Performance, Key Operational Parameters, and Design Targets For Nitrogen Removal During Post Aerobic Digestion (PAD): Analysis From Two Full-Scale PAD Reactors Over Multiple Years of Operation

Background
Sequential anaerobic-aerobic digestion is an under-studied, under-used process that offers potential for improved solids digestion with simultaneous nutrient removal. Previous studies have highlighted the importance of post-aerobic digestion (PAD) following anaerobic sludge digestion and reported high volatile solids removal (VSR) and chemical oxygen demand (COD) reduction along with improved sludge dewaterability (Parravicini et al. 2008, Zupancic and Ros 2008). In addition, nitrogen removal can occur via nitrification and dentrification resulting in reduced nitrogen load recycled to the head of the plant (Novak et al 2011). Limited information is available on the PAD process for full-scale systems. Thus future design and current understanding of parameters that drive this process are not well documented or understood. The PAD process has been a critical component of overall nitrogen removal at the Northern Treatment Plant (NTP) for the Metro Water Recovery in Denver and the WRRF in Boulder. Analysis of these systems is needed to help fill this critical knowledge gap.
Status These full-scale reactors have been in operation for several years. Full-scale data have been collected for multiple years at both plants during times of healthy operation and poor performance. Analysis has been conducted on this data set. The presentation and proceeds will present work on this completed study. Findings Overall, the PAD process at both Boulder and Denver resulted in substantial removal of N and reduced the returned N burden. Boulder and Denver had similar average influent ammonia concentrations of approximately 1,600 mg/L (NH4-N). Denver had a longer SRT (Table 1) than Boulder, and slightly more ammonia removal than Boulder, but both plants achieved ≥ 80% ammonia removal. The VSR was slightly higher at Denver than Boulder, but both were near 10% which has been observed in lab-scale studies as well (Novak 2011). Denver had very low nitrite and nitrate levels, both less than 5 mg/L, implying substantial denitrification following nitrification. Denver's 91% TIN removal is substantial in terms of minimizing nitrogen that otherwise would be returned to the head of the plant. Boulder had higher levels of nitrite, over 50 mg/L, resulting in 80% removal of TIN, and thus a major reduction in nitrogen that would be returned to the head of the plant. While the average ammonia and TIN removals were similar between the two plants, the nitrogen loading rate and the nitrogen rate of removal was much greater at Boulder. Boulder operated at a lower SRT and consequently had an ammonia loading rate to the PAD reactor that was double that of Denver. The ammonia removal rate per volume of reactor and per kg VS was nearly double in the Boulder reactor compared to the Denver reactor. These data represent the first data set on two full-scale PAD operators and indicate that PAD can operate well over a wide range of feed rates and SRT values. At both Denver and Boulder, higher removal rates occurred at SRT values below 15 days (Figure 1). All removal rates above 125 mgN/L-day (indicated by blue line) occurred at SRT values less than 15 days. Conversely, when SRT values were above 15 days, the ammonia removal rate never surpassed 124 mgN/L-day. Lab-scale experiments revealed more than 80% removal and 10% VSR at an SRT value of 5 days (Novak et al. 2011). Based on these selected healthy operational data, an SRT value of 6 to 8 days would be an appropriate operational target. Ammonia removal rate was linearly correlated to nitrogen loading rate (Figure 2). Boulder had higher ammonia removal rates and higher nitrogen loading rates than Denver. The linear relationship between nitrogen loading rate and ammonia removal rate did not taper off across the ranges used. Therefore, it's possible that nitrogen loading rate could be increased even more to achieve higher ammonia removal rates. This result is similar to the SRT results above, i.e. that SRT could be decreased even more which would result in a higher nitrogen loading rate. For the Boulder data set, 94% of ammonia removal rate variance was explained by nitrogen loading rate. Loading rate is therefore a major driver of performance. Denver had a lower, but still strong R2 value of 69%. Slow changes to operation rates could reveal how much beyond the 0.3 gN/L-day threshold the loading rate could be increased. Both treatment plants had extended periods of healthy reactor operation, as evidenced above. Both plants had periods of poor performance (Figure 3). At Denver, ammonia removal rate had a steady decline beginning Fall, 2020 and lasting through mid 2021. The Boulder PAD reactor was off-line from September, 2018 until mid-summer, 2019. The functional data from all of these periods were evaluated together to determine the impact of operational parameters (SRT, pH, temperature) on process performance based on N removal. In general, the reactors operated well at pH values lower than 7.5 (Figure 4). At higher pH values and higher ammonia concentrations, free ammonia (NH3) toxicity and inhibition of nitrification can become an issue. Other data not shown here revealed that sudden changes to multiple parameters such as dropping pH and temperature can have confounding impacts. The PAD system is fundamentally driven by biology; when the microbial community is perturbed process upset occurs, leading to a rise in pH and process failure. The complete set of functional data will be presented at WEFTEC, including aeration rates, anoxic/oxic cycling times, SRT, and alkalinity.
Significance of the Investigation This is the first report to compare multiple full-scale PAD reactors relative to design criteria. Information on PAD reactors even at the lab-scale is scarce in literature. This technology, though, can be readily applied at WRRFs that have available tanks. The data presented in this proceedings and presentation can be used to determine how much additional ammonia removal could be achieved via PAD systems. Several key design parameters to consider for PAD design: - Ammonium loading rate: volumetric removal rates of up to 300 mg/L-day and were strongly correlated to loading rate - Sludge Retention Time (SRT): higher removal rates were observed at lower SRTs. - pH: ammonium removal was achieved at a pH range of 6 to 7.5. Bicarbonate alkalinity is the larger driver, which will be discussed during the presentation. - Volatile solids reduction: the VSR was consistently 9 to 10% - Operational stability: one key area to highlight is the importance of SRT stability in PAD For the Denver facility, it can be seen that periods of time where the SRT is changed rapidly correspond to effluent ammonium stability (Figure 5). There may be a tendency to think of PAD as an equalization volume before dewatering, but SRT stability is stressed.