Economic Viability of Dissolved Oxygen Amelioration

in Tailwater of Hydroelectric Dams to Ensure Wildlife

Indemnity

CEE 361: Environmental Engineering

Date Submitted: April 9th, 2015

Course Instructor: Arjun Venkatesan

Teacher Assistant: Maurissa Charles

Author: Hugo Ponsonnet, Ali Ibrahim, David Resler, Roy Sims, Pinquan Li, Christian Castillo, Kachun Tsui & Matt Starks

Key words: Lake turnover, eutrophication, dissolved oxygen, lake stratification, hypolimnion layer, tailwater

Abstract

Dissolved oxygen consists of microscopic bubbles of gaseous O2 mixed in liquid

water and is required for marine life respiration. Any anthropogenic process that results

in the depletion of DO in the surrounding water is called cultural eutrophication. If DO

drops below minimum levels in a body of water, the water quality is compromised and

aquatic life will perish. This paper specifically addresses water released from

hydroelectric dams. The goal of this paper is to determine the most cost effective

method of increasing dissolved oxygen (DO) levels in the tailwater of hydroelectric

dams which subsequently minimizes the negative impacts on aquatic organisms

downstream. A literature review was conducted to summarize the current knowledge of

DO quality issues related to hydroelectric dams. Table Rock Dam, located on the White

River in Arkansas, will be used as a case study for this paper.

Literature data shows two widely used processes by which low DO levels in a

dam’s tailwater can be increased. The first method of influencing DO levels consists of

injections of liquid oxygen (LOX) into the tailwater of a dam. The second method

involves the introduction of atmospheric oxygen via vents in the turbines housed within

the dam, which is then mixed with the water as it passes through the turbines. Both

methods are effective with regards to raising DO levels in a dam’s tailwater. The

majority of hydroelectric dams possess both of the previously mention DO manipulation

methods. It is expected that LOX injections will rectify low DO levels more economically

when compared to turbine venting due to the loss of turbine efficiency associated with

venting.

Introduction/Background

DO is an issue to companies that operate hydroelectric dams due to the fact that

fish kills are highly visible and make great news on a slow day. Dams, by their very

nature, are highly prone to creating low DO conditions in their tailwater due to the

physical nature of the project. A dam create a large, slow-moving body of water that

serves as an accumulation point for cultural eutrophication processes. In addition, a

natural phenomenon known as lake stratification creates a hypolimnion layer where low

DO conditions can persist for an extended duration. Unfortunately, dams must use

water from the hypolimnion layer to generate electricity because that layer provides the

most head to the turbine units. The low DO water is run through the turbines and into

the dam’s tailwater where fish tend to accumulate. Fish tend to die when DO drops

below 2 ppm resulting in large kills when this condition is met. Hydroelectric companies

use several methods to increase tailwater DO, the two most prevalent being turbine

venting and LOX injections. Turbine venting results in efficiency losses and LOX

purchases can run a large bill. The goal of this report is to determine which method is

more cost effective.

Lake Turnover

Lake Turnover is the process of a lake’s water turning from top (epilimnion) to bottom

(hypolimnion) (1). During the fall, the warm surface water starts to cool. As the water cools

down, it become more dense and therefore causes to sink. This dense water forces the water

of the hypolimnion (bottom surface) to rise, turning over the layers. During the summer, the

epilimnion or surface layer is the warmest as it is heated by the sun. The deepest layer, the

hypolimnion is the coldest as the sun’s radiations do not reach this cold, dark layer. The most

efficient way to determine if a lake is turning over it to check the temperature with an electric

thermometer. When the turnover is in progress, the entire water temperature readings will be

the same,

give or take a

couple of

degrees.

Figure 1: Lake Turnover Illustration (2)

The set of diagram above well illustrates the year-round lake turnover. In our project, we

will only focus on the summer and winter season, as the turnover does not occur during spring

or fall. Lake turnover happens depending on many factors. Large reservoirs or natural lakes

that have constant wind or a frequent current will not typically turn over (3). The wind and

current keep the water mixed at all time, leading to similar temperatures at the bottom and top

layers. In our case, the Little Rock Lake does turn over, as the current and wind are fairly weak.

