Non-airtight fermentation of sugar beet pulp with anaerobically digested dairy manure to provide acid-rich hydrolysate for mixotrophic microalgae cultivation

doi:10.1016/j.biortech.2019.01.075

Bioresource Technology

Volume 278, April 2019, Pages 175-179

https://doi.org/10.1016/j.biortech.2019.01.075 Get rights and content

Highlights

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Non-airtight fermentation of sugar beet pulp with digested dairy manure was tested.
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The hydrolysis process of non-airtight fermentation produced up to 8.1 g/L VFAs.
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Acid-rich hydrolysate was used for microalgae cultivation.
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Microalgal growths in hydrolysate and original digested dairy manure were compared.
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Logistic models were established for microalgal growths in different wastewaters.

Abstract

Non-airtight fermentation of lignocellulosic agricultural residues with animal wastes is an emerging pretreatment method to produce acid-rich substrates in two-phase anaerobic digestion. Acid-rich hydrolysate could be an excellent feedstock for cultivating microalgae, therefore, the feasibility of a two-step process combining non-airtight fermentation of sugar beet pulp with anaerobically digested dairy manure and mixotrophic microalgae species Chlorella cultivation in the hydrolysate was explored in this study. The hydrolysis and acidification process of 8-day non-airtight fermentation produced up to 8.1 g/L volatile fatty acids under mesophilic condition. Microalgal growths in diluted hydrolysates were compared with that in diluted digested dairy manure (DDM) as a control using experimental data and fitted logistic models. Chlorella grown in the 10-fold diluted DDM showed an exponential decay, while Chlorella cultured in the 3-fold diluted hydrolysate demonstrated the best performance in terms of biomass density, which reached 2.17 g/L within a short period of time.

Introduction

Idaho ranks second in the production of sugar beets in the US, providing 20% of the total national yields (Idaho State Department of Agriculture, 2018). About 175,000 acres of sugar beets were planted and harvested, generating more than 6,000,000 tons of beets each year. Sugar beet pulp (SBP) is a by-product of refined sugar production from beets. After the sugar extraction process, the dry-matter content of the pressed SBP lies in the range of 18–23% and crude fibers, especially, cellulose constitute about 15–35% of the total dry weight of SBP (McCready, 1966). The crop of beets harvested in Idaho yielded more than 1.3 million tons of pressed SBP (calculated based on 22% dry content) annually, according to Legrand (2015) who reported 1 ton of sugar beets yielded approximately 150 kg of sugar and 50 kg of drum dried dehydrated SBP with around 11% moisture. To date, SBP has been mostly used as animal feed or wasted in landfill dumping in regions with no livestock farming (Kühnel et al., 2011). The combined feature of high lignocellulosic content and low protein content of SBP makes its commercial value relatively low, therefore, other value-added usages of SBP are worthy of deployment in order to diversify its application and provide an optional disposal route for waste SBP.

Dairy is another booming industry in Idaho, with more than 500 dairies and nearly 579,000 milk cows within the state as of March 2016, generating huge amounts of dairy manure. Anaerobic digestion (AD) is a recommended treatment method for dairies at operation scales of thousands of cows to recover energy residing in the high contents of organic matters in manure and generate electricity that can be put into grid for residential uses. Since there are still abundant nitrogen and phosphorus left over after AD, anaerobically digested dairy manure (DDM) can be used as a suitable nutrient supplement for cultivating microalgae, which can be further used as valuable biomass feedstocks for fuel, feed or fertilizer (Wang et al., 2010, Ruiz et al., 2016, Markou et al., 2018). However, it has been noticed that the growth rates of microalgae in DDM were not competitive enough to produce significant amount of biomass in a short time period (Mulbry et al., 2008, Wang et al., 2010), due to the deficiency of available carbon sources. A substantial portion of carbonaceous materials, measured in the form of chemical oxygen demand (COD), has been degraded by miscellaneous bacteria in digestion process to produce biogas through AD. Employing the unique feature of utilizing organic carbon as the carbon source and light as the energy source simultaneously, mixotrophic microalgal growth is influenced by many cultivation parameters (Zhu et al., 2016, Stiles et al., 2018), such as the characteristics of the wastewater, inoculum species and density, light intensity and penetration, ambient temperature, bioreactor configuration and operation strategy, etc. The characteristics of the wastewater, referring to the nutrient or elemental composition, could play the most important role in the algal culture given that other parameters have to be set in real applications.

