Enhanced Anaerobic Digestion of Sewage Sludge by Addition of Food Waste

Li HAN 1,a, Ruying LI 1,b and Min JI1,c 1School of Environmental Science and Engineering, Tianjin University, Tianjin, China

ahanlitju@163.com, bliruying@tju.edu.cn, cjimin@tju.edu.cn

Keywords: sewage sludge; food waste; anaerobic digestion; methane

Abstract. In order to improve the methane yield and removal efficiency of organic matters in

anaerobic sludge digestion, effects of addition of food waste were investigated at mesophilic

condition. Results showed that the optimal TS ratio between sewage sludge and food waste was 4:1,

with a methane yield of 592.7 ml/g-VS, methane content of 66.84% and the VS removal efficiency

of 31%, which were 47%, 50% and 55% higher than those of sole sludge digestion, respectively.

Introduction

Management of sewage sludge is one of the main environmental issues in China, since the

amount of sludge has been increasing dramatically during the past decade. Anaerobic digestion can

achieve removal of organic pollutants in sludge, and simultaneously produce methane energy.

However, the C/N ratio of sludge is only about 5:1, which limits its digestion performance. Food

waste contains great amount of organics and has much higher C/N ratio than sludge.

Co-fermentation of sewage sludge and food waste can contribute to the balance of nutrients in the

substrate[1], dilution of toxic components in the sludge, neutralization reaction between volatile

fatty acids (VFA) and ammonia in order to maintain the stability of the pH value in the fermentation

process[2], which can improve the digestion efficiency.

In this study, effects of addition of food waste on mesophilic anaerobic sludge digestion were

investigated in batch experiments. Methane yield, methane content of biogas, and VS removal

efficiency were examined to select an optimal mixing ratio of sewage sludge and food waste for

enhancement of digestion. Variation of organic matters in the mixed liquor such as

soluble-carbohydrate and soluble-protein were also investigated.

Materials and Methods

Feedstock and Seed Sludge. The feedstock for anaerobic digestion was mixture of sewage sludge

and food waste. Sewage sludge was obtained from Jizhuangzi Wastewater Treatment Plant in

Tianjin China. The total solid (TS) content of sludge was 3.6% and the VS/TS ratio was 59.3%.

Food waste, sampling from dining hall in Tianjin University, was crushed by a blender and diluted

to TS content of 21.0%. The seed sludge was obtained from an anaerobic digestion reactor with TS

of 9.7% and VS/TS of 43%.

Experimental Conditions. Batch experiments were conducted using 300 ml serum bottles filled

with 150 ml of substrates. The initial concentration of sludge was 10g-VS/L. The sewage sludge

and food waste were mixed by TS ratio of 1:1, 2:1, 3:1, 4:1 and 5:1, respectively, and the sole

sewage sludge and sole food waste served as controls. Batch experiments were conducted in

parallel, and the seed sludge in each bottle was 6 g. All the bottles were purged with N2 before

sealing. The initial pH was 7.0-7.2. Experiments were conducted for three cycles. At the end of each

cycle, 50 ml of residual substrate was reserved and 100 ml of fresh substrate was added for the next

cycle.

Methods of Analysis. Volume of biogas produced was measured by a glass syringe. Composition of

biogas was analyzed by a gas chromatograph (BEIFEN 3040, China) equipped with a thermal

conductivity detector and a stainless steel packed column (TDX-01, 2 m). The soluble carbohydrate

was analyzed using anthrone-sulfuric acid method [3], and the soluble protein was analyzed by

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Lowry method [4]. The samples for analysis of soluble organics were filtrated by 0.45 µm

membrane before detecting. TS, VS and ammonium were determined according to standard

methods [5].

Results and Discussion

Methane Production and Yield. The cumulative methane production under different mixing ratio

between sewage sludge and food waste is shown in Figure 1. The methane volume produced in the

first cycle was higher than the other two cycles because the amount of substrate was only 2/3 of that

in the first cycle. Figure 1 indicated that the cumulative methane production increased with food

waste proportion and co-digestion had much higher methane production than sole sludge or food

waste digestion. During the three cycles of anaerobic digestion process, the methane content in

co-digestion was 66.9% -76.9%, which was 50% -200% higher than those of sole sludge or food

waste digestion, and there was no obvious difference among different mixing ratios. Therefore,

co-fermentation could obviously improve the methane production and methane content in biogas.

