Sludge Solution

Bryen Woo and Chuck Thompson

Bellefonte Wastewater Treatment Plant (WWTP) in Bellefonte, Pa., currently operates an Ovivo Pre-thickened Aerobic Digester using a Mechanical Thickener (P.A.D.-M) Process and was commissioned on July 7, 2009.

The city of Bellefonte will increase the capacity of its wastewater treatment plant from a 2-million-gal-per-day (mgd) flow rate to a 3.2-mgd flow rate. Nittany Engineering & Associates LLC was contracted by the city of Bellefonte to design a Class B biosolids handling facility for land application. Nittany Engineering proposed to retrofit an existing equalization tank into aerobic digestion tanks to minimize costs. Ovivo recommended the P.A.D.-M Process to Nittany Engineering. By pre-thickening the waste-activated sludge (WAS) up to 4% solids with the P.A.D.-M Process, the construction of additional tanks was not required because it allowed for the existing equalization tank volume to provide the necessary solids retention time (SRT) to comply with the time/temperature Class B Pathogen Reduction criteria at the 3.2-mgd flow rate.

System Design

The Bellefonte WWTP P.A.D.-M Process design converted the existing equalization tank into three aerobic digestion tanks to be operated in series. Each aerobic digester tank was designed with an Airbeam Cover integrating Ovivo ’s MS diffusers and shear tubes to provide maximum mixing and aeration efficiency of thickened solids, minimize odors and provide optimum temperature control during winter operations when Class B biosolids are most difficult to achieve.

Temperature Control

Temperature control is essential to a successful aerobic digestion process. Ovivo provided an Airbeam Cover design for each aerobic digester tank in the Bellefonte P.A.D.-M Process to aid in temperature control, primarily during winter operations. Temperatures between 20°C and 35°C provide the most favorable aerobic digestion conditions. Temperatures lower than 10°C inhibit nitrification and can create odor issues, while temperatures greater than 37°C create thermophilic conditions. With average ambient temperatures in Bellefonte reaching as low as -3.37°C during the 2010 operations, the aerobic digesters were able to sustain average temperatures no lower than 17°C from this period. Figure 2 shows the average temperature maintained in each aerobic digester tank comparing with the average ambient temperature each month from February to December 2010. As shown in Figure 2, each aerobic digester tank consistently maintained a temperature that achieves optimum aerobic digestion performance. The data also demonstrate excellent temperature control in each digester, most notably when average ambient temperatures are lower than 10°C.

Results

WAS is pre-thickened up to 4% solids from a gravity belt thickener (GBT) before entering the aerobic digesters to provide a longer SRT for enhanced volatile solids reduction (VSR). Excellent VSR was achieved at this facility, ranging from 40% to 59% during 2010 operations. This VSR is possible due to longer SRT, complemented by outstanding temperature control maintained in the aerobic digester tanks as noted above. This performance exceeds the minimum CFR 40 Part 503 Class B Vector Attraction regulations of 38% VSR.

The Bellefonte P.A.D.-M Process was able to produce a sludge with pathogens ranging as high as 1,040 to as low as 364 colony-forming units (CFU) per dry gram for the entire 2010 operations based on a geometric mean from seven repetitions of each sample analyzed. These results exceed the CFR 40 Part 503 Class B Pathogen Reduction regulations of less than 2,000,000 CFU per dry gram.

Due to temperature control and the enhanced mixing efficiency of thickened solids from the Airbeam Cover aeration system, the Bellefonte WWTP P.A.D.-M Process consistently produces a Class B treated sludge for land application.