CE3503 Expectations –Water Quality
Overview
Understand the various beneficial uses of surface waters (drinking water,
recreation, wastewater discharge) and how their location within a community
and/or watershed can lead to conflicts.
Lakes - Thermal Stratification
Understand the mechanism of thermal stratification and turnover, including the
significance of the temperature of maximum density.
Be able to draw profiles of temperature versus depth for summer stratification,
winter stratification and turnover.
Be able to identify the completely-mixed and thermally stratified periods on a 2-D
temperature-depth-time plot.
Epilimnion
Metalimnion
Hypolimnion
Thermocline
Lakes - Trophic State
Understand the Limiting Nutrient Concept and be able to identify the nutrient
which is typically limiting in freshwater.
Be able to differentiate between oligotrophic and eutrophic lakes in terms of
phosphorus and algae content, transparency, hypolimnetic oxygen and the
fisheries that they support. Why is a mesotrophic lake often considered a ‘happy
medium’?
Be able to define eutrophication, differentiating the natural and cultural forms and
explaining how the interplay of eutrophication and thermal stratification leads to
water quality problems.
Be able to draw profiles of oxygen and temperature versus depth for oligotrophic
and eutrophic lakes.
Understand how anaerobic metabolism exerts negative influences on water
quality through iron-phosphorus interactions and through the sulfur cycle.
Be familiar with the seven engineering approaches to lake management applied in
mitigating the impacts of phosphorus on trophic state.
Rivers
Be able to explain what BOD is and why it is a good example of ‘representation
by effect’.
Be able to define and differentiate between 5-day and ultimate BOD.
Be able to write the differential equation describing a mass balance on oxygen in
rivers and describe the formation and behavior of the dissolved oxygen sag curve
based on that equation.
Understand the value of benthic macroinvertebrates as indicators of stream water
quality and be able to explain the ‘diversity-abundance’ relationship.
Surface Water Quality Problems
River Management: river flow upstream of the mixing basin = 10 m3·s-1; wastewater
treatment plant effluent flow = 2 10 m3·s-1; 5-day CBOD of the river upstream of the
mixing basin = 2 mg∙L-1; 5-day CBOD of the wastewater effluent = 80 mg∙L-1; CBOD
decay rate coefficient = 0.15 d-1; oxygen saturation value = 9.0 mg∙L-1; reoxygenation
coefficient = 0.3 d-1.
1. Calculate the ultimate CBOD of the wastewater effluent and the river upstream of the
mixing basin. [152 mg∙L-1; 4 mg∙L-1]
2. Calculate the ultimate CBOD of the mixing basin. [28 mg∙L-1]
3. Calculate the dissolved oxygen concentration and the dissolved oxygen deficit of the
mixing basin. [7.8 mg∙L-1; 1.2 mg∙L-1]
4. Calculate the location of the critical point. [time of travel downstream, 4.3 d]
5. Calculate the dissolved oxygen deficit and the dissolved oxygen concentration at the
critical point. [7.4 mg∙L-1; 1.6 mg∙L-1]
6. If the water quality standard for oxygen in the river was 4.0 mg∙L-1 would that
standard be violated under these discharge conditions?
7. Note that the iterative process used to determine the required CBOD removal
efficiency at the wastewater treatment plant, as well as the system’s sensitivity to
environmental forcing conditions, e.g. temperature and flow, are best examined using a
tool such as the River DO simulator.