Water quality modelling of the River Almond, Scotland, UK

Transcription

1 Atkins Lectures Water quality modelling of the River Almond, Scotland, UK IAHS-IAPSO-IASPEI joint assembly, Gothenburg

2 Vera Jones, senior environmental scientist, Water and Environment 25 July 2013

3 Introduction Background to the project

4 Catchment understanding at the start of the project River Almond, SE Scotland Large part of the flow from wastewater discharges and untreated discharges from combined sewer overflows Pollution from septic tanks and other private discharges Diffuse pollution pressure associated with historic mining A number of surface water outfalls industrial estates.

5 Key objectives of this study Water quality survey and development of a modelling tool Help to assess the impact of Scottish Water assets on the water quality of the River Almond and its tributaries Development of strategic options to meet requirements of the Water Framework Directive and the Urban Wastewater Treatment Directive.

6 Study area Cramond Weir Summer 2011 survey Upper boundary

7 Modelling Model development, calibration and validation

8 Space Space Model description (Mike 11 Ecolab by the DHI Group) Biogeochemical interactions Advection/dispersion Air temperature, air humidity, wind speed Heat Time Time Hydraulic model Advection Dispersion d[bod]/dt= - BOD Decay - Sedimentation + Resuspension Photosynthesis = Pmax*suninp/depth Respiration= DO2/(DO2+ mdo)*resp* teta2(temp-20)/depth

9 Model description Biogeochemical interactions Advection/dispersion Heat Hydraulic model Determinants modelled: Dissolved oxygen Temperature Ammonia Nitrate Ortho-phosphate Particulate phosphorus Biochemical oxygen demand

10 Model description Biogeochemical interactions Advection/dispersion Heat Hydraulic model Boundaries: 31 rural subcatchments Seven wastewater treatment works Six private discharges ~120 combined sewer overflows modelling by MWH

11 temperature ( ºc) Calibration and validation summary

12 Calibration and validation summary Excellent match when considering daily average temperature

13 Calibration and validation summary Key parameter in this study: overall good match with observed data

14 Calibration and validation summary Some spikes in the model record potentially high values not picked up by the monitoring

15 DO% station 19 : Downstream of Newbridge DO% saturation station 19 : Downstream of Newbridge WwTW, Almond Crucial to fish Many interactions with other parameters Model Observed observed Max: day modelled /08/ /08/ /08/ /08/ /09/ /09/ /09/ /09/ /09/2011 Min: night

16 Methodology for the needs assessment

17 Methodology for needs assessment 10-year runs stochastic baseline Two years selected for further scenario testing : poor and average water quality Results extracted and processed for every model node: Water Framework Directive standards 99th percentile (99 th ile) standards* Fundamental Intermittent Standards (FIS)* *FWR (2012). Urban Pollution Management Manual Urban Wastewater Treatment Directive

18 Methodology for needs assessment Programme developed for processing results at every node against the relevant standards. Output in a suitable format to produce maps of classification at model node. High Good Moderate Poor Bad For all results analyses: Good or High Status is required.

19 Needs assessment: stochastic baseline runs

20 Results of needs assessment: summary High Good Moderate Poor Bad Ammonia WFD assessment (10-year) Summary of 10-year baseline stochastic runs: No FIS failures WFD failures for ortho-phosphate and ammonia 99 th ile failures for ammonia and BOD

21 Needs assessment: scenarios runs

22 Results of needs assessment: analysis against WFD standards Wastewater Treatment Works (WwTW) discharges Ortho-phosphate baseline Ortho-phosphate 2024 no waste water treatment works scenario Works are a key cause of failure to meet the Water Framework Directive standards High Good Moderate Poor Bad

23 Results of needs assessment: analysis against 99 th ile standards Wastewater Treatment Works (WwTW) discharges BOD baseline BOD 2024 no waste water treatment works scenario Works dilute intermittent untreated combined sewer overflow inputs High Good Moderate Poor Bad

24 Results of needs assessment: analysis against WFD standards Combined Sewer Overflows Ortho-phosphate baseline Ortho-phosphate 2024 no combined sewer overflows scenario Small improvement combined sewer overflows have a small effect on compliance with Water Framework Directive standards High Good Moderate Poor Bad

25 Results of needs assessment: analysis against 99 th ile standards Combined Sewer Overflows BOD baseline BOD 2025 no combined sewer overflows scenario Combined sewer overflows have a noticeable impact on compliance with 99 th ile standards High Good Moderate Poor Bad

26 Results of needs assessment: analysis against WFD standards Best Available Technology Ortho-phosphate baseline Ortho-phosphate 2025 best available technology scenario Best available technology treatment at WwTW is not sufficient to achieve required status High Good Moderate Poor Bad

27 Summary and Conclusion

28 Summary and Conclusion The results of the modelling portray a complex picture of water quality dynamics in the River Almond system Key parameters of concern with regards to the Water Framework Directive and Urban Wastewater Treatment Directive compliance ortho-phosphate and BOD Most significant cause of poor water quality: continuous discharges from wastewater treatment works However wastewater treatment works also have a beneficial effect - diluting the intermittent untreated combined sewer overflow inputs Operation of key wastewater treatment works at Best Available Technology treatment levels: not sufficient to bring the whole system to the required status key for strategic decisions Assuming the current wastewater load to the system, achieving requirements through investment in wastewater treatment work will be challenging Model currently being used to test different options in discussion with the regulator (Scottish Environment Protection Agency) aiming to achieve best possible balance of compliance, also considering impacts on flow.

29 For more information contact: Vera Jones

30 Celebrating 75 years of design, engineering and project management excellence.