Collection System Maintenance & Inflow/Infiltration (I&I)
Most of a collection operator's job is preventing two bad outcomes: blockages (which cause backups and sanitary sewer overflows) and excess flow (inflow and infiltration that overwhelms pipes and the treatment plant). This guide covers the maintenance programs that keep sewers flowing and the I&I work that keeps them from flooding.
TL;DR
- Blockages come mostly from grease (FOG), roots, and debris/grit — each has its own control program.
- Sewer cleaning (jetting, rodding, bucketing) is the routine defense; high-frequency spots get cleaned more often.
- CCTV inspection finds defects and is scored with PACP coding to prioritize repairs.
- Inflow (stormwater in directly) and infiltration (groundwater seeping in) inflate wet-weather flow and cause SSOs — finding and fixing them is a major program.
- Pair with gravity sewers & lift stations.
What causes blockages
- FOG (fats, oils, grease) congeals on pipe walls and narrows the sewer until it blocks — the leading cause of sanitary sewer overflows. Controlled with grease-trap ordinances for restaurants, public education, and targeted cleaning of grease-prone lines.
- Roots seek the moisture and nutrients in sewers, entering through joints and cracks and forming masses that snag debris. Controlled with mechanical cutting and chemical root treatment, and ultimately by sealing or replacing the pipe.
- Debris and grit — rags, wipes ("flushable" wipes are not), sand, and gravel settle in flat or low-velocity segments and build up.
Sewer cleaning methods
- Hydraulic jetting (high-velocity water) — the workhorse; a nozzle propels itself upstream and scours the pipe as it's pulled back. Best for grease, grit, and general cleaning.
- Rodding — mechanical rods with cutters, used for roots and tough blockages.
- Bucket machines — pull a bucket through to remove heavy debris and grit from large lines.
Operators run a scheduled cleaning program, cleaning the whole system on a cycle and hitting known trouble spots (grease lines, flat runs, root areas) more frequently.
CCTV inspection and PACP
A remote camera ("CCTV") is sent through the sewer to record its condition. Defects — cracks, breaks, root intrusion, offset joints, corrosion, infiltration — are logged using PACP (Pipeline Assessment Certification Program) codes, a NASSCO standard that puts a consistent severity score on each defect. PACP scoring lets a utility prioritize which pipes to clean, repair, or rehabilitate, and feeds the capital plan.
Inflow and infiltration (I&I)
- Infiltration — groundwater entering through cracked pipe, bad joints, and deteriorated manholes. It's worst where the water table is high and rises seasonally.
- Inflow — stormwater entering directly and fast through illegal roof/yard drains, sump pumps, cross-connections, and leaky manhole covers. It spikes immediately when it rains.
Why it matters: I&I uses up pipe and treatment-plant capacity with clean water that didn't need treating, and during big storms it causes sanitary sewer overflows (SSOs) — illegal discharges of raw sewage that bring regulatory penalties. Operators find I&I by comparing dry- vs wet-weather flows, smoke testing (smoke pushed into the sewer reveals inflow connections and defects), dye testing, and CCTV. Fixes range from sealing joints and lining pipe to rehabilitating manholes and disconnecting illegal inflow sources.
Rehabilitation methods
When repair beats replacement, common trenchless methods include cured-in-place pipe (CIPP) lining, manhole coatings/liners, and joint sealing (grouting). These restore the pipe and seal out infiltration without digging up the street.
Safety note
Sewer work means confined spaces and hydrogen sulfide — atmospheric testing, ventilation, and confined-space entry procedures are mandatory before entering manholes or wet wells. (Full treatment on the collections hub.)
Practice it
Tie this together with gravity sewers & lift stations and the collections operator math guide. The Wastewater Class I practice test covers collection-system fundamentals.