Recently, non-airtight fermentation of lignocellulosic agricultural residues with animal wastes has emerged as a pretreatment step to produce acid-rich substrates for more efficient biogas generation in two-phase anaerobic digestion. Yu et al. (2017) studied effects of solid content, temperature, and mixing mode on hydrolysis and acidification of the mixture of rice straw and cow dung in a non-airtight acidogenic system. Their results indicated that high solid content played a key role in promoting non-airtight hydrolysis, while the temperature (35 °C and 55 °C) and mixing mode (unmixed, intermittent mixing and continuous mixing) had no obvious effect on the whole process of hydrolysis and acidification. Lim and Wang (2013) demonstrated that microaeration applied to brown water and food waste led to higher COD solubilization, greater volatile fatty acids (VFA) accumulation and more acetate in total short chain fatty acids. Díaz et al. (2011) studied the effect of limited oxygen supply in the degradation of cellulose and subsequent anaerobic methane production. Their results indicated that the microaerobic treatments presented a higher specific methanogenic activity compared to the anaerobic ones and the period taken to reach the maximum methane production was shortened from 19 days to 15 days in the microaerobic tests compared to the anaerobic test, confirming the positive effect of microaeration on cellulose degradation.

In this study, SBP was used as an example of lignocellulosic biomass to be non-airtightly fermented with digested dairy manure in order to provide more organic carbon sources for microalgae cultivation. Mixotrophic culture of microalgae could be very efficient if there are readily usable organic carbon sources in the culture media. Chlorella is a competent microalgae species that can utilize broad ranges of small molecular organic acids, such as acetic acid, propionic acid, butyric acid, citric acid, lactic acid, etc. (Eny, 1951, Liu et al., 2013). Acid-rich hydrolysate is also an excellent feedstock for cultivating protein-rich microalgae, therefore, the feasibility of a two-step process combining non-airtight fermentation of SBP with DDM and mixotrophic microalgae species Chlorella cultivation in the acid-rich hydrolysate was explored in this study. The hydrolysis and acidification process of the non-airtight fermentation was monitored. Microalgal growths in 3- and 10- fold diluted hydrolysates were compared with that in 10-fold diluted original DDM as a control using experimental data and logistic models.

Section snippets

Substrate and inoculum

Sugar beet pulp was collected from a local sugar production factory having the capacity of processing ∼7000 tons of beets and generating ∼1000 tons of sugar per day. Wet SBP with a moisture content of 75–78% was produced with the sugar processing and most of them was sold to local farms as animal feed. Anaerobically digested dairy manure was collected from a dairy manure based anaerobic digester operated on a commercial dairy farm located in Southern Idaho. The collected SBP was shredded to

Hydrolysis and acidification process of non-airtight fermentation of the mixture of SBP and DDM

Fig. 1 presents the clear trends of TS and VS reductions during the 8-day experimental period of non-airtight SBP fermentation with DDM. The original TS of the SBP and DDM mixture was about 5.1%, which was composed of about 54.5% SBP total solids and 45.5% DDM total solids. At the end of the fermentation period, TS and VS of the mixture were reduced by 34.4% and 41.1%, respectively. The reduction rates were higher than those in the study conducted by Yu et al. (2017), who achieved VS reduction

Conclusions

A two-step process combining non-airtight fermentation of SBP with DDM and mixotrophic microalgae species Chlorella cultivation in the acid-rich hydrolysate was demonstrated to be feasible in this study. The hydrolysis and acidification process of the non-airtight fermentation produced up to 8.1 g/L VFAs under mesophilic condition, which was essential to support vigorous microalgal growth. Chlorella grown in the 10-fold diluted DDM showed an exponential decay, while Chlorella cultured in the

Acknowledgements

This study is supported by the USDA National Institute of Food and Agriculture (NIFA), Western Sustainable Agriculture Research and Education project-SW18-015.

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Non-airtight fermentation of sugar beet pulp with anaerobically digested dairy manure to provide acid-rich hydrolysate for mixotrophic microalgae cultivation

Highlights

Abstract

Introduction

Section snippets

Substrate and inoculum

Hydrolysis and acidification process of non-airtight fermentation of the mixture of SBP and DDM

Conclusions

Acknowledgements

Bioresour. Technol.

Waste Manage.

Bioresour. Technol.

Biotechnol. Adv.

Bioresour. Technol.

Food Res. Int.

Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

Waste Manage.

Bioresour. Technol.

Waste Manage.