Semi-industrial production of methane from textile wastewaters
© Opwis et al; licensee Springer. 2012
Received: 11 October 2011
Accepted: 17 January 2012
Published: 17 January 2012
The enzymatic desizing of starch-sized cotton fabrics leads to wastewaters with an extremely high chemical oxygen demand due to its high sugar content. Nowadays, these liquors are still disposed without use, resulting in a questionable ecological pollution and high emission charges for cotton finishing manufacturers.
In this paper, an innovative technology for the production of energy from textile wastewaters from cotton desizing was developed. Such desizing liquors were fermented by methane-producing microbes to biogas. For this purpose, a semi-industrial plant with a total volume of more than 500 L was developed and employed over a period of several weeks.
The robust and trouble-free system produces high amounts of biogas accompanied by a significant reduction of the COD of more than 85%. With regard to growing standards and costs for wastewater treatment and disposal, the new process can be an attractive alternative for textile finishing enterprises in wastewater management, combining economic and ecological benefits.
Moreover, the production of biogas from textile wastewaters can help to overcome the global energy gap within the next decades, especially with respect to the huge dimension of cotton pretreatment and, therefore, huge desizing activities worldwide.
In the manufacturing of cotton yarns to textile fabrics, so-called sizing agents are needed in order to protect the warp threads against huge mechanical stress in the weaving process. Apart from synthetic agents, starch is still the most important sizing agent, and more than 1 million tons of starch are used per year worldwide. After weaving, the starch has to be removed from the raw cotton fabric to avoid impairments in the following finishing steps such as scouring, bleaching, and dyeing. Since many decades, the desizing of starch-sized cotton has been conducted by the use of α-amylases, which are able to hydrolyze the water-insoluble starch to water-soluble oligosaccharides under moderate conditions compared to the harsh oxidative procedures [1, 2]. However, the enzymatic desizing procedure leads to wastewaters with very high sugar content and therefore, an extremely high chemical oxygen demand [COD]. Nowadays, these liquors are still disposed without use, resulting in a questionable ecological pollution and high emission charges for cotton finishing manufacturers.
In principle, the sugars from desizing are ideal substrates for the biological fermentation to biogas. The biogas process itself is well known, using various microbes for the step-wise transformation of native biomass such as carbohydrates, proteins, and fats to methane. Besides energy from wind, water, and sun power, the generation of energy from organic materials - especially undissolved solid residuals from agriculture - plays the most important role in the production of regenerative energy [9, 10]. Apart from the obvious wastes from domestic homes and especially economic plants such as corn, wheat, rice, and all kinds of vegetables and fruits, various industry-specific recycling strategies have been developed to generate an added value in the production chain and to minimize pollutions. This includes the biogas generation from wastewaters coming from, e.g., food, chemical, and pharmaceutical industries [11–17]. In the textile industry, and especially here in the cotton manufacturing, various scientific groups are investigating the biogas or bio-ethanol production starting from solid residuals such as willow dusts or fiber wastes with a subsequent enzymatic or chemical hydrolysis of the cellulosic material [18–21].
Here, the pre-hydrolyzed saccharides coming from the enzymatic desizing step in cotton pretreatment are already in the liquefied state and can be applied directly to anaerobic fermentation. In our previous work, we described this process (Figure 1, right), dealing with the biological transformation of such sugar-containing wastewaters from textile desizing liquors to methane-containing biogas . After showing the principle feasibility of the new innovative strategy, here, the change to more industrial relevant continuous reactor conditions and the upscaling is described and completed by an economic and ecological outlook of the overall process.
Especially in these days of climate change, where huge polluting catastrophes such as the oil disaster in the gulf of Mexico (2010) and the nuclear accident in Japan (2011) are querying more and more the energy supply by fossil fuels and nuclear power, alternatives from renewable resources, wind, water, and solar energies, are essential to overcome the energy gap within the next decades. Therefore, the present investigation is another contribution to make this world cleaner and safer.
Semi-industrial biogas plant
The COD, the total organic carbon [TOC], and the total nitrogen of the desizing liquors were measured using the testing kits HT-COD LCK 014, TOC LCK 387, and LatoN LCK 338 (HACH LANGE GMBH, Duesseldorf, Germany), respectively. The element contents of the initial desizing liquor were determined according to DIN EN 13346. About 100 mL of desizing liquor was liberated from water by heating and followed by drying for 6 h at 80°C. About 300 mg of the waterless residue was treated with 6.0 mL of concentrated HNO3 (65%) and 2.0 mL of concentrated HCl (37%) digested in a microwave digester (MARSXpress, CEM, Kamp-Lintfort, Germany) at 180°C. The completely digested samples as a clear solution were transferred to a 25-mL volumetric flask and subsequently diluted with distilled water. The samples thus prepared were analyzed using an inductively coupled plasma optical emission spectrometer [ICP/OES] (720-ES, Varian Medical Systems GmbH, Darmstadt, Germany) to determine the element concentrations.
