Transcript An Overview of the Safe Drinking Water Act
Water Treatment Processes
Water Treatment Plant Operation
Water Treatment Processes
Section 1: Water Treatment Concerns Section 2: Well Considerations Section 3: Conventional Water System Processes Section 4: Disinfection By-Product Control Section 5: Corrosion Control Section 6: Demineralization Processes Section 7: Coagulation Process Control Section 8: Water Softening Florida Rural Water Association Water Treatment Plant Operation 2
Section 1: Water Treatment Concerns
Microbial Contamination Concerns Barriers to Contaminants Reaching the Public Where Contamination Comes From Bacterial Indicators and Pathogens Primary Standards Secondary Standards Florida Rural Water Association Water Treatment Plant Operation 3
Microbial Contamination is Primary Concern of Water Operators
Coliform bacteria
Common in the environment and are generally not harmful but their presence in drinking water indicates that the water may be contaminated and can cause disease.
Fecal Coliform and E coli
Bacteria whose presence indicates that the water may be contaminated with human or animal wastes. Microbes in these wastes can cause short-term effects, such as diarrhea, cramps, nausea, headaches, or other symptoms.
Turbidity
Has no health effects. However, turbidity can interfere with disinfection and provide a medium for organisms that include bacteria, viruses, and parasites that can cause symptoms such as nausea, cramps, diarrhea, and associated headaches.
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Multiple Barrier Approach
Source: Selection and Protection Treatment: Methods and Efficiencies Florida Rural Water Association Water Treatment Plant Operation Distribution: Maintenance and Monitoring 5
Where Contamination Comes From
Condition Reoccurring Gastro-illness* Test For: Coliform in Drinking Water Pipeline Failure Nearby Agriculture Nearby Mining Nearby Landfill pH, Lead, and Copper Nitrates, Pesticides and Coliform Metals and pH VOCs, TDS, Chlorides, & Sulfate Nearby Fueling Bad Taste/Odors VOCs Hydrogen Sulfide and Iron Stains Clothes/Plumbing Hydrogen Sulfide and Iron Scaly Residue Hardness
* Multiple Sources, ie. runoff, septic tanks, CAFOs Florida Rural Water Association Water Treatment Plant Operation 6
Microbial Contaminants found in Surface Water or UDI Sources
Cryptosporidium and Giardia
Parasites that enters lakes and rivers through sewage and animal waste. These typically cause mild gastrointestinal diseases. However, the disease can be severe or fatal for people with severely weakened immune systems. EPA and CDC have prepared advice for those with severely compromised immune systems who are concerned about
these organisms.
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Some Facts About Bacteria
Bacteria are widely distributed on earth They have been found 4 miles above earth and 3 miles below sea sediments.
One gram of fertile soil contains up to 100,000,000 bacteria.
Bacteria are inconceivably small and measured in microns. One micron is equal to 1/1,000,000 of a meter.
During the rapid growth phase bacteria undergo fission (cell division) about every 20 to 30 minutes.
One bacterial cell after 36 hrs of uncontrolled growth, could fill approximately 200 dump trucks
.
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Total
Bacteria and Pathogenic Indicators in Water Treatment
Ferment Coliform Lactose @ 35
O
C Include Species of Genera Fecal Coliform E. Coli HPC Citrobacter Enterobacter Klebsiella E. Coli Grow at 44
O
C Produce Enzyme More Specific Indicator of Contamination < 500 colonies/ml Photo: CDC. E. coli 0157:H7 11 of 140 cause gastrointestinal disease
Identifying Source of Contaminants
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Primary or Inorganic Contaminants
Mineral-Based Compounds These include metals, nitrates, and asbestos. These contaminants are naturally-occurring in some water, but can also get into water through farming, chemical manufacturing, and other human activities. Potential health effects include learning disorders, kidney and liver damage. EPA has set legal limits on 15 inorganic contaminants.
