Michael Brown, Jr, Town of Hopedale, Hopedale, Massachusetts
Carl Stone, PE, Weston & Sampson, Carmel, New York
Rebecca Mongada, Weston &Sampson, Portsmouth, New Hampshire
Escargot may be enticing while dining at a café in Paris, but when snails invaded a wastewater treatment facility (WWTF) in Hopedale, Massachusetts, it was far from an enjoyable delicacy.
The WWTF was plagued for years by recurring operational and maintenance issues related to worm and snail infestations in their aeration tanks. The plant’s operators installed integrated fixed film activated sludge (IFAS) media in their aeration tanks in 2005 per the recommendation of a prior consultant to increase the amount of nitrifying bacteria within their process without the need for a footprint expansion. Shortly after the IFAS system was installed, worms and snails dominated the media, resulting in repeatedly taking the system down for re-seeding.
In 2013, the community began working with a new consultant that determined how the facility could operate at higher mixed liquor suspended solids (MLSS) with improved secondary clarification, while eliminating the need for the IFAS media.
Background
In 2005, the Town of Hopedale, Massachusetts began an effort to remove ammonia nitrogen from the existing wastewater treatment process at their WWTF to come into compliance with their National Pollutant Discharge Elimination System (NPDES) permit. As a result of the process of piloting and analysis, the town determined that the use of IFAS media would allow the WWTF to increase the biomass in the aeration basins, increase the solids retention time (SRT), and treat ammonia nitrogen by increasing the surface area for the growth of nitrifying bacteria.
Photo 1a. Shortly after IFAS installation, the system experience a shift in biological organisms, with predatory worms and snails dominating the media
The selected IFAS media incorporated fixed structure sheet media panels as a surface for biofilm/biomass growth. Since the media and subsequent biofilm is fixed within the aeration basins, the SRT increases without an increase in MLSS that would have increased the solids loading rate on the downstream secondary clarifiers. Shortly after installation however, the system experienced a shift in biological organisms, with predatory worms and snails dominating the media (Photos 1a and 1b) and consuming the heterotrophic bacteria responsible for wastewater treatment. This resulted in permit compliance issues for the facility, ultimately requiring re-occurring and expensive maintenance activities. This maintenance included a very labor-intensive process to remove, clean, and replace the media and re-seed the WWTF each time plugging issues with snails and worms occurred. Even planned maintenance activities to reduce the likelihood of permit compliance issues were expensive.
Photo 1b. Closeup of snails on the ground
With an eye towards addressing the troublesome process, the town’s new wastewater consultant began working on the WWTF in 2013 and immediately began considering ways to improve system operations. The technical team identified the existing secondary clarifiers as a key limitation on the process (small diameter with seven-foot side water depth). By refocusing on the entire process stream, new options for the plant were considered. A comprehensive WWTF evaluation report was prepared and submitted to the town in 2014. This report helped inform town leadership and the community on the reasons why an upgrade project was required at the WWTF. The town successfully gained support for the project and began working with their consultant in 2015 to begin the process of designing improvements to the WWTF and obtaining State Revolving Fund (SRF) loan approval to fund the project.
The consultant submitted a Preliminary Engineering Report to the Massachusetts Department of Environmental Protection (MADEP) in 2016. This report explained that the utilization of two new 50-foot (ft) (15.2-meter (m)) diameter secondary clarifiers (ultimately put into service in 2019), operating at maximum 30-day conditions of 1.3 million gallons per day (mgd) (4.9 million liters per day (ML/d)) and 3,460 milligrams per liter (mg/L) (3,460 parts per million (ppm)) MLSS, in conjunction with the existing aeration tanks without IFAS media, could maintain summertime nitrification with a SRT of eight days.
Under these design operating conditions, the hydraulic loading rate (HLR) for the clarifiers is 318 gallons per day per square foot (gpd/sf) (12,957 liters per day per square meter (L/d/m²)). The corresponding solids loading rate (SLR) is approximately 14.1 lbs. of total suspended solids per day per square foot (TSS/d/sf) (68.9 kilograms per day per square meter (kg/d/m²)). This is consistent with typical industry design recommendations. After review, these calculations continued to appear appropriate, and it was projected that the facility should be capable of meeting its permit requirements without the IFAS media.
