Chemical Feed Systems Best Practices NJAWWA Annual Conference Atlantic City, NJ March 18th, 2015 John Perry Director of Public Works and Water and Sewer Utilities Montville Township, NJ What are Chemicals used for? • • • • Coagulants – they bridge particles together pH/Corrosion Adjusters Oxidants & Disinfectants Polymers – • Coagulant • Expensive (compared to typical coagulants) • Critical to dewatering equipment and high rate clarification processes • Can be very damaging to membranes Different Phases Solid, liquid and gas phases Solids Less expensive on a per pound basis But more complex to feed Liquids Bulk (delivered) concentration Different feed concentrations Gas Done through eductor using water Systems are more reliable Safety concerns Components Delivery – dry, liquid, gas Storage – tanks, bags and batch, cylinders Conveyance – Pumps, feeders and eductors Pipes – materials and diameter Suction Discharge Dilution Water Injectors Delivery • Delivery – dry, liquid, gas • Secondary containment for delivery area. (Regulated under DPCC if total storage is above 20,000 gallons) • Quick Connect with in-line check valve. • Sampling valve • Alarms at delivery area • Safety showers at delivery area Chemical Delivery NJ DPCC/DCR (Discharge Prevention Containment and Countermeasure) Requires plans for Hazardous Materials storage. Requires secondary containment The most common water treatment chemicals regulated by DPCC are alum, hypo, KMnO4, ferric chloride and fuel oil. NJ TCPA N.J.A.C. 7:31 The Toxic Catastrophe Prevention Act was established in 1985. Identifies and regulates Extremely Hazardous Substances. (EHS) Chlorine, Sulfur Dioxide and Ozone are the most common water treatment chemicals regulated by TCPA. Other Risk Management Programs for EHS: OSHA Process Safety Management - 29 CFR 1910.119 US EPA 40 CFR 68 - Federal Chemical Accident Prevention Provisions Main Requirements of a Plan Site Overview Process Safety Information Process Hazard Analysis (Risk Assessment) Standard Operating Procedures Operator Training Mechanical Integrity (Preventative Maintenance) Management of Change Safety Review Main Plan Requirements Cont.. Compliance Audits Accident Investigation Employee Participation Hot Work Permits Contractor/Subcontractor Safety Emergency Response Annual Reports Chemical Storage Tanks Bulk Chemical Hoppers Positive Displacement Pumps Positive displacement pumps are commonly used for pumping chemical solutions. They are not used for water distribution purposes due to their pulsating type flow and their tendency to develop excessive pressure build up when pumping against a closed valve. Unlike centrifugal pumps, positive displacement pumps must always have an unobstructed discharge. Extensive damage can occur to the pump and piping if the positive displacement unit is operated against a closed valve. Positive Displacement Pumps Two broad classifications of positive displacement pumps are reciprocating and rotary. Of the reciprocating type pumps, the piston and diaphragm type are the one most commonly used Positive Displacement Pumps: Reciprocating Piston/Plunger Diaphragm Piston Pumps As the piston retracts, the discharge check valve is pulled closed, and the suction check valve is pulled open. Fluid is drawn into the pump cavity. As the piston pushes forward, the suction valve is pulled closed and the discharge valve is pushed open. Fluid is expelled. Diaphragm Pumps Diaphragm pumps use the reciprocating action of a diaphragm to move the liquid. Similar to piston pumps, check valves are provided on the inlet and outlet sides to control the flow of liquid in the pump chamber. Positive Displacement Pumps: Rotary (Single Rotor) Flexible Tubing/ Peristaltic (Wave) A vacuum is created by the rollers compressing the hose and moving along Liquid is drawn into vacuum Rollers capture liquid between them and move liquid towards discharge Positive Displacement Pumps: Rotary (Single Rotor) Screw Progressive Cavity Pumps A progressive cavity pump is another type of positive displacement pump. Progressive cavity pumps are equipped with a helical or spiral rotor. The rotor consists of a shaft with a helical surface, which rotates in a rubber sleeve. As the shaft turns, it traps fluid between the shaft and