Sludge-Drying Lagoons: a Potential Significant Methane Source in

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Sludge Drying Lagoon - a Potential Significant Methane Source in Wastewater Treatment Plants Yuting Pan, Liu Ye, Ben van den Akker, Ramon Ganigue, Ronald Ssemiganda Musenze, and Zhiguo Yuan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04844 • Publication Date (Web): 07 Dec 2015 Downloaded from http://pubs.acs.org on December 14, 2015

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Environmental Science & Technology

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Sludge Drying Lagoon - a Potential Significant Methane

2

Source in Wastewater Treatment Plants

3 4

Yuting Pan1, 2, Liu Ye1,3, Ben van den Akker4,5,6,Ramon Ganigué Pagès1,7, Ronald S.

5

Musenze1, Zhiguo Yuan1,*

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1

8

QLD, Australia

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2

Advanced Wastewater Management Centre, The University of Queensland, St. Lucia,

Department of Environmental Science and Engineering, School of Architecture and

10

Environment, Sichuan University, Chengdu, Sichuan 610065, China

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3

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QLD 4072, Australia

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4

Australian Water Quality Centre, Adelaide, 5000, South Australia, Australia

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5

Health and Environment Group, School of the Environment, Flinders University,

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Bedford Park, 5042, South Australia.

16

6

17

University of South Australia, Mawson Lakes, 5095, South Australia.

18

7

19

Spain

School of Chemical Engineering, The University of Queensland, St. Lucia, Brisbane,

Centre for Water Management and Reuse, School of Natural and Built Environments,

LEQUIA, Institute of the Environment, University of Girona, Girona, Catalonia,

20 21

*Corresponding author: phone + 61 7 3365 4374; fax +61 7 3365 4726; email:

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[email protected];Address: Office 420, Level 4 Gehrmann Building (60), The

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University of Queensland, Brisbane, Australia, 4072.

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ACS Paragon Plus Environment


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TOC Art

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29

Abstract

30

“Sludge drying lagoons” are a preferred sludge treatment and drying process in

31

tropical and sub-tropical areas due to the low construction and operational costs.

32

However, this process may be a potential significant source of methane (CH4), as

33

some of the organic matter would be microbially metabolised under anaerobic

34

conditions in the lagoon. Quantification of CH4 emissions from lagoons is difficult

35

due to the expected temporal and spatial variations over a lagoon maturing cycle of

36

several years. Sporadic ebullition of CH4, which cannot be easily quantified by

37

conventional methods such as floating hoods, is also expected. In this study, a novel

38

method based on mass balances was developed to estimate the CH4 emissions, and

39

was applied to a full-scale sludge drying lagoon over a three year operational cycle.

40

The results revealed that the sludge drying lagoon process would emit 6.5 kg CO2-e

41

per megalitre of treated sewage. This would represent a quarter to two-thirds of the

42

overall greenhouse gas (GHG) emissions from wastewater treatment plants (WWTPs).

43

This work highlights that sludge drying lagoons are a significant source of CH4 that

44

adds substantially to the overall GHG footprint of WWTPs, despite being recognised

45

as cheap and energy efficient means of drying sludge.

46 47

1 Introduction

48

Excess sludge is an abundant by-product of WWTPs. The treatment and disposal of

49

this sludge is one of the most challenging and costly components of the wastewater

50

treatment process, which can represent up to 60% of the total treatment cost1.

51

Therefore, it is natural for WWTP managers and operators to utilise economical

52

means of sludge treatment that are suited to local conditions. In tropical and sub-

53

tropical regions, where the climate is hot and dry and land is inexpensive, a “sludge

54

drying lagoon” is the preferred sludge treatment and drying process due to its low

55

construction and operating cost2. A sludge drying lagoon can be a simple earth basin

56

or a concrete reservoir equipped with some auxiliary works such as bottom liner to

57

prevent groundwater contamination and a supernatant decanting facility 3.

