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The OD consists of a "ring or oval shaped channel" equipped with mechanical aeration devices. Screened wastewater, which enters the ditch is aerated and circulated. ODs typically have long detention times and are capable of removing between 75% and 95% of the Biological Oxygen Demand (BOD).
The proprietary "Orbal System" uses three channels or ditches concentrically placed. Each channel is independently aerated and can be configured to act in parallel or series with the other channels, depending upon the degree of treatment required.
After screening and grit removal, sewage enters the outer channel where most of the biological reaction takes place. The second channel is held at a slightly higher dissolved oxygen content for further BOD and nutrient reduction. The innermost channel is used for polishing the effluent before it passes to a clarifier.
Typical figures for ODs are as follows:
|(mg/L)||Raw Sewage||Effluent||DOE Standard B|
|Biological Oxygen Demand||200-400||10-30||20|
The ODs can be easily adjusted to meet most combinations of incoming sewage and effluent standards. This system achieves both high BOD reduction and some nutrient removal.
ODs require more land than other processes but can be cheaper to construct and operate. There are currently approximately 30 ODs in Malaysia. The new modified ODs in Malaysia are located in Sg. Besi - Kuala Lumpur, Bayan Baru - Penang and Cyberjaya.