NITRITE-DOMINATED CENTRATE HEADWORKS RECYCLE: SIDESTREAM AMMONIA OXIDATION

For wastewater treatment plants with required nitrogen removal, 15% or more of the ammonia loading on secondary treatment can come from centrate recycle from in-plant solids handling facilities. This “extra” load can be difficult to negotiate, particularly if available carbon is already marginal. The 91st Avenue Multi-Cities WWTP (179.25 mgd capacity) employs a single-stage denitrification/nitrification/denitrification process that achieves 80% total nitrogen removal at current flow and load. 91st Avenue has the ability to apply advanced primary treatment chemicals to assist in maintaining aeration SRT, and methanol for reducing final effluent nitrate concentration, but no direct means for reducing the centrate ammonia load prior to headworks recycle, or for maximizing nitrification rate in the liquid stream aerators.91st Avenue has decided to convert its 27 million gallons of mesophilic anaerobic digester volume to a multi-phase process (a mesophilic acid phase, followed by a thermophilic gas phase, and possibly followed by a mesophilic gas phase). This conversion will produce class A digested sludge, and reduce the need for digester expansion. It is also hoped that it will increase volatile destruction, and improve dewaterability. It is also predicted that digested sludge dewatering centrate ammonia concentration will increase from the current peak of 650 mg/L to 1,300 mg/L as a result of the new process. This would increase the overall ammonia load on secondary treatment by about 10%.The design for the next upgrade and expansion project is ongoing. Instead of building more liquid stream aeration capacity, the design team has incorporated the 10% increase in ammonia loading from multi-phase digestion into the design by planning for a centrate treatment facility. The team selected two sidestream treatment schemes for consideration: 1) a high temperature, high pH, short HRT approach that would oxidize ammonia to nitrite, and 2) a more conventional SRT approach that would oxidize the ammonia to nitrate. The effluent from either approach would be recycled to the headworks to assist with odor control, and for at least partial denitrification prior to aeration. Based upon the evaluations of the two approaches, a conceptual design for the centrate treatment facility is in progress that will have sufficient flexibility to allow plant staff to operate in either mode.