Environ Eng Res > Volume 24(3); 2019 > Article
Shin, Lee, Im, and Kim: Effect of seaweed addition on enhanced anaerobic digestion of food waste and sewage sludge

### Abstract

To investigate the effect of seaweed (SW) addition on anaerobic co-digestion of food waste (FW) and sewage sludge (SS), batch experiments were conducted at various substrate concentrations (2.5, 5.0, 7.5, and 10.0 g volatile solids (VS)/L) and mixing ratios ((FW or SS):SW = 100:0, 75:25, 50:50, 25:75, and 0:100 on a VS basis). The effect of SW addition on FW digestion was negligible at low substrate concentration, while it was substantial at high substrate concentrations by balancing the rate of acidogenesis and methanogenesis. At 10 g VS/L, CH4 production yield was increased from 103 to 350 mL CH4/g VS by SW addition (FW:SW = 75:25). On the other hand, SW addition to SS enhanced the digestion performance at all substrate concentrations, by providing easily biodegradable organics, which promoted the hydrolysis of SS. khyd (hydrolysis constant) value was increased from 0.19 to 0.28 d−1 by SW addition. The calculation showed that the synergistic CH4 production increment by co-digesting with SW accounted for up to 24% and 20% of total amount of CH4 production in digesting FW and SS, respectively.

### 1. Introduction

Due to the high content of organic matters and moisture, organic solid wastes such as food waste (FW) and sewage sludge (SS) generate huge amount of odor, leachate, and greenhouse gases during their decomposition. Thus, they should be handled through appropriate treatment methods including anaerobic digestion (AD), which was proven as an environmentally friendly technology capable of recovering bioenergy in the form of methane (CH4) [13]. AD is a biological process, consisting of four steps of reaction: hydrolysis, acidogenesis, acetogenesis, and methanogenesis, and the balance among each step is known to be critical for the successful digestion performance [4].
The characteristics of FW might vary depending on the region and season, but it is generally characterized to have a high C/N ratio (20 to 50) and salinity concentration (2 to 5%), and insufficient trace element for the stable long-term operation [5, 6]. Also, FW contains a huge amount of easily biodegradable carbohydrates, causing the excessive hydrolysis and acidogenesis [7]. It could result in the accumulation of organic acids with pH drop under high-strength condition [8]. To avoid this failure, anaerobic digesters fed with FW should be operated at a low organic loading rate (OLR) range, from 1 to 4 g volatile solids (VS)/L/d, or long hydraulic retention time (HRT) up to 80 d [9, 10]. On the other hand, SS has a poor hydrolysis rate due to having microbial cells and extracellular polymers substances (EPS). The EPS are composed of various substances such as protein, carbohydrates, lipids, nucleic acids, and humic acids [11]. It forms a tight extracellular matrix to prevent the penetration of hydrolytic enzymes during AD [12]. To overcome this barrier, physical, thermal, and biological pretreatment methods have often been applied to SS, but their economic benefit is still questioned [13].
Extensive researches have been carried out on co-digestion, which can overcome some problems in the mono-digestion such as lack of trace elements, imbalanced C/N ratio, and high or low biodegradability [1416]. For example, Zhang et al. [14] reported a 40% increase in a total CH4 production yield (MPY) compared to the mono-digestion of FW, by co-digesting with cattle manure at a FW/cattle manure ratio (on a VS basis) of 2. They mentioned the supply of buffer capacity and lowering C/N ratio, as the main cause of the increased performance. Higher amount of CH4 was produced by adding microalgae to SS, which was ascribed to the fact that the easily biodegradable portion in microalgae helped the degradation of SS [17].
Since 2000, the third-generation biomass has gained a lot of attention as a bioenergy feedstock, owing to its negligible lignin content, and higher CO2 uptake rate than terrestrial biomass. In particular, seaweed (SW) does not require land for its growth, and is easier to cultivate compared to the fresh-water algae. SW, for example brown algae, is, in general, composed of 30 to 40% carbohydrates, 35 to 40% of minerals, 15% of proteins, and 5% of lipids. It has a higher content of carbohydrates than SS, and its main carbohydrate unit is cellulose while it is starch in case of FW [18, 19]. Starch and cellulose are well known polysaccharides consisting glucose units, while their binding patterns are different with each other. In starch, each glucose is bound with α-linked connection, while each glucose is bound to β-linked connection in cellulose [20]. This structural feature can lead to the big differences in the biodegradability because β-linked is stronger than α-linked.
In the present work, we hypothesized that SW has a lower anaerobic biodegradability than FW, while higher biodegradability than SS, which in turn, a certain amount of SW addition to these organic solid wastes might enhance AD performance. To investigate this, batch anaerobic co-digestion experiments were performed at various substrate concentrations (2.5, 5.0, 7.5, 10.0 g VS/L) and mixing ratios ((FW or SS):SW = 100:0, 75:25, 50:50, 25:75, 0:100 on a VS basis). The synergistic increment by co-digestion with SW was calculated, and the reasons were revealed by monitoring pH and organic acids during digestion, and by applying first-order kinetic model.

