Flow paths, sources of wastewater, service connections, and collection system maps

Follow the wastewater

Collection operation starts with knowing where flow comes from and where it goes. The WPI/ABC collection outline includes interpreting blueprints, GIS, sketches, and system records, and locating manholes, laterals, force mains, sanitary sewers, combined sewers, outfalls, and cross-connections. Expect practical map-reading, not just vocabulary.

Typical sanitary flow path

1. Building plumbing discharges to a building sewer or service lateral.
2. The lateral connects at a tap, wye, or tee to a public gravity main.
3. The gravity main carries flow through manholes and junctions.
4. Flow enters a larger trunk sewer or interceptor.
5. Where gravity cannot continue, a lift station pumps the flow through a force main.
6. The force main discharges to a gravity sewer, interceptor, or directly toward treatment.
7. Wastewater reaches the treatment plant headworks.

Sources of flow

Infiltration versus inflow (I/I)

Infiltration is groundwater entering through defects: cracked pipe, offset joints, failed gaskets, defective lateral connections, and porous or cracked manholes. It tracks groundwater level, often rising gradually and continuing after rain stops. Inflow enters quickly through direct pathways: roof drains, downspouts, yard and foundation drains, sump pumps, catch-basin cross-connections, missing cleanout caps, and holes in manhole covers. Inflow creates a fast flow spike during a storm and recedes soon after. On a hydrograph, inflow is the sharp, short peak; infiltration is the broad, delayed rise.

Combined versus separate systems

A separate sanitary sewer is intended to carry sanitary wastewater only, while a separate storm sewer handles runoff. Connecting roof drains or sump pumps to a separate sanitary sewer is usually prohibited because it consumes capacity and can cause basement backups, surcharging, and treatment-plant overload. A combined sewer carries sanitary wastewater and stormwater in one pipe, common in older cities. During heavy wet weather a combined sewer may exceed capacity and discharge through a permitted, regulated combined sewer overflow (CSO).

A sanitary sewer overflow (SSO) from a separate system is an unpermitted release of untreated sewage; it is typically reportable to the regulator and demands immediate containment.

Reading maps and asset records

Operators use system maps, GIS layers, record drawings, field sketches, and asset records to answer operational questions:

• Which way does flow travel from a complaint location?
• What upstream tributary area contributes to this manhole?
• Is the next reach 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 segment should be televised, cleaned, bypassed, or flow-monitored?

Good practice is to update records after locating missing structures, finding unmapped laterals, collecting GPS points, repairing pipe, or discovering a cross-connection. An accurate map can keep a small backup investigation from becoming an SSO response.

Diurnal flow, peaking factors, and design flows

Sanitary flow is not steady; it follows a diurnal pattern. The lowest flow occurs in the early morning hours (roughly 3 to 5 a.m.), and the highest occurs after people wake and again in the early evening. A typical residential community generates on the order of 100 gallons per capita per day (gpcd) of wastewater, though the figure varies by service area. Operators describe variation with a peaking factor: the ratio of peak hour flow to average daily flow, commonly 2 to 4 for small systems and lower for large ones, because many tributaries average each other out.

The exam may give an average flow and a peaking factor and ask for the peak design flow, which is simply the average multiplied by the factor.

Knowing the diurnal pattern is practical. Crews schedule isolation, bypass pumping, and CCTV work during the early-morning low-flow window so a temporary blockage of the line is least likely to cause a backup. A flow reading that is high at 4 a.m., when domestic flow should be at its minimum, is a strong sign of infiltration or a stuck pump, not normal use.

Cross-connections and illicit connections

A cross-connection is any unintended link between systems, for example a storm drain tied into a sanitary sewer or a sanitary lateral discharging into a storm drain. In separate-sewer communities these are illegal because a storm-to-sanitary cross-connection floods the sanitary system during rain, while a sanitary-to-storm connection sends raw sewage to a waterway untreated. Smoke testing (blowing non-toxic smoke into an isolated reach and watching where it surfaces) and dye testing are the standard field tools to find them: smoke rising from a roof gutter, yard area drain, or downspout reveals a direct inflow connection.

The map and asset record should be updated the moment a cross-connection is confirmed so the next operator knows it exists.