Download - CIV3202-A3
13/0933/1323
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
13/0933/1323
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.
13/0933/1323
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
13/0933/1323
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.
13/0933/1323
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.
13/0933/1323
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
13/0933/1323
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].