of 48
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
1/48
WATER QUALITY CONTROL.WATER TREATMENT.
WASTE WATER TREATMENT.
Two components to water quality:
1. Safe drinking treatment of surface or subsurface
water for consumption2. Safe release treatment of municipal sewage and
industrial wastewater
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
2/48
Water quality concern:
1. Health of people who drink the water
avoidance of cholera, typhoid fever,
gastroenteritis, etc.
2. Aesthetics: water color, hardness, taste, odor
3. Quality of water in the environment:
dissolved oxygen, salt content, habitat
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
3/48
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
4/48
Drinking water standards:
Primary and Secondary
Primary standards - health-related criteria
Secondary standardsaesthetics (such as taste,
odor, and color) and nonaesthetic (corrosivity and
hardness)
Iron and manganese are objectionable because of taste
and their ability to stain laundry
Excessive fluoride causes a brownish discoloration ofteeth
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
5/48
Drinking water standards: primary standard
1) Synthetic organic chemicals (SOCs) are compounds used in the
manufacture of a wide variety of agricultural and industrial products
(insecticides, herbicides);2) Volatile Organic Chemicals (VOCs) are synthetic chemicals that
readily vaporize at room temperature (carbon tetrachloride; 1,1,1,-
trichloroetahne (TCA); trichloroethylene (TCE) and vinyl chloride).
3) Disinfectant byproduct (DBPs) are the byproducts formed when adisinfectant reacts with chemicals in the water to form a toxic
product. Trihalomethanes (THMs) are the byproducts of water
chlorination. The most common solution is to remove the DBP-
forming compounds from the water before the disinfection.4) Radionuclides concentrations in drinking water are expressed in
Pico curies per liter (pCi/L). 1 pCi= 2.2 radioactive decays per
minute (1 Ciis the decay rate of 1 gram of radium). Radon occurs
naturally in some groundwater, inhaled radon gas is thought to be a
major source of lung cancer.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
6/48
Water treatment (drinking water):
Disinfection
of effluent toeliminate
harmful
pathogens.
Screening:
removal of
large floating
/suspended
debris, grit
and sand.
Coagulationadding
chemicals and agitation to
promote suspended solids
to form/coagulate into
larger particles.
Flocculation - gentle
mixing of water with
chemicals to form
larger flocs.
Sludge processing -
mixture of solids andliquids collected from
the settling tank is
dewatered and
disposed of.
Treatment for surface water
Settling/ clarification
remove particles that
settle out by gravity
Filtrationremoval ofparticles and floc by
gravity settlement
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
7/48
Treatment for groundwater
In general, underground water is much clearer from particulate matter compared to
surface water, therefore the main steps in the treatment include:
1.Aeration (removes excess and objectionable gases)2.Flocculation (precipitation) to bind Ca and Mg ions
3.Sedimentation (gravity settling of particulate matter)
4.Recarbonation readjust pH and alkalinity (to reduce the pH back to near-neutral)
5.Filtration, disinfection, and solids processing
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
8/48
1. Sedimentation
During this treatment step, the particles are simply allowed to settle due to gravity effect.
Particles may have very different, irregular shapes and when describing particles, an
equivalent to a sphere' diameter is used. It is hydrodynamic diameter (in water) andaerodynamic diameter (in air).
The drag force is a function of the particles Reynolds number:
where =density of water; vs= particle settling velocity; dp=particle
hydrodynamic diameter; =absolute viscosity of water.
For the laminar flow: Re < 1
For transition flow: 1 < Re < 10 000
For turbulent flow: Re > 104
psdRe
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
9/48
In most environmental engineering cases, only laminar flow is considered, therefore:
FD=3 vsdp
The gravitational force: FG=mg=Vppg;
The buoyancy force: FB=mwg=Vpg;
where g=gravitational acceleration (9.8 m/s2); m=the particle mass; mw=the mass of the
water displaced by the particle; Vp=particle volume=dp3/6; p=particle density;
=density of water.
