Treated Sewage Effluent Research Articles

Water and sludge resource planning for sustainable agriculture: An energy-water-food-waste nexus approach

The population rise and expected increase in global food demand will stress natural freshwater resources. It will also result in increased emissions if proper resource planning is not implemented and unconventional water and energy resources are not exploited. There is a need to assess the potential of these unconventional resources, such as wastewater and sludge in alleviating carbon emissions and water stress. This study aims to do so by assessing the use of treated wastewater and sludge for food production and optimizing their planning using a multi-objective optimization model embedded within an energy-water-food-waste nexus approach applied to a case study on the agricultural sector in the state of Qatar. It is demonstrated that the use of treated sewage effluent as a water resource can supply the fodder farms in Qatar with all their nitrogen fertilization requirements almost throughout the year. Furthermore, the use of both treated sewage effluent and sludge as a source of energy has the potential to reduce the carbon footprint and water stress level by 62 % and 57 %, respectively. It is thus recommended to include treated sewage effluent in the agricultural water budget and to suspend outdoor farming not only during the summer but also in the hot month of March, as the latter can result in an additional 7 % and 30 % reduction of carbon footprint compared to the base-scenario when the water stress level is set to 0 % and 75 %, respectively.

Effectiveness of the tropical plants Rhynchospora corymbosa and Coix lacryma-jobi in vertical flow constructed wetlands for municipal primary sewage effluent treatment

The performance of two tropical plants, Rhynchospora corymbosa L. (RC) and Coix lacryma-jobi, L (CL) in treatment of primary sewage effluent in lab-scale vertical-flow constructed wetlands (VFCW) along with no plant control wetland was investigated. A batch-flow VFCWs were operated under batch fill and drain hydraulic loading system with hydraulic retention times (HRT) of 0.5, 1, and 2 days and fill rate of 8 L/day. Removal of solids, organics, nutrients, and pathogens were monitored. The volumetric contaminant removal rates were best described by 1st order kinetics except for ammonia and phosphate, which was best described by Stover-Kincannon kinetics. Influent TSS, PO4 3−, COD, BOD5, and total coliform concentration were low but high in NH4 + concentration. CL was better in nutrient removal as HRT increases compared to RC. RC was more efficient at TSS, turbidity, and organics removal. Pathogen removal was independent of plant type but HRT. Solids and organic removal were lower in CL planted CWs due to preferential flow paths created by their bulky root. CL planted CWs removed more nutrients followed by RC planted CWs and then no-plant control CWs. The results of these tests demonstrate that both CL and RC are suitable for the treatment of municipal wastewater in VFCW system.

Synergetic Water Demand and Sustainable Supply Strategies in GCC Countries: Data-driven Recommendations

The Gulf Cooperation Council (GCC) countries, characterized with limited water resources and high oil/gas revenues, rely heavily on energy-intensive seawater desalination and non-renewable groundwater abstraction. The need to shift solutions to demand-side practices and sustainable supply alternatives has been long advocated; yet this study is the first to “quantify” the impacts of such solutions on the water management system of Qatar – considered a study case of GCC countries. In this research, a scenario-based approach was utilized to predict the impact of water demand control and wastewater reuse (and the resulting synergies) on consumption of desalinated water, extraction of groundwater resources, and development needs of water and wastewater infrastructure. To this effect, country-specific models for Qatar were developed to project annual household water demand, wastewater generation and residential construction growth, up to year 2050. The outcomes showed that tariff reforms and regulated greywater reuse would reduce the annual household demand for desalinated water by up to 27% and 7%, respectively. Also, intensive reuse of Treated Sewage Effluent (TSE) would reduce 40–80% of total groundwater abstraction for irrigation by 2050. Finally, adopting an integrated water strategy, with combined demand and supply management targets, creates synergies that would: (1) limit groundwater abstraction to rates close to the aquifers safe yield; and (2) delay the need for expansion of the water and wastewater infrastructure by more than a decade. Data-driven recommendations were provided accordingly.

