Download PDF Version

Date: April 2011
Authors: Stephen Nutt, XCG Consultants Ltd.
Patrick Devlin and Krista Thomas, City of Peterborough
Vincent Nazareth, R.V. Anderson
Presented at: WEAO 2011 Technical Conference

ABSTRACT
Upgrades to the Peterborough Wastewater Treatment Plant (WWTP) are currently being completed to increase the average day flow (ADF) capacity from the current rated capacity of 60,000 m3/d to an ADF capacity of 68,200 m3/d. The upgrades to the secondary treatment process include the conversion of the conventional activated sludge (CAS) bioreactors into Integrated Fixed Film Activated Sludge (IFAS) bioreactors by the installation of free floating plastic media into each bioreactor, along with other upgrades. Although the IFAS process has been demonstrated at pilot-scale and full-scale at several Ontario WWTPs, it is relatively new in Ontario. As a result, when the MOE issued the amended Certificate of Approval (CofA) for the Peterborough WWTP, it required the City of Peterborough to undertake a “comprehensive process audit” to demonstrate that the upgraded plant will have the capability to effectively treat average day flows up to 68,200 m3/d and peak flows up to 190,900 m3/d.

In response to this requirement, a comprehensive process audit was carried out at the Peterborough WWTP to demonstrate the ability of the liquid treatment train to meet the effluent targets specified in the amended CofA under the proposed re-rated design ADF and peak flows. The long term testing covered a period from January 2009 to March 2010, a test period of approximately 15 months including two winter seasons to assess cold weather performance. All stress testing carried out for the comprehensive process audit indicated that the plant will have the ability to meet the effluent objectives specified in the amended C of A under the proposed re-rated ADF and peak flows.

KEYWORDS
Integrated Fixed Film Activated Sludge (IFAS), Re-rating, Process Audit

INTRODUCTION AND STUDY OBJECTIVES
The Peterborough Wastewater Treatment Plant (WWTP) is a conventional activated sludge plant that had been expanded to incorporate two parallel treatment trains (Plants 1 and 2), providing an average day flow (ADF) capacity of 60,000 m3/d and a peak flow capacity through primary treatment and UV disinfection of 190,900 m3/d. After extensive study of the Integrated Fixed Film Activated Sludge (IFAS) process that had been retrofitted into Plant 1, it was decided to retrofit the Plant 2 bioreactors to operate as IFAS reactors in order to cost-effectively increase the plant capacity and meet new effluent limits.

Plant 1 had been retrofitted with free-floating rigid plastic media in the first half of the first pass of the three pass aeration tanks, and equipped with mesh screens to retain the media and air scouring headers. As the IFAS process was relatively new in Ontario, MOE required that the City of Peterborough undertake a “comprehensive process audit” to demonstrate that the upgraded plant incorporating the IFAS process in Plants 1 and 2 would have the capability to effectively treat average day flows up to 68,200 m3/d and peak flows up to 190,900 m3/d when both Plants 1 and 2 had been modified.

In response to this requirement, a comprehensive process audit was carried out at the Peterborough WWTP with the primary objective of demonstrating the ability of the liquid treatment train to meet the effluent targets specified in the amended CofA under the proposed design ADF (68,200 m3/d) and peak flows. In support of the comprehensive process audit, the following testing was carried out:

  • Long term stress testing of the upgraded Plant 1 running as an IFAS process at a flow equivalent to the proposed ADF of 68,200 m3/d;
  • Long term stress testing of the primary clarifiers at a flow equivalent to the proposed ADF of 68,200 m3/d;
  • Peak flow stress testing of Plant 1 at a simulated flow equivalent to the secondary treatment peak flow capacity of 120,000 m3/d;
  • Peak flow stress testing of the primary clarifiers and UV disinfection to simulate an equivalent peak flow of 190,900 m3/d and collimated beam testing to further confirm the disinfection capacity of the UV disinfection system; and
  • Desktop review using the BioWin™ modelling to support the process audit results.

PETERBOROUGH WWTP DESCRIPTION
The Peterborough WWTP, following completed and scheduled upgrades, will provide full secondary treatment for average day flows of 68,200 m3/d and peak flows up to 120,000 m3/d, with primary treatment for flows up to 190,900 m3/d. Effluent disinfection using UV irradiation is provided for the total peak flow of 190,900 m3/d.

