water engineering mid 1

challenges with clean water

diminishing sources

contamination

conservation

qanats

Persia - 700 BC

channels/tunnels for supply groundwater to cities / towns

aquaducts

Rome - 300 BC

stairway of fountains

inca - 1450 AD

2000 BC Greece and India

treating drinking water necessary

sand+gravel filtration, boiling

better tasting drinking water

1500 BC Egypt

discovered coagulation

applied alum for removing suspended particles

1627 francis bacon

applied sand filtration for seawater desalination

1676 antonie van leeuwenhoek

observed water microbes

1804 robert thom

designed first municipal water treatment plant in scotland

1854 john snow

applied chlorine for water disinfection - cholera

water supply

source, collection distribution

hydrology, hydraulics

pipe network analysis

sources of freshwater

surface waters - lakes, reservoirs, rivers

groundwater

sea - desalination

conservation

US, Oregon

diuron leftover before treatment and after treatment

organic compounds and materials

C, N, O, F, P, S, Cl, Br, I

inorganic chemicals and materials, heavy metals/trace elements

Na, Mg, K, Ca, Fe, Al, Hg, Pb

atomic number

ID of element, # protons

Atomic weight

weight of an atom in atomic unit (a.u.)

molecular weight

weight one molecular or compound in atomic unit

molar weight

weight of one mole of chemical species in g/mol

mole-based

counting molecules or species

mass-based

weighing molecules or species

mole

basic quantity for counting atoms, molecules, compounds

1 mole =

6.022 × 10²³ atoms or molecules

concentration

how many of an atom/molecule/compound in a given volume (i.e. density)

entities per volume

C = n_i/V

mol/L or M ( or molar)

relevant for reaction stoichiometry

mass per volume

C = m_i/V

mg/L, ug/m³

easy to measure (aqueous phase)

partial pressure (gas phase)

C = P_i=n_iRT/V (assuming ideal gas applies)

Pa, bar, atm

easy to measure (gas phase)

1 bar =

10⁵ Pa = 100 kPa = 1 atm = 1.01325 bar = 760 mmHg

1 M = 1 molar = 1 mol/LE

ppm

1 part per million

1 part in 10⁶ parts

1 ppm = 1 mg/L

ppb

1 part per billion

1 part in 10⁹ parts

1 ppb = 1 ug/L

ppt

1 part per trillion

1 part in 10¹² parts

1 ppt = 1ng/L

unit conversion

Example: Determine the concentration of water molecule (H₂O) in 1 L of water in (a) mol/L and (b) mg/L

Open system

exchange of mass energy:

• wastewater treatment

• continuous

• steady state

• dynamic

  

closed system

no exchange of mass/energy:

• batch reactors, biological treatment/pharmaceutical

• BOD/COD test

• equilibrium

chemical equation

what species (reactants) change into what (products)?

stoichiometry (molar or molecular ratios)

what is reactant-product conversion ratio?

chemical equilibrium question

what is the theoretical maximum “extent” of reaction (partially converted, close to full conversion, hardly conversion)

dynamics or transient

are things changing with time or not?

homogeneous vs heterogeneous

is the reaction occurring the same everywhere (homo) or not (hetero) within system of interest

steady state question

reaction may take place at conditions such that compositions and other conditions are not changing with time

chemical reaction

a, b, c, d are stoichiometric coefficients of A, B, C, D

A, B reactants, C, D, products

Reaction types

equations should be balanced with respect to atoms and charge (same number or net charge in reactant and products)

chemical equilibrium

chemical state where potential of reactants to transform into products and potential of reverse reaction equal:

chemical equilibrium rate derivation

batch reactor example

chemical reaction at equilibrium

steady state

when system properties (including composition) remain constant with respect to time

chemical equilibria vs steady state

all chemical equilibria are steady states, but not all steady states are equilibria

Law of Mass Action

law of mass action rules

EX. equilibrium expressions

ex. determine concentration of dissolved oxygen in water at 25C in ambient atmosphere PO2 = 0.2 atm. Given Henry’s law constant for O2(g) is 1.28×10^-3 M/atm

delta g and differential comparison

chemical speciation

different forms of the same species

ex.

