Water Pollution Water contaminants Water supply Water treatment

Name: Water Pollution Water contaminants Water supply Water treatment
Uploaded: 2017-06-23T18:35:02+00:00
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Description: Water Pollution Water contaminants Water supply Water treatment

Water Pollution Water contaminants Water supply Water treatment
Contents: Water contaminants Water supply Water treatment Wastewater collection Wastewater treatment Sludge treatment

Sources of Water Pollution
industrial pollution: chemicals municipal pollution: combined sewage agriculture sediment erosion petroleum products mine leaching A pollutant is a substance that is “out of place” in its environment. Pollutants include toxic substances, and also subtances that are harmless or essential to living systems at specific concentrations but that are harmful at higher concentrations. Industrial waste is the greatest single water pollution problem. New chemicals are constantly being created; their decomposition and reactivity are in many cases poorly understood. Toxicity of byproducts or products of decomposition also may not be known. Effect of heated discharges: the increase in water temperature reduces oxygen solubility, and at the same time increases metabolism of aerobic life. Increased metabolism means more growth and higher oxygen demand, but oxygen is more readily depleted because of reduced solubility, and aquatic aerobes (eg. fish) die. Municipal waste is the second major source of water pollution. Originally, municipal systems used combined sewers, which carry storm water and sanitary waste through the same pipelines. Modern municipalities have built two separate systems. Older cities with combined sewers have built treatment plants, but these only work when it doesn’t rain: stormwater causes the systems to overflow, and untreated sewage is carried to rivers, etc. Pollutants in municipal waste include organic material (nutrients) and microorganisms. Agricultural waste includes pesticide and herbicide residues and animal waste from feedlots, where animals are packed into small spaces. Sediment erosion contributes inorganic pollution, which affects fish by altering light penetration, plugging gills, interfering with spawning. Petroleum products from oil spills: acute effects on birds, fish. Effects on salmon spawning. Mine leaching. Sulphur leaching in water from mines combines with air to form acid.

Effect of Pollution on Rivers
When high energy organic material is discharged to a river, several changes can occur downstream: decomposition of organics causes drop in dissolved oxygen (DO). If DO>0 can get oxygen sag curve. If DO=0; anaerobic. get changes in biodiversity. Effect of pollution on streams: as organics are decomposed [dissolved ox] drops, some but not enough oxygen is absorbed from air - recovery depends on stream flow dissolved oxygen sag curve. calculating 10 year, 7 day low flow sludge and bubbles (ammonia and H2S) indicate anaerobic downstream from point of pollution, types and numbers of species are affected due to incr in turbidity, settled solid matter, and low dissolved O2. Diversity index as indicator of organic pollution. nitrogen compounds as indicators of organic pollution: as org nitrogen decreases, ammonia increases, then nitrates increase Attach pages to be discussed in class.

Water Supply and Treatment
hydrological cycle groundwater supplies surface water supplies water transmission Treatment Methods Coaggulation and flocculation Settling Filtration Disinfection Sufficient water supplies exist for the world, but are not evenly distributed. Therefore it is necessary to engineer supply and transmission of clean water from area to area, and clean it up when it arrives. The hydrologic cycle and water availability Hydrologic cycle = precipitation of water from clouds, and infiltration into ground or runoff into surface water, followed by evaporation and transpiration (plants) back into the atmosphere. Rates of precip. and evap./transp. help define the baseline quantity of water available for human consumption. Influenced by solar heat, ambient air temp, humidity and wind speed, and amount of soil moisture available to plants.

Source of Water - 1 Potential drinking water sources: groundwater
surface water Groundwater comes from underground aquifers, into which wells are bored to recover the water. Water in some aquifers is non-renewable. There is therefore a risk of depleting the aquifer. Also possibly for the city to sink as the water is withdrawn

Source of Water - 2 Surface water is drawn from a lake or a river.
For major rivers (e.g. Mississippi River, Rhine, Danube), the water is used by many communities along the river. Groundwater tends to be less contaminated than surface water because organic matter in the water has time to be decomposed by soil bacteria. The soil itself acts as a filter so that less suspended matter is present in groundwater than in surface water.

Contamination of Groundwater
Inorganic contaminants: The most important inorganic contaminant is nitrate ion, NO3-. Excess nitrate ion in drinking water is a potential health hazard: risk of methemoglobinemia (or “blue-baby” syndrome). Sources of nitrate in groundwater: nitrogen fertilizers atmospheric deposition human sewage deposited in septic systems

Purification of Drinking Water

Conventional water treatment
Flow diagram Conventional water treatment Sedimentation Drinking water Filtration Removal of unsettled hardness precipitate and residual floc Chemical additions Water supply Flocculation Disinfection Chlorine, to establish residual 0.3 mg/L preventing bacterial growing Removal of suspended materials and precipitation by gravity

Steps in a typical water treatment plan Coagulation (settling and precipitation) Hardness removal Disinfection

Coagulation - 1 Coagulation (or settling and precipitation)
The finest particles, such as colloidal minerals, bacteria, and pollen do not settle in the raw water. Removal of this colloidal particles is necessary: to give the finished water a clear appearance because they contain viruses and bacteria that are resistant to later disinfection.

