Introduction to Water Treatment PDF

**INTRODUCTION TO WATER TREATMENT** **REASONS FOR WATER TREATMENT** - Two main reason (1. remove contaminants harmful to health and 2. remove contaminants that affect the waters' look, taste, or smell bad) - ***Aesthetics*** - pleasant in appearance, color, and taste - ***Turbidity...

**INTRODUCTION TO WATER TREATMENT** **REASONS FOR WATER TREATMENT** - Two main reason (1. remove contaminants harmful to health and 2. remove contaminants that affect the waters' look, taste, or smell bad) - ***Aesthetics*** - pleasant in appearance, color, and taste - ***Turbidity*** - caused by the presence of suspended matter, results in scattering and absorpotion of light rays - ***Hydrogen Sulfide Gas***- from bacterial anaerobic decay. Strong rotten egg odor - ***Hardness*** - caused primarily by calcium and manganese ions - ***Acute***- rapid with low levels of exposures - ***Chronic*** - prolonged and repeated exposures - ***Disinfection by products***- formed by the reaction of disinfectant with contaminants in water **MOST COMMON CONTAMINANTS** - ***Giardia*** -- excreted by animals (6-18 micrometers) - ***Cryptosporidium*** -- " (4-6 micrometers) - ***Total Trihalomethanes (TTHM)*** -- formed when natural organic substances from decaying vegetation and soil react with chlorine - ***HAA5 (five Haloacetic Acids)*** **BASIC WATER TREATMENT UNIT PROCESSES** Biological Activated Carbon -- biological process removes contaminants from potable water - ***Oxidation*** -- add oxy, removes hydrogen -removal of inorganic contaminants (***iron, manganese, and arsenic***) -prior to other process to improve the removal of inorganics, particulates, taste or odor \- most commonly used oxidants (**chlorine and potassium permanganate**) \- chlorine is supplied in gas,solid, and liquid form \- potassium permanganate is supplied as a fine granular solid material that is dissolved in water \- in operation, **free chlorine** is used \- oxidants are injected as gas or liquid \- the speed or reaction time depends on pH, type of oxidant, contaminants, and water temp -**lower pH or temp = slow oxidation** \- rate slowed/demand increased by the presence of iron/manganese -used of chlorine as oxidants can formed a disinfection byproducts (TTH or total\ trihalomethane or HAA5 or Haloacetic acids (5) \- ammonia = chloramines - a much weaker oxidant and slow oxidation \- the proper amount of oxidant is visible in slight pink - ***Coagulation*** -- destabilization or decreasing the stability of colloids in water \- results from adding salts in iron, aluminum, or cationic polymer \- colloidal material (**hydrophobic** - water fearing, **hydrophilic** - water loving and common source of color in water) \- organic material that will pass through a 0.45 micrometer membrane filter can be dissolved ( **humic and fulvic** acids which causes water color) \- organic carbon - comes from plant/animal sources \- **Total Organic Carbon (TOC)** - both larger and smaller than 0.45 micrometers \- **Dissolved Organic Carbon (DOC)** - fraction of organic material smaller than 0.45 micro \- opposing forces\ \> **Stability factors**- help to keep colloids dispersed \> **Instability factors** - contribute to natural removal of colloids \- **Polymer** is high molecular weight that forms ions when dissolved in water (polyelectrolytes) \> primary coagulant and is long sting-like chain molecules \> Polymeric - small molecules \> **Cationic Polymers** - positive charged polymeric \> **Anionic Polymers** - negative charged polymeric \> **Nonionic Polymers** - no charge \- Common coagulants (**aluminum sulfate, sodium aluminate, ferric sulfate, ferrous sulfate, ferric chloride, Polyaluminum chloride, cationic polymers**) \- **simplest coagulation process** (Alum placed in water then chemical reaction occurs which produces positive charged ions and it will attached to the negative charged colloid then the overall result in reduction of negative surface charge and subsequent formation of agglomerate) \- 4 factors that influence the coagulation is **pH, turbidity, temperature & alkalinity** \- **Alkalinity** - waters ability to neutralize an acid \- Metal salts must be thoroughly dispresed into the stream within 1-2 seconds for effective treatment \- Most common type of flash mixers \> **Pump Diffusion systems** - uses jets to inject coagulant into raw water stream \- produces no additional headloss \- additional electric power consumption and maintenance \> **Inline static** - No electric power and very little maintenance \- mixing efficiency varies with flow rate and headloss can be 2 ft or more \- appropriate detention times are required for completion before water is filtered or additional chemicals (**10-20 mins**) and detention occurs in piping, reaction vessels and in the head space in the filter \- **Gang mixer or jar test apparatus** is used to determine correct chemical dosage (6 mixers connected together and 6 one-liter beakers) \- Samples of the water and various dosages of coagulants are added to the jars, the jars then is stirred to duplicate the flash mix then slowly \- proper dosage is determined by observing best performing **floc** (small gelatinous masses formed liquid by coagulant + thereto) , pH and turbidity settled/ filtered - ***Disinfection*** - control pathogenic organisms - ***Flocculation*** -- gentle stirring either mechanical or hydraulic means \- slowly mixing the coagulated water to increase the probability of particle collision \- forms the floc and it is snowflake-looking material made up of colloidal particles, microorganisms, and precipitate \- occur with addition of only the primary coagulant, addition of chemicals can be added to improve the settling and filtering floc \- polymers can increase speed of floc formations, strength floc, and weight floc \- **Baffled Channel Mixers** - rely on hydraulics to provide energy, the energy varies with changes in water flow rate or temp \- **Paddle Mixers** - greatest level of operational