Sludge thermal oxidation processes: mineral , energy impact, and greenhouse effect gases release.

Eric Guibelin. V. V.w. , France

Abstract. Different treatment routes have been studied for a mixed sludge: the conventional agricultural use is compared with the thermal oxidation processes, including (in gaseous phase) and wet air oxidation (in liquid phase). The interest of a sludge digestion prior to the final treatment has been also considered accrding to the two major criteria, which are the fossil energy utiisation and the greenhouse effect gases (C02, CH4, N20) release. Thermal energy has to be recovered on thermal processes to make these processes environmentally friendly, otherwise their main interest is to extrct or destroy micropollutants and pathogens frm the carbon cycle. In case of continuous , incineration can produce more energy than it consumes. Digestion is especially interesting for agriculture: accrding to these two schemes, the energy final balance can also be in exces. As to wet air oxidation , it is probably one of the best way to minimize greenhouse effect gases emission. Key words: digestion, grenhouse effec gases, incineration, land spreading, recycling, wet air oxidation.

Introduction New regulations or recommendations encourage WWP manufactuers to consider processes which not only elimiate the nuisance caused by but also recover energy and/or recycle the material. Another major concern is the global warng. Many goverents have already recommended to tae into account the Greenhouse Effect Gases (GEG), essentially CO , ClL and N emitted durng the life cycle of the system.

Energy and material recycling Nowadays, land application remains the main final disposal route in many countres but it can be called "a way to get rid of sludge" anymore, but "a way to recycle safely nutrents , especially N andP. Landfilling is not yet considered as a sustainable disposal route in spite of improvements in the way to built and to operate new landfills with a safe containment and a treatment of the liquid and gaseous effuent: up to 90% of the biogas could be indeed recovered in modem landfills considered as "long residence time bioreactors" (Diot-Morcet M., Aran C, & al. 2002). In the same approach, the thermal oxidation route, mainly sludge incineration, must prove that there is a way to recycle some compounds. For instance, the Swedish regulation compels to recover nutrents such as phosphorus in case of incineration (Hultman B. & aI., 1997), but other minerals from the remaining ash can be reused for different puroses such as road constrction or brick manufactue (Tay J-H. Show K-Y; 1997). But a major interest of the thermal oxidation routes is the ability to produce energy, due to the sludge energy potential: A rough estimate of the sludge production (depending mainly on the wastewater treatment trin) gives indeed a value in the range of 40 to 80 g/ of dr solid per day and per capita, tyically containing 2/3 of organics or more with an initial concentration close to 1 % DS in the raw sludge. Therefore, the energy potential on a dr fuel basis can achieve 15 W / capita. In spite of the energy required to remove the mainly by sedimentation and fitration, mainly electrcity, a huge amount of energy is thus recoverable through thermal oxidation processes. Anaerobic sludge digestion, that include heat and electrcity generation using a gas engine is probably the best way to recover energy because it produces a storable fuel with a high calorific value (6.5 kWhm ). However, sludge is far to be completely mineralised since an important par of the organic matter (slowly biodegradble) remains. Therefore digestion alone canot be contemplated as a final disposal route if land application is not possible.

317 Greenhouse effct gases emission Whatever the sludge treatment considered, GEG are produced due to the fossil energy required to produce the electrcity and chemicals or consumed for trnsporttion. Conversely, processes producing energy or substituting natual sludge fertilsers to chemical fertilisers diminish the final impact. The 3 gases considered are CO , ClL and N20. Due to different absorption capacities, each gas is converted in C02 equivalents - 100 years according to the rule proposed by the Intergovernmental Panel on Climate Change (2002): 1 ClL = 23 * CO and 1 N 0 = 296 * CO

Thermal oxidation processes These processes consist in a mineralisation of the sludge using at high temperatue (;:850 C) in gaseous phase (Incineration) or at "low" temperatue (230-350 C) in liquid phase (Wet Air Oxidation WAO). Consequently, these thermal processes are considered as final disposal routes since the organic matter is volatilised and an iner mineral residue is produced.

Sludge incineration

Figue 1 depicts a tyical way to incinerate dewatered sludge in a dedcated fluidised bed fuace (FBF). An appropriate flue gas treatment (dr or half dr) enables to meet the present gas emssion standads such as the European Directive 2000/76/CEE (Guibelin E. , 2002).

enerQV recovery system pressurised water 180

160

air cooler anti plume reheater cool down heat exchanger 640 wet scruber fan hot flue gas (from freeboard) 120 870 dry ash

water flue gas/air heat exchanger stack

fluidizing air

Figure 1 PYROFLUIDCI FBF and half-dry flue gas treatment including energy recovery

Considering an "easy sludge" to treat (i.e. mixed sludge), it is possible to incinerate diectly a pasty sludge after dewatering (tyically by centrfuge) with a Dry Solid content high enough to operate without extra-fuel when the steady state has been reached. This thermal balance is achieved than a flue gas/air heat exchanger. Downstream, a heat exchanger cools down the flue gas prior to dust removal. Then the cooling fluid (tyically pressursed water) can recover up to 40% of the sludge calorific value. This thermal energy can be used for many puroses such as buildings heating, or even steam generation to deliver electrcal power through a tubine but in this last case the total efficiency is prett low, around 5% of the input thermal power. Lastly, it means that 4-6 W as heat or 0.5- 8 W as electrcity can be produced per PE (Population Equivalent).