Coagulation - 2 The capture of the fine particles is done by adding to the water either iron(III) sulfate, Fe2(SO4)3, or aluminum sulfate, Al2(SO4)3. In the case of aluminum sulfate, Al(OH)3 is formed (in the pH range 6-8): At this pH values, Al(OH)3 is close to its minimum solubility and at equilibrium very little aluminum is left dissolved in the water.

Coagulation - 3 Aluminum hydroxide forms a very gelatinous precipitate, which settles very slowly and which incorporate the colloidal particles. With iron(III) sulfate the chemistry is analogous: Fe3+ forms gelatinous iron hydroxide Fe(OH)3. These reactions consume hydroxide pH decreases (neutralize alkaline water)

Hardness Removal - 1 Hardness is characterized by the concentration of Ca2+ and Mg2+. Major problem caused by hard water: formation of mineral deposits. Calcium can be removed by addition of phosphate (see later). A more common way is by precipitation and filtering of insoluble CaCO3

Hardness Removal - 2 When the calcium is present primarily as “bicarbonate hardness” (intermediate pH), it can be removed by direct addition of Ca(OH)2 alone: When bicarbonate ion is not present at substantial levels, a source of CO3- must be provided at a high pH to prevent conversion of most of the carbonate to bicarbonate.

Hardness Removal - 3 Source of carbonate ion: sodium carbonate, Na2CO3. The precipitation of magnesium as the hydroxide requires a higher pH than the precipitation of calcium as the carbonate The high pH required may be provided by the basic carbonate ion from soda ash (Na2CO3)

Hardness Removal - 4 There are two main problems:
Supersaturation effect: Some CaCO3 and Mg(OH)2 usually remain in solution. They need to be removed. Use of highly basic sodium carbonate, which gives the product water an excessively high pH, up to pH 11. Solution: water is recarbonated by bubbling CO2 into it. The carbon dioxide converts the slightly soluble calcium carbonate and magnesium hydroxide to their soluble bicarbonate forms

Disinfection Disinfection is the most essential part of water treatment Disinfectant used in water treatment: chlorine chlorine dioxide ozone ultraviolet radiation

Disinfection/Chlorine - 1
Chlorine dissolves in water: Aqueous chlorine rapidly hydrolyzes to form hypochlorous acid: Hypochlorous acid is a weak acid that dissociates according to the reaction: Ka = 3×108 mol/ L

Speciation of active chlorine as a function of pH

Sometimes (e.g. swimming pools) hypochlorite salts, Ca(OCl)2, are substituted for chlorine gas as a disinfectant. The hypochlorite salts are safer to handle than gaseous chlorine. Sodium hypochlorite, NaOCl, can also be used as a substitute for chlorine. The hypochlorite ions is then converted to hypochlorous acid:

The two chemical species formed by chlorine in water, HOCl and OCl-, are known as free available chlorine. Free available chlorine is very effective in killing bacteria (in particular HOCl). HOCl(aq) about 10× more effective than ClO-(aq) –result of the more lipophilic HOCl crossing bacterial membranes more easily water with pH > 7.5 requires more chlorine – or longer disinfection time – than water with pH < 7.5 In the presence of ammonia chloroamines are formed. Alkaline pH will prevent the formation of these chloroamines. The chloroamines are called combined available chlorine. Breakpoint (Cl:N (wt/wt) =8:1)

important terms: chlorine dose = concentration originally used chlorine residual = concentration in the finished water chlorine demand = concentration consumed by oxidizable substances present in the water free available chlorine = sum of concentrations of HOCl(aq) and ClO-(aq) combined available chlorine = the concentration of chloroamines

Chlorine Demand Curve applied chlorine chlorine demand breakpoint
Chlorine - most commonly used disinfectant for killing bacteria in water supply systems. When chlorine is added to water, it hydrolyzes rapidly to give hypochlorus acid: Cl2 + H2O <===> H+ + Cl- + HOCl K = [H+] [Cl-] [HOCl] = 4.5 x and, [Cl2 ] HOCl <===> H+ + OCl- K = 2.7 X 10-8 From the above it can be calculated that the concentration of Cl2 is negligable at equilibrium above pH 3, when added to water at concentrations below 1.0 g/L. Hypochlorite salts such as Ca(OCl)2 are sometimes substituted for chlorine gas because they are safer to handle. HOCl and OCl- are known as free available chlorine, and are very effective antibacterial agents. They react with ammonia to produce chloroamines, collevtively known as combined available chlorine. Chlorination practice frequently provides for formation of combined available chlorine, which is a weaker disinfectant than free available chlorine, but is more readily retained in the water distribution system. Too much ammonia in water is considered undesirable because it exerts excess demand for chlorine. At sufficiently high C:N molar ratios in water containing ammonia, some HOCl and OCl- remain unreacted, and some NCl3is formed. The ratio at which this occurs is called the breakpoint. Chlorination beyond the breakpoint ensures disinfection. As well, it destroys the common odour and taste-causing materials in water. At moderate levels of ammonia-N (20 mg/L), when pH is 5.0-8, chorination with a minimum of 8:1 weight ratio Cl:N produces good denitrification, but when organic waste content is too high (eg. wastewater) production of chloroform is a problem. Refer to Droste, pp combined residual free residual

