Wastewater Treatment Processes

Transcription

1 Wastewater Treatment Processes CEL212 Environmental Engineering (2 nd Semester ) Dr. Arun Kumar Department of Civil Engineering Indian Institute of Technology (Delhi) Courtesy: Dr. Irene Xagoraraki (USA) and Dr. Arvind K. Nema (India)

2 Wastewater Treatment 2

3 Wastewater Management wastewater.gif 3

4 Municipal Wastewater Treatment Systems Preliminary treatment (removes materials that can cause operational problems, equalization basins are optional) Primary treatment (remove ~60% of solids and ~35% of BOD) Secondary treatment (remove ~85% of BOD and solids) Advanced treatment (varies: 95+ % of BOD and solids, N, P) Final Treatment (disinfection) Solids Processing (sludge management) 4

5 Pre-Treatment of Industrial Wastewaters Industrial wastewaters must be pretreated prior to being discharged to municipal sewer system Pretreatment requirements set by regulatory bodies Why: remove materials that will not be treated by municipal system, remove materials that inhibit the biological processes in secondary treatment 5

6 3 stages of water treatment Primary solids are separated Secondary dissolved biological matter is converted into a solid mass by using water-borne bacteria 95% of the suspended molecules should be removed Tertiary biological solids are neutralized then disposed, and treated water may be disinfected chemically or physically 6

7 Bar racks Purpose: remove larger objects Solid material stored in hopper and sent to landfill Mechanically or manually cleaned 7

8 Grit Chambers Purpose: remove inert dense material, such as sand, broken glass, silt and pebbles Avoid abrasion of pumps and other mechanical devices Material is called grit 8

9 Equalization Basins Wastewater flow has daily fluctuations Purpose: To dampen the variation in wastewater flow into a WWTP Flow equalization is not a treatment process Improves effectiveness of primary & secondary treatment Usually achieved by large basins to collect wastewater and pumped to treatment plant at a constant rate Adequate aeration and mixing need to be provided to prevent odors and deposition of solids 9 Source: Davis and Cornwall, Introduction to Environmental Engineering, 2008

10 Treatment stages - Primary treatment typical materials that are removed during primary treatment include fats, oils, and greases (aka FOG) sand, gravels and rocks (aka grit) larger settleable solids including human waste, and floating materials 10

11 Settling/Sedimentation Solid liquid separation process in which a suspension is separated into two phases Clarified supernatant leaving the top of the sedimentation tank (overflow). Concentrated sludge leaving the bottom of the sedimentation tank (underflow). Purpose of Settling To remove coarse dispersed phase. To remove coagulated and flocculated impurities. To remove precipitated impurities after chemical treatment. To settle the sludge (biomass) after activated sludge process / tricking filters 11

12 Some basic definitions Sedimentation, also known as settling, may be defined as the removal of solid particles from a suspension by settling under gravity. Clarification is a similar term, which usually refers specifically to the function of a sedimentation tank in removing suspended matter from the water to give a clarified effluent. In a broader sense, clarification could include flotation and filtration. Thickening in sedimentation tanks is the process whereby the settled impurities are concentrated and compacted on the floor of the tank and in the sludge-collecting hoppers. Concentrated impurities withdrawn from the bottom of sedimentation tanks are called sludge, while material that floats to the top of the tank is called scum. 12

13 Primary Sedimentation Tank Remove grease, oil Fecal solid settle, floating material rise to the surface Produce a homologous liquid for later biological treatment Fecal sludge are pumped to sludge treatment plant 13

14 Primary Settling Basins 14

15 Secondary Treatment (Biological)

16 Secondary Treatment Secondary treatment is a biological treatment process that removes dissolved organic matter from wastewater. Sewage microorganisms are cultivated and added to the wastewater. The microorganisms use organic matter from sewage as their food supply. This process leads to decomposition or biodegradation of organic wastes. Two approaches are used to accomplish secondary treatment; fixed film, and suspended film systems. 16

17 Secondary Treatment Basic approach is to use aerobic biological degradation: organic carbon + O 2 microorganisms CO 2 + new cells Objective is to allow the BOD to be exerted in the treatment plant rather than in the stream 17

18 How is this accomplished? Create a very rich environment for growth of a diverse microbial community 18

19 Basic Ingredients High density of microorganisms (keep organisms in system) Good contact between organisms and wastes (provide mixing) Provide high levels of oxygen (aeration) Favorable temperature, ph, nutrients (design and operation) No toxic chemicals present (control industrial inputs) 19

20 Activated sludge mixed community of microorganisms Both aerobic and anaerobic bacteria may exist Biological floc is formed 20

21 Activated Sludge Process in which a mixture of wastewater and microorganisms is agitated and aerated Leads to oxidation of dissolved organics After oxidation, separate sludge (mostly microbial cells, water, and other contaminants) from wastewater Induce microbial growth Need food, oxygen Want Mixed Liquor Suspended Solids (MLSS) of 3,000 to 6,000 mg/l 21

22 Return Activated Sludge (RAS) Activated Sludge Process Air Mixed Liquor raw wastew ater Ai r Waste Activated Sludge (WAS) Secondary clarifier treated wastewa ter Discharge to River or Land Application 22

23 Biological Treatment Microorganism Gr ravity Soluble and colloidal organics + = For further treatment 23

24 Physical components of activated aeration tank sludge process oxygen is introduced into the system 24

25 aeration source ensure that adequate oxygen is fed into the tank provided pure oxygen or compressed air 25

26 secondary clarifiers activated-sludge solids separate from the surrounding wastewater 26

