OC San P1 and P2 Operating Range Values (ORVs) and plant performance during sampling period.

AS = activated sludge, TF = Trickling Filter, SC = Solids Contactor, MCRT = mean cell residence time, BOD-T = total biological oxygen demand. MCRT for all secondary treatment processes are calculated daily by OC San using the below equations. OCWD proposes to use the OC San-calculated values of MCRT for assessment of ORVs related to LRV credit value. BOD-T (P1 TF) is measured by OC San as a daily composite. Mean Cell Residence Time (MCRT) Calculations: P1 Activated Sludge Processes MCRT = (Volume of reactor x MLSS) ÷ [(WAS flow x WAS MLSS) + (Effluent Flow x Effluent TSS)]; where MLSS = mixed liquor suspended solids, WAS = waste activated sludge, and TSS = total suspended solids. P2 Trickling Filter Solids Contactor MCRT = [(Volume of reactor x MLVSS) + (Volume of reactor RSS VSS)] ÷ [(Waste Flow x RSS VSS) + (Effluent Flow x Effluent VSS)]; where MLVSS = mixed liquor volatile suspended solids, RSS VSS = raw sewage sludge volatile suspended solids, and VSS = volatile suspended solids. (XLSX)</p

Statistical attributes of the log removal values calculated for each microbial target using the covariance-based approach and the modified Monte Carlo approach.

Statistical attributes of the log removal values calculated for each microbial target using the covariance-based approach and the modified Monte Carlo approach.</p

Summary of the log removal values (LRVs) generated by the covariance analysis approach for cultivable enteric viruses, enterovirus (ddPCR) and norovirus GII (ddPCR), MS coliphage, and SOM coliphage.

Summary of the log removal values (LRVs) generated by the covariance analysis approach for cultivable enteric viruses, enterovirus (ddPCR) and norovirus GII (ddPCR), MS coliphage, and SOM coliphage.</p

Plant performance, operational parameters, and Operating Range Values (ORVs).

Operating range values, or ORVs, were defined in this study as parameter values that describe typical and normal treatment process operations, as observed during this study over the microbial monitoring period. ORVs were derived for each secondary treatment process supplying influent to OCWD AWPF, i.e., P1 TF, AS1, AS2 (current) and P2 TF/SC (future). An exceedance of a designated ORV represents a deviation from the normal treatment performance. This deviation could be related to an unexpected event but also due to planned activities such as operational maintenance of treatment systems or flow adjustments by the OC San operators. These ORVs are proposed to be used for contingent LRV virus credits for GWRS, as with prior pathogen crediting schemes from wastewater treatment for potable reuse in California. This approach is preferred by California regulators such that any virus removal observed from a one-time, past approved study that is used as the basis for ongoing WWTP virus removal credit is only awarded if the WWTP day-to-day performance is normal and acceptable (i.e., within the established ORVs). It should be noted that the ORVs are not directly related to any benchmark, performance goal, or effluent limitation stated in the OC San National Pollutant Discharge Elimination System (NPDES) permit. Thus, failure to meet the ORVs does not signify secondary effluent is of poor quality or unsuitable for reclamation, but rather that the effluent does not meet the normal conditions observed during the microbial sampling study associated with the observed virus removal value and therefore the credit may not be applicable. As a result of the present study, OCWD has proposed to DDW the herein described ORVs framework where each of the four OC San effluents serving OCWD GWRS features an ORV, as does the combined (blended) effluents in the form of Microfiltration Feed (MFF) and Microfiltration Effluent (MFE) at GWRS, all of which must meet their respective ORVs in order to receive virus LRV credit. To determine a recommended ORV for each OC San treatment process, performance data from P1 and P2 were collected as part of routine plant monitoring during the LRV study sampling period. Parameters chosen to develop operational envelope ORVs are shown further below in S3 Table. Performance data were reviewed and analyzed by calculating 30-day averages and interquartile ranges. In addition to ORVs from OC San’s treatment processes, GWRS AWPF MFF and MFE monitoring locations were also selected to represent normal GWRS microfiltration influent and effluent quality. ORVs for each selected parameter were calculated using a baseline threshold equation from 30-day average data as follows: (Eq S1) (Eq S2) where: Q1 = 25th percentile of the 30-day running average dataset, Q3 = 75th percentile of the 30-day running average dataset, and IQR = Interquartile range, defined as Q3 –Q1. The baseline threshold approach (i.e. defined by interquartile ranges) was used to define excursions of baseline conditions of a treatment process using a statistical model derived from a large dataset. Lower thresholds are defined by values below the 25th percentile by 1.5 times the IQR, while upper thresholds are above the 75th percentile by 1.5 times the IQR, respectively. (DOCX)</p

Summary of percent recoveries for all virus targets.

Average recovery percentages shown in bold were used to correct each respective native dataset, e.g., 120% recovery was used to correct all native P1 raw influent samples for SOM coliphage. The average percent recovery used to correct the native cultivable enteric virus dataset was obtained by taking an average of SOM, MS and Poliovirus recoveries, which are shown as the bold mean values for Poliovirus. An average recovery value was not used to correct the native molecular (enterovirus and norovirus GII) datasets. (XLSX)</p

OC San simplified flow diagrams.

Simplified flow diagrams illustrating secondary treatment trains and the study sampling locations. (A) Two parallel secondary treatment trains at OC San Reclamation Plant No. 1. Plant secondary clarifiers that follow TF, AS1, and AS2 have engineering differences illustrated simplistically in the diagram. (B) Trickling Filter/Solids Contactor (TF/SC) secondary treatment train at OC San Treatment Plant No. 2. (TIF)</p

Fig 2 -

Probability distributions for cultivable enteric virus concentrations obtained from raw influent and secondary effluent samples taken at OC San P1 (left) and P2 (right). Each point represents one sampling event and the solid line represents a best-fit regression. The coefficient of determination (R2 value) is also shown. Raw influent and secondary effluent cultivable enteric virus data obtained from both P1 and P2 are lognormally distributed.</p

Monte Carlo simulation.