The balance of forces on a particle when it is at its terminal velocity is F G= FD + FB
Substitution of the above equations results in the simplified form of Stokes law:
1. Sedimentation
18
2
pp
s
dgv
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
10/48
1. Sedimentation
Figure shows the trajectory of a particle that is being carried by the horizontal flow of
water from one end to the other of a rectangular settling basin. The particle settles at
distance, hp:
where =the hydraulic detention time of the basin; Vb=the basins volume; Q=the volumetric flow rate
of water through the basin.
In order to achieve higher efficiency of particles settling, the settling velocity (vs) should be
equal or greater than a critical settling velocity (vo):
where Ab=the surface area of the rectangular basin.
Q
Vvvh bssp
bbb
o
A
Q
hA
hQ
V
hQhv
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
11/48
1. Sedimentation
The critical settling velocity is also known as surface loading rate or overflow rate.
If we want to design a clarifier to remove all particles of a size, d, from a water streamwith a flow rate, Q, the surface area,Ab, of the rectangular basin must be
The same formula is applicable for circular settling basin too. However, the influent enters
circular settling basins at the centre and the overflow rates are within
1.0-2.5 m3/m2h.
Also the detention time influences the efficiency of the clarifier. The hydraulic detention
time in any tank is its volume divided by the influent flow rate (typically from 1 to 4
hours).
218
pp
bdg
QA
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
12/48
Silt Removal in a Clarifier
A drinking water treatment plant uses a circular sedimentation
basin to treat 3.0 MGD of river water. (MGD stands for million
gallons per day and is a common U.S. measure of flow rate used
for water and wastewater treatment. 1.0 MGD = 0.0438 m3/s).
After storms occur upstream, the river often carries 0.010 mm
silt particles with an average density of 2.2 g/cm3, and the siltmust be removed before the water can be used. The plants
clarifier is 3.5 m deep and 21 m in diameter. The water is 15oC.
(water density and viscosity at 15oC are found to be 999.1kg/m3
and 0.00114 kg/m.s)
a. What is the hydraulic detention time of the clarifier?
b. Will the clarifier remove all of the silt particles from the
river?
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
13/48
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
14/48
2. Coagulation and Flocculation
Particles that are too light or small require longer time period to settle.Many of such particles are colloids (0.001 to 1 m).
These particles have overall charge and repulsion is present between theparticles that prevents aggregate formation; therefore a chemical agentis required to stimulate the floc formation.
Coagulationis a chemical treatment that destabilizes particles surfacemaking them sticky so the particles adhere to each other andconsequently removed by settling and filtration.
Flocculation, sedimentation, filtration are physical treatment processes.
The usual coagulant is alum Al2(SO4)3*18H2O; also FeCl3, FeSO4, andpolyelectrolytes are used.
The overall reaction:
Al2(SO4)3*18H2O + 6HCO3- 2Al(OH)3 + 6CO2+ 18H2O + 3SO4
2-
alum aluminium hydroxide sulfates
If it is necessary to increase pH then Ca(OH)2or Na2CO3is added.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
15/48
Coagulants are added to the raw water in rapid mix-coagulation tank; the detention time is about minute.
Flocculation is followed up and provides gentle agitation forabout hour, during this period, the precipitating aluminiumhydroxide forms floc.
Parameter, G, the mixing intensity is used by engineers to
maximize the rate of collisions between particles, without thebreaking up the flocs.
where P=the power input to the paddles; Vb=the volume of thevessel; and =the viscosity of water.
2. Coagulation and Flocculation
bV
PG
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
16/48
3. Filtration It is a very common procedure with a few well established techniques in
use.
The rapid depth filtration: the filter consists of layers of carefully sievedmaterials such as sand, anthracite coal, diatomaceous earth and a bedof graded gravel.
Processes involved: adsorption, flocculation, sedimentation. Such filtersare cleaned by backwashing the medium.
The typical filtration rates (va), (loading rate or superficial velocities) are
between 525 m3
/ m2
h).
The filter efficiency (production efficiency): f= (VfVbVr) / Vf
The effective filtration rate: ref=(VfVbVr) / Af*tcwhere Vf=volume of water filtered per filter cycle (m
3)
Vb=volume of water used to backwash the filter (m3)
Vr=volume of water used to rinse the filter after backwash (m3)
tc=duration of time for a complete filter cycle (hr)
Af = filters cross-sectional area
va= Q / Af
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
17/48
4. Disinfection
Disinfection is to kill any pathogens and prevent pathogens from growingin the treated water.