Reduction of Chlorine Disinfection Dosage through Optimal Jet Design in the Hong Kong Harbor Area Treatment Scheme

Chlorine is commonly used in disinfection processes in wastewater treatment plants. The chlorine solution is typically dosed by turbulent mixing through multiport jet diffusers. The chlorine reacts with the inorganic and organic nitrogenous compounds at a fast rate (in the order of 1 s or less), and a significant portion of the dosed chlorine can be rapidly consumed by the reactions instead of destroying the pathogens. Despite extensive previous research, the chlorine consumption in the complex sewage flow remains an elusive problem. Field-scale experiments have been carried out to study the chlorine consumption in the turbulent mixing of dense chlorine jets with the primary treated sewage effluent of the Stonecutter’s Island Sewage Treatment Works (serving a population of 5.7 million). Based on jet theory and the field tests, it is proposed to discharge the chlorine jets at a location of highest velocity to minimize the contact time of the sewage with high concentration chlorine. Full-scale in-plant experiments demonstrated that significant savings in chlorine can be achieved with optimal jet diffuser design and placement; at nominal applied chlorine dosages of 12 and 20 mg/L in winter and summer, respectively, the chlorine demand can be reduced by up to 30% compared with the conventional dosing design. The relatively simple dosing reconfiguration results in significant savings in chlorine and energy, reduces harmful impact to the environment, and improves plant operation reliability. The chlorine demands from daily operations are well-correlated with the results of the field-scale models. Predictions of the chlorine demand as a function of applied dosage and temperature were obtained from the collective field data.

EOF and target PFAS analysis in surface waters affected by sewage treatment effluents in Berlin, Germany

Per- and polyfluoroalkyl substances (PFAS) are emerging organic pollutants and can occur in surface and groundwater. To identify the degree of pollution in surface water with PFAS, often targeted HPLC–ESI–MS/MS has been employed in which commonly 30–40 compounds are analyzed. However, other PFAS and organofluorines remain undetected. We sampled surface water of the river Spree and the Teltow Canal in Berlin, Germany, which are affected by the effluent discharge of wastewater treatment plants. Here, we employed high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS-GFMAS) for measuring extractable organofluorines (EOF) and compared in a mass balance approach the total fluorine to the identified and quantified PFAS from the targeted analysis. The analysis highlights that the EOF are in the range expected for an urban river system (Winchell et al. in Sci Total Environ 774, 2021). However, downstream of an effluent discharge, the EOF increased by one order of magnitude, e.g., 40.3 to 574 ng F L−1, along the Teltow Canal. From our target analytes, mostly short-chained perfluorinated carboxylic acids and sulfonates occur in the water, which however makes up less than 10% of the EOF. The increase in EOF in the Teltow Canal correlates well with the increase of perfluorohexanoic acid (PFHxA), indicating that PFHxA is characteristic for the discharged EOF but not responsible for the increase. Hence, it points to PFHxA precursor discharge. The study highlights that EOF screening using HR-CS-GFMAS is necessary to identify the full scale of pollution with regard to PFAS and other organofluorines such as pharmaceutical compounds from the effluent of WWTPs.

Comparison of Membrane-Based Treatment Methods for the Removal of Micro-Pollutants from Reclaimed Water

