Oxidation ditches are very popular wastewater treatment processes for small to mediumsized municipalities that have to meet stringent water quality standards. Oxidation ditches provide cost-effective treatment while requiring relatively simple operational skills. Because carbonaceous BOD removal, nitrification, and de-nitrification can be accomplished within a single reactor, the process can produce a high quality effluent.
Obviously, it is important to adequately mix the wastewater with the biomass in the ditch to ensure effective pollutant removal. Mixing is promoted by ensuring that wastewater velocities in the reactor are sufficient to create turbulence which keeps the biomass and solids in the mixed liquor suspended. Generally, average sustained velocities of about 1 fps are considered sufficient for this task. TNRCC also has a requirement of 100 HP/mil gal as a secondary requirement to ensure adequate mixing. Some oxidation ditches have old, poorly performing fixed brush aerators, which provide poor mixing and poor oxygen transfer.
One way to solve this problem is to install floating horizontal brush aerators to improve oxygenation and mixing capacities. Operating data from an existing ditch where floating brush aerators have been installed will be presented. Most oxidation ditches are designed to achieve CBOD removal and nitrification. However, many NPDES permits may become more restrictive in the future with respect to the discharge of total nitrogen or total inorganic nitrogen. Therefore, modifying oxidation ditches to include an anoxic zone to achieve de-nitrification is an excellent way to meet the more stringent discharge standards. In this paper, the specifics of enhancing nitrogen removal by establishing an anoxic zone will be discussed. Data from existing oxidation ditches will be reviewed to demonstrate the feasibility of adding the anoxic zone.
Another approach for achieving de-nitrification in oxidation ditches is to implement onoff operation of the aeration system. The details, advantages, and disadvantages of on-off operation will be discussed in this paper.