dissolved oxygen (do) - philadelphia university · biochemical oxygen demand (bod) : - is the...

Section VIII Water Treatment- Introduction

Dissolved Oxygen (DO):

- The source of D.O in water is photosynthesis and aeration

- It is one of important parameters to measure the water quality.

- It gives pleasure taste to water

- As the temp D.O

- If the D.O concentration decreases to less than 4mg/l all fish die

- If the D.O concentration is less than 2 mg/l all organism dies and the water is

called septic water

- Best D.O concentration is between 8-10 ppm. Optimum is 9ppm.

- The maximum naturally accrued is 14 mg/l.


Biochemical Oxygen demand (BOD) :

- is the quantity of oxygen that is used by microorganism to stabilize the wastewater,

Usually measured after 5 days.

- a BOD test can be used to measure waste loadings to treatment plants, plant

efficiency and the effects of a discharge on a receiving stream, and to control the

plant process.

- It is indicator for the required aeration amount.

- The main equation describes the process is:

DO + organic matter CO2 + biological growth

- Drinking water usually has a BOD of less than 1 mg/L

- Ordinary domestic sewage may have a BOD of 200 mg/L.

-Any effluent to be discharged into natural bodies of water should have BOD less than

30 mg/L.

Test Summary:

1- The sample is filled in an airtight bottle and incubated at 20 oC for 5 days.

2- The dissolved oxygen (DO) content of the sample is determined before and

after five days of incubation at 20°C

3- and the BOD is calculated from the difference between initial and final DO.

The initial DO is determined shortly after the dilution is made; all oxygen uptake

occurring after this measurement is included in the BOD measurement


Calculations:

BOD5 mg/l = (Initial DO - DO5) x Dilution Factor

Dilution Factor =Bottle Volume (300 ml)

𝑠𝑎𝑚𝑝𝑙𝑒 𝑉𝑜𝑙𝑢𝑚𝑒

BOD At any time:

BODt = L(1-10-kt

)…. Or BODt = L(1-exp-kt

)….

Where:

BODt : BOD at any time

L: ultimate bio-oxygen demand

k: oxygen decay constant, [day-1

]

t: time, [days]

However: The BOD reaction rate constant (K) is dependent on the following:

1. The nature of the waste:

- degradability of the organic matter. For example , Simple sugar

and starches are rapidly degraded

2. The ability of the organisms in the system to utilize the waste

3. The temperature

- the water temperature may vary from place to place for the same river; hence, the

BOD rate constant is adjusted to the temperature of receiving water using

following relationship:

KT = K20 θ (T-20)

Where

T = temperature of interest, oC

KT = BOD rate constant at the temperature of interest, day-1

K20 = BOD rate constant determined at 20oC, day

-1

θ = temperature coefficient. This has a value of 1.056 in general and 1.047 for higher

temperature greater than 20oC


BOD incubator

Example:

Determine ultimate BOD for a wastewater having 5 day BOD at 20oC as 160 mg/L.

Assume reaction rate constant as 0.23 per day (base exp).

Solution

BOD5 = Lo ( 1 – exp-k.t

)

160 = Lo (1 – exp-5 x 0.23

)

Therefore, Lo = 234.1 mg/L

Example:

A BOD test is done by pipiting 5 ml of waste water into 300 ml testing bottle. If the

initial DO was 8.4 mg/l and the DO after 5-days of incubation at 20 oC was 3.7mg/l,

calculate the BOD and estimate the 20-days BOD value assuming the reaction decay

constant k = 0.1 day-1

. (USE base 10)

Solution:

Dilution factor = 300/5 = 60

BOD5 = (8.4 – 3.7)*60 = 282 mg/l ….*

b- to determine the BOD after 20 days:

from (*) calculate L


282 = L (1-10−0.1 ×5 ) L = 412 mg/l

Thus BOD20 = 412 (1-10−0.1 ×20 ) BOD20 = 407.8 mg/l

Try the solution using Base (Exp ) but in this case the K = 0.23 day-1

Example:

The wastewater is being discharged into a river that has a temperature of 15oC. The BOD

rate constant determined in the laboratory for this mixed water is 0.12 per day. What

fraction of maximum oxygen consumption will occur in first four days? (Base Exp)

Solution

Determine the BOD rate constant at the river water temperature:

K15 = K20 (1.056) (T-20)

= 0.12 (1.056) (15-20)

= 0.091 per day .....

note 1: per day mean day-1...

note 2: as the temp decrease the reaction rate (K) decreases

Using this value of K to find the fraction of maximum oxygen consumption in four days:

BOD4 = Lo (1 – e-0.091x4

)

Therefore, BOD4 / Lo = 0.305


Chemical oxygen demand (COD):

-Measure the amount of organic compounds in water that can be oxidized by strong

oxidant like mixture of sulfuric and chromic acids.

