Other Sewage Treatment Methods

OTHER TREATMENT METHODS

• It is a modified activated sludge process (ASP). It eliminates primary sedimentation and sludge digestion.

• As the aeration time is longer, it is called extended aeration process.

• Oxidation-ditch plant is economical for population upto 1.5 lakh, compared

• Land requirement is less.

• Oxidation ditch plant involves the construction of number of ditch channels, placed side by side having depth of 1-1.5 m, width of 1-5 m based on the rotors from 150 m to 1000 

• The oxidation ditches may be constructed either in earthwork with earthern embankments (or) in brick (or) in stone masonry walls.

• Each ditch channel is equipped with a horizontal axis rotor for agitating and circulating the sewage and thereby oxygenating the sewage and keeping the sewage-solids in suspension.

• The velocity of sewage is 0.3 m/s.

• The aerated sewage is then settled in a settling tank by stopping the rotors for 2 hours.

• When rotors are stopped, the supernatant liquor is taken out.

• A part of settled sludge is recirculated and the excess sludge (stabilised) is easily dried in sand beds and disposed suitably.

Types of Oxidation Ditches:

(i) Intermittent Flow Type:

• There is no separate settling tank.

• When the rotor is stopped, the sludge get settled and the supernatant is 2009d withdrawn.

• The surplus sludge is removed and disposed after treatment.

(ii) Continuous Flow Type:

• There is continuous operation. The mixed liquid gets settled in a separate settling tank.

Design Considerations:

• Long aeration basin to carry MLSS conc - 3000 to 8000 mg/l.

• F/M ratio (loading factor) is low - 0

• Efficiency - 95% for suspended solids removal,

- 98% for BOD removal.

• Settled sludge which is mineralised is dried without odours.

• Detention period = 12 to 15 hours (or)

0.8 to 2.5 m3 per kg of BOD, load in sewage.

• Volume of ditch = 150 m3 per metre length of rotor.

Kinetic Equations (similar to ASP):

Normally MLVSS is taken as the mass of active micro-organism in reactor, but in oxidation ditch, it includes inactive volatile suspended solids also.

Influent soluble BOD S₀ = L_a - BOD of suspended solids in influent

↓

(0.25 kg/kg of SS).

BOD of SS escaping with effluent = 0.45 kg/kg of SS.

Microbial mass conc = 60% MLSS.

SS - Suspended Solids

MLVSS - Mixed Liquor Volatile Suspended Solids

Microbial mass in excess sludge, 

Wasting of excess sludge =

Active microbes in return sludge.

Hydraulic retention time = V/Q

Process loading  kg total BOD/kg of MLSS.