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Primary Standards and their Maximum Contaminant Levels (MCLs)
Contaminant Arsenic Asbestos Fluoride Mercury Nickel Nitrate Nitrite 10 1 Total Nitrate+Nitrite 10 Sodium 160 MCL (mg/l) 0.010
7 (MFL) 4.0
0.002
0.1
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Disinfectants and Disinfection By-Products
Disinfectants are water additives that are used to control microbes Disinfection By-products are created when chlorine is added in the presence of naturally occurring low levels of organic materials found in drinking water Both are regulated because of health concerns Florida Rural Water Association Water Treatment Plant Operation 13
Secondary Standards and Concerns
These compounds cause aesthetic concerns such as taste, odor and color. EPA recommends MCL limits Some states such as Florida have set regulatory limits on these contaminants Florida Rural Water Association Water Treatment Plant Operation 14
Secondary Standard Maximum Contaminant Levels
Contaminant MCL (mg/l) Chloride Sulfate TDS Copper Fluoride Iron Manganese Silver pH (MRCL) Color (MCRL)
Florida Rural Water Association Water Treatment Plant Operation
250 250 500 1.0
2.0
0.30 0.05
0.1
6.5 to 8.5
15 cfu
15
Protecting Well by Grouting
Prevent movement of water between aquifer formations Preserve quality of producing zones Preserve Yield Prevent water intrusion from surface
Pressure Testing of Grout Seal @ ~10 psi for 1 hr. Should be Performed.
Protect Casing against Corrosion!
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Section 2 Well Considerations
Floridan Aquifer Well Contaminants Preventing Contamination at the Well Head Florida Rural Water Association Water Treatment Plant Operation 17
Floridian Aquifer Across Florida
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Well Source Water Parameters
Quality and Quantity Dictates Depth of Well TDS Total Hardness Total Fe and Mn Chlorides & Sulfates Total Alkalinity Nitrate pH Corrosivity CO 2 H 2 S Fluoride Florida Rural Water Association Water Treatment Plant Operation 19
2 3 4 # 1
Preventing Contamination at the Well Head
Observation Septic tanks, broken storm or san. pipes, ponds Drainage up-hill Likely Pathway Through Surface Strata Surface water runoff Well subject to flooding Surface water transport of contaminants Casing termination Must be 1’ and above 100 yr flood plane 20
6 7 8 5 #
Preventing Contamination at the Well Head (continued)
Observation Likely Pathway Area around well is wet Possible Abandoned wells in area Sanitary condition unacceptable Corroded Casing Pipe Surface water intrusion from contaminated source Contaminated water intrusion Cracking in Well Slab Contaminated water intrusion Florida Rural Water Association Water Treatment Plant Operation 21
9 #
Preventing Contamination at the Well Head (continued)
Observation Likely Pathway 10 11 12 Evidence of Algae or Mold on Slab Poor Drainage Birds and insects attracted by moist conditions Surface water intrusion from contaminated source Seal water Draining into well head Contaminated water entering borehole Well Seal damaged Contaminated water intrusion Florida Rural Water Association Water Treatment Plant Operation 22
#
Preventing Contamination at the Well Head (continued)
Observation Likely Pathway 13 14 15 16 Fittings pointing upward Well vent not properly installed Check Valve absent or not working Cavitation or water hammer Contaminated Water intrusion into casing Contaminated Water intrusion into casing Contaminated water back flowing into casing Ck. Valve damage & water back-flowing into casing Florida Rural Water Association Water Treatment Plant Operation 23
#
Preventing Contamination at the Well Head (continued)
Observation Likely Pathway 17 18 19 20 Well Site Security Compromised Livestock or wild animals close by Surface water evidence ID Several wells available Contaminated Water from undesirable activities Animal source of Contamination Indicator organisms, color, temp and TOC contributing One well is more likely to contribute than others Florida Rural Water Association Water Treatment Plant Operation 24
#
Preventing Contamination at the Well Head (continued)
Observation Likely Pathway 21 22 Intermittent Well Operation Wet or extreme weather events Contaminated occurring from long-term biological activity Contamination from run-off or from higher pumping levels.
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Section 3: Conventional Water System Processes
TOC in Source Water Disinfection and Uses of Chlorine Aeration and Aerator Types Iron and Hydrogen Sulfide Control Filtration Sedimentation Florida Rural Water Association Water Treatment Plant Operation 26
Organic Carbon (TOC) in Natural Waters mg/l
Mean Surface Water 3.5 Sea Water Ground Water Surface Water Swamp Wastewater Wastewater Effluent .1 .2 .5 1.0 2 5 10 20 50 100 200 500 1000 Florida Rural Water Association Water Treatment Plant Operation 27
Disinfection with Chlorine
The primary methods of disinfection is the use of chlorine gas, chloramines, ozone, ultraviolet light, chlorine dioxide, and hypochlorite.