-Most applications of COD determine the amount of organic pollutants found in

surface water.

- It indicates of the strength (degree of pollution) of industrial WW that are not

biodegradable

- the test is faster than BOD test.

PH

- Very important parameter that affects treatment processes, especially

coagulation, anddisinfection

- any unusual change may reflect a major event

H2O H+ + OH

-1,

A- kw = [H+] [OH

-1];;;; kw = 1 × 10−14

B- PH = -log[H+] [H

+] = 10

-PH

https://en.wikipedia.org/wiki/Organic_compound

https://en.wikipedia.org/wiki/Water

https://en.wikipedia.org/wiki/Organic_compound

https://en.wikipedia.org/wiki/Pollutant

https://en.wikipedia.org/wiki/Surface_water


Conventional water treatment:

is a combination of coagulation, sedimentation, filtration and disinfection process.

1- Primary treatment involves : pumping, screening and grit removal

- Screening: aims to remove large objects, such as stones or sticks, that could

plug lines or block tank inlets.

- grit chamber- slows down the flow to allow grit to fall out

- sedimentation tank (settling tank or clarifier)- settleable solids settle out and

are pumped away, while oils float to the top and are skimmed off

2- Secondary treatment : can remove up to 90% of the organic matter in

wastewater by using biological treatment processes. It performed by attached

or suspended growth processes.

A- Activated Sludge: is the most common option with aeration and agitation, then

allows solids to settle out.

- Bacteria-containing “activated sludge” is continually recirculated back to the

aeration basin to increase the rate of organic decomposition.

B- Trickling Filter: is bed of coarse media (often stones or plastic) 3-10 ft.

deep.

- Wastewater is sprayed into the air (aeration), then allowed to trickle through

the media.

- Microorganisms, attached to and growing on the media, break down organic

material in thewastewater.

- Trickling filters drain at the bottom

C- Lagoons: These are slow, cheap, and relatively inefficient, but can be

used for various types of waste water.

- They rely on the interaction of sunlight, algae, microorganisms, and oxygen

(sometimes aerated).


3- Tertiary treatment as ultra filtration

Aerobic conversion of organic matter: the general simplified equation is

C6H12O6 + 6O2 6CO2 + 6H2O + energy

In reality there are more complex equations.

Un-aerobic conversion


C6H12O6 3CH4 + 3CO2 + energy + bio mass

It is two step reaction (i) Acidogenic phase where the organic matter converted to

organic acids (ii) Methanogenic phase where organic acids are concerted into methane

and carbon dioxide.

The typical function of each water treatment units can be summarized as the

following:

Treatment unit Function (removal)

Screening Floating matter

Sedimentation Suspended matter

Coagulation Small suspended matter, part of colloid and

bacteria

Filtration Remaining colloidal , dissolved matter and

bacteria

Softening Hardness

Chemical treatment Iron, manganese, heavy metals..etc

Aeration Color, Odor and taste

Disinfection Pathogenic bacteria, organic matter

The type of treatment required for various water sources are as the following:

Water source Recommended treatment

Ground and spring water fairly free from

contamination

No treatment or chlorination

Ground water with chemical, minerals

and gasses

Aeration, coagulation (if necessary)

filtration and disinfection

Lake, surface water reservoirs with low

amount of pollution

Disinfection

Other surface water such as river, canals

and impounded reservoirs with

considerable amount of pollution

Complete treatment

Section IX Water Treatment - Aeration

Aeration:

- It removes odor, color and taste due to reducing the concentration of volatile

gasses like H2S (hydrogen sulfide) , and algae and other related organisms.

- It oxidizes iron and manganese, increases [DO], removes CO2 , CH4 and other

flammable gasses, reduces corrosion.

- Its work principle is based on the fact that the atmospheric oxygen will replace

the volatile gasses in water , while it will escape into atmosphere.

- The replacement will continue till reaching the equilibrium depending on the

partial pressure of each specific gasses.

