Treatment of wastewater using an oxidation ditch is relatively similar to wastewater treatment in a packaged plant. But the oxidation ditch replaces the aeration basin and provides better sludge treatment.
The only pretreatment typically used in an oxidation ditch system is the bar screen. After passing through the bar screen, wastewater flows directly into the oxidation ditch.
The oxidation ditch is a circular basin through which the wastewater flows. Activated sludge is added to the oxidation ditch so that the microorganisms will digest the B.O.D. in the water. This mixture of raw wastewater and returned sludge is known as mixed liquor.
Oxygen is added to the mixed liquor in the oxidation ditch using rotating biological contactors (RBC's.) RBC's are more efficient than the aerators used in packaged plants. In addition to increasing the water's dissolved oxygen, RBC's also increase surface area and create waves and movement within the ditches.
Once the B.O.D. has been removed from the wastewater, the mixed liquor flows out of the oxidation ditch. Sludge is removed in the clarifier. This sludge is pumped to an aerobic digester where the sludge is thickened with the help of aerator pumps. This method greatly reduces the amount of sludge produced. Some of the sludge is returned to the oxidation ditch while the rest of the sludge is sent to waste.
Comparison to a Packaged Plant
As you can see, the treatment of wastewater in an oxidation ditch is similar to treatment in a packaged plant. The two main differences between the processes are the retention time and the type of organisms which digest the wastewater.
Retention time is much longer in an oxidation ditch. A packaged plant usually has a retention time of two to four hours while an oxidation ditch retains the wastewater for two days.
Since the D.O. is higher in the oxidation ditch than in a packaged plant, a greater variety of microorganisms live in the oxidation ditch. In contrast, packaged plants usually depend upon only a few types of microorganisms to eat the sewage.
Variations from the Typical Process
In this section we will answer the following question:
How can the typical design be varied to promote ammonia removal and prevent washouts?
Ammonia Removal
Oxidation ditches can be set up to remove ammonia very effectively. Wastewater can be sent through two sets of ditches, each of which has a different pH. The different pH in the two ditches creates a niche for certain microorganisms. These microorganisms are very efficient at removing B.O.D and converting ammonia to nitrates.
Oxidation ditches are much more efficient at ammonia removal than packaged plants are. As a result, most new treatment facilities are designed as oxidation ditches.
Diversion Basins and Washouts
Some oxidation ditches, like the one located in Big Stone Gap, Virginia, have a diversion basin to hold the influent when flows increase because of excessive rainfall. The diversion basin holds the excess influent and allows more time for treatment.
Without a diversion basin, heavy rains could cause a washout to occur. A washout occurs when a large influx of influent rushes into the oxidation ditches. The ditches are unable to contain the extra water, so microorganisms, sludge, and wastewater are pushed through the plant and out into a river or stream before being properly treated.
Without diversion basins, total washouts can be prevented by shutting off the inner ditches and allowing the outside ditch to circulate the influent, providing primary treatment to the water before it is released. In periods of excessive rainfall, oxidation plants can be operated on high flow settings for a month at a time.
Advantages and Disadvantages
In this section we will answer the following question:
What are the advantages and disadvantages of using an oxidation ditch system?
Advantages and Disadvantages
The greatest advantage of an oxidation ditch is the efficiency of sludge removal. In an oxidation ditch, only about 15% of the original B.O.D. ends up as sludge, compared to a packaged plant where about 60% of the B.O.D. becomes sludge.
However, oxidation ditches are expensive to maintain. The monetary cost is very high per ton of B.O.D. removed. In some cases, the cost may reach nearly 350 dollars per ton.
Oxidation ditches have an additional environmental drawback. The water is moved through the ditches using rotors, and these rotors in turn use electricity. The electricity used to operate the plant causes sulphur dioxide and other contaminants to be released into the atmosphere from coal-burning electrical plants.
Oxidation ditches provide the most thorough process for treating sewage, but oxidation ditches are also one of the most costly forms of treatment.