Wastewater Sources, Characteristics, and Flow
Core concepts
Understand where flow comes from (domestic, commercial, industrial, infiltration, inflow) and how strength varies. Key parameters: BOD, TSS, pH, ammonia, and for collection operators especially hydrogen sulfide generation, fats/oils/grease, and debris. Fresh wastewater is relatively low in sulfide; as it turns septic in slow or long detention, sulfate reduces to sulfide and sulfuric acid corrodes concrete above the waterline (crown corrosion).
Flow concepts
Know the difference between average dry-weather flow, peak wet-weather flow, and minimum flow. Peaking factors decrease as population served increases. Infiltration enters through defects (groundwater), while inflow is direct stormwater connection (downspouts, sumps, manhole covers). Understand diurnal curves: morning and evening peaks. Velocity matters: too low (below about 2 ft/s) allows solids deposition; too high (above about 10 ft/s) scours and abrades.
Traps
Do not confuse infiltration with inflow. Do not assume BOD and TSS are the same; typical domestic wastewater runs roughly 200 mg/L BOD and 200-250 mg/L TSS. Watch for questions equating high flow with high strength (dilution from I/I actually lowers strength).
Rule of thumb
Self-cleansing velocity is about 2 ft/s at design flow; keep velocities between roughly 2 and 10 ft/s. For a quick strength check, domestic BOD ~200 mg/L and TSS ~220 mg/L are reasonable planning values.
Worked example
A basin receives 1.5 MGD average dry-weather flow; apply a peaking factor of 2.5 for peak hourly design: 1.5 x 2.5 = 3.75 MGD peak.