Clifton's Innovative Treatment Plant

One of the best kept secrets in the wastewater engineering field at the moment is the sequencing batch reactor for sewage treatment plants. Originally developed in the 1840's as a “draw and fill” system, the treatment process fell out of favor with plant operators who had to open and close valves on a fixed schedule to either fill or drain treatment basins. The advantage was that the single basin could be used to pre-settle, aerate, and then final-clarify the sewage. The only problem seemed to be what to do with the sewage coming in while the valve was closed. For that reason multiple basins were always employed so that one was filling, one was aerating, one was settling and one was decanting or drawing off the clear liquid

In the 1980's with the advent of the personal computer and programmable electronic devices, the process once again gained favor.

That is the root of the plant built in 2000on the banks of the Bosque River for the city of Clifton, about 65 miles northwest of Waco in central Texas.

The original plant was a a small 400,000 gallon per day “race track” oxidation ditch, a process brought to the U.S. in the 1960's from The Netherlands. At peak flow the old plant could handle 1.2 million gallons per day for short periods of time, usually during rainfall events when the sewerage system was overloaded. During extreme rainfall periods the river overflowed its banks and inundated the small plant.

It was this unusually large reaction to rainfall and susceptibility to flooding that made the sequencing system ideal.

The Austgen Biojet System (ABJ), originally developed in Australia, handles peak loads very well because of its built in capacity to hold large amounts of excess flow and cycle it out slowly. The ABJ system is not really a true sequencing batch reactor because the flow into any one basin is not ever shut off. Instead the flow enters into a pre-react basin and is directed under a wall through a slot along the floor causing the influent to rise up through the activated sludge blanket whether it is being aerated or is settling at the time. Slow vertical velocities make short circuiting an improbability and mixing does not take place rapidly enough to overcome gravity and force the settling blanket of sludge up into the clarifying layers on top.

Clifton's new plant has a capacity of 650,000 gallons per day based on a 30-day average of flows and it has a peak capacity of over 2.0 million gallons per day. Effluent parameters for the plant were much more stringent than for the old plant.

Instead of constructing a pre-clarification basin, an aeration basin, and a post clarifier, one basin serves all three purposes. The basin fills quietly for one hour, aerates for one hour, settles for one hour and decants for one more hour making a total cycle of four hours and six cycles per day. Of course if additional air is needed, up to the first two hours can be aerated providing more than enough oxygen for the extended aeration process even when nitrification/denitrification is needed.

During the ending of the decant process, sludge pumps in the bottom of the reactor basin come on for a specified period of time depending on the sludge production going on at that season of the year. The timing of the sludge withdrawal pump can be adjusted so that just th e proper amount of sludge (Waste Activated Sludge ) is drawn off each 4-hour cycle. Conventional multi-basin systems are usually adjusted for sludge weekly or even monthly meaning the process is not constantly being monitored and sludge can become too concentrated or too dilute for proper treatment.

The plant footprint is about half of the old plant even though it is nearly 80% bigger in capacity.

Another advantage of the ABJ system is the lack of sludge recycling or Return Activated Sludge (RAS). The sludge from a conventional clarifier has to be collected with rotating or sliding rakes and pumped back into the front of the plant. In the ABJ plant, there is no need to pump it back. . . its already there!

We have noticed some reluctance on the part of operators to accept the new plant during the planning process. However, almost without exception, operators who have previously operated other types of extended aeration or contact stabilization plants invariably fall in love with the simplicity of the ABJ system. One operator who operates a race track and an ABJ plant in Georgia, told representatives of The Brannon Corporation in a private interview, he spent 80% of his time on the conventional plant and only 20% on the ABJ plant.

Another advantage of the ABJ system is that the basin rises and falls about five feet every cycle making the growth of algae virtually nil. On conventional plants, v-notch weirs and rotating clarifier components have to be hosed down daily to scour off algae and keep it from the discharge weir.

Effluent from ABJ plants typically runs less than 5 mg/L carbonaceous biochemical oxygen demand (CBOD) and 5 mg/L total suspended solids ( TSS) when influents are in the 250 mg/L range for each. Permit parameters are 10 mg/L and 15 mg/L respectively.

The Clifton plant uses a ½-meter Roediger belt press for sludge thickening after gravity thickening in a sludge holding tank.

The plant was built with a low interest loan from the office of Rural Development of the Department of Agriculture. Total price for the plant, office and mechanical building, sludge handling shelter, treatment plant, influent lift station, influent interceptor lines, and associated parking and fencing facilities was $2.1 million. The project was completed in May, 2000.

Project Engineer for The Brannon Corporation was Jesse Whitten, PE. Others on the design team included Terry Brannon, PE and Kirk Bynum, PE. For more information write whitten@brannoncorp.com.

We have similar designs in Atlanta, Livingston, and Rio Vista, all in Texas. If your plant operator has better things to do with his time than fight with his plant, you might want to consider an ABJ facility on your next plant. Call us, we can help!