Generally Chlorine will be used by small systems and may be applied as a gas, solid or liquid.
The most common chlorine application is sodium hypochlorite or bleach.
Primary Disinfectants are used to inactivate microbes and Secondary Disinfectants are used to provide for a residual chlorine concentration that prevents microbial regrowth
.
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Reactions of Chlorine with Water Constituents
Reducing Compound (inorganics) Production of Chloramines Production Chlororganics Combined Chlorine Breakpoint Chlorination Free Chlorine Residual Florida Rural Water Association Water Treatment Plant Operation 29
0.6
Fe Mn H 2 S 0 Add NH 3 Chloromine Dichloromine DISINFECTION BYPRODUCTS REMAIN
Breakpoint Chlorination Curve Florida Rural Water Association Water Treatment Plant Operation
0.2
0
30
Other Chlorine Uses
Chlorine is often used as an oxidant to remove inorganic impurities such as iron and hydrogen sulfide When used in this manner particulate matter is formed that often must be removed.
Chlorine is also used to prevent the growth of algae on tank walls and other surfaces exposed to sunlight and to prevent bacteria from growing inside filters and tanks Chlorine has been used to remove color, taste and odors but will produce disinfection by-products which are regulated
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Aeration
Aeration is generally used in small systems to remove naturally occurring dissolved gasses from the water such as CO2 and H2S.
Aeration may also be used to oxidize iron which then drops out as precipitate and must be filtered.
Special aerators called Packed Towers are sometimes used to remove VOCs Florida Rural Water Association Water Treatment Plant Operation 32
Cascade Tray Aerator
Even distribution of water over top tray Loading Rates of 1 to 5 GPM for each sft. of Tray area.
Trays ½” openings perforated bottoms Protection from insects with 24 mesh screen Florida Rural Water Association Water Treatment Plant Operation 33
Forced Draft Aeration System
Includes weatherproof blower in housing Counter air through aerator column Includes 24 mesh screened downturned inlet/outlet Discharges over 5 or more trays Florida Rural Water Association Water Treatment Plant Operation 34
Packed Tower Odor Removal System
Uses Henry’s Law constants for mass transfer Usually requires pilot testing Used to Remove VOCs below MCL Col to Packing >7:1 ratio Air to water at pk >25:1 with max 80:1 Susceptible to Fouling from CaCO3 > 40 PPM
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Iron Problems - Most Prevalent in Unconfined, Surficial, and Biscayne Aquifers
Iron dissolved by reaction with CO 2 Iron from well sources will be in a dissolved state When exposed to O 2 precipitants form Visible as red and brown color Will stain fixtures and clothes Imparts taste and odor
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Iron, Turbidity/TOC Relationships
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Dissolved Iron Problems
Soluble iron passing into the water distribution system will encourage the growth of iron bacteria Precipitates will form in the distribution system Iron particles will stain clothes and fixtures (Red Water Complaints) Florida Rural Water Association Water Treatment Plant Operation 38
Treatment of Dissolved Iron
Type of Treatment Removal Considerations Oxidation w/ Chlorine Max. 0.1 mg/l w/o filtration Greensand Filter 0 – 10 mg/l w/ pH > 6.8
Ion Exchange Softener 0 – 10 mg/l Phosphate Addition 0 – 2 mg/l
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Fe
++
Aeration
Plot of pH vs. Time for Iron Removal at 90% Efficiency (min 30 minutes detention) Florida Rural Water Association Water Treatment Plant Operation 40
Filtration Requirements for Iron and Manganese
Requires bé DEP at > 1.0 mg/l Fe Turbidity must be no more than 2 NTUs above Source Water Oxidized particles must generally be removed Anthracite filters are frequently employed with higher iron content Florida Rural Water Association Water Treatment Plant Operation 41
Hydrogen Sulfide Removal Techniques (DEP)
Sulfide (mg/l) Recommended Treatment Process Achievable Range of Removal
< 0.3
> 0.3
0.3 to 0.6
0.6 to 3.0
> 3.0
Direct Chlorination Direct Chlorination (requires filtration) Conventional Aeration Forced Draft Aeration Packed Tower Aeration Florida Rural Water Association Water Treatment Plant Operation 100% 100% 50% 90% > 90% 42
Hydrogen Sulfide Removal Dynamics
Gas Soluble
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Clarification
Clarifiers are often used in water treatment to allow particles to settle prior to filtration.