318 Wet air oxidation (WAO) Mild" conditions of temperatue (250 C) and pressure (50 bar) encountered in the ATHOS process depicted in figue 2 have proved effcient. The oxidat is pure oxygen, stored on-site under liquid form. Over 80% of total COD is oxidised after one hour of retention in the reactor. The remaining 20% is mainly a soluble and highly biodegradable COD. The exhaust gas is harless and a solid mineralised by-product is recovered after an easy dewatering (;:50% DS without polymer addition). Moreover, the liquid frction, which contains biodegradable COD, can be conveniently used as a carbon source on the wastewater plant for denitnfication purose instead of methanol (Djafer M. & aI., 2000). The organic nitrogen from sludge is firstly degraded into soluble amonia. This ammonia is parially strpped then converted into atmospheric N through a catalytc reactor.

exhaust oxygen gases HP pump thickened sludge

enerc:1V recove stem

filter press hot water ao.

Figure 2: ATHOSCI wet air oxidation process Usually, the thermal self-sustainabilty is achieved with a COD concentration around 50 g/l in the thckened sludge. However, the energy and GEG balance have to tae into account two importnt factors: the electrcal consumption, in the range of 500 to 800 kWhtDS (ton of dr solid) and the oxygen consumption, around 800 kg 02 / tDS.

Comparative study Incineration and W AO have been compared to land application regarding the fossil energy consumption and GEG. The influence of a digestion upstream has also been considered. Others important and debatable parameters such as human or environmental toxicities have not been considered because the actual impact is diffcult to assess, especially the transmission coeffcients (e.g.. pathogens or micropollutats transmission from sludge to crop and crop to bodies). But in any case, thermal oxidation processes remove all the pathogens and micropollutants from the consumer-sludge-crop cycle. The system considered is a 15 tDS/d capacity (250 000 PE) mixed sludge. After dewatering, 28% Dr Solid (DS) content is expected. Y olatile Solid (VS) is supposed to be 72% of the DS and the low calorific value (LCY) is 24 MJ/kg YS.

319 Fossil energy consumption and GEG emission

Assumptions. A simplified method consists in considering system, and estimating in plus and minus the fossil energy and GEG impacts (plus) and substiution (minus) for each treatment. In order to be consistent with previous studies, the system considered is the dewatered sludge. Thus: Polymer and energy consumptions for dewatering appear as a minus for W AO. N & P brought by the sludge save chemical fertilisers, which is a minus for agrcultue.

able 1 Energy and GEG impact for chemicals, fuel and electricity Chemicals: MJ/k produced g C02/kg Gas emittedfossil energy produced C02/MJ quicklime 200 soda 1500 polymer 1800 oxygen 300 Ferters: MJ/kS sludge gCOl/k DS slud nitrogen + phosphoru 2,4 101 Transportation(l) and heatig: MJII consumed g COlli

fuel oil (for engine or boiler) Electricity: kWh/Wh g C02/kWh roduced produced European average (2) 540

(I) diesel oil, 1 trck 20 m 35l/ (2) other studies could consider a different figue according to the national par of hydrulic or nuclear plants.

Table 2 : specific performance considered for each process Agriculture Incineration WAO Polymer rate for dewaterig kg/ Yearly dr mass after lime addition IDS/year 8371 5479 5479 Sludge calorific value (VS) MJ/kgVS Long storage month Organic matter reduction 20% of which anaeobically degraded 80% Soda used kg/IDS Oxygen used kg/IDS 800 Electrc energy requied kWhe/IDS 110 290 500 Theral recovery yield(* MJ/M 37% 35% Heatig perod duration monthyear N20 emitted g/GJ 100 incinerted Electrcal gy power recovered from biogas kW 400 300 370 ) expressed as thennal power, the 2 rates are very close. The level of energy is far better with incineration because the energy is recovered on hot flue gas around 500 C, against 1000C for W AO.

