Continuous Anaerobic Digestion of Food Waste and Paper Waste under Mesophilic-Dry Condition

ABSTRACT With increasing concerns regarding the limited capacity of landfill, conservation of resources, and reduction of CO2 emissions, dry anaerobic digestion of organic solid waste has recently been gaining considerable attention. However, there have been few reports on continuous operation and most have involved operation under thermophilic condition. In the present study, a continuous dry anaerobic digestion system treating a mixture of food waste and paper waste was operated under mesophilic condition. For easy injection of a solid type substrate, the feed was diluted six-fold with the sludge inside the reactor, and then, fed into the reactor. During the operation, hydraulic retention time (HRT) decreased as follows: 150, 100, 60, 40, and 30 d at a fixed substrate concentration of 30% total solids (TS), corresponding to a solid loading rate (SLR) of 2.0, 3.5, 5.0, 7.5, and 10.0 kg TS/m3/d, respectively. Up to 40 d of HRT, biogas production proportionally increased as SLR increased, but at 30 d of HRT, biogas production decreased. At further operation, instead of controlling HRT, substrate concentration was increased to 40% TS, which was found to be a better option for increasing the treatability. The system could achieve a stable CH4 production yield of 0.27 m3 CH4/kg TSadded and 0.25 m3 CH4/g CODadded, and over 75% of volatile solids (VS) reduction.

Journal of Water and Environment Technology, Vol 8, No.3, 2010

Address correspondence to Sae-Eun Oh, Department of Environmental Engineering, Hanbat National

Continuous Anaerobic Digestion of Food Waste and Paper Waste under Mesophilic-Dry Condition

Dong-Hoon KIM*, Mo-Kwon LEE**, and Sae-Eun OH**

* Bioenergy Research Center, Korea Institute of Energy Research, Daejeon 305-343, Republic

of Korea

** Department of Environmental Engineering, Hanbat National University, San 16-1,

Duckmyoung-dong, Yuseong-gu, Daejeon 305-719, Republic of Korea

ABSTRACT

With increasing concerns regarding the limited capacity of landfill, conservation of resources, and reduction of CO2 emissions, dry anaerobic digestion of organic solid waste has recently been gaining considerable attention However, there have been few reports on continuous operation and most have involved operation under thermophilic condition In the present study, a continuous dry anaerobic digestion system treating a mixture of food waste and paper waste was operated under mesophilic condition For easy injection of a solid type substrate, the feed was diluted six-fold with the sludge inside the reactor, and then, fed into the reactor During the operation, hydraulic retention time (HRT) decreased as follows: 150, 100, 60, 40, and 30 d at a fixed substrate concentration of 30% total solids (TS), corresponding to a solid loading rate (SLR) of 2.0, 3.5, 5.0, 7.5, and 10.0 kg TS/m 3 /d, respectively Up to 40 d of HRT, biogas production proportionally increased as SLR increased, but at 30 d of HRT, biogas production decreased At further operation, instead of controlling HRT, substrate concentration was increased to 40% TS, which was found to be a better option for increasing the treatability The system could achieve a stable CH4 production yield of 0.27 m 3 CH4/kg TSadded and 0.25 m 3 CH4/g CODadded, and over 75% of volatile solids (VS) reduction

Keywords: dry anaerobic digestion, hydraulic retention time, mesophilic, methane

INTRODUCTION

In order to mitigate the effects of climate changes, the Kyoto Protocol was announced in

1997, dictating that industrialized countries should reduce their total greenhouse gas (GHG) emissions by 5.2% by the end of 2012 from the level of emissions in1990 This target can only be met with a significant transition from fossil fuels to alternative energy

sources that are cheap, renewable, and not causing pollution (Saxena et al., 2009) Tidal,

geothermal, hydroelectric, and wind power could be the suitable candidates in some countries; however, they are not expected to become the dominant sources inthe future

(Zidansek et al., 2009) Meanwhile, biomass is widespread throughout the world, and

thereby is not subject to world price fluctuations or supply uncertainties, in contrast with

imported fuels In addition, it is a carbon neutral resource in its life cycle (Fortman et al.,

2008)

The use of landfill has been the main final disposal method of organic solid wastes to date in most nations However, as it creates a large amount of polluted leachate, emits GHG, and requires a long time (30-100 yrs) for degradation, the need to avoid landfill is

now shared by all technical communities (Gioannis et al., 2009) Considering these

aspects, the choice of a biological process seems obvious In this regard, anaerobic

digestion perhaps offers the best solution in terms of energy and mass balance (Pavan et

al., 2000) Throughanaerobic digestion, biomass including organic solid waste can be

stabilized in a closed reactor where biodegradation is highly accelerated relative to that

in a landfill Furthermore, clean biogas can be obtained, which can be utilized for heat

or electricity generation In addition, the effluent sludge may be composted or may be

used for soil conditioning depending on its characteristics (Vallini et al., 1993)

