The City of Graceville, Florida Advanced Wastewater Treatment Facility was placed into service January 1998. Florida Department of Environmental Protection (FDEP) sets the effluent requirements for this facility, as well as all other wastewater treatment facilities in the state of Florida. The permitted effluent limits include: Total Nitrogen (TN) of 3.0 mg/L, Total Phosphorus (TP) of 1.0 mg/L, Carbonaceous Oxygen Demand (CBOD) of 5.0 mg/L, and Total Suspended Solids (TSS) of 3.0 mg/L. Note that TN consists of organic nitrogen, ammonia nitrogen, and nitrate/nitrite nitrogen. In addition, the sum of the organic nitrogen and the ammonia nitrogen is equal to Total Kjeldahl Nitrogen (TKN).

In the spring of 2009, damaged airlines were repaired and the required aeration time for efficient nitrification returned to those prior to 2008 (average 30%). Examination of the operational data of September 2008 results revealed that SND had occurred for the entire month. Examination this data also revealed that despite constant aeration accompanied with low DO concentrations, the permitted TN concentration limit of 3.0 mg/L was met via SND. In early Summer 2014, air leaks were discovered once again adjacent to the blower room foundation. Further inspection revealed air leaks at joints of airline under the foundation of the blower room located at each airline exit to the SBRs. Figure 2 provides a plan view of sequential batch reactors (SBRs) and blower room at the wastewater treatment facility, and displays that air lines used for aeration are buried. The airline used in the design consists of 10” ductile iron pipe (bell end with rubber gaskets). It was discovered that the rubber gaskets at each joint of the airline had deteriorated resulting into a loss of air seal. This air loss resulted into a reduced amount of air flow into the reactors and low Dissolved Oxygen (DO) concentrations during aeration. Due to this leak, the required aeration time for efficient nitrification increased to 68%, more than double of the average required aeration time prior to 2008.

In April 2013, FDEP submitted to the Environmental Protection Agency (US EPA) its adopted nutrient standards for streams, spring vents, and lakes. The

states chose to use the hierarchal approach to determine the nutrient criterion. In doing so, the department approach gave preference to site specific analyses to provide numeric interpretation. In addition to the above, the FDEP set a goal to maintain the nutrient concentration of the water body as to prevent the imbalance in the natural populations of aquatic flora or fauna. In the case of lakes, the nutrient criterion for TP and TN was determined by the strong stressor response between those values, and the production of Chlorophyll a by phytoplankton. As a result, a criterion was established for using the calculated annual geometric mean for both nutrients and Chlorophyll a. The criteria of a Chlorophyll a, TN, and TP in general were determined by the long term geometric mean lake color and alkalinity. In the case of spring vents, FDEP used the strong stressor-response relationship between Nitrate-Nitrite and the presence of nuisance algal mats and as such the criterion for Nitrate-Nitrite was established to prevent the growth of such mats. The nutrient criterion for streams was established according to region. The criterion for Total Phosphorus and Total Nitrogen as established for using the calculated annual geometric mean for both nutrients. Note that the criterion limits for each region were general in nature. Figure 3 provides a map for the delineated regions.

The design of this process within the wastewater treatment plant consists of two sequential batch reactors (SBRs) sharing positive displacement blower aeration. Coarse diffusion is utilized for aeration coupled with AquaDDM^® mixing (Aqua-Aerobics Systems Inc. patented mixing). The design implements primarily six phases of operation which are as follows:

● Mix Fill: Default time of 60 minutes. Phase in which the influent is mixed with activated sludge; aeration is not provided in that air valve is closed. During this time period, anaerobic conditions are promoted to allow phosphate accumulating bacteria to release phosphorus.

● React Fill: Default time 120 minutes. Phase in which the influent is mixed with activated sludge to include on and off cycling of aeration in order to promote aerobic and anoxic conditions whereby nitrification and denitrification may take place. In addition, CBOD removal and phosphorus removal (luxury uptake) occurs. Note influent valve and air valve to reactor is open.

