRIT CHAMBERS: - It is an enlarged channel or long basin in which the cross-section is increased -------to reduce the velocity of the flowing sewage sufficiently ------- to cause heavy inorganic matter such as grit, sand and gravel of size 0.2mm and larger to settle, with the lighter organic matter remaining in suspension. Types of Grit Chambers Mechanically cleaned and Manually cleaned.
DESIGN CRITERIA : In case of PST both Surface Overflow Rate and Detention Period are important design considerations. This is because of the flocculent nature of the settling particles. In case of Secondary Settling Tanks, Solid Loading Rate and Overflow Rates are important for the design
1) Detention period: 2 to 3 hours for PST and 1.5 to 2 hours for SST. Longer periods result in higher efficiency but too long a period induces septic condition and should be avoided.2) Velocity of Flow: About 30cm./mt.3) Depth of Flow: 2.5 to 3.0 mts. for PST and 3 to 4 mts. For SST. Depth influences sludge thickening in SST.
Surface Loading or Overflow Rate: It is thehydraulic loading per unit of surface area oftank per day and is expressed as litres/m2/day.Solid Loading is due to sludge solidscontained in the mixed liquor emanating fromaeration tanks/trickling filters and is referred asMixed Liquor Suspended Solids(MLSS).Weir Loading should be such as to ensureuniform withdrawal over the entire perophery ofthe tank to avoid short-circuiting.
LOADING RATES FOR SETTLING TANKS Design Primary Settling Tank Secondary Settling Tank Parameter Primary Primary Secondary Secondary Settling Only Settling settling after settling after followed by trickling filter activated secondary sludge process treatmentOverflow Rate 25,000 – 35,000 – 15,000 – 50,000 15,000 – 50,000 50,000 80,000(litres/m2/day) Solid Loading - - 70 - 180 70 - 210 (kg/m2/day) Weir Loading Not to Exceed 1,50,000 Not to exceed 1,85,000(litres/m/day)
SEDIMENTATION TANK DESIGN Problem: Design a rectangular sedimentation tank to treat 2.4 million litres of raw water per day. The detention period may be assumed to be 3 hours. Solution: Raw water flow per day is 2.4 x 106 l. Detention period is 3h.1. Volume of tank = Flow x Detention period = 2.4 x 103 x 3/24 = 300 m32. depth of tank = 3.0 m.3. Surface area = 300/3 = 100 m24. L/B = 3 (assumed). L = 3B. 3B2 = 100 m2 i.e. B = 5.8 m L = 3B = 5.8 X 3 = 17.4 mHence surface loading (Overflow rate) = 2.4 x 106 = 24,000 l/d/m2 < 40,000 l/d/m2 (OK)