Production of volatile fatty acids from wastewater screenings using a leach-bed reactor

Accepted Manuscript

Production OF volatile fatty acids from wastewater screenings using A leach-bedreactor

Luz Stella Cadavid-Rodríguez , Nigel J. Horan

PII: S0043-1354(14)00343-1

DOI: 10.1016/j.watres.2014.05.001

Reference: WR 10652

To appear in: Water Research

Received Date: 16 October 2013

Revised Date: 18 March 2014

Accepted Date: 1 May 2014

Please cite this article as: Cadavid-Rodríguez, L.S., Horan, N.J., Production OF volatile fatty acidsfrom wastewater screenings using A leach-bed reactor, Water Research (2014), doi: 10.1016/j.watres.2014.05.001.

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http://dx.doi.org/10.1016/j.watres.2014.05.001

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Recirculation of Leachate

Screenings

VFA-rich stream

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PRODUCTION OF VOLATILE FATTY ACIDS FROM WASTEWATER 1

SCREENINGS USING A LEACH-BED REACTOR 2

Luz Stella Cadavid-Rodrígueza,1 and Nigel J. Horanb 3

aNational University of Colombia, Engineering Department, Carrera 32 No 12-00 Palmira, Colombia 4 bUniversity of Leeds, School of Civil Engineering, Leeds LS2 9 JT, United kingdom 5 6

Abstract 7

Screenings recovered from the inlet works of wastewater treatment plants were 8

digested without pre-treatment or dilution using a lab-scale, leach-bed reactor. 9

Variations in recirculation ratio of the leachate of 4 and 8 L/Lreactor/day and pH 10

values of 5 and 6 were evaluated in order to determine the optimal operating 11

conditions for maximum total volatile fatty acids (VFA) production. By increasing 12

the recirculation ratio of the leachate from 4 to 8 L/Lreactor/day it was possible 13

to increase VFA production (11%) and soluble COD (17%) and thus generate 14

up to 264 g VFA/kg-dry screenings. These VFA were predominantly acetic acid 15

with some propionic and butyric acid. The optimum pH for VFA production was 16

6.0, when the methanogenic phase was inhibited. Below pH 5.0, acid-17

producing fermentation was inhibited and some alcohols were produced. 18

Ammonia release during the hydrolysis of screenings provided adequate 19

alkalinity; consequently, a digestion process without pH adjustment could be 20

recommended. The leach-bed reactor was able to achieve rapid rates of 21

screenings degradation with the production of valuable end-products that will 22

reduce the carbon footprint associated with current screenings disposal 23

techniques. 24

Keywords: wastewater screenings, anaerobic digestion, acidification, 25

hydrolysis, volatile fatty acids, pH 26

1 Corresponding author. Tel.: +57 2 2868888 Ext. 35709; fax: +57 2 2868808.

E-mail address: [email protected] (L.S. Cadavid-Rodríguez)

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1. Introduction 27

Screens are used in wastewater treatment plants (WWTP) to remove large 28

materials from the influent and thus protect downstream unit processes. In the 29

UK, 6 mm screens are commonly used prior to primary settlement, but smaller 30

screens as low as 1 mm may be used to protect other, more sensitive unit 31

processes, such as biological aerated filters or membrane bioreactors. The daily 32

production of screenings in the UK is between 488 and 1464 wet-tonnes and 33

the organic fraction is around 90% on a dry basis (Cadavid and Horan, 2012). 34

This material is unpleasant to handle and typically disposed of to landfill. 35

However, due to the high organic content, its GHG potential is high when 36

disposed of via this route. 37

Screenings will readily digest anaerobically with methane yields of around 0.3 38

m3 CH4/kg VSappied (Le Hyaric et al., 2010; Cadavid and Horan, 2012). 39

Nevertheless, given its heterogeneity and high content of sanitary items, it is an 40

unsuitable feedstock for conventional mesophilic anaerobic digestion (MAD), a 41

process commonly used at wastewater treatment plants, where it would cause 42

solids accumulation, stratification and foaming (Le Hyaric et al., 2010). 43

To make anaerobic treatment feasible, an alternative approach would be to 44

utilize an optimised and flexible two-stage process with the first-stage 45

generating a VFA rich stream. This could then be fed to existing on-site MAD 46

assets for methane production or alternatively utilized directly in the wastewater 47