### 2.1. Seeding Inoculum and Feedstock

The granular sludge taken from the anaerobic digester in a wastewater treatment plant of local beer factory in Cheongju, South Korea, was used as an inoculum after grinding less than 2 mm of diameter. Prior to seeding, it was kept at 35°C for two weeks to remove residual biogas. FW was collected from the cafeteria in Inha University, where around 1,000 people eat two times of meal every day. It was also ground with blender having less than 2 mm of diameter. Laminaria japonica, which collected at Yeosu of South Korea, was chosen as a SW, and it was washed with tap water to remove salt, and dried at 105°C for 2 h. Then, the dried SW was pulverized with blender, and diluted with distilled water. The SS used in this study was the mixture of primary sludge and secondary sludge at the same volume ratio, taken from the local wastewater treatment plant. The characteristics of seeding inoculum and feedstock are shown in Table 1.

### 2.2. Experiment

The co-digestion experiments were conducted by using 280 mL serum bottle, with a working volume of 100 mL. The seeding inoculum was injected into each serum bottle at a concentration of 7.5 g VS/L. Substrate concentrations of co-substrates (1. FW and SW, and 2. SS and SW) were set to 2.5, 5.0, 7.5, and 10.0 g VS/L at the mixing ratios of 100:0, 75:25, 50:50, 25:75, and 0:100 on a VS basis. The corresponding I/S (inoculum to substrate on a VS basis) ratios were 3.0, 1.5, 1.0, and 0.8 at each substrate concentration of 2.5, 5.0, 7.5, and 10.0 g VS/L, respectively. In addition, an experiment was conducted to produce CH4 from the inoculum itself, and all results presented exclude this CH4 production. The initial pH was adjusted between 7.8 and 8.0 with 2 N KOH solution, and then sufficiently replaced with nitrogen gas (99.999%) to provide anaerobic condition. Nutrients media including buffer were not added to clarify the role of SW in the co-digestion. The fermentation was carried out in an incubator (VS-8480SRN, VISION Co., Korea) set at 35°C and at the agitation speed of 150 rpm. All experiments were conducted duplicates, and the results were averaged. Gas and liquid samples were taken every 2–5 d to analyze biogas amount, CH4 content, pH, and organic acids.

### 2.3. Analysis

The concentration of total solids (TS), VS, total nitrogen (TN), alkalinity (as g CaCO3/L), and total chemical oxygen demand (COD) were measured according to standard methods [21], while carbohydrate concentration was determined by the colorimetric method [22]. pH value was measured using a pH meter (HM-3R, DDK-TOA). The amount of biogas produced was measured using a gas syringe, and was converted to standard temperature and pressure (STP). CH4 gas content was measured by taking 0.5 mL of the gas phase in serum bottle with Gas-tight syringe (HAMILTON CO., USA) and using gas chromatography (GOWMAC series 580, USA) equipped with a thermal conductivity detector. The temperature of injector, column, and detector was set to 50, 80, and 70°C, respectively. The column was a 183 cm (Length) 0.32 cm (Diameter) stainless steel column with porapak Q (80/100 mesh). Nitrogen (N2, 99.999%) gas was used as the carrier gas and the flow rate maintained at 30 mL/min. The organic acids were analyzed by HPLC (Model VP, Shimadzu Co., Japan) after filtered the supernatant in the reactor with a 0.2 μm membrane.