Special clarifiers called “Upflow Clarifiers” are used in surface water treatment plants that used coagulants and in softening plants that use lime. These types of clarifiers perform several treatment processes in one tank Florida Rural Water Association Water Treatment Plant Operation 44
Causes of Poor Clarifier Performance
If Surface water plant flocculators are not adjusted for rate of flow Sludge removal is not routine There is no test to control sludge quantities
Settled water turbidities are not measured or are not measured routinely (e.g., minimum of once per shift)
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Filtration
Filters are primarily used to remove particulate matter and turbidity from the water.
The primary types of filters used in water treatment are Rapid Sand or gravity and Pressure Filters
Special Membrane Filters are used for Particulate and Microbial removal.
Special Filters employ Resins and Media such as greensand and are used to remove select contaminants such as iron and manganese. Activated carbon filters are used to remove organic compounds
.
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Nanofiltration Florida Rural Water Association Water Treatment Plant Operation
Filter Applications
47
Media Configurations for Gravity Filters Single media (sand) Dual Media (sand and anthracite) Mixed or multi media (sand, anthracite and garnet) Florida Rural Water Association Water Treatment Plant Operation 48
Characteristics of Various Filters
Filter
Slow Sand Rapid Sand
Media
Fine Sand Course Sand
Sz
(mm) 0.2
0.35 – 1.0
Spec Grav
2.6
2.6
Depth
(in) 36 – 48 24 – 36
Flow Flow
gpm/sf Gravity Gravity .05 - .03
2 – 4 Dual Media Mixed Media Diatom. Earth Pressure Anthracite Sand Anthracite Sand, Garnet Diatomaceous All Media 0.9 – 1.2
0,4 – 0,55 0.9 – 1.2
0,4 – 0,55 0.2
0.005 to 0,125 1.4 – 1.6
2.6
1.4 – 1.6
2.6
4.2
Application 18 – 24 6 – 10 16.5
9 4.5
1/16 to 1/8 Gravity Gravity 4 – 5 5 Pressure or Vacuum Pressure 0.5 – 5 2 – 4
49
Calculating Filter Flow Rate
1.
2.
Determine Surface Area of Filter Measure Filter Rise with stopwatch and tape measure (often meters are out of calibration) Example: 150 sft surface area, 10.7” rise in 20 seconds
(10.7 in / 12 in/ft) x 150 sft x 7.48 gal/cft = 1000 gal.
(20 seconds / 60 min ) = 0.333 min Flow Rate = 1000 gal / 0.333 min = 20 gpm / sft 150 sf
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Causes of Poor Filter Performance
Filter Problems: operational, mechanical equipment failure, media failure Turbidity Errors: calibration, air bubbles, debris Chemical Feed Failures: coagulant, coagulant aid, filter aid Poor Water Quality: increased turbidity, algae Operating Plant intermittently exceeding peak loading capacity Florida Rural Water Association Water Treatment Plant Operation 51
Common Filter Operation Deficiencies
Filters are started dirty (i.e., without backwashing Increases in plant flow rate made with no consideration of filtered water quality Filter to waste capability is not being used or not monitored if utilized Filters removed from service without reducing plant flow, resulting in overload No testing of filters resulting in media loss, underdrain or support gravel damage Operations staff backwash the filters without regard for filter effluent turbidity Significant build up of mudballs in filter media Backwash rate too low for longer period or stopped early to conserve water Individual filtered water quality is different and quality is not monitored Performance following backwash is not monitored or recorded. There are no records available which document performance Calibration procedures are not practiced
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Filter Integrity Testing
Evaluates filter media, support gravel and underdrains Check for filter depth, surface cracking, mudball and segregation Media is checked by excavation Steel rod is used to probe support gravel location and uniformity (should vary < 2”) Observe clearwell for evidence of media Check for uneven flow splitting to filters Florida Rural Water Association Water Treatment Plant Operation 53
Backwash Parameters
Typically at about 24 hour intervals Rate: 15 gpm/sft – 20 gpm/sft Expand at min. 25% Backwashing Duration: 5 - 10 min.
Filter to waste for 3 - 5 min.