treatment for biological phosphorus or nitrogen removal (Cadavid-Rodríguez 48

and Horan, 2013). A dry process in a leach bed reactor (LBR) appears to offer 49

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an attractive option as the first stage, since that would overcome the operational 50

problems associated with solids and reduce the need for pre-treatment. 51

The LBR has been employed to digest organic materials with a high solids 52

concentration and a large fraction of contaminants (Dogan, et al., 2008). It is 53

operated to optimize the reactions of hydrolysis in which particulate organic 54

matter is hydrolysed to smaller, soluble compounds. These are then reduced to 55

VFA by the reactions of acidogenesis and acetogenesis. The resultant VFA-56

rich liquid stream, largely free from particulate material, can then be passed 57

forward to a more conventional methanogenic reactor for the production of 58

methane. This technique has proven to be effective for the digestion of the 59

Organic Fraction of Municipal Solid Waste (OFMSW) (Chugh et al., 1999; 60

Vieitez et al., 2000), fruit and vegetables waste (Mtz-Viturtia et al., 1995), food 61

waste (Xu et al., 2011), animal manure (Demirer and Chen, 2008), as well as 62

biohydrogen production from food waste (Shin and Youn, 2005). 63

The real challenge for a successful operation of a LBR is to ensure optimum 64

hydrolysis by overcoming the mass transfer limitation which appears to be the 65

true rate-limiting process step (Martin et al., 2003). Hydrolysis is a complex 66

process that is function of pH, type of substrate, nature of the biomass, size of 67

particles and residual concentration of biodegradable matter (Elefsiniotis et al., 68

1996). It is driven by physico-chemical reactions and microbial metabolism 69

where solid-liquid mass transfer plays a key role. One parameter that can be 70

manipulated to increase the solid-liquid mass transfer in the LBR is the 71

recirculation ratio of the leachate. However, there are no clear criteria for 72

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selecting this parameter in the literature and therefore a wide range of values 73

(0.2-14.4 l/lreactor/day) can be found (Table 1). 74

Table 1 Recirculation ratio of leachate in LBR reported in literature 75

Reference Recirculation ratio of leachate (l/lreactor/day)

Cycle

O’Keefe and Chynoweth (2000) Rajeshwari et al. (2001) Babel et al. (2004) Cysneiros et al. (2008) Lehtomaki et al. (2008) Myint and Nirmalakhandan (2009) Nizami et al. (2010) Jagadabhi et al. (2011)

12* 7.2* and 14.4*

4* 0.5* 0.75 4**

1* and 2* 0.2* and 0.4*

continuously continuously

1-hour-on, 5-hours-off continuously

n.r. 30-min-on, 150-min-off

n.r. n.r.

*values calculated from the data reported 76 **l/day 77 78

But in addition, pH has also been reported as one of the major parameters 79

affecting the VFA production in anaerobic acidogenesis. This parameter plays 80

an important role in determining the type of anaerobic fermentation pathway in 81

the acidification process. Specifically, researchers have confirmed that 82

Clostridium sp. shifts its metabolism from the acid-producing to alcohol-83

producing by modifying pH value (Dong et al., 2010), although there are 84

contradictory reports for the optimum pH for acidification. Whereas some 85

authors report it in the range 5.0 to 6.0 (Ghosh, 1991) or even 4.0-5.0 (Chen et 86

al., 2007) when using activated sludge as substrate; others, using pineapple 87

waste (Babel et al., 2004) and kitchen waste (Dong et al., 2010), have found 88

that a pH below 6 is inhibitory for the acidification process and suggested a pH 89

between 6 and 7. It is, therefore, necessary to generate further information 90

about the role of pH on hydrolysis/acidogenesis processes. An LBR has the 91

potential to provide a simple, technological solution to harness the energy 92

potential of screenings and other similar waste types. So it was the aim of this 93

study to investigate the role of the leachate recirculation ratio and operating pH 94

in enhancing the production of VFAs from wastewater screenings and thus 95

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evaluate its potential as a full-scale process option when retrofitted upstream of 96

existing MAD assets. 97

98

2. Materials and methods 99

2.1 Feedstock 100

Screenings from a local WWTP in the Yorkshire region (UK), treating mainly 101

domestic wastewater, were collected regularly throughout the year with an 102

average of four samples each season, and stored at -20oC. Representative 103

samples were removed as required and characterised (Table 2). 104

Table 2 Characterisation of screenings used in this research 105

Parameter Value Standard deviation TS (%) VS (% TS) C (% dry) N (% dry) C:N Ash (%) P (% dry)