sleeve, and forces toward the upper end of the sleeve. Positive Displacement Pumps: Rotary (Multiple Rotors) Gear Gear pumps External gear pumps come in single or double configurations. Handle viscous and watery-type liquids. Reduced speeds with high-viscosity liquids results in greater efficiency. Two gears come into and out of mesh to produce flow. Uses two identical gears rotating against each other. Chemical Eductors Volumetric Feeders o The bulk material is discharged from a hopper with a constant volume per unit of time by regulating the speed of a feeding device. o The actual volume of material fed is determined through calibration. The feeding accuracy is dependent on the uniformity of the material flow characteristics and the bulk density. o Volumetric feeding can, however become inaccurate if the bulk density of the solid that is being handled varies. The feeder cannot recognize a density change because it simply discharges a certain volume per unit of time. o Examples of volumetric type feeders include screws, belts, rotary valves and vibratory. Gravimetric Feeders o The bulk material is discharged from a hopper by weighing the material being fed. o The weighing system with control compensates for non-uniform material flow characteristics and variations in bulk density, and therefore provides for a high degree of feeding accuracy. o A gravimetric feeder relies on weighing the material to achieve a required discharge rate or batch weight. Disinfection Options Disinfection Agents • Chlorine • Compressed Chlorine Gas - Cl2 • Solution of Sodium Hypochlorite - NaOCl • Dry Calcium Hypochlorite (HTH) - Ca(OCl)2 • Chloramine • Chlorine + Ammonia = NH2Cl (monochloramine) • Chlorine Dioxide • Chlorine + Sodium Chlorite + Acid = ClO2 • Ozone - O3 • UV • Sonic and Electromagnetic Pulse • Membranes Disinfection Primary Disinfection The destruction or inactivation or removal of pathogens Secondary Disinfection Maintain residual Control regrowth Prevent nitrification (NH2Cl) Oxidizing Potential of Various Reagents Oxidizing Reagent Ozone Hydrogen Peroxide Permanganate Chlorine Dioxide Hypochlorous Acid Chlorine Gas Hypobromous Acid Oxygen Bromine Hypoiodous Acid Hypochlorite Chlorite Iodine Oxidizing Potential 2.07 1.77 1.67 1.57 1.49 1.36 1.33 1.23 1.09 0.99 0.94 0.76 0.54 Chlorine is the Most Commonly-Used Disinfectant/Oxidant Effective at inactivating bacteria, viruses Less effective at inactivating protozoan cysts (Giardia, Cryptosporidium) Provides a relatively long-lasting residual in distribution system Inexpensive compared to hypo, ozone, UV Reacts with organic material in water to form halogenated disinfection by-products (trihalomethanes, haloacetic acids) Compressed Chlorine Gas Basic Chemistry Cl2 + H2O ---> HOCl + HCl HOCl <---> H+ + OCl- Chlorine Species HOCl Hypochlorous acid pH free chlorine residual Up to 100 times more effective than hypochlorite ion OCl– - Hypochlorite ion Compressed Chlorine Gas Advantages Least costly of approved disinfectants Relatively easy to use Provides residual Disadvantages Extremely hazardous chemical Produces DBPs Tastes and odors Highly reactive-requires booster stations Chlorine Properties Liquid is 1.5 times as heavy as water Gas is 2.5 times as heavy as air Liquid vaporizes readily at atmospheric pressure and normal temp Effect of pH Concentration of species Dissociation of Hypochlorous Acid 100% 80% 60% [OCl-] [HOCl] 40% 20% 0% 5 6 7 8 pH 9 10 What Does this Mean Chlorine Feed Three states Gas under pressure – cylinder to chlorinator Gas under vacuum – chlorinator to ejector Chlorine Solution – ejector to diffuser Each state requires a different method of handling Chlorine Handling Issues Storage: compressed gas – 85% liquid, 15% gas Max Withdrawal Rate 150 lb cylinder = 40 lb/day 1 ton container = 400 lb/day Absorbs heat – freezing of lines at high rates Higher rates require evaporator Common Problems with Chlorine Gas Operator Safety Public Safety TCPA/RMP Storage Emergency Response Equipment - Training Toxic Gas Release Solution of Sodium Hypochlorite Advantages Non-hazardous Easy to use Provides residual Disadvantages Can be costly (location dependent) Produces DBPs Solution strength decays Tastes and odors Sodium Hypochlorite More expensive than chlorine