Continuous-flow laboratory simulation of stream water quality changes downstream of an untreated wastewater discharge

Abstract

In regions of the world with poor provision of wastewater treatment, raw sewage is often discharged directly into surface waters. This paper describes an experimental evaluation of the fate of two organic chemicals under these conditions using an artificial channel cascade fed with a mix of settled sewage and river water at its upstream end and operated under continuous steady-state conditions. The experiments underpin an environmental risk assessment methodology based on the idea of an "impact zone" (IZ) - the zone downstream of wastewater emission in which water quality is severely impaired by high concentrations of unionised ammonia, nitrite and biochemical oxygen demand (BOD). Radiolabelled dodecane-6-benzene sulphonate (DOBS) and aniline hydrochloride were used as the model chemical and reference compound respectively. Rapid changes in (14)C counts were observed with flow-time for both these materials. These changes were most likely to be due to complete mineralisation. A dissipation half-life of approximately 7.1 h was observed for the (14)C label with DOBS. The end of the IZ was defined as the point at which the concentration of both unionised ammonia and nitrite fell below their respective predicted no-effect concentrations for salmonids. At these points in the cascade, approximately 83 and 90% of the initial concentration of (14)C had been removed from the water column, respectively. A simple model of mineral nitrogen transformations based on Michaelis-Menten kinetics was fitted to observed concentrations of NH(4), NO(2) and NO(3). The cascade is intended to provide a confirmatory methodology for assessing the ecological risks of chemicals under direct discharge conditions.

Integrated presentation of ecological risk from multiple stressors

“…One can also be interested in a scenario accounting for the seasonal dynamics of food or photo period (e.g. for algae) or even for more complex ecological interactions such as the impact of competition and/or predation on the response of a population to a chemical stressor or the impact of a reduced dissolved oxygen in areas receiving untreated wastewater64.…”

Occurrence, accumulation, attenuation and priority of typical antibiotics in sediments based on long-term field and modeling studies

“…In order to investigate past contamination, one sampling campaign was carried out in 2008 before dredging. The other campaigns were conducted at 1,12,14,16,18,20,22 and 24 months after dredging until April of 2011, respectively. Twelve typical antibiotics were measured in all samples.…”

“…The first order kinetic model has successfully described the attenuation of organic compounds in the water of rivers or channels [22,23]. First order kinetics was assumed to also occur in the sediments in this work.…”

Estimating surface water concentrations of “down-the-drain” chemicals in China using a global model

“…Derivation of a statistical distribution of PECs allows probabilistic risk (sensu Cardwell et al, 1999) to quantified by integrating the product of the exposure probability density function (essentially the relative frequency distribution of PECs) and the species sensitivity distribution (representing the effect vs concentration relationship). That said, the extent to which conventional risk assessment for "down-the-drain" chemicals is a useful indicator of ecological impact is questionable in systems receiving significant volumes of untreated waste water (direct discharge) because the ecosystem is often so impacted by high concentrations of sanitary determinands, such as BOD, ammonia and nitrite Finnegan et al, 2009) In the scenario in which 100% provision of secondary sewage treatment was assumed, surface water exposure to LAS is significantly reduced (Figure 2b). In this scenario, predicted LAS concentrations approach the PNEC only in those river reaches where there is no dilution (vertical sections in the cumulative distributions shown in Figure 3).…”