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Effect of Mixing on Bio Gas Production from Cow Dung

R Nandi
Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

CK Saha*
Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

MS Huda
Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

MM Alam
Department of Farm Power and Machinery, Bangladesh Agricultural University, Mymensingh-2202, Bangladesh

Abstract/ Executive Summary:

Mixing in an anaerobic digester keeps the solids in suspension and homogenizes the incoming feed with the active microbial community of the digester content. The effect of mixing on anaerobic digestion of cow dung was evaluated in lab-scale experiment at 35°C.The study was carried out at the biogas laboratory of Green Energy Knowledge Hub (GEKH) at Bangladesh Agricultural University, Mymensingh. The effect of continuous mixing (mixing for 5min at 15 min interval at 100 rpm)on methane production was investigated in three lab-scale continuously stirred tank reactors. In addition to gas production and composition measurements various analyses were conducted periodically on the digester contents.In comparison to without mixing and ambient condition, biogas production was increased by 2.20 % and 18.85%, respectively in the reactor with mixing. Methane productions in the reactor with mixing were improved by 3.21% and 18.23%, respectively in comparison to without mixing and ambient condition. The study thus indicated that mixing has no considerable effect on anaerobic digestion of cow dung. Further research should be aimed at studying the effects of mixing on a chemical and microbial level and on the different stages of anaerobic digestion.

Keywords: Anaerobic digestion, Mixing; Continuously stirred tank reactor, Biogas.

Location: Biogas laboratory of Green Energy Knowledge Hub (GEKH) at Bangladesh Agricultural University

Start Date:

End Date:

Program Area: Resource Development and Management

Commodity: Bio-gas

Objectives:

This research work is intended to evaluate the effect of mixing on gas production from cow dung at lab-scale.

Methodology:

Experimental site The experiment was conducted at the biogas laboratory of Green Energy Knowledge Hub (GEKH) at Bangladesh Agricultural University. Substrate collection The main substrate used in this research work was cow dung. Fresh cow dung was collected from the dairy farm of the university. The cow dung was always collected from the same farm in order to ensure as uniform feed characteristics as practically possible. Experimental set-up The biogas test plants system (BTP2-control, UIT, Germany) consists of three digesters units, heating unit,agitation system, module SENSO control and a gas analyzing unit. Every digester has a diameter of 27 cm and a height of 42cm resulting in a maximum volume of 17.2 L with a net capacity of 15 Land is equipped with different heating systems.The agitator is connected with agitator shaft by a flexible coupling installed in the tank. On the shaft there are two stirring tools (propeller and stirrer) installed, which can be changed by height. The agitator (Allowed environmental temperature 5–40 ⁰C, 80 % RH) with stirrer can be adjusted by data logging and control module SENSO control. Biogas generated in each digester was collected in a gas bag of capacity 10 L. Feed was prepared by diluting the manure with water, whenever necessary, to attain a consistent TS of 7.5 –10 %. The digester was fed from top of the reactor.The digester was fed daily by an amount of 500 ml influent and was discharged with the same amount considering 30 days HRT (Hydraulic Retention Time). Effluent was removed from bottom of the reactor. The reactor had one outlet at the bottom for sediment removal. Experimental consideration Reactor 1 (R1) was operated with stirring,reactor 2 (R2) was operated without stirring and reactor 3 (R3) was operated on ambient condition without stirring. The temperature of reactor 1 and reactor 2 was maintained at 35°C by T-controller and temperature of reactor 3 varied between 26.9–33°C as it was operated at ambient condition. The reactor (R1) was stirred for 5 minutes at 15 minutes interval at 100 rpm. The temperature of the fermenting substrate, gas quality and biogas temperature are measured continuously in every digester. Samples of fermenting substrate were taken on a weekly basis and analyzed for TS, VS, pH, ammonium (NH4-N) and volatile fatty acids (VFA) content in the biogas laboratory. Measurement of different parameters Gas volume was measured by a drum meter. Biogas composition (CH4, CO2, H2S, O2) was auto analyzed by a gas analyzing unit (SSM 6000 LT, PRONOVA, Germany). pH was continuously monitored in the CSTR. VFAand alkalinity was measured by the method developed by Møller and Ward (2011), TS and VS was measured according to standard method (APHA, 1998). Ammonium was measured by a photometer (NOVA 60, Memmert, Germany) according to manufacturer procedure. Data analysis After finishing all the experiments, accumulated data in the CPU (central processing unit) of CSTR were collected and analyzed with the help of MS excel 2010.

Source of Information: Eco-friendly Agril. J. 10 (02):07-13, 2017

Article Link (Online Journal): www.efaj-international.com

Funding Source:

Budget:

Total Budget (Tk.) :

Achievement/Findings:

Lab-scale experimental results showed that mixing had improved biogas production by 2.20 % and 18.85 %, respectively than unmixed reactor and the reactor at ambient condition.Reactor with mixing at fixed temperature increased gas production considerably than reactor at ambient condition without mixing. The difference in the performance of mixed reactor and unmixed was not considerable. In long term operation of digester, crust formation in reactor is a common problem. Crust formation hampers effective reactor operation and biogas formation. Mixing can be an engineering approach to solve this problem.However, it would be interesting to examine if the mixing patterns inside the digesters changed with the applied physical changes in the mixing conditions or not. The effects of mixing on the digestion process can not be appropriately evaluated in the laboratory, future mixing studies should be performed on field digesters. Further research should be carried out to investigate the effects of mixing on a chemical and microbial level and on the different stages of anaerobic digestion. The above experiment was conducted for one HRT (Hydraulic Retention Time). So in future a two HRT study should be carried out for better performance of mixing effect.

Knowledge Management: Journal