8.2 Wastewater Collection Systems, Inflow & Infiltration

8.2 Wastewater Collection Systems, Inflow & Infiltration

A wastewater collection network is an underground transportation utility engineered to convey municipal domestic sewage and industrial pretreated wastewater from individual customer laterals to the wastewater treatment facility. In Illinois, collection systems are regulated under 35 Ill. Adm. Code Subtitle C (Water Pollution), Part 306, along with the Recommended Standards for Wastewater Facilities (Ten States Standards). Because raw municipal sewage contains dense inorganic grit (sand, gravel, coffee grounds) and organic particulate matter, collection systems must be designed, maintained, and operated to prevent solids deposition, structural blockages, sanitary sewer overflows, and biogenic crown corrosion.

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1. Gravity Sewer Hydraulics & Minimum Self-Cleansing Slopes

The fundamental hydraulic design parameter for all municipal gravity sanitary sewers is the minimum self-cleansing velocity:

• Standard Criterion: Gravity sewers must maintain a minimum scouring velocity of $2.0\text{ ft/s}$ ($0.61\text{ m/s}$) when flowing full or half-full.
• Consequences of Low Velocity ($<2.0\text{ ft/s}$): When velocity drops below $2.0\text{ ft/s}$, dense mineral grit (specific gravity $\approx 2.65$) settles out of suspension, accumulating on the invert. These deposits constrict hydraulic cross-section, impede flow, trap fecal matter, and foster thick anaerobic biological slime blankets that generate toxic, foul-smelling hydrogen sulfide ($H_2S$) gas.
• Upper Velocity Limits: Maximum velocities are typically restricted to $10\text{ to }15\text{ ft/s}$ ($3.0\text{ to }4.5\text{ m/s}$) to prevent abrasive wear of the pipe invert from rolling grit and to avoid severe hydraulic jumping and turbulence inside receiving manholes.

Manning's Equation for Gravity Flow

Open-channel gravity sewer flow velocities are governed by Manning's Equation:

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

Where:

• $V$ = Mean fluid flow velocity in feet per second ($\text{ft/s}$)
• $n$ = Manning's roughness coefficient (dimensionless; $n=0.013$ for standard concrete and vitrified clay pipe; $n=0.009\text{ to }0.010$ for smooth PVC)
• $R$ = Hydraulic radius in feet ($R = A / P$, cross-sectional flow area $A$ divided by wetted perimeter $P$; for a circular pipe flowing full or half-full, $R = D/4$)
• $S$ = Hydraulic slope of the pipe invert in feet per foot ($\text{ft/ft}$)

Illinois Minimum Sewer Slopes

Under 35 Ill. Adm. Code 306 and Ten States Standards, all new public sanitary sewers must have a minimum diameter of 8 inches (200 mm) (except building service laterals, which are typically 4 or 6 inches). To achieve the mandatory $2.0\text{ ft/s}$ scouring velocity at a design roughness of $n=0.013$, minimum allowable pipe slopes are strictly codified:

Note on Terminal Runs: Because the initial upstream reach (terminal run) of a sewer network carries only a few homes, the hydraulic flow depth is low, making it difficult to achieve $2.0\text{ ft/s}$. Designers often steepen terminal 8-inch lines to $\ge 0.60%$ to prevent premature solids stranding.

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2. Collection Piping Materials: Performance & Applications

Sewer pipe materials must endure external earth loads, abrasive grit slurry, and biological sulfuric acid attack.

Vitrified Clay Pipe (VCP)

Manufactured from blended surface clays and shales extruded, dried, and vitrified in high-temperature kilns ($2,000^{\circ}\text{F}$), VCP is a rigid ceramic material.

• Advantages: Exceptionally inert. It is 100% immune to biological sulfuric acid crown corrosion, solvents, petroleum hydrocarbons, and industrial chemicals. It never deforms or defUtility ovalizes under deep earth burial.
• Disadvantages: Highly brittle, low tensile and beam strength, and manufactured in short laying lengths ($3\text{ to }6\text{ feet}$). Because of the high number of joints per mile, older VCP networks are exceptionally vulnerable to root intrusion, cracked bells, offset joints, and excessive groundwater infiltration.

