The treatment and the correct disposal of waste is a global concern. It may generate numerous consequences for the environment and for society. It is estimated that the Brazilian leather industry generates more than 202,000t of leather waste tanned with chromium a year.

Anaerobic digestion (AD) can be a promising alternative for the treatment of this waste since it converts waste into energy (biogas). Biogas is composed of methane (60%), carbon dioxide (40%), and trace amounts of water vapour, hydrogen sulphide and ammonia. It is an odourless and colourless gas that produces a blue flame when burned, similar to the one of liquefied petroleum gas. Among the main applications of biogas, heat production, steam production, and combined electricity and heat production can be mentioned.

The conversion of complex organic matter to methane by the AD process requires a consortium of microorganisms. It comprises several groups of anaerobic microorganisms that interact in a metabolic manner.

The substrates most often employed for waste conversion into energy are manure (54%), sewage sludge (22%) and municipal solid waste (11%). In order to improve the AD process, other residues are added to the process: industrial waste (41%), agricultural waste (23%) and municipal waste (20%) being the most common ones.

Co-digestion is defined as the anaerobic treatment of a mixture of at least two different substrates in order to increase the efficiency of the AD process. It makes it possible to adjust the C/N ratio.

The process

AD can be divided into four main phases: hydrolysis, acidogenesis, acetogenesis and methanogenesis. In the first stage (hydrolysis), large molecules of protein, fat and carbohydrate polymers (such as cellulose and starch) are hydrolysed in water-soluble monomers (amino acids, long-chain fatty acids and sugars). Hydrolysis is caused by extracellular enzymes (hydrolases) such as lipases, proteases and cellulases.

In the second stage, the acidogenic bacteria produces intracellular enzymes that convert the hydrolysis products in volatile fatty acids and alcohol. In the third stage, acetogenic bacteria (or homoacetogenics) convert the product of simple organic phase into acids, carbon dioxide and hydrogen, allowing the production of acetic acid, butyric acid, propionic acid and ethanol.

In the fourth stage, the methane can be produced by two routes: from acetate – the conversion of methyl and carboxyl groups of acetate to methane and carbon dioxide by the action of methanogenic bacteria (a process known as direct methanogenesis); and by oxidation of the acetate to carbon dioxide and reduction of the methane. The latter route occurs through oxidising and methanogenic hydrogenotrophic bacteria.

Factors such as pH, temperature and total solids content directly influence the AD process and its products. Low temperatures (12–16°C) can result in the exhaustion of microbial cell energy. In contrast, high temperatures (above 40°C) lead to a low yield of methane in the gas generated due to the production of volatile gases, such as ammonia, which suppresses the methanogenic activity. The optimum pH for the methanogenic bacteria was set around 7.0. There are reports that the hydrolysis and acidogenesis occurs at pH values of 5.5–6.5.

The total solids content of the substrate influences the AD process. Tests conducted with total solids content progressively varying 10–35% reduced the methane production. The efficiency of the AD process can be affected by pre-treatments. They aim to increase the kinetics of the process wastes’ AD and are responsible for an increase in biogas production efficiency. Ultrasound is one of the most commonly used pre-treatments (33%), followed by thermal pretreatment (24%) and chemical pretreatments (21%). The chemical pretreatment conducted in the waste of leather tanned with chromium salts and vegetable tannins provided a higher yield in biogas production than the samples without pretreatment.

The objective of this study was to evaluate the influence of an inoculum addition in leather-industry waste for biogas generation.

Study materials and methods

Parboiled rice waste was provided by Caxiense Company of Biological Control (CCBC). It was obtained from a culture of the fungus trichoderma, which is used for preventive and curative control of diseases caused by phytopathogenic fungi in plants. The tannery sludge (TLS) was obtained from a local tannery wastewater treatment plant (WWTP). The same company provided chromium-tanned leather shavings. The sludge from the University of Caxias do Sul (UCS) WWTP was made available by the institution.

The determination of the percentage of moisture and total solids content (TS) of rice and leather wastes were based on the method ASTM D3790-12. The total solids content of WWTP sludge samples was analysed based on the standard ABNT NBR 14550. The organic carbon content was determined by means of the sample carbon oxidation in acidic medium followed by titration with ferrous sulphate.

