Normal Secondary Reactor & Clarifier + disinfection constitutes a "secondary treatment plant".

Transcription

1 Secondary Treatment! secondary treatment: < removal of BOD < some removal of ammonia! method: < biological reactors/clarifiers (harnessing of micro-organisms)! devices: < suspended growth " ponds " lagoons " activated sludge < attached growth " trickling filters " rotating biological contactors! Removal efficiencies (depends on specific system): < 80-95% BOD 5 removal common! resulting problems: < secondary sludge < odors < TAC emissions < potentially sensitive system < aeration requirements < cost NOTE: Normal Secondary Reactor & Clarifier + disinfection constitutes a "secondary treatment plant". Wastewater Microbiology:! Microorganisms of interest in WW treatment: < bacteria " most important group, but none is selected as best " essential in nutrient cycle of ecosystem " use soluble food, ideal for WW Pseudomonas - degrade organics and denitrification Methanococcus - produce methane gas in digester Nitrobactor & Nitrosomonas - convert nitrite to nitrate Sphaerotilus - (filamentous) sludge bulking < fungi " useful in biological treatment, wspecially low ph " when nitrogen deficient, can predominate over bacteria " play an important role in breakdown of organic matter wastewater_ii.wpd Page 1 of 11

2 < algae " beneficial in oxidation ponds as they produce O 2 < protozoa " most are aerobic chemothrophs that consume bacteria " desirable in wastewater effluents as they polish the bacteria left behind < rotifers/crustaceans " consume organic matter and bacteria! classification of microorganisms by carbon source: < heterotrophic (organic material as carbon source) < autotrophic (CO 2 as carbon source)! classification of microorganisms by energy source: < phototrophic (use light energy) < chemotrophic (chemical energy - organic or inorganic) photoautotrophs light CO2 photohetrotrophs light organic matter chemautotrophs inorganic matter CO2 chemoheterotrophs organic matter organic matter! oxygen also important < aerobic or anaerobic < facultative anaerobes can grow in presence or absence of O 2! temperature also important < Psychrophilic 10 to 30 optimium 12 to 18 C < Mesophyllic 20 to 75 optimum 20 to 50 C < Thermophilic 35 to 75 optimum 55 to 65 C Monod Equation Review Section 17.3 in text wastewater_ii.wpd Page 2 of 11

3 ! classification of microorganisms by relationship to oxygen*: * ability to use D.O. as terminal e - acceptor < obligate aerobes (in aerobic ww) - use O 2 as terminal e - acceptor < obligate anaerobes (in anaerobic ww) - cannot survive without O 2 - need other e - acceptor (usually SO 2-4 or CO 2) < facultative anaerobes - can use O 2, but not necessary < anoxic Denitrifiers - - v - use NO 2 and NO 3! types of decomposition in wastewater: < aerobic decomposition: " requires O 2 for degradation of substrate (catabolism) " end products = CO 2 + H 2 O + new cells (stable!) " diverse spectrum of substrates " fast production of new cells = rapid substrate use " a lot of sludge (wasted biomass) produced " good for BODs < mg/l " problem of BODs > mg/l (O 2 requirements) < anaerobic decomposition " - O 2 and NO 3 cannot be present " SO 2-4, CO 2,... as terminal e - acceptors " 2- reduce SO 4 v H 2 S = odors! " two step process: produce low mw fatty (volatile ) acids degrade to CH4 (a) ph problems. " products = CO 2 + CH 4 +H 2 O + new cells + H - s +... " slow production of cells = slow substrate use " minor sludge production " higher temp. requirements < anoxic Decomposition: " NO 3 - used as terminal e - acceptor (denitrification) " end products = CO 2 + H 2 O + new calls + N 2 (g) " relatively fast production of cells = rapid substrate use " good for ammonia removal wastewater_ii.wpd Page 3 of 11

4 Activated Sludge Process (ASP)! major features of ASP system: < biological reactor < aeration system < biomass clarifier < biomass recycle! additional notes: < entire system = ASP (complicated) < X depends on S S depends on X < X & thus S depends on O 2 < reactor depends on clarifier clarifier depends on reactor! oxygen requirements & input systems < oxygen utilization rate by ASP microbes = mg (L - hr) " natural reaeration will not cut it " need forced O 2 input air (20.9% O 2 ) pure oxygen (99% + O 2 ) < types of oxygen input: " diffused bubble aeration: coarse bubble fine bubble air vs. oxygen vs. foul air stripping of constituents in ww " Mechanical aerators: (agitation of surface) surface impeller dual surface/submerged (impeller - spurger) surface brush! Some ASP configurations: < conventional ASP: " CFSTR " PFR " uniform aeration < tapered aeration " for PFR type " more air at front " diff. spacing of diffuses < step feed input: " equalize F/M " lowers peak O 2 demand " lessons problems with filamentals bulking " flexibility & controls wastewater_ii.wpd Page 4 of 11

5 < oxidation ditch: < equencing batch reactors (SBRs) < pure-oxygen reactors: " same as above but with pure O 2! biomass concentration in reactor: < true biomass concentration difficult to measure < use TSS or VSS as measures of cell concentration (X) < wastewater + microbial culture = mixed - liquor < mixed - liquor suspended solids (MLSS) < mixed - liquor volatile suspended solids (MLœSS)! apply mass balance over entire ASP System, i.e., BES! final notes on activated sludge: < Not cheap! " aeration costs " WAS generation, treatment and disposal < aeration plugging (rare, but not unheard of) < importance of secondary settling " " and decoupling from analysis < bulking and foaming < requirement of operational knowledge < most important biological treatment system " most applied " most research on < biodegradation of specific contaminants (e.g., toxics) " primary utilization " secondary utilization " acclimation CSTR See Section 17.4 in course text or class handout Plug Flow Reactor See Section 17.7 in course text or class handout wastewater_ii.wpd Page 5 of 11

