DEPARTMENT OF CIVIL ENGINEERING

COLLEGE OF SCIENCE AND TECHNOLOGY

COVENANT UNIVERSITY, OTA, OGUN STATE, NIGERIA

Copyright David O. Olukanni Covenant University, Ota

CVE 410WATER AND WASTEWATER ENGINEERING (2) Module 2

Elements of Water Chemistry, Treatment Processes for Surface and Groundwater

Water is a clear colourless liquid with an insipid taste. Usually recognized in the laboratory by its capacity to turn anhydrous copper II sulphate (white) to a blue colour

CUSO4 (s) + 5H2O (l) CUSO4.5H2O(s)

Copper (II) Sulphate Water Hydrated Copper II

Sulphate (blue)

Water freezes @ 00c, boils @ 1000c when barometer stands @ 760mm, pure water will boil away completely with no change in temperature. Maximum density is 1gcm-3 @ 40c.

Analysis and Examination of WaterBy water examination, we refer to the determination of the following:

1. Physical - Colour,

- Turbidity

- Odour

- Taste

- Solids

2. Chemical - (performed to detect the quality of chemical substances in water). Chemical substance can conveniently be discussed under the following:

i. Substance which affect portability

ii. Substance which are hazardous to health Fluoride and Nitrate. Fluorides are beneficial for prevention of dental clinics when less than 2mg/liter in water.

Above this level, it causes complexity like dental, crispling and skeletal flourosis. Nitrate above 45mg/liter causes blue baby (methaemoglobinaemia) under one year of age

iii. Substance which are toxic: This accumulates poisons, lead, calcium, cyanide, arsenic, mercury and celenium.

iii. Substance which are indicators of pollution e.g. BOD, COD, Carbon, chloroform, Ammonia and grease.

Methods of Chemical Examination

Basically, there are 2 available methods, namely:1. Titration method: Conventional method used in water and

wastewater examination laboratories. The practice is to choose the normality of the reagent so that 1ml of the titration agent is equivalent to a unit weight of the substance to be determined. In titration, the reagent are prepared to facilitate calculations of the concentration of the substance required.

2. Colourimetric method: The practice is to prepare standard colour to correspond to non different concentration of the substances. The intensity of the colour is proportional to the concentration of the substance present. It is also called comparator method.

3. Bacteriological Examination and other microscopiccharacteristics of water.

This is performed to ensure that water does not transmitdisease to the consumer. There are 2 broad methods

1. Multiple tube method

2. Filter membrane method

The most important parameter of drinking water quality is thebacteriological quality. It is not possible to test raw water for allorganism that it might contain, instead, the water is examined fora specific type of bacteria which originates in large number fromhuman and animal excreta and whose presence in water isindicative of faecal contamination.

Reasons for examination of water quality are grouped under the following Headings:

1. To ascertain the suitability of water for the intended use

2. To determine the most appropriate treatment desired or required

3. To determine the efficiency of treatment processes

4. To ascertain the source, type and extent of pollution and contamination

5. To ensure continuity and uniformity and otherwise in the characteristics of the water.

Suspended load (water bodies)

Chemical loadMicrobial load

Pathogenic microbesSuspended particles

(Colloids)

Organic Inorganice.g. Pesticides and

DDT

Biodegradable Non biodegradable

e.g. Domestic wastewater & industrial w/w

e.g. Phenols, plastics etc

Anthropogenic activity

Anthropogenic activity: known as man made pollution as a result of human activity

Source River

SinkIntake Structure

Transmission line

Water treatment plant

Treated water

Distribution system

Sewerage system Untreated w/w

Wastewater treatment plant

Treated effluent

We have point source pollution (Domestic and Industrial W/W) and the non-point source pollution (Agricultural runoff)

Before water is supplied to community, quality should be enhanced to meet water quality standard.

