Thickening with membranes is independent of the WAS settling characteristics in contrast to a standard aerobic digestion process using decanting, where thickening performance can fluctuate greatly.
Reusable Quality Permeate
High nutrient concentrations in recycled water from a solids handling process are widely recognized as a leading cause for high nitrogen and phosphorus loadings. In order to remove phosphorus and nitrogen from the effluent, chemical addition of alum or ferric chloride is required which can add substantial cost and can increase sludge loads up to 40%.
The permeate produced from a P.A.D.®-K system contains a very minimal amount of total nitrogen, total suspended solids, and phosphorus and if recycled to the head of the plant will protect the effluent quality of the BNR biological process without requiring chemical addition. The permeate can also be sent directly to disinfection or combined with the plant effluent.
The system uses nitrification and denitrification processes to remove nitrogen in both the solid and liquid phase. The process also can minimize the release of the three forms of phosphorus: inorganic phosphorus, polyphosphorus, and organic phosphorus.
The continuous nitrification and denitrification sequencing allows excellent pH control, preventing the dissolution of inorganic phosphorus. Polyphosphorus accumulating organisms (PAOs) can store carbon compounds as a source of energy in the absence of oxygen or nitrate, which are common energy sources in biological processes. The storage of carbon compounds by PAOs results in a polyphosphorus release. The incoming WAS into the Anoxic Tank provides a fresh carbon source for PAOs to release polyphosphate following the exhaustion of nitrate, which also occurs in this tank. The aerated MBT tank, where permeate is collected, allows for the PAOs to grow and uptake the released polyphosphorus while burning their stored carbon reserves resulting in reduced phosphorus levels in permeate.
PAO decay causes organic phosphorus release into the permeate due to substantial biomass destruction in the P.A.D.®-K process. However, organic phosphorus release in permeate is minimal since Van Haandel and Van der Lubbe found the decay rate of PAO is approximately six times slower than non-PAO bacteria, and there are approximately twice as many non-PAO bacteria than PAO in a typical biomass. Although non-PAO bacteria are more common in a biomass and have a faster decay rate they contain approximately 15 times less polyphosphorus than PAO bacteria (Van Haandel and Van der Lubbe, 2007).
Case Study
The wastewater treatment plant at Dundee, MI, required a process that could provide class B sludge for subsurface injection which wouldn't negatively impact the plant's BNR activated sludge system. Since sub-surface injection timing was limited due to regular heavy snowfall, the facility was required to have a holding time of approximately 180 days with very limited sludge hauling.
In order to increase the sludge retention time to 180 days, Arcadis Engineers integrated the P.A.D.®-K process with existing aerobic digester tanks. Only a new membrane thickening tank (MBT) was added.