Polyvinyl Chloride (PVC SDR 35)

PVC sewer pipe (ASTM D3034 for $4\text{ to }15\text{ inches}$; ASTM F679 for larger sizes) is the dominant material in modern collection systems.

• Characteristics: Flexible thermoplastic pipe with standard laying lengths of $14\text{ to }20\text{ feet}$. Joined using elastomeric push-on gasket bells. Exceptionally smooth interior ($n=0.009$), low weight, high abrasion resistance, and chemical immunity.
• Structural Requirements: As a flexible pipe, SDR 35 ($OD/t = 35$) relies on the structural support of surrounding compacted granular backfill (Class I or II bedding) to resist overburden. Post-installation mandrel deflection testing must verify that internal vertical pipe deflection does not exceed 5% of base inside diameter.

Ductile Iron Pipe (DIP)

DIP with mechanical or push-on joints is specified for sewer installations subject to high structural stress: shallow burials under heavy highway traffic, deep cuts exceeding 20 feet, stream crossings, bridge crossings, or parallel utility corridors lacking adequate separation.

• Internal Lining Mandate: Raw, unlined ductile iron or standard cement-mortar lined DIP is rapidly destroyed by biogenic sulfuric acid in wastewater collection systems. Ductile iron sewer pipe must be factory-lined with an impermeable ceramic epoxy (e.g., Protecto 401) or high-build polyurethane lining (minimum 40 mils dry film thickness) to seal the metal from microbial acid corrosion.

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3. Sewer Manholes: Architecture, Benching & Drop Structures

Manholes provide surface access for sewer inspection, cleaning, flow monitoring, and emergency clearing.

                     [Manhole Frame & Pickhole Cover]
                                   |
                           [Eccentric Cone]
                                   |
                         [Precast Concrete Riser]
                                   |
        [Inlet Sewer] --->         |          ---> [Outlet Sewer]
                           [Invert Benching (U-Channel)]

• Spacing and Siting: Manholes must be placed at all changes in sewer horizontal alignment (bends), changes in slope/grade, changes in pipe diameter, at all pipe intersections, and at the dead-end terminus of each line. On straight sewer reaches, maximum spacing must not exceed $300\text{ to }400\text{ feet}$ for sewers $\le 15\text{ inches}$ diameter, and no more than $500\text{ feet}$ for sewers $>15\text{ inches}$.
• Construction: Constructed from precast reinforced concrete barrel sections (ASTM C478) sealed with flexible butyl rubber joint gaskets, coated externally with bituminous waterproofing to limit infiltration. The top section utilizes an eccentric cone to center the cast-iron frame and cover over the vertical rung ladder.
• Invert Benching: The concrete floor of the manhole must never be a flat slab. A smooth, semicircular U-shaped invert channel matching the diameter and grade of the connected sewer pipe must guide wastewater through the structure. Concrete benching on either side of the channel must slope upward at $1.0\text{ inch per foot}$ (8.3% slope) toward the manhole walls to prevent wastewater stranding and solids deposition during high surcharged flows.

Drop Manholes: Criteria and Purpose

When a lateral or incoming branch sewer enters a manhole at an elevation significantly higher than the exiting invert, a drop manhole structure is mandatory:

• Threshold Criterion: A drop assembly is required whenever the invert of the incoming sewer is $2.0\text{ feet}$ ($24\text{ inches}$) or more above the manhole invert.
• Configuration: An external drop stack is installed outside the manhole barrel (or internally with an approved stainless-steel channel shield). It consists of a tee fitting on the incoming sewer, a vertical drop pipe extending down the outside of the wall, and a 90-degree curved elbow at the base discharging smoothly into the bottom channel. The upper horizontal run of the tee penetrates the manhole wall, providing a cleanout plug for inspection and rodding.
• Engineering Purpose: Dropping wastewater directly $2\text{ to }10\text{ feet}$ through the open air of a manhole creates violent splashing. This splashing damages concrete benching, coats inspection rungs in grease and feces, creates severe safety hazards for operators entering the structure, and violently strips dissolved hydrogen sulfide ($H_2S$) out of solution into the ambient atmosphere, triggering toxic atmospheric conditions and rapid crown corrosion.