Drinking water quality

Time / Distance

Qua

lity

Water treatment

Water quality

wastewater treatment

Self purification capacity curve

River water quality

Town B

Town A

Water quality is poor

Before water is supplied to the community, quality should be enhanced to meet water quality standard

Class Description SourceA No treatment Some borehole water

Occasional upland water

B Disinfection only Some borehole waterOccasional upland water

C Standard water treatment Lowland rivers and reservoirs

D Special water treatment Some rural supplies (Fe and Mn)Colour removal Trace element removalIndustrial waterElectronics industry requirementAlgae removal Organics removal

Water treatment processesThere are four classes of water treatment

General Treatment Processes1. Aeration2. Coagulation3. Flocculation4. Sedimentation5. Filtration6. Disinfection7. Chlorination8. Storage

Assignment: Discuss the impurities typically removed in treating groundwater for a municipalsupply. What are the most common pollutants extracted from surface water?

Illustration of a typical treatment process

Pretreatment Treatment

Residual disinfectant

DisinfectantProduces water free of microorganism

i. Chlorinationii. UV radiationTurbidity

(

Selection of treatment processes

The selection of the set of treatment processes is preceded by

detailed raw water quality analysis.

The analysis should run over a period of a minimum of one year

and, where possible, longer.

It should sample the raw water at periods of low, medium and

high flows from a surface water source.

The report on the raw water quality analysis should be evaluated

in conjunction with other engineering reports on site regarding

suitability, availability and continuity of water supply, proximity to

the consumers and available land and its suitability for structures.

Parameter Treatment processFloating matter Coarse screens, fine screens

Suspended matter Microscreens

Algae Microscreens, pre-chlorination, carbon adsorption, rapid filtration

Turbidity Coagulation, Sedimentation, post-chlorination

Colour Flocculation, coagulation, filtration

Taste and odour Activated carbon

Hardness Coagulation, filtration, lime softening

Iron and manganese> 1mg/l< 1mg/l

Pre-chlorinationAeration, coagulation, filtration

Pathogens, MPN/100ml

< 2020 -100> 100

Post-chlorinationCoagulation/filtration/post-chlorinationPre-chlorinationCoagulation/filtration/post-chlorination

Free ammonia Post-chlorinationAdsorption

Recommended treatment for specific impurities

Mixing and flocculation equations Chemical reactors in water treatment and biological aeration

basins in wastewater processing are designed as either completely mixed or plug-flow basins.

In an ideal completely mixed unit, the influent is immediately dispersed throughout the volume and the concentration of reactant in the effluent is equal to that in the mixing liquid.

For steady-state conditions the first-order reaction kinetics is given as:

Where: t = detention timeV = volume of basinQ = quantity of flowK = rate constant for first-order kineticsC0= influent reactant concentrationC1= effluent reactant concentration

1......................11

1

-==

CC

kQVt o

In an ideal plug-flow system, the water flows through a long chamber at a uniform rate without intermixing. The concentration of the reactant decreases along the direction of flow, remaining within the imaginary plug of water moving through the basin.

For steady-state conditions the relationship between detention time and concentration, applying first-order kinetics, is given as:

Where:t, V, Q, C0, and C1 are the same as in equation 1L = length of rectangular basinv = horizontal velocity of flow

In practice, ideal plug flow is very difficult to achieve because of short-circuiting and inter-mixing caused by frictional resistance along walls, density currents, and turbulent flow

2................................log1

1

===

CC

kvL

QVt Oe

QC0

vC1

Q, C1

C0

C1QC0

Q C1

(a)

(b)

Ideal completely mixed and plug-flow tanks. (a) Completely mixed (b) Plug flow

Exercise 1:

1. Based on laboratory studies, the rate constant for a chemicalcoagulation reaction was found to be first-order kinetics with a k equalto 75 per day. Calculate the detention times required in completelymixed and Plug-flow reactors for an 80 percent reduction, C0 =200 mg/land C1 = 40 mg/l.