Influence of digestion Figure 3 shows the calculation results with no digestion and 6 months heat recovery

320 63 239 100 000

10000

1000 El GJ/year 100 8 tC02lyear

agriculture incineration WAO

Figure 3: No digestion, 6 months heat recovery As long as thermal energy is recovered, incineration doesn t impact more than agrcultual spreading. GEG release is even lower as already demonstrated (Davis R.D. & aI., 2002), in spite of 0 emission, since no ClL is released durng storage. Concerning W AO, the fossil C utilisation due to the electrcity and oxygen needs is ver high but GEG is paricularly low due to the total absence of CIi and N20 and no need for dewatering chemicals.

With digestion Figure 4. shows the reduction of energy needs and GEG emissions.

100000 56 706

10000

1000 El GJ/year 100 18tC02/year I

104

agriculture incineration WAO

Figure 4: With digestion, 6 months heat recovery

In spite of a higher investment cost, digestion wil dratically improve the mass and energy balance since: around 1/3 of the sludge is destroyed, reducing the equipment capacities downstream (liming facility, incinerator or W AO) and the final disposal volume. A fuel , the biogas, is produced enabling many energetic recovery schemes. A biogas production around 4800 Nm /d is expected (1280 kW as fuel).Obviously the best way to recover energy is to supply a CHP engine (Combined Heat and Power) which enables not only the heating of a 5000 m3 digester but also the supply of electrcal power with a rate of 400 kW if all the biogas is used as fuel in the engine at 90% of availability.

321 In case of incineration, a par of the biogas (roughly 1000 /d) wil be re-injected in the fuace to maintain the thermal self-sustainability. In case of W AO, a par of the electrcity produced is used for a post thickening of digested sludge by centrfugation (roughly 2 kWhm ) in order to get the good value of the COD ( circa 50 g/I) enabling the theral self-sustainability of the W AO unit. The digestion enhances paricularly the energy and GEG balances for agrcultue that has been already shown (Remelle P. , 1995). GEG balance is also significantly improved for incineration and less for W AO because for incineration, the impact of a smaller FBF capacity (actually 1.5 MW against 3 MW) is fairly high: 0 release is twice less. But the thermal energy recovered is also halved and not compensate for biogas, which has been used, according to our assumption, for electrcity production, not heat.

Influence of the cake dryness and heating period The cake dress governs obviously the thermal balance in case of incineration, but the availability of heating demand durng the year impacts more as shown in table 3: beyond 6 months of heat utilisation, the energy benefit is significant.

Table 3 Influence of the heating period and cake dryness on incineration performance

Cake d ness Heatin2 period riculture Incineration WAO 28%DS 6 months 2278 GJ/yr 2412 GJ/yr 63 239 GJ/yr 3 332 tC02/ 2 180 tC02/yr I 643 tC02/yr 28%DS 3 months 2 278 GJ/yr 12920 GJ/yr 73 179 GJ/yr 3 332 tC02/yr 3 021 tC02/ 2 438 tC02/yr 28%DS 9 months 2 278 GJ/yr 8 097 GJ/yr 53 298 GJ/yr 3 332 tC02/ I 339 tC02/yr 847 tC02/yr 26% DS 6 months 2 278 GJ/yr 5 820 GJ/yr 3 332 tC02/ 2 439 tC02/yr 31%DS 6 months 2 278 GJ/yr 996 GJ/yr 3 332 tC02/yr I 926 tC02/ Discussion It is very importnt to consider not only the sludge treatment line but also the wastewater train which can impact a lot the global energy and GEG balance. Operation methods, especially concerning the heat utilisation, also influence the conclusion. It demonstrates that the environmental balance has to be established for each specific case and never automates the decision-making. Neverteless, tendencies arse: oxidation processes have to integrte an energy recovery scheme to be environmentally frendly. due to methane release durng storage and CO emission due to lime production, the agrcultul solution without preliminar digestion is not very favourable regarding GEG emission compared with oxidation processes. It remains however the best regarding fossil carbon utilisation as long as the energy recovery duration is low. Digestion is really worthwhile in case of agrcultual use. The W AO uses a lot of primar energy for the required oxygen and electrcal power. Neverteless, the W AO (including an energy recovery) presents the best GEG balance since neither ClL nor N are released in the atmosphere. Moreover, a part of - pH dependent - is precipitated as carbonate in the solid phase. This CO entrapped, biogenic not anthropogenic, has not been taken into account in our balance. It would improve more the GEG balance for W AO.

322 g/)

We could consider another wastewater train treating nitrogen: the conclusion could be completely different because: the expected cake dress after dewatering could be only 20% makng the thermal energy recovery in case of incineration very low. N treatment maes digestion less interesting, since more or less 50% of the N entering into digestion retus to the headwork after dewaterig Conversely, in case of high nitrogen removal stadad requiring an extra carbon source, the W AO could appear more interesting. A calculation (Houillon G., 2003) would show that in this last case, the W AO would impact less than incineration.

