Michael J. Gardner

Results are presented for 170 wastewater treatment works sites (20 per substance in influent, effluent and 36 per substance in river water upstream and downstream of the WwTW discharge) over a period of two years between 2015 and 2017; this comprises data for approximately 3000 samples for effluent and 6000 for river samples taken downstream of effluent discharges. Seasonal trends in contaminant concentrations for several substances are reported. Two clear patterns of seasonal variation are proposed over and above all of the variables associated with environmental data including process technology, dilution and geography. Firstly, variation of riverine concentrations caused by seasonal fluctuations in river flow (sewage flow being relatively consistent) resulting in summer maxima and winter minima. Alternatively, variation is observed that is attributable to the improved performance of wastewater treatment processes under warmer conditions. This leads to the lowest concentrations in autumn when surface water/sewage treatment temperatures tend to peak. Seasonality for trace contaminants is more difficult to characterise than that of sanitary parameters owing to the higher variability in the concentration of the substances of interest. The data also provide an insight into the amplitude of such variations. This makes it possible to assess the likely effects of seasonality and its impact on aquatic life. For example, the existence of seasonality (perhaps due only to dilution effects) might be demonstrated, but the amplitude might be too small in relation to the potential ecotoxicological effects to be of any consequence. GRAPHICAL ABSTRACT

One of the most important functions of environmental monitoring is the detection of change. This can be the delineation of deteriorating circumstances or the identification of the success of remedial measures. The design of effective monitoring of change (and hence the optimisation of resources devoted to monitoring) relies on appropriate replication – knowing how many samples are required. Lack of information on the variance of the measured parameter is often a barrier to determining the optimum sampling strategy. An important new information resource on within-site variance of the concentrations of over 60 trace substances in wastewater treatment works effluents has been provided by the UK water industry research programme. This paper makes use of this resource in order to explore the potential to design monitoring programmes that will be capable of demonstrating the success of planned remedial measures that will be implemented in the coming years. Two approaches to experimental design (simple before-and-after sampling and detection of trends via correlation) are examined. It is concluded that for programmes involving numbers of samples of less than 30 the detection of a change in concentration of less than 50% might be very challenging for many of the trace substance of greatest interest. Knowledge of the difficulty of the task in hand should make it possible to design programmes that optimise the use of resources and the approaches taken, such that effects of interest are detected as soon and as economically as possible.

Tightening quality standards for European waters has seen a move towards enhanced wastewater treatment technologies such as granulated organic carbon treatment and ozonation. Although these technologies are likely to be successful in degrading certain micro-organic contaminants, these may also destroy compounds which would otherwise complex and render metals significantly less toxic. This study examined the impact of enhanced tertiary treatment on the capacity of organic compounds within sewage effluents to complex copper and zinc. The data show that granulated organic carbon treatment removes a dissolved organic carbon (DOC) fraction that is unimportant to complexation such that no detrimental impact on complexation or metal bioavailability is likely to occur from this treatment type. High concentrations of ozone (>1 mg O3/mg DOC) are, however, likely to impact the complexation capacity for copper although this is unlikely to be important at the concentrations of copper typically found in effluent discharges or in rivers. Ozone treatment did not affect zinc complexation capacity. The complexation profiles of the sewage effluents show these to contain a category of non-humic ligand that appears unaffected by tertiary treatment and which displays a high affinity for zinc, suggesting these may substantially reduce the bioavailability of zinc in effluent discharges. The implication is that traditional metal bioavailability assessment approaches such as the biotic ligand model may overestimate zinc bioavailability in sewage effluents and effluent-impacted waters.

Sampling and analysis of Water Framework Directive priority chemicals were undertaken in nine urban catchments across the UK. Over 9000 samples were collected from a number of different catchment sources including tap water, domestic waste water, surface water runoff, trade discharges, town centre and light industrial estate wastewaters. Determinands included trace metals, polyaromatic hydrocarbons (PAHs), persistent organic pollutants and a number of common pharmaceuticals. Loads of the chemicals from each catchment entering the local wastewater treatment works (WwTW) were estimated and were shown to be relatively consistent between different catchments, after taking population into account. A Monte Carlo mixing model was used to combine the concentrations and flows from the different catchment sources and to predict concentrations and loads entering the WwTW. Based on the model output, the significance of the different sources could be evaluated. The study highlighted the importance of domestic wastewater as a source of contaminants, including metals and trace organic substances (such as ethylenediaminetetraacetic acid (EDTA), bisphenol A, nonylphenol and tributyl tin (TBT)). Concentrations in trade discharges were important in some locations in the case of nonylphenol, EDTA, TBT, as well as for some metals such as copper, zinc and nickel. Contributions to the total load from town centre and light industrial estate sources were generally less than 10% of the total.

It is important to understand the statistical distribution of monitoring data for them to be of value in determining the parameters of environmental models. No such distributional information has been available for many trace contaminants in sewage effluents. This paper applies the data of a major UK sewage works’ effluent monitoring programme to determine the validity of the common assumption that data are lognormally distributed. Effluent quality was monitored at 162 wastewater treatment works over 1 year, generating over 3,000 results for each of over 40 substances, including metals, trace organic substances and pharmaceuticals. It is demonstrated that the lognormal assumption is clearly justified for the great majority of substances in the spatial case—for annual average effluent concentrations across different treatment works. In the site-specific, temporal case—for individual determinations of concentration at a single site over an annual period—lognormality is generally supported but not demonstrated so unequivocally for all site/substance combinations. The principal source of uncertainty was lack of sufficient numbers of observations reported to adequately low reporting limits.