INFLUENCE OF A CATIONIC FLOCCULANT ON THE ANAEROBIC DIGESTION OF - PowerPoint PPT Presentation

INFLUENCE OF A CATIONIC FLOCCULANT ON THE ANAEROBIC DIGESTION OF WASTE ACTIVATED SLUDGE Bolzonella D. (1), Pavan P. (2), Battistoni P. (3), Cecchi F. (1) (3) Marche (1) University (2) University Polytechnic of Verona. of Venice. University. Italy Italy Italy

The anaerobic stabilisation of waste activated sludge produced in D-N or BNR processes presents some problems. In fact: • The absence of primary settlers to preserve the COD availability for nutrients removal processes And • The application of large SRTs (> 10 days) used to preserve the nitrification capability Determine …..

the absence of easily biodegradable matter and a partial sludge stabilisation during the wastewater treatment process so biogas production is limited. Because of this situation the heat balance of the reactor is often negative ….

Heat balance for a digester where sludge with a different TS concentration is fed 7500 5000 Well pre-thickened sludge (6% TS) and SGP = 0,15 m3/kgVSfed Well pre-thickened sludge (6% TS) 2500 with a CHP unit Heat balance, kWh/d 0 0 2,5 5 7,5 10 12,5 15 17,5 20 22,5 25 27,5 30 32,5 35 -2500 Present situation (2,6-4% TS) -5000 -7500 Fed Sludge Temperature, °C Bolzonella et al. (2002), WST 46(10): 199-208

Therefore, in order to sustain the process, the sludge thickening to solid concentration > 4% is generally a necessity To obtain this, organic flocculants are generally mixed with sludge to improve their dewaterability….

Type of commercial flocculants: 1.Neutral 2.Anionic 3.Cationic

Most used are cationic flocculants…. Flocculated Cationic Sludge sludge flocculant flocs Scheme of the cationic flocculant action on sludge flocs

Literature on this topic is scarce and often contradictory … • Dentel and co-workers reported the anaerobic biodegradability of flocculants…… Wat. Sci. Technol. (2001) 44(2-3): 461-468 • Lee and co-workers reported the partial inhibition of sludge anaerobic digestion due to the presence of flocculants…… Chemosphere (2003) 53: 757-764

In this study the influence of an organic cationic flocculant, the PRESTOL K233L, on the anaerobic digestion process for sludge stabilization was investigated at lab-scale

Materials and Methods Lab scale reactor Basic structure of for anaerobic the cationic digestion tests flocculant PRAESTOL K233L

The study considered: • The cationic flocculant characterization • Batch studies for the determination of the first order kinetic constant at different flocculant doses and the flocculant biodegradability • Continuous studies where sludge conditioned with increasing amounts of flocculant where fed to the reactor

Flocculant characterization Parameter pH 6,6 Charge density, millieq/g 2,7 COD, g/l 2,8 TKN, g/l 0,1 TSS, g/l 2,2 TVSS, g/l 2,1 Characteristics of the 1.5 ‰ solution of PRAESTOL K233L

Batch studies 1500 1250 Cumulative Biogas, ml Batch biogas 1000 production of the 750 waste activated 500 sludge 250 0 0 3 6 9 12 15 18 21 24 27 30 33 Time, days 1,2 Calculation of the 1 -ln((B0-SMP)/B0) value of the 0,8 y = 0,12x - 0,006 constant for the 0,6 first order 0,4 kinetic reaction 0,2 0 0 1 2 3 4 5 6 7 8 9 10 Time, days

Values of the ultimate biogas production, B 0 , and the first order kinetic constant, K, determined in the batch experiments for different doses of flocculant Run 1 2 3 4 Flocculant Dose, g/kg 0 5 15 30 B 0 , l/gVS fed 0,16 0,16 0,18 0,22 K, day -1 0,13 0,12 0,11 0,13

Biodegradation test 500 Praestol K233L Cumulative Biogas, ml 400 Inoculum only 300 200 100 0 0 12 24 36 48 60 72 84 96 108 120 132 144 156 Time, hours

Continuous experiments Performances and yields…. Run 1 2 3 4 Floccluant dose, g/kg 0 5 15 30 Avg S.Dev. Avg S.Dev. Avg S.Dev. Avg S.Dev. OLR, gVS/l? d 1,15 0,09 1,16 0,14 1,16 0,09 0,93 0,12 pH 7,30 0,04 7,30 0,04 7,37 0,03 7,46 0,04 GP, l/d 1,8 0,140 1,9 0,102 1,7 0,302 2,1 0,152 SGP, l/gVS 0,15 0,016 0,16 0,014 0,15 0,027 0,23 0,039 GPR. l/l reactor d 0,18 0,014 0,19 0,010 0,17 0,030 0,21 0,015 Alk pH 4, gCaCO 3 /l 3,8 0,23 3,9 0,33 4,3 0,18 4,3 0,14 Alk pH 6, gCaCO 3 /l 2,7 0,19 2,9 0,23 3,1 0,16 3,2 0,13 VS removal, % 45,6 8,9 40,2 8,6 44,4 3,9 32,2 11,6 OLR: organic loading rate; GP: gas production; SGP: specific gas production; GPR: gas production rate; VS: volatile solids; Alk: alkalinity.

Gas production in different steady state conditions… 3 Run 2 Run 3 Run 4 Run 1 2,5 2 1,5 Reactor problems 1 0,5 0 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 Days The flocculant dose did not affect the biogas production

Mass balances Run 1 2 3 4 TS in , g/d 15,1 14,4 14,2 13,5 TS out , g/d 9,4 9,6 8,8 8,6 TS removal, % 37 33 38 36 TVS in , g/d 11,6 11,7 11,5 9,7 TVS out , g/d 6,3 6,9 6,4 6,2 TVS removal, % 46 40 44 32 COD in , g/d 14,5 14,0 13,2 12,1 COD out , g/d 8,2 9,0 10,0 8,4 COD removal, % 43 34 24 30 TVS and COD removal…. Biogas production…. Because of the partial biodegradation of the flocculant !!!

New issue: presence of acrylamide and acrylate groups in digester supernatants…… Transmittance spectrum for the pure flocculant solution (1,5 ‰) Acrylate and acrylamide presence Transmittance spectrum for the digester supernatant

Conclusions • the presence of a cationic flocculant in concentrations up to 30 g/kg of sludge (dry weight) did not affect significantly the anaerobic digestion process of waste activated sludge • batch tests revealed a similar value of the first order reaction constant, K, at some 0,12 day -1 , regardless the flocculant presence and its concentration • spectrophotometric determinations in the infra-red field revealed the presence of the specific functional groups of the flocculant in the anaerobic supernatants

ACKNOWLEDGEMENTS SIXTH FRAMEWORK PROGRAMME PRIORITY 6 Sustainable development, global change and ecosystems Sustainable Energy Systems CROPGEN Renewable energy from crops and agrowastes