Production of Biogas From the Anaerobic Co-digestion of Tannery Fleshing and Cow-dung Enhanced by Sodium and Calcium Alginates

Production of Biogas From the Anaerobic Co-digestion of Tannery Fleshing and Cow-dung Enhanced by Sodium and Calcium Alginates

Chapter One

Aim and Objectives

The primary aim of this investigation is to produce biogas by the anaerobic co-digestion of tannery fleshing and cow-dung; and enhance it by introducing sodium and calcium alginates. This can be achieved through the following objectives:

To produce biogas from fleshing with cow-dung as carbon booster and carrier of anaerobic microbes.
To increase methane production by introducing sodium and calcium alginates into the digesters.
To ascertain desulphurizing properties of sodium and calcium alginates within the digesting system.

CHAPTER TWO

LITERATURE REVIEW

Background

In Nigeria, tanning and finishing of leather in the mechanized way started in Kano in 1949, and currently not less than 32 tanneries have been established in the country. The installed capacity of the tanneries for the production of wet-blue leather is above 8.5 million kilograms (17 million pieces) per annum. If processed to finished leather, this will earn the country about 12 billion naira per annum at the selling price of 400 naira per square foot (Paiko, 2002). Nigeria‘s leather industry export potential is currently valued at 6.9 trillion naira in comparison to the world‘s current production value of 11.5 trillion naira (Siaka, 2012). The economic benefits derived from the leather industries are enormous; however, the negative environmental impact of its effluent is a major setback (Rao et al, 1997). During a typical tanning process at least 300kg chemicals (lime, salt etc.) are added per ton of hides. The beam house is the source of all non-limed and limed solid wastes such as fleshing, trimming and waste split (Buljan, 1994). According to Srinivasan et al., (2010), 15% of total thickness of raw skin/ hides is removed as fleshing; this translates to about 37.5kg of fleshing daily from Nigerian Institute of Leather and Science Technology, Zaria, with installed capacity of 250kg.

The Tanning Process (a general overview)

The processing of hides and skins in a tannery can be split into the following four main headings:

Preservation of hides and skins.
Beam house operations.
Tanning operations.
Post-tanning and finishing operations.

Beam house operations

Cleaning and conditioning of hides and skins takes place at this stage and it produces the largest part of the effluent load. It involves soaking, unhairing and fleshing (liming), deliming and bating, and pickling.

Pollutants in a Tannery

Solid waste is inherent to manufacture of leather from skin and hide. Solid wastes generated at various unit operations of the tanning process considerably vary in quantity and composition. Statistically, the quantity of the world leather produced is about 1.5 x 10¹⁰ kg per day. Out of 1000kg of raw hide, nearly 850kg is generated as solid wastes in leather processing. Only 150kg of the raw material is converted into leather. Tannery generates huge amount of solid waste with the following approximate composition (Kanagaraj et al., 2006):

fleshing 50-60%;
chrome shaving, chrome splits and buffing dust, 35 -40%;
skin trimmings, 5-7%; and
hair, 2-5%.

Solid waste generation from tannery process is estimated at 6 x 10⁹kg per year (Rajamani et al., 2009). Solid waste includes salt (sodium chloride) from preserved raw skin/hide, dusting from raw skin/hide trimmings; hair from the liming/unhairing process which may contain lime and sulphide; and fleshing from raw skins/hides.

Fleshing

Fleshing is a type of animal tissue waste generated during the preparatory leather processing stage in relatively large quantities as compared to other types of solid waste in the tanning industry. Fleshing mainly contains fat and protein and residual chemicals such as lime and sulphide used in the ‗unhairing‘ process. Fleshing contain a significant quantity of volatile solids amenable to biodegradation. Biomethanation of fleshing and solid sludge from primary and secondary treatments is an economically viable option for secured disposal (Colak et al., 2005). Different proportions of waste fleshing and primary sludge can be subjected to anaerobic digestion in a laboratory scale reactor with the aim of developing an appropriate technology for recovery of bio-energy from the waste and subsequently ensure their safe disposal.