speciation diagrams

• identifies dominant or most abundant species as a function of system properties of characteristics (e.g. pH, pe) “choose most dominant to make assumptions”

• constructed from chemical equilibrium constants (i.e. K’s)

Ex. carbonic acid speciation diagram (H₂CO_3(aq))

Ex. Iron (Fe) speciation diagram

objectives of conventional water treatment

• to treat water sufficiently clean for drinking (and other purposes)

• conventional water treatment: convert raw water from natural sources to potable water

• (different levels of cleanliness needed for different applications

when is water dirty?

• has color, appears hazy

• coarse particles

• very fine sized particles (i.e. colloids “diameter less than 1 um”

• polluted (with chemicals)

• biological agents

what exactly is pollution?

• organic pollutants, inorganic pollutants, others

• dose/concentration and harm

• mobility of pollutants

components in conventional water treatment system

• preliminary screening

• chemical mixing basin

• flocculation basin

• sedimentation basin

• rapid sand filter

• disinfection

• storage and distribution

• sludge treatment

basic water treatment diagram

7-step water treatment diagram

for water with high levels of hardness, softening of water can be one of main treatment objectives

groundwater - water in equilibrium with minerals of hard metals (e.g. Ca-, Mg-minerals)

minor modification of treatment operations:

why disinfection is always placed at end of water treatment

colloids can protect microbes, so disinfection is placed at end, after colloids are broken down

organic matter examples

lipids, fats, carbohydrates, proteins

problems of oxygen demands

1. organic matter in water causing high oxygen demand

2. organic matter can degrade and consume dissolved oxygen

3. effluents with high organic matter have higher oxygen demand

4. these streams can harm aquatic life in receiving water bodies

5. (oxygen consumption exceeds supply, causing anaerobic conditions)

solubility of O₂ in water

• DO = Z = O_2(aq)

• essential for aquatic life

• poorly soluble in water

• aqueous solubility of O₂ in air at 1 atm in freshwater (salinity = 0%)

  

• [O_2(aq)]^sat = saturated dissolved oxygen concentration

• [O_2(aq)]^sat decreases with increasing temperature

methods to maintain good level of DO in water

• a supply = demand problem

• supply side - dissolution of atmospheric oxygen into water

• demand side - organisms or matters that can use up oxygen

• organisms need oxygen for energy (i.e. respiration)

• organic matters: consume oxygen as they undergo oxidation

• want to know the rate at which oxygen is being consumed = oxygen demand

types of oxygen demand

• theoretical oxygen demand (ThOD)

• Biochemical oxygen demand (BOD)

• Chemical oxygen demand (COD)

Theoretical oxygen demand (ThOD) characteristics

• calculated, theoretical value

• need composition of organic materials (i.e. stoichiometry or elemental composition) e.g. glucose (C₆H₁₂O₆), atrazine (C₈H₁₄IN₅)

Theoretical oxygen demand (ThOD) definition

ThOD = oxygen demand for complete decomposition of organic matter into inorganic products:

ThOD composition

C-ThCOD = oxygen demand due to decomposition of carbonaceous material

N-ThOD = oxygen demand due to decomposition of nitrogenous material

Ex. What is the ThOD for a 1.67 × 10^-3 M solution of glucose (C₆H₁₂O₆)?

Ex. What is the ThCOD in liters of air for a 50 mg/L solution of acetone (CH₃COCH₃)?