Water used for backwashing: 2% - 4% per filter of total water produced Florida Rural Water Association Water Treatment Plant Operation 54
Sand Filter ~40% Multimedia ~25% Deep Bed ~50% 15 to 20 gpm/sft Min. Expansion 25%
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Determining Backwash Expansion in Plant
Can be made with tin can lid
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Visual Identification of Filter Problems
Mudballs – Formed by chemical deposits of solids during backwashing (leads to coating of media surfaces) Surface Cracking – Caused by compressible matter around media at surface Media Boils – Caused by too rapid of backwash and displaces gravel support below Air Binding – Caused by excessive headloss (infrequent backwashing) allowing air to enter media from below Florida Rural Water Association Water Treatment Plant Operation 57
Section 4 Disinfection By-Product Control
Disinfection By-Product Formation Factors Affecting By-Product Formation Locating THM and HAA5 Areas Formation of THMs and HAA5s Controlling Disinfection By-Products Importance of Water Age Flushing Methods and Benefits Florida Rural Water Association Water Treatment Plant Operation 58
Disinfection By-Product (DBP) Formation
Disinfection Byproducts (DBP) are produced by the reaction of free chlorine with organic material found in natural waters.
The amount of organic materials in a natural water called NOM can be approximated by the amount of Total Organic Carbon (TOC) present in the water source.
NOM consists of various chemical compounds containing carbon, originating from decayed natural vegetative matter found in water. Florida Rural Water Association Water Treatment Plant Operation 59
Factors Affecting Disinfection By-Product Production
Turbidity and the type of NOM present Concentration of Chlorine added pH of water Bromide Ion Concentration Temperature Contact Time Florida Rural Water Association Water Treatment Plant Operation 60
Locating TTHM Areas
High Water Age Storage Tanks do not fluctuate No / Few Customer Areas Stagnant Areas Dead Ends Bad Pipe Regrowth Areas Florida Rural Water Association Water Treatment Plant Operation
Pipe Tuberculation with Bacterial Growth producing Organic Precursors
61
Locating HAA5 Areas
Low Demand Areas Toward Middle System Areas w/ Stagnant / Low Water Age Areas with No / Little Regrowth – Eliminate Biodegradation Locations – Free Chlorine Residuals < 0.2 mg/L – HPC Data No Dead Ends Florida Rural Water Association Water Treatment Plant Operation 62
Formation of DBP in a Water System
63
Disinfectant and DBP Production in a Typical Water System 64
DBP Reduction Techniques in a Water Distribution System
Reducing detention time in storage tanks, Ensuring turnover in distribution system Flushing dead-end lines.
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Typical Distribution System Water Age (Days) in Pipelines
Population > 750,000 < 100,000 < 25,000 Miles of WM > 1,000 < 400 < 100 Water Age 1 – 7 days > 16 days 12 – 24 days AWWA: Water Age for Ave and Dead End Conditions Florida Rural Water Association Water Treatment Plant Operation 66
There are Two Types of Flushing Used by Water Distribution Systems
Conventional Flushing
&
Unidirectional Flushing < 2.5 fps velocity that reduces water age, raises disinfectant residual removes coloration > 2.5 fps velocity that removes solid deposits and biofilm from pipelines
67
How Often to Flush
• Dead-end mains at least monthly • Other flushing points at least twice annually (DEP requires quarterly flushing) • At intervals necessary to maintain consistent water quality throughout the distribution system • Often enough to maintain adequate disinfection residuals throughout the distribution system • Whenever Customer complaints of bad taste, odor, clarity or turbidity are received (DEP requirement) Florida Rural Water Association Water Treatment Plant Operation 68
Flushing Benefits Summarized
• • • • • • •
Restores disinfectant residual Maintains or improves water quality a. Reduces bacterial growth b. Reduces customer complaints Restores flow and pressure in the distribution system a. Reduces sediment b. Reduces corrosion and tuberculation in mains Reduces DBP problems and lowers disinfection costs Reduces pipeline maintenance costs Increases life expectancy of the distribution system Typically results in a fire hydrant maintenance program
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Section 5 Corrosion Control
Corrosion Control Methods Factors Affecting Corrosion Corrosion Tuberculation Example pH and Alkalinity Relationships Langerlier Index Troubleshooting Corrosion Complaints Basics of Sequestering Florida Rural Water Association Water Treatment Plant Operation 70
Corrosion and Chemical Activity
Most all forms of corrosion are chemical reactions (erosion is the exception) that require three things:
1.
2.
3.
A carrier such as Water that allows the movement of positively charged ions (from Anode+ to Cathode-) A condition (water metal contact) that allows metals to disassociate (ionize) and allows electrons to flow An imbalance that favors the transport of metals or ions to achieve a chemical balance in a water solution.