### 2.4. Synergistic Effect

In this study, the amount of CH4 production at mono-digestion of each substrate was used to determine the increased CH4 production at co-digestion. The amount of CH4 production increase by the synergistic effect was calculated by defining as follows (Eq. (1)):
##### (1)
$Synergistic CH4=Ma-((A×C1)+(B×C2))$
where ‘Ma’ was the actual amount of CH4 production, while ‘A’ and ‘B’, were the amount of CH4 production from the mono-digestion of FW or SS, and SW, respectively. ‘C1’ and ‘C2’ indicates the mixing ratios in co-digestion of FW or SS and SW, respectively. In other words, the value of ‘((A × C1) + (B × C2))’ indicates the expected amount of CH4 production in the co-digestion of FW or SS with SW.

### 2.5. Kinetic Model Applied

First-order kinetic model has been widely applied in AD of organic matter, in particular, SS degradation [23, 24]. The amount of cumulative CH4 production in the anaerobic co-digestion of SS and SW was fitted using the first-order kinetic model as shown below (Eq. (2)):
##### (2)
$M(t)=Fd×(1-ekhydt)$
where ‘M(t)’ is the cumulative CH4 production at time (mL CH4); ‘Fd’ is maximum CH4 production potential (mL CH4); ‘khyd’ is hydrolysis rate constant (d−1); and ‘t’ is digestion time (d).

### 3.2. Co-digestion of FW and SW

#### 3.2.1. Performance and synergistic effect

The amount of CH4 production, MPY, and MCE are arranged in Table 2. Biogas production was completed within 14 d, and hydrogen production was negligible during whole experimental period (Fig. S1). The MPY of FW (FW:SW = 100:0) decreased as substrate concentration increased. In particular, there was a huge drop at 10 g VS/L, exhibiting only 103 mL CH4/g VS. In cases of low substrate concentration range (2.5, 5.0 g VS/L) in FW digestion, SW addition did not show any improvement. It rather decreased the MPY, probably due to the less biodegradable characteristics of SW compared to FW. However, in cases of high substrate concentration range (7.5 and 10.0 g VS/L) of FW digestion, the MPY significantly increased, by up to 1.2 and 3.5 times at 7.5 and 10.0 g VS/L, respectively. In terms of MCE, SW addition increased MCE from 60% to 74% and 22% to 76% at 7.5 and 10.0 g VS/L, respectively. By applying Eq. (1), it was found that the ‘Synergistic CH4’ was negligible at low concentration, while it was increased at high concentration with SW addition increase. In particular, 85 mL CH4/g VS of ‘Synergistic CH4’ was produced at a substrate concentration of 10.0 g VS/L and FW:SW = 25:75, accounting for the 24% of total amount of CH4 produced. To explain the reasons for the synergistic effect, some analytical results are shown in the next chapter.

### 3.3. Co-digestion of SS and SW

#### 3.3.1. Performance and synergistic effect

In co-digestion of SS and SW, biogas production was completed within 8 and 14 d, and hydrogen production was negligible during whole experimental period (Fig. S2). Compared to FW and SW, the MPY of SS was lower, ranging 170–180 mL CH4/g VS (Table 4). However, the inhibition at high substrate concentration was negligible, probably indicating that initial steps (hydrolysis or acidogenesis) rather than the final step (methanogenesis) in AD were the rate-limiting step in using SS as a feedstock. Under all substrate concentrations, SW addition to SS substantially increased the digestion performance. For example, MPY and MCE at 10.0 g VS/L increased from 180 to 308 mL CH4/g VS, and from 33 to 64%, respectively. However, it could not be a surprising result since SW had a higher biodegradability than SS as seen in Fig. 1.
Therefore, it is very important to see the synergistic effect in co-digestion of SS by employing Eq. (1). It was found that the ‘Synergistic CH4’ increased with SW addition increase at all substrate concentrations. In particular, 51 mL of ‘Synergistic CH4’ was produced at 10.0 g VS/L and SS:SW = 75:25, accounting for the 20% of the total amount of CH4 produced. Assuming that MPY of SW was equal in all cases, this meant that SW addition triggered the anaerobic biodegradation of SS. To explain the reasons for the synergistic effect, some analytical results are shown in the next chapter.

