Presentation on theme: "Sewage Discharges and Oxygen Depletion in Natural Waters"— Presentation transcript:
1 Sewage Discharges and Oxygen Depletion in Natural Waters -- find out how many die each year because of water pollution (largely pathogens)
2 Outline of Topics Introduction Oxygen Depletion Sewage Treatment overview of problems due to human and animal waste pollutioncombined sewer overflowspathogensOxygen Depletionsaturated DOoxygen demand (BOD, COD)sources of BOD wasteoxygen sag curvesSewage Treatmentprimary and secondary treatmenttertiary treatmentdisinfectionsewage sludge
3 Pollution due to Human & Animal Waste Sources?Municipal wastewater (ie, sewage)Especially from combined sewer overflows (CSOs)Treated sewage discharge from POTW (publicly owned treatment works)Septic tanksSubsurface disposal/treatment system for each homeWhen sewer lines are not available (eg, sparsely populated areas)Consists of a buried septic tank and either a leaching field or seepage pitFrom EB: “Solids are decomposed by the anaerobic bacterial action of the sludge. After several years the accumulation of sludge interferes with the action, and a scavenger unit must pump the sludge out of the tank for disposal elsewhere.”Livestock wasteAny agricultural operation that includes livestockAnimal feeding operation (AFOs)Confined livestockNo cropsLarge AFOs are a particular concernConcentrated animal feedlot operations (CAFOs) are defined by the number of animals in the AFOAs of 2003, regulated as point sources under the NPDES portion of the Clean Water Actdefinition (from Encyclopedia Brittanica): a septic tank is a “sewage-disposal unit used principally for single residences not connected to sewer lines. It consists ordinarily of either a single- or double-compartment concrete or steel tank buried in the ground, so arranged that settled sludge is in contact with the waste water as it flows through the tank and thence to a tile drain field. Solids are decomposed by the anaerobic bacterial action of the sludge. After several years the accumulation of sludge interferes with the action, and a scavenger unit must pump the sludge out of the tank for disposal elsewhere.”more from Encyclopedia Brittanica:In sparsely populated suburban or rural areas, it is usually not economical to build sewage collection systems and a centrally located treatment plant. Instead, a separate subsurface disposal system is provided for each home. For subsurface disposal to succeed, the permeability, or hydraulic conductivity, of the soil must be within an acceptable range. The capacity of the soil to absorb settled wastewater is determined by a “percolation test.”A subsurface disposal system consists of a buried septic tank and either a leaching field or seepage pits. A septic tank serves as a settling tank and sludge storage chamber. Although the sludge decomposes anaerobically, it eventually accumulates and must be pumped out periodically. Floating solids and grease are trapped by a baffle at the tank outlet, and settled sewage flows out into the leaching field or seepage pits. A leaching field includes several perforated pipelines placed in shallow trenches. The pipes distribute the effluent over a sizable area as it seeps into the soil. If the site is too small for a conventional leaching field, deeper seepage pits may be used instead of shallow trenches. Both leaching fields and seepage pits must be placed above seasonally high groundwater levels.LAS structure: p-substituted benzene ring; on one side is a straight-chain alkane with Cs; on the other is the anionic sulfonate group, SO3-. The LAS cpds are usually isomers, differing in their point of attachment to the interior carbon on the alkane. See Rand for more details. All LAS cpds are moderately soluble in both water and organic solvents.LAS compounds are the major anionic surfactant used in detergentsDBPs: disinfection by-products. Most prevalent are the trihalomethanes (Cl- and Br- substituents) formed during chlorination of water.PPCPs: pharmaceuticals and personal-care products
4 The Nature of Sewage Discharges Lecture QuestionWhat pollutants are present in raw sewage discharges? What are the resulting effects on ecosystem and human health?PathogensColiform count is usually 105 – 106 /mL in raw sewageCause various waster-borne diseases (see later slide). The most serious water pollution problem in developing countries.Degradable Organic Pollution“High BOD waste.” BOD of raw sewage: mg/L (typically 200 mg/L)Causes oxygen depletionNutrientsNitrogen, phosphorus in inorganic and organic formsAssociated with eutrophication and related problems.Suspended SolidsIncreases turbidity and siltation in receiving water bodyToxic ChemicalsToxic organicsDisinfection byproducts (DBPs) in treated sewagePharmaceuticals and personal-care products (PPCPs)Surfactants (especially linear alkyl sulfonates, LASs)Anything else poured down the drain (toxic metals, etc)Sometimes combined with industrial wasteStormwater runoff (in combined systems)
5 Combined Sewer Overflows Case Study: Seattle1212 overflow events in 2001Over 500 million gallons storm water + sewage flowed into Puget Sound
6 Typical Pathogens in Human Waste WHO: pathogens in water from human waste kill 3.4 million people every yearMany of these are children under 5 years
7 Saturated Oxygen Levels in Fresh Water Like all gases, O2 is less soluble as temperature increasesSaturated level of water exposed to the atmosphere is typically 8-14 mg/L
8 Effects of Oxygen Depletion DO level (mg/L)Qualitative effect6 – 15OK4 – 6Stressed2 – 4Choking1 –2Dying0 – 1DeadEffects of low DO on ecosystem communities and populationsHow much DO is enough?Rapid decrease in DO can cause massive fish killsSo-called dead zones form if DO level falls enoughEffects of low DO on chemical compositionConverts chemicals to their to reduced stateMethane (CH4), hydrogen sulfide (H2S), and ammonia (NH3) instead of carbon dioxide (CO2), sulfate (SO42-) or nitrate (NO3-)Reduced forms of metals frequently more solubleMetals can become more mobileIncreases exposure of humans and animals to toxic metals
9 BOD Waste Water Question What is “high-BOD” waste water? What are the major sources of high-BOD waste water?BOD = biochemical (or biological) oxygen demandWhen organic material is decomposed (mostly microbial aerobic respiration) it “demands” oxygenOxygen demand represents a potential loss of DO in a water bodyImportant factor: relative rates of oxygen loss and replenishmentIf the rate of oxygen loss due to decomposition exceeds the rate of oxygen replenishment (eg due to dissolution of gaseous O2), then the DO level fallsOxygen demand can be quantified by measurement of BOD, COD (or TOC)BOD measurement: (i) collect a sample of known volume; (ii) measure the DO level; (iii) seal and incubate at constant temperature for 5 or 7 days; (iv) measure the DO level again.The BOD is the difference between the two DO measurements.BOD is determined by the amount of degradable material present in the water. Usually, it is mostly due to organic material (“CBOD”) but can also be due to other chemicals in their reduced state (eg, ammonium).