26.5 93.2 50.2 2.6 17.6 1.8 0.32

1.61 0.44 1.92 0.89 3.75 1.61 0.03

Values presented are average from triplicate analyses 106

107

2.2 Reactors and operation 108

Two lab-scale LBRs were used, each with a total volume of 5 L and a working 109

volume of 4 L. The reactors were primed with 1.2 kg of screenings and 110

inoculated with an acidogenic seed culture that had been taken from a reactor 111

operating for four months with a retention time of 4 days and a pH of 5.5. The 112

seed culture was well-mixed with the screenings at a ratio of 0.78 (w/w) 113

inoculum:screenings. A sufficient volume of water was then added to exceed 114

the field capacity of the waste (Vieitez et al., 2000) and reach a liquid to solids 115

ratio (L/S) of 10 (w/w). A stainless steel 1.5 mm mesh supported the solids and 116

prevented coarse solids entering the leachate. Finally, the LBRs were flushed 117

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with N2 gas to ensure anaerobic conditions, before being completely sealed and 118

then operated at 37(±2)°C using a heating tape. 119

The recycled leachate was homogeneously applied over the bed through a 120

sprinkler placed at the top of each reactor. The daily recirculation cycle was 121

intermittent with 5-hours-on and 19-hours-off, at the appropriate recirculation 122

rate. The VFA concentration in the leachate was allowed to increase until no 123

further VFA production was observed. Dilution was undertaken by removing 124

50% of the volume of leachate and replacing this with distilled water, as 125

described by Xu et al. (2011). In total nine dilutions were performed over a 126

period of 35 days. 127

Initially experiments were conducted using two leachate recirculation rates: a 128

low recirculation rate (LRR) of 4 l/lreactor/day and a high one (HRR) of 8 129

l/lreactor/day. The pH during this stage was uncontrolled. In a second set of 130

experiments pH values were controlled at 5 and 6 whilst the leachate 131

recirculation rate was maintained constant. 132

133

2.3 Analytical methods 134

Leachate samples were centrifuged in 50 ml tubes, at 4000 rpm for 20 min. 135

prior to analysis. Total VFA (mg acetic acid/l), ammonia (mg NH3-N/L), pH value, 136

COD, TS, VS, alkalinity (mg CaCO3/l), TKN and total phosphate were analysed 137

following the methods outlined in Standard Methods (APHA, 2005). 138

Individual volatile fatty acids and alcohols were analyzed using an Agilent gas 139

chromatograph equipped with a flame-ionization detector (FID) and an NB-351 140

NORDION column (25m X 0.32mm). Operating conditions were: injector 141

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temperature 150°C; FID temperature 240°C; oven temp erature program: 95-142

140°C (10°C/min), 140-200°C (40°C/min) held for 5 m inutes. Helium was used 143

as a carried gas. 144

A one-way ANOVA with blocks, being time the block, was used to determine 145

statistical differences between the recirculation ratios and pHs evaluated. 146

Additionally, in order to discern differences and rank treatments a t (LSD) test 147

was performed. P-values less than 0.05 were considered statistically significant. 148

149

3. Results and discussion 150

3.1 Effect of the recirculation rate of the leachate 151

3.1.1 Production of total VFAs 152

Volatile fatty acids were produced very rapidly following the onset of digestion 153