4. Inflow and Infiltration (I&I): Sources & System Impacts

Extraneous, non-sewage water entering sanitary collection systems is categorized into two distinct hydrological components: Inflow and Infiltration.

HYDROGRAPH RESPONSE TO RAINFALL EVENT:
Flow (MGD)
  ^
  |            /\  <-- INFLOW PEAK (Rapid, sharp spike matching storm intensity)
  |           /  \
  |          /    \       /-----------------\  <-- INFILTRATION (Delayed, prolonged)
  |         /      \     /                   \
  |  ______/        \___/                     \________  <-- BASE SANITARY FLOW
  +----------------------------------------------------> Time (Hours/Days)

Inflow (Direct Stormwater Connections)

Inflow is clear stormwater runoff that enters the sewer system directly through deliberate, unpermitted, or deteriorated surface connections:

• Sources: Roof downspouts (gutters) plumbed into house laterals; residential sump pumps discharging basement seepage; yard and driveway catch basins; outdoor stairwell drains; low-lying manhole covers with open pickholes located in ponding street gutters; submerged manhole frames in floodplains; and direct cross-connections with municipal storm sewers.
• Hydrograph Response: Inflow creates a rapid, immediate, sharp peak on the collection system hydrograph. Flow rates surge within minutes of intense rainfall and decline rapidly following storm cessation.

Infiltration (Subsurface Groundwater Seepage)

Infiltration is clear groundwater that enters the sewer piping network indirectly through subsurface defects when the local water table rises above the pipe elevation:

• Sources: Cracked, crushed, or sheared pipe barrels; unsealed, offset, or deteriorated pipe joints; tree root penetrations; cracked building service laterals; and deteriorated, leaky manhole walls, mortar joints, and riser rings.
• Hydrograph Response: Infiltration generates a delayed, gradual, prolonged elevation in baseline flow. The hydrograph rises slowly as rainfall percolates through the soil mantle to raise the regional water table, and elevated flows persist for days, weeks, or months after precipitation ends.

Detrimental Impacts on Wastewater Infrastructure

1. Sanitary Sewer Overflows (SSOs): Surging hydraulic volumes overwhelm pipeline carrying capacities, pressurizing gravity sewers into surcharged states. Raw sewage erupts from low-lying manholes into streets, parks, and storm ditches, or backs up through residential basement floor drains, creating acute public health crises. In Illinois, SSOs are strict violations of 35 Ill. Adm. Code 306.305 and trigger mandatory 24-hour IEPA notification.
2. Treatment Plant Hydraulic Washouts: Surging wet-weather flows flood the headworks, overwhelming preliminary grit basins, cutting primary clarification detention times below 30 minutes, and scouring biological solids blankets out of secondary clarifiers across effluent weirs, destroying plant compliance.
3. Inflated Municipal Costs: Cities spend tens of thousands of dollars in electrical pumping energy and treatment chemicals (chlorine, coagulants, polymer) to collect, lift, and treat unpolluted rainwater and groundwater.

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5. I&I Detection & Collection System Assessment Techniques

Municipal utilities execute systematic field evaluation programs to identify, quantify, and eliminate I&I defects.

NASSCO PACP Standards

Condition assessment data collected during Closed-Circuit Television (CCTV) inspections is standardized using the National Association of Sewer Service Companies (NASSCO) Pipeline Assessment Certification Program (PACP). Operators assign alphanumeric codes and severity grades from Grade 1 (Minor defect) to Grade 5 (Severe defect / imminent structural collapse) for structural defects (cracks, fractures, hole voids, broken pipe) and operational maintenance defects (grease, roots, scale, encrustation).

