Extraneous ﬂows in sewers; simply, inﬂow is water discharged into sewer pipes or service connections from such sources as foundation drains, roof leaders, cellar and yard area drains, cooling water from air conditioners, and other clean-water discharges from commercial and industrial establishments. Deﬁned by Metcalf & Eddy as follows:
- Inﬁltration water entering the collection system through cracks, joints, or breaks.
- Steady inﬂow water discharged from cellar and foundation drains, cooling water discharges, and drains from springs and swampy areas. This type of inﬂow is steady and is identiﬁed and measured along with inﬁltration.
- Direct ﬂow those types of inﬂow that have a direct stormwater runoff connection to the sanitary sewer and cause an almost immediate increase in wastewater ﬂows. Possible sources are roof leaders, yard and areaway drains, manhole covers, cross connections from storm drains and catch basins, and combined sewers.
- Total inﬂow the sum of the direct inﬂow at any point in the system plus any ﬂow discharged from the system upstream through overﬂows, pumping station bypasses, and the like.
- Delayed inﬂow stormwater that may require several days or more to drain through the sewer system. This category can include the discharge of sump pumps from cellar drainage as well as the slowed entry of surface water through manholes in ponded areas.