(Fig. 1). This increased to a concentration of 11,310 mg/L after 4 days when 154

production stopped. At this point half of the leachate was removed and replaced 155

with dilution water and VFA production recommenced. Again the production 156

reached a plateau on day 9 when the leachate was replaced. This procedure 157

continues until the end of the experiment. 158

By removing and performing dilutions of leachate it was possible to maintain a 159

high rate of VFA generation, ranging between 255 and 520 mg/L/day during the 160

first 17 days of the operation. This range is higher than the range 205-440 161

mg/L/day reported by Babel et al. (2004) for pineapple peel under similar 162

conditions (batch mode, performing dilutions of leachate, temperature 31-37°C 163

and 4 l/lreactor/day of recirculation ratio of the leachate). This higher value is 164

evidence of a successful hydrolysis/acidification process. As pointed out by 165

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Sandoval et al. (2009), the increase in VFA concentrations with high values in 166

the acidogenic reactor can be due to a balance of fermentative bacteria 167

followed by acetogenic bacteria. This in turn can be attributed to an adequate 168

inoculation of the system with an acidic inoculum, already acclimatised to 169

screenings. 170

In general, the rate of total VFA production was greater in the first two weeks of 171

the process for both recirculation ratios. This was also reported by Ghanem et 172

al. (2001) who found a maximum leaching rate on day 10, for kitchen solid 173

waste. After these initial two weeks the VFA production started to decrease until 174

the end of the process, when acid production was reduced by 60% for the 175

higher ratio (HRR) and 80% for the lower (LRR). The production of VFA was 176

higher in the experiment with the higher ratio, resulting in an 11% greater total 177

VFA production compared with the lower ratio. Correspondingly, a one-way 178

ANOVA yielded significant differences between the two recirculation ratios (P = 179

0.0001), as well as with the time (P < 0.0001). Post hoc comparisons using the t 180

(LSD) test revealed that HRR showed a significantly greater VFA production 181

than LRR (α = 0.05). 182

The total VFA generation, calculated from the amount of leachate withdrawn 183

and its concentration, was 264 and 240 g/kg-dry screenings for HRR and LRR, 184

respectively. These values are higher than the values reported for potato 185

wastes (34 and 53 g/kg), cattle manure (132-154 g/kg) and waste activated 186

sludge (210 g/kg), in similar studies (Parawira et al., 2004; Myint and 187

Nirmalakhandan, 2009; Chen et al., 2007). However, the values found here for 188

screenings are very similar to the values of 220 g/kg and 280 g/kg reported for 189

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pineapple peel at pH 5.0-6.0 and pH 6.0-7.0, respectively (calculated from the 190

data reported by Babel et al., 2004). This shows that screenings as VFA 191

producers compete well with other highly biodegradable organic waste such as 192

fruit waste. 193

194

3.1.2 pH, ammonia and alkalinity 195

The pH of the leachate was not adjusted and it was between 5.3 and 7.0 (Fig. 196

2), a range that is considered optimum for acid production (Khanal et al., 2008). 197

The pH values of leachate strongly decreased during periods of greater VFA 198

generation (Fig. 1). After that, there was a period of very little change until the 199

last 10 days of the process, when the pH increased above 6. Thus it appears 200

that the pH profile was a reflection of VFA production. However, since there was 201

no pH control, the system tended to buffer itself towards a constant pH (around 202

6), which has been also observed by Guerrero et al. (1999).The buffering 203

capacity exhibited by the system might be due to the buffering by ammonia 204

released from amino-acid fermentation (Eastman and Ferguson, 1981). There 205

was a significant release of ammonia together with the acid production and the 206

concentration of ammonia during the first 10 days ranged between 1,000 and 207

1,600 mg/l (Fig. 3a). Even though as the experiment progressed there was a 208

decrease in the ammonia concentration, the value still remained above 400 mg/l 209

until day 20. This ammonification provides a relatively high alkalinity and 210

sufficient to maintain the VFA/Alk ratio at around 2 for most of the experiment, 211

decreasing only when the production of VFA was exhausted (Fig. 3b). It is 212

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important to point out that the level of alkalinity was also slightly higher in the 213

HRR system. 214

215

3.1.3 Hydrolysis 216

During the leaching process, soluble substrates that are hydrolyzed from 217

insoluble substrate accumulate in the liquor. The concentration of this 218

hydrolyzed substrate is represented by the mass of COD in the liquid (Nizami et 219

al., 2010). When the COD yield was measured, values of 439 and 376 g 220

COD/kg-dry screenings were obtained for HRR and LRR, respectively. This 221

means there was a 17% higher COD production from HRR compared with LRR. 222

The higher concentration of COD in leachate with the higher recirculation ratio 223

means that more solubilisation took place and so a higher level of hydrolysis. 224

This, in turn, is evidence that a high recirculation ratio of leachate has a positive 225

impact on the solid-liquid mass transfer. 226

The values found in this research are higher than the range of 172-186 g 227

COD/kg-dry manure reported by Myint and Nirmalakhandan (2009) in a similar 228

leach bed reactor. The difference in the COD yield, apart from the different 229

nature of the substrates, might result from the replacement of leachate by 230

water, which prevents accumulation of organic substances and thus boosts 231

further hydrolysis and COD solubilisation (Jagadabhi et al., 2011). It means that 232

the mass transfer, from solid to liquid phase in the LBR, may be optimized by 233

removing VFA and diluting the leachate. 234

The efficiency of the hydrolysis process can be measured in terms of the 235

solubilisation of the particulate organic matter initially fed into the digester and 236

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expressed as the soluble COD (sCOD)/initial COD ratio. This parameter reflects 237

the development of the solubilisation of the particulate organic matter in the 238

feedstock and its conversion to VFA. In this case, the analysis for system HRR 239

(Fig. 4) is presented due to its better performance. Start-up of hydrolysis was 240

rapid and within the first 17 days, 18% of the initial COD mass of the organic 241

fraction of screenings was hydrolysed to sCOD. The hydrolysis process 242

continued, although at a slower rate, reaching a value of 30% by the end of the 243

operation. This percentage is known as the hydrolysis yield. 244

A similar result was reported by Lai et al. (2001), who found that the value of 245

hydrolysis yield was 30-36% in the LBRs fed with unsorted coarsely shredded 246

MSW, over a period of 53 days. The value observed in our research, over a 247

shorter time period, shows the potential of screenings to be digested if right 248

operating conditions are met. 249

250

3.1.4 Acidification and VFAs composition 251

The VFA/COD ratio is a measure of the degree of success of acidogenesis, 252

representing the amount of solubilised matter which has been converted to VFA 253

(Maharaj and Elefsiniotis, 2001). In this experiment, VFA (converted to COD 254

equivalent) accounted for most of the sCOD in both reactors, from an initial 50% 255

to over 70% by day 12, then varying between 70% and 80% until the last 10 256

days of the process. The profile of the main VFA produced also showed little 257

difference between the two recirculation ratios investigated and so only the 258

results for the HRR system are shown (Fig. 5). The total VFA concentration in 259

the leachate reached a maximum value of 11.7 g HAc/L in the first 9 days, after 260

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which it decreased, falling to below 2 g HAc/L at the end of 35 days. The 261

increase and decrease in acid production is supported by the variations in pH 262

(Fig. 2) and the hydrolysis efficiency in the LBR. The main VFAs produced were 263

acetic, propionic and butyric. The leachate generated during the period of 264

greater acid production also contained the highest concentration of acetic acid 265

(7.6 – 8.2 g HAc/l). Dilution of leachate appears to give the acetic, propionic and 266

butyric acid-producing bacteria an advantage over the rest of the acid-producing 267

bacteria leading to less acidification via proton removal, and thus resulting in an 268

extension of the period during which such bacteria are active. 269

Alcohols were only detected during the first four days of the process, up until 270

the first dilution. Although ethanol was the main alcohol it was found only during 271

the first 2 days, whereas methanol was observed for the first 4 days of the 272

process. The average concentration of these alcohols was 275 mg/l and 63 mg/l 273

for the HRR and 104 mg/l and 72 mg/l for the LRR, respectively. Only traces of 274

butanol (16 mg/l) were detected on the second day with the experiment at the 275

LRR. The poor production of alcohols is due to the pH of the process that 276

favoured acid production, since the optimum pH for alcohol production is 277

between 3.0 and 4.5 (Dogan et al., 2008; Grupe and Gottschalk, 1992). 278

The acidification yield is the ratio of cumulative total VFA produced during the 279

operation of the LBR and the initial COD of the feedstock, which in this study 280

was 18%. The difference between the percentage of hydrolysis and acidification 281

is due to the production of various alcohols (ethanol, methanol, butanol, 282

propanol, etc), ketones (glycerol, acetone, etc.), CO2 and H2 that are also taken 283

into account when hydrolysis yield is calculated. The extent of acidification 284

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found in this study is much higher than the value reported for the OFMSW by 285

Dogan et al. (2008), who found only 4.4% of acidification at the end of 80 days. 286