However, as the conventional anaerobic digestion method proceeds under a slurry state (<5% total solids – TS), a large amount of external water is required for diluting solids This will not only increase the energy consumption for digester heating and feed slurry

pumping, but also the volume of digester effluent that should be dewatered (Radwan et

al., 1993) To overcome these drawbacks, dry digestion or “high-solid digestion” can be

employed, in which a solid substrate having over 20% of TS concentration is directly

fed to the reactor (Bolzonella et al., 2003)

During the 1990s, dry digestion prevailed over wet digestion, and several commercialized dry digestion systems e.g DRANCO (Six and De Baere, 1992),

KOMPOGAS (Willinger et al., 1993), and VALORGA (Laclos et al., 1997) were

developed Various kinds of solid wastes such as agricultural residues, sludge cake, and organic fraction of municipal solid wastes have been treated Recently, with increasing concern over the limited capacity of landfill, conservation of resources, and reduction of

CO2 emissions, dry digestion is gaining much attention However, most researches have been limited to batch tests or investigation of the start-up period, and all the systems have been operated under thermophilic condition, based on thermophilic operation

(50-60°C) being favorable in terms of hydrolysis and microbial kinetics (Forster-Carneiro et

al., 2007; Shuguang et al., 2007; Forster-Carneiro et al., 2008a; Forster-Carneiro et al.,

2008b) It is important, however, to attain the maximum treatability of the system, and it

is clear that the operation at mesophilic condition (30-40°C) would be less energy consumptive, thereby enhancing economic viability

In Korea, the amount of waste dumped to landfill has reached 10.5 million tons per year, accounting for 11.2% of total waste produced in 2007 Considering the increasing trend

of waste production and the current remaining landfill capacity of 185 million tons, it is expected that the nation’s landfill will be filled within 10 years Also, Korea is a highly energy-dependent country, fulfilling 97% of its energy consumption needs by import, and will be forced to reduce CO2 emissions in the near future under the Kyoto Protocol Therefore, implementation of anaerobic digestion for the treatment of organic solid waste is an urgent issue

In the present work, the performance of a continuous dry anaerobic digestion process under mesophilic condition was investigated A mixture of food waste and paper waste, the main sources of municipal solid wastes, was used as a feedstock During the operation, hydraulic retention time (HRT) and the solid concentration of substrate were controlled

MATERIALS AND METHODS

Feedstock and seeding source

Food waste collected from a school cafeteria, and paper waste comprised of toilet paper,

newspaper, and copy paper were shredded by a hammer crusher (TOP-03H) and cut

crusher (TOP-03-CC), respectively, to a size less than 5 mm Both crushers are

manufactured by Korean Mechanics Engineering Corp The mixing ratio of food waste

and paper waste was 7:3 on aweight basis and a certain amount of water was added to

adjust the TS concentration The TS concentration of food waste and paper waste was

20% and 99%, respectively Volatile solids content (VS/TS), total nitrogen (TN), and

chemical oxygen demand (COD) concentration of the mixed feedstock were 94.5±0.9%

(VS/TS), 0.014±0.004 g/g TS, and 1.09±0.10 g COD/g TS, respectively

Asa seeding source, a mixture of dewatered sludge cake and anaerobic digester sludge

taken from the same local wastewater treatment plant was used The characteristics of

these two different types of sludge are presented in Table 1 Dewatered sludge cake and

anaerobic digester sludge were mixed at a 4:1 ratio by volume basis, resulting in initial

TS and VS concentration of 17.3% and 7.6%, respectively

Table 1 - Characteristics of seeding inoculum used in this study

Item Dewatered sludge cake Anaerobic digester sludge

TS concentration (%) 20.2 5.8

VS content (VS/TS, %) 40.2 95.0

Reactor operation

As shown in Fig 1, a horizontal-type cylindrical reactor was used for dry anaerobic

digestion The total volume of the reactor was 60 L with a diameter and length of 320

mm and 750 mm, respectively The broth was agitated by four impellers at 25 rpm

Thirty litersof seeding source was added to the reactor, and purged with N2 for 10 min

in order to provide anaerobic condition After three days of adaptation period (no feed

injection), 0.27 L of substrate (30% TS), corresponding to 150 d HRT, was fed daily

There was no sludge injection until the inside sludge volume reached an effective

volume of 40 L At further operation, HRT was decreased to 100, 60, 40, and 30 d at a

fixed substrate concentration of 30% TS The substrate was added once daily until60 d

HRT, but the injection time was increased to twice daily at further HRT decrease As a

decline in system performance was observed at 30 dHRT, theHRT was again increased

to 40 d for performance recovery Substrate concentration was subsequently increased to

