Municipal Wastewater Engineering

Transcription

1 Institute of Chemical and Environmental Engineering Municipal Wastewater Engineering (7) Sludge treatment, utilisation and disposal Prof. Ján Derco, DSc. Faculty of Chemical and Food Technology, SUT, SK

2 Outline 1. What are sludge treatment processes? 2. What it does mean sludge stabilisation? 3. What processes are applied for sludge stabilisation? 4. What are differences in WWTP lines for various methods of sludge stabilisation? 5. How to minimise operational costs in sludge management? 6. What are tendencies in sludge disposal? 2

3 Sludge production sludge inevitable waste product of water and wastewater treatment unfavourable components of WW are removed and concentrated in the most important by-product by volume - sludge sludge 1 to 2 % of WW volume 50 to 80 % of WW pollution 3

4 Sludge production main goal of sludge treatment to anticipate adverse impact on human health and environment content / concentration of favourable and unfavourable compounds in sludge and related health risk - initial WW composition and the level of WW treatment 4

5 Sludge management at WWTP Sludge treatment 50 % of operational costs for WW treatment reduction of sludge volume and mass, sludge stabilisation and sanitary treatment. Processes thickening, stabilization, dewatering. 5

6 Stabilisation of sludge elimination of sensorial problems reduction / elimination of pathogens reduction of volume and mass of sludge chemical stabilisation temporary effect not reducing sludge amount biological stabilisation permanent effect 6

7 Chemical stabilisation of sludge Lime mainly as additional method after biological stabilisation hygienization improvement of dewatering acid soils increase solid content (till 30 %), better manipulation / handling enhanced metals bounding in sludge Problems smell / malodour looses of N 7

8 Chemical stabilisation of sludge ph increase local rise of T CaO + H 2 O = Ca(OH) 2 + H over 5 % (CaO / dry content of sludge) ph nad % ph 12 - more than 72 hours Lime dosing application / utilization of sludge - composting, soil, kind of soil, etc. 8

9 Chemical stabilisation of sludge Soil application of sludge - only biologically or chemically or termically treated - low fermentation activity - without healthy risks - dry content over 18 % 9

10 Chemical stabilisation of sludge heavy metals (As, Cd, Cr, Cu, Hg,Ni, Pb, Zn) microbial indicators (thermotholerant coliforms + fecal streptococs CFU / g (colony-forming unit) e.g. in Czech Republic two categories of sludge: - less than less than Salmonella = 0 in SR sufficient up to now anaerobic mezophilic (35 o C) or aerobic stabilisation or chemical stabilisation 10

11 Chemical stabilisation of sludge Directive 86/278 EEC (draft) sludge is applicable without restrictions - progressive treatment methods (including hygienization): drying at temperature over 80 o C to dry content 90% termophilic processes (55 o C) thermical treatment of wet sludge (in suspension) at 70 o C sludge condicioning with lime to ph = 12 and maintaining such treated sludge at T = 55 o C (or higher) at least for 2 hours sludge condicioning with lime to ph 12 and sludge storage for 3 months at temporary storing place 11

12 Prospective of chemical stabilisation of sludge conventional processes of sludge stabilisation - termophilic processes (55 C) - mezophilic anaerobic processes (35 C) - extended aerobic stabilisation / aeration sludge applicable only to selected agricultural soil and with limited dosing sludge condicioning with lime to ph 12 and maintaining of a such treated sludge at least 24 hours 12

13 Biological stabilisation of sludge anaerobic stabilisation: cold (psychrophilic) warmed up (mezophilic) warmed up (thermophilic) o C o C aerobic stabilisation directly in aerated tank SRT 25 days smaller WWTPs usually up to p.e. thermophilic aerobic stabilisation 13

14 Schematic of small WWTP with aerobic stabilisation of sludge Pretreatment Biological treatment Tertiary treatment I E DB S SK Sludge management 10 14

15 Description of small WWTP aerobic stabilisation I influent WW, E effluent / treated WW, DS dewatered sludge, S - sand, DB - debris, BG - biogas 1 - screens, 2 grit chamber, 3 aeration tank, 4 secondary sedimentation tank (clarifier), 5 - tertiary treatment, 6 recirculation of returned activated sludge, 7 excess activated sludge withdrawal, 8 storage tank, 9 pumping of sludge water to beginning of WWTP, 10 dosing of lime 15

16 Termophilic aerobic stabilisation of sludge C MLSS sufficient amount of released heat pasteurising conditions aeration heat looses due to evaporation and water vapor exhausting by air pure O 2 heat looses reduction process feasibility at MLSS 4 % 16

17 Anaerobic sludge digestion / stabilisation p.e. - recommended cold (psychrophilic) anaerobic digestion / stabilisation larger WWTPs - recommended warmed up mezophilic anaerobic digestion / stabilisation - the most frequently used - mezophilic stabilisation processed at 33 to 40 o C 17

