Why We Need Improved Characterization of Solids in Wastewater, Stormwater, and Combined Sewer Flows

The vast majority of pollutants in urban water are associated with sediments and sewer solids. Sediment has been identified as the most widespread pollutant in US Rivers and streams. In addition to sediment itself being a major pollutant, many priority pollutants such as heavy metals and toxic organics (e.g. PAHs) are associated with sediments. Becker et al., (1996) for example found pollutants to be associated with the suspended solids that have settling velocities in the range of 0.04 – 0.90 cm/sec. Research and operational experiences have shown that considerable sediment and sewer solids deposition occurs in most sewerage (collection) systems as self-cleansing velocities are hardly attained during dry-weather flow conditions. The re-mobilization of gross sewer solids, the settled sediments and their associated pollutants during wet-weather events accounts for the observed first-foul flush during storm events especially after a prolonged period of dry weather. This highly polluting material including what has been described as the "fluid sediment" (Ashley et al., 1999) can result in banks of detritus with varying amounts of organic pollutants and heavy metals accumulating immediately below outfalls or where the first slack length of receiving water occurs causing ecological impairments (especially during low summer flow conditions). The discharge of previously captured sediments and their associated pollutants into receiving environments has been described as a missing dimension in urban water (Andoh et al., 2009). Total Suspended Solids (TSS), Total BOD, Total COD and other related parameters have been widely used as surrogate measures of the fine particulate fraction in wastewater streams and have routinely been measured in water quality studies and analysis. By their very nature, however, these parameters do not provide any information on (or differentiation of) the particulates in the wastewater in terms of their size, relative density, propensity to agglomerate or their settling characteristics. TSS for example only depicts the mass of solid material above ~1 μm (micron) in size in a water or wastewater sample. Because traditional sampling methods based on small-bore tube samplers normally sample only the suspending fluid, most urban water monitoring and evaluation schemes have taken no account of the readily settleable solids, particularly the sediment fractions and their associated pollutants though these are probably the most polluting portions of urban sewer solids particularly in terms of chronic impacts on receiving watercourses. This coupled with the very nature of the sampling and analytical test procedures associated with their measurements, results in a considerable bias and the missing out of a significant fraction of the solids spectrum in urban water and wet-weather flows. The relevance and use of wastewater settling characteristics in design has been described by Andoh and Smisson (1996) with the need for taking wastewater characteristics particularly particle size distribution and settleability into account in Wet-weather including CSO pollutant studies and control, increasingly being recognized and advocated (Shin et al., 2001). The paper reviews current approaches to wastewater settling characterization and presents results of on-going work comparing settling velocity distributions from North American sites with European sites highlighting the importance of both settling characterization and particle size distribution analysis in determining appropriate control measures to mitigate adverse impacts of CSOs and other polluted urban wet-weather impacted flow sources.