Material recovery The mineral par of fly ash from incinerator or residue from W AO has been analysed regarding both composition and leachate content.

Incineration residue

Table 4: Sludge ash composition compared with other ashes Si0 Ah0 CaO ash from slud 35-45% 15% 3-4% 15-20% 20% silico-alum 43-54% 22-32% 15% ash from power plants silico- calcic 39% 26% sulfo-calcic 27-32% 10- 16% 10% 35-46% 10% ash from MSW 23,

Regarding the ash composition, ash from sludge is very similar to ash from power plants and can pretend to the same application.

WAO residue

Table 5 WAO Results of the leaching tests of silts from sludge oxidation (pH 7)

Leaching Leaching Leaching Total leached French Circular No. No. No. fraction (( clinker )) 9/5/94 gl) (mg/g DS) Cadium bdl* bdl* bdl* bdl* Total chromium bdl* bdl* bdl* bdl* 1.5 for Mercur 0.0013 0015 0006 0.14 Lead bdl* bdl* bdl* bdl* Arsenic bdl* bdl* bdl* bdl* Sulphates 2484 10000 TOC -:500 1500 ) below detection limit

All metals are firmly immobilized within the silts matrx. Other analyses gives a total P value of 4.5% and a residual TOC content: -: 3% (w/w DS basis). Considering theses compositions, the mineral residues either from FBF or W AO can be: reused in road constrction or in brick manufactue (Canto A ,2001). In France, the Rouen s community has thus decided to reuse the 1800 t/yr of ash produced by the sludge incinerators; as for the W AO residue, it has been recognised by the French Senate to be appropriate for similar reuses or for dumping in inert material landfills. treated to recover the P, for example according to the Biocon process (Hultman B. & al. 2002).

323 , .

Conclusions The previous study is considering a specific tye of sludge and should be remade for different sludge characteristics. Considering the mixed sludge, conclusions are the following: In spite of some uncertainties regarding the calculation of ClL and N20, we confirm that the GEG emission is not very different between incineration (with energy recovery) and agrcultue. However, W AO seems to impact less the GEG balance, even without considering carbonate entrapment. But the energy balance is often better with agrcultue, especially in case of digestion. W AO remains very energy consuming and incineration is only interesting regarding this energetic standpoint if the thermal energy of flue gas is recovered durng the major par of the year. Regarding mineral matter recycling, it appears that fly ash from incineration or residue from W AO can be easily reused in brick industr or in road constrction, making the oxidation processes safe and environmentally frendly. References Diot-Morcet M. , Aran C. , Hebe I. , Bogner J., Chanton J. , Spokas K. , Graff, C. " Evaluation of the seasonal variation on the methane mass balance at a French landfill" - Proceedings from the SWNANA, 25th Annual landfill gas symposium, pp 69-S0, 2002 Hultman, B. , Levlin, E. , Mossakowska, A. and Stark, K (2001). Effects of wastewater treatment technology on phosphorus recovery from sludges and ashes. Paper presented at 2nd Intemational Conference on Recovery of Phosphates from Sewage and Animal Wastes, CEEP, Nordwijkerhout, NL, 12-13 March 2001. Tay J-H. Show K-Y;, "resource recovery of sludge as a building and construction material-a future trend in sludge management", Wat. Sci. Tech. Vol 36, No11 pp 259-266 1997 IPCC 3rd assesment report" Climate Change 2001: The Scientific Basis " Cambridge University Press. (2002) Guibelin E. . sustainability of thermal oxidation processes: strenghts for the new Milenium" Wat. Sci. & Tech. ; p. 259 -267 Vol 46, N , 2002 Djafer M., Luck F. , Rose J. , Cretenot D. " Transforming sludge into recyclable solids and a valuable carbn source by wet oxidation" Wat. , Sci. & Tech. , Vol. 41 , n , pp. 77- , 2000 Guibelin E. , Houilon G. "Impacts energetiques et environnementaux des fiieres de devolution des boues M. N , feb. 2003, pp 45-50. Remelle P. Okobilanz fur Klarschlamm, Vorprojekt: Evaluation geeingneter Methoden, Systemgrenzen und Parameter" 1 . Globalanalyse, Praktikumbericht, Teilprojekt 1 , Interner Bericht zuhanden des verbandes bernischer Klaranlagen 1995 (1996) Davis R. D. " environmental implications of incineration and other advanced sludge-treatment processes J.lCEW 2002, 16 August pp 157-163 Houilon G. , Jollet O. Life cycle assessment of processes for the treatment of wastewater urban sludge: energy and global warming analysis , Journal of cleaner production , 2003, (accepted). Canto A. "des cendres de boues d'epuration pour Ie baton , Ie Moniteur, oct. 2001 , P 101

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