For the Monte Carlo approach, LRVs for each microbial target were calculated using the MATLAB software (1984–2020 MathWorks, Inc., version R2020a 9.8.0.1359463), equipped with the Statistics and Machine Learning Toolbox. Microbial concentration data were imported into the MATLAB software using a simplified tab-delimited file. Once imported, a statistical model for each influent and effluent dataset for a given microbial target was generated using the maximum likelihood estimates function. Briefly, influent and effluent microbial concentrations were used to generate a statistical distribution. From these modeled influent and effluent distributions, one independent and random value was selected from each distribution and subsequently paired to calculate an LRV as shown in Eq S3: (Eq S3) Where Ceff is the concentration of the microbial target taken from the secondary effluent distribution, and Craw is the concentration of the microbial target taken from the raw wastewater distribution. This calculation was performed 10,000 times to generate a distribution of n = 10,000 LRVs. All LRVs were then sorted from low to high and assigned a rank, i, over the total number of data points, n. A cumulative probability, p, for each value was assigned as shown in Eq S4. (Eq S4) Where p is the cumulative probability, i is the rank assignment, and n represents the total number of calculated data points (10,000). The Monte Carlo simulation approach was also modified for the present study to generate only non-negative LRVs (n = 10,000+ non-negative log removal values). This was done because the standard Monte Carlo simulation approach generated a fairly large number of negative LRVs as a portion of the total 10,000 LRVs, which reduced the resulting 5th percentile LRV. To address the negative LRVs, approximately 1,000 additional LRVs were calculated for a total of 11,000 samples. Negative LRVs within the n = 11,000 dataset were removed such that the remaining number of positive LRVs were at least n = 10,000. This modified (censored) Monte Carlo approach was used to determine process-specific LRVs, imposing a condition of reality on the statistically determined outcome. A modified Monte Carlo simulation was executed to address the negative LRVs calculated for the P1 TF and P2 TF/SC distributions when using the standard Monte Carlo approach. While this only occurs a fraction of the time, these negative values are recorded as a possible LRV at the low end of the percentile distribution. The calculated negative LRVs were believed to be a mathematical artifact of the Monte Carlo’s random pairing of influent-effluent values, specifically attributed to the large overlap in concentration values observed between the raw influent distribution and OC San P1 TF effluent distribution. The modified (censored) Monte Carlo approach was used to determine process-specific LRVs, imposing a condition of reality on the statistically determined outcome. Negative LRVs generated with the modified Monte Carlo simulation were removed such that the remaining number of positive LRVs were at least n = 10,000. It is physically impossible to generate a negative LRV for an enteric virus during wastewater treatment, as virus cannot be created within primary or secondary treatment processes due to the lack of a host organism. Despite the attempt to resolve this issue by censoring data to remove negative LRVs, this estimation is not representative of the actual low-end virus log removal observed for all four treatment processes (Table 1). (DOCX)</p

OC San sampling location description.

A total of six (6) sampling locations from OC San P1 and P2 were monitored. Sampling locations for OC San P1 were raw wastewater influent, trickling filter (TF) secondary effluent, activated sludge 1 (AS1) secondary effluent, and activated sludge 2 (AS2) secondary effluent, while sampling locations from OC San P2 consisted of raw wastewater influent and trickling filter/solids contactor (TF/SC) secondary effluent. Sampling at P2 was limited to characterizing the TF/SC process and not other parallel treatment processes. Raw wastewater entering OC San P1 is treated through preliminary screening and primary clarification with chemical addition and is then diverted into one of two secondary treatment trains that operate in parallel (trickling filter process or activated sludge process). Raw wastewater from P1 was collected after primary bar-screening but before primary clarification and chemical addition. Secondary effluents generated by three parallel treatment processes at P1 were sampled in this study. The first P1 treatment train routes the primary effluent through a trickling filter (TF) process followed by secondary clarification. Treated effluent from the TF process was sampled. P1 primary effluent is also sent through two parallel trains of the activated sludge (AS) treatment trains, designated separately as AS1 and AS2. Secondary effluent samples taken from each AS process following secondary clarification was sampled. Both AS trains operate in the nitrification-partial denitrification (NDN) mode. The major difference between the AS1 and AS2 processes is that AS1 does not receive mixed liquor return, while the newer AS2 facility does receive it. Microbial concentrations of both the AS1 and AS2 effluent streams from P1 were of interest due to the operational differences between the two processes described above. (DOCX)</p

Performance monitoring and ORV results.

Performance data from OC San P1 and P2 were collected to determine a recommended operating range value (ORV) for each OC San treatment process. Total biological oxygen demand (BOD-T) was selected as the ORV for the OC San P1 TF process. An upper baseline threshold for BOD-T is proposed because higher-than-normal BOD-T (30-day running average of > 26 mg/L) in this sampling location would suggest deviations from the typical performance that was documented during the enteric virus sampling. Lower baseline thresholds of Mean cell residence time (MCRT) was selected and proposed as the ORV for OC San P1 AS1, P1 AS2, and P2 TF/SC treatment processes. Lower thresholds were chosen because a lower-than-normal MCRT (Giardia and Cryptosporidium removal credit. In addition to this, OC San effluent must meet a specified turbidity requirement to provide blended secondary effluent to the AWPF. Using the described ORVs framework for the overall potable reuse project, each of the four OC San effluents serving GWRS features an ORV, as well as the combined (blended) effluents in the form of MFF and MFE, must meet their respective ORVs to receive virus LRV credit. (DOCX)</p