9.4 Wastewater Collection Systems, SSMP & FOG Management

Gravity Sewer Hydraulics & Manning's Open-Channel Flow

Municipal sanitary collection networks are designed to operate primarily as gravity open-channel conduits, where raw wastewater flows downhill along sloped pipe inverts with a free water surface exposed to atmospheric air.

                                  FREE WATER SURFACE
                                    ~~~~~~~~~~~~~
                                  /               \
                                 /                 \
                                /   WET CROSS-      \
                               │    SECTION (A)      │
                               │                     │
                                ╲                   ╱
                                 ╲                 ╱
                                  ▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
                                  WETTED PERIMETER (P)

Manning's Equation for Gravity Sewers

Flow velocity and volumetric discharge in partially full circular gravity pipes are calculated using Manning's Equation:

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

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

Where:

• $V$ = Mean flow velocity (feet per second, $\text{ft/s}$)
• $Q$ = Volumetric flow rate (cubic feet per second, $\text{cfs}$)
• $n$ = Manning's Roughness Coefficient (dimensionless):
  • PVC / Vitrified Clay Pipe (VCP) / HDPE: $n = 0.009\text{ to }0.011$ (standard design baseline $n = 0.010$)
  • Smooth Precast Concrete: $n = 0.012\text{ to }0.013$
  • Aged, Corroded Concrete / Tuberculated Ductile Iron: $n = 0.015\text{ to }0.018$
• $R$ = Hydraulic Radius (feet) $= \frac{\text{Cross-Sectional Flow Area } (A)}{\text{Wetted Perimeter } (P)}$. For a circular pipe flowing full: $R = \frac{\pi D^2 / 4}{\pi D} = \frac{D}{4}$.
• $S$ = Slope of the hydraulic grade line / pipe invert slope (feet vertical fall per foot horizontal length, $\text{ft/ft}$).

The 2.0 ft/s Cleansing Velocity Rule

California collection system design standards require all gravity sewer mains to be laid at sufficient slope to achieve a minimum self-cleansing velocity of $2.0\text{ ft/s}$ ($0.6\text{ m/s}$) when flowing full or half-full.

┌────────────────────────────────────────────────────────────────────────┐
│                     Sewer Velocity Dynamics & Impact                   │
├──────────────────────────┬─────────────────────────────────────────────┤
│ Low Velocity (< 2.0 ft/s)│ Heavy fecal solids, grit, and grease settle │
│                          │ on pipe invert. Anaerobic fermentation forms│
│                          │ deadly H2S gas and Sulfuric Acid (MIC).     │
├──────────────────────────┼─────────────────────────────────────────────┤
│ Scouring Velocity        │ Keeps organic solids and grit suspended in  │
│ (2.0 to 3.0 ft/s)        │ stream; scours invert sediment clean.       │
├──────────────────────────┼─────────────────────────────────────────────┤
│ High Velocity (> 10 ft/s)│ Suspended abrasive grit scours and erodes   │
│                          │ pipe invert; high turbulence releases H2S.  │
└──────────────────────────┴─────────────────────────────────────────────┘

Hydrogen Sulfide ($H_2S$) & Microbiologically Induced Corrosion (MIC)

When sewage velocity drops below $2.0\text{ ft/s}$, settleable organic solids accumulate on the pipe bottom, creating a thick anaerobic slime layer. Sulfate-reducing bacteria (Desulfovibrio) in the slime reduce dissolved sulfate ($\text{SO}_4^{2-}$) into hydrogen sulfide gas ($H_2S$).

1. Gas Partitioning: $H_2S$ gas releases out of the sewage into the warm, humid sewer crown atmosphere (lethal toxic gas, IDLH = 100 ppm, paralyzes olfactory nerve at 100+ ppm).
2. Acid Oxidation: On the damp concrete pipe crown above the waterline, aerobic sulfur-oxidizing bacteria (Thiobacillus) oxidize the $H_2S$ gas into concentrated Sulfuric Acid ($\text{H}_2\text{SO}_4$).
3. Crown Rot: The sulfuric acid dissolves the alkaline calcium carbonate matrix of concrete pipes and unlined ductile iron crowns, causing Microbiologically Induced Corrosion (MIC), structural thinning, and eventual catastrophic roadway sinkholes/sewer collapses.