40% TS at 40 d HRT

In this study, for easy injection of a solid-type substrate, the feeding pump was turned

on after the solid content of the substrate was reduced by dilution with some sludge

inside the reactor The same screw-type pumps were used for feeding the substrate and

recycling the sludgeinside the reactor Further details are provided in Table 2 In order

to optimize this process, 0.3 L (= 1Q) of substrate was mixed with 2-6 times recycled

sludge (2Q-6Q), and thepump was then turned on This test was conducted during the

100th-120th day of continuous operation All the systems were installed at a temperature-controlled (35±1°C) room

Fig 1 - Schematic diagramof anaerobic dry digestion system

Table 2 - Details of screw-type pump for feed injection and sludge recycling

Model Output

(W)

Voltage (V)

Frequency (Hz)

Current (A)

Starting torque (N·m)

Rated torque (N·m)

Max speed (rpm) K9IP200FH 200 220 50 1.3 3.0 1.45 1,350

Analysis

Measured biogas production was adjusted at standard temperature and pressure (STP), 0°C and 760 mmHg The contents of CH4, N2, and CO2 were determined by gas chromatography (GC; Gow Mac series 580) using a thermal conductivity detector and a 1.8 m  3.2 mm stainless-steel column packed with porapak Q (80/100 mesh) with helium as a carrier gas The temperatures of injector, detector, and column were kept at

80, 90, and 50C, respectively Volatile fatty acids (VFAs, C2-C6) and lactate were analyzed by a high-performance liquid chromatograph (HPLC; Finnigan Spectra SYSTEM LC, Thermo Electron Co.) with an ultraviolet (210 nm) detector (UV1000, Thermo Electron) and a 100 mm  7.8 mm fast acid analysis column (Bio-Rad Lab.) using 0.005 M H2SO4 as a mobile phase The liquid samples were pretreated with a 0.45

m membrane filter before injection to the HPLC Alkalinity, pH and concentrations of

TS, VS, COD, TN and ammonia were measured according to Standard Methods (APHA,

at el 1998)

RESULTS AND DISCUSSION

Continuous operation performance

In order to treat solids with high concentration, highly concentrated biomass should also

be prepared Generally, however, wet digester sludge having less than 5% VS

concentration has often been used as a seeding source, requiring a long period to build

up a highly concentrated microbial consortium (Forster-Carneiro, 2007; Fernandez et al., 2008; Montero et al., 2009) Instead, dewatered sludge cake having 8% VS

concentration was used as the main seeding source in this study It appears that this strategy was successful; CH4 production was observed from the first day, and biogas production was stabilized within 30 d (Fig 2)

Time (day)

0

50

100

150

200

250

3 /d)

0 2 4 6 8 10 12

Biogas SLR

HRT

= 150d

HRT

= 100d

HRT

= 60d = 40d HRT

HRT = 30d

HRT = 40d HRT

= 40d

Substrate conc = 30% TS

Substrate conc

= 40% TS

Fig 2 - Daily biogas production of mesophilic-dry anaerobic digestion of organic solid

wastes at various operating conditions

However, in employing this strategy, special care should be taken with regard to ammonia inhibition, because dewatered sludge cake is rich in nitrogen, being degraded into ammonia during the digestion, and there does not exist any nitrogen removal mechanism in anaerobic digestion In this study, fortunately, ammonia concentration during the start-up period did not exceed 3,500 mg NH4-N/L, and as the substrate was continuously supplied, its concentration gradually decreased to a range of 1,500-2,500

mg NH4-N/L If the dewatered sludge cake contains a high concentration of ammonia, its removal by stripping or applying other tools prior to seeding is strongly recommended

Until the 336th day of operation, HRT was decreased to150, 100, 60, 40, and 30 d at a fixed solid concentration of 30% TS, corresponding to SLR of 2.0, 3.5, 5.0, 7.5, and 10.0 kg TS/m3/d, respectively At the 171st-175th d period, immediately after the HRT transition from 60 d to 40 d, a sudden decrease of biogas production was observed It

wassuspected that this failure was related to the number of feeding time Feeding of the substrate corresponding to 7.5 kg TS/m3/d at one time fell outside the treatable range In order to recover the performance, the substrate injection time was increased to twice a day, which worked efficiently Biogas production increased and stabilized with an average value of 143 L/d

When the HRT was decreased from 40 d to 30 d, there was an increase of biogas production, but it was not sustained From the 250th day, drastic biogas production drop was observed along with a decrease of CH4 content in the produced biogas (Fig 3) CH4

content was in a range of 50-55% until40 d HRT decrease, but it dropped below 50% at

