Test: Oxidation Ditch - Civil Engineering (CE) MCQ

The ditch is built on the surface of the ground and is lined with an impermeable lining. This allows the waste water to have plenty of exposure to the open air for the diffusion of oxygen.

An oxidation ditch is a large holding tank in a continuous ditch with an oval shape similar to that of a racetrack. The ditch is built on the surface of the ground and is lined with an impermeable lining.

The MLSS concentration in the oxidation ditch generally ranges from 3,000 mg/L to 5,000 mg/L. This is dependent upon the surface area provided for sedimentation, the rate of sludge return and the aeration process.

The liquid depth in the ditches is very shallow, 0.9 to 1.5 m, which helps to prevent anaerobic conditions from occurring at the bottom of the ditch.

Long solids retention times (SRTs) are used by oxidation ditch to remove biodegradable organics. The typical oxidation ditch is equipped with aeration rotors or brushes that provide aeration and circulation.

The surface area provided for sedimentation, the rate of sludge return and the aeration process affects MLSS concentration. The MLSS concentration in the oxidation ditch generally ranges from 3,000 mg/L to 5,000 mg/L.

Oxidation ditch:

• It is a particular type of extended aeration process, where the aeration tank is constructed in the ditch shape (oval shape) as shown in the figure below:

• An oxidation ditch is a modified activated sludge biological treatment process that utilizes long solids retention times (SRTs) to remove biodegradable organics
• In the extended aeration process, low organic loading rate (F/M) and long aeration time are used to operate the process at the endogenous respiration phase of the growth curve
• Since the cells undergo endogenous respiration, the excess sludge generated in this process is low and the sludge can directly be applied on the sand drying beds where aerobic digestion and dewatering of the sludge occurs

An oxidation ditch is a large circular basin equipped with aerators that are used to remove organic matter and pollutants from sewage through the processes of adsorption, oxidation and decomposition.

The conventional activated sludge plant is modified to eliminate the primary sedimentation tank and sludge digestion tank, in a process, called extended aeration process (also called oxidation ditch).
In this, an aeration tank of longer aeration time is provided.  A some part of settled sludge is re-circulated back and excess settled sludge which is well stabilised due to long detention period of 12-15 hours  can be easily dried on sand beds and disposed of suitably without using any sludge digestion tank.
Important Point
Design parameters for an oxidation ditch are:

• Detention time: 12-15 hours.
• Volume of Ditch - 120 to 150 cum per metre length of rotor.
• Mixer Liquor suspended solids (MLSS) concentration: 4000-5000 mg/l.
• BOD removal: 98 % and Suspended solids removal: 95 %.

The velocity of the wastewater within the ditch is 1 to 2ft/s. This process may be adaptable to the fluctuations in flows and loadings associated with recreation area wastewater production.