Free chlorine is the most common disinfection procedure. It involves anyof the following compounds: gas Cl2, sodium hypochlorite (NaOCl),calcium hypochlorite (Ca(OCl)2).
For Secondary disinfection, ammonia is added, which reacts with freechlorine and forms chroloamines that have longer residence time in the
treated water. Disinfectant byproducts (DBPs) include Trihalomethanes (THMs),
chloroform (CHCl3), and haloacetic acids (HAAs).
Presence of organic matter promotes formation of the DBPs, therefore thedisinfection is most efficient when carried out as the last treatment
process step. Alternative disinfectants are chlorine dioxide (ClO2) and ozone (O3). Both
agents are effective against cysts and viruses but more costly comparedto chlorination. Additionally, ozone does not leave any protective residualsin the water after the treatment.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
18/48
5. Hardness and Alkalinity
Hardness is defined as the concentration of Ca2+ and Mg2+ in
solution. Groundwater is especially prone to excessive hardness.
Hardness causes problem of soap curd (reaction between
hardness and soap produces a sticky, gummy deposit) and
scaling (precipitation of CaCO3 and Mg(OH)2 that clogs hotwater pipes)
Alkalinityacid buffering capacity of water. In most natural
water, the total amount of H+ that can be neutralized isdominated by the alkalinity provided by the carbonate buffering
system.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
19/48
5. Hardness and alkalinity
EW = AW / n
e.g. EW (CaCO3) = (40+12+3x16) / 2 = 50 g/eq = 50 mg/meq
for EW (Ca2+) = 40 / 2 = 20 mg/meq
The general expression is meq/L (X) = [X, mg/L] / EW (X, mg/meq)
Alkalinity = [HCO3-
] + 2[CO32-
] + [OH][H+
]
Hardness is expressed in terms
of equivalent weight, EW,
which is atomic or molecular
weight of a substance divided
by a number, n, that relates to
its valence or ionic charge.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
20/48
6. Softening
The surface waters seldom have hardness exceeding 200 mg/L
and softening is not regular part of the water treatment. For groundwater waters with hardness ~ 1000 mg/L the
treatment is required. It involves lime-soda ash and the ion-
exchange processes.
1. Lime-soda ash processing, Ca(OH)2is added to water and pH
goes up to about 10.3. At this higher pH Mg++and Ca++ionsprecipitate and are removed as solids.
2. In the ion-exchange process, the water flows through a
column packed with solid resin beads which are filled with
sodium ions that are exchanged for Mg++
and Ca++
ions as thewater passes through the resin. The resin must be
regenerated once the sodium is depleted from it.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
21/48
A sample of water having a pH of 7.2 has the following
concentration of ions.
Calculate the total hardness, the carbonate hardness, thealkalinity and the total dissolved solids for the following ions.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
22/48
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
23/48
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
24/48
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
25/48
6. Membrane Processes
All membranes can be considered semipermeable physical
barrier, they allow passage of water, while severely restrictingthe permeation of contaminants in water (including pathogen,
hardness and dissolved solids, organic and disinfectant
byproduct precursors, metals, and suspended solids)
Classification of membrane: Particulate separation and solute
separation
1. Particulate separation (reject particles, including pathogens)
microfiltration and ultrafiltration
2. Solute separation (dissolved substance)nanofiltration andreverse osmosis
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
26/48
Size ranges and types of contaminants removed by membrane processes.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
27/48
Wastewater treatment
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
28/48
Objectives of wastewater treatment:
Wastewater treatment process is divided into 3stages:
1. Primary treatment to reduce content of
solids2. Secondary treatment to reduce BOD
3. Tertiaryor advanced treatment to remove
nutrients, nitrogen and phosphorus. Residuesof pharmaceutical substances (e.g.
antibiotics) require additional treatment.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
29/48
1. Primary treatment
Screening: to remove large objects, debris
Grit chamber allows the heavy stuff to settle,
a few min detention time
Primary settling tank or primary clarifier: the
flow is sufficiently reduced,
detention time is ~ 1.53 hours during which
about 50-65% of suspended solids and 25-40%
of BOD are removed.