control and speed of paddles can changed to compensate changes in water temp, turbidity, or flow rate \- Tapered energy is critical in preparing the flocculated material for efficient filtration \- Higher mixing energies form smaller denser floc = filtering \- Lower mixing energies form larger heavier floc = settling \- **15 to 45 minutes** of mixing and the time is based on the chemistry of water, temperature and mixing intensity \- Temperature is the key component in determining the amount of time required for good floc formation \- **Jar test apparatus** is used to determine the proper dosage of flocculant (anionic polymer) \- proper timing between the addition of coagulant and flocculant \- adding flocculant too early or late reduces effectiveness \- **turbidimeter** is used in determining proper dosage and turbidity \- Addition of too much polymer will result in flocculated material settling in jars - ***Clarification and Sedimentation*** - removal of solid particles from water by settling induced by gravity \- clarification of water involves removing contaminants through simple gravity sedimentation or through solids contact processes that operate in either a down-flow or up-flow configuration -3 most common types of clarifiers used in small system \> **Down-Flow Contact Clarification** - this process uses large diameter media \- media is 2-3mm in diameter and can consist of sand, anthracite \- less complicated and lest costly \> **Up-Flow Contact Process**- uses floating media or simply the sludge blanket \> **Gravity Sedimentation and sludge blanket clarification** - designed and constructed as part of a conventional packaged water treatment system. \- Contact clarifiers are more commonly custom-designed and resemble a roughing filter or prefilter in a two-stage filtration process \- **Gravity sedimentation** units incorporate tube settlers to improve removal efficiencies \- **Tube Settlers** are typically two-inch-square or oval-shaped tubes on a 7.5 to 60 deg \- In improving the sedimentation process is to use the sludge blanket as a solids contact media \- the term roughing filter is often used to describe a down-flow contact clarifier \- **Surface Loading** is the term to describe the up-flow velocity -up-flow velocity depends on the settling characteristics of the flocculated particles and the temperature of water \- surface loading rate for sedimentation basin that incorporates inclined tube settlers is expressed as gallons per minute per square foot of water surface area and usually ranges 2-3.5 \- when gravity sedimentation is used, the settled particles form sludge \- **Hydraulic Loading** - is the loading rate of the flow rate of a unit and expressed in gal per min per square foot of media/bed area and can vary from less than 1 to 8 And it is the flow rate per surface or cross sectional area \- Backwash is reversal of flow through a filter to clean the filter by removing material trapped by the media in filtration \- 25-35 gpm/ft2 is required to dislodge and remove accumulated solids \- **Clarification** is to reduce the solids loading on the down stream process (filter) and thus increase the length of the filter cycle \- performance of clarifier can be measured by the turbidity of the clarifier effluent or visual observation of the clarified water - ***Granular Media Filtration*** \- **Gravity filters** rely on the depth of water above the filter media to provide the driving force to pass water through the media as it clogs and can operate with 4-6 ft of available head \- **Pressure Filters** enclosed in pressure vessels and can operate with much higher driving forces and can operate with 10-20 ft of head -water can be treated under pressure and pumped to a water storage tank at higher elevation without the need to pump the water after filtration \- **Filter Media** consist of silica sand, greensand, anthracite coal, activated carbon \- can be used by themselves as a single media filter or mixed to provide improved filtration characteristics \> **Dual-media filter** - consist of anthracite coal and silica sand \> **Triple- media filter** -anthracite coal, silica sand and fine garnet \- **Greensand Media** used in conjunction with anthracite coal in a dual media configuration. It is to remove inorganic contaminants (manganese and iron) \- **Activated Carbon** used as a topping for silica sand and used to absorb organic contaminants \- **Direct Filtration** gravity or pressure filter that includes coagulation, flocculation, filtration, and disinfection \- Recommended hydraulic loading rates for raid sand filters ranges from **1-5 gpm/ft2** - ***Membrane Filtration* -** membrane processes commonly used in water treatment include - - - - - **- MCF** process includes using Bag Filters and Cartridge filters \- **MF and UF** are effective at removing turbidity, particles, and pathogens from water \- **NF** provides higher level of treatment than MF/UF and has added capability of removing dissolved organic contaminants\ - **RO** provides the highest level of treatment\ - **Alternative Filtration** used to meet the requirements of Surface Water Treatment Rule\ - Bag or cartridge filter are considered to be Alternative Filtration and must be approved\ by Alaska Department of Environmental Conservation\ - Required pressure can be as low as 20 psi for bag or catridge filter to more than 1000 psi for reverse osmosis\ - Backwashing Membranes requires using chemicals to extend the time between cleanings\ - Chemical cleaning is used to control membrane fouling\ - Chemical cleaning is the primary means of restoring the membranes because they cannot be effectively cleaned by backwash alone. - ***Adsorption*** - gathering a gas or dissolved substance onto the surface of a solid \- organic and inorganic contaminants can be removed from water using this\ - Adsorbents can include stationary media, such as, activated carbon, ion exchange resins, or metal oxides - Precipitate - substance in a liquid phase fall or passes out into a solid form