Primary treatment is provided through mechanical screening, aerated grit chambers as well as primary clarification. Secondary treatment will be ultimately provided in two parallel treatment trains of IFAS bioreactors and secondary clarifiers, termed Plant 1 and Plant 2. UV irradiation is utilized to provide disinfection prior to discharge into the Otonabee River. At the time of the process audit, only Plant 1 had been upgraded to the IFAS. Unit process sizes (bioreactors and secondary clarifiers) in Plants 1 and 2 are marginally different. This difference was taken into account in developing the process audit methodology and work plan.

Management of wet weather flows in excess of the secondary plant peak flow capacity of 120,000 m3/d is provided by a peak flow attenuation facility that is comprised of two parallel 4,000 m3 overflow retention tanks. These tanks receive primary treated effluent when flows exceed 120,000 m3/d and store the effluent to be returned to the secondary plant when flows decline to less than 120,000 m3/d. If the wet weather flow event is prolonged and the stored flows exceed the storage capacity, the excess flow from the peak flow attenuation facility overflows to the UV system for disinfection prior to discharge.
Biosolids stabilization of the combined primary and waste activated sludge (WAS) is provided by two primary anaerobic digesters, each with a volume of 2,444 m3, and two secondary digesters, each with a volume of 1,445 m3. Liquid biosolids were originally stored on-site in two storage lagoons. Storage of liquid biosolids has been phased out and biosolids are dewatered with centrifuges. Centrate from the dewatering process is returned to the plant headworks. The Peterborough WWTP also receives and treats hauled sewage, which is off-loaded from trucks into a receiving facility and pumped at a controlled rate to the plant headworks.

The plant currently doses ferric sulphate upstream of the aerated grit tanks to facilitate chemical precipitation of phosphorus in the primary clarifiers. Provisions for the addition of alum to bioreactor effluent also exist; however, alum is dosed on an “as needed” basis, typically at the discretion of plant operating staff when dissolved phosphorus levels in the primary effluent are noted to be elevated above typical concentrations.

Figure 1 provides a process flow diagram of the Peterborough WWTP at the time of the process audit.

Process Flow Diagram of the Peterborough WWTP

Figure 1: Process Flow Diagram of the Peterborough WWTP

TESTING METHODOLOGY
Prior to initiating the process audit, a detailed Work Plan was developed and submitted to MOE for approval. The Work Plan identified stress testing methodologies to characterize the performance of the key components of the liquid treatment train under both ADF and peak flow conditions.
Stress testing to demonstrate performance at the proposed plant ADF of 68,200 m3/d, required that flows equivalent to the re-rated plant ADF be directed through Plant 1, the plant that had been converted to the IFAS process. As noted previously, the aeration tanks in Plant 1 (5,680 m3) are marginally larger in size than those in Plant 2 (5,068 m3). As a result, in order to adequately meet stress testing conditions, more flow was diverted to Plant 1. Based on the ratio of the aeration tank sizing, a flow of 36,000 m3/d [68,200 m3/d x 5,680/(5,680 + 5,068)] was targeted for the long term stress testing of Plant 1. For peak flow testing of secondary treatment, a flow of 63,250 m3/d [120,000 m3/d x 5,680/(5,680 + 5,068)] was directed to Plant 1 based on bioreactor sizing.
Baseline monitoring of the primary clarifiers was undertaken to provide benchmark performance data for the clarifiers under current ADF conditions. This performance benchmark, as well as historical performance data, was used to assess primary clarifier performance data, was used to assess primary clarifier performance under increased hydraulic and organic loadings associated with stress testing.

The effluent objectives used to assess the performance of the plant under stress testing conditions are presented in Table 1. These target concentrations are the effluent limits currently contained in the amended CofA for the Peterborough WWTP at the re-rated capacity of 68,200 m3/d with the exception of the total ammonia nitrogen (TAN) concentrations. At the re-rated capacity, no TAN limit is specified. Therefore, the TAN objectives specified at the ADF capacity of 60,000 m3/d were used to assess performance. An un-ionized ammonia objective of 0.1 mg/L is also specified at a capacity of 60,000 m3/d. A non-toxic effluent limit (non-acutely lethal to Rainbow Trout and Daphnia magna) will also apply to the re-rated plant; therefore, monthly toxicity testing of the Plant 1 effluent was included in the Work Plan.

Table 1: Plant 1 Comprehensive Process Audit Effluent Objectives

Table 1: Plant 1 Comprehensive Process Audit Effluent Objectives

In addition, collimated beam testing was conducted to verify the ability of the UV disinfection system to meet the target level of effluent disinfection at the Peterborough WWTP under peak flow conditions.

Table 2 presents a brief summary of the sampling and analysis conducted during the comprehensive process audit.

