Continuously-stirred Anaerobic Digester to Convert Organic Wastes into Biogas: System Setup and Basic Operation

Summary

Laboratory-scale anaerobic digesters allow scientists to research new ways of optimizing existing applications of anaerobic biotechnology and to evaluate the methane producing potential of various organic wastes. This article introduces a generalized model for the construction, inoculation, operation, and monitoring of a laboratory-scale continuously stirred anaerobic digester.

Abstract

Anaerobic digestion (AD) is a bioprocess that is commonly used to convert complex organic wastes into a useful biogas with methane as the energy carrier ^1-3. Increasingly, AD is being used in industrial, agricultural, and municipal waste(water) treatment applications ^4,5. The use of AD technology allows plant operators to reduