Transcript WATER POLLUTION ASSESSMENT

CIV 913 Environmental Assessment and
Sustainability
Eutrophication
Eutrophication of Freshwaters - Harper D
Freshwater Ecology - various
Limnology - various
Eutrophication
• Objectives
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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 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)
Eutrophication
Limnology
• Trophic classification of Lakes
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Ultraoligotrophic
Oligotrophic
Mesotrophic
Eutrophic
Hypertrophic
see OECD
categories
• Numerical Classification
– Trophic State Index (TSI)
• scale 0 - 100
• by Secchi depth - 64m= 0; 32m= 10; 16m=20; etc
Eutrophication
Limnology
• OECD Trophic Categories
CATEGORY
P
Chl.
Max
Secchi Secchi (min)
Chl.
(m)
(m)
4
1
 2.5
12
6
Oligotrophic
 10
 2.5
8
6
3
Mesotrophic
10 - 30
2.5 - 8
8 - 25
6-3
3 - 1.5
Eutrophic
35 - 100
8 - 25
25 - 75 3 - 1.5
100
25
Ultraoligotrophic
Hypertrophic
75
 1.5
1.5 - 0.7
 0.7
Eutrophication
Productivity
• Rates of Primary Production in Lakes.
Oligotrophic
Eutrophic
Natural
Polluted
300-1000
3000 - 15000
75-250
350-700
Mean rates in
growing season.
(mgC/m2/d)
Annual Rates
(mgC/m2/d)
30-100
7-25
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
TP 
• L is surface loading of P
• z is depth
• p is flushing (renewal per year)
• O is sedimentation rate coefficient of P
 
L
z (O  p)
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
see UWWT Directive
• 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
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”)