Table 2: Comprehensive Process Audit – Stress Testing Summary

Table 2: Comprehensive Process Audit - Stress Testing Summary

Sampling Locations and Analyses
To characterize the performance of the various process components of the liquid treatment train, sampling was conducted at various locations at the plant. Autosamplers were used to collect all liquid stream samples for laboratory analysis. Flow measurements were taken from the SCADA system. With the exception of toxicity testing, all laboratory analyses were conducted at the City’s Analytical Laboratory which is certified for all the relevant analytical methodologies.

RESULTS OF PROCESS AUDIT

Long Term Stress Test
The Plant 1 long term stress test began January 1, 2009 after an acclimation period at the target flow in December 2008. Monitoring of Plant 1 was continued until March 30, 2010. Flows were diverted to Plant 1 during this period to maintain the target ADF of 36,000 m3/d. The overall ADF for the 15 month comprehensive process audit was 35,562 m3/d, or 99 percent of the target ADF. Table 3 presents the monthly average day flows (ADF) to Plant 1 during the test period.

Table 3: Average Day Flow to Plant 1 – Long Term Stress Test

Average Day Flow to Plant 1 - Long Term Stress Test

The target flows were not met in several monthly periods during the long term stress test, most notably July, August, September and October 2009. This was due to dry weather conditions during these months. The total raw sewage flow to the Peterborough WWTP during these months was in excess of the long term stress testing objective of 36,000 m3/d for Plant 1 as shown in Table 3; however, some flow had to be diverted to Plant 2 to maintain its operation.

Figure 2 presents the daily flow to Plant 1 during long term stress testing. The long term stress testing met the target ADF for the extended period of 15 months.

Average Day Flows to Plant 1 during Long Term Stress Testing

Figure 2: Average Day Flows to Plant 1 During Long Term Stress Testing

Table 4 presents the raw wastewater characteristics measured during the long term stress test and compares these with historic plant data for the period from January 1, 2006 and June 30, 2008. The measured raw wastewater concentrations do not include the effects of septage or centrate sidestream loadings.

Table 4: Raw Sewage Received at Peterborough WWTP During Stress Testing

Raw sewage concentrations observed during stress testing were consistent with raw influent sewage concentrations measured historically with the exception of TKN which during the stress test period significantly increased from concentrations observed historically at the plant. This may reflect the impact of centrate return to the plant from biosolids dewatering.

Table 5 presents the monthly average effluent quality from Plant 1 for the long term stress test. Early complications associated with the partial washout of nitrifying bacteria and the elevated TAN concentrations in January, February and March 2009 excluded, the plant was in compliance with all parameters and produced a non-toxic effluent over the entire comprehensive process audit period. During the winter/spring period of 2009 when TAN effluent concentrations were elevated, the effluent was non-toxic and the un-ionized ammonia limit of 0.1 mg/L was met. During the long term stress test, target effluent objectives for cBOD5, TSS and E. coli were not exceeded in any measured effluent sample.

Table 5: Plant 1 Effluent Quality – Long Term Stress Testing of Plant 1

Plant 1 Effluent Quality - Long Term Stress Testing of Plant 1

The TP objective was exceeded in one effluent sample (0.44 mg/L on April 29, 2009); however, the monthly average concentration for TP was met every month during the long term stress testing period.

TAN effluent objective concentrations were exceeded during January, February and March. A large peak flow event that occurred in late December 2008 at the end of the acclimation period is suspected of partially washing out the nitrifying biomass inventory in Plant 1. The sustained period of high flows and low temperatures in January, followed by several peak flow events in February and March likely inhibited the plant’s ability to re-establish the nitrifier population. Figure 3 illustrates the effluent TAN concentrations measured during the test period. Following the re-establishment of the nitrifier population in late April 2009, all monthly TAN requirements were within the seasonal effluent objectives.

Figure 3: Long Term Stress Testing Plant 1 Effluent Quality – TAN

Toxicity testing on both rainbow trout and Daphnia magna was conducted monthly during the 15 month long term stress test. Plant 1 produced a non-acutely lethal effluent for both organisms during the entire long term stress test.

Figure 4 presents un-ionized ammonia concentrations in Plant 1 effluent samples collected over test period. The un-ionized ammonia effluent objective of 0.1 mg/L was never exceeded at any time during the long term stress testing.