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Corrosion Control Methods
Corrosion Control is employed in water treatment to protect pipeline materials, appurtenances and fittings from leaching problematic (iron) and/or dangerous inorganic chemicals (lead and copper).
Three types of treatment are generally used: 1.) Chemical Adjustment, Water Treatment and Sequestering Protection Measures in water system include the use of sacrificial metals and electronic cathodic protection.
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Factors Affecting Corrosion
Water’s pHs Water alkalinity Solids content Temperature Materials Used for pipes and other fittings.
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Cathodic Action Resulting in Tuberculation in Water Pipelines Inside Pipe Wall
1.5”
74
Effects of pH on the Rate of Corrosion of Iron in Water Florida Rural Water Association Water Treatment Plant Operation 75
Relationships between Alkalinity, pH
A Water can be Corrosive or Depositing based upon it’s pH and Alkalinity. Florida Rural Water Association Water Treatment Plant Operation 76
Affects of Raising or Lowering Alkalinity and CO 2 by Chemical Addition Florida Rural Water Association Water Treatment Plant Operation 77
Determining pH of Water
pH = log CaCO3) } { 2.2 x 10 6 X (Alkalinity in mg/l as (CO 2 in mg/l) Measured Alkalinity 60 mg/l as CaCO 3 Measured CO 2 = 7.4 mg/l pH = log { 2.2 x 10 6 X 60/7.4 } = 7.25
Approximate pH between 7.0 to 8.0
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Use of the Langerlier Index for Determining Water Stability
Every water has a particular pH value where the water will neither deposit scale nor cause corrosion. A stable condition is termed saturation. Saturation (pHs), varies depending on calcium hardness, alkalinity, TDS, and temperature. The Langerlier Index = pH – pHs Corrosive < LI = 0 > Scale Forming Florida Rural Water Association Water Treatment Plant Operation 79
Recommended Treatment for Corrosive and Scaling Water based on LI
Saturation Index - 5 - 4 - 3 - 2 -1 -0.5
0 0.5
1 2 3 4 Description Severe Corrosion Severe Corrosion Moderate Corrosion Moderate Corrosion Mild Corrosion None- Mild Corrosion Near Balanced Some Faint Coating Mild Scale Coating Mild to Moderate Coatings Moderate Scale Forming Severe Scale Forming General Recommendation Treatment Recommended Treatment Recommended Treatment Recommended Treatment May Be Needed Treatment May Be Needed Probably No Treatment No Treatment Probably No Treatment Treatment May Be Needed Treatment May Be Needed Treatment Advisable Treatment Advisable
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Troubleshooting Customer Complaints caused by Corrosion Water Characteristic Likely Cause
Red/reddish-brown Water Blueish Stains on fixtures Black Water Foul Tastes and Odors Loss of Pressure Lack of Hot Water Reduced Life of Plumbing Tastes Like Garden Hose Distribution Pipe Corrosion Copper Line Corrosion Sulfide Corrosion of Iron By-Products of Bacteria Tuberculation Scaling Pitting from Corrosion Backflow From Hose
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Sequestering Action of Poly and Ortho Phosphates
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Polyphosphates for Sequestering Soluble Iron and Manganese after Treatment The Polyphosphate, Hexametaphosphate is commonly used for Sequestering Soluble Iron and Manganese Sequestering is used when soluble Iron and Manganese exists after treatment; The Agent is added after sedimentation Large doses (>5 mg/l) will soften rust deposits in pipelines which are transported into homes Proper dose is to keep soluble iron and/or manganese tied up for 4 days Florida Rural Water Association Water Treatment Plant Operation 83
Use of Orthophosphates for Sequestering
Orthophosphate is used to sequester iron ions at pipe surfaces The Sequestering forms a protective coating that prevents further iron migration Ortho/Poly Blends provide both sequestering of soluble iron and iron movement from pipelines under corrosive conditions Florida Rural Water Association Water Treatment Plant Operation 84
Section 6: Demineralization Processes
Basic Demineralization Systems RO Operating Considerations Pretreatment; Fouling and Scaling Issues Ion Exchange Considerations Sodium/Calcium Exchange Florida Rural Water Association Water Treatment Plant Operation 85
Ion Exchange, Membrane Filtration and Electrodialysis
Several special treatment processes are used to remove selected mineral contaminants from the water. These include Ion Exchange, Membrane Filtration and Electrodialysis. These systems remove selected salts such as sodium, hardness consisting of Calcium and Magnesium and removal of selected contaminants such as Nitrate or Arsenic Florida Rural Water Association Water Treatment Plant Operation 86
Reverse Osmosis (RO) Treatment Considerations
Used to Remove Highly Concentrated Salts (TDS) Operating pressure < 400 psi Salt Rejection Rates of < 95% Turbidity <1 NTU Flux Range 15 – 32 GFD (gallons Flux per day per sq. ft. membrane surface) Florida Rural Water Association Water Treatment Plant Operation 87
Pretreatment Requirements for Reverse Osmosis Systems Suspended Particulates Colloidal materials Microbiological Matter Chlorine Carbonates Sulfate Silica Iron Hydrogen Sulfide Blockage Filtration Fouling Coagulation/Filtration Fouling Chlorine Failure GAC or Dechlorination Scaling pH adjust or Softening Scaling Inhibitor or Cation Rem.