The degree of acidification was favoured in our study by performing 287

recirculation of leachate as well as by an optimum pH. That is, the degree of 288

acidification may be improved if pH is maintained in optimum values and if an 289

efficient recirculation of leachate is performed (Rajeshwari et al., 2001). 290

291

3.2 Effect of pH on acidogenesis in the LBR 292

In order to determine which pH value leads to more effective acid production, 293

the extent of acidification was measured in the leachate in terms of the ratio 294

VFA (converted to COD equivalent)/sCOD (Fig. 6a). At an operating pH value of 295

6.0, acidification increased with time, reaching 78% by day 26. By contrast at 296

pH 5, acidification was much lower and also decreased with time reaching a 297

minimum of 23.5% by day 26. VFA production was also significantly higher at 298

pH 6 with a reduction in acid production at pH 5.0 of 58% (Fig. 6b). This was 299

confirmed by the ANOVA which revealed significant statistical differences 300

between two operating pH values with regard to VFA production (P < 0.0001), 301

as well as with the time (P < 0.0001). Post hoc comparisons using the t (LSD) 302

test revealed that pH 6 showed significantly higher VFA levels compared with 303

pH 5 (α = 0.05). 304

Veeken et al. (2000) also observed that hydrolysis of biowaste was inhibited at 305

pH 5.0 and similarly Russell and Wilson (1996) reported that cellulose 306

degradation was inhibited at pH below 6.0. In general, the LBR operated at pH 307

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6.0 showed a better performance in all aspects (Table 3), including VS removal 308

(57%) and recovery of nutrients (60% of TKN and 36% of total phosphorus). 309

Both Dong et al. (2010) and Babel et al. (2004) found the optimal pH for 310

acidification was between 6 and 7 and stated that at lower pH values, 311

undissociated acids are inhibitory for the acidogenesis. However, these findings 312

contradict those reported by Ghosh (1991) and by Chen et al. (2007), who 313

claimed that the best pH for acid production was in the range 5-6, and 4-5, 314

respectively. The reason for this discrepancy may lie in the fact that different 315

substrates were studied. Kitchen waste (Dong et al., 2010) and fruit and 316

vegetables (Babel et al., 2004) are easily hydrolysed whereas activated sludge 317

(Ghosh, 1991 and Chen et al., 2007) is more difficult to hydrolyse. Thus 318

although a higher pH (6-7) might lead to a higher production of acids, it is also 319

likely that it would encourage production of methane through methanogenesis. 320

Table 3. Performance of the LBR process and composition of the resultant leachate 321

322 Parameter pH 5 pH 6 VFAs produced (g/kg VS) VS removal (%) Volume reduction (%) TKN recovery (%) Total phosphorus recovery (%)

90 25 16 48 54

213 57 40 60 36

Average composition of VFAs detected (%)

Acetic Propionic Butyric Isobutyric Valeric Isovaleric

63 11 12 2 6 6

67 12 10 2 4 5

323

When the total VFAs production at pH 6.0 was compared with that of 324

uncontrolled pH (5.3-6.5), no significant difference was found (P = 0.493). Thus 325

acid digestion of screenings in the LBR under uncontrolled pH, besides being 326

less expensive, presents an adequate equilibrium for the production of VFAs 327

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whilst inhibiting efficiently the methanogenic phase. It means that, for a cost-328

effective VFA production, the acid digestion of screenings without pH control 329

may be recommended. 330

Contrary to expectation the production of alcohols was no higher at pH 5 331

compared to either pH 6 or the uncontrolled pH, perhaps because this pH value 332

is still far from the optimum for alcohol production, which occurs between pH 3.0 333

and 4.5. However, it was observed that methanol was produced at pH 5, 334

although at a low concentration (45 mg/L, on average) and only until day 11 of 335

the experiment. 336

337

3.3 General discussion 338

The role of recirculation within the leach bed is to provide efficient mixing 339

between the substrate and biomass, promoting an effective solid-liquid mass 340

transfer, and thus leaching capacity. At the same time, recirculation will pose an 341

energy demand, so it is important to ensure the lowest rate compatible with 342

good performance. The leaching capacity depends on the size of the particles 343

and porosity of the bed and in this study the leaching characteristics were 344

excellent due to the heterogeneous composition of the screenings. As a result 345

bed blockage was not observed at any time, and it was not necessary either to 346

treat the screenings to reduce their size or to add a bulking agent to increase 347

porosity. In fact the leaching capacity of the bed was observed to improve over 348

time as hydrolysis of the solids fraction took place. 349

The better performance of the higher leachate recirculation ratio (8 L/Lreactor/day) 350

compared to the lower one (4 L/Lreactor/day) is explained by the fact that a higher 351