Fig. 1 Cumulative methane production under different mixing ratios between sewage sludge and

food waste

Fig. 2 Methane yield at different mixing ratios between sewage sludge and food waste

Figure 2 illustrates the methane yield at different mixing ratios between sewage sludge and food

waste. Figure 2 showed that methane yields of co-digestion were higher than that of sludge or food

waste fermentation alone, and showed increased by cycles, which may be due to the increased

activity of anaerobic sludge in the later cycles after cultivating. Compared to sludge or food waste

digestion, methane yields of co-digestion were increased by 37%-57% and 112%-606%. Similar

results were also observed by Zhao [6] and Fu [7], which were 327 mL/ g-VS and 373 mL/ g-VS at

the mixing ratio of 1:1.

140 Environmental Biotechnology and Materials Engineering (2013)

The methane yield of 89.7 ml/g-VS for food waste digestion was much lower than its theoretical

yield. This is likely due to accumulation of fatty acid and rapidly pH decrease in the system. The

system could not be recovered by adjusting pH value. Gao [8] and Beno [9] reported similar

findings that food waste digestion ceased in 24 hours and little methane was produced due to

inhibition by fatty acids.

VS Removal Efficiency. Figure 3 shows the VS removal efficiency at different mixing ratios

between sewage sludge and food waste. VS removal efficiency showed similar trends in three

digestion cycles, which increased with the proportion of food waste. The highest VS removal

efficiency was 55.8%, at the mixing ratio of 1:1 in the first cycle, which was nearly two times of

those for sole sludge or food waste digestion. Thus, addition of food waste could also improve the

degradation efficiency of organic matters.

Fig. 3 VS removal efficiency at different mixing ratios between sewage sludge and food waste

Soluble Organic Matters. Table 1 indicates the concentration of soluble organic matters in the

mixed liquor under different mixing ratios between sewage sludge and food waste. It was shown

that the concentration of the soluble carbohydrate dramatically decreased and the degradation

efficiency for all different mixing ratios were more than 95%, which indicated that the

carbohydrates contributed most for methane production during digestion. Table 1 also shows that

the concentration of ammonia increased at the end of digestion because of the degradation of

protein. The concentration of soluble protein also increased at the end of digestion, which suggested

that dissolution rate of protein was higher than its degradation rate. It was reported that 1500～3000

mg/l of ammonia would inhibit anaerobic digestion [10]. The ammonia concentration in this study

was 180-290 mg/l, which was much lower than the reported inhibition concentration.

Tab.1 Concentrations of soluble organics under different mixing ratios between sewage sludge and

food waste

Mixing

ratio

Soluble carbohydrate

（mg/l）

Soluble protein

（mg/l）

Ammonia

（mg/l）

Input Output Input Output Input Output

1:1 2300.7 78.1 277.9 690.1 75.2 286.1

2:1 1783.7 38.2 180.2 475.0 60.23 242.9

3:1 1278.5 40.7 145.8 392.9 60.5 228.2

4:1 987.0 43.7 130.6 321.2 52.7 215.0

5:1 799.2 37.4 110.7 275.0 48.8 179.9

sludge 109.7 50.2 80.3 233.0 35.9 180.3

food waste 2542.6 50.6 251.4 650.8 80.9 215.8

Advanced Materials Research Vol. 777 141

Summary

Addition of food waste as a co-substrate to sewage sludge can enhance the performance of

anaerobic digestion. The optimal mixing ratio of sewage sludge and food waste based on TS was

4:1,with the methane yield and methane content in biogas of 592.7ml/g-VS and 66.8% respectively,

which were 47% and 50% higher than those in anaerobic digestion of sole sludge. Soluble

carbohydrate was almost consumed for digestion and the soluble protein and ammonia increased

during digestion.

Acknowledgements

Authors are grateful to the financial support from the National High Technology Research and

Development Program (863 Program), China (Project No. 2012AA063502).

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142 Environmental Biotechnology and Materials Engineering (2013)

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