Problems with the use of chlorine as a disinfectant: Simultaneous production of some toxic chlorinated organic compounds (e.g. chlorinated phenol). Production of trihalomethanes (THMs), CHX3. Of particular concern is the formation of chloroform, CHCl3 (carcinogen, suspected to affect reproductive system) chlorination byproducts, notably trihalomethanes. Example: chloroform CHCl3 which is often present at 10ppb or more. Source is natural substances (humic acids) >C(=O)CH3 + 3HOCl —> –CO2- + CHCl3 + 2H2O

Disinfection/Chlorine Dioxide - 1
Chlorine dioxide, ClO2, is an effective water disinfectant In the absence of impurity Cl2, it does not produce THMs in water treatment. Chlorine dioxide oxidize organic molecules by extracting electrons from them. Chlorine dioxide is a gas that is violently reactive with organic matter and explosive when exposed to light.

Chlorine dioxide is generated on-site, for example by the reaction of chlorine gas with solid sodium hypochlorite: unstable, must be made in situ an oxidizing agent, not a chlorinating agent — no taste and odour problems no residual effect – rapidly decomposes – must add Cl2 afterward Some concern has been raised over possible health effects of its main degradation byproducts, ClO2- and ClO3- (chlorate) ions.

Disinfection/Ozone - 1 Ozone (stronger oxidizer than O2) is sometimes used as a disinfectant instead of chlorine, particularly in Europe. unstable, must be made in situ – by electric discharge on dry O2 (air) Process: air is filtered, cooled, dried, and pressurized, then subjected to an electrical discharge of approximately 20,000 volts. 3O2 —> 2O3 formed as a dilute mixture in air The ozone produced is then pumped into a contact chamber where water contacts the ozone for minutes.

Disinfection/Ozone - 2 ozonation equipment is expensive, only economic on a large scale an oxidizing agent, not a chlorinating agent – no taste and odour problems, but cannot be used like ClO2 as a temporary replacement for chlorine no residual in the water, decomposition is pH dependent (also faster at higher water temperature) rate  [OH-]0.55•[O3]2

Disinfection/Ozone - 2

A typical ozone water-treatment system
air ozone generator corona discharge air dried air filter water desiccator contact chamber pump 20,000 volts Ozone is sometimes used as an alternative to chlorine for disinfecting drinking water, particularly in Europe, where it was first used in More than 2000 treatment works in Europes and over 50 plants in Canada use ozone. Principle: Ozone is created by an electrical discharge in a gas containing oxygen. (Alternatively, the gas containing oxygen can be subjected to UV irradiation.) The rate of ozone formation is a function of oxygen concentration. 3O2 ---> 2O3 In a treatment system, air is filtered, cooled, dried, and pressurized, then subjected to an electrical discharge of 20,000 volts. The oxone produced is then pumped into a contact chamber where water contacts ozone for minutes. Ozone is more destructive to viruses than is chlorine. Ozone reacts with most organic matter. Byproducts with human health significance are organic peroxides, unsat. aldehydes, and epoxides. On the other hand, ozonation destroys the precursors of most halogenated byproducts such as trihalomethanes and haloacetic acids, that can be formed by subsequent chlorination. Its disinfecting power is limited by its relatively low solubility in water. The decomposition of ozone to oxygen is fast and it is impossible to maintain free ozone residuals in water. The half-life of ozone is about 20 min. See Droste, pp oxygen cooled air compressed air purified water refrigeration

Disinfection/Ozone - 3 Interest in ozonation arises from the possible production of toxic organochlorine compounds by water chlorination. ozone is more destructive to viruses than is chlorine. Unfortunately, the solubility of ozone in water is relatively low, which limits its disinfective power.

Disinfection/Ultraviolet light - 1
Ultraviolet radiation having wavelenghts below 300 nm is very damaging to life, including microorganisms by photochemical cross-linking of DNA, which absorbs strongly at this wavelength Mercury lamps (germicidal lamps) are available, having their output radiation principally at 254 nm (UV-C at 254 nm – major output of a low pressure) Advantage of UV method: Short contact time: 1-10 s. Ozone and chlorine both require contact of minutes, therefore construction of a large reaction tank. UV disinfection can be run on a simple “flow-through” system (no holding tank)

Advantage of UV method (cont’d): Low installation costs. Ozone generators are complex and expensive to install; chlorine equipment is less so. Not influenced by pH or temperature. Chlorination and ozonation work best at lower pH, chlorine because more of it is in the HOCl rather than the OCl- form, ozone because it decomposes more rapidly at high pH. applicable to large and small scale installations, even domestic use No toxic residues. water must be clear and free of absorbing solutes

Cost comparison between the various disinfectants: • small installations: UV is cheapest, then chlorine • large installations: chlorine is cheapest by a wide margin

The Treatment of Wastewater and Sewage

Pollutants in Sewage - 1 Typical municipal sewage contains oxygen-demanding materials, sediments, grease, oil, scum, pathogenic bacteria, viruses, salts, algal nutrients, pesticides, refractory organic compounds and heavy metals. Major disposal problem with sewage: the sludge produced as a product of the sewage treatment process.

