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)

(2) University

• f Venice.

Italy (3) Marche Polytechnic University. Italy (1) University

• f Verona.

Italy

The anaerobic stabilisation of waste activated sludge produced in D-N or BNR processes presents some problems. In fact:

Determine …..

• 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

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

• f the reactor is often negative ….

• 7500
• 5000
• 2500

2500 5000 7500 2,5 5 7,5 10 12,5 15 17,5 20 22,5 25 27,5 30 32,5 35 Fed Sludge Temperature, °C Heat balance, kWh/d Well pre-thickened sludge (6% TS) and SGP = 0,15 m3/kgVSfed Well pre-thickened sludge (6% TS) with a CHP unit Present situation (2,6-4% TS)

Heat balance for a digester where sludge with a different TS concentration is fed

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,

• rganic flocculants are

generally mixed with sludge to improve their dewaterability….

Type of commercial flocculants:

1.Neutral 2.Anionic 3.Cationic

Scheme of the cationic flocculant action on sludge flocs Cationic flocculant Sludge flocs Flocculated sludge

Most used are cationic flocculants….

Literature on this topic is scarce and

• ften 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,

• n the anaerobic digestion

process for sludge stabilization was investigated at lab-scale

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

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

250 500 750 1000 1250 1500 3 6 9 12 15 18 21 24 27 30 33

Time, days Cumulative Biogas, ml

Batch biogas production of the waste activated sludge Calculation of the value of the constant for the first order kinetic reaction

y = 0,12x - 0,006 0,2 0,4 0,6 0,8 1 1,2 1 2 3 4 5 6 7 8 9 10

Time, days

• ln((B0-SMP)/B0)

Run 1 2 3 4 Flocculant Dose, g/kg 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

Values of the ultimate biogas production, B0, and the first order kinetic constant, K, determined in the batch experiments for different doses of flocculant

Biodegradation test

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

Continuous experiments

OLR: organic loading rate; GP: gas production; SGP: specific gas production; GPR: gas production rate; VS: volatile solids; Alk: alkalinity.

Run 1 2 3 4 Floccluant dose, g/kg 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/lreactord

0,18 0,014 0,19 0,010 0,17 0,030 0,21 0,015 Alk pH 4, gCaCO3/l 3,8 0,23 3,9 0,33 4,3 0,18 4,3 0,14 Alk pH 6, gCaCO3/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

Performances and yields….

Gas production in different steady state conditions…

0,5 1 1,5 2 2,5 3 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 Days

Run 2 Run 3 Run 1 Run 4 Reactor problems

The flocculant dose did not affect the biogas production

Mass balances

Run 1 2 3 4 TSin, g/d 15,1 14,4 14,2 13,5 TSout, g/d 9,4 9,6 8,8 8,6 TS removal, % 37 33 38 36 TVSin, g/d 11,6 11,7 11,5 9,7 TVSout, g/d 6,3 6,9 6,4 6,2 TVS removal, % 46 40 44 32 CODin, g/d 14,5 14,0 13,2 12,1 CODout, 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 ‰) Transmittance spectrum for the digester supernatant Acrylate and acrylamide presence

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
• rder 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