58 59

A typical operational cycle of a sludge-drying lagoon is usually more than one year

60

depending on the local climate, and consists of the following major phases: “filling”,

61

“drying” and “desludging”4. During the filling phase, digested sludge from WWTPs is



62

pumped into the sludge drying lagoon. When the designed sludge depth is reached,

63

the filling stops and the drying phase commences. The solids settle and form a thick

64

sludge layer at the base of the lagoon, as the sludge is dried by evaporation and is

65

stabilized by microbial processes. Usually, a supernatant water layer of more than 0.5

66

m is maintained over the digesting sludge layer forming a “cap”. This allows the

67

oxidation of odorous compounds that are generated and released from the sludge layer

68

during anaerobic digestion 5.Once the dewatered sludge reaches the desired solids

69

concentration (e.g.25%~30% dry sludge content), the dried sludge is removed in a

70

single desludging event.

71 72

Although sludge drying lagoons provide a cost effective and energy efficient method

73

to treat excess sludge from WWTPs, they may be a significant source of CH4. This is

74

because part of the organic matter treated in the lagoon would be transformed by

75

microorganisms under anaerobic conditions, leading to CH4 production. In general,

76

anaerobic digestion is likely to happen at the bottom of the sludge drying lagoon in

77

the sludge layer where oxygen or other external electron acceptors are depleted (see

78

Figure 1).In addition to the anaerobic digestion process, organic matter can also be

79

removed by other microbial processes such as aerobic respiration, denitrification and

80

sulphate reduction. For example, aerobic oxidation of organic matter would occur

81

within the upper "water cap" layer, using oxygen that is continuously diffusing from

82

the atmosphere or produced in-situ by algae. This oxygen supply would also drive

83

nitrification, producing nitrate and/or nitrite, which would facilitate the oxidation of

84

organic matter in anoxic zones via denitrification. Other electron acceptors such as

85

sulphatecan also facilitate the oxidation of organic matter to carbon dioxide (CO2) via

86

sulphate reduction.

87 88

Understanding CH4emissions is of great concern, because CH4is an important GHG

89

has with a global warming potential of around 25 times that of CO2-equivalents

90

6

91

reported due to a number of challenges: 1) a typical sludge drying lagoon cycle is long

92

(up to several years, depending on the local conditions) and therefore emissions are

93

expected to vary over time;2) Significant spatial variation would exist due to sporadic

94

CH4 and CO2 gas ebullition5, and is unlikely to be captured by the traditional gas hood

95

method that is widely applied for quantifying emissions from wastewater treatment

.However, the CH4 emissions from full-scale sludge drying lagoons are rarely being


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plants; and 3) access to sample sludge drying lagoons is difficult and presents

97

occupational hazards.

98 99

The overall aim of this study was twofold: 1) to develop a method for quantification

100

of methane emissions from sludge drying lagoons and 2) to determine the contribution

101

of sludge drying lagoon treatment processes to the overall carbon footprint of a

102

WWTP. The methodology is demonstrated through its application to a full-scale

103

sludge drying lagoon in Australia, over a three year operational cycle.

104 105

2 Method Development

106

A quantification method based on mass balance was developed to estimate the CH4

107

emissions by considering the key physical and biological processes relevant to

108

chemical oxygen demand (COD) balance, total nitrogen (TN) balance and oxygen

109

balance within a sludge-drying lagoon. The detailed mass balance is schematically

110

shown in Figure 2.