Pollutants in Sewage - 2

Sewage Treatment - 1 Three main categories:
primary treatment: primary settling, mechanical treatment secondary treatment: biological treatment, include the related problem of disposal of sewage sludge tertiary treatment: include advanced treatment.

Sewage Treatment - 2

Primary Treatment - 1 Primary treatment of waste water consists of the removal of insoluble matter such as grit, grease, and scum from water. First step: screening to remove or reduce the size of trash and large solids that get into the sewage system. The solids are collected on screens and scraped off for subsequent disposal. Second step: Grit removal.

Primary Treatment - 2 In the second step the sewage enters a large lagoon and moves through slowly enough that any solid particles settle out. Some materials float at the surface of the sewage (Those materials are called grease). They are removed by a skimming device. The effluent form the primary settler is almost clear, but has a high BOD (several hundred milligrams per liter).

Secondary Treatment Objective of the secondary treatment: to reduce the BOD to acceptable level (below 100 mg/L). The basic principle consists of the action of microorganisms provided with added oxygen degrading organic material in solution or in suspension. Two main systems: Trickling filter Activated sludge

Trickling Filter The simplest biological waste treatment.
The trickling filter is a large round bed of sand and gravel. A rotating boom sprinkle sprays the wastewater over rocks or other solid support material covered with microorganisms. Main threat: presence of toxic substances, which would kill the microorganisms. Disadvantage: require large space Advantage: low energy consumption

Activated Sludge Reactor - 1
Very effective wastewater treatment. Require less land and, being enclosed, can be maintained at the optimum temperatures for biological activity. The reactor is a large tank in which the wastewater is agitated and aerated to provide the oxygen required by the microorganisms. A portion of the sludge of microorganisms is removed from the exit stream of the reactor and recycled into the influent stream.

Schematic of activated sludge reactor:

Activate Sludge Reactor - 3
BOD may be removed by: oxidation of organic matter to provide energy for the metabolic process. Synthesis, incorporation of the organic matter into cell mass.

The water content in the sludge may be removed by some drying process and the resulting dewatered sludge may be incinerated or used as landfill. To a certain extent, sewage sludge may be digested in the absence of oxygen by methane-producing anaerobic bacteria to produce methane and carbon dioxide A well-designed plant may produce enough methane to provide for all of its power needs.

Tertiary Treatment - 1 In some cases a portion of the drinking water is actually water that has been discharged from a municipal sewage treatment. Tertiary waste treatment (also called advanced waste treatment): term used to describe a variety of processes performed on the effluent from the secondary waste treatment.

Tertiary Treatment - 2 The contaminants removed by tertiary treatment fall into three general categories: suspended solids: responsible for residual biological oxygen demand in secondary sewage effluent waters. dissolved organic compounds: they are potentially the most toxic dissolved inorganic materials: the major problem: nitrates and phosphates (nutrient for algae). Also, potentially hazardous toxic metals may be found among the dissolved inorganics.

Removal of Solids Some of the colloidal particles are removed using aluminum salt which forms Al(OH)3. Similar to the process described during the purification of the drinking water.

Removal of Dissolved Organics
The standard method for removal of dissolved organic material is by adsorption on activated carbon. Activated carbon is characterized by a very large surface area. The carbon is regenerated by heating it to 950C in a steam-air atmosphere. Adsorbent synthetic polymers can also be used instead of activated carbon. They are regenerated by using solvent such as isopropanol and acetone.

Removal of Dissolved Inorganics
The effluent of secondary waste treatment generally contains mg/L more dissolved inorganic material than does the municipal water supply. The most cost-effective methods of removing inorganic material from water is currently membrane processes. Methods considered: reverse osmosis, electrodialysis, and ion exchange.

Reverse Osmosis Basic principle: force water through a semipermeable membrane that allows the passage of water but not other material.

Electrodialysis - 1 Basic principle: apply a direct current across a body of water separated into vertical layers by membranes alternately permeable to cations and anions. Cations migrate toward the cathode and anions toward the anode. Layers of water enriched in salts alternate with those from which salts have been removed.

Electrodialysis - 2

Ion Exchange Basic principle: passing the water successively over a solid cation exchanger and a solid anion exchanger, which replace cations and anions by hydrogen ion and hydroxide ion, respectively. The cation exchanger is regenerated with strong acid and the anion exchanger with strong base.

Phosphorus Removal - 1 Even when water is not destined for immediate reuse, the removal of the inorganic nutrients phosphorus and nitrogen is highly desirable to reduce eutrophication downstream. Organic phosphorus is converted to orthophosphate (H3PO4, H2PO4-,HPO42-,PO43-). The main sources of phosphate are polyphosphates in detergents, raw sewage, and the runoff from farms that used phosphate fertilizers.

