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UNIVERSITY of GUYANA FACULTY OF TECHNOLOGY

NAME: Jaikeshan TAKCHANDRA

REG #: 13/0933/1323

COURSE: CIV 3202 – Water & Wastewater Engineering

ASSIGNMENT 3 – Email Questions

LECTURER: Ms. S. Eastman

DATE: 30th March, 2014

Department of Civil Engineering 2015

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Disadvantages of prolonged storage of water

Disadvantage Solution / treatment

Microbial proliferation: Macrophytes (rooted aquatic plants) and free floating plankton plants (phytoplankton such as algae), and bacteria may become colossal in number in stored water supplies over extended periods of time, especially if the water is exposed to even small amounts of sunlight. If the water contains sufficient amounts of nutrient minerals to support these aquatic plant growths, the plant growth may become faster. This is characteristic of most lake water sources, containing igneous rock minerals. The plant and pathogens can alter the water’s quality by changing its color, taste and safety over the storage period. They can also attach themselves to the wall of the storage medium and feed on any form of organic matter, minerals and planktons in the water. This fuels their reproduction and a balanced system can be well established within a few months.

Disinfection prior to storage with sparing amounts of chlorine. Also, filtering the water may remove algae and planktons since they are quite macroscopic and will be trapped in the filter. It the storage medium is a conservancy, disinfection can be done prior to use.

Chemical alteration: Water in close and extended contact with the material of the storage container (e.g. plastic or paint) may experience alterations in taste and odour. This is as a result of chemical intrusion from the material and may pose health risks if the material is toxic or carcinogenic nature. Increased risks exist where the container is exposed to direct sunlight – direct sunlight can foster the breakdown of polymeric hydrocarbon chains resulting and increased release of chemicals in the water.

International health organisations have placed emphasis on the types and composition of certain types of plastic materials that are suitable and recommended for water storage. Storage medium made with these materials are recommended for usage.

Deoxygenated water: Water that has been stored for long period of time in a closed system may experience a reduction in the oxygen content to the point where the water has less than 5 parts per million oxygen. This is not a matter of concern but it does result in a profoundly noticeable flat taste in the water.

Mixing of the water before use will eliminate the taste and introduce oxygen in the water again

Risk of contamination: Stored water is always at a risk of contamination since there is no special need and emphasis placed on monitoring stored water on a routine basis. As such, there is a potential risk of contamination of the water that

Using a disinfecting agent in excess will prove to be effective since it will remain within the water and remove any forms of contamination. The disinfectant can be introduced such that it remains even after a few weeks.

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will propagate throughout the storage period. Water placed next to sewage lines, fertilizers, and even livestock (such as pigs and cows) is at risk for contamination. There is no simple way of testing for such contaminations and the question of whether or not the water was contaminated could be problematic.

Changing cycle: Some storage medium for water are such that the water needs to be changed and replenished within a certain time frame (usually 6 to 12 months). This can incur a lot of time and money if is required that the water be stored in large quantities.

Research on the storage container before implementation will help to reduce the effects of this problem.

Unexpected damage: Stored water that has been left unattended for long periods is at risk for unexpected damage which can potentially deplete all the water supplies in the container (e.g. a large storage tank that serves a desert village in drought season). Mechanical damage from a passing vehicle, or even a leaky container can cause a reduction in the water supplies.

Some containers can be strategically placed in secured locations (e.g. a cellar) or can be equipped with some form of external reinforcement or securing mesh. Also, durable and impact resistance materials can be used.

Lake Stratification

Epilimnion, thermocline and hypolimnion are characteristics of a climatic event that occurs

in the southern hemisphere of the earth each year. This phenomenon is called stratification and it

takes effect in lakes and reservoirs, whereby the water separates in several density-temperature

profiles. Changes in temperatures and density in the water in these systems are the main cause for

stratification, and as a result, the term thermal stratification is more common. The relationship

between water density and temperature is such that an increase in temperature will result in a

decrease in water density – until it reaches 4°C where the density decreases for deceasing temperature, enabling ice to float on water.

To understand how these profiles form, seasonal changes must be considered:

Pre-spring: Before the beginning of the spring season, the ice from winter has just begun to

melt on the lake or reservoir surface (lake will be used for further discussion). This melting of ice will continue until spring.

Spring: After all the ice has been melted, the water in the lake will have generally a constant

temperature throughout the lake – from the surface to the bottom. Wind action allows slow

circulation and mixing of the water via currents that move from the surface to the bottom

and back up. This allows for the transport of oxygen to the bottom of the lake and is termed

spring overturn. As spring progresses, the sun begins to warm the water’s surface (per se

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the first several feet since solar radiation decreases with depth). Due to the temperature-

density relationships mentioned before, this warm layer of water remains on top of the

cooler water below it, forming is what is known as the epilimnion – and it has similar

temperature throughout. The cooler water layer below this is called the hypolimnion. These

two layers are separated by a layer of water which changes rapidly in temperature with depth. This changing layer is called the thermocline (or metalimnion).