After approval, the WWTF was upgraded to address some ageing components and to add the two new larger secondary clarifiers. These improvements to the WWTF were completed by September 2019.
Approaching Regulators Again
The town and the consultant team then approached MADEP again in early 2023 to discuss the possibility of demonstrating that the WWTF could effectively operate within permit limits by increasing MLSS concentrations within the aeration tanks. The concept was to remove the troublesome IFAS media from the aeration tanks, increase the MLSS concentrations, and fully utilize the new larger secondary clarifiers.
Working with MADEP, the town and consultant team proposed to conduct a side-by-side demonstration with the IFAS media removed from one aeration tank but remaining in the adjacent tank during both cold (six-week pilot) and warm (four-week pilot) weather seasons. This was a means of evaluating the effectiveness of nitrification during seasonal temperature changes. In consultation with MADEP, sampling was planned three times per week during both periods. Sampling was also planned once per week during the period in between the cold and warm weather pilot periods as a means of having more data for evaluation. Through written correspondence with the town in January 2023, MADEP approved the pilot testing for the facility with the following approved testing protocol:
Side-by-side sampling of the aeration tank’s contents three times per week (Monday, Wednesday, Friday) just upstream of the aeration tank effluent weir.
MLSS samples were to be drawn from each tank at roughly the same time each sampling day.
Monitor and log dissolved oxygen (DO) at each aeration tank at the time of sample collection.
Perform 30-minute settling tests on the collected MLSS samples and nitrogen series analysis.
Record MLSS, sludge volume index (SVI), and clarity observations on the supernatant after 30 minutes.
Filter the supernatant with a standard TSS filter and collect the filtered liquid in a clean test tube or Erlenmeyer flask.
Test the filtered supernatant for ammonia utilizing an Orion Ion Selective Probe and inorganic nitrogen (NOx) using a Hach DR6000 spectrophotometer.
Pilot Testing
As part of the pilot testing, the town replaced the effluent weirs on the aeration tanks, which were original to the facility and had become severely degraded (Photo 2). New stainless-steel weirs, approximately 12 inches (in.) (30.5 centimeters (cm)) taller than the original weirs, were Figure 2 goes here 5 installed in each aeration tank to improve flow control and increase the volume of mixed liquor that could be stored in each tank. Following the installation of the new stainless-steel weirs, the town removed the IFAS media from aeration tank #2 and re-established the MLSS in both aeration tanks in preparation for the pilot sampling and testing.
Photo 2. As part of the pilot testing, the town replaced the effluent weirs on the aeration tanks, which were original to the facility and had become severely degraded
Cold weather pilot testing began in late January 2023, with three samples collected and tested per week for eight weeks through mid-March. This pilot period provided two additional weeks’ worth of data compared to the minimum six-week period approved by MADEP.
Warm weather pilot testing period began in mid-July that year and continued through early August. Weekly interim testing was also completed between the end of the cold weather pilot and the beginning of the warm weather pilot. This interim period spanned 17 weeks and resulted in 17 additional data points for comparing the tanks both with and without the IFAS media (Figure 3).
Figure 1. NH3 + NOx Winter & Summer: weekly interim testing resulting in 17 additional data points for comparing the tanks both with and without the IFAS media
Unfortunately, the town’s sludge hauling contractor was unable to provide reliable and consistent liquid sludge removal during the pilot testing. This resulted in the town being unable to complete the normal annual springtime cleaning of the installed IFAS media in aeration tank #1. However, during the pilot testing, normal daily visual inspections of the tanks were conducted and the IFAS media was air-scoured as needed. Based upon the visual inspections, there were no indications of media clogging during the pilot testing period.
Permit Compliance
The town and its consultant reviewed data from the facility’s discharge monitoring report (DMR) during the months that the pilot testing was occurring. The only issues noted were a failed toxicity test in July and an earlier aluminum violation in February. To provide phosphorous removal, poly-aluminum chloride (PAC) is flow-paced at the facility with manual adjustment, and elevated flows were noted in February. The slightly elevated aluminum level is believed to be due to a manual dose adjustment required by the high flows. There were no impacts to the biomass on the IFAS media from aluminum, so this exceedance was deemed to not be relevant to the use of IFAS media.