Phosphorus Removal - 2 Algae may grow at PO43- (phosphate ions) levels as low as 0.05 mg/L. Growth inhibition requires levels well below 0.5 mg/L. Municipal wastes typically contain approximately 25 mg/L of phosphate (as orthophosphates, polyphosphates, and insoluble phosphate) The efficiency of phosphate removal must be quite high to prevent algal growth.

Phosphorus Removal - 3 This removal may occur in the sewage treatment process in the primary settler in the aeration chamber of the activated sludge unit. Activated sludge treatment removes about 20% of the phosphorus from sewage. after secondary waste treatment. Phosphorus is most commonly removed by precipitation, which are capable of at least 90-95% phosphorus removal at reasonable cost.

Phosphorus Removal - 4 Some chemical precipitants and their products:
Lime, Ca(OH)2, is the most commonly chemical used for phosphorus removal The products formed are insoluble calcium phosphate, such as Ca5OH(PO4)3 and Ca3(PO4)2.

Nitrogen Removal - 1 Organic nitrogen is converted to ammonium ion and nitrate Ammonia is the primary nitrogen product produced by most biological waste treatment processes. One method is to strip ammonia in the form NH3 gas from the water by air. Another method is nitrification followed by denitrification.

Nitrogen Removal - 2 Nitrification - denitrification:
First step: conversion of ammonia and organic nitrogen to nitrate under strongly aerobic conditions: These reactions occur in the aeration tank of the activated sludge plant.

Nitrogen Removal - 3 Nitrification-denitrification (cont’d)
Second step: reduction of nitrate to nitrogen gas. This reaction is also bacterially catalyzed and requires a carbon source and a reducing agent such as methanol, CH3OH: In pilot plant operation, conversions of 95% of the ammonia to nitrate and 86% of the nitrate to nitrogen have been achieved.

Waste Water Treatment Methods
aeration to remove volatile solutes precipitation of divalent cations coagulation and flocculation settling filtration disinfection Alphabetical summary of treatment options for water operations 1. Activated carbon. broad-scale adsorbant of dissolved substances. Attracts dissolved, colloidal, and particulate substances. Used to remove taste- and odour-causing compounds, and toxic organic compounds, and for dechlorination. Units can be batch or continuous, or as part of filtration unit. 2. Aeration. exposes water to air to remove volatile dissoved components that exceed their saturation concentration. eg. reduces [CO2] in groundwater. Process increases [dissolved oxygen], which increases conc. of insoluble metal oxides, which can then be removed in sedimentation basins and filtration units. 3. Air flotation. Separates suspended matter, used to thicken sludge suspension; may be used instead of sedimentation. Under pressure, air is injected into water, then released to vessel at atmospheric pressure, releasing dissolved air as small bubbles that form nuclei to buoy up suspended particulates, which are skimmed from surface. 4. Biological treatment. New technology, used to convert ammonia nitrogen to nitrate, to remove nitrogen by denitrification (as N2 gas), and to remove organics. Removal of organic matter reduces the quantity of disinfection byproducts formed from disinfection, and reduces microbial growth. Uses filters, fluidized beds, or packed beds containing activated carbon. 5. Chemical feed mixers. Used to disperse treatment additives (see Droste p.777 for list). 6. Coagulation. Colloidal particles are agglomerated or flocculated with other suspended particles to form larger more readily seetled partilces. This is done by dispersing coagulation chemical coagulant very fast so that it is not consumed in side reactions with water. 7. Coagulant recovery. Sludge generated with alum and iron coagulant salts is regenerated by addition of acid. Calcium oxide is recovered from calcium carbonate sludge by heating to drive of CO2, then converted to calcium hydroxide by adding water.

A typical municipal water treatment plant
lime coagulant Raw water first basin: insoluble Mg2+ Ca2+ precipitate second basin: flocculation aerator filter 8. Disinfection. The last treatment applied; removes or inactivates pathogens. Chorine ir its derivatives, ultraviolet irradiation, ozone. 9. Filtration. Follows sedimentation. Water moves through tanks that contain sand and other media. Fine solids that escape sedimentation basin are entrapped in filter. Some bacteria and all larger organisms such as protozoa are removed by filter. Slow sand filters use only sand, and are cleaned by scrpaing off top layer as filter clogs. Rapid filters contain various media including sand and anthracite, and have much higher loading rates than slow sand filters. They are cleaned by regular backwashing at a flow rate sufficient to expand the media. These are gravity filters. Pressure filters are also used in some smaller operations. 10. Flocculation. Hydraulic or mechanical flocculators provide gentle agitation of water that has been coagulated to promote particle contact and formation of larger particles. Precedes sedimentation and filtration. 11. Fluoridation. Added as a chemical feeder at end of treatment process; promotes dental health. When natural [fluoride] is too high, it is reduced by ion exchange using alumina. 12. Ion exchange. Used to remove hardness ions, nitrates, other inorganics. Consist of high molecular weight compounds (resins). 13. Prechlorination. First operation. High concentrations of Fe, Mn, or metals that may be toxic are oxidized to more insoluble states, then removed by coagulation, flocculation, etc. Also used to oxidize sulphides, and retard microbial growth during treatment processes. Chorine is not used here because it will form carcinogenic chloro-organics with organic matter not yet removed. Choramines or other oxidants are used instead. carbon dioxide chlorine Clean water sludge lagoon