111 112

2.1 The COD, Nitrogen and Oxygen Balances of a Sludge Drying Lagoon

113

CH4 production and COD mass balance

114

As shown in Figure 2, the organic matter contained within the influent sludge

115

(_ ) has three possible fates: 1) the COD contained in the remaining dried

116

sludge would be transported out of the sludge lagoon by sludge decanting at the end

117

of the operational cycle (_ ); 2) the COD suspended or dissolved in the

118

supernatant

119

(_ ); and 3) The gap between the COD transported in and the COD

120

transported out will be the “lost” COD ( _ ). The “lost” COD is due to

121

microbial degradation under aerobic, anoxic and anaerobic conditions. Under aerobic

122

or anoxic conditions, the COD is lost from the system in the form of CO2, while under

123

anaerobic conditions the COD loss is attributed to CH4 production. Therefore, the

124

amount of CH4 produced during lagoon treatment can be estimated through Eq-1 and

125

Eq-2, as shown below:

would

be

removed

in

the

supernatant

decanting

process

126 127

_ = _ − _ −_

(Eq-1)

128

__ = _ − _

(Eq-2)

!"#$_

− _

%&'#$_



129 130

Where:

131

_ (tonnes TCOD): the total COD loss during one lagoon cycle;

132

_ (tonnes TCOD): the total COD loaded into the lagoon;

133

_ (tonnes TCOD): the total COD removed in desludging;

134

_ (tonnes TCOD): the total COD contained in the supernatant

135

drained;

136

__ (tonnes TCOD): the amount of CH4 produced measured as

137

COD ;

138

_()_ (tonnes TCOD): the COD loss through anoxic reactions;

139

_*)_ (tonnes TCOD): the COD loss through aerobic reactions.

140 141

In Eq-1, _ , _ and

_ can be directly

142

calculated based on the lagoon operational data, which was illustrated in this detailed

143

case study. In Eq-2, _ is the output of Eq-1, while _()_ and

144

_*)_ is estimated based on nitrogen balance and oxygen balance, as

145

shown below.

146 147

Nitrogen mass balance

148

COD consumption by microorganism within anoxic zones is mainly achieved via

149

denitrification. Therefore, the nitrogen balance is incorporated to estimate COD loss

150

via denitrification, i.e. _()_ . Similar to the COD mass balance, the total

151

nitrogen contained within the influent sludge (_ + ) is removed via three

152

possible mechanisms: 1) transported out of the sludge lagoon by the desludging

153

process at the end of a sludge drying lagoon operational cycle (_,); 2)

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removed from the supernatant decanting process ( _ + ); and 3) The

155

difference between the TN transported in and the TN transported out would be the

156

“lost” TN ( _ + ) caused by microbial denitrification that transforms solid or

157

dissolved nitrogen into nitrogen gas, mainly N2(N2O emissions are expected to be

158

negligible, and was confirmed during the sampling campaign. Therefore, the “lost”

159

nitrogen matter through microbial denitrification processes can be calculated

160

according to Eq-3:


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_ + = _ + − _, −_ +

(Eq-3)

163 164

Where:

165

_ + (tonnes TN): the total nitrogen loss during the lagoon operation;

166

_ + (tonnes TN): the total amount of nitrogen matter loaded into the

167

lagoon;

168

_, (tonnes TN): the nitrogen matter removed in desludging;

169

_ + (tonnes TN): the nitrogen matter contained in the supernatant

170

drained.

171 172

The stoichiometric COD consumption for denitrification is 2.86g COD/g N.

173

Assuming nitrate is the electron acceptor, the COD loss through anoxic reactions can

174

be calculated as:

175 176

_()_ = 2.86 × _ +

(Eq-4)

177 178

Nitrite could be an alternative intermediate form of oxidized nitrogen that occurs

179

during nitrification and denitrification. In this case, the reaction stoichiometry would

180

be 1.71g COD/g N. Therefore, the true ratio would be between 1.71 and 2.86g COD/g

181

N, depending on the contribution of each intermediate. However, as shown in the

182

oxygen balance below, the uncertainty associated with the COD:N ratio does not

183

affect the estimated amount of CH4 emitted.