The consequences of lake turnovers are greater than just a change in water density and

temperature. It eliminates oxygen barriers. After the lake has stratifies in early summer and

water in the depths no longer mixes, a stagnation process starts. Decaying organic materials at

the lake bottom, along with living organisms in the water, consumes the dissolved oxygen. With

no circulation to restore the level of oxygen and no aquatic plants to produce it, this leads to the

oxygen levels in the hypolimnion layer to decline. Furthermore, in our particular case, the Table

Rock dam pumps the bottom water in which contains a deficiency of oxygen and pumps it out

on the other side of the dam in the river (4). This river is then contaminated with low

oxygenated water, which drastically impacts its eco-system.

DO Limits and Reactions

One of the most important elements for a healthy water quality is the amount of

dissolved oxygen present (DO). The amount of DO in water affects the water activity. Nethers

too high level of DO nor too low DO are good to the environment because both too high and too

low level of DO will be harm aquatic life. Therefore, it is important to understand what are the

factors affect the amount of DO available.

Some of the major factors are list as the following:

● Volume and velocity of water flow

● Climate and season

● Type and amount of organisms

● Organic wastes

Some of the factors might seem to be negotiable; however, they are very important. A small

change of the above factors affect significantly in DO levels. Since there are so many factors

can cause the effects of DO levels, a healthy water depend on the amount of DO present.

The minimum amount of Oxygen in lakes depend on the environment. Different kind of

species required different DO levels. Some species can easily tolerate DO level below 1 ppm;

while some other species can survive at 2 ppm DO levels for only a short period of time. In

order to determine what is the DO levels best for certain types of living environment, we have to

know what kind of species do live in the area. In the case study of the Table Rock Dam

tailwater, some of the fish species live in that area is listed below:

1)Bass, Hybrid Black

2)Bass, Spotted

3)Bass, White

4)Spotted/smallmouth hybrid (see bass, hybrid black)

(Figure of minimum dissolved oxygen requirements of fish species)

Base on the above information, most of the fish species that live in the Table Rock Dam

tailwater belong into a group of bass fish. The bass fish requires at least a minimum of 5 ppm

DO in order to grow and thrive. DO of a level 5 ppm is a most steady state for the species to

live.

Consequences of fail DO levels leads to rise fish mortality rates. For example of Trout

and Salmon, from the figure Salmon has to live in a DO level of 6 ppm. Once the DO levels drop

below the DO levels requirement. Those kind of fish species will no longer to be able to

reproduce. The reason is because the Trout eggs are not only delicate, but the Trout fry (baby

Trout fish) are also very sensitive and require higher DO levels. Therefore, those kind of fish has

to have above 7.0 ppm in order to reproduce.

Additionally, the DO levels drop below a certain level, fish kill may happen. A sudden

appearance of dead fish is to alarm people for the unusual activities. A fish kill is a larger

amount of fish die, and float on the water surface is usually associated the low DO levels.

The DO levels in Table Rock Dam tailwater is definitely not a good environment for fish

to live. In order to sustain the DO levels on health level, the turbine vent operate runs more

oxygen dissolve into the water. If the DO continuous drop to, once the DO levels reach less than

4 ppm, the LOX injection and generation limitations will run. All of the operations included

turbine vent, LOX injection and generation limitations are intended to manage the DO levels to

keep on a level of 6 ppm.

In order to have the operations occupy when the DO levels reach below 4ppm, a

prediction of sudden drop of DO levels will be helpful. This involves some of the chemical

reaction that happens in the tailwater.

The Dissolved Oxygen in the tailwater is mainly depleted by the following chemical and

relative reactions. Three major chemicals are: manganese sulfate (Mn(SO4)), potassium

hydroxide (KOH), and sulfuric acid (H2SO4)(5). The main processes of the reactions are shown

below(6).

First, an excess of Mn(SO4), hydroxide (OH-) ions is added to a water causing a white

precipitate of Mn(OH)2 to form. This precipitate is then oxidized by the dissolved oxygen in the

water sample into a brown manganese precipitate:

MnSO4 + 2KOH ↔ Mn(OH)2 + K2SO4

Manganese Sulfate + Potassium Hydroxide ≡ Manganese Hydroxide + Potassium Sulfate

In the next step, a strong acid (either hydrochloric acid or sulfuric acid) is added to acidify the

solution, this acid provide huge number of free protons and create a complete redox reaction:

2Mn(OH)3 + 3H2SO4 ≡ Mn2(SO4)3 + 6H2O

Manganese Hydroxide + Sulfuric Acid ≡ Manganese Sulfate + Water

Also, the oxygen in the water oxidizes an equivalent amount of the manganous hydroxide to

brown-coloured manganic hydroxide. For every molecule of oxygen in the water, four molecules

of manganous hydroxide are converted to manganic hydroxide. Chemically, this reaction can be

written as:

4Mn(OH)2 + O2 + 2H2O ↔ 4Mn(OH)3

(Manganese Hydroxide + Oxygen + Water ≡ Manganese Hydroxide)

This last reaction is the final process that the dissolved oxygen being depleted. Also,

because it is colorimetric modification, where the trivalent manganese produced on acidifying

the brown suspension is directly reacted with EDTA to give a pink color, so these reactions can

be easily monitor and the volume of depleted dissolved oxygen can be recorded.

