Pressure-loss computation procedures for non-Newtonian pipe flow

Wastewater sludge with solids concentrations above about 2 percent exhibit non-Newtonian behavior, i.e., the sludge apparent viscosity changes with the shear rate (flow velocity). To determine pipeline head loss for sludge streams to and from anaerobic digesters, samples of primary, secondary and digested sludge were collected from Orange County Sanitation District Plant No. 1 and analyzed to determine their rheological parameters. The results were used in non-Newtonian hydraulic calculation procedures to estimate head loss under future operating conditions with sludge streams containing high solids concentration. This paper presents the basic sludge rheology theory, provides some details on the sludge sampling and viscosity measurements, and describes the non-Newtonian hydraulic theory and calculation procedures used to estimate head loss for sludge streams having high solids concentrations.

Wastewater treatment plants using sludge streams containing higher solids concentrations than typically used in the past are becoming more common. In the UK, the wastewater treatment plants are working to maximize the capacity of existing anaerobic digestion facilities by using higher design loadings. Southern Water adopted a design volatile solids (VS) loading of up to 0.28 lb VS/ft3/day. The Nigg plant has a design loading of 0.25 lb VS/ft3/day. The Lowestoft
plant has an average design loading of 0.22 lb VS/ft3/day (Cumiskey, 2004). These loadings in excess of 0.22 lb VS/ft3/day compare with typical US design loadings in the range of 0.12 to 0.16 lb VS/ft3/day (WEF Manual of Practice 8, 1998).

The Orange County Sanitation District (OCSD) is expanding their wastewater treatment plant located in Fountain Valley, California to provide full secondary treatment. The expansion includes increasing the capacity of the existing anaerobic digestion facilities to match the future capacity for treating wastewater. Instead of building new digesters, feed sludge to the process will be thickened by centrifugation to 6 percent dry solids concentration. This will increase volatile solids loading to the digesters while maintaining a minimum 15-day detention time in accordance with EPA 40 CFR Part 503 Regulations for Class B biosolids. The current operation thickens primary sludge in the clarifiers and pumps primary sludge (PS) to the digesters at a solids concentration of about 4 percent. Waste activated sludge (WAS) is thickened using dissolved air flotation (DAF) to approximately 4 percent.

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