184 185

Oxygen mass balance

186

The oxygen balance was also introduced to estimate the COD loss due to aerobic

187

respiration, i.e. M_*)_3 . Oxygen for aerobic oxidation of organic matter and

188

nitrification will be continuously diffused from the atmosphere into the liquid phase

189

due to air circulation. As discussed below, oxygen production from phototrophic

190

processes does not affect the COD balance and therefore was not included. The mass

191

of oxygen transferred during the operational life of a lagoon can be estimated basing

192

on water-air gas transfer models (further described in Section 2.2).Once the quantity

193

of oxygen diffused into the liquid(M_4 , tonnes O2) and the oxygen consumed



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194

by nitrification ()_4 , tonnes O2) were quantified, the aerobic heterotrophic

195

respiration was estimated using Eq-5:

196 197

_*)_ =

%&'#$_ 4

= _4 − !#5#$

5#!_ 4

(Eq-5)

198 199

Where:

200

201

aerobic COD

202

_4 (tonnes oxygen): the total amount of oxygen diffused from the

203

atmosphere into the sludge drying lagoon;

204

)_4 (tonnes oxygen): the amount of oxygen consumed for

205

nitrification;

%&'#$_ 4 (tonnes

oxygen): the amount of oxygen consumed through the oxidation process;

206 207

The oxidised nitrogen transformed by denitrification is mainly provided by

208

nitrification. Assuming that ammonium is oxidized all the way through to nitrate and

209

then denitrified to nitrogen gas, then the amount of oxygen consumed for nitrification

210

is 4.57gO2/g N7. The amount of oxygen used by nitrification can then be estimated

211

usingEq-6:

212 213

)_4 = 4.57 × _ +

(Eq-6)

214 215

If nitrite was the end product of nitrification, the oxygen consumption coefficient

216

would be 3.43gO2/g N. Therefore, the true oxygen consumption for nitrification

217

would be between 3.43 and 4.57gO2/g N.

218 219

Introducing Eqs- 4, 5 and 6 into Eq-2, we have,

220 221

__ = _ − ,;< + 1.71 × _ +

(Eq-7)

222 223

The coefficient 1.71g COD/g N in Eq-7 is independent of the intermediate (nitrate or

224

nitrite) between nitrification and denitrification.

225 226

In the above balance analysis, we have ignored COD oxidation by sulphateand also


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227

the potential for oxygen formation by algal photosynthesis, as these processes are not

228

expected to affect the overall estimation of __ . While consuming COD,

229

sulphate reduction forms hydrogen sulphide. The lagoon is capped by water to avoid

230

hydrogen sulphide emission. This water layer leads to the oxidation of sulphide to

231

sulphate, and thus oxygen is consumed. The amount of oxygen consumed for sulphide

232

oxidation is equivalent to the amount of COD consumed for sulphate reduction, and

233

therefore the overall COD and oxygen balance is not affected. Similarly, any oxygen

234

produced by algae is accompanied by CO2 fixation to organic carbon. Therefore algal

235

photosynthesis does not affect the overall COD and oxygen balances.

236 237

Another reaction we have ignored is the anammox reaction, which could happen at

238

the interface of the aerobic and anaerobic zones in the sediments. In this zone, nitrite

239

formed by ammonium oxidation could react with ammonium to produce N2. The

240

combined nitritation and anammox reaction is 2NH3 + 3/2O2 -> N2 + 3H2O. This

241

means that the net oxygen consumption for NH3 conversion to N2 is still1.71 ×

242

_ + . In other words, the presence of the anammox reaction does not affect Eq-7.

243

In the above combined reaction, for simplicity, we have ignored nitrate production

244

from nitrite by the anammox reaction. As discussed above, denitrification from nitrite

245

or nitrate does not affect the oxygen balance either.

246 247

2.2 Estimation of Oxygen Diffusion Using Wind Models

248

The amount of oxygen transferred (_4 ) during one operational cycle of a

249

sludge drying lagoon can be quantified based on Eq-8:

250 251

F

_4 = ?G @(B) × D × EB

(Eq-8)

252 253

Where F(t) (g/m2/d) is the oxygen transfer flux from air to water; A (m2) is the area of

254

the sludge drying lagoon; T (d) is the duration of one operational cycle of a sludge

255

drying lagoon. For the estimation of F, we used the thin boundary layer equation that

256

is detailed below.