Dual membrane hybrid systems generally produce reclaimed water for non-potable uses by blending microfiltered biologically treated sewage effluent (BTSE) and reverse osmosis (RO) permeate. This reclaimed water is found to contain a significant amount of micro-pollutants, which possibly cause toxicity effects to aquatic organisms and plants when exposed to it. Therefore, removing such pollutants from the reclaimed water before reaching the community is highly emphasized nowadays. The currently used treatment of the RO treatment of microfiltered BTSE is energy intensive and not cost effective. This paper focuses on less costly and efficient membrane-based hybrid treatment systems such as the microfiltration-adsorption (MF-GAC) hybrid system, Nano filter (NF) and RO in the removal of micro-pollutants from the microfiltered BTSE. Both the MF-GAC hybrid system and NF (with NTR 729HF membrane) removed 70 to 95% of micropollutants from microfiltered BTSE. The removal depends on the hydrophobicity, charge, and size of the micropollutants. RO was excellent in removing more than 90% of pollutants, while MF was inefficient, as the latter primarily depends on the size exclusion mechanism. Based on the finding, it is suggested to treat only a portion of microfiltered BTSE through the MF-GAC or NF membrane before blending with RO permeate to enhance the removal of micro-pollutants from reclaimed water. The development of sustainable hybrid systems for the removal of all micropollutants of different chemical and physical properties is the key for the water reclamation.

Soil self-cleaning capacity: Removal of organic compounds during sub-surface irrigation with sewage effluent

Globally, the reuse of treated sewage effluent for irrigation purposes is increasingly encouraged as a practical solution against the mismatch between the demand for and availability of freshwater resources. The reuse of sewage effluent for sub-surface irrigation (SSI) in agriculture serves the dual purpose of supplying water to crops and diminishing emissions of contaminants of emerging concern (CoECs) into surface water. To investigate such reuse, in a real scale cropland with SSI using sewage effluent, from September 2017 to March 2019 including the extremely dry year 2018, residues were followed of 133 CoECs as related to their physicochemical properties and quantified by liquid chromatography coupled to high-resolution mass spectrometry. Of the 133 target CoECs, 89 were retrieved in the field, most non-detect CoECs have low persistency. During the growing season with sub-surface irrigation, CoECs spread to the shallow groundwater and rhizosphere. Significantly lower concentrations are found between infiltration pipes as compared to directly next to the pipes in shallow groundwater for all persistency-mobility classes. CoECs belonging to the class pm (low persistency and low mobility) or class PM (high persistency and high mobility) class show no change amongst their removal in the rhizosphere and groundwater in a dry versus normal year. CoECs belonging to the class pM (low persistency and high mobility) show high seasonal dynamics in the rhizosphere and shallow groundwater, indicating that these CoECs break down. CoECs of the class Pm (high persistency and low mobility) only significantly build up in the rhizosphere next to infiltration pipes. Climatic conditions with dry summers and precipitation surplus and drainage in winter strongly affect the fate of CoECs. During the dry summer of 2018 infiltrated effluent is hardly diluted, resulting in significantly higher concentrations for the CoECs belonging to the classes pM and Pm. After the extremely dry year of 2018, cumulative concentrations are still significantly higher, while after a normal year during winter precipitation surplus removes CoECs. For all persistency-mobility classes in the shallow groundwater between the pipes, we find significant removal efficiencies. For the rhizosphere between the pipes, we find the same except for Pm. Next to the pipes however we find no significant removal for all classes in both the rhizosphere and shallow groundwater and even significant accumulation for Pm. For this group of persistent moderately hydrophobic CoECs risk characterization ratio's were calculated for the period of time with the highest normalized concentration. None of the single-chemical RCRs are above one and the ΣRCR is also far below one, implying sufficiently safe ambient exposures. Overall the deeper groundwater (7.0–11.8 m below soil surface) has the lowest response to the sub-surface irrigation for all persistency-mobility. When adopting a SSI STP effluent reuse system care must be taken to monitor the CoECs that are (moderately) hydrophobic as these can build up in the SSI system. For the deeper groundwater and for the discharge to the surface water, we find significant removal for the pM and the PM class but not for other classes. In conclusion, relatively high removal efficiencies are shown benefiting the surface waters that would otherwise receive the STP effluent directly.