For the resources of these two main consumers of dissolved oxygen, Magnesium's

effect on the environment results from the emission of hazardous air pollutants from magnesium

industrial plants. Potassium hydroxide KOH is a colorless, odorless, corrosive, deliquescent

crystalline solid. It readily absorbs water and carbon dioxide from air. KOH has a good solubility

in water, 49.4% wt at 0°C, its solvation is highly exothermic. It also dissolves in methyl alcohol

(35.5% wt at 28°C), ethanol (27.9% wt at 28°C)(7). Normally, Potassium hydroxide is found in

varied items such as liquid soaps, lotions, shampoos, hairsprays, and denture cleaners, but is

also found in more industrial compounds such as oven cleaners, drain cleaners, driveway and

concrete cleaners, in non-phosphate detergents, and in drain and pipe cleaners(8).

Engineering solutions/Technology options

There are many different methods to ameliorate DO levels in a dam’s tailwater.

The three methods that are commonly used and that will be discussed are aerating

turbines, liquid oxygen (LOX), and natural aeration. Dams can either use the turbines

that are already installed and simply retrofit them or use turbines that are already able to

aerate.

The most popular and widely used type of turbine used for hydropower are

Francis turbines. These turbines have varying designs, the two predominant being

active and passive, or automatic, aeration [23,19]. Active turbines mechanically force

oxygen into the water passing through, either by motorized blowers or air compressors.

There are a few variants of the passive turbine designs. In Hydro Performance

Processes Incorporated’s report [19] to the U.S. Department of Energy, they suggest

the best passive turbine design is done by modifying the runner or draft tube, creating

localized zones of low pressure that draw atmospheric air into the turbine, aerating the

water. This is the cheapest and easiest way to retrofit an older turbine that does not

have an active aeration design.

Aeration by passivated turbines are primarily done by three different means,

central aeration, peripheral aeration, and distributed aeration [22]. In central aeration,

air bubbles are brought down through the draft tube, it is inexpensive but is most

effective at partial loads, and is inefficient at raising the DO levels at full load. In

peripheral aeration, oxygen bubbles are added from the inner walls of the draft cone,

this method more evenly distributes the oxygen into the water than does central

aeration, it is good with medium to high loads but decreases the turbines efficiency. In

distributed aeration, oxygen is injected into the runner bucket and utilizes the low

pressure area at the very edge of the turbine’s blades, injecting the air directly into the

runner, this allows for most thorough mixing and most efficiency. All passive aeration

injection areas are located on Figure ###.

Figure ###: Francis Turbine [19]

The cost associated with turbine venting is the loss of power production

efficiency due to cavitation. Cavitation is the largest sources of inefficiency in a hydraulic

turbine. The loss occur when bubbles of air are formed in the turbine and collide with

the runner [24]. When this occurs, the bubbles collapse, sending waves through the

runner and surrounding water, at the high flow turbines see and multiplied by the

number of bubbles in the turbines, this can destroy runner blades. The efficiency loss is

generally less than 1.3% [13].

Certain hydroelectric facilities are outfitted with the ability to augment discharged

turbine water with liquid oxygen injections (LOX). LOX is typically stored onsite in large

tanks, used to supplement the tailwater during periods of exceptionally low DO levels.

For Table Rock Dam (see Case Study), two, 20 short ton tanks, approximately 4200

gallons each, are kept onsite. The company that owns the project will typically have a

standing order with a contracted provider.

Figure xxx: LOX storage at dams [26]

When the DO is low in the tailwater of the dam, LOX will be released directly to

the tailwater to aerate it. This method requires either a substantial amount of storage

space for multiple LOX tanks or frequent refilling of the two current tanks. Using either

method, the cost to use LOX are expensive and will have to be compared to turbine

efficiency losses.

