英文资料

ass Balance Approach Successful in Controlling Bulking Sludge

Author(s) : Steven Woodard Stephen Fuller



The wastewater treatment plant (WWTP) at Mead Paper, Rumford, ME, historically struggled with sludge bulking episodes. Mill changes, including conversion to elemental chlorine-free (ECF) bleaching and alkaline papermaking, exacerbated the problem, and microscopic examinations revealed that organic acids were the primary cause of bulking.

The solution is an organic acid control program that results in a more stable, manageable secondary treatment system that can tolerate higher levels of filamentous organisms, while needing appreciably less sodium hypochlorite addition to the return activated sludge (RAS) for filament control.

The facility is an integrated bleached Kraft pulp and paper mill, 75 acres in western Maine, on the Androscoggin River. Total production is roughly 1700 T/D market pulp and finished paper. Products are coated grades such as magazine, catalog, insert and commercial printing papers and market pulp. The mill operates four coated paper machines and one pulp dryer, along with a hardwood and softwood Kraft pulp mill and bleach plant and a groundwood mill. The mill also includes power generation from a hydro facility (40 MW), co-generation plant (100 MW), and two other power boilers (12 MW). Sludge, wood waste, and tire derived fuel is burned in the co-gen facility, as well as oil and coal.

Wastewater generated by the mill averages 30 mgd. The wastewater is treated using coarse screens, primary sedimentation, and a complete mix activated sludge system. Primary treatment consists of a single, 220-foot-diameter clarifier. Primary effluent flows by gravity to two aeration basins, operated in parallel. The basins provide a total volume of 8.85 million gallons and have a liquid depth of 14 feet. Coarse bubble diffused air supplies oxygen to the aeration basins. There are three rectangular secondary clarifiers, with a total volume of 6.35 million gallons and a side water depth of 15 feet. Primary and secondary sludges generated by the WWTP are combined and dewatered on gravity belt thickeners and screw presses.

The WWTP has several physical limitations. Since there is only one primary clarifier, much of the maintenance can only be performed during periodic mill shutdowns, particularly cold outages when there is very little wastewater flow. The aeration basins are marginally sized for the average primary effluent biochemical oxygen demand (BOD) loading of 70,000 lb/day. A mixed liquor suspended solids (MLSS) concentration of 4,500 to 5,500 mg/l is needed to maintain a suitable F/M to prevent overgrowth of 1701, a filamentous bacteria caused by high F/M, low dissolved oxygen (DO) conditions. The diffused aeration system is also marginally sized, often resulting in mixed liquor DO concentrations of less than 1.0 mg/l.

The secondary clarifiers are undersized with respect to solids loading rate, which averages 1.4 lb/ft2-hr. This means that the secondary clarifiers are being loaded 75% higher than design guidelines recommend for pulp and paper mill secondary clarifiers. This is mostly due to the relatively high mixed liquor concentrations that must be carried to discourage the overgrowth of 1701. The secondaries have also had problems with hydraulic short-circuiting currents and poor flow split between the clarifiers.
Historical Treatment Challenges

The combination of factors described above, coupled with the inherent variability associated with pulp and paper mill wastewater loadings, makes this a challenging plant to operate. The WWTP has little buffering capacity, due to the marginally sized aeration basins, secondary clarifiers and aeration system. As a result, operating conditions can change rapidly, and operators must react quickly and maintain a close watch on a number of operating parameters.

The mill also has tigh