Wastewater Lift Stations & Wet Well Engineering

Where topography prevents continuous gravity drainage, wastewater lift stations collect sewage and pump it under pressure through force mains to higher elevation gravity sewers or directly to the treatment plant headworks.

                                SUBMERSIBLE DUPLEX LIFT STATION
┌────────────────────────────────────────────────────────────────────────┐
│ Influent Gravity Sewer ──► [ WET WELL SUMP ]                           │
│                                  │                                     │
│      ── High Level Alarm Elevation (Level Transducer)                  │
│      ── Lag Pump On Elevation                                          │
│      ── Lead Pump On Elevation   ▲                                     │
│                                  │ Operating Working Volume (V_eff)    │
│      ── All Pumps Off Elevation  ▼                                     │
│                                  │                                     │
│      [ Pump 1 (Submersible) ] [ Pump 2 (Submersible) ]                 │
│                   │                         │                          │
│                   └────────────┬────────────┘                          │
│                                │ (Force Main)                          │
│                                ▼                                       │
│                 [ Air Release / Vacuum Valve at Summit ]               │
└────────────────────────────────────────────────────────────────────────┘

Wet Well Sizing & Pump Cycle Time Calculations

Electric motors driving submersible non-clog centrifugal pumps generate tremendous internal heat during startup (inrush current). To prevent motor winding insulation breakdown, wet wells must be sized to limit pump starts to a maximum of 6 to 10 starts per hour ($T_{\text{min}} \ge 6\text{ to }10\text{ minutes}$).

The theoretical cycle time ($T$, in minutes) for a single operating pump is governed by:

$T = \frac{V_{\text{eff}}}{Q_p - Q_{\text{in}}} + \frac{V_{\text{eff}}}{Q_{\text{in}}}$

Where:

• $V_{\text{eff}}$ = Usable operating liquid volume between "Lead On" and "All Off" floats (gallons)
• $Q_p$ = Pump discharge capacity (gpm)
• $Q_{\text{in}}$ = Inflow rate to the wet well (gpm)

Minimum cycle time occurs when influent flow rate exactly equals half the pump capacity ($Q_{\text{in}} = 0.5 \times Q_p$), yielding the classic sizing equation:

$\mathbf{V_{\text{eff}} = \frac{T_{\text{min}} \times Q_p}{4}}$

Example Sizing Calculation

Problem: A lift station pump discharges $500\text{ gpm}$. Motor manufacturer specifications restrict the motor to a maximum of 6 starts per hour ($T_{\text{min}} = 60 / 6 = 10\text{ minutes}$). Calculate the minimum required effective wet well volume ($V_{\text{eff}}$).

$V_{\text{eff}} = \frac{10\text{ min} \times 500\text{ gpm}}{4} = \mathbf{1,250\text{ gallons}}$

Wet Well Maintenance & Force Main Management

• Bottom Slope: Wet well floors must be steep-sloped (minimum 1:1 or $45^\circ$) toward pump suction intakes to prevent grit and fecal solids from accumulating in dead corners and turning septic.
• Sewage Air Release Valves: Sewage force mains must be equipped with specialized sewage air release valves (featuring elongated conical bodies, 316 stainless steel trim, and isolation flush valves) installed at all high points along the force main profile to continuously vent entrapped sewer gas pockets without fouling.

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Inflow & Infiltration (I&I) Assessment & CCTV Inspection

Inflow and Infiltration (I&I) is extraneous clear stormwater and groundwater that enters separate municipal collection systems, consuming valuable hydraulic capacity and triggering sanitary sewer overflows (SSOs).