Water Quality Improvement Features of Aquaponic Systems and Their Economic Feasibility

Aquaponics is an evolving sector with significant presence in arid regions or areas that have scarce arable land. Since they are a combination of aquaculture and hydroponics, naturally the focus of such systems will be the amount and quality of the food produced. Nevertheless, aquaponic systems also contain large quantites of various microorganisms that helps with fish growth and the assimilation of nutrients by plants. In a way, they are cleaning up the water as well but this aspect of aquaponic systems is largely ignored. In this study, effluent from a sewage treatment plant was fully applied in a media bed unit aquaponic system in place of natural freshwater to investigate the capability of the system in improving the quality of the water. On top of that, economic feasibility of such a system using the benefit-cost ratio method was also studied. It was found through this study that the first 12 hours upon application of the contaminated water is when the rate of change in the parameters were greatest. The water quality parameters tested (biochemical oxygen demand, chemical oxygen demand, total suspended solids, ammonia-nitrogen, nitrate-nitrogen, phosphorous, oil & grease) showed good improvement by the end of each trial. Economic feasibility of the treated sewage effluent aquaponic system was also studied using the benefit-cost ratio and it was found that the system is able to deliver a positive net present value. In short, aquaponic systems is a feasible alternative to traditional farming and agriculture.

Bird Communities and the Rehabilitation of Al Karaana Lagoons in Qatar

Qatar, a peninsular country in the Persian Gulf, is significant to avian species due to its location along the African–Eurasian Flyway, a key migratory path. Receiving untreated domestic and industrial liquid waste from Qatar in the past, Al Karaana Lagoons have since been reconstructed as an artificial wetland to address the growing environmental concern posed by contamination build-up. This study documents the changes in biodiversity at Al Karaana Lagoons following their environmental remediation. Data collected (2015 and 2017) by Ashghal (Public Works Authority) prior to project implementation was analyzed alongside data collected independently following project completion (2019–2021). There was a marked increase in bird biodiversity following remediation, including substantial use by migratory species and resident breeders. Further analysis of water quality data of the TSE (treated sewage effluent) ponds shows that they are eutrophic but still support substantial bird life. The project’s success demonstrates how reclaimed lands can provide important habitats to local and migratory birds and encourages similar restoration efforts in the future in both Qatar and elsewhere. We call for the continued monitoring of the site and the implementation of guidelines for the use of the site that balance human activities and habitat quality.

Dust mitigation by the application of treated sewage effluent (TSE) in Iran

Considering the presence of 274 dusty days in 2021 in Zabol city, Iran, the present study aimed to evaluate the feasibility of using treated sewage effluent (TSE) for dust mitigation with natural methods of increasing land cover. Hence, first of all, the identification of sewage treatment facilities along with the volume and chemical status was carried out and compared to the various national and international legislation. Then, field investigation on land use and land cover, along with literature review on dust origins, sand detachment areas, and sand corridors in the study area will be assisted for optimal area suggestion. Note that, in the present study it was assumed that the application of TSE for wetting the surface to vegetation restoration resulted in wind erosion control in critical foci. The results showed that, so far, a total of 39,000 m3/day could be treated, in the whole study area. The TSE volume calculated based on two scenarios consisting, (1) data obtained from the related organization, and (2) based the capacity of the wastewater plant is 2.8 and 5.1 mcm/year, respectively. Additionally, the study of TSE quality and its comparison to various regulation such as FAO, USEPA, INS, and CWQI indicated the applicability of transforming TSE to 14 km away from the WWT planet daily for rehabilitation of Hammon Hirmand through irrigation of T.stricta to increase the vegetation cover to above 30%.

The Dutch chain approach on pharmaceuticals in water: Stakeholders acting together to reduce the environmental impact of pharmaceuticals.