Type of aeration:

1- Gravity aerators:

In this type water is allowed to fall by gravity, such that large area of water will

exposed to atmosphere.

2- Fountain (Spray) aerators

A special nozzle is used to produce a fine spray. Each nozzle is 2.5- 4cm diameter.

Discharging about 18-36 L/h

Nozzle spacing should be such that each meter cube of water (m3) has aerator area of

0.03 – 0.09 m2 for one hour.

3- Injection or Diffused aerator :

- It consists of tank with perforated pipes, tube or diffuser plates, fixed at the

bottom to release fine air bubbles from compressor unit.

- The tank depth is 3-4m and the tank width is 1.5 times of tank depth.

- If the depth is more the diffusers must be placed at 3-4m below the water

surface


- Aeration time is 10-30 min and 0.2-0.4 litter of air is required per litter of

water.

- Increase the diffuser depth increases the aeration rate and efficiency

- Decrease the orifice size increases the aeration rate and efficiency

4- Mechanical aerator:

Mixing paddle as in flocculation are used.


The puddle may be submerged or at surface.


Settling (Sedimentation)

it is a type of solid liquid separation process in which a suspension is separated into two phases:

a- Clarified supernatant leaving the top of the sedimentation tank (overflow).

b- Concentrated sludge leaving the bottom of the sedimentation tank (underflow).

Purpose of Settling

To remove coarse dispersed phase. To remove coagulated and flocculated impurities.

To remove precipitated impurities after chemical treatment. To settle the sludge (biomass) after activated sludge process /

tricking filters.

Principle of Settling

Suspended solids present in water having specific gravity

greater than that of water (1) tend to settle down by gravity as soon as the turbulence is retarded by offering storage.

Basin in which the flow is retarded is called settling tank. Theoretical average time for which the water is detained in

the settling tank is called the detention period. Sedimentation tanks are circular or rectangular.

Sedimentation types:

itexist, based on characteristics of particlesa. 1- Discrete or type 1 settlingparticleswhose size, shape, and specific gravitydo not

change over time(has a density (ρ) = 2000 – 2200 kg/m3).

2- Flocculating particles or type 2 settling; particles that change size, shape and

perhaps specific gravity over time(has a density (ρ) = 1030 – 1070 kg/m3).

3- Hindered settling or type 3 settling: blanket sedimentation occrues at Lime

softening sedimentation and Sludge thickeners in water treatment

4- Compressed settling or type 4 settlingoccruesSludge thickeners in water treatment


Applications in Wastewater Treatment:

1. grit removal

2. suspended solids removal in primary clarifier

3. biological floc removal in activated sludge

Place of Sedimentation in various WW plant:

Or


For treating hard water to removes flocculated solids. The sedimentation tank comes after the flocculation tank.


Sedimentation Theory:

If a particle is suspended in water , it initially has 2 forces acting upon it.

The forces of gravity Fg = mg .... but m = ρ . Vp

Thus Fg= g* ρp * Vp

The buoyant force quantified by Archimedes Fb =

Thus Fb= g* ρw * Vp

Once motion has been initiated, a third force is created due to viscous friction that is

drag force Fd

Fd= CD *Ap * ρw * 𝑣𝑠

2

2


CD = 24/Re For laminar Re < 1

CD = 24/Re + 3/√Re +0.34 For transitional 104> Re > 1

CD = 0.4 ForturbulentRe > 104


Sedimentation process:

It is a physical treatment that allows for particles having specific gravity higher than

water to settle under its own weight.

Factor affecting the sedimentation:

1- Water prosperities:

b- Sed. Decreases when water viscosity increase

c- Sed. Decreases when water density increase

d- Sed. Increase when water temp. Increase.

2- Suspended solids concentration, size and shape

3- Detention time: sed. Efficiency increase by increasing the water retention at

the tank , but after time the precipitation is decreased sharply thus the time

should be determined correctly. (2-4hr)

4- Flow velocity: decrease the flow velocity increase the sed. The max.

Allowable velocity in the tank is 0.3 m/sec

5- Tank shape: circular tanks are more efficient for sedimentation

Ideal sedimentation tank:

1- The flow is laminar

2- There are no dead zones

3- The horizontal velocity is constant

4- Good arrangement of inlet and outlet weirs.

dissolved oxygen (do) - philadelphia university · biochemical oxygen demand (bod) : - is the...

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