Quiz 4 Flashcards
(33 cards)
Why add oxygen to lakes and reservoirs (2)
Deposition of carbonaceous materials, decomposition of algae, and chemical reactions reduce oxygen demand in lakes and reservoirs
Adding oxygen meets the increased oxygen demand that this creates in stratified eutrophic lakes
Whole lake oxygen depletion (2)
Occurs in winter in eutrophic lakes because wind and diffusion inputs can’t penetrate the ice cover
Results in loss of suitable habitat for cold water fish (salmonids) as oxygen goes less than 3-5mg/L
What is a fish kill called and why? (2)
In the summer it is called a summerkill or a “thermocline squeeze” and this is because, fish are squeezed in the epilimnion during stratification as oxygen is too low at depth.
In the winter it is called a winterkill. These occur usually in mid-late February or at spring circulation.
What causes a winterkill? (2)
Mid to late February it is caused by fish running out of oxygen as they can only go ~90 days without
In the spring it is caused by epilimnion layer mixing into lower oxygen depleted layers, causing dilution of oxygen in upper layers, as well as release of nutrients and chemicals from the sediment layer
What negative effect can spring mixing cause? (3)
During the winter, anoxic conditions cause 10X more toxic compounds and nutrients to be released from the sediment (lid is removed because of lack of chemical reactions)
This causes winterkills in spring because those toxins are circulated throughout the lake
Therefore: lack of oxygen causes more nutrients causes more lack of oxygen and it is a positive reinforcement cycle
What are the toxic compounds that can be released from the sediment layer? (3)
NH3, H2S - very toxic
Mn and Fe ions - less toxic
P and NH3 - algal nutrients
Internal loading
Increased nutrient loading caused by the removal of the oxic layer at the bottom of a eutrophic lake
Results of internal loading (4)
Grows more algae from nutrient input
Further depletes oxygen in the hypolimnion
Causes fish kills and releases even more nutrients from their bodies
= runaway process that leads to an alternate stable state
Two categories of artificial circulation (2)
Destratification - aerates through removing stratification
Hypolimnetic aeration - aerates only the hypolimnion, preserving stratification
Destratification (3)
Type of artificial circulation that breaks up lake layers
Only a cosmetic procedure, unless underlying source of excess nutrient loading has been identified and reduced
Uses compressed air or mechanical mixers
Compressed air destratification (3)
Type of destratification system
Oxygen added to lake from rising bubbles (from diffusers at bottom of lake), but majority is on the surface when rising bubbles come in contact with the atmosphere and transfer the air to the surface water
Winter aeration creates an open ice area when the system is operating ( need fence)
Mechanical surface aerators (4)
Type of destratification system
Adds oxygen by:
Propeller turbulence causing internal motion
Water spray contact with atmosphere at surface
Requires an underwater electrical cable and voltage of electricity to operate propellor = major limitation
What is a major limitation of mechanical aerators?
They require electricity and long cables, making them expensive and difficult to implement in rural/wilderness areas
Solar and wind powered mechanical aerators
Not usable as mechanical destratification systems they don’t have enough power due to lack of sun and wind
Garton pumps (2)
Type of destratification system that uses a propellor and electricity
Very common in drinking water reservoirs in Australia
Electricity need (3)
Mechanical aerators are the preferred method for many lake destratification projects
Therefore we need electricity
In B.C., electricity is 60 hertz and single or three phase
Single phase voltage (5)
115 or 230 volts
As voltage doubles, the amperage decreases
On a “virgin line” the max power you can operate is two 230 volt, 7.5hp (5.6kW) electric motors
Because of the rise and falls in voltage, power is not delivered at a constant rate and single phase motors larger than 10kW are rare
Single phase motors need additional circuits for starting because single phase supply does not produce a revolving magnetic field
Watts equation
Watts (ie. power) = volts* amps
Ex. 0.746kW electric motor running on 115 volt single phase electricity, requires 746W/115v= 6.5 amps
If 230 volt single phase electricity with same kW electric motor, 746W/230v= 3.2 amps
Relationship between volts and copper wire (6)
Higher volts means longer, thinner copper wire
115 volts = 49m distance
230 volts = 120m distance
Why? Because increasing voltage decreases the amps and decreases resistance
Therefore, always use highest available voltage to decrease amperage, weight, and cost of cable
Unwritten rule: on a “virgin line” can exceed these distances by 50%
Can also use multiple units, as long as you don’t exceed 11.2kW/230v limit
Three phase power (4)
In three phase power, the currents reach their peak values sequentially, first one, then second one; then third one.
This is in contrast to the single phase that has two wires reaching their peak at the same time.
Therefore, they have much higher voltage than the single phase (240, 480, 600 and higher)
Three phase motors are self starting because they produce a magnetic field
When should you switch to three phase? (2)
When you require more than 11.2kW of power
Bottom line: you cannot install a destratification system from a single phase power line if your energy requirements exceed 2 x 7.5hp (2 x 5.6kW) of energy (two 230 volt motors)
Phase converters (2)
Phase converters are meant to create three phase electricity out of single phase electricity
Generally don’t work
Fossil fuel electricity (2)
Can’t justify, causes climate change
Diesel, propane, liquified natural gas all options, but worth it?
Possible energy sources for power generation (4)
For destratification: Electricity Wind power Solar power Fossil fuels