Aeration Reasons for aeration systems: Types of aeration systems:
to reduce [dissolved CO2] to reduce [dissolved H2S] to promote oxidation of Fe and Mg to remove volatile organic compounds Types of aeration systems: gravity aerators, eg. cascade of steps spray aerators - require large land area 14. Recarbonation. After sedimentation of Ca and Mg salts, excess hydroxide is neutralized by addition of carbon dioxide. 15. Reverse Osmosis and Electrodialysis. Used to remove high concentrations of dissolved solids. Reverse osmosis forces water through a membrane by applying greater pressure than the osmotic pressure of the solution. Elecrodialysis uses a current to attract ions across membrane. These technologies are used t desalinate brackish waters, or for water softening. 16. Sedimentation. Settling tanks are used to remove solids by gravity. Sludge may be disposed of in landfills, or downstream in the water supply. Sedimentation follows coagulation and flocculation, and in some cases is also carried out before, when sediment load is high. Groundwater often needs no sedimentation. 17. Super-chlorination. After higher-than-usual chlorination, water is held in a contact basin for required contact time. Chorine is then removed with sulfite or activated carbon. Used when taste or odour-causing compounds are a problem, or when there is no filtration system, and protozoa are a risk. 18. Water stabilization. pH, alkalinity, and Ca content is adjusted so that some CaCO3 is deposited in distribution pipes to form a protective barrier against corrosion. *for more on Aeration Systems, see Droste, pp

Coagulation and Flocculation
Coagulation and flocculation. The objective of the coagulation-flocculation process is to make larger particles out of smaller particles. Larger particles are easier to remove by sedimentation or by filtration. The mechanisms of the process are as follows: Small particles, or colloids, are usually negatively charged. Since they have like charges, they are repulsed by each other and are very stable. A chemical coagulant with a positive charge and high valence (eg. aluminum sulphate (alum) as a source of trivalent cations) is mixed into the water. The coagulant acts as a destabilizer, and allows the colloids to come together to form particles (coagulation). The coagulant precipitates at neutral pH. In some cases colloidal particles are joined by polmeric bridging groups called flocculants that join the particles together. The precipitates, called flocs, act as seeds for forming larger particles, or capture particles by enmeshment; this is flocculation (see figure). Mixing the water increases the chance of contact between particles. At this stage, the mixing must be gentle because of the delicate nature of the flocs. For details on Coagulation and Flocculation, see Droste, Chapter 13. Settling Flocs formed from procedure above are separated from water in gravity settling tanks. Alum sludge is not highly biodegradable, therefore will not decompose at bottom of the tank. Accumulations are periodically removed through mud valve at bottom of tank. Filtration Soil particles naturally help to filter ground water, and this process has been applied in water treatment systemsuch as the Rapid Sand Filter (see figure in V/P p. 82). Filter has two phases: filtration and backwashing. Water from settling basins enters filter and seeps through the sand and gravel bed, through a false floor, and out into a clear well that stores the finished water. Filter beds are classified as single medium, dual media, or tri-media. The latter 2 are used to treat wastewater. For details on Filtration, see Droste, Chapter 14.

Filtration Chemical Feed Injections - alum coagulant followed by polymeric flocculent Depth Clarifier - flocs adhere to the sand as water passes through filter Depth Filter - consists of three different media The following was extracted from an advertisement on the Internet - there are all kinds of similar technologies. Multi-Tech Filtration Process (example illustrated in figure above) Alum is fed into the raw water supply to act as a base for coagulating turbidity particles. Conventional technology uses a dosage of 30 to 50 mg/L of alum so that a large floc is formed that will readily settle with adequate retention. The Multi-Tech process uses mg/L of alum which is sufficient for a pin floc formation. This is also adequate for adsorption of organic contaminants. Additionally, a cationic polymer is injected into the raw water supply to help in the charge neutralization. The long chain polymeric chemical also strengthens the floc that forms as the turbidity particles gather together. The chemically treated water flows through the clarifier tank in a downflow manner. Turbidity clings to the surface of clarifier media as it passes through the 3- 1/2 foot deep bed of coarse sand. Additional coagulation and flocculation occur as the water travels through this torturous path. Water clarity is improved 50 to 85 percent, depending upon raw water conditions. Clarified water then enters the tri-media depth filter tank and flows downward. The clarity improvement after filtration is commonly in the 98 to 99.5 percent range unless the influent turbidity is quite low; below 5 NTU. The Multi-Tech filtration process improves clarity so that surface water supplies can be treated and comply with the USEPA turbidity standardof 0.5 NTU. University studies have demonstrated that on cold water with less than 1 NTU, this process was also capable of 99 percent Giardia removal. Post-chlorination is used for disinfection purposes on these applications requiring a safe drinking water supply. When the filter effluent quality is no longer satisfactory, or the headloss through the equipment increases by 5 to 10 psi, it is time to recondition the Multi-Tech filtration system. Water is directed upflow through the filter backwash the bed. This backwash water passes into the clarifier tank in series and is then used for backwashing the clarifier. The backwash water from the filter passes through an eductor which can draw air. After a few minutes of backwashing with water only, the eductor valve is opened to allow air scouring of the clarifier bed. This high velocity air/water mixture carries out heavy turbidity. The air valve is closed to continue the upflow cycle and displace any accumulated air. The entire upflow cycles typically operates for 10 minutes. The unit then shifts to a downflow rinse-to-waste mode to settle the beds and "ripen" the media with chemically-treated water. Water passes through the clarifier and through the filter in series. When the water is of acceptable quality, the unit is returned to service. The rinse cycle can vary from minutes but is typically about 5 minutes. The entire cycle takes minutes.