Overflow rate and detention time are the key
parameters.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
30/48
2. Secondary treatment, biological
Two approaches: suspended growth treatment
(microorganism are suspended in and move with the water)and attached growth treatment processes (microorganism are
fixed on a stationary surface, and the water flows past the
microorganism)
Wastewater contains organic compounds such as glucose
(C6H12O6). Microorganisms in the water use these organic
compounds as food while degrading these organic
compounds, microorganisms consume oxygen dissolved in the
water, the O2used by the organisms is replenished by mass
transfer from air
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
31/48
Microbial kinetics
Microorganisms consume organic matter (substrate) that is
measured in mg/L of BOD. Mass of m/orgs fluctuates (depending on growth condition and
availability of substrate) and their biomass is measured in mg/L of
Volatile Suspended Solids (VSS or VS). See more details in Experiment #2 of the lab. manual.
Activated sludge tank works as a bioreactor.
The rate of substrate entering and leaving the reactor is affected by
the water rate entering and leaving, and the rate of microbial growth
is affected by changes in the mass of substrate available.
For a particular substrate and growth conditions, a proportion
between substrate mass consumed and new microbial cell mass
should be achieved to maintain efficiency of the w/water treatment.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
32/48
TS = total solid
TSS = total suspended solid
VSS = volatile suspended solid
TDS = total dissolved solid
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
33/48
rg=microbial mass growth rate
X=concentration of microorganisms (mg VSS/L)
= specific biomass growth rate constant (time-1)
The Monod equation (1949)
When substrate concentration, S (mg BOD5/L), is
0, the growth rate is also 0.
When substrate is present in excess, the growth
rate reaches maximum rate of microbial
reproduction, m.
During the exponential phase microbial growth, the increase in the microbial mass
growth rate, rgcan be modeled by the first-order rate expression
Kssubstrate concentration when the growth rate constant is half
of its maximum value
Rate of microbial growth depends on the substrate concentration and the
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
34/48
Rate of microbial growth depends on the substrate concentration and the
amount of generated biomass should be proportional to the amount of
substrate consumed, i.e. certain proportion of the substrate consumed should
be converted into a predictable amount of new microbial cells.
Y= proportionality constant = yield coefficient , mg VSS/mg BOD5Yrelates the rate of substrate consumption, rsu, under particular conditions to
the rate of microbe growth:
Maximum specific substrate
utilization constant, krelates
maximum specific growth
rate,m, to the yield
coefficient:No death of microbial
cells is considered in
this equation.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
35/48
= net rate of change in
microbe concentration.
Microbial growth rate is proportional to the rate of substrate
consumption (rsu) minus the rate microbes die (rd).
kd=death /decay rate constant, time-1
X = concentration of microorganism(mg VSS/L)
The death rate for microbes, rd
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
36/48
Example 6.9 BOD Consumption in Pond
The shallow pond depicted in Figure below stays well mixed due to the wind and
the steady flow through of a small creek
If the microbes in the pond consume the inflowing biodegradable organic matter
according to typical kinetics, determinea. The BOD5 leaving the pond
b. The biodegradable organic matter removal efficiency of the pond
c. The concentration of volatile suspended solids leaving the pond
Given Ks= 60 mg BOD5/L; kd= 0.06 d-1, k = 5 mg BOD5/(mg VSS.d)
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
37/48
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
38/48
Suspended growth treatment:
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
39/48
Suspended growth treatment:a) Activated sludge
Two conditions:
1. Supply of oxygen, i.e. aeration
2. Promoted growth of microbial biomass
Mixed liquor
1. Bioreactor: BOD consumption
2.
(return activated sludge)
(waste activated sludge)
Retention time:
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
40/48
The microbial cells mass settles in the secondary clarifier
and then is returned to the activate sludge tank to maintainsufficient microbial activities.