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recirculation ratio implies a more efficient solid-liquid mass transfer that, in turn, 352

should provide an improved hydrolysis process. It also confirms the important 353

role of solid-liquid mass transfer in the hydrolysis phase. However, the 354

recirculation ratio of leachate cannot be increased indefinitely since high ratios 355

cause serious reductions to the porosity of the solid bed, and may even 356

completely stop the flow of liquid. Thus it is likely that the optimum recirculation 357

ratio that gives the most effective hydrolysis will be also a function of the type of 358

substrate and its particle size. Nonetheless, in addition of taking into account 359

technical factors it is equally important to take into account economic factors in 360

order to select the more cost-efficient recirculation ratio. Therefore, despite its 361

better performance, it is not possible to suggest that the higher recirculation 362

ratio of the leachate studied in this research is the optimum for the process, 363

without an economic analysis that considers the additional energy requirements 364

for recirculation. 365

From the results it is clear that pH has a greater influence on the VFA 366

production than recirculation ratio. This is because the pH directly determines 367

the fermentation type, i.e. acid-producing or alcohol-producing (Babel et al., 368

2004) whereas the recirculation ratio of the leachate only influences the solid-369

liquid mass transfer and the biomass contact. 370

In addition to generating VFAs, the LBR also proved to be very effective in the 371

destruction of volatile solids and in the release and recovery of nutrients, 372

making this reactor a very effective at undertaking the hydrolysis/acidogenesis 373

processes. The leachate produced that is rich in VFAs, can be used not only to 374

produced methane in a second digestion stage but also may be used as an 375

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internal carbon source for the biological removal of nutrients in a WWTP 376

(Manyumba et al., 2009; Maharaj and Elefsiniotis, 2001) or even as a raw 377

material for the production of butanol or biopolymers. 378

379

4. Conclusions 380

This study demonstrated the effectiveness of the LBR system for the digestion 381

of screenings without pre-treatment, exhibiting a high efficiency of 382

hydrolysis/acidification processes. By increasing the recirculation ratio of the 383

leachate from 4 to 8 L/Lreactor/day it was possible to increase 11% the VFA 384

production, enhance 17% COD solubilisation and generate up to 264 g VFA/kg-385

dry screenings; however, the higher energy demand has to be taken into 386

account to select the more cost-effective recirculation ratio. On the other hand, 387

the acidogenesis was inhibited at pH 5 and an uncontrolled pH, ranging from 388

5.3 to 6.5, was found to be adequate for the production of VFA and the effective 389

inhibition of methanogenesis. The LBR can make feasible the recovery of 390

resources from screenings reducing its current carbon footprint. 391

392

Acknowledgements 393

The authors want to thank to COLCIENCIAS for giving financial support to the 394

first author, and to the Public Health Laboratory at Leeds University for all the 395

collaboration received from staff. 396

397

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Figure 1. Effect of the recirculation ratio of the leachate on VFA generation (Vertical lines mark each dilution performed).

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Figure 2. The effects of the recirculation ratio on the pH profile of the leachate

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Figure 3. Ammonia production (a) and VFA:Alk ratio (b) with two recirculation ratios of leachate

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Figure 4. Cumulative sCOD/initial COD ratio values, system HRR.

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Figure 5. Quantitative and qualitative analysis of VFAs during operation of the LBR.

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Figure 6. Grade of acidification (a) and total VFA production (b) at two pH conditions

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Highlights

• A leach bed reactor showed to be effective for the acid digestion of screenings. • VFA production was optimized to produce 264 g/kg-dry screenings. • At pH 5.0 acidogenesis was inhibited, the optimum pH found to VFA production

was 6. • Valuable end-products such as VFAs, alcohols and nutrients can be obtained. • The LBR might be an alternative for reducing the carbon footprint of screenings.

Documents

Production of volatile fatty acids from wastewater screenings using a leach-bed reactor