Extended Aeration and Packaged Plants

Hello, Iím Jay Blevins from Mountain Empire Community College. Detention time is the main difference between an extended aeration plant and a package plant. In both plants, the influent is first filtered through a bar screen to remove large objects. After passing through the bar screen, the influent enters the grit chamber and the velocity is reduced to allow the solids to settle. Once
the influent has passed through the grit chamber, the remaining influent reaches the comminutor. The comminutor or grinding pump grinds the organic matter into small particles in order for the microorganisms to be able to utilize the food. From this point, the influent is pumped to the aeration basin. In the aeration basin, the B. O. D. or the organic matter within the water comes into the chamber where the B.O.D. is digested. The organic material is seeded with microorganisms, which begins a period of feeding and multiplying. The microorganisms multiply very rapidly and consume the B.O.D. in a short period of time, usually within the first two hours of entering the aeration basin. After the food is consumed in an extended aeration plant, air is pumped to the microorganisms for two additional hours. In this process, the microorganisms start dying for lack of food and the microorganisms feed on each other, thus reducing the sludge. A packaged plant produces more sludge than an extended aeration basin plant or oxidation ditch.

Packaged plants do not allow time for oxygen to produce a major reduction in the sludge. The influent leaves the aeration chamber and enters the clarifier. The microorganisms, which have food, grit, and other particles stuck to the enzyme coating, are heavy and floc out in the clarifier. The clarifier is located at the back of the extended aeration chamber or the packaged plant. The clarifier is an important part of the whole operation. The main distinction between a clarifier and extended aeration is the use of forced air. The air helps keep the microorganisms suspended; keeping the microorganisms from settling. In the clarifier, the microorganisms floc out (settle) and the supernate (clear liquid) rises to the top. The supernate is then chlorinated and de-chlorinated before being released. The microorganisms that have flocked out in the clarifier are re-circulated to become seed for the new influent. All of the microorganisms, however, are not re-circulated because of the food to microorganism ratio concept. An easy way to visualize this concept is to imagine having 10 hungry football players trying to eat 1000 sandwiches. There are too many sandwiches for the number of players and there will be a large portion of the sandwiches left over. Conversely, if there are 100 hungry football players and only 10 sandwiches available, the players would likely fight and the sandwiches would be torn apart and probably not eaten anyway. Though this is a simple visualization, this is what essentially happens with microorganisms. The food to microorganism ratio must to be matched on order to have efficient B.O.D. removal. One manufacturer, Clow, suggests the food to microorganism ratio be about 0.6.  The remaining sludge not used for re-circulation that is produced per day of treatment is wasted. The re-circulation rate is controlled by a pump and valve combination. The valve can be opened or closed as needed in order to secure the optimum food to microorganism ratio.

In both the packaged plant and the extended aeration plant, the sludge must be aged and stressed to achieve maximum benefits. Each dayís floc or sludge is added to the top of the previous days sludge and the oldest layer is retained about 10 days before being pumped as seed to the aeration chamber. The 10-day cycle is important because the microorganisms have been stressed enough so that they are ready to feed again and are able to reduce the B.O.D. in the water in a short period of time. The stressed microorganisms not only eat rapidly, the microorganisms also multiply rapidly. A packaged plant and an extended aeration plant removes B. O. D. in about two hours or less because the developed microorganisms provide optimum food removal and a good quality effluent.

Though the standard ten years ago, packaged plants and extended aeration plants are no longer in large demand because of poor ammonia removal. New regulations limit the amount of ammonia that can be released into a waterway system and are listed on the National Pollution Discharge Elimination System (NPDES) permit issued to a plant. A trickling filter or a rotating
biological contactor must be added to remove ammonia. Most communities are now using oxidation ditches instead of packaged plants or extended aeration plants because oxidation ditches remove both B.O.D. and ammonia. Only a few packaged plants and extended aeration plants still operate near the college.