### 4. Conclusions

The anaerobic biodegradability of SW was found to be between FW and SS, which acted as a suitable co-digesting material for enhanced AD of FW and SS. At low substrate concentration of FW digestion, SW addition showed a negligible effect on AD performance, while it synergistically enhanced the performance by balancing acidogenesis and methanogenesis at high substrate concentrations. At 10 g VS/L and FW:SW = 25:75, MCE increased from 22 to 76% by SW addition. On the other hand, SW addition to SS synergistically enhanced the digestion performance at all substrate concentrations, by providing easily biodegradable organics to anaerobic consortium, which promoted the hydrolysis of SS.

### Acknowledgments

This work is supported by the “R&D Center for Reduction of Non-CO2 Greenhouse gases” (2017002410003) funded by Korea Ministry of Environment (MOE) as “Global Top Environment R&D Program”.

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##### Fig. 1
Cumulative CH4 production yield in the anaerobic digestion of food waste (FW), sewage sludge (SS), and seaweed (SW) at 2.5 g VS/L.
##### Fig. 2
The variation of pH in anaerobic co-digestion of food waste (FW) and seaweed (SW) at (a) 2.5 g VS/L, and (b) 10.0 g VS/L.
##### Fig. 3
The variation of pH in anaerobic co-digestion of sewage sludge (SS) and seaweed (SW) at (a) 2.5 g VS/L, and (b) 10.0 g VS/L.
##### Table 1
Characteristics of Seeding Sludge, Food Waste, Sewage Sludge, and Seaweed
Item Units Seeding sludge Food waste Sewage sludge Seaweed
TSa g/L 31.3 83.6 24.2 26.3
VSa g/L 27.4 77.2 18.9 20.7
CODa g/L 37.8 104.6 29.9 26.9
Alkalinity g CaCO3/L 4.1 N.D.a 1.1 N.D.a
Carbohydrate g COD/L - 62.2 2.0 12.5
TNa g/L - 2.9 2.3 0.6
C/N ratioa - - 36.0 13.0 44.8

a TS = Total solids;

VS = Volatile solids; COD = Chemical oxygen demand; N.D. = Not detectable; TN = Total nitrogen; C/N ratio = COD/TN ratio

##### Table 2
Batch Performances in the Anaerobic Co-digestion of Food Waste (FW) and Seaweed (SW) at various substrate Concentrations and Mixing Ratios
2.5* (100:0)a 2.5 (75:25) 2.5 (50:50) 2.5 (25:75) 2.5 (0:100) 5.0 (100:0) 5.0 (75:25) 5.0 (50:50) 5.0 (25:75) 5.0 (0:100) 7.5 (100:0) 7.5 (75:25) 7.5 (50:50) 7.5 (25:75) 7.5 (0:100) 10.0 (100:0) 10.0 (75:25) 10.0 (50:50) 10.0 (25:75) 10.0 (0:100)
Actual CH4 production (mL) 106 101 97 93 86 198 187 177 167 160 212 226 240 255 245 103 168 265 350 319
CH4 production yield (mL CH4/g VS) 422 402 390 373 344 395 375 353 334 320 282 300 319 340 327 103 168 265 350 319
CH4 conversion efficiencyb (%) 89 76 84 81 76 83 80 76 73 71 60 64 69 74 72 22 36 57 76 70
Expected CH4 productionc (mL) 100 95 91 189 179 170 220 228 237 157 211 265
Synergistic CH4d (mL) 1 2 2 −2 −2 −3 6 12 18 11 55 85