These three layers of water with distinct temperature characteristics is what is happens in thermal stratification.

Lake stratification can be limited and managed by installing several aeration equipment in

the lake medium. They function and reduce lake stratification by mixing air in the water in

the stratified layers by making the layers more thermally stable. Aeration will also eliminate

some of the problems of eutrophication and will act as a water quality enhancer.

The following table is the summarized characteristics for the epilimnion, thermocline and hypolimnion.

Layer Average location

(depth) Oxygen level Description

Epilimnion <25 ft. Balanced with

atmosphere

Some amount of turbidity due to presence

of algae brought up by water currents from

lake bottom – ideal for algae growth.

Algae growth usually results in greenish

hue to this layer

Hypolimnion >40 ft. Very low

(stressful)

Bacterial decay of nutrients and organic

matter aiding in low oxygen levels

Sparingly turbid and lack of sunlight and

photosynthesis

Thermocline (25-40) ft. Adequate

Maybe temporary layer or may not occur at

all.

Has high entropy and its agitated nature

allows for the up drafting of algae and

nutrients to the epilimnion

Water in the hypolimnion would prove to be most suitable for water use in my opinion. This is

because of the absence of most planktons and algae. However, the layer is known to have low

oxygen levels and amounts of iron, manganese is expected to be present. These constituents might

be a limiting factor in the use of water from this layer. If this occurs, water may be taken from the thermocline.

Water from the epilimnion is not selected since the presence of large amounts of algae and

planktons might cause eutrophication almost year round (except in winter). This makes the

purification process of the water timelier.

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FIG 1 – lake stratification in the summer showing expected temperature differences Source:

(Shaw, 2014)

Selecting optimum coagulant dosage

The optimal dosage of coagulant required in the clarification processes is dependent on a

few parameters, mainly:

The pH of the water to be treated (essentially taken as the raw water alkalinity)

The amount of turbidity present

The nature of the turbid particle

This means that the water quality assessment is essential before the commencement of any

coagulation process. These factors will then determine the subsequent mixing energies

required to carry out the coagulation process, and whether there exist a need for coagulant

aids. Typically, low turbidity waters require coagulant aids in addition to the use of

primary coagulants, and requires more turbulence or mixing to achieve maximum charge

neutralization of the particles. Higher turbidity waters can be effectively treated with just

the use of a primary coagulant. Furthermore, additives may need to be added to stabilize

the pH water during the coagulation process in order to optimize the functioning

conditions of the coagulant throughout the process – the pH of the raw water will need

determine the extent to which this is needed.

The evaluation of the aforementioned coagulant parameters can be achieved by using the

jar test. The jar test is one of the most popular means of conducting these tests. The tests

conducted are as follows:

Member re-filtration experiment

Jar test filtration experiment

Jar settling experiment

In addition, the pH is tested using a cation exchange experiment such triple-titration.

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FIG 2 – Surface water treatment process chart

FIG 3 – Groundwater treatment (conventional) process chart

Intake

• retrieve water for supply system

Screeing

• prevent damge to equipment

• prepare water for treatment

Coagulation

•necessary for preparing colloidal particles for flux formation

• removal of colloidal size particles

Flocculation

• very importation for the formation of large enough fluxs to foster sedimentation

• allows patriculate flox to sediment easily

Sedimentation

• final step of clarification that removes particle from the previous steps

•make water palatable

Disinfection

• important to make the water safe to drink

Storage

• allows for buffering of distribution system

• keeps water in potable state until distribution

Distribution

•provide water to consumers

Intake

•Retrieves water from deep well

Aeration

•Softens water by removal of iron and manganese

•Removes carbon dioxide, hydroden sulfide and other constituents

Filtration

•Aids in the removal of organic matter and other particles that affect turbidity

Chlorination

•Renders the water safe to drink by dissinfection

Storage

•Provides buffer for distribution system by safely storing water

Distribution

•Takes water to consumers

Rapid mixing

Chlorine

Filtration

Intake structures

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References

Lee, G. F., 1965. Effects on Intake Location on Water quality. [Online]

Available at: http://www.gfredlee.com/intake.html

[Accessed 27 3 2015].

Loosdrecht, M. v., 2014. Coagulation & Floocculation in Water and Wastewater

Treatment. [Online]

Available at:

http://www.iwawaterwiki.org/xwiki/bin/view/Articles/CoagulationandFlocculatio

ninWaterandWastewaterTreatment

[Accessed 28 03 2015].

Shaw, B., 2014. Understanding lake data. [Online]

Available at: http://www.uwsp.edu/cnr-ap/weal/Documents/G3582.pdf

[Accessed 27 3 2015].