27 Activated sludge outflow line Pump activated sludge back to the aeration tank Effluent outflow line discharged effluent into bay or tertiary treatment plant 27

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29 Lagoon Systems hold the waste-water for several months natural degradation of sewage Usually reeds are preferred 29

30 Dispersed Growth vs Fixed Growth Systems Dispersed growth (suspended film) systems stir and suspend microorganisms in wastewater. As the microorganisms absorb organic matter and nutrients from the wastewater they grow in size and number. After the microorganisms have been suspended in the wastewater for several hours, they are settled out as a sludge. Some of the sludge is pumped back into the incoming wastewater to provide "seed" microorganisms. The remainder is wasted and sent on to a sludge treatment process. 30

31 Dispersed Growth vs Fixed Growth Systems Fixed film systems grow microorganisms on substrates such as rocks, sand or plastic. The wastewater is spread over the substrate, allowing the wastewater to flow past the film of microorganisms fixed to the substrate. As organic matter and nutrients are absorbed from the wastewater, the film of microorganisms grows and thickens. 31

32 Dispersed (suspended) growth vs Fixed growth Dispersed Growth (suspended organisms) Activated sludge Oxidation ditches/ponds Aerated lagoons, stabilization ponds Fixed Growth (attached organisms) Trickling filters Rotating Biological Contactors 32

33 Advanced Wastewater Treatment

34 Nutrients Removal Nitrogen removal Ammonia (NH 3 ) nitrite (NO 2- ) nitrate (NO 3- ) Phosphorous removal Precipitation with iron or aluminums salt Lead to eutrophication May cause algae bloom 34

35 Disinfection Processes (refer Notes from Water Treatment) remove disease-causing organisms from wastewater 3 different disinfection process Chlorination UV light radiation Ozonation 35

36 What can effluent use for? discharged into a stream, river, bay, lagoon or wetland used for the irrigation of a golf course, green way or park If it s sufficiently clean, it can be used for groundwater recharge 36

37 Summary 37

38 38

39 Sludge Treatment and Disposal CEL212 Environmental Engineering (2 nd Semester ) Dr. Arun Kumar Department of Civil Engineering Indian Institute of Technology (Delhi) Courtesy: Dr. Irene Xagoraraki (USA) and Dr. Arvind K. Nema (India)

40 Sludge Treatment and Disposal 40

41 Sludge Types Bar screens Primary sludge Grit chambers 3 to 8% solids About 70% organic material odor Not true sludge, not a fluid. Since it can be drained easily and is relatively stable, it can be disposed of directly in a municipal landfill. Secondary sludge Consists of wasted microorganisms and inert materials About 90% organic material WAS: 0.5 to 2% solids Trickling filter sludge: 2-5% solids Tertiary sludge If secondary clarifier is used to remove phosphate, this sludge will also contain chemical precipitates (more difficult to treat) Denitrification sludges - similar to WAS sludge 41

42 Goals of Sludge treatment Goals of treatments are: Reduce odors Remove water reduce volume Decompose organic matter 42

43 Treatment processes include: Sludge Treatment Thickening separates water from solids by gravity or flotation Stabilization converts organic solids to more refractory forms through digestion Conditioning treats sludge with chemicals or heat so that the water can be readily separated Dewatering separates water by vacuum, pressure or drying Reduction decreases the volume of sludge by incineration 43

44 Sludge Treatment: 1) Thickening Flotation Especially effective on activated sludge Increases solids content from 0.5-1% to 3-6% Primary Sludge Secondary Sludge Gravity Thickening Flotation Gravity thickening Best with primary sludge Increases solids content from 1-3% to 10% Further processing 44

45 Thickening Gravity Belt Thickener Flotation Gravity Thickener (From: 45

46 Aerobic Digestion Sludge Treatment: 2) Extension of activated sludge Accomplished by aeration of sludge then followed by sedimentation Supernatant goes back to head of plant (high in BOD, TKN, total-p) Treated sludge is 3% solids Stabilization Anaerobic Digestion 2 stage: acid fermentation followed by methane production Advantages: produce methane do not add oxygen As with aerobic digestion, supernatant goes to headworks 46

47 Stabilization Aerobic Digestion Anaerobic Digestion 47

48 Composting aerobic process requires the correct mix of carbon, nitrogen, oxygen and water with sludge Generate large amount of heat 48

49 Chemical Conditioning Add lime, ferric chloride, or alum Sludge Treatment: 3) Can also add polymers Chemicals are added just prior to dewatering stage Conditioning Heat Treatment High temperatures ( o C) High pressure (10 to 20 atmospheres) Advantages bound water is released and sludge is easily dewatered Disadvantages complex process highly concentrated liquid stream 49

50 Sludge Treatment: 4) Dewatering Sludge Drying Beds Most popular method Simple Low maintenance Effected by climate Filtration Apply vacuum to pull out water Force out water by essentially squeezing water between two moving filter belts 50

51 De-watering Sludge Drying Beds (From: Vacuum Filtration (From: 51

52 De-watering: Belt Filter Press Fairhaven, MA (From: East Lansing, MI 52

53 Sludge Treatment: 6) Volume Incineration Complete evaporation of water from sludge Requires fuel Solid material is inert Exhaust air must be treated prior to discharge Reduction Wet Oxidation Treated sludge is wet Requires energy Solid material is inert Exhaust air must be treated prior to discharge 53

54 Volume Reduction: High Temperature Fluidized Bed Incineration (From: (From: 54

55 Sludge Disposal Method depends on regulations Land Spreading lawns, gardens agricultural land forest land golf courses and other public recreational areas Municipal Solid Waste Landfill Utilization in other materials 55

56 Summary 56