● React: Default time 60 minutes. Considered to be polishing phase the remaining ammonia, CBOD, P, and nitrate ( NO 3 − 1 ) not removed during React Fill is reduced to permitted levels. Air valve is open, however, influent valve is closed.

● Settle: Default time 74 minutes. Mixing is halted and activated sludge is allowed to settle were upon suspended and settable solids are removed (clarification). Influent valve and air valve is closed.

● Decant: Default time 46 minutes. Supernatant is removed and the reactor is decanted to predetermined low water level.

● Waste: Default time is 13 minutes. Activated sludge is removed to maintain steady Mean Cell Residence time (MCRT or sludge age).

Default times of above indicate a total of 8 batches a day for this particular facility. The number of batches may be increased or decreased based upon flow.

The two SBRs treat the domestic wastewater with a design flow of 4164 m³/day utilizing eight batches per day (each batch volume consisting of 514 m³) while sharing three positive displacement blowers. During the aeration period only two blowers would be placed into operation at one time. Two different phase times (Table 1) were utilized for optimum treatment efficiency and resulted in a two-month average daily flow was 1813 m³/day with an average hydraulic retention time (HRT) of 41.1 hours which is similar to the Orbal processes described in [9]. The average Mixed Liquor Suspended Solids (MLSS) concentration was 4200 mg/L and the average Mixed Liquor Volatile Suspended Solids (MLVSS) was 3288 mg/L. The Sludge Retention Time (SRT) over the two months averaged 33 days, also comparable to processes detailed by [9]. The Phase Time 1 adjustments resulted into a batch fill time of 396 minutes with 3.63 batches/day or 1.82 batches/basin daily. This phase time consisted of an anaerobic period of 276 minutes (mix fill phase) followed by a 396-minute treatment time (React Fill Phase 120 min. & React Phase 276 min.) with the overall process is summarized

in Figure 4(a). Due to increased influent flows, a Phase Time 2 was implemented with its required associated air structure (summarized in Table 1 and Figure 4(b)).

Eight (one per week) 24-hour flow-proportioned influent and effluent composite samples were taken during a two-month period at the wastewater treatment facility. Influent and effluent composite samples were analyzed per Standard Methods for the Examination of Water and Wastewater (SM), EPA Methods for Chemical Analysis of Water and Wastes, and HACH^© methods. Influent composite samples were analyzed for CBOD (SM5120B), Total Phosphorus as P (SM4500P E), Ammonia as N Salicylate Acid method (HACH^©), cadmium reduction method (HACH^©), pH, and Total Suspended Solids (SM2540D). Effluent composite samples were also analyzed for NO₃/NO₂ as N (SM4500NO3 E), Total Phosphorus as P (SM4500P E), Ammonia as N Salicylate Acid method [12], pH, and TKN (EPA 351.2). Influent grab samples were conducted once per shift (twice daily) and were analyzed for pH and temperature using a Fisher scientific model 15 Accumet pH meter, Reactive Phosphorus as P (SM4500P E), NO₃/NO₂ as N cadmium reduction method (HACH), Ammonia as N Salicylate Acid method (HACH^©). Dissolved oxygen and oxidation reduction potential (ORP) measurements were taken within the reactor in at the time of each sample using the HACH sc200 DO meter and the PinPoint ORP meter manufactured by American Marine Inc. Fifteen batches were sampled (7 from SBR1 & 8 from SBR2) during the two-month period. Sampling of batches with Phase Time 1 included samples being at the start and end of the batch, specifically the start of the Mix Fill phase and end of React phase, respectively. Next, was sampling at the end of the Mix Fill and React Fill phase and then at the end aeration periods. Sampling of batches with Phase Time 2 were taken at start of the Mix Fill phase and end of React phase, the end of the Mix Fill and React Fill phase, and the start of the 2nd aeration period.