The microorganisms previously isolated (five in total) were used to prepare the inoculum. Aliquots of isolates were added to the LB culture medium (liquid medium – LM) and placed in a thermostatic bath at 35°C until medium optical density (OD), measured at 600nm, equal to 1.0.

The influence of the addition of the inoculums was evaluated through the volume and molar gas fraction. The gas samples were generated by leather pretreated with a chemical and thermal process, and by TLS added to rice waste. The thermal treatment of leather waste consisted of placing it in an airtight glass container and autoclaving it (in Prismatec, model CS, equipment) at a pressure of 1atm for 15 minutes. The chemical pretreatment method used 200% distilled water, 0.003g of oxalic acid and 0.003g of ethylene diamine tetra acetic acid (ETAA).

The samples were placed in flasks with a total volume of 100ml. The test was conducted at 35°C for as long as biogas was being generated. Inoculum or LM was added after 24 hours of testing and then every seven days, totalling five additions. The volume of inoculum or LM was added such that the final TS content did not exceed 9%. After the tests, the reduction of organic carbon and TS was evaluated.

The volume and the molar fraction of the biogas generated in each sample were measured daily. The verification of the generated gas volume was performed using the volumetric method. For the chromatographic analysis, nitrogen was used as carrier gas. Thermal conductivity detector and capillary column Supelco Carboxen TM 1006 (30m×0.53mm) were employed. For sample collection, a Gastight 1ml Hamilton syringe was used.

Study results

It may be observed that the leather waste with addition of inoculum generated a larger volume of biogas than the samples with the addition of LM and to the ones of leather waste without the addition of inoculum. The gas generated was predominantly composed of carbon dioxide. The molar fraction of hydrogen in the samples with addition of LM (0.10) decreased when compared with the samples with the addition of inoculum (0.24). This result confirms the importance of the presence of microorganisms that assist in the AD process.

The predominant molar fraction of carbon dioxide in the gas produced from leather waste and its combinations causes a contamination in the biogas, reducing its calorific value. However, the addition of CO2 in digesters to treat food waste and sludge from municipal WWTP was evaluated. The authors added different partial pressures of CO2 in a reactor. They noticed an increase of 13% for methane production and consequent carbon dioxide reduction on the gas obtained, ranging 8–34% for sludge of municipal WWTP. According to the authors, with CO2 enrichment, the acetoclastic microbial activity was increased, which resulted in an increase in the molar fraction of methane.

It can be seen from the results that the sample containing rice and TLS presented similar gas volumes and hydrogen molar fraction close to 1.0, independently of the addition of inoculum or LM.

Acetogenic bacteria typically produce hydrogen and, for each four moles of hydrogen consumed by methanogenic hydrogenotrophic microorganisms, one mole of carbon dioxide is converted to methane.

The fermentation process produces hydrogen from the decomposition of carbohydrates present in waste. polysaccharides, such as starch, are hydrolysed and converted into low molecular weight compounds that are transported into the interior of the bacterial cells and converted into products of interest, such as hydrogen.

The step of AD that produces hydrogen is enabled by acidogenic microorganisms, which decompose the substrates in hydrogen, carbon dioxide and volatile fatty acids.

It is possible to confirm that the carbon contents and TS, after the process, reduced. The rice waste with TLS and inoculum addition showed greater reduction in carbon (77.88%) and in TS content (90.34%).

It indicates an effective AD process of the wastes studied in this work and shows the importance of inoculum addition to improve the process.

The addition of the inoculum to wastes of leather chemically and thermally pretreated resulted in an increase in the generated gas volume. The molar fraction of hydrogen generated in samples with added inoculum was higher than the samples with addition the of LM. The predominant molar fraction of the biogas generated from the addition of tannery sludge to rice waste was hydrogen (approximately 1.0) independently of the addition of inoculum. The reduction in organic carbon and TS showed an effective anaerobic digestion process in the tests performed in this study.

Full paper with references available upon request.