257 258

Estimation of oxygen transfer flux (F) using the thin boundary layer equation

259



260

@ = H (I' − I 5 )

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(Eq-9)

261 262

Where k (m/d) is the normalized gas transfer velocity; Cobs (g/m3) is surface water

263

dissolved oxygen concentration and Csat (g/m3) is atmospheric equilibrium/saturation

264

concentration at the prevailing in situ temperature. Cobs can be measured (as detailed

265

in the case study), while Csat can be calculated using Henry’s Law from the

266

atmospheric concentration of oxygen. For the estimation of k, we used wind -based

267

models, as explained below.

268 269

Estimation of the gas transfer velocity (k)

270

While many factors such as wind, waves, bubbles and water currents can influence

271

gas exchange across the water–air interface,8 wind is clearly recognised as being key

272

for stationary water bodies such as the water layer in a sludge drying lagoon.9, 10To

273

estimate the oxygen transfer velocity due to wind (k), three commonly used wind

274

models, including Ro and Hunt (2006)9(hereafter RH06), Liss and Merlivat (1986)11

275

(hereafter LM 86), and Cole and Caraco (1998)10 (hereafter CC98) were used (see

276

supplementary information for details). RH06 is a unified model for wind-driven

277

surficial oxygen transfer into stationary water bodies developed by combining

278

coefficients from relationships for gas transfers derived in investigations in controlled

279

wind tunnels, floating re-aerators in open water and natural open water bodies. LM86

280

was developed through laboratory experimental approaches using wind/water tunnels

281

and validated with measurements from natural open water bodies while CC98 was

282

developed using wind measurements and gas fluxes in freshwater lakes. Details

283

regarding these models including variability in their estimates have been previously

284

discussed.12-14

285 286

3 Implementing the Method Developed to a Full Scale Sludge Drying

287

Lagoon

288

3.1 The Sludge Drying Lagoon Used for the Case Study

289

The sludge drying lagoon treatment process used is this case study receives

290

anaerobically digested sludge from two Australian WWTPs, with design capacities of

291

160ML/day and 45 ML/day. The sludge comprises both digested primary and

292

digested secondary (waste activated sludge) sludge. The climate in the area is


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classified as subtropical, with hot, dry summers and mild winters with moderate

294

rainfall. There are eight evaporation sludge drying lagoons, with a total surface area of

295

120 hectares and a design depth of 1.5m.Lagoon filling is spread across several

296

lagoons until one is full. The filled lagoon is moved offline and a new lagoon replaces

297

its rotation. Sludge in the offline lagoon then undergoes drying and stabilization

298

before desludging at the end of the operational cycle.

299 300

We investigated one of the eight lagoons. The lagoon operation involves a three-year

301

operational cycle, including a 27 month intermittent filling, eight months of drying

302

and one month of desludging. The initial “water cap” depth was1m, however the

303

volume could change due to evaporation and rainfall. Excess supernatant was

304

collected and returned to the head of the plant. Sludge removed from the lagoons is

305

stockpiled for a three year period to be reused on farmland or trucked offsite for

306

landfill disposal.

307 308

3.2 Data Collection

309

Operation regime and the parameters of the sludge drying lagoon

310

The operation information of a sludge drying lagoon was collected from the operator,

311

which included the following information that was need to calculate the TCOD mass

312

and TN mass required by Eq-1 and Eq-3:

313 314

a) During the sludge “feeding period”: parameters relevant to the TCOD mass and

315

TN mass fed into the sludge drying lagoon collected included: 1) the monthly sludge

316

influent flow rate (Qinf, ML/month); 2) the dry sludge (total suspended solid) content

317

in the influent (Sinf_TS, % of dry sludge weight/the total influent weight); 3) the

318

influent TCOD concentration (Sinf_TCOD, kg COD /kg dry sludge); 4) the influent TN

319

concentration (Sinf_TN, kg TKN /kg dry sludge).

