Flow paths, sources of wastewater, service connections, and collection system maps
Key Takeaways
- A collection system conveys domestic, commercial, institutional, and permitted industrial wastewater to treatment; stormwater should be separate unless the utility operates a combined sewer.
- Infiltration is groundwater entering through defects, while inflow is stormwater entering through direct or rapid pathways such as downspouts, area drains, sump pumps, and manhole covers.
- Combined sewers carry sanitary flow and storm runoff in one pipe; separate sanitary sewers are intended to carry wastewater only.
- Maps, GIS, as-built drawings, sketches, and asset records help operators trace flow direction, locate structures, plan isolation, and respond to backups or overflows.
- Service connection details matter because laterals, taps, protruding laterals, cleanouts, and cross-connections often explain local blockages or wet-weather flow spikes.
Follow the wastewater
Collection system operation starts with knowing where flow comes from and where it goes. WPI's current collection exam outline includes interpreting blueprints, GIS, sketches, and system records, along with locating manholes, laterals, force mains, sanitary sewers, combined sewers, outfalls, and cross-connections. The exam expects practical map-reading, not just vocabulary.
Typical flow path
A simple sanitary flow path looks like this:
- Building plumbing discharges to a building sewer or service lateral.
- The lateral connects at a tap or wye to a public gravity main.
- The gravity main carries flow through manholes and junctions.
- Flow may enter a larger trunk sewer or interceptor.
- If gravity cannot continue, a lift station pumps the wastewater through a force main.
- The force main discharges to another gravity sewer, interceptor, or directly toward treatment.
- The wastewater reaches the treatment plant headworks.
Sources of flow
| Source | Normal in sanitary sewer? | Operator concern |
|---|---|---|
| Domestic wastewater | Yes | Daily peaks, solids, rags, wipes, grease from homes |
| Commercial wastewater | Yes, if allowed | Food service grease, car wash grit, laundries, high flow variation |
| Industrial wastewater | Only if permitted and controlled | pH, toxic pollutants, high strength waste, pretreatment requirements |
| Groundwater infiltration | No | Defective pipe, joints, laterals, and manholes; wet-weather capacity loss |
| Stormwater inflow | No in separate sanitary systems | Downspouts, sump pumps, area drains, roof drains, leaky covers |
| Stormwater in combined sewers | Yes by system design | Combined sewer overflows and high wet-weather loads |
Infiltration versus inflow
Infiltration is groundwater entering through defects such as cracked pipe, offset joints, leaking gaskets, defective lateral connections, and porous or cracked manholes. It often rises more gradually with groundwater level and can continue after rain stops.
Inflow is water that enters quickly through direct or rapid pathways. Examples include roof drains, downspouts, yard drains, foundation drains, sump pumps, catch basin cross-connections, missing cleanout caps, and manhole cover holes. Inflow often creates a fast flow spike during storms.
Combined and separate systems
A separate sanitary sewer is intended to carry sanitary wastewater only. A separate storm sewer handles storm runoff. In this setting, roof drains or sump pumps connected to the sanitary sewer are usually prohibited because they consume capacity and can cause basement backups, surcharging, sanitary sewer overflows, and treatment plant overload.
A combined sewer carries sanitary wastewater and stormwater in one pipe. Combined systems exist in some older cities. During heavy wet weather, a combined sewer may exceed capacity and discharge through a permitted or regulated combined sewer overflow. This is different from a sanitary sewer overflow from a separate sanitary system.
Reading maps and asset records
Operators use system maps, GIS layers, record drawings, field sketches, and asset management records to answer operational questions:
- Which way does flow travel from a complaint location?
- What upstream area contributes to this manhole?
- Is the next pipe gravity, force main, siphon, or pressure sewer?
- Where are isolation valves, air release valves, cleanouts, and pump stations?
- Does this service connect to a sanitary sewer, combined sewer, or storm drain?
- Which pipe segment should be televised, cleaned, bypassed, or flow monitored?
Good field practice is to update records after locating missing structures, finding unmapped laterals, collecting GPS points, repairing pipe, or discovering a cross-connection. A correct map can prevent a small backup investigation from becoming an overflow response.
A sewer crew sees a sharp flow increase within minutes of a rainstorm, then flow drops soon after the rain stops. Which source is most likely?
A restaurant lateral connects to a public gravity main. Repeated blockages occur downstream of the tap, and CCTV shows greasy deposits. What should the operator suspect first?
During an emergency backup call, why should the operator check the collection system map before selecting a downstream manhole for inspection?