18 Cold / psychrophilic anaerobic stabilisation Functions of Emscher tank: - stabilisation - thickening Emscher tank (gap tank) - akumulation attainable MLSS of stabilised sludge 3 to 4 % (30 40 g/l) SRT 150 days (anaerobic mezophilic digestion d) mixing / homogenisation 18

19 Layout of small WWTP - anaerobic stabilisation Pretreatment Mechanical treatment Biological treatment Tertiary treatment I WW E WW DB S 7 StSl I WW influent WW E WW effluent / treated WW StSl stabilized sludge S sand DB - debris 1 - screens 2 grit chamber 3 Emscher tank 4 aeration tank 5 secondary sementation tank (clarifier) 6 - tertiary treatment 7 recirculation of returned activated sludge 19

20 Layout of large WWTP anaerobic stabilisation Sludge management BG 15 ODS Primary treatment I DB S 9 Mechanical treatment E Tertiary treatment Biological treatment

21 Description of large WWTP anaerobic stabilisation I influent WW, E effluent / treated WW, DS dewatered sludge, S - sand, DB - debris, BG - biogas 1 gravel tank, 2 - screens, 3 grit chamber, 4 - primary sedimentation tank, 5 aeration tank (biofilter), 6 secondary sedimentation tank (clarifier), 7 - tertiary treatment, 8 recycle of returned activated sludge, 9 excess activated sludge withdrawal, 10 raw sludge (primary + excess activated sludge) withdrawal, 11 dewatering and storage tank, 12 anaerobic stabilisation reactor, 12 - mechanical dewatering of stabilised sludge, 14 - pumping of sludge water to beginning of WWTP, 15 - gasholder 21

22 Mezophilic anaerobic digestion / stabilisation production of biogas and methane production of heat and energy (cogeneration) - solid retention time - 11 days 40 C - 18 days 33 C operation and control requirements dosing intensity of raw sludge (minimal twice a day) sufficient mixing maintenance of optimal temperature (33 40 o C) ph in neutral region alternative energy source in winter period higher investments 22

23 Biogas m 3. kg -1 (biogas/sludge) l biogas.hab -1.d -1 about 60 % decomposition efficiency of organic portion of sludge anaerobic stabilised sludge about 50 % organic content (VSS) 23

24 Thickening of raw sludge Minimalization of energy looses increase of retention time in sludge management system mainly in anaerobic digestion / stabilisation reactor higher production of biogas gravitation thickening / tanks flotation decanting centriguges thickening sieves / screens / belt press» mechanical - mainly for excess sludge - possibly also for primary sludge concentration about 4 %, mechanically also 5-6 % (but higher organic floculants doses, i.e. 2-4 g/kg MLSS / SS / dry content) 24

25 Sludge thickening Rotaing thickener Screw press Belt press Centrifuge 25

26 Dewatering of stabilised sludge Minimazing of operational costs less sludge to transport from WWTP less transport costs sludge conditioning application mainly organic flocculants - app. 5-6 g/kg of sludge dry content Sludge bed sludge height cm days till months mechanical dewatering pressure filtration belt press centrifuging separated and closed process 26

27 Sludge dewatering Sludge beds Centrifuge Belt press Dewatered sludge 27

28 Sludge policy of EU stifle sludge deposits sludge dumping unsustainable sludge deposition in sea - stopped from the end of 1998 minimise sludge production and increase sludge recycling sludge production can not be avoid it can be decreased proper method and technology. Introduction to AWWT 28

29 Sludge policy of EU more stringent effluent standards increase sludge production by 40 to 60 % possibilities to reduce recycling soil organic fertilizer, improvement of soil quality, recultivation destructive methods incineration with or without energy utilisation gasification utilisation as process fuel produced ash utilised or dumped 29

30 Sewage sludge disposal The crucial point not the real risk decides about the public opinion, but the risk perception. as long as sludge is a waste and waste is linked with negative aspects in public opinion difficult to convince people that agricultural use of sludge is a sustainable way of disposal. BIOSOLIDS 30

31 Sewage sludge disposal 90% of phosphorus found in private households ends up in the sewage, only 9% in the solid waste. the same applies to nitrogen - 89% ends up in WW in the context of regional material fluxes mass flow of nutrients (N, P) in sewage sludge is relatively small in comparison to the losses of nutrients from agriculture. 31

32 Sewage sludge disposal In the context of regional material fluxes - sewage sludge has a low resource and pollution potential despite that it can be characterised as a sink concentrating also the hazardous waste water compounds. Health risk linked with the agricultural utilisation of sewage sludge. Sludge analysis - important monitoring tool for source control even if sludge is not used in agriculture or on soils. 32

33 Sewage sludge disposal 90% of phosphorus found in private households ends up in the sewage only 9% in the solid waste. the same applies to nitrogen - 89% ends up in WW in the context of regional material fluxes mass flow of nutrients (N, P) in sewage sludge is relatively small in comparison to the losses of nutrients from agriculture. 33