320 321

b) During the “desluding period”:parameters relevant to the TCOD mass and TN

322

mass removed from the sludge drying lagoon collected included:1) the weight of the

323

sludge removed in the desludging process (Mdes, tonnes); 2) the dry sludge (total

324

suspended solids) content in the removed sludge (Sdes_TS, % of dry sludge weight/

325

Mdes); 3) the TCOD concentration in the removed sludge (Sdes_TCOD, mg TCOD /kg

326

dry sludge weight); 4) the TN concentration in the removed sludge (Sdes_TN, mg TN



327

/kg dry sludge weight).

328 329

c) Sludge supernatant parameters: parameters relevant to the TCOD mass and TN

330

mass content in the supernatant collected included: 1) volume of the supernatant (Vsup,

331

ML); the supernatant TCOD concentration (mg TCOD/L); the supernatant TN

332

concentration (Ssup_TN, mg TKN/L).

333 334

Wind data collection

335

Three different wind models, which all require wind speed data were used to estimate

336

the oxygen diffusion. The wind speed data was spanning around two years was

337

obtained from two Bureau of Meteorology automatic weather and climate stations

338

located closest to the lagoon. Wind data was measured every 10 seconds and averages

339

logged at 15 minutes intervals throughout the study period. The 15 minutes averages

340

were then used for the estimation of k, using the three models15, which was

341

subsequently used for the estimation of F and MO2,diffused using Eq.-9 and Eq.-8,

342

respectively.

343 344

Determination of the surface water dissolved oxygen concentration Cobs

345

A remote controlled boat (Figure S1) equipped with a DO meter (YSI Professional

346

Plus, United States) was used to monitor the surface DO concentrations. The remote

347

controlled boat could travel approximately 50 meters away from the bank of the

348

sludge drying lagoon, to record DO measurement at different locations. The DO meter

349

was mounted at the front of the boat and programmed to record the DO concentration

350

and temperature at 5 minute intervals. The DO sensor was submerged ~2 cm below

351

the sludge drying lagoons water surface.

352 353

4 Results

354

4.1 The Operational Regime and Parameters of the Sludge Drying Lagoon

355

The parameters used to calculate the mass of TCOD and TN that was i) fed into the

356

sludge drying lagoon, ii) removed from sludge drying lagoon during the desludging

357

period; and iii) removed when decanting the supernatant are summarized in Table 1,

358

which shows parameter range, average and standard error. Average values were used

359

to calculate the mass of TCOD and TN transported in and out of the sludge drying


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lagoon. To calculate the overall volume of sludge fed into the sludge drying lagoon

361

during the sludge “feeding period”, we integrated the monthly influent flow rate data

362

(Qinf).The volume of supernatant removed from the sludge drying lagoon was not

363

collated because routine decanting does not occur during normal operation.

364

Occasional ad hoc decanting of supernatant does occur, however the flow was not

365

recorded by the plant operator. We assumed that 15 ML was removed from the plant,

366

because the surface area of the lagoon studied is 15 hectare sand the initial water was

367

1m. The potential impact of this uncertainty associated with the volume of supernatant

368

removed was shown to be negligible, as detailed in section 4.3.

369 370

The dissolved CH4 and N2O concentrations in the surface water were measured in two

371

sludge drying lagoons, with Lagoon A representing a maturing lagoon, while Lagoon

372

B represented a matured lagoon(Figure S2).The CH4 concentration varied

373

significantly, with concentrations of 0.02 mg CH4/L and 0.9 mg CH4/L, respectively.

374

Such variation was probably closely related to the level of maturity of a sludge drying

375

lagoon. In contrast, no large difference was observed in N2O concentrations, which

376

were on average was 0.025 mg N/L within both lagoons. These concentrations were

377

measured to confirm the presence of CH4 and N2O in the lagoon and were not

378

required to calculate emissions.

