Online Phosphorus Stability Analyzer: Quantifying The Risk Of BPR Upset

INTRODUCTION
Biological phosphorus removal (BPR) has been a cornerstone of the Clean Water Services (CWS) effluent phosphorus permit compliance strategy for decades. The BPR process has historically been observed to operate stably over long periods of time, only to become upset during critical times of the phosphorus permit compliance season. For CWS, an upset condition is defined as the secondary effluent orthophosphate (OP) daily average concentration > 0.1 mg/L P. To reliably meet a stringent effluent phosphorus permit with limited chemical use, it is useful to have an early warning metric for detection of upsets in the secondary effluent. We have demonstrated the effectiveness of an online orthophosphate analyzer coined the 'Stability Analyzer' to better understand BPR Stability (Menniti et al, 2016, Bushee et al, 2018). The analyzer has been operating on aeration basin one (AB1) at the Durham Wastewater Resource Recovery Facility (WRRF) since 2017. Its location was intentionally determined at about halfway through the aerobic zone, which we define as the stability point, where generally all phosphorus uptake has occurred by this point during stable operation. The purpose of this paper is to quantify the risk of an upset event using historical stability analyzer data. A new aeration basin (AB5) recently came online at the Durham facility. This paper will also demonstrate the process of strategically locating a stability analyzer on AB5, which has a different physical geometry to AB1 (Figure 1, Table 1).
UPSET RISK QUANTIFICATION RESULTS
Through empirical observation, we found that BPR operates stably when the Stability Analyzer reads 'zero,' which we define as a daily average concentration < 0.2 mg/L P, and that the BPR process is moving toward an upset event when the analyzer becomes non-zero (Figure 2). Not all instances of non-zero daily average Stability Analyzer measurements result in upset events. Therefore, we have quantified the risk of upset using historical data. Through data analysis from summers 2018-2020, we have quantified the risk of near-future upset events. All analysis is based on daily average analyzer concentrations and excludes time periods when alum was applied to the secondary clarifiers, isolating periods of phosphorus removal solely by biological means. We observe two types of conditions where the Stability Analyzer offers early warning of upset events: A. The daily average Stability Analyzer OP concentration has read zero for at least three days followed by an increase into non-zero daily average territory (Figure 3A, 3B). B. The daily average Stability Analyzer OP concentration is already non-zero and the daily average concentration increases the next day (Figure 4). There are 18 instances of Condition A and 139 instances of Condition B. Of the 18 Condition A instances, 9 resulted in a secondary effluent upset event within the next 2 to 6 days. This equates to a 50% risk of upset. Within these 9 scenarios, the average early warning of upset is 2.8 days. Of the 139 Condition B instances, 107 resulted in a secondary effluent upset event within 3 days. This equates to a 77% risk of upset. To divert an upset, the operations analysts can re-direct VFA flow from fermentate, and if necessary, add alum in the primary and/or secondary clarifiers. The Stability Analyzer has informed process decision making on a day to day basis and has proven useful in predicting BPR stability.
FINDING THE STABILITY POINT
After success with the Stability Analyzer on AB1, CWS wanted to demonstrate that the process could be replicated for basins with different geometries. AB5 is configured with a larger anaerobic and anoxic volume than AB1 (Figure 1, Table 1). In order to find the stability point in AB1, basin OP profiling efforts began in 2013 on all four identical aeration basins at the Durham WRRF (Figure 5). The profiles measure OP concentrations in multiple locations throughout the aerobic zone, with the goal of locating the stability point (Menniti et al, 2016). The stability point for AB1 was found 58% down the aerobic zone, referred to as Cell 7B (Figure 5). However, the stability analyzer was installed in the middle of Cell 7B, 52% down the aerobic zone, rather than where the sampling point was located due to inaccessibility for an analyzer to be placed. The profiling procedure was repeated for AB5 during its first summer of operation (2021) to determine the stability point, therefore informing stability analyzer installation location (Figure 6). The sampling points focused on for AB5 were accessible and appropriate locations for installing an analyzer. It is imperative to collect the profile samples at the same time of day due to diurnal patterns observed in carbon and phosphorus loading (Figure 7). Throughout summer 2021, AB5 was profiled once a week between 9-9:30am for 3.5 months, through stable times and during upset events. Determining the stability point and therefore the location for a stability analyzer is informed by where uptake is complete during time periods when no secondary effluent upset is observed. The OP Basin Profiling efforts on AB5 suggest the ideal location for predicting stability is at Cell 6C, where 45% of the aerated volume is completed (Figure 6). Figure 8 includes the profile results from the anaerobic and anoxic zones in addition to the aerobic zone. These profiles demonstrate that there is limited correlation between anaerobic zone release and corresponding strong aerobic zone uptake. In fact, the anaerobic release is highest during a secondary effluent upset event. We will be installing the stability analyzer on AB5 in January 2022.
CONCLUSIONS
The Stability Analyzer on AB1 provides an indication of BPR stability, and has enabled us to calculate for risk of secondary effluent upset given two prominent conditions, which have been empirically observed to provide early warning of upset events. Operations analysts have used the stability analyzer data in combination with other bioassay data to determine the need to re-distribute fermentate flow and decide on alum dosing approaches to divert a major upset event during stringent phosphorus permit limits. Despite the difference in anaerobic and anoxic volumes, we conclude that the stability points for the two basins with different geometries to be about halfway through the aerobic zone, with AB1 stability analyzer located at 52% and the AB5 stability point found at 45% down the aerobic zone of the basins.
<bF>UTURE WORK
We are currently performing OP Basin Profiling on two step feed basins at our Rock Creek WRRF. The results of this effort will be included in the full paper. Operational experience with the AB5 stability analyzer will also be included from its first 9 months in service, specifically focusing on summer operation when upsets are likely to occur while under strict phosphorus permit limits.