Long Term Stress Testing Plant 1 Effluent Quality - Un-ionized Ammonia

Figure 4: Long Term Stress Testing Plant 1 Effluent Quality – Un-ionized Ammonia

TAN Removal by IFAS Media
Additional samples were collected to assess the ammonia removal occurring through the IFAS media. Samples were originally taken at three sample points within the bioreactors. These sample points are shown in Figure 5 and labelled as Pass 1, Pass 2 and Pass 3. Prior to September 10, 2009, the influent TAN concentration was estimated based on the primary effluent TAN concentration. In subsequent tests, the aeration tank influent was sampled and analyzed for TAN. The calculated TAN removal accounts for the dilution effect of the RAS flow into the bioreactors. The recycle stream loading was calculated based on the reported Tank 1 and Tank 2 bioreactor RAS flows and the concentration of ammonia leaving the bioreactors (Pass 3).

Ammonia Sampling Locations

Figure 5: Ammonia Sampling Locations

Table 6 presents a summary of the ammonia removal in the bioreactor passes in each treatment train. Based on these data, the IFAS media, although located in the first pass of the bioreactors, is contributing significantly to the nitrification achieved in Plant 1. TAN removal in the first pass of the bioreactor that contains the IFAS media ranged from 25 percent to 90 percent.

Table 6: TAN Removal by IFAS Media

TAN Removal By IFAS Media

Peak Flow Testing
The objective of the Plant 1 peak flow stress testing was to simulate the design peak flow through Plant 1 and monitor the plant response. For each peak flow test, the primary clarifier effluent gates were adjusted to direct approximately 63,500 m3/d (the equivalent of the peak instantaneous secondary plant flow of 120,000 m3/d) through Plant 1.

Peak flow tests were conducted in April, May and December 2009 and in March 2010. The test undertaken in March 2010 simulated peak flow during colder weather conditions typical of a spring thaw and the target flow was sustained for approximately 14 hours. The effluent from Plant 1 was monitored for a total of 20 hours to measure the response after the peak flow event had passed. The results of the peak flow testing are summarized in Table 7 which presents the plant influent and the Plant 1 effluent quality during both the 14 hour peak flow period and the complete monitoring period of 20 hours.

Table 7: Peak Flow Stress Testing Results – March 17, 2010

Peak Flows Stress Testing Results - March 17, 2010

Of particular interest is response of the effluent TAN concentration to the peak flow. Figure 6 shows TAN concentration during the test. The effluent ammonia concentrations increased during the first eight hours of stress testing, but averaged 4.1 mg/L for the period of sustained high flow. Ammonia concentrations decreased during the final two hours of stress testing and averaged 3.6 mg/L during the 20 hour event. The short-term increase in effluent TAN concentration may, at least in part, be due to the higher concentration and loading of TKN in the influent during this event compared to an actual extreme wet weather event.

Peak Flow Stress Test - Plant 1 Effluent TAN

Figure 6: Peak Flow Stress Test – Plant 1 Effluent TAN

Figure 7 presents the un-ionized ammonia levels throughout the March 17, 2010 peak flow test. The un-ionized ammonia levels never exceed the 0.1 mg/L objective during the peak flow test, peaking at a concentration of 0.044 mg/L before declining to pre-stress test levels.

Peak Flow Stress Test - Plant 1 Effluent TAN and Un-Ionized Ammonia

Figure 7: Peak Flow Stress Test – Plant 1 Effluent TAN and Un-Ionized Ammonia

CONCLUSIONS
All testing carried out during the comprehensive process audit indicated that the Peterborough WWTP will have the ability to meet the effluent objectives specified in the amended C of A under the proposed ADF and peak flows.

Flows were diverted to Plant 1, which had been converted to operate as an IFAS process, to maintain the target ADF of 36,000 m3/d for a period of 15 months. As a result of wet weather and extreme high flow events during late December 2008 and January, February and March of 2009, elevated concentrations of TAN were observed in the Plant 1 effluent during this period. Following the re-establishment of the nitrifying bacteria biomass, TAN levels met the effluent objectives for the remainder of the long term stress test.

All other parameters including cBOD5, TSS, TP, E. coli and unionized ammonia were observed to be well below the effluent objectives for the duration of the long term stress test. The effluent was also demonstrated to be non-acutely toxic to rainbow trout and Daphnia magna.

The plant has demonstrated the ability to meet effluent objectives operating as an IFAS process at the proposed ADF throughout the long term stress test, which included extreme weather conditions related to both increased hydraulic and organic loading as well as temperature.

ACKNOWLEDGEMENT
The authors acknowledge the operating staff at the Peterborough WWTP for their diligence and assistance throughout the testing period.