* Substrate concentration unit = g VS/L

a Mixing ratios = (FW:SW) on a VS basis

b Actual CH4 production/theoretical CH4 production (COD basis, 1 g COD = 350 mL CH4)

c It was obtained by “((A × C1) + (B × C2))”, where ‘A’ and ‘B ‘ are the amount of CH4 production from mono-digestion of FW and SW, while ‘C1’ and ‘C2’ are the mixing ratios in co-digestion of FW and SW, respectively.

d Actual CH4 production – Expected CH4 production

##### Table 3
Cumulative Organic Acids Concentration in the Broth in Co-digestion of Food Waste (FW) and Seaweed (SW) at 2.5 and 10.0 g VS/L (Samples were taken after 12 d and 14 d of operation.)
2.5* (100:0)a 2.5 (75:25) 2.5 (50:50) 2.5 (25:75) 2.5 (0:100) 10.0 (100:0) 10.0 (75:25) 10.0 (50:50) 10.0 (25:75) 10.0 (0:100)
Organic acids (mg COD/L) 40 20 50 70 60 7,430 6,530 2,330 1,020 320

* Substrate concentration unit = g VS/L

a Mixing ratios = (FW:SW) on a VS basis

##### Table 4
Performances in the Anaerobic Co-digestion of Sewage Sludge (SS) and Seaweed (SW) at Various Substrate Concentrations and Mixing Ratios
2.5* (100:0)a 2.5 (75:25) 2.5 (50:50) 2.5 (25:75) 2.5 (0:100) 5.0 (100:0) 5.0 (75:25) 5.0 (50:50) 5.0 (25:75) 5.0 (0:100) 7.5 (100:0) 7.5 (75:25) 7.5 (50:50) 7.5 (25:75) 7.5 (0:100) 10.0 (100:0) 10.0 (75:25) 10.0 (50:50) 10.0 (25:75) 10.0 (0:100)
Actual CH4 production (mL) 42 55 70 78 86 87 108 130 154 160 133 169 199 233 245 180 267 278 308 319
CH4 production yield (mL CH4/g VS) 170 220 276 310 344 174 215 261 309 320 177 224 265 310 326 180 267 278 308 319
CH4 conversion efficiencyb (%) 31 42 55 65 76 31 41 52 64 71 32 42 52 65 72 33 50 55 64 70
Expected CH4 productionc (mL) - 53 64 75 - - 105 124 142 - - 161 189 217 - - 216 251 285 -
Synergistic CH4d (mL) - 2 5 2 - - 3 6 12 - - 7 10 16 - - 51 26 23 -

* Substrate concentration unit = g VS/L

a Mixing ratios = (SS:SW) on a VS basis

b Actual CH4 production/theoretical CH4 production (COD basis, 1 g COD = 350 mL CH4)

c It was obtained by “((A × C1) + (B × C2))”, where ‘A’ and ‘B ‘ are the amount of CH4 production from mono-digestion of FW and SW, while ‘C1’ and ‘C2’ are the mixing ratios in co-digestion of FW and SW, respectively.

d Actual CH4 production – Expected CH4 production

##### Table 5
Estimated Parameters Obtained through the First-order Kinetic Model
2.5a (100:0)b 2.5 (75:25) 2.5 (50:50) 2.5 (25:75) 2.5 (0:100) 10.0 (100:0) 10.0 (75:25) 10.0 (50:50) 10.0 (25:75) 10.0 (0:100)
R2 0.98 0.99 0.99 0.98 0.99 0.98 0.99 0.98 0.99 0.99
Fdc 57 66 77 86 92 218 296 302 340 363
khydc 0.40 0.44 0.46 0.50 0.51 0.19 0.26 0.28 0.27 0.24

a Substrate concentration unit = g VS/L

b Mixing ratios = (sewage sludge:seaweed) on a VS basis

c Fd and khyd were the values obtained through the first-order kinetic equation of M(t) = Fd × ((1 − ekhydt), where ‘M(t)’ is the actual CH4 production at time (mL CH4); ‘Fd’ is maximum CH4 production potential (mL CH4); ‘khyd’ is hydrolysis rate constant (d−1); and ‘t’ is digestion time (d).

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