Solution:Using equation 1 for complete mixing,

Using equation 2 for plug flow,

min771/40/200min1440

75=

-=

lmglmg

daydayt

t

min317.03.22.19

)5log3.2(2.1940

200log75

1440

==

=

= et

Exercise 2:

The in-line system of a flocculator sedimentation tank has the following sized units: flash mixing chamber with a volume of 5.7m3; a flocculator 4.6 m wide, 21 m long, and 2.5 m liquid depth; and a settling tank 23 m square, 3.6m liquid depth, and 90 m of effluent weir. Calculate the major parameters used in design of these units based on a water flow of 11,400m3/d

Solution:Flow = 11, 400m3/d = 7.92m3/min = 0.132m3/sDetention time in flash mixer;

Detention time and horizontal velocity in flocculator

ssm

mt 43/132.0

7.53

3

==

min30min/92.7

5.2216.43 =

=

mmmmt

smmmm

mmmsmAQv /5.11

5.26.4/1000/132.0 3

=

==

hhm

mmmt 0.4min/60min/92.76.32323

3 =

=

Settling time, weir loading, and overflow rate in clarifier

Weir loading:

Overflow rate:

dmmm

dm ./12790

/400,11 33 =

dmmmm

dmV o ./6.212323/400,11 233 =

=

Example on chlorination:

Chlorine usage in the treatment of 20,000 m3/d of water is 8.0kg/d. The residual after 10 min contact is 0.20mg/l. Compute the dosage in milligrams per liter and chlorine demand of the water.

Solution:

Chlorine demand = 0.40 0.20 = 0.20 mg/l

lmgdm

kggdkgDosage /40.0/000,20

/1000/0.83 =

=

Assignment:1. The filter unit of a dual gravity filter has dimension 4.5m by 9.0m. After

filtering 10,000m3/d in a 24-h period, the filter is backwashed at a rate of 10l/m2.s for 15min. Compute the average filtration rate and the quantity andpercentage of treated water used in backwashing.

2. A new water main is disinfected using a 50mg/l chlorine dosage by applying a2.0 percent hypochlorite solution.

(a) How many kilograms of dry hypochlorite powder containing 70 percentavailable chlorine, must be dissolved in 100 liters of water to make a 2.0percent (20,000mg/l) solution?

(b) At what rate should this solution be applied to the water entering the mainto provide a concentration of 50mg/l?

(c) If 34,000 liters of water are used to fill the main at a dosage of 50 mg/l, howmany liters of hypochlorite solution are used?

Groundwater contamination Groundwater contamination results from human activities

where pollutants, susceptible to percolation, are stored and

spread on or beneath the land surface.

Almost every known instances of groundwater

contamination has been discovered only after a drinking

water supply was affected

Typically pollutant sources are industrial wastewater

impoundments, sanitary landfill, storage piles, absorption

fields, septic tanks, improperly constructed wastewater

disposal wells, and application of chemicals on

agricultural lands.

The figure below illustrates idealized flow from a wastewater pond overlying an unconfined aquifer forming a recharge mound on the water table that flows laterally outward.

Natural contamination ---- Principal natural chemicals found in groundwater are dissolved salts, iron and manganese, fluoride, arsenic, radio nuclides, and trace metals.

In arid regions with limited water recharge, slow percolation results in mineralized, poor-quality water high in sodium chloride while in humid climates, weathering of sedimentary rock releases calcium and magnesium, creating excessive hardness and often dissolved iron and manganese

Water table

Wastewater pond

Unsaturated zone

Original water table

Contamination from domestic source

Treated wastewater, if discharged to a dry streambed canincrease infiltration of dissolved salts.

Burial of solid waste can result in degradation of subsurface water

through the generation of leach ate caused by water percolation the

refuse fill.

Industrial and commercial source Hazardous wastes from manufacturing are disposed of on the land,

mainly because it is the cheapest waste management option.

All forms of mining create waste and changes in hydro- geological

conditions that can contribute to degradation of groundwater.

Accidental spills of toxic fluids, gasoline and oil can migrate through

the unsaturated soil zone to groundwater