Primary Treatment bar rack primary clarifier grit chamber influent
treated effluent Primary treatment Settling tank (also called sedimentation or clarifier): for as much solid matter as possible. Retention time in settling tank is long, and turbulence is low to facilitate this. Figure in text. 1. Primary clarifier: the settling tank that follows pretreatment - removes raw sludge. Before it can be disposed of, it must be stabilized to prevent further decomposition, and the water must be removed. (See wastewater treatment, next chapter). An example of characteristics of wastewater after primary treatment follows: Raw Wastewater After Primary Treatment BOD 200 mg/L 150 mg/mL SS P 8 7 waste grit waste sludge

Secondary Treatment (remove the BOD)
Secondary treatment Removes BOD. Aeration lagoons: wastewater is sprayed in air to assist aerobic microbial action. Alone, not enough to reduce BOD of sanitary sewage. Trickling filter: waste is trickled over filter bed of fist-sized rocks, microbial growth forms on rocks, fed by wastewater. Activated sludge: same as above except no rocks: Aeration tank in which microbes float freely, fed by wastewater and bubbled air. Secondary or Final Clarifier: When organic material is used up, microbes are settled out from the liquid in a settling tank. The liquid escapes over a weir. Return-activated sludge: the now-hungry microbes are recycled by pumping to the head of the aeration tank for more food. Waste-activated sludge: some of the microbes must be disposed of in order to maintain the correct population - disposal of this stuff is difficult. Design factors: mixed liquor: combo of liquid and microbes in aeration tank. mixed liquor suspended solids (MLSS) microbes in tank. food-to-microbe ratio (F/M ): these are approximated by BOD and susp. solids, respectively, in aeration tank. also known as the loading on the system, calculated as mass of BOD per day per mass of MLSS in aeration tank. See V/P pp for details. The secondary treatment plant in Seattle (shown in previous photo and in figure above), was built thirty years ago and has undergone several expansions. By the end of 1997 it will treat up to 103 million gallons of wastewater per day (mgd). The digester is being enlarged - more space is needed to reduce and stabilize increasing amounts of biosolids produced at the plant during the wastewater treatment process. Digesters subject wastewater solids to intensified bacterial action and reduce the digestion mass by 50 percent.

Tertiary Treatment Wastewater receiving tertiary treatment is unable to support microbial growth, and can be of such high quality that it can be pumped directly into the water supply. The most popular means of removing BOD is with biological treatment. Physiochemical processes are used to remove inorganic nutrients, especially phophate and nitrate. Tertiary treatment Treatments up to this point do not remove nutrients such as P, or toxic substances. Contaminants removed by tertiary treatment processes may include (1) suspended solids (2) dissolved organic compounds, (3) dissolved inorganic materials. Polishing Pond (Oxidation Pond): most popular advanced treatment method for BOD removal - aerobic pond, needs light and large surface area. Uses combination of photosynthetic algae, aerobic and anaerobic bacteria (see figure in V/P p 113). Activated Charcoal Adsorption: also removes BOD, and has advantage that inorganics as well as organics are removed. Mechanism is chemical and physical: colloidal particles are caught in crevices. Column is regenerated by heating in absence of oxygen, and adding some new carbon. Nitrogen is removed by first treating the waste thoroughly enough to produce nitrate ions. Eg. longer detention time in Secondary treatment so that bacteria eg. Nitrobacter, Nitrosomonas, convert ammonia to nitrate (nitrification). see text for equations. Nitrate is reduced by facultative and anaerobic bacteria, eg. Pseudomonas (denitrification) - requires C-source, often MeOH is used. (see V/P for equations) Phosphorous is removed chemically with lime and alum at high pH. Calcium and phosphate form an insuoluble precipitate. For a thorough description of physical-chemical tertiery treatments, see Droste pp Raw Wastewater After 1o Treatment After 2oTreat. After 3o Treat. BOD 200 mg/L SS P