Toxicological studies (TTC test)
The dehydrogenase activities of living microorganisms reduce the redox indicators such as the colorless triphenyl tetrazolium chloride [TTC] to red formazan (absorption maximum at 480 nm). Therefore, 4.5 mL of desizing liquor (i.e., 4.5 mL of 2% aqueous glucose solution as a blank test) was inoculated with 4.5 mL of bacterial population from the methane reactor and 1.0 mL TTC (51 mg/mL). After an incubation time of 2, 4, and 24 h, the color generation was measured. The red color indicates living cells, i.e., a nontoxic behavior of the medium.
Results and discussion
The outgassing starts rapidly with a strong generation of biogas within the first 120 h, where approximately two thirds of the total biogas amount was accumulated. Afterwards, the curve flattened significantly, and after 360 to 400 h (ca. 15 to 16 days), the biogas generation was finished to the greatest possible extent. The experiment was interrupted after 524 h. At this moment, 27 m3 of biogas was generated per cubic meter of textile desizing liquor. The methane content within the produced biogas amounted to 51%.
The hydraulic design of the semi-industrial plant was aligned with the results from the lab-scale reactor and the static gas emission experiments. The measured dwell time (15 to 16 days) was supplemented by a conservative overhead of 50%, leading to a maximal expected dwell time in the new reactor of 24 days. The main fermenter has an absolute volume of 430 L, and therefore, a needed input of 18 L of desizing liquor per day resulted in a calculated biogas production of 486 L/day.
Chemical composition of biogas generated in the semi-industrial fermentation plant
Method of determination
According to EN ISO 69746
According to EN ISO 69746
According to EN ISO 69746
According to EN ISO 69746
According to EN ISO 69746
According to DIN 518558
Moreover, a high reduction of the COD within the drain outlet was detected. Figure 8 demonstrates a relative decrease of COD over the regarded period of 72 days. Compared to the lab-scale reactor, the semi-industrial plant exhibits a further improvement of the degradation performance to average levels of more than 85% due to the higher dwell time of 24 days (instead of 15 days within the lab-scale reactor).
To implement an economic and ecological sustainable textile finishing of cotton, innovative technologies are becoming more and more important which enable the recycling process of water, textile auxiliaries, or energy. The creation of such in-house cycles leads to an increased cost-effectiveness and a minimization of emissions.
In this context, the aim of our work was the development of an easy and inexpensive strategy for the minimization of high COD loads in textile wastewaters occurring in the enzymatic desizing of cotton fabrics accompanied by the generation of energy. This has been succeeded by the biological transformation of the sugar-containing wastewaters to biogas with methane-producing microbes.
After demonstrating the general feasibility of the envisaged process on a lab-scale, a semi-industrial plant was developed and employed over a period of 72 days under continuous and therefore, praxis-relevant conditions. The results show that also the upscaled system produces huge amounts of biogas with a high methane content of almost 60 vol.% in a robust and trouble-free way. The COD reduction in the wastewater of more than 85%, on one hand, and the production of a valuable energy source, on the other, yield two economic advantages.
Thus, an innovative technology for the production of energy from textile wastewaters was developed. With regard to growing standards and costs for wastewater treatment and disposal, the new process can be an attractive alternative for textile finishing enterprises and is accompanied with economic and ecological benefits.
Moreover, the production of biogas from textile wastewaters is another brick to overcome the global energy gap within the next decades, especially with respect to the huge dimension of cotton pretreatment (global annual output 25 million tons) and, therefore, huge desizing activities worldwide.
Target values and hydraulic dimensions of a pilot plant for biogas production from textile wastewaters
Dwell time target value
7,200 L/working day
Average wastewater amount
27 m3 gas/m3 wastewater
Volume of the collection container
Volume of the pre-chamber
Volume of the main fermenter
Volume of the post-fermenter
Volume of the gas tank
The authors would like to thank the Deutsche Bundesstiftung Umwelt e.V. for the financial support of the project DBU Az 26589 'Entwicklung einer Verfahrenstechnik zur Generierung von Methan aus Stärkeschlichte in der textilen Vorbehandlung von Baumwolle'.
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