Scaling Softening Scale/Foul Greensand (no aeration) Scale/Foul Degasification Florida Rural Water Association Water Treatment Plant Operation 88
Operating Considerations Ion Exchange Softening
Iron and Manganese Corrosiveness of Brine Solution Pump Strainer Fouling of Resin Florida Rural Water Association Water Treatment Plant Operation 89
Optimal Water Characteristics for Ion Exchange
pH NO 3 SO 4 TDS Turbidity 6.5 – 9.0
< 5 mg/l < 50 mg/l < 500 mg/l < 0.3 NTU
Selectivity Considerations S04 -2 > NO3 -2 > CO3 -2 > NO2 -2
Florida Rural Water Association Water Treatment Plant Operation
> CL -1
90
Sodium Exchange MCL Considerations
Sodium provides 100% exchange for Ca ++ and Mg ++ NaZeolite + Ca ++ NaZeolite + Mg ++ --> CaZeolite + Na + and --> MgZeolite + Na + For every grain (17.1 grains = 1 mg/l) of hardness removed from water, about 8.6 mg/1 of sodium is added.
Sodium MCL = 160 mg/l - Initial Na water concentration + NaOCl 5 grains needed for corrosion control (86 mg/l) thus: source water hardness limit ~ 350 mg/l hardness (~20 grains) ie. 100% x 5 grains , or 15 grains removed x 8 = 134 mg/l Na 20 grains Provides 134 mg/l Na and 5 grains or 86 mg/l Hardness Florida Rural Water Association Water Treatment Plant Operation 91
Section 7 Coagulation Processes Control
Metal Charges and Electron Attraction Elemental Weights and Chemical Formulas Particle Chemistry and Colloidal Particles The Floc Building Process Optimizing the Coagulation Process Use of a Jar Test Florida Rural Water Association Water Treatment Plant Operation 92
Periodic Table of the Elements
Valances are shown at the top of the Periodic Table, F is one electron short and Mg has two extra electrons
93
The Periodic Chart also Provide the Atomic Weight of an Element Atomic Number 8
Includes Isotopes Use 16
O Oxygen 15.99
Symbol Name Atomic Weight 94
Solids and Colloidal Material
Suspended Solids Colloids Turbidity Zeta Potential Suspended in the Water and can be Removed by Conventional Filtration Finely Charged Particles that do not Dissolved The Cloudy Appearance of Water caused by Suspended Matter and Colloids Electrical Charge of a suspended particle Florida Rural Water Association Water Treatment Plant Operation 95
Primary Coagulants
Primary coagulants are lime, aluminum sulfate (alum), ferrous sulfate, ferric sulfate and ferric chloride.
These inorganic salts will react with the alkalinity in the water to form insoluble flocs which will trap the suspended matter in them.