Sources of Sludge Two kinds of sludge are generated in a waste treatment plant: - organic sludge from activated sludge, trickling filter, or rotating biological reactors. inorganic sludge from the additional of chemicals such as those used for phosphorous removal. Sludge treatment accounts for 50% of treatment costs in typical secondary treatment plant. Sources of sludge 1. Susp. solids that enter treatment plant and are partially removed in the primary settling tank or clarifier. About 60% of SS become raw primary sludge (96% water). 2. Waste-activated sludge: excess microbial mass removed during secondary treatment, expressed as yield: kg SS produced/kg BOD removed. 3. P removal processes produce excess solids in the form of calcium carbonate and calcium hydroxyapatites, Al hydroxides and phosphates. Oxidation ponds require periodic harvesting of algae, water plants, fish etc. Most commonly, sewage sludge is subjected to anaerobic digestion in a digester designed to allow bacterial action to occur in the absence of air. This reduces the mass and volume of sludge, and ideally results in the formation of a stabilized humus. Disease agents are also destroyed in the process. After digestion, sludge is generally conditioned and thickened to concentrate and stabilize it, and to facilitate dewatering. Sludge dewatering converts the sludge from an essentially liquid material to a damp solid containing no more than 85% water. Sewage sludge may contain undesirable constituents including biorefractory organic compounds such as PCBs; heavy metals such as cadmium, lead and mercury; and pathogenic microbes including viruses and fecal coliforms, so ultimate disposal may be a problem.

Anaerobic Sludge Digestor
gas outlet methane scum layer scum removal supernatent sludge inlet supernatent supernatent removal activated digesting sludge 1. Sludge Stabilization: reduces odour and tendency to decay; reduces pathogens. a. Aerobic digestion: waste-activated sludge is put in aeration tanks for a long time, bugs start eating each other, reduces volatile solids. Resulting aerobic sludge is more difficult to de-water than anaerobic sludge. b. Anaerobic digestion: (figure in text)requires acid formers followed by methane formers, who are strict anaerobes, and v. fussy. Treatment plants may have 2 anaerobic digesters: primary and secondary. the first is covered, heated and mixed to increase reaction rate. The second is used to store gas and concentrate the sludge by settling. Supernate is pumped back to main plant. Gas produced is used to heat primary digester. How to calculate digester volume for anaerobic digesters, other production details. See text. An anaerobic digester is not hot enough to destroy pathogens, ie, does not sterilize. stabilized sludge sludge outlet

Sludge Thickening 99% water 80% of water removed 100% 1% solid
2. Sludge Thickening: sludge is concentrated but still behaves like a liquid. a. Gravity thickener: looks like a circular settling tank. waste enters at centre, water moves to the outside, eventually leaves as clear effluent over weirs. Sludge is removed from bottom. Can remove 80% of volume as water (see figure above). b. Flotation Thickener: air is forced to dissolve in return flow under pressure. Pressure is released as return flow is mixed with sludge feed. Pressure release causes air to come out of solution, tiny bubbles attach themselves to the solids and carry them upwards to be scraped off. 3. Sludge Dewatering: Produces sludge that behaves as a solid. Usually the final volume reduction method prior to ultimate disposal (except in cases where sludge is to be burned). a. Sand Beds: most cost-effective way to dewater when land is available and labour is cheap. Sludge is poured on beds of tile drains in gravel; solids are separated from water by seepage and evaporation (3 months drying time). b. Pressure Filter: uses positive pressure to force water through a filter cloth. c. Belt Filter: both a pressure filter and gravity drainage. Sludge is introduced onto moving belt, free water drips through belt, solids are retained. Slidge is then squeezed between 2 belts. d. Centrifugation: became popular ofter organic polymers were available for sludge conditioning x g. 1% solid 5% solid

Ultimate Disposal Some alternatives for ultimate disposal of sludge have included ocean dumping, land spreading, and incineration. Since 1992, there has been legislation prohibiting ocean dumping of sewage sludge. Accumulation of heavy metals is of concern when sewage sludge is used on cropland. Prior control of heavy metal contamination from industrial sources enables sludge to be used more extensively. Incineration is an expensive alternative to sludge disposal, and there is no benefit from its potential use as a fertilizer. Ultimate disposal Incineration: strict controls on air pollution make it expensive. Disposal in deep water: used less because of unknown effects on ecology. Land disposal: landfills or land spreading - spreading sludge over land and allowing natural biodegradation. In practice, only incineration and land disposal are used at present. Land spreading - relatively new - best soils are sandy with lush vegetation, low rainfall, gentle slope. Mixed digested sludges are spread from tank trucks; activated sludges are sprated from both fixed and moving nozzles. Used in silviculture successfully; sludge is also treated and packaged as fertilizer. Transportation is expensive; therefore volume reduction by dewatering is needed. Chemical fixation: sludge solids are chemically bonded so that the mixture sets in a few days; used in industry. Toxicity: most domestic sludges do not contain enough toxins such as heavy metals to harm vegetation. Total body burden is of some concern, though. Metals can be precipitated out during sludge treatment, but most effective means is to prevent metals from entering sewage system by regulations. Pathogenic organisms persist in sludge, and present a risk when the sludge is applied to soil. Ways to reduce pathogen numbers include aerobic agitation of sludge for days, followed by air drying for at least 3 months. Anaerobic digestion or composting at degr. C are other methods. See Droste, pp for Sludge Processing and Land Application. See pp for description and comparison of treatment processes. Model for aerobic sludge digestion, pp

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