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CIV 913 Environmental Assessment and Sustainability

CIV 913 Environmental Assessment and Sustainability. Eutrophication Eutrophication of Freshwaters - Harper D Freshwater Ecology - various Limnology - various. Eutrophication. Objectives Causes Limnology and Lake Ecology Effects Control Strategy Definition

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CIV 913 Environmental Assessment and Sustainability

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  1. CIV 913 Environmental Assessment and Sustainability Eutrophication Eutrophication of Freshwaters - Harper D Freshwater Ecology - various Limnology - various

  2. Eutrophication • Objectives • Causes • Limnology and Lake Ecology • Effects • Control Strategy • Definition • The enrichment of waters by inorganic plant nutrients.

  3. Eutrophication • Cause - sources of Nitrogen and Phosphorous. • External • Municipal and Industrial wastewaters. • (Main source of Phosphorous) • Land run-off. • (Main source of nitrogen) • Atmospheric Deposition. • Internal • Nutrient regeneration from bottom sediments. • Groundwater seepage (sub-surface flow) • Historical incidence • recent (demographic growth - consumerism)

  4. EutrophicationLimnology • Lake vs River • renewal time years vs days • Stratification in Lakes. EPILIMNION THERMOCLINE HYPOLIMNION

  5. EutrophicationLimnology • Stratification. • Formed by temperature gradient. • Most of the heat from light penetration is absorbed in top 1 or 2 metres. • Wind gives rise to mixing to form: • epilimnion at the top • hypolimnion at the bottom • a transitional zone, the metalimnion. in which a thermocline exists. • Temperature range may be: • 20`C to 4`C in temperate lakes. • 29`C to 25`C in tropical lakes (but can be equally stable stratification)

  6. EutrophicationLimnology • Nutrients in lakes • Nitrogen • fixation, sediment denitrification • Internal Phosphorus Cycling Forms of P • bound to Ferric hydroxides • bound to Calcite (CaCO3) or hydroxyappetite (Ca5OH(PO4)3 • bound to clay • released by extreme pH, change in redox (anaerobic)

  7. EutrophicationLimnology • Trophic classification of Lakes • Ultraoligotrophic • Oligotrophic • Mesotrophic • Eutrophic • Hypertrophic • Numerical Classification • Trophic State Index (TSI) • scale 0 - 100 • by Secchi depth - 64m= 0; 32m= 10; 16m=20; etc see OECD categories

  8. EutrophicationLimnology • OECD Trophic Categories CATEGORY Ultraoligotrophic Oligotrophic Mesotrophic Eutrophic Hypertrophic Max Chl.  2.5  8 8 - 25 25 - 75 75 Secchi (m) 12 6 6 - 3 3 - 1.5  1.5 Secchi (min) (m) 6 3 3 - 1.5 1.5 - 0.7  0.7 P 4  10 10 - 30 35 - 100 100 Chl.  1  2.5 2.5 - 8 8 - 25 25

  9. EutrophicationProductivity • Rates of Primary Production in Lakes. Oligotrophic Eutrophic Natural Polluted Mean rates in growing season. (mgC/m2/d)30-100 300-1000 3000 - 15000 Annual Rates (mgC/m2/d) 7-25 75-250 350-700

  10. Eutrophication • Prediction of Water Treatment Plant Problems. • UK study in 1960s by Lund to predict effects: • Winter maximum PO4 > 5g/l • Winter maximum NO3 > 300 g/l • Produces Algae > 3000 cells/ml • Models in 1960’s by Vollenweider • where • TP is total phosphorus • L is surface loading of P • z is depth • p is flushing (renewal per year) • O is sedimentation rate coefficient of P

  11. Eutrophication Predicting Permissible P Loading Using OECD Formulae. • Developed relationships between: • Chlorophyll A (annual mean and maximum),[Chl] • P inlet concentration [P]i and • hydraulic residence time Tw [Chl]mean = [P]i / (1+(Tw)0.5) mg/m3

  12. Eutrophication • Effects. • Freshwater. • Fish diversity reduced. • Low/no DO in hypolimnion, hence reduced fauna and flora diversity. • Algal blooms and adverse aesthetics. • Algal blooms and water treatment difficulties. • affects drinking water quality and treatment costs.

  13. Eutrophication • Effects • Seawater. • Algal blooms. • Red tides (phaecocystis) and toxins affect coastal fisheries. • Corals. • Suffocated by algal sedimentation. • Macrophytes in shallow coastal waters. • Increased biomass (fish).

  14. Eutrophication • Adverse Effects of Algae in Water Treatment • physical blocking of filters • 3000cells/ml detrimental • polysaccharides • chelate Fe and Al ions (enter treated water) • THM production • Taste and odour • toxins • animal infestation in distribution system • industrial • ion exchange poisoned • deposits block valves

  15. Eutrophication • Water Quality Objectives for Lakes. • Must take account of intended use. • Develop a nutrient load control strategy. • Using algal biomass as a trophic response indicator: • set target for mean algal biomass • set target for peak algal biomass • Determine phosphorous load to be removed. • Control point sources, then diffuse sources.

  16. Eutrophication • Typical Controls. • Municipal sewage treatment. • chemical precipitation • biological removal • combinations. • Pre-reservoirs (>15 day HRT, aerobic) • Chemical precipitation in the lake. • High flow-through lake. 3 - 5 day HRT. • Hypolimnetic aeration. • Artificial water circulation. • Land use practices. • Removing polyphosphates from detergents • Flushing • Dredging see UWWT Directive

  17. Nutrient Removal - Standards - UWWT Directive (1991): Pop >10,000 N<15mg/l P<2mg/l Pop >100,000 N<10mg/l P<1mg/l or 80% removal of Total P 70 - 80% removal of Total N (The above applies to “sensitive waters”)

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