Report did not consider primary treatment in its design analysis though this is a widely used process unit to reduce settleable solids in the treatment system.
Experienced wastewater treatment plant designers know that it is extremely unusual for utilities of the District’s size to have primary treatment. First, primary treatment robs the downstream bacteria of the energy (as measured by biochemical oxygen demand) they need to remove nitrogen and phosphorus to the very low levels that are targeted here. This would require supplemental carbon sources be hauled in to feed the nutrient removing bacteria. Second, anaerobic digestion is typically used to treat the solids that are generated by primary treatment. Anaerobic Digestion is rarely seen at treatment plants smaller than 5 mgd (the District’s design is for 0.91 mgd) as it is very expensive to construct, generates odorous, corrosive and explosive gases that trigger extensive and expensive NFPA and NEC code compliance. There’s not enough biogas generated at such small plants to practice cogeneration, so the biogas must be captured and flared. Lastly, there is very little area at the existing WRF site, and this area should be preserved to the maximum extent possible for logical treatment processes. Primary treatment would require much more space and is not a logical treatment process for the design conditions.