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6. Sewer Maintenance Practices: Hydraulic & Mechanical Cleaning

Routine preventive cleaning dislodges settled solids, cuts intrusive roots, and strips biological grease coatings to maintain full pipe capacity.

Hydraulic Cleaning (High-Velocity Jetting)

High-velocity water jetting (vactor trucks) is the primary method for maintaining modern municipal collection systems:

• Operating Parameters: Triplex positive displacement water pumps deliver $35\text{ to }80\text{ gpm}$ at operating pressures of $1,500\text{ to }2,500\text{ psi}$ through a heavy-duty reinforced thermoplastic hose.
• Nozzle Dynamics: Specialized nozzles utilize rear-facing propulsion orifices (drilled at angles of $15^{\circ}\text{ to }35^{\circ}$). Water jets shooting backward generate high forward thrust that propels the nozzle and hose upstream through the sewer line toward the next manhole. As the operator slowly rewinds the hose reel under hydraulic power, the high-energy water jets scour the pipe perimeter, dislodging grease, cutting minor roots, and dragging heavy sand and gravel down the invert back to the starting manhole. A continuous positive-displacement vacuum hose (vactor tube) extracts the accumulated debris from the manhole benching for landfill disposal.
• Forward-Penetrating Nozzles: Feature front-facing cutting jets designed to bore through total solids blockages, ice plugs, or heavy grease plugs.

Mechanical Cleaning Equipment

• Power Rodders: Continuously rotating spring-steel rods ($5/16\text{ to }3/8\text{ inch}$) driven by an engine. Outfitted with augers, bullet bits, and circular root-cutter blades, rodders bore through dense tap-root blockages, solid grease plugs, and mineral deposits that resist hydraulic jetting.
• Bucket Machines: Dual motorized winches positioned over adjacent manholes pull an expandable steel clam-bucket back and forth along the sewer floor. Used exclusively for older, large-diameter trunk sewers choked with massive deposits of rocks, gravel, and heavy construction silt.
• Sewer Balls: Heavy pneumatic rubber balls with exterior spiral ribs inflated inside the sewer. Wastewater heads up behind the ball; as water squirts through the narrow annular gap beneath the ball at high velocity, it scours the pipe floor clean.

Chemical Root Control

Tree roots penetrating pipe joints expand, catching wipes and paper to form massive choke-points. Chemical root control utilizes dense, expanding chemical foam containing registered contact herbicides (e.g., diquat dibromide and metam-sodium). The surfactant foam coats the pipe circumference and penetrates joint crevices, chemically cauterizing root tips and causing them to rot away over 3 to 6 months without poisoning the above-ground tree or disrupting biological wastewater treatment processes downstream.

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7. Microbial Mechanism of Hydrogen Sulfide ($H_2S$) Crown Corrosion

Biogenic crown corrosion (microbiologically induced concrete corrosion) is the primary chemical destruction mechanism destroying concrete gravity sewers, manholes, and lift station structures worldwide.

                      [CONCRETE PIPE CROWN (AIR HEADSPACE)]
          =============================================================
          1. Condensation moisture forms on crown concrete.
          2. H2S gas rises from wastewater into crown headspace.
          3. Aerobic Thiobacillus bacteria oxidize H2S to Sulfuric Acid (H2SO4).
          4. H2SO4 reacts with Ca(OH)2 in concrete ---> Soft, mushy Gypsum (CaSO4).
          =============================================================
                                ^ H2S Gas Stripping ^
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          [WASTEWATER FLOW]
          - Dissolved Oxygen = 0.0 mg/L (Anaerobic conditions)
          - Sulfate-reducing bacteria (Desulfovibrio) in bottom slime
            reduce Sulfate (SO4^2-) to Dissolved Sulfide (H2S / HS-).
          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          =============================================================
                     [PIPE INVERT (SUBMERGED SLIME LAYER)]

