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– The enrichment of waters by inorganic

plant nutrients.

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)

Eutrophication

Limnology

• Lake vs River– renewal time years vs days

• Stratification in Lakes.

EPILIMNION

THERMOCLINE

HYPOLIMNION

Eutrophication

Limnology

• 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)

Eutrophication

Limnology

• Nutrients in lakes– Nitrogen

• fixation, sediment denitrification

– Internal Phosphorus CyclingForms 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)

Eutrophication

Limnology

• 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 OECDcategories

Eutrophication

Limnology

CATEGORY

Ultraoligotrophic

Oligotrophic

Mesotrophic

Eutrophic

Hypertrophic

• OECD Trophic Categories

P

4

10

10 - 30

35 - 100

100

Chl.

1

2.5

2.5 - 8

8 - 25

25

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

Eutrophication

Productivity

• 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

Eutrophication

• Prediction of Water Treatment Plant Problems.

– UK study in 1960s by Lund to predict effects:

• Winter maximum PO4 > 5g/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

)( pOz

LTP

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

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.

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).

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

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.

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

Nutrient Removal - Standards -

UWWT Directive (1991):

Pop >10,000 N<15mg/l P<2mg/lPop >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”)