Sewage treatment effluent with pharmaceutical residues is discharged into surface waters, raising societal concerns. The aim of this paper is to describe the Dutch chain approach on medicinal residues in water that has been implemented by the Dutch government. We show how stakeholders from both the health and water sectors have got actively involved. Within this chain approach, source measures as well as end-of-pipe measures are identified and, where feasible and effective, implemented. Descriptive paper on the Dutch chain approach. Getting the water and health care sectors to talk with each other instead of about each other was the key accomplishment. Comprehension of each other's viewpoints, terminology, policy goals and span of control, was pivotal in setting shared goals, creating perspective about possible measures and actually taking (joint) action. Together, stakeholders agreed to act within their own possibilities, without pointing at others, and to focus on pragmatic measures. In this article, we provide examples of measures taken, pilot projects performed, and of measures that were not implemented. Besides this, we discuss the most important barriers encountered during this process and how they were overcome. The issue of pharmaceuticals in the environment is a wicked problem, which makes it necessary to work together with many stakeholders on possible solutions, avoiding paralysis by complexity. Most importantly, stakeholders need to invest in mutual understanding, keep an open communication, and feel invited to bring in solutions for their part of the chain.

Evapotranspiration and crop coefficients using lysimeter measurements for food crops in the hyper-arid United Arab Emirates

In the hyper-arid United Arab Emirates (UAE) irrigation is needed for food crops because there is limited rainfall of less than100 mm y−1 in the crop-growing parts of the UAE. Groundwater is the main source of irrigation water, yet it is a declining resource both in terms of quantity, and quality as a result of rising salinity. Law 5 has been passed by the Government of Abu Dhabi to manage groundwater extraction by limiting water takes to an amount that is considered to be sufficient to grow a given crop. In order to allocate these water takes, the Environment Agency – Abu Dhabi sought experimental data and a model platform to calculate equitable allocations for each vegetable crop. We designed simple weighing lysimeters to measure directly the water-use lettuce, capsicum, tomato, cucumber and zucchini grown in the field, as well as in a shadehouse and a cooled greenhouse for capsicum, cucumber and tomato. Total crop yields were measured via progressive harvests through to the end of the growing season. The yields in the greenhouse were the highest. The crop water-use efficiency (WUEC, kg crop per m3 crop evapotranspiration ETC) was three fold higher in the greenhouse (≈ 30 kg m−3) than in the shadehouse or field (≈ 10 kg m−3). However, when the water used to cool evaporatively the greenhouse was accounted for in the water productivity (WPI kg crop m−3 water for irrigation and cooling) there were no differences between the field, shadehouse and greenhouse. So whereas the farmer benefits from higher yields in the greenhouse, there is no advantage in terms of water productivity. However, use of alternative waters for greenhouse cooling, such as treated sewage effluent, could be used to protect groundwater. Our water-use data were used to calculate the seasonal pattern of the FAO56 crop coefficient, KC, through the piecewise linear phases of crop growth, the initial (KCini), middle (KCmid), and the end (KCend) of the growing season. These KC values were compared with recent reviews of FAO56 for vegetables. Due to the various training systems for the crops in this study, the KC values varied considerably, with KCini ranging from 0.2 to 0.5, KCmid from 0.5 to 1.35, and KCend from 0.2 to 0.7. These KC values will be used in our Crop Calculator Decision Support Tool (DST) to implement Law 5.

Efficient Irrigation Decision-Making with the Use of Soil Tensiometers: A Case Study in the Maltese islands

The Maltese islands face challenges of water scarcity owing to a lack of rainfall and limited water resources (RDP 2021). The main sources of water for agriculture in Malta come from the mean sea-level aquifer, the perched aquifer, rainwater harvesting, and treated sewage effluent. However, most of these are dependent on precipitation which is another problem in itself due to the scarcity of rainfall in more recent years. Although soil tensiometers have been in use in agriculture for decades, in Malta there is no evidence of their use. Research was done on the potato crop variety Cara which was divided into two groups: the control which was irrigated by observation of the crop and soil, and the experimental group which was irrigated using soil tensiometers. The irrigation volume was recorded for each group and, during harvesting, the weight and number of tubers were recorded. This experiment demonstrated that the water use efficiency can be improved with the use of soil tensiometers and that the application of soil tensiometers in potato cultivation can reduce the amount of water consumed whilst attaining good yields.

The Geological Characteristics of the Vadose Zone Influence the Impact of Treated Wastewater on the Groundwater Quality (SCA.Re.S. Project 2019-2020).