34 Sewage sludge disposal Nitrogen contained in sewage sludge low as compared to the N flow in the WW very low as compared to the losses of nitrogen from agriculture. Nitrogen is not a limited resource. Nitrogen contained in the sludge - important if sludge is put to landfill long term ammonia leaching. 34

35 Sewage sludge disposal Organic matter of the sludge very low as compared to the organic material flows in agriculture. The value of the organic material in the sludge can only play a local role mainly in hot arid climates. 35

36 Sewage sludge disposal The most valuable element in sludge is phosphorus availability of phosphorus minerals for production of low cost market fertiliser is limited. The recovery of P from the sludge - topic of international research. P recovery from the sludge is a technological (organic matrix, iron) and an economic problem. 36

37 Sewage sludge disposal Up to 90% of phosphorus of WW can be recovered in the sludge. The easiest way of P-recycling is agricultural sludge application. 37

38 Sewage sludge disposal Every sludge disposal option - has to be reliable for the treatment plant operator at any time sludge is a necessary by-product of fulfilling the legal requirements for WW treatment. Practical experience with sludge disposal - different in European countries (A, S, DK, NL, G, F, CH) For large treatment plants in agglomerations (e.g. > 1 Mio. PE) 38

39 Sewage sludge disposal raw sludge incineration with ash disposal can be an economic and ecologically sound solution. Development of low cost small incineration plants (for digested sludge) meeting advanced off gas standards. Co-incineration of digested sludge with cement and solid waste can result as economically sound solutions. regard to P-recovery it does not represent a sustainable solution. 39

40 Sewage sludge disposal Sludge disposal of dewatered sludge in landfills should be avoided it results in long term monitoring requirements especially for ammonia leaching. Small treatment plants (e.g. < PE) agricultural use of sewage sludge seems to be the most economic and sustainable solution as long as source abatement of potentially hazardous substances is successful. 40

41 Biosolids 41

42 Biosolids 42

43 Energetic efficiency of sludge treatment processes 1. Wet oxidation in supercritical water - supercritical water gasification - SCWG - the highest energy utilisation T cr = 374 ºC 2. Pyrolysis 500 to 600 ºC, 25 MPa 3. Anaerobic stabilisation 4. Direct combustion p cr = 22 MPa 43

44 Supercritical water Supercritical water - a supercritical fluid above T cr = 374 ºC P cr = 22 MPa the ability to dissolve materials not normally soluble in liquid water or steam and to promote some types of chemical reactions. These properties - make supercritical water a very promising reaction medium for the conversion of biomass to value-added products. 44

45 Supercritical water oxidation organic compounds - including lignocellulosic material such as solid biomass - readily dissolve in supercritical water, once dissolved - supercritical water efficiently breaks cellulose bonds, the reactions - generally are not selective - resulting in the rapid formation of gaseous products biomass gasification 45

46 Supercritical water oxidation organic compounds - including lignocellulosic material such as solid biomass - readily dissolve in supercritical water, once dissolved - supercritical water efficiently breaks cellulose bonds, the reactions - generally are not selective - resulting in the rapid formation of gaseous products biomass gasification 46

47 Supercritical water oxidation biomass gasification producing hydrocarbon fuels for use in an efficient combustion device or producing hydrogen for use in a fuel cell. hydrogen yield can be much higher than the hydrogen content of the biomass,» due to steam reforming where water is a hydrogen-providing participant in the overall reaction. 47

48 SCWG process chemistry with temperature increasing to 600 C - water becomes a strong oxidant complete disintegration of the substrate structure by transfer of oxygen from water to the carbon atoms of the substrate. as a result of the high density - carbon is preferentially oxidized into CO 2 48

49 SCWG process chemistry also low concentrations of CO are formed hydrogen atoms of water and of the substrate form H 2 typical overall reaction for glucose: 2 C 6 H 12 O H 2 O => 9 CO CH 4 + CO + 15 H 2 49

50 SCWG process description SCWG process consists of a number of unit operation feed pumping, heat exchanging, reactor, gas-liquid separators and if desired product upgrading. reactor operating temperature is typically between 600 and 650 o C operating pressure is around 30 Mpa residence time of up to 2 minutes is required to achieve complete carbon conversion (depending on the feedstock) 50

51 SCWG process description heat exchange between the inlet and outlet streams from the reactor essential for the process to achieve high thermal efficiency. two-phase product stream is separated in a highpressure gas-liquid separator (T = o C) significant part of the CO 2 remains dissolved in the water phase. 51

52 SCWG process description possible contaminants like H 2 S, NH 3 and HCl captured in the water phase due to their higher solubility in-situ gas cleaning is processed. gas from the high pressure separator contains mainly the H 2, CO and CH 4 and part of the CO 2. 52

53 SCWG process description in a low pressure separator second gas stream is produced containing relative large amounts of CO 2, but also some combustibles. can be used for instance for process heating purposes. 53