CHAPTER THREE

MATERIALS AND METHODS

Introduction

This chapter presents the materials and methods used in the production of biogas from co-digesting tannery fleshing and cow-dung; and reduction of its sulphide concentration. The values of BOD, COD, Chloride, Sulphide, Ammonia-N and Calcium were preliminarily determined for the beam house liquors. These liquors were sourced from the tannery in Nigerian Institute of Leather and Science Technology (NILEST), Zaria, Kaduna state. The liquors characterized were soaking liquor, liming liquor and deliming liquor. The unhairing liquor was not considered due to its inherent high sodium sulphide load and hairs, both of which are inhibitory to methanation. This was followed by a selection of the most favorable liquor for digesting with the substrates. The substrates used were cow-dung (sourced from the Abattoir, Zango Zaria) and fleshing sourced from NILEST, Zaria). The digestion experiments were carried out in the Multiuser Laboratory, Ahmadu Bello University, Zaria in different batches, and every cycle was for a period of 50 days. Biogas concentration readings were taken at intervals of one day using a Biogas Analyzer at the Multiuser Laboratory, Ahmadu Bello University, Zaria. This work adopted the sulphide reduction method using sodium alginate as recommended by Dublein and Steinhauser (2008). The detailed methodology carried out in this order is further explained below.

CHAPTER FOUR

RESULTS AND DISCUSSION

Introduction

This study focuses on the concentrations of methane and hydrogen sulphide gases obtained from co-substrates in various anaerobic digestion cycles which were analyzed via a gas analyzer, Biogas 5000. Batch systems were applied for each anaerobic digestion process according to Gregor and Viktor (2012) who proffered that a single digester is sufficient for good performance for digestion of protein rich substrates like animal fleshing. Temperature was maintained at room conditions between 29^oC and 31^oC throughout the digestion processes, pH between 6.5 and 7.4 were averagely experienced. The volatile fatty acid was also measured 3 times in a cycle following Kapp, (1984) method, and was found to be below inhibitory concentration of 2,000 mg/l. Microsoft Excel graph-plotting, bar chart tools and tables were used to represent the data obtained.

CHAPTER FIVE

CONCLUSION AND RECOMMENDATIONS

This chapter presents a summarized conclusion on this study, recommendations and its contributions to knowledge.

Conclusion

From the work, the following conclusions were made:

The soaking liquor obtained from the beam house of Nigerian Institute of Leather and Science Technology, Zaria‘s tannery, is best suited for use as diluent in the anaerobic digestion. The composition of biogas obtained from using the soaking liquor gives the highest methane and lowest hydrogen sulphide concentrations.
Cow-dung is concluded to be a viable co-substrate in the anaerobic digestion of tannery fleshing. It was able to boost methane concentration about 8 (eight) times. The ratio 1:2 of fleshing to cow-dung can be used to produce biogas of 45% v/v methane concentration.

The use of sodium alginate and calcium alginate beads in the ratio 0.01% wt/v boosted methane production (concentration) above 20% and reduced sulphide concentration reaching 0ppm. Though, addition of sodium alginate gave highest methane concentrations in shorter retention time and higher biogas volume than addition of calcium alginate beads. The calcium alginate beads were generally limited due to mass transfer phenomenon.

Contributions to Knowledge

The following are the contributions this study has provided to knowledge

The soaking liquor amongst other beam house liquors is better suited as diluent in the anaerobic co-digestion of tannery fleshing and cow-dung without any form of treatment.
Sodium alginate and calcium alginate beads enhanced methane production and inhibited sulphide in biogas produced from the anaerobic co-digestion of tannery fleshing and cow-dung; with sodium alginate performing better.

Recommendations

The following are some recommendations made from this study.

Further research should be carried out on the use of sodium alginate in the anaerobic digestion of other substrates, to further ascertain methane boosting and desulphurizing potentials.
Equipment for detecting all anaerobic digesting parameters should be readily available and accessible for proper monitoring of the digesting system.
A pilot plant that will employ the data obtained from the use of sodium and calcium alginates in the anaerobic co-digestion of tannery fleshing and cow-dung should be designed to fit into tanneries for solid waste treatment.

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