Two natural aeration processes that will be discussed, but not compared to either

turbines or LOX, are siphons and weirs. Some projects use what is known as a siphon

to foster freshwater habitats. For example Norfork Dam in Arkansas uses a 42-inch-

diameter pipe to carry water through the dam and down the structure's face, discharging

it into the Norfork tailwater (North Fork River) [10].

Figure xxx: Norfork Dam Siphon (photo by Roy Sims)

Another option to aerate the water are weirs. A weir is a man made structure

that imitates a natural waterfall or rapids. As the water runs over the weir, it is agitated

and mixes with the surrounding atmosphere. Both help increase DO levels, although

the siphons are not very affective and weirs aerate downstream water and not tailwater,

the water immediately leaving the turbines. Because of this, both options are seen as

additional DO amelioration steps that go with either turbine venting or LOX injection, but

not alone, as far as dams are concerned.

Figure xxx: Example of a weir [27]

Additionally a project can self-limit generation which is referred to as

recommended maximum generation rates (RMGR). The reduced influx of low DO

outflow into the dam’s tailwater is a common practice, but all water must eventually be

released in order for flooding attenuation requirements.

● Tennessee Valley Authority Study

In the late 1980’s the Tennessee Valley Authority (TVA) began an extensive, five year

study on improving dissolved oxygen levels in their hydrologic power production

facilities. The study included 20 different dam locations, each with specific requirements

for minimum DO levels and initial conditions. Each location was evaluated

independently based on environmental conditions, tailwater and upstream water DO,

and feasibility of improvement techniques. The result of the project was individual

improvement plans for each location which included several different types of DO

improvement processes among other improvements. These DO improvement

processes included; turbine venting, oxygen diffusers, surface water pumps, air

blowers, and liquid oxygen injection which are summarized in Figure XXX below.

Figure XXX: Composition of different aeration methods. (13)

Each location, having its own unique needs and conditions, was fitted with the most

appropriate DO improvement method, as selected by the TVA. After improvements for the

locations were completed, each location was monitored for DO, along with other critical

information, to determine whether the methods were successful. The study gathered historical

data, when available, taken from each location from 1971 through 1995 and averaged for the

historical data. The historical average was then compared against the data taken in the four

years after the completion of the projects, in 1994 through 1997 and is summarized in Figure

XXX below.

Figure XXX shows days below dissolved oxygen target vs year by dam location.

As seen in Figure XXX, the number of days when the dissolved oxygen fell below the

target limit by location decreased significantly in most locations when compared to the

respective historical average. This effect was directly attributed to the improvement

projects undertaken by the TVA.

● Table Rock Dam (TRD) Case Study

○ TRD reservoir volume: 3,462,000 Acre-ft

○ TRD reservoir maximum depth: 220 ft

○ TRD surface area: 43,100 Acres

○ Power generation capabilities: 4(50 MW vertically mounted turbines)

○ TRD low DO season typically begins in July and ends in December.

○ 2012 low DO season:

■ Energy production: 73,289.5 MWh

■ Average turbine release: 1380.9 cfs

■ DO amelioration methods used:

● Vents opened: 7/6/2012

● Recommended Maximum Generation Rates (RMGR) begins

at 70% = 140 MW: 10/19/2012

● RMGR ends: 12/19/2012

● Vents Closed: 12/20/2012

○ Costs associated ($/season)

The conditions that warrant these actions are detailed in the Plan of Operation, and are

intended to keep DO levels in the tailwater above 4 ppm during periods of generation.

No action is required at TR when the tailwater DO drops below 4 ppm during times of no

generation.

Discussion

– Comparison of different solutions/technologies

- Which method was more cost effective at removing low DO at TRD (cost

analysis)

- Cost analysis table

- Will all other areas benefit the same from the most cost effective option?

Conclusions

It was expected that the liquid oxygen injection would be a more economic

approach to solving the low tailwater DO issue. This was due to a comparatively low

cost of liquid oxygen when compared to the loss of efficiency in the turbine generation,

which translates into a loss in revenue. This expectation was entirely incorrect, the cost

“Two findings guided the selection of alternatives. First, no single alternative or set of solutions is

appropriate for all projects. Each dam is physically different and requires a unique set of facilities and

operations. Second, some redundancy or mix of alternatives is

needed to provide operational flexibility. These principles are apparent in the diversity and combinations

of facilities installed at the 20 projects “ Reword and cite for conclusion paragraph.

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