During the sampling period, the City of Graceville had a census reported population of 2000 residents; its service area also included a 1500 bed State correctional facility, and a 500-bed work camp. All wastewater received was municipal in nature. Table 2 summarizes the influent 24-hour composite monthly averages for the two-month sampling period. As stated earlier, influent samples were taken twice per day once shift and analyzed for, pH, Ammonia-Nitrogen, Reactive-Phosphorus as P, NO₃/NO₂ as N, and temperature. Table 3 summarizes the average influent grab results for both months as well.

Beyond the data taken from the wastewater treatment plant samples, the effectiveness of the entire simultaneous nitrification denitrification process was also evaluated in this research. The efficiency was calculated based on the following equation [13].

SND Efficiency = NH 4 + -N oxidized -NO x -N produced NH 4 + -N oxidized × 100 % (1)

where NH 4 + -N (oxidized) is the amount of ammonia-nitrogen oxidized after the nitrification process, NO_x-N (produced) is the concentration of NO 2 − -N and NO 3 − -N.

The processes of nitrification and denitrification involve multiple bacterial communities. Pyrosequencing was used to identify the microorganism communities found in different stages of the SBR tanks, and the SBR 1 and SBR 2 tanks were evaluated both at the beginning and end of their treatment cycles. Prior to treatment, both SBRs’ microbial communities were dominated by Proteobacteria and Bacteroidetes, combined being over 80% of the totals in each. In SBR 1 before treatment (Figure 5(a)), the Proteobacteria was 44% compared to 41% of Bacteroidetes; while prior to treatment in the SBR 2, the percentages

were 49% and 33% (Figure 5(b)), respectively. These percentages of the Proteo-bacteria also paralleled what was seen in other municipal wastewater systems [14] [15]. In the initial SBRs, the predominance of Proteobacteria was expected as it is a phylum that includes bacteria that contributes to and involved in nitrogen fixation [14] [16]. After treatment, the percentage of the overall microbial communities remained ~80% Proteobacteria and Bacteroidetes, however, the Bacteroidetes became the larger component. After treatment, SBR 1 had levels of 48% Bacteroidetes and 32% Proteo-bacteria (Figure 5(c)); while these levels were 42% and 36% for the SBR 2 (Figure 5(d)), respectively. This does not mean that the Bacteroidetes’ amount increased, but rather, research has shown this phylum is not completely eliminated by the activated sludge technology when compared to other bacterial groups [17]. In each of these situations, the microbial results noted from the examined SND system were parallel to those seen in similar systems [15].

The data related to the 1^st aeration period of both Phase Times was examined for SND as the aeration periods were either preceded and/or followed by anoxic conditions (halt in aeration). The DO concentration at the end of the React Fill i.e., the end of the 1^st aeration period for Phase Time 2 or two hours into the 1st aeration period of Phase Time 1 within both SBR1 and SBR2 was below detectable levels (<0.01 mg/L). DO concentration after the 1^st aeration period for Phase Time 1 (180 minutes) was also <0.01 mg/L. The Ammonia-Nitrogen ( NH 4 + -N) concentration at the end of the Mix Fill phase (end of the anaerobic period) and the NO₃-NO₂–Nitrogen (NO₃-NO₂–N) concentration at the end of the 1^st aeration period was used to calculate the SND efficiency. The average SND efficiency for 17 batches sampled during the two-month period was 52.8%, as compared to the SND rates of 7.7% and 44.9% respectively within the lab-scale sequencing batch reactors described in [18]. This average SND efficiency did, however, parallel the SND efficiencies of 50.8% and 61.2% seen in another municipal wastewater treatment SBR system as detailed by [19].

The NH 4 + -N % removal was also calculated for each batch during the 1^st aeration period as shown in Figure 6. It was noted that the SBR-2 (Figure 7), the influent NH 4 + -N concentration was in the range between 4 - 6 mg/L during the

normal operation period. Though the NH 4 + -N concentration varied moderately on the influent side, the effluent NH 4 + -N concentration was relatively stable at concentrations below 0.15 mg/L. Despite this spike, for the SBR-2, the removal efficiencies for NH 4 + -N concentration were always >99%. This level of removal agrees with similar results shown by Baek and Pagilla [20] and Wang [21] similar influent concentrations. This data displayed the effectiveness of this system to remove NH 4 + -N regardless of the usual variability often seen in influent waste to a treatment facility. In addition, the average effluent was found to have CBOD and TSS concentrations of 4.0 mg/L and 1.1 mg/L, respectively. Based on the reported criteria, it can be noted that SND removed the necessary amounts of these components as well.

