7.2 Sanitary Sewer Collection System Maintenance

8.1 Sanitary Sewer Collection System Maintenance

Sanitary sewer collection systems form the foundational infrastructure responsible for conveying raw municipal wastewater from residential, commercial, and industrial sources to treatment facilities. Proper collection system operation and maintenance (O&M) are essential to protect public health, prevent sanitary sewer overflows (SSOs), maintain structural integrity, and ensure smooth hydraulic conveyance.

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1. Gravity Sewer Collection System Hydraulics & Slope Requirements

Gravity sewer lines are designed as open-channel flow conduits driven by gravitational force. The hydraulic performance of gravity sewers is governed by Manning's Equation:

$V = \frac{1.486}{n} R^{2/3} S^{1/2}$

Where:

• $V$ = Flow velocity ($\text{ft/s}$)
• $n$ = Manning's roughness coefficient (dimensionless; e.g., 0.009–0.011 for PVC, 0.013 for smooth concrete)
• $R$ = Hydraulic radius ($\text{ft}$), defined as cross-sectional flow area ($A$) divided by wetted perimeter ($P$)
• $S$ = Slope of the energy grade line ($\text{ft/ft}$)

Velocity Thresholds & Self-Cleansing Standards

• Minimum Self-Cleansing Velocity: Gravity collection mains must achieve a minimum velocity of 2.0 feet per second (2.0 ft/s) (0.6 m/s) when flowing full or half-full at design capacity. At velocities below 2.0 ft/s, heavy inorganic grit, sand, suspended organic solids, and fats, oils, and grease (FOG) settle out of the wastewater stream. Accumulated solids cause localized blockages, septicity, anaerobic degradation, and the generation of hydrogen sulfide gas ($H_2S$), leading to odor complaints and severe crown corrosion.
• Maximum Scouring Velocity: Flow velocities should not exceed 10.0 ft/s (3.0 m/s). Velocities above 10.0 ft/s generate abrasive grit scouring that erodes invert materials, damages pipe linings, and creates excessive turbulence at downstream manholes.

Minimum Pipe Slopes by Diameter

To guarantee the minimum 2.0 ft/s self-cleansing velocity, South Carolina regulatory standards mandate minimum pipe slopes based on nominal pipe diameter:

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2. Pipe Materials & Manhole Appurtenances

Collection Pipe Materials

Selection of sewer pipe materials depends on chemical resistance, structural strength, soil loading, and joint integrity:

• Polyvinyl Chloride (PVC - SDR 35 / SDR 26): The most widely used gravity sewer pipe material. Features a low Manning's $n$ coefficient (0.009), excellent chemical and corrosion resistance, flexible joint gaskets, and high impact resistance.
• Ductile Iron Pipe (DIP): Utilized in high-load structural applications, shallow cover under roadways, stream crossings, and force main discharges. Must be lined internally with cement mortar, polyurethane, or ceramic epoxy to resist $H_2S$ acid corrosion.
• Vitrified Clay Pipe (VCP): Highly inert material immune to acid corrosion and industrial solvents. Extremely rigid, but susceptible to brittle cracking under ground shifting and root intrusion at unsealed mechanical joints.
• Reinforced Concrete Pipe (RCP): Preferred for large-diameter trunk sewers and interceptors ($>24\text{ inches}$). Requires sacrificial concrete cover or protective coatings (alkali-resistant liner) to withstand biogenic sulfuric acid attack.

Manhole Structures & Inverts

Manholes are access structures placed at maximum intervals of 400 feet for pipes $\le15\text{ inches}$ (and up to 500 feet for larger pipes), as well as at every change in pipe alignment, slope, diameter, or pipe material.

• Bench & Invert Shaping: The invert channel at the base of the manhole must be smoothly U-shaped, matching the pipe radius. Benching sloped toward the channel at $1\text{ in/ft}$ (8.3%) prevents wastewater and solids from pooling on the floor.

Drop Manholes

Whenever the invert elevation of an incoming sewer pipe is 2.0 feet (24 inches) or higher above the main manhole invert channel, a Drop Manhole must be installed.

• Purpose: Free-falling sewage splashing onto the manhole bench causes severe hydraulic turbulence, releases toxic $H_2S$ gas into the atmosphere, accelerates biogenic corrosion of concrete structures, and causes solids deposition on the bench.
• Configuration: Drop manholes incorporate an external or internal vertical drop pipe assembly that channels incoming wastewater directly down into the bottom invert channel smoothly.

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3. Inflow and Infiltration (I&I) Dynamics

Uncontaminated water entering the collection system reduces hydraulic capacity, causes sewer surcharging, triggers sanitary sewer overflows (SSOs), and drastically increases wastewater treatment plant operational costs.

Inflow vs. Infiltration Comparison

Diagnostic & Inspection Methods

1. Closed-Circuit Television (CCTV) Inspection: Remotely operated crawlers equipped with video cameras navigate pipe segments. Defect severity is coded using PACP (Pipeline Assessment Certification Program) standards developed by NACE/PCCP.
2. Smoke Testing: Non-toxic, non-staining chemical smoke is blown into isolated sewer mains under low pressure using a surface blower. Smoke emerging from roof gutters, yard drains, catch basins, or ground cracks pinpoints illicit inflow connections and structural breaks.
3. Dye Testing: Non-toxic fluorescent dye solutions are poured into storm drains, ditches, or suspected inflow sources while monitoring downstream manholes to confirm direct cross-connections.
4. Sub-Basin Flow Monitoring: Electronic open-channel flow meters record continuous diurnal hydrographs to isolate sub-catchments exhibiting high wet-weather peaking factors.

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4. Sewer Line Cleaning & Maintenance Operations

High-Velocity Hydro-Jetting

Hydro-jetting uses high-pressure water pumps operating at $\ge 2,000\text{ psi}$ (typically 2,000 to 3,500 psi at 30 to 80 gpm) connected to heavy-duty hose reels and rear-facing propulsion nozzles.

• Direction of Travel: Cleaning MUST proceed upstream (from the lower manhole toward the upper manhole, against the direction of sewage flow). As the nozzle travels upstream, high-pressure water jets scour pipe walls and break up grease. When retrieved downstream, the high-velocity flow flushes dislodged debris into the lower manhole where a vacuum truck captures the solids. Jetting downstream risks forcing sewage up domestic lateral lines into building basements.

Mechanical & Chemical Cleaning Methods

• Mechanical Rodding: Motorized machines push flexible steel continuous rods or coupled section rods equipped with augers, root cutters, or bullet heads to clear rigid blockages, mineral scale, and heavy root intrusion.
• Balling: Inflated heavy rubber balls with spiral ridges are introduced into sewer lines. Wastewater head built up behind the ball forces high-velocity water underneath, scouring sand and heavy grit along the pipe invert.
• Chemical Root Control: EPA-registered herbicidal foams (e.g., metam-sodium or dichlobenil combined with foaming agents) are injected to coat root masses. The chemical kills root tissue inside the pipe without damaging the parent tree.

Fats, Oils, and Grease (FOG) Control Programs

FOG represents one of the primary causes of collection system blockages and SSOs. Municipalities establish mandatory FOG ordinances requiring Food Service Establishments (FSEs) to install properly sized Grease Interceptors (minimum 1,000-gallon capacity outdoor underground tanks) or grease traps. Interceptors must be pumped when grease and solids occupy 25% of the total liquid volume (the 25% Rule).