gas Deteriorates rapidly in a warm/hot environment 12.5 or 15% solution typically used Deterioration rate increases with higher solution strength Dilution by 50% will reduced decomposition rate by a factor of 4 Used because of safety concerns with chlorine gas and EPA Risk Management Program, NJ TCPA Easier to handle than chlorine gas Hypo Concerns Common Problems with NaOCl Pump stoppage due to out-gassing (loss of prime) Leaking pipe joints Degradation of concentration Unstable residual Ambient corrosion DPCC (containment) Operator safety Sodium Hypochlorite Degradation – Average Manufacturer @ 70 Degrees F Time of Manufacture - 12.5 Trade Percent 2 Days Later – 12.43 Trade Percent 7 Days Later – 12.25 Trade Percent 14 Days Later – 12.01 Trade Percent 21 Days Later – 11.78 Trade Percent 28 Days Later – 11.55 Trade Percent 35 Days Later – 11.34 Trade Percent Case Study Largest chlorine gas leak occurred at WTP using hypo. Truck delivered FeCl3 (pH=4) into hypo tank by mistake. Hypo pH dropped from 12 to 5 almost instantly, releasing 12,000 lbs of chlorine gas Incompatible with acid – do not store near hydrofluosilic acid or sulfuric acid. Dry Calcium Hypochlorite Advantages Calcium Hypochlorite Tablet Feed Systems Gravity Feed Non-hazardous chemical, but is an oxidant Pressure Feed Does not degrade as long as kept dry First approved by NJDEP mid1990’s Disadvantages Feed rates up to 650 lb/day chlorine More difficult to use Produces DBPs Advantages of Tablet Systems Safety - Operator and Neighborhood friendly Easy to handle and operate Low Maintenance Compact Chemical Storage Dry, Stable Chlorine Source No Risk Management Plan Requirements No Secondary Containment Convenience Separate room not required Chlorine Dioxide Advantages Strong disinfectant Effective at Inactivating Bacteria, Viruses, and Protozoa Cysts Maintains a good residual in the distribution system Does not form DBPs Disadvantages Requires specialized mixing equipment Requires filtered water pH of 8 Hazardous chemical MCL on chlorine dioxide = 0.8 ppm MCL on chlorite = 1.0 ppm Tastes and odors with new household carpeting Chloramines Cl2 • Advantages • • • • • Stops DBP formation Little taste and odor when monochloramine Provides persistent residual Does not react with distribution materials More effective in penetrating biofilms • Disadvantages • • • • NH3 Weaker disinfectant Requires more operational control Reaction with free chlorine residual Potential for nitrification Ozone – What is it? Created by a high intensity reaction which splits O2 into separate O molecules, which attach to other O2 molecules and form O3. Extremely reactive. It is the most powerful disinfectant in the world. Residual ozone dissipates quickly, with a half life of about 30 minutes. Ozone Advantages Very effective disinfectant and oxidant No DBPs of concern No tastes and odors Disadvantages Hazardous chemical Costly -requires specialized equipment No residual - need chlorine in some form Forms bromate in presence of bromide (bromate MCL = 10 ppb) Ozone Oxidation and Disinfection As a comparison based on 99.99% of bacterial concentration being killed and time taken: Ozone is 25 times of that of HOCl (Hypochlorous Acid) 2,500 times of that of OCl (Hypochlorite) 5,000 times of that of NH2Cl (Chloramine). Further more, ozone is at least 10 times stronger than chlorine as a disinfectant. COMPONENTS OF AN OZONATION SYSTEM INFLUENT OZONE OZONE GENERATOR AIR/OXYGEN PREPARATION AIR CONTACT CHAMBER OZONE DESTRUCTOR VENT OTHER OXYGEN APPLICATIONS EFFLUENT Alternative paths for enriched oxygen recycle Basic Ozone Application Pre-ozonation Raw Water Screening Clarification Ozone Disinfection Filtration Distribution Residual Disinfection Ozone Generators – Large Scale Ozone Generators – Small Size Ozone GeneratorsMedium Size Inside the Unit Oxidation: Why? Iron removal Manganese removal Taste and odor control Algae Hydrogen sulfide Color removal VOC removal (advanced oxidation) Potassium Permanganate (KMnO4) • Strong Oxidizer • Purple in color • Form - Crystals • Density - 90 to 100 lbs./ft3 KMnO4 Applications (Drinking Water) • Taste and Odor • Manganese removal • THM reduction • Fe, H2S, Ra and As Control • Zebra mussel control • Biofilm control Standard Dosage Estimates • 1 part of Fe requires 0.94 parts of KMnO4 for treatment. • 0.5 part of Mn requires 1.00 part of KMnO4 for treatment. Coagulation Since colloidal and microbial particulates tend to be negative in overall charge, introduction of a positively (cationic) charged metal based coagulant is necessary for charge neutralization. Once charge neutralization occurs, particulates collide and adsorb to one another to form larger floc particles. Adsorption of positively charge species on to negatively charged particle Raw Water Negatively charged particles Precipitation of metal hydroxide which absorbs suspended particles Coagulated particles •- •- •- •Coagulant addition Flash Mix or Rapid Mix Flocculation Flocculation simply defined is the agglomeration of neutrally charged (coagulated) particles to form amorphic settable floc. Flocculation allows particles to “bridge” with one another forming larger and more dense floc. Coagulation is a very quick chemical reaction, accomplished between 1 and 3 seconds. High mixing rates and proper dispersion of coagulant are essential. Lack of proper mixing or improper chemical dispersion will lead to coagulant overdosing and poor floc formation. Chemicals used for Coagulation Iron Inorganic Metal Species Aluminum Aluminum Sulfate [Al2(SO4)3] Sodium Aluminate [ NaAlo2] Ferric Chloride [FeCl3] Ferrous Chloride [FeCl4] Ferrous Sulfate [Fe2(SO4)3] Ammonia Alum [Al2(SO4)3·(NH4)2SO4·24 H2O] Poly Aluminum Chloride (PAC) Organic Polymers Cationic Anionic Nonionic Sometimes in-line or static mixers are used for rapid-mixing. This type of mixer is illustrated here: Mechanical Flocculator L H Cross flow Flocculator (sectional view) W Plan (top view) Hydraulic Flocculation L • Horizontally baffled tank The water flows horizontally. The baffle walls help to create turbulence and thus facilitate mixing W Plan view (horizontal flow) • Vertically baffled tank The water flows vertically. The baffle walls help to create turbulence and thus facilitate mixing H L Isometric View (vertical flow) Hydraulic Flocculation: Pipe Determining Proper Floc Size There are many ways to determine if you are coagulating/flocculating properly. Jar Testing Zeta Potential Streaming Current Monitors Pilot Studies Jar Tests The jar test – a laboratory procedure to determine the optimum pH and the optimum coagulant dose A jar test simulates the coagulation and flocculation processes Determination of optimum pH Fill the jars with raw water sample (1000 or 2000 mL) – usually 6 jars Adjust pH of the jars while mixing using H2SO4 or NaOH/lime (pH: 5.0; 5.5; 6.0; 6.5; 7.0; 7.5) Add same dose of the selected coagulant (alum or iron) to each jar (Coagulant dose: 5 or 10 mg/L) water treatment Jar Test 67 3/6/2015 pH/Corrosion Adjusters To drop pH for coagulation – Sulfuric acid To raise pH for corrosion or for faster oxidation of manganese Sodium Hydroxide (caustic soda) – liquid concentration Lime – dry – quick or hydrated? Soda Ash - dry Figure Courtesy of Environment Canada (www.ns.ec.gc.ca) pH Scale Caustic Soda Choice 25% or 50%. 25% More expensive Crystallization Temp 0 Deg F 50% Less expensive Crystallizes at 55 Deg F – Heat trace recommended Higher pH – More calcification Lime Choice quicklime (CaO) or Hydrated Lime (Ca(OH)2) Lime Feed Handbook and ZMI/Portec Quicklime Less expensive Need to slake - exothermic Hydrated lime More expensive Slaker not required Instrumentation - Instruments are commonly used to measure and/or control the following parameters in water treatment plants Location Raw water influent Pump well Pump header Electrical equipment valves Parameter/Function Flow, pH, pressure Tank Level Pressure, flow Run time meters Limit switches Control Control systems can be manual, semi-automatic or automatic. Turning a hand wheel to open or close a valve is an example of manual control. Semi-automatic control requires manual initiation of an automatic function. A typical example would be pressing a control button at a panel, which in turn causes an electric valve to open or close. The automatic control method uses instruments to automatically control the process or equipment.A typical example would be a variable speed pump which automatically speeds up or slows down to maintain a set pressure.
© Copyright 2017