30 d HRT Also, the VS concentration in the reactor clearly showed an increasing trend, suggesting an overloading condition for solid hydrolysis In addition, as shown in Fig 4, both pH and alkalinity concentrations, which were maintained over 7.5 and 9,000 mg CaCO3/L, respectively, dropped significantly Total organic acid concentration was lower than 200 mg COD/L until 40 d HRT, but it increased to 2,500 mg COD/L in which most of the acids consisted of propionate This indicates that the balance was broken between the production of acids and their consumption by methanogenesis at 30

d HRT Accumulation of acids decreased the pH, resulting in the failure of the whole system The growth rate of the acidogenic bacteria is much higher than that of methanogenic archea, and the bacteria can be active at a weak acidic condition whereas the archea cannot Therefore, the balance between the generation of acids and their conversion to CH4 is important in a single-stage anaerobic digestion process

(Mata-Alvarez et al., 2000; Ward et al., 2008)

Time (day)

30

35

40

45

50

55

60

6.0 6.5 7.0 7.5 8.0 8.5 9.0

VS (%)

HRT

HRT

= 40d

Fig 3 - Change of CH4 content and VS concentration of the system

Time (day)

6.5

7.0

7.5

8.0

8.5

4000 6000 8000 10000 12000 14000

pH Alkalinity

HRT

HRT

= 40d

HRT

Substrate conc = 30% TS

Substrate conc

= 40% TS

Fig 4 - Change of pH and alkalinity of the system

At further operation, HRT was increased again to 40 d in order to see the performance recovery The productivity of biogas recovered, and all important indicators including

CH4 content, VS concentration, pH, and alkalinity showed a recovering trend

From the 337th day, instead of controlling HRT, substrate concentration was increased to 40% TS in order to increase SLR At this time, stable biogas production was observed for 100 days The CH4 content, pH, and alkalinity concentration did not show a decreasing trend At first glance, VS concentration in the reactor seemed increasing, but this did not indicate a decrease of VS reduction efficiency as the substrate concentration increased The VS reduction efficiency achieved in both feeding 30% TS and 40% TS of substrate was over 75%

The CH4 production yield (MPY) is one of the important parameters determining the success of an anaerobic digestion system The average MPY achieved at 10 kg TS/m3/d

of SLR (40% TS fed at HRT 40 d) was 0.27 m3 CH4/kg TSadded Based on the input COD, MPY was 0.25 m3 CH4/kg CODadded, indicating that 71% of the energy content in the organic solid waste was converted to clean bioenergy, CH4 The MPY value achieved in this study was comparable with the ones achieved in conventional wet

digestion processes (Mata-Alvarez et al., 2000), and dry digestion process operating under thermophilic condition Montero et al (2008) continuously operated thermophilic

dry digestion, and achieved 80% of VS removal efficiency and 0.24 m3 CH4/kg

Solid-type substrate injection

Recirculated sludge volumeis an important parameter affecting the economic aspect of this system When the recirculated sludge volume was 2Q, injection of feed was

impossible, as it clogged the pump line When the recirculated sludge volume was increased from 3Q to 6Q, the required time to pump Q was reduced from 30.5 to 8.9 s However, the total required time for feed injection was minimal when 5Q was recirculated, as shown in Fig 5 In this study, the same pumps were used for recirculating and feeding, meaning that the consumption of electricity only depends on the total required time in running the pump Therefore, 5Q recirculation was found to be the most economically feasible condition The proper volume of recirculated sludge can vary according to the type of reactor, characteristics of the substrate and pumps used, and system performance, but it is obvious that this kind of test is very informative for scale-up, and to our knowledge, this is the first ever report of such an attempt

Recirculation Volume

0 20 40 60 80 100

120

140

160

Substrate injection Recirculation Total

Fig 5 - Required time for feed injection at various recirculation volumes

CONCLUSIONS

Continuous mesophilic-dry anaerobic digestion of food waste and paper waste was successfully conducted for 420 days For easy injection of a solid type substrate, the feeding pump was turned on after the sludge inside the reactor was recirculated and mixed with the substrate When the HRT was decreased to 30 d, corresponding to 10 kg TS/m3/d of SLR, system failure was observed The change of CH4 content, pH, VS and alkalinity concentration all reflected poor anaerobic digestion performance The performance was recovered when HRT was increased to 40 d again At further operation, instead of controlling HRT, substrate concentration was increased to 40% TS at a fixed HRT of 40 d At this time, stable performance was achieved with a high MPY of 0.27

m3 CH4/kg TSadded and 0.25 m3 CH4/g CODadded, and over 75% of volatile solids (VS) reduction The MPY of 0.25 m3 CH4/g CODadded indicates that 71% of the energy content in the organic solid waste was converted to clean bioenergy In optimizing

solid-type substrate injection, the least time was required when the volume of recirculated sludge was five times that of thesubstrate

ACKNOWLEDGEMENT

This study was supported by Korean Ministry of Environment as “The Eco-technopia

21 project”

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