The average cell retention time (solids retention time), SRT,cis
Activated sludge tanks are relatively inexpensive, have lessproblems with insects and odour, provide higher BOD
removal rates. However, activated sludge reactors require higher expertise
to operate and more energy for pumps and blowers.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
41/48
Approximate concentrations of BOD, suspended solids, total
nitrogen and total phosphorus as wastewater passes through
a secondary wastewater treatment plant.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
42/48
b) Membrane bioreactors (MBRs) draw water fromthe mixed liquor into hollow fiber membranessubmerged in the activated sludge aeration tank,thus avoiding the need for a secondary clarifier.
Microfiltration fibers have a pore size is ~ 2micrometer and effective in producing low TSS
effluent.MBRs application is very effective where
wastewater reuse and reclamation is desired.
MBRs have fouling problems, are more expensive to
build and operate; and their longevity is alsoquestioned as it is a relatively new technology.
c) Aerated lagoons and oxidation ponds
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
43/48
c) Aerated lagoons and oxidation ponds.
Typically, oxidation ponds
are large, 1-2 m deep
where sewage isdecomposed by
microorganisms.
The decomposition near
the surface is aerobic
and at the ponds bottom
is anaerobic; such ponds
are termed facultative
ponds.
The ponds are easy to
build and they
sufficiently reduce BOD;however the effluent
may contain undesirable
concentrations of algae
and unpleasant odour.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
44/48
Attached Growth Treatment is often used as the sole secondary
treatment process and as pre-treatment step before an activated
sludge process.
a) Trickling filters are successfullyused since 19thcentury.
A trickling filter consists of a rotating
distribution arm that sprays the feed
w/water over a circular bed ofplastic packing or other coarse
materials.
Tall trickling filters filled with plastic media are called biotowers. The spacesbetween the packing allow air to circulate easily so that aerobic conditions are
maintained. The media is covered by biological slime populated with
microorganisms, insects, fungus, protozoa, worms, snails, etc. that are
responsible for the w/water decomposition.
What is the principal difference b/w suspended and attached growth treatment?
b) R t ti bi l i l t t (RBC) i t f i f
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
45/48
b) Rotating biological contractors (RBC) consist of a series of
closely spaced, circular, plastic disks, typically 3.6 m in diameter
that are attached to a rotating horizontal shaft.
40% of each disk is submerged in wastewater.
Microorganisms populated on the surface of the rotating disks
decompose the wastewater.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
46/48
Sludge treatment: Sludge is a mixture of solids and water that
remains to be disposed of. The important objective of the sludge
treatment is to separate the water from the solids as much as
possible. The traditional method is anaerobic digestion. It isslower than aerobic decomposition but has an advantage that a
small % of the waste is converted into new bacterial cells. Most of
the organics are converted into CO2and CH4.
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
47/48
Many treatment plants use a two-stage digester shown below.
In the 1ststage, the sludge is thoroughly mixed and heated to increase the rate of
digestion (detention time 10-15 days)The 2ndstage is characterized by stratification (no mixing): liquid, solids and gas. The
liquid part (supernatant) is returned for BOD removal; the sludge is dewatered and
disposed of; the gas is potential fuel used for heating purpose at most of the
treatment plants. The digested and dewatered sludge is potential soil conditioner but
mostly it is disposed in a landfill.
Advanced treatment involves removal of nutrient
7/24/2019 201501_UEMX_3613_Topic2-2_R1_
48/48
Advanced treatment involves removal of nutrient
elements such as nitrogen and phosphorus.
Nitrogen
Aerobic bacteria converts ammonia (NH4+) to nitrate (NO3
-) which is nitrification; then
anaerobic bacteria converts nitrate to nitrogen gas (N2) which is denitrification.
The aerobic part of the reaction : NH4++ 2O2+ bacteriaNO3
-+ 2H++ H2O
For the nitrification process the detention time of at least 15 days is required.
For the anaerobic part, denitrification, the reaction:
2NO3-+ organic matter + bacteriaN2 + CO2+ H2O
Because the denitrification takes place after waste treatment, there may be not enough
organic material for the bacteria and therefore additional nutrient is required, which is
typically methanol (CH3OH).
Phosphorus
Phosphorus in w/water exists as orthophosphate (H2PO4-; HPO4
2-, and PO43-), and it is
removed by reaction with added coagulant such as alum or lime.
The reaction: Al2(SO4)3+ 2PO42-2AlPO4 + 3SO4
2-
