Simultaneous Nitrification and Denitrification (SND) and Ammonia-Based Aeration Control (ABAC) were popular topics at the WEF’s Nutrient Removal and Recovery Conference in June in Charlotte NC. SN. Both are sound aeration control strategies, particularly for total nitrogen removal and minimizing energy consumption that have been around since at least the 1980’s.
SND reliably achieves very low total nitrogen levels. It is particularly effective with long detention time, complete-mix processes such as oxidation ditches and their variants including the Schreiber counter-current process. Like many great innovations, SND came about mostly by accident through undersized aeration systems.
The first several BNR projects that Mr. Bassett worked on, beginning in 1986, were three barrier oxidation ditches at Broadneck and Patuxent in Anne Arundel County and Berwick, PA. This type of oxidation ditch, patented by Inova-Tech, had a barrier wall that completely stooped flow from circulating in the ditch. It relied on an 8-foot impeller to force all mixed liquor in the ditch down a U-shaped draft tube roughly 20 feet below the bottom of the ditch, and then back up on the other side of the barrier wall. Coarse bubble air was introduced by a round sparge ring beneath the impeller, so the aerated mixed liquor would be force down the draft tube, and back up. The theory was that this would double the oxygen transfer efficiency. This of course never happened. As a result these oxidation ditches were constantly starved for oxygen, to the point where DO was normally less than 0.5 mg/l, and rarely above 1.0 mg/l, when the goal was to be at least 1.2 mg/l, ideally 2.0 mg/l. To everyone’s surprise, this “problem” produces excellent performance, with TN always below 5.0 mg/l, and averaging 3.0 mg/l. We recognized that the low DO conditions were causing simultaneous nitrification and denitrification – SND. It only happened because draft tube aeration could not supply the oxygen that was promised.
Two notable downsides were:
- SND produced a very poor-settling sludge and promoted filament growth, but these plants had generously-sized clarifiers with very surface overflow rates (SOR’s). Filaments did provide the benefit of eliminating pin floc, producing TSS and BOD consistently below 5 mg/l w/o filters.
- Ultra-low DO levels caused a lot of stress for operators who had minimum 2.0 DO pounded into their heads since operator training school. Once we all realized we were producing some of the best effluent in the country, everyone relaxed – a little.
The fourth similar plant was a Schreiber process at Maryland City, Anne Arundel County, MD, which Mr. Bassett also worked on beginning in 1988. The Schreiber counter-current process circulates mixed liquor in a continuous loop. It really is a round oxidation ditch. Schreiber had a number of innovations which proved very sound including ammonia based aeration control (ACAB). Aeration blowers could be turned on and off in response to either DO or ammonia concentrations. Low DO was the normal mode of operation at Maryland City, which also produced ultra-low TN’s, averaging 3.0 mg/l.
SND and ABAC produce very low total nitrogen concentrations, and they have the significant side benefit of minimizing electricity consumption through aeration.