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Removal of Colloidal Particles by Coagulation & Flocculation
Floc Building Process : Neutralization of repulsive charges Precipitation with sticky flocs Bridging of suspended matter Providing “agglomeration sites” for larger floc Weighting down of floc particles Florida Rural Water Association Water Treatment Plant Operation 97
Polymers and Ionic Charges
Cationic + *Anionic Nonionic
Bridging Action of Cationic Polymer with Colloidal Particles
* Used with Metal Coagulants in water treatment Florida Rural Water Association Water Treatment Plant Operation 98
Factors Affecting the Coagulation Process
pH (pH Range: Al, 5 – 7 ; Fe, 5 – 8) Alkalinity of water (> 30 PPM residual) Concentration of Salts (affect efficiency) Turbidity (constituents and concentration) Type of Coagulant used (Al and Fe salts) Temperature (colder requires more mixing) Adequacy of mixing (dispersion of chemical) Florida Rural Water Association Water Treatment Plant Operation 99
Jar Test Plot for Low Alkalinity or Low Turbidity Water
Alum initially reacts with low alkalinity With Ferric Chloride requires chemical to reach optimal pH before reacting Adding too much coagulant increases turbidity Florida Rural Water Association Water Treatment Plant Operation 100
Section 8: Hardness and Water Softening
Hardness Removal by Softening Treatment Methods Used to Remove Hardness Alkalinity Definitions Alkalinity/Acidity Relationships pH and Lime Treatment Removal of Color and Organics Importance of Recarbonation Florida Rural Water Association Water Treatment Plant Operation 101
Water Hardness
Hardness in Water causes scaling, causes fibers in clothes to become brittle and increases the amount of soap that must be used for washing Hardness in water is caused by the water’s Calcium and Magnesium Content Water is considered hard when it has a hardness concentration of > 100 mg/l expressed as calcium carbonate equivalent Water that hardness < 100 mg/l expressed as CaCO3 is considered soft Hardness can either be removed by water treatment or sequestered using phosphates
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Methods of Removing Hardness
Treatment Method Hardness Levels Retained
Lime Softening
(Chemical Precipitation) RO (Nanofiltration) (Membrane Filtration)
Ion Exchange
(Chemical Exchange) Solubility Level of about 35 mg/l (CaCO 85 – 90% removal Florida Rural Water Association Water Treatment Plant Operation 3 ) Basically Zero Water must be blended 103
Alkalinity Definitions
The capacity of water to neutralize acids.
The measure of how much acid must be added to a liquid to lower the pH to 4.5
It is caused by the water’s content of carbonate, bicarbonate, hydroxide, and occasionally borate, silicate, and phosphate.
In natural waters, Alkalinity = Bicarbonate Hardness = Total Carbonate Hardness Florida Rural Water Association Water Treatment Plant Operation 104
CO2 T=0 pH
Relationships among pH, Alkalinity and Indicators
0% P=0 100% Bicarbonate Bicarbonate and Carbonate Carbonate and Hydroxide T Alkalinity P Alkalinity CaCO3 9.4
100% Florida Rural Water Association Water Treatment Plant Operation
10.2
10.6
Mg(OH)2 105
Types of Alkalinity that can be Present at pH Values
Below 4.5 only CO 2 present, no Alkalinity Between 4.5 to 8.3 only Bicarbonate present Between 8.3 to 10.2 Bicarbonate & Carbonate. Between 10.2 to 11.3 Carbonate & Hydroxide At 9.4 Calcium Carbonate becomes insoluble and precipitates At 10.6 Magnesium Hydroxide becomes insoluble and precipitates Florida Rural Water Association Water Treatment Plant Operation 106
Removal of Organics by Lime Softening Precipitation
Calcium Carbonate 10% to 30% of Color, TOC & DBP Magnesium Hydroxide 30% to 60% of TOC & DBP and 80% of Color Addition of Alum/Ferric +5% to +15% of Color, TOC & DBP Sequential Treatment Additional Removal Color, TOC and DBP Florida Rural Water Association Water Treatment Plant Operation 107
Recarbonation in Lime Softening
Because water has unused lime (calcium hydroxide) and magnesium hydroxide in solution at high pH (pH 11), these must be converted to a stable forms.
CO CO 2 2 is added to reduce Ca(OH) 2 to CaCO 3 which precipitates at about pH 10; additional is added to convert Mg(OH) 2 to soluble Mg(HCO 3 ) 2 which occurs at a pH of 8.4.
Reaction must be completed before filtration so that calcium carbonate will not precipitate in the filters or carry into distribution system.
Florida Rural Water Association Water Treatment Plant Operation 108
Water Treatment Summary
Effective Water Treatment Requires the application of accepted principles Most Process Problems in Water Treatment are the result of failure to recognize the symptoms that result from improper application or adherence to these factors Most treatment plant problems can be resolved by application of the techniques presented Florida Rural Water Association Water Treatment Plant Operation 109