11 Oxygen-Sag Curves Question What is an oxygen sag curve? An oxygen sag curve is the dip in dissolved oxygen observed when BOD waste water is discharged continuously into a river.The extent of the sag is determined by BOD level in the wastewater stream, by the rate of discharge, and by other factors such as temperature and river characteristics (flow rate, turbulence, etc).An oxygen sag curve is also observed due to a one-time discharge of BOD waste into a lakeIn that case, the DO drop is with time instead of distance downriver.Continuous discharge of BOD waste into a lake results in a decrease in steady-state DO level (not a “sag” followed by a recovery).
13 More Oxygen Sag Curves Effect of BOD Level: sag becomes more severe longer distance (or time) at unhealthy DO levelsEffect of temperature:sag deepens and shortensmay cause a portion of river to have unhealthy DO levels
14 Dynamics of Oxygen Depletion & Dissolution Figure on left shows a model framework to calculate:DO as a function of distance downstream from a point source discharge; orDO as a function of time after a single “spike” discharge of BOD wastewaterDO falls when decomposition rate > dissolution rate and DO rises when decomposition rate < dissolution rateRate of decomposition (deoxygenation)Linearly proportional to BOD levelBOD falls exponentially with timeRate of oxygen dissolution (reaeration)Linearly proportional to the oxygen deficit: DOsat - DOactual
15 Sewage Treatment Lecture Question How does the Clean Water Act regulate sewage discharges?CWA passed in 1972Actually, they were comprehensive amendments to an existing statuteregulates sewage discharges (among other things) as point sources of pollutionA major problem at the timeRequires all Publicly-Owned Treatment Works (POTWs, ie “sewage treatment plants”) to obtain discharge permitsUnder the National Pollutant Discharge Elimination System (NPDES)Requires that all POTWs meet a minimum of secondary treatment level of sewageA big help in reducing BOD, butProblems remainBOD still a little highNutrient levels not reduced very much with only secondary treatment levelsAnd neither are toxics (metals, organics)
16 Sewage Treatment Plants (POTWs) Lecture QuestionDescribe in some detail the processes in the primary and secondary treatment of sewage.Primary treatment: physical separation for removal of bulky solids and oil/greaseSecondary treatment: bioreactor primarily intended to reduce BOD
19 Effects of Primary and Secondary Treatment Levels
20 Advanced (Tertiary) Treatment Lecture QuestionWhat are the methods used in advanced (tertiary) sewage treatment? How do they help safeguard water quality?Advanced treatment consists largely of chemical treatment methods designed to do a number of things:Remove nutrient pollutionPhosphate removed by treatment with lime, Ca(OH)2Ammonia removed by basification followed by sparging (accelerated outgassing)Further reduce BODCoprecipitation, activated charcoal, further decompositionRemove toxic organicsActivated charcoal filterRemove toxic metalsIon exchange resin filter
22 Disinfection Lecture Question What are the main methods of disinfection used in sewage treatment?ChlorinationApplied either as chlorine gas or as a hypochlorite (OCl-) saltpH control is importantAdvantagesCheapResidual disinfectionDisadvantagesMany disinfection byproducts (DBPs): THMs, HAAs, chloraminesAlternativesOzonationuv lightWith alternative treatments, a lesser amounts of chlorine often also used for residual disinfection (or as a backup if coliform counts get too high).
25 Sewage Sludge (Biosolids) Lecture QuestionWhat is sewage sludge (‘biosolids’) and what is done with it?Wastewater treatment generates large quantities of solid wasteCollectively this is called “sludge” or, more euphemistically, “biosolids”Contains all solid material removed from the waste stream, includingHuman waste, microorganisms, and toxic chemicalsVolume dwarfs that of municipal solid waste (ie, “trash”)Sludge is very wateryLooks essentially like muddy water in original formOnly 1-10% solidUsually ‘dewatered’ at the treatment plantTexture of a wet sponge11-40% solid at this point
26 Disposal of Sewage Sludge Eventual Fate?Land application/recycling (40-50%)67% of that used as fertilizer on cropsMust be treated to remove pathogensContinued uncertainty over health effects due to pathogens and pollutants in the sludge12% of that to publicGiven or sold9% of that applied to damaged landsUsually to revitalize closed mines3% of that sprayed onto forestsSlope cannot exceed 10-20%Sanitary landfill (50%)DirectIncinerationResulting ash is landfilled