This study evaluated whether some chemical and microbial contaminants in treated sewage effluents from two wastewater treatment plants (WWTP) reached the groundwater when they drained through a fractured karst vadose zone (WWTP-K) and a porous vadose zone (WWTP-P). Forty-five samples of sewage water (SW), treated water (TW), and monitoring well (MW), collected from WWTP-P (24) and WWTP-K (21), were analyzed for a range of microbiological and chemical properties. The E. coli and Salmonella counts were below the limits outlined in the Legislative Decree 152/06 in effluents from both types of WWTP. Enteric viruses were found in 37.5% and 12.5% of the SW and TW from WWTP-P, respectively. The percentages of Pepper mild mottle virus isolated were higher in TW (62.5% in WWTP-P, 85.7% in WWTP-K) than in SW and MW. The residual concentrations of contaminants of emerging concern (CEC) of each drug category were higher in the MW downstream of WWTP-K than of WWTP-P. Our results showed that the porous vadose zone was more effective at reducing the contaminant loads than the fractured karst one, especially the CEC, in the effluent. The legislation should include other parameters to minimize the risks from treated effluent that is discharged to soil.

Towards Sustainable Application of Wastewater in Agriculture: A Review on Reusability and Risk Assessment

The use of marginal-quality waters, not limited to brackish/saline and treated sewage effluent (TSE), is called reclaimed water. Reclaimed water is a sustainable source in the future for use in agriculture, essentially required to offset the food demand of a rapidly growing population. Moreover, the sustainable recovery of reclaimed water is essential for humanity to satisfy extreme sanitation and water-supply demands. To increase access to water supply, alternate water resources’ use, existing water resources’ degradation, and improved water-use efficiency are imperative. There is a high potential to address these factors by using reclaimed water as an alternative source. The reclaimed water treated at a tertiary level has the potential for use in crop production, especially for forage crops, irrigating urban landscapes, recreational and environmental activities, industry, and aquifer recharge to increase strategic water reserves in water-scarce countries. This way, we can save precious freshwater that can be utilized for other purposes. Eminently, freshwater applications for industrial and agronomic sectors account for 20% and 67%, respectively, depleting freshwater resources. The use of reclaimed water in agriculture can significantly reduce pressure on freshwater. However, if the quality of reclaimed water does not comply with international standards, it may cause serious health risks (diseases) and soil pollution (heavy metals).

Influence of Aquatic Plants and Flow Rate on Physico-chemical Characteristics of Sewage Effluent Treated through Constructed Wetland Technology

Aims: To evaluate the influence of aquatic plants and flow rate on physico-chemical properties of sewage effluent with constructed wetland technology.
 Study Design: Randomized block design
 Place and Duration of Study: Department of Environmental Sciences, Tamil Nadu Agricultural University, Coimbatore - 03 and June 2020 – Dec 2020
 Methodology: Constructed wetland (CW) model was designed and three different aquatic plants viz., Canna indica, Arundo donax, and Xanthosoma sagittifolium were used for the study. Two different flow rates viz., 5ml/min and 10 ml/min were compared for assessing their influence in treating sewage effluent in the constructed wetlands. 
 Results: Among the aquatic plants, the pollutants like BOD, COD,TDS and TSS in the sewage effluent were declined and found to be 320, 1220, 666, 22 mg L-1 in Canna indica and 340, 1380, 866, 36 mg L-1 in Xanthosoma sagittifolium and 340, 1380, 866, 36 mg L-1 in Typha angustifolia utilized treatments at flow rates 5 ml/min respectively during 7th day retention time. Similar decreasing trend was observed in BOD, COD, TDS and TSS of the sewage effluent at flow rates of 10 ml/min.
 Conclusion: Based on the study, it was evident that constructed wetland (CW) with Canna indica, Xanthosoma sagittifolium and Typha angustifolia found to perform better under flow rate of 5 ml/min for treating sewage effluent at the retention time of 7 days.