The Step-by-Step Biogeochemical Reaction Cascade

1. Anaerobic Sulfide Generation in Submerged Slime: In slow, flat, or long collection mains, dissolved oxygen in the wastewater is rapidly depleted ($DO = 0.0\text{ mg/L}$). In the anaerobic biological slime layer attached to the submerged pipe invert, obligate anaerobic sulfate-reducing bacteria (SRB), primarily belonging to the genus Desulfovibrio, utilize organic carbon while reducing inorganic sulfate ($SO_4^{2-}$) in the wastewater as an electron acceptor:

$SO_4^{2-} + 2\text{CH}_2\text{O} \xrightarrow{\text{Desulfovibrio (SRB)}} H_2S + 2\text{HCO}_3^-$

1. Headspace Volatilization: Dissolved hydrogen sulfide exists in equilibrium between hydrosulfide ions ($HS^-$) and un-ionized dissolved hydrogen sulfide gas ($H_2S$). At wastewater $pH < 7.0$, the equilibrium shifts heavily toward dissolved un-ionized $H_2S$. Where wastewater experiences hydraulic turbulence (e.g., steep slopes, drop manholes, or force main discharges), gaseous $H_2S$ rapidly volatilizes (strips out) into the air headspace above the water surface.
2. Biological Sulfuric Acid Synthesis on Pipe Crown: Moisture from warm sewage condenses onto the damp concrete pipe crown and manhole ceiling. Aerobic autotrophic bacteria, specifically Thiobacillus (including Acidithiobacillus ferrooxidans and Thiobacillus thioparus), colonize the moist, dark concrete surface. Utilizing oxygen from sewer ventilation, these chemolithoautotrophic bacteria oxidize gaseous $H_2S$ into liquid sulfuric acid ($H_2SO_4$):

$H_2S + 2\text{O}_2 \xrightarrow{\text{Thiobacillus}} H_2SO_4$

1. Chemical Concrete Destruction: The concentrated biogenic sulfuric acid (which frequently drives crown surface $pH$ down to an extraordinarily corrosive $pH\ 1.0\text{ to }2.0$) chemically attacks the alkaline calcium hydroxide ($Ca(OH)_2$) and calcium silicate hydrates within the Portland cement matrix:

$Ca(OH)_2 + H_2SO_4 \rightarrow CaSO_4 \cdot 2H_2O\ \text{(Gypsum)}$

1. Structural Collapse: The reaction converts hard, durable concrete into soft, mushy, expansive gypsum ($CaSO_4 \cdot 2H_2O$) and ettringite. Gypsum possesses zero structural compressive strength; it sloughs off the ceiling into the sewage flow, exposing structural steel rebar to direct acid attack. The pipe crown thins continuously until the sewer undergoes structural crown collapse, generating massive street sinkholes.

Prevention & Corrosion Control Strategies

• Hydraulic Maintenance: Maintain scouring velocities $\ge 2.0\text{ ft/s}$ to minimize detention time and prevent anaerobic sludge buildup.
• Chemical Addition: Dose chemical oxidants or precipitants at upstream pump stations:
  • Calcium Nitrate ($Ca(NO_3)_2$): Provides an alternative electron acceptor, keeping the oxidation-reduction potential (ORP) high and preventing sulfate reduction.
  • Iron Salts (Ferric/Ferrous Chloride): Precipitates dissolved sulfide as insoluble iron sulfide ($FeS$) slurry.
  • Sodium Hydroxide ($NaOH$): Raises wastewater $pH > 8.5$, locking sulfide in the non-volatile ionic $HS^-$ form.
• Acid-Resistant Materials: Construct gravity sewers using vitrified clay pipe (VCP), PVC, HDPE, or install spray-applied 100% solids structural epoxy or calcium aluminate mortar liners inside concrete manholes.