Summary
Photo 3. Following authorization from MassDEP, the Hopedale removed the IGAS media in aeration tanks in November 2023
According to the results of the pilot study, air supply to the tanks was not an issue. DO in excess of 0.017 lb/gal (2,039 mg/L) was maintained to allow for biological treatment and to maintain nitrification. Operations staff worked to provide air scour to the tank with IFAS media when necessary via valve operation. They also temporarily directed all air flow to the tank with IFAS for 10-15 minutes once per week. This was done mainly following observation of worms within the tank during settleability testing.
In addition to maintaining DO levels and nitrification, the results of the pilot testing showed that the WWTF was able to maintain similar MLSS concentrations within each tank during pilot testing. This is largely the result of return activated sludge (RAS) being introduced and subsequently thoroughly mixed in the flow upstream of the split to the aeration tanks.
Based upon the lack of DMR violations related to parameters that the IFAS media would be expected to impact during testing, combined with the data obtained during the testing period and illustrated in this article, the team believes that the pilot testing successfully demonstrated that the WWTF does not need the IFAS media to nitrify and meet permit limits. The existing IFAS media have been a headache for operations staff almost from the beginning. They have also proven to be detrimental to the facility by forcing staff to pull and maintain the media whenever they plug. This resulted in limited treatment capacity and often required additional time, effort, and expense to re-seed the facility.
The data from pilot testing supported the position that the WWTF could remove the IFAS media and still maintain operational performance within permit parameters. Subsequent discussion with MADEP following the completion of the pilot study resulted in their concurring that the IFAS media could be removed from the aeration tanks.
Post IFAS Removal
Following authorization from MADEP, the Town of Hopedale removed the IFAS media in the aeration tanks in November 2023 (Photo 3). The WWTF has met ammonia nitrogen discharge permit limits and has not seen any exceedances for this parameter (Table 1).
Ammonia Nitrogen (mg/l) avg. monthly
Ammonia Nitrogen (mg/l) avg. monthly
NPDES Permit Limit
Hopedale Effluent
September 2023
2.0
0.08
PASS
October 2023
2.0
0.07
PASS
November 2023
8.6
1.03
PASS
**iFAS media removed
December 2023
8.6
0.08
PASS
January 2024
8.6
0.41
PASS
February 2024
8.6
0.06
PASS
March 2024
8.6
0.09
PASS
April 2024
8.6
2.79
PASS
May 2024
2.7
0.08
PASS
June 2024
2.0
0.10
PASS
July 2024
2.0
0.08
PASS
August 2024
2.0
0.06
PASS
September 2024
2.0
0.06
PASS
October 2024
2.0
0.53
PASS
November 2024
8.6
0.10
PASS
December 2024
8.6
0.08
PASS
Table 1. NPDES compliance data before and after removal of iFAS media in November 2023
The removal of the media, in conjunction with the new larger secondary clarifiers, has eliminated a re-occurring headache for the facility operators. By eliminating this recurring annual headache, operations staff have been able to focus more on daily operations and maintenance rather than reacting to emergency conditions.
Thoughts of snail and worm infestations impacting the WWTF operations are gone and now, run-ins with escargot only occur on the menu at upscale restaurants.
About the Authors
Corey Repucci, PE, is a Team Leader and Senior Associate at Weston & Sampson in Reading, Massachusetts. He earned his Bachelor of Science in Civil Engineering Technology from the Wentworth Institute of Technology and is a Registered Professional Engineer in Maine.
Michael Brown, Jr., is the Chief Wastewater Operator with the Town of Hopedale, Massachusetts. Michael has 21 years of experience as a wastewater treatment operator and has been Chief Operator in Hopedale since 2020.
Kent Nichols, Jr., PE, is a Vice President and Wastewater Practice Leader at Weston & Sampson in Reading, Massachusetts. He earned his Bachelor of Science in Civil Engineering from Northeastern University and is a Registered Professional Engineer in Massachusetts, Rhode Island, and Vermont.
Carl Stone, PE, is a Senior Technical Leader at Weston & Sampson in Carmel, New York. He earned his Master of Science in Environmental and Civil Engineering from Cornell University and his Bachelor of Science in Civil Engineering from Clarkson University. He is a Registered Professional Engineer in Connecticut, New Jersey, New York, and Virginia.
Rebecca Mongada is a Project Engineer at Weston & Sampson in Portsmouth, New Hampshire. She earned her Bachelor of Science in Environmental Engineering from the University of Vermont.