Mechanisms of impact on the soil environment of drip irrigation applying treated sewage effluent: a review in China

Abstract Due to the severe shortage of water resources, more than 33.3% of treated sewage effluent (TSE) is used for agricultural irrigation in China. There are significant advantages of using drip irrigation of TSE. However, it is still not known how TSE drip irrigation influences the soil environment. It is known that drip irrigation technical parameters determine the distribution of TSE in soil and have a substantive effect on the soil environment, while the magnitude of these impacts depends on the TSE quality. Since the application of conventional water resources is limited, TSE is expected to be used more frequently for agricultural irrigation. The growing concern of soil environmental risk due to TSE drip irrigation requires further study of the interaction and coupling of drip irrigation and TSE. This summary will help understand TSE drip irrigation in China, and guide the practice of reclaimed water utilization in the future.

Enhanced sewage effluent treatment with oxidation and adsorption technologies for micropollutant control: current status and implications

Enhanced sewage effluent treatment with oxidation and adsorption technologies for micropollutant control: current status and implications

A mini review of recent progress in the removal of emerging contaminants from pharmaceutical waste using various adsorbents.

The presence of emerging contaminants (ECs) originating from pharmaceutical waste in water, wastewater, and marine ecosystems at various geographical locations has been clearly publicised. This review paper presents an overview of current monitoring data on the occurrences and distributions of ECs in coastal ecosystem, tap water, surface water, ground water, treated sewage effluents, and other sources. Technological advancements for EC removal are also presented, which include physical, chemical, biological, and hybrid treatments. Adsorption remains the most effective method to remove ECs from water bodies. Various types of adsorbents, such as activated carbons, biochars, nanoadsorbents (carbon nanotubes and graphene), ordered mesoporous carbons, molecular imprinting polymers, clays, zeolites, and metal-organic frameworks have been extensively used for removing ECs from water sources and wastewater. Extensive findings on adsorptive performances, process efficiency, reusability properties, and other related information are thoroughly discussed in this mini review.

Biogeochemical impacts of sewage effluents in predominantly rural river catchments: Are point source inputs distinct to background diffuse pollution?

Discharge of treated sewage effluent to rivers can degrade aquatic ecosystem quality, interacting with multiple stressors in the wider catchment. In predominantly rural catchments, the river reach influence of point source effluents is unknown relative to complex background pressures. We examined water column, sediment and biofilm biogeochemical water quality parameters along river transects (200 m upstream to 1 km downstream) during summer at five wastewater treatment works (WWTW) in Scotland.Treated sewage effluent (subset, n = 3) pollutant concentrations varied between sites. Downstream concentration profiles of water and sediment biogeochemical parameters showed complex spatial changes. A hypothesised point source signature of elevated concentrations of pollution immediately downstream of WWTW then a decaying pollution ‘plume’ did not commonly occur. Instead, elevated soluble reactive phosphorus (SRP), ammonium and coliforms (maximum 0.23 mgP/l, 0.33 mgN/l and >2 × 106 MPN/100 ml) occurred immediately downstream of two WWTW, whereas some downstream pollutant concentrations decreased. Microbial substrate respiration responses only differed 1 km downstream. Significantly greater concentrations of sediment metal occurred >500 m downstream, likely due to the redeposition of historic contaminated sediments. Significantly lowered chlorophyll-a downstream of one WWTW coincided with elevated metals, despite water SRP and sediment P increases. Overall, stress caused to microbes and algae by effluent contaminants outweighed the subsidy effect of WWTW nutrients.We observed variable effluent flows to the rivers limited localised pollution downstream of WWTW and overall influence of arable land cover on river water quality. Together, this challenges views of consistently discharging point sources impacting low dilution sensitive rivers in summer contrasting with ‘diffuse’ sources. Thus, river water column and benthic compartments are altered at varying scales by point source effluents in combination with rural catchment pollution sources, both discrete (e.g. farmyards and septic tanks) and diffuse.