Patterns of Wastewater Infrastructure along a Gradient of Coastal Urbanization: A Study of the Puget Sound Region

Abstract

Abstract:The aim of this paper is to explore patterns of wastewater infrastructures (sewers vs. septic tanks) in urbanizing watersheds across a coastal metropolitan region. This research combines an urban-rural gradient with spatial metrics at the patch and watershed scale (proportion of parcels on a treatment system, septic density, lot size and percent imperviousness) to analyze wastewater patterns in the Puget Sound, WA, USA. Results show that most urban residential parcels are hooked up to a sewer, although there remain urban residences on a septic tank with small lots. I find a complex arrangement of wastewater treatment in suburban watersheds representing a patchwork of parcels on sewers and septic tanks. Sewers dominate in total numbers, while the density of septic tanks is highest in this portion of the urban gradient. Lot size decreases from rural to urban; however, it varies depending on the type of wastewater treatment system. In urban watersheds, lots on septic tanks are significantly smaller than lots in suburban and rural watersheds and of a similar size compared to lots on sewers. I also find a significant difference in the amount of impervious surfaces in watersheds dominated by sewers vs. septic tanks. In the urban portion of the gradient, the amount of paved surfaces in parcels with septic tanks is also similar in level as parcels with sewers. I discuss how these patterns emerge from the interplay of biophysical, socio-economic and technological factors and how different regulatory regimes for septic tanks and sewers may further induce these patterns.

Development of Effective Procedures for Illicit-Discharge Risk Mapping

“…Aquatic habitats and organisms are impacted by higher peak flows, lower base flow, and higher pollution levels due to increased runoff from new development and impervious surfaces (Booth and Jackson, 1997;Wang et al, 2001). Urban sprawl has contributed to the development of a patchwork of aging wastewater infrastructure, complicating the protection of local waterways (Spirandelli, 2015). Furthermore, the interactions between aging wastewater and stormwater infrastructure systems and the natural water environment have contributed to increased public health hazards including bacteria, pathogens, and heavy metals in drinking water supplies and recreational waters (Gaffield et al, 2003;Swann, 2001).…”

“…Since the 1900s urban design has shifted to using separate sewer systems (SSS) to transport wastewater to treatment facilities, while municipal separate storm sewer systems (MS4s) convey stormwater runoff to local waterbodies. Additionally, rural fringes of urban centers which were previously served by septic systems are now becoming urbanized, resulting in a patchwork of piped sewer service expansions and septic treatment systems (Spirandelli, 2015).…”

Screening European market potentials for small modular wastewater treatment systems – an inroad to sustainability transitions in urban water management?

“…Urban form which is used to estimate wastewater management costs is closely related to population density (ECOPLAN 2000). Spirandelli (2015) also finds different patterns of wastewater infrastructures depending on the urban gradient. Finally, Bieker et al (2010) write that 'wherever certain thresholds of population density are exceeded,' semi-centralised approaches offer new flexible solutions, and Kerstens et al (2015) confirm that wastewater systems can be planned on a national scale on the basis of key parameters such as urban/rural features.…”

The cost of hybrid waste water systems: A systematic framework for specifying minimum cost-connection rates

“…For regional or medium-scale analysis, data aggregation is generally required to reduce computational complexity (Haggag and Ayad, 2002). To run the heuristic sewer generation algorithm efficiently (Section 2.2.3), we choose an aggregation technique based on urban structural units (USU), as sanitation planning is closely linked to urban patterns (Spirandelli, 2015;Bach et al, 2015). USU are defined as 'areas with a physignomically homogeneous character, which are marked in the built-up area by a characteristic formation of buildings and open spaces' (Wickop, 1998).…”