Inflow and infiltration in coastal wastewater collection systems: effects of rainfall, temperature, and sea level

Wastewater collection and treatment systems are vital to public health, economic growth, and environmental quality, but do not receive as much consideration for upgrades and improvements as other forms of public infrastructure. Extraneous inputs to a wastewater collection system are caused by rainfall and submergence ("Inflow and infiltration," I&I) and other factors, such as variation in sea level in coastal settings. These factors all pose risks for system degradation, sanitary system overflows (SSOs), and water quality impairment, but remain poorly quantified. Multiple regression analyses of total flows through 19 wastewater collection systems in coastal North Carolina, all using gravity collection systems, over a 2-year period (2010-2011) demonstrated statistically significant effects of rainfall, temperature, and sea level as drivers of extraneous flows. Rainfall effects were significant for 18/19 (95%) of these systems. Temperature effects were also significant for 18/19 (95%) of these systems. Sea level effects, primarily driven by spring-neap tidal oscillations, were significant for 11/19 (58%). Further, single factor regression analyses of the effects of temperature and sea level on system flows demonstrated significant effects for 16/19 (84%) and 18/19 (95%) systems, respectively. These collective results demonstrate the potential vulnerability of coastal wastewater collection and treatment systems to breaches in system integrity that allow extraneous flows, primarily through groundwater elevation, to drive further infrastructure degradation and environmental pollution. PRACTITIONER POINTS: Heavy rainfalls drive statistically significant inflow and infiltration (I&I) in over 90% of central wastewater systems in coastal North Carolina. Temperature, most likely as effects of seasonal variation in groundwater levels, also had a significant effect on I&I in over 90% of these systems. Sea level, expressed as daily high-high tide, drove significant effects on flow through over 90% of these systems.