Ex. What is the ThOD in liters of air for a 300 mg/L solution of methylamine (CH₃NH₂)?

limitations of ThOD

• ThOD calculation needs chemical formula of organic materials/wastes

• organics in natural waters and wastewaters are mixture of different organic materials with unknown composition

• need to measure oxygen demand, rather than calculate theoretically

• use microbes that convert and mineralize organic matter to determine oxygen demand of given water sample

Biochemical oxygen demand (BOD)

• measures oxygen use or potential use

• measure O₂ needed by microbes to degrade organic matter in a defined time

standard BOD test (BOD₅)

1. run in dark at T = 20C for 5 days

   1. algae may be present

   2. algal growth can interfere BOD measurement by producing O₂ via photosynthesis in presence of light

2. stndard BOD bottle (300 mL, non-reactive glass + glass stopper)

3. air-tight (prevent O₂ influx)

BOD₅, BOD_t BOD_u (or L)

BOD₅ = amount of O₂ consumed in first 5 days = difference between initial DO and final DO attributed to decomposition of organic matter:

BOD_t (BOD after t-d of incubation):

BOD_U = ultimate BOD = L = BOD_30:

Ex. Determine BOD for samples A, B, C, in figure below:

BOD₅ with sample dilution:

Ex. water sample diluted into three solutions for BOD₅ analysis. The volumes of the original sample and the dilution water are provided. Determine the dilution factors and BOD₅ of original from three diluted samples

Estimated BOD of the original sample ranges from 750 to 1000 mgO2/L.

This example illustrates the problem of diluting samples with clean water, which is known as the “sliding scale” problem.

sliding scale problem and seed solution

• previous example illustrates the need to seed samples with microbes

• using clean water for diluting water sample can lead to measurements artifacts or biases

• want to seed the sample with microbes to ensure a baseline decomposing capacity is present

• seeding – the process in which decomposing microorganisms are added to the BOD bottle with the sample to ensure aerobic degradation of organic materials will occur

Ex. An unknown sample is diluted with a seed solution prior to standard BOD₅ analysis. The seed solution has a BOD₅ of 6 mg/L. A diluted sample is prepared by mixing 100 mL of the unknown sample with 200 mL of the seed solution. The diluted sample is found to have a BOD₅ of 5 mg/L. Determine BOD₅ of the original unknown solution.

Kinetics of BOD

• DO changes with time during BOD test

• mass balance of DO in BOD bottle:

  • rate of change DO = rates of (IN - OUT + PRODUCED - CONSUMED)

• BOD is closed system, no exchange of DO (IN/OUT = 0)

• BOD in dark, no photosynthesis, no O₂ produced (PRODUCTION = 0)

• CONSUMPTION of DO occurs:

  • rate of change in DO = - rate of DO consumed

BOD figure (DO vs time)

Ex. Derivation of y = L(1 - e^-kt)

Ex. Determine BOD₅ if BOD₃ = 148 mg/L and a deoxygenation rate constant of 0.25 d^-1 may be assumed

Extending BOD incubation beyond 5 d will bring in a change in total BOD curve

Limitations of BOD measurements: low BOD reflects -

• low organic content in water

• microbes present incapable of consuming organic materials present

• inactive microbes (dead, dying, dormant, etc.)

Chemical oxygen demand (COD)

quick measurement that quantifies (potential oxygen demand)

better than BOD, which takes 5 days to complete

Standard COD test procedure

1. sample mixed with a strong chemical oxidizing agent (K₂Cr₂O₇) in the presence of a strong acid (H₂SO₄) and then heated

2. COD reflected as consumption of K₂Cr₂O₇

3. chemicals oxidize all organic matter (both biodegradable and non-biodegradable)

4. thus, COD > BOD

Interpretation of COD test results

• COD results are more consistent; ready in 3 hours

• COD may also be used to estimate BOD_u and to indicate the presence of biologically resistant organics

result after organic waste is released into streams

• organic waste oxidized - exerting oxygen demand on receiving water

• transport along stream (physical process)

• oxygen resupplied to stream (dissolution of atmospheric O₂)

• if O₂ reaeration rate < O₂ consumption rate, [O_2(aq)] = DO = z will decline

• when [O_2(aq)] is totally depleted, stream is anaerobic

oxygen sag curve

tracking DO or [O_2(aq)] over the length of the stream following the introduction of organic waste

Dissolved oxygen (DO) vs Oxygen Deficit (D)

DO not the same as D

Time vs dissolved oxygen

Ex. Determine oxygen deficit for the following water samples:

DO and D after waste introduced into a stream (DO and D immediately after mixing point?)

suppose organic-rich effluent introduced into pristine stream: