Civica Infrastructure Inc. is a leading consultant in the wastewater management industry specializing in inflow & infiltration (I&I) reduction.
Civica was hired by two developer groups to investigate and fix I&I issues within the Town of Newmarket. The goal was to reduce clean rainwater entering the sanitary system and improve sewer system capacity for future developments.
During the investigation, it was discovered that clean water from roads was entering the sanitary sewer system through pick holes on the maintenance hole (MH) covers, causing unwanted inflow. As a solution, Civica proposed sealing the sanitary MHs to prevent surface inflow and assigning appropriate capacity for future developments.
To address concerns about pressure and gas buildup, a MH gas monitoring project was conducted in Newmarket. The program involved installing sealed frame and cover systems on pilot MHs and conducting an eight-month study on sewer gas buildup in those MHs, as well as the immediate upstream and downstream MHs. This study was necessary for the remediation of other target MHs as per the Town’s requirement.
Objective of MH Gas Monitoring Project (Town of Newmarket)
The project’s objective was to install, operate, and analyze data from nine gas monitoring locations. The central aim was to assess the long-term impacts of sealed MH systems (Hamilton Kent or Trumbull, specifically) on sewer gas buildup. With the information obtained, Civica evaluated whether further MH sealing would be feasible for targeted low-lying MHs that are susceptible to high inflow.
Gas Monitoring Locations
The sewer gas generation monitoring program included three sealed MH monitoring locations and six MH monitoring locations at the nearest accessible upstream and downstream unsealed MHs.
MH Gas Monitoring Methodology
Sealed MHs
For monitoring in fully sealed MHs, gas meter chambers were excavated and installed near the target sealed MH. These chambers, at road grade or in the boulevard, housed the monitoring equipment and sampling compartment. Gas samples were taken through an intake hose that connected the sampling compartment to the MH. The samples were then returned through an exhaust hose running from the sampling compartment to the sealed MH.
Monitoring was done within one to two meters of the lid. Hydrogen sulfide (H2S) concentration and air pressure were automatically sampled hourly. Oxygen (O2), methane (CH4), and carbon monoxide (CO) were manually sampled using the intake and exhaust hose every month during maintenance visits.
In July and August, when there was a special interest in hot weather impacts on gas generation, sampling was performed every 10 days. When maintenance was required for the gas meter, the gas meter chamber was opened, and the intake and exhaust hoses were clamped shut. All maintenance and calibration work was completed without opening the sealed MH lid or introducing fresh air into the MH.
Unsealed MHs
H2S monitors were installed in the control (unsealed) MHs at nearby upstream and downstream sites of sealed MHs. These monitors automatically took samples on an hourly basis within one to two meters of the lid. O2, CH4, and CO were manually sampled using an intake hose inserted through the pick hole during monthly maintenance visits.
Since these MHs were not sealed and open to the atmosphere, opening the MH lids for maintenance and calibration did not affect the results. The pressure was not monitored in non-sealed MHs.
Monitoring Equipment
- Manual Grab Samples: H2S gas monitors are known to have cross-sensitivity to continuous exposure to high temperature and high humidity. Therefore, grab samples were taken to verify H2S readings. Industrial Scientific Ventis MX4 multi-gas monitors were used to collect grab sample concentrations of H2S, CH4, and CO.
- Continuous H2S samples: To ensure redundancy, two types of industrial H2S gas monitors were used for hourly sampling of H2S in the selected MHs: SPEC Sensors HYD SULF and Acrulog PPM LD200. Like most continuous H2S gas monitors, the sensors require removal and drying every 30 days when used in humid environments, which limits their effectiveness. As a result, monthly equipment swaps, along with maintenance and calibrations, were conducted.
- Hourly Pressure Samples: The BMP280 measured absolute air pressure in the three sealed MHs. To convert it to gauge pressure, or the pressure differential between the surface and sealed MH, the surface’s absolute pressure was identified and subtracted from the MH pressure. This required an additional BMP280 in an unsealed MH.
Exposure Limits*
- For H2S: Currently, safe working levels for H2S are between 0 and 15 parts per million (ppm) with a recommended eight-hour exposure limit of 10 ppm.
- For O2: Oxygen concentrations between 19.5% and 23.5% is considered acceptable.
- For CH4: When asphyxiants like methane are present, it is recommended to maintain atmospheric oxygen levels above 19.5% volume. Methane, being explosive, should be kept below the lower explosive limit of 5% volume or 50,000 ppm.
- For CO: The safe working levels for CO are between 0 and 25 ppm.
- Corrosion Concerns: To avoid corrosion related to H2S, it is advised to maintain average hydrogen sulfide levels below 5 ppm. Pressure, oxygen, methane, and carbon monoxide levels do not pose a corrosion concern.
- For the Trumbull Bolt-Down Lid: It is recommended to keep the pressure below 113 PSI to prevent the pull-out of threaded inserts.
Summary of Gas Monitoring Results
Temperature
During the monitoring period, several significant heat waves were observed, contributing to the understanding of gas concentration responses in different conditions. Around six heat waves with durations of four days or more, where the average daily high temperature surpassed 27° C, were recorded. The monitoring period also captured an extreme temperature of -16° C in late fall.
Hydrogen Sulfide Concentration
H2S generation in sanitary sewers is typically associated with stagnant water, debris buildup, and nutrient-rich wastewater. Atmospheric H2S levels above 5 ppm can lead to corrosion, while sustained exposure to levels above 15 ppm can pose health risks. However, grab sampling did not detect any H2S traces at the tested sites, and all collected data remained below specified thresholds.
Oxygen, Methane, and Carbon Monoxide
Civica measured oxygen and methane levels using manual grab sampling. All collected data was well below the specified thresholds. The slightly reduced oxygen levels in sealed MHs compared to typical air composition suggested minimal sewage decomposition and slower but still sufficient ventilation compared to unsealed MHs.
Pressure
Pressure in the three sealed MHs was continuously monitored and closely mirrored the local atmospheric pressure. The differential pressure remained within instrument error margins. This data indicated that there was an air connection between the sealed MHs and the open air, allowing pressure equalization as atmospheric pressure changed with weather.
Conclusions
Based on the report’s findings:
- No elevated levels of H2S or CH4 were observed in either sealed or unsealed MHs during the monitoring period. Air pressure levels were also insignificant, and O2 levels remained within the safe range.
- Continuous monitoring showed near-zero average levels of H2S in all MHs, with peak levels within the recommended threshold, highest in unsealed MHs.
- Gas concentration levels dissipated quickly in both sealed and unsealed MHs, indicating sufficient ventilation. Ventilation primarily occurs through the headspace or air space in the sewer pipe connecting the MHs, with secondary ventilation through the MH riser and sealed MH lid, which are not airtight nor completely watertight.
- A total of six heat waves with average daily highs above 27° C were recorded, with the period from July 18, 2019, to July 21, 2019, being the hottest of 2019, reaching an average daily high of 32° The most extreme temperatures measured were 32.9° C on July 20, 2019, and -16.1° C on November 13, 2019.
Recommendations Proposed by Civica’s Wastewater Management Consultants
Civica made the following recommendations from the project findings:
- Proceed with further MH sealing projects to reduce surface inflows to the sanitary system, as no harmful levels of noxious gas or air pressure have been detected and measured under varying weather conditions.
- Compare ventilation rate or air change rate between sealed and unsealed MHs using CFD or injection of benign gas into a test MH and monitoring concentration samples to measure decay rate.
- Continue monthly grab samples in sealed MHs after the eight-month monitoring period.
- If necessary, install pressure relief vents in sealed MHs to prevent pressure buildup under surcharge conditions.
- Install manual sampling ports in future sealed MHs to allow regular grab samples without opening the lid or introducing fresh air, for further assessment of performance under different site conditions.
- Implement a health and safety policy stating that sealed MHs must have air sampling ports and air quality must be tested before opening the lid.
- Analyze flow at proposed sealed MH locations to ensure adequate hydraulics within the system.
Contact Civica for Your Wastewater and Rainwater Management Needs
Civica is at the forefront of water management solutions in southern Ontario, with extensive experience in sewage water management. Civica specializes in a wide range of field and consulting services including sanitary manhole hole sealing, MH maintenance, sewer inspection services, sewer flow monitoring, wastewater monitoring, and wastewater system modeling.
For more information, please contact Civica today.
*We recommend reading the applicable guidance from the province. Civica are not experts in gases and the requirements could be subject to change.
Learn more at:
Sanitary Maintenance Hole Sealing and Approaches
Methodology of Sanitary Maintenance Hole Inflow and Infiltration
Inflow and Infiltration Inspection Case Studies