6 Settling of Wastewater! Type 1: Grit Chamber and Primary Clarification (no chemical additives)! Type 3: settling of activated sludge! Type 4: thickening of sludge Settling in a Secondary Clarifier (Reynolds and Richards, 1995) Settling of Activated Sludge (Reynolds and Richards, 1995) Procedure For Type III Suspension being tested is placed in a graduated cylinder with height H o, having a known concentration C o. The position of the interfaces changes with time and is plotted as below: where t u = time to reach desired underflow concentration, determined by calculating and reading off graph c u = desired underflow concentration c 2 = midpoint between hindered and compression settling wastewater_ii.wpd Page 6 of 11

7 Then compare area of thickening to area for clarification. Larger of the two applies. Area for thickening Area for clarification Find slope of tangent starting from H o, gives interfacial subsidence velocity v s. Also need clarification rate, Q c, which is based on height of water column above H u, or Typical Parameters! sludge loading: kg/m 2 /d! overflow rate: m/d! depth: 3-5 m! detention time: 1.5 to 2 h Advanced Wastewater Treatment (AWT)! AWT = "tertiary" treatment < for water reclamation purposes or < discharge to sensitive ecosystems < common for small rivers, lakes < rare for ocean disposal! Goals may be: < Removal of nutrients (ammonia and phosphorus) < Removal of biological solids (and residual BOD) < Removal of metals and COD (refractory organics) < Removal of toxins wastewater_ii.wpd Page 7 of 11

8 ! Nutrient Removal (Phosphorus): < Chemical addition: Alum = Al(SO 4 ) 3 Al (SO 4 ) PO 4 3- è 2 Al PO SO 4 2- settles " employ just prior to primary or 2nd clarifiers < Biological Removal: gave outline! Nutrient Removal (Ammonia): (_ for O 2 demand, toxic and algal growth) < Physical: NH 4+ W NH 3 + H + " lower ƒh + + = increase ph > 11, get NH 4 6 NH 3 " pass through stripping toner (Q g /Q 1 = ) " Q g requirements = cost " Not good in cold weather " Recall: < Chemical: (primarily as R x N with chlorine) HOCl + NH 4+ NH 2 Cl + H 2 O + H + HOCl + 2 NH 2 Cl N HCl + H 2 O Requires 9 mg/l chlorine / mg/l of NH 3 At 30 mg/l NH 3, must use mg/l (too high!) (wow!) < Biological (preferred, if possible - particularly in cold climates). Step 1: NH 4 + NO 2 - NO 3 - (nitrification) " nitrosomonas & nitrobacter. " need O 2 " need low organic matter " recall nitrogenous $OD = 4 $OD Step 2: NO 3 - N 2 (dentrification) " need low O 2 " need organic matter prim. effluent upfeed (lower quality of effluent) methanol etc. injection wastewater_ii.wpd Page 8 of 11

9 Two-step process described above = nitrification/denitrification. Sometimes just get step 1: " Usually get some nitrification in PFR ASP, TF, RBCs. " Get more with TF in series or more RCBs in series! Filtration < Solids polishing (get TSS < 1-2 mg/l) " unsettled bacteria " incidental $OD < Single-media versus multi-media filters! Carbon Adsorption: < Removal of refractory COD & metals " toxic organics (pesticides, solvents, etc.) < High internal surface area: activated carbon < Adsorb to walls < Collect carbon particles " thermally regenerate on-site or off-site < Granular Activated Carbon (GAC) - like filtration system " fixed < Powdered Activated Carbon (PAC) " distributed in ASP " adsorption with degradation? Final Effluent Disposal! Rivers/streams: < bank or pipe diffuser? < size of river < uses of river < ecology or river < top three affect requirements for BOD 5, TSS, NH 3, phosphorous etc.! Lakes: < effluent pipeline < Lake Ontario, mostly 2' treatment + (advanced for NH 3 and phosphorous)! Ocean Disposal: < pipeline (length = ƒn (current, beaches) < generally less treatment wastewater_ii.wpd Page 9 of 11

10 ! Evaporation/Percolation Ponds: < warm areas/arid < a lot of land < groundwater! Reuse: < agricultural (human vs. animal crops) < golf courses < highway/parks < recreational: lakes & marshes < industrial < drinking water? Sludge Treatment & Disposal! Potentially most complex and costly part of WWTF! Major Processes Involved With Sludge Treatment: < Thickening (removing water) " flotation " gravity thickening < Stabilization (reducing BOD; i.e., reduces mass of sludge) " aerobic digestion " anaerobic digestion < Conditioning (treating with chemical or heat; i.e.,improves ability to remove water) " chemical coagulants " heat < Dewatering (separation of water) " sludge drying beds " vacuum filtration " belt filter presses < Reduction (decrease solid volume) " incineration! Sludge Disposal: < Incineration " residue " by-product < Ocean discharge " now banned in U.S. < Landfilling wastewater_ii.wpd Page 10 of 11

11 < Land spreading " nutrients recovery < Utilization " composting " co-firing with MSW " formation of activated carbon " oil - from - sludge wastewater_ii.wpd Page 11 of 11