379 380

4.2 Estimation of the Amount of Oxygen Transferred

381

The measured DO concentration of the lagoon surface water was 3.7 ± 0.3 mg/L, with

382

no pronounced spatial or temporal variations. This was probably because the lagoon

383

is shallow and the sludge is evenly distributed at the bottom of the lagoon. Three

384

wind-based models were used to estimate the gas transfer velocities and eventually

385

fluxes, with the estimated yearly average oxygen transfer fluxes (F) shown in Figure 3.

386 387

The estimated F varied between the three models, ranging between 2.2 to 2.8kg

388

O2/(m2×year). The average Fvalue (2.4±0.35kg O2/(m2×year)that was derived from

389

the three models was applied to Eq-7, together with 1) the surface area of15hectare,

390

and 2) the sludge filling and drying period of35 months. The oxygen transfer from air

391

to sludge drying lagoon (_4) was estimated to be 1.1 ± 0.35kilo tonnes O2.

392



393

4.3 Mass Balance of the Sludge Drying Lagoon Investigated and CH4Emission

394

With the plant operation data acquired (Table 1), the component required by the

395

TCOD and TN balance in Eq-1 and Eq-3 can be calculated, with the results shown in

396

Table 2.For example, #!6J%!5_ isthe sum of the TCOD origin from Plant A and

397

Plant B. Take Plant A as an example, the COD input was calculated by multiplyingthe

398

overall volume of sludge fed into the sludge drying lagoon during the sludge “feeding

399

period” (Qinf), the density of the influent sludge ( ρ ), the dry sludge content in the

400

influent, (Sinf_TS), and the influent TCOD concentration (Sinf_TCOD). As shown in Table

401

1, the Sinf_TS and the Sinf_TCOD for Plant A were 2.0 ± 0.022% and 0.87 ± 0.025kg

402

TCOD/kg dry sludge, respectively. The density of the sludge was assumed to be equal

403

to the density of water. Therefore, the COD input from Plant A was estimated to be

404

3.9 kilo tonnes TCOD. Following the same method, the COD input for Plant B was

405

estimated to be 2.8 kilo tonnes TCOD. #!6J%!5_ is the sum of the TCOD origin

406

from Plant A and Plant B, which was 6.7 kilo tonnes TCOD, as shown in Table 2.

407

Similarly, the amount of TCOD and TN removed during the desludging process

408

( _ , _, ), and supernatant decanting ( _ ,

409

_ + ) was calculated based on parameters listed in Table 1, with the results

410

shown in Table 2. The difference between the COD/TN transported in and the

411

COD/TN transported out represents the “lost” COD ( _ )/TN ( _ + ),

412

which was determined using Eq-1 and Eq-3, respectively.

413 414

Once _ + was determined, the the amount of oxygen consumed for

415

nitrification )_4 was calculated using Eq-6, and the COD lost through

416

anoxic reactions (_()_ ) was calculated using Eq-4. The amount of oxygen

417

used by heterotrophs aerobically

418

Eq-5.

%&'#$_ 4 can

be estimated by the oxygen balance,

419 420

Finally, using Eq-2, the mass of CH4 generated in the sludge drying lagoon was

421

estimated to be 2.86 ± 0.32 kilo tonnes COD over a period of three years. Considering

422

that 1g CH4 is equivalent to 4 g COD, and the global warming effect of 1g CH4

423

correspond to 25 g CO2equivalentswith time horizon of 100 years, the CH4 emission

424

from the sludge drying lagoon investigated was 6.1 ± 0.67 kilo tonnes CO2-e/year.

425

There are eight similar sludge treatment lagoons operated in parallel, therefore, the


Page 14 of 25

Page 15 of 25


426

overall CH4 emission from the sludge treatment lagoon system was estimated to be

427

48.9 ± 5.5 kilo tonnes CO2-e/year.

428 429

With the influent COD considered as 100% bioavailable COD, Figure 4 shows that 32%

430

of the influent COD was removed by the desludging process, where as

Sludge-Drying Lagoons: a Potential Significant Methane Source in

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