Most notably, comparison of the theoretical NO₃-NO₂–N concentration to the actual NO₃-NO₂–N concentration indicated that the combination of both SND processes provided a synergistic effect on the treatment of waste. Based on the samples examined, the SND process resulted in an average NO₃-NO₂–N concentration that was 82% lower than the theoretical concentration under the same circumstances. Furthermore, it was shown that overall nitrogen removal was generally > 99% (Figure 7), as all major nitrogen species in the effluent concentrations were less than 2 mg/L. TN was mostly comprised of the TKN, the sum of organic nitrogen and ammonia nitrogen, and NO₃-NO₂–N. The presence of these components clearly corresponds to one another as noted in the analyses of the wastewater samples seen in Figure 7. This overall relationship in the present study is similar to that of other SND systems [9] [21], but with higher removal of the total nitrogen; and the TN removal efficiency in this present study was higher than what was noted in other municipal wastewater treatment SBRs [19]. This could be attributed to the wastewater treatment design and the coupling of SND and the traditional nitrogen removal process.

The effluent composite average for TP was calculated as 0.77 mg/L and its presence in the effluent paralleled that of the TN, as noted in Figure 7. All of the effluent sample amounts were below the level of 1.0 mg/L which is/was the current concentration permitted by the state of Florida (current NPDES permit #: FL0038555 from https://floridadep.gov/). However, the averages were considerably higher than the proposed Geometric mean for TP for lakes with a range of 0.05 - 0.01 mg/L and for streams with a range of 0.06 - 0.49 mg/L [22]. Consequently, it should be noted that the facility primarily removes phosphorus biologically via the luxury uptake process. The use of metallic salts such aluminum sulfate would lower the TP concentration to that of the proposed criterion of lakes and streams to alleviate effluent concerns regarding that nutrient. However, the lower the NO₃-NO₂–N concentrations that were observed facilitates the luxury uptake process in that the lower the nitrate concentrations at the start of each batch. Shorter denitrification periods allow for longer anaerobic periods which optimize the luxury uptake process in the facility.

The average dissolved oxygen concentration at the end of the batches was 2.1 mg/L (Figure 8); however, three of the batches had a dissolved oxygen concentration of 0.0 mg/L which is contrary to the accepted dissolved oxygen concentration of 1.5 mg/L required for efficient nitrification [10]. It should be noted that the recommended DO residuals to remove ammonia require substantial aeration thus, in increase in overall operational cost and increase carbon footprint as compared to conventional facilities. Therefore, reducing the minimum DO residual required for nitrification without having a deleterious effect on overall treatment would reduce both cost and greenhouse gas emissions.

The threshold limit submitted to the EPA for TN for water bodies defined as streams was delimited according to region with the state of Florida [22]. The allowable TN within the state highlights the importance of SND efficiency to reduce these amounts in discharges to these water bodies which is greatly affected by effluents from municipal wastewater treatment plants. Table 4 summarizes the threshold limits submitted to the USEPA for TN in regional water bodies within the state of Florida defined as streams compared to the effluent TN from the examined SND process. As noted, of the 6 Florida regions evaluated, the explored SND process met the criteria for 5 of them. Only the region denoted as Panhandle West, which is known for shellfish harvesting, would require further treatment to meet the criteria.

Overall, the data suggests that the City of Graceville Wastewater Treatment plant may be able to meet the proposed FDEP’s minimum TN annual geometric mean. Also, the coupling of the SND had no adverse effect on this facility to meet the current effluent permitted criterion for CBOD, TSS, and TP.