┌────────────────────────────────────────────────────────────────────────┐
│                     Inflow vs. Infiltration Comparison                 │
├──────────────────────────┬─────────────────────────────────────────────┤
│ Inflow (Direct Stormwater│ Rapid, immediate wet-weather surge into     │
│ Surface Entry)           │ sewer mains. Directly correlates with rain. │
│                          │ Sources: Manhole pickholes, vented covers,  │
│                          │ illegal roof downspouts, yard drains.       │
├──────────────────────────┼─────────────────────────────────────────────┤
│ Infiltration (Groundwater│ Slower, sustained groundwater seepage into  │
│ Subsurface Seepage)      │ pipes. Correlates with elevated water table.│
│                          │ Sources: Cracked/crushed pipe barrels,      │
│                          │ offset joints, root intrusions, laterals.   │
└──────────────────────────┴─────────────────────────────────────────────┘

I&I Diagnostic Tools & CCTV Inspection

1. Smoke Testing: Non-toxic white smoke is blown under low pressure into sewer segments using a gasoline-powered blower. Smoke surfacing from roof gutters, yard drains, or street storm grates quickly pinpoints illegal direct inflow connections.
2. Dye Water Flooding: Environmentally safe fluorescent dye (uranine or Rhodamine) is introduced into suspected storm ditches or catch basins while monitoring downstream manholes.
3. Closed-Circuit Television (CCTV) & NASSCO PACP: Robotic motorized crawler cameras equipped with pan-and-tilt zoom heads travel through sewer mains, recording structural and operational defects. Defects are standardized and scored per the NASSCO PACP (Pipeline Assessment Certification Program) on a 1 to 5 scale (Grade 1 = minor cosmetic defect; Grade 5 = imminent structural collapse requiring emergency trenchless lining or dig-and-replace).

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Fats, Oils & Grease (FOG) Management & The 25% Rule

Commercial Fats, Oils, and Grease (FOG) discharged by Food Service Establishments (FSEs—restaurants, bakeries, fast food kitchens) represents the single largest operational cause of collection system blockages and SSOs across California.

Hot Kitchen Wastewater ──► [ Commercial Gravity Grease Interceptor (GGI) ]
                                      │ (750 to 2,000+ Gallon Underground Vault)
                                      ▼
    ┌─────────────────────────────────────────────────────────────────┐
    │ Floating Grease Layer (F)   ──► Cooled FOG floats to surface    │
    ├─────────────────────────────────────────────────────────────────┤
    │ Clear Water Zone            ──► Clarified water flows through   │
    ├─────────────────────────────────────────────────────────────────┤
    │ Settled Food Solids (S)     ──► Heavy food scraps sink to floor │
    └─────────────────────────────────────────────────────────────────┘

The 25% Rule for Grease Interceptors

Under standard California municipal Sewer Use Ordinances (SUO), all commercial grease interceptors must be pumped out, cleaned, and fully evacuated whenever the combined thickness of the floating grease layer ($F$) plus the bottom settled solids layer ($S$) exceeds 25% of the total operating liquid depth ($D$):

$\frac{\text{Floating Grease Thickness } (F) + \text{Bottom Solids Thickness } (S)}{\text{Total Operating Liquid Depth } (D)} \ge 0.25 \quad (25\%\text{ Rule})$

• Operational Failure: When total accumulated grease and solids exceed $25%$, the effective hydraulic retention time drops below the minimum 30 minutes required for gravity separation. High-velocity kitchen flows sweep molten grease directly into public sewer mains, where it cools, saponifies with calcium into hard "fatbergs," and causes major main blockages.

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California Statewide General WDR (Order WQ 2022-0103-DWQ) & SSMP

In California, collection system management is strictly regulated by the State Water Resources Control Board under the Statewide General Waste Discharge Requirements (WDR) for Sanitary Sewer Systems (SWRCB Order WQ 2022-0103-DWQ), which superseded the historic Order 2006-0003-DWQ.

The 11 Core Sewer System Management Plan (SSMP) Elements

Every public collection agency operating more than 1.0 mile of sewer main must develop, formally certify, and implement an 11-Element Sewer System Management Plan (SSMP):

1. Goals: Measurable targets to prevent SSOs and maintain system reliability.
2. Organization: Designates the Legally Responsible Official (LRO), operational chain of command, and SSO emergency notification coordinators.
3. Legal Authority: Enforceable Sewer Use Ordinance authorizing FOG inspections, mandatory grease interceptor pump-outs, lateral repair mandates, and civil penalty enforcement.
4. Operations and Maintenance (O&M): Preventative hydro-jet cleaning schedules, mechanical rodding, CCTV inspection schedules, and lift station maintenance.
5. Design and Performance Provisions: Engineering standards for pipe slopes, materials, joint watertightness, trench compaction, and lateral connections.
6. Overflow Emergency Response Plan (OERP): Standard operating procedures for immediate spill containment, vacuum recovery, traffic control, public warning signage, and water sampling.
7. FOG Control Program: Source control permitting for all commercial FSEs, grease trap sizing, disposal tracking, and 25% rule compliance audits.
8. System Evaluation & Capacity Assurance Plan (SECAP): Hydraulic modeling to identify wet-weather hydraulic bottlenecks and prioritize Capital Improvement Projects (CIP).
9. Monitoring, Measurement & Program Modifications: Annual performance tracking (e.g., total SSOs, miles of pipe cleaned, I&I reduction).
10. SSMP Program Audits: Mandatory comprehensive audits conducted every two years (biennially) to evaluate SSMP effectiveness and implement regulatory updates.
11. Communication Program: Public outreach and educational campaigns regarding FOG disposal, flushable wipes hazards, and lateral maintenance.

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Sanitary Sewer Overflow (SSO) Reporting Mandates & CIWQS

Under California Order WQ 2022-0103-DWQ, any spill of raw sewage from a collection system or lateral is strictly prohibited. When an SSO occurs, the agency must immediately contain and recover the sewage and comply with strict electronic reporting timelines via the California Integrated Water Quality System (CIWQS).

┌────────────────────────────────────────────────────────────────────────┐
│                     California SSO Categorization & Reporting          │
├────────────┬─────────────────────────────────┬─────────────────────────┤
│ Category   │ Overflow Criteria               │ Mandatory Reporting Time│
├────────────┼─────────────────────────────────┼─────────────────────────┤
│ Category 1 │ ANY volume reaching surface     │ - 2 Hours: Notify Cal   │
│            │ water, storm drain, or drainage │   OES (if ≥1,000 gal or │
│            │ channel without 100% recovery;  │   surface water contact)│
│            │ OR any spill ≥ 1,000 gallons.   │ - 3 Business Days: CIWQS│
│            │                                 │   Draft Spill Report    │
│            │                                 │ - 15 Days: Certified Rpt│
├────────────┼─────────────────────────────────┼─────────────────────────┤
│ Category 2 │ Spill ≥ 1,000 gallons that is   │ - 15 Calendar Days:     │
│            │ 100% contained and recovered    │   Certified CIWQS Report│
│            │ before reaching surface waters. │                         │
├────────────┼─────────────────────────────────┼─────────────────────────┤
│ Category 3 │ Spill < 1,000 gallons that is   │ - 30 Calendar Days after│
│            │ 100% contained and recovered    │   end of calendar month:│
│            │ without reaching surface waters.│   Certified CIWQS Report│
├────────────┼─────────────────────────────────┼─────────────────────────┤
│ No-Spill   │ Calendar month with zero SSOs   │ - 30 Calendar Days after│
│ Cert.      │ across entire service network.  │   end of calendar month │
└────────────┴─────────────────────────────────┴─────────────────────────┘

[!WARNING] Immediate 2-Hour Cal OES Notification: For any Category 1 SSO that equals or exceeds 1,000 gallons, or any spill that enters a surface water body (lake, river, ocean, bay, or flowing storm channel), the utility must notify the California Governor's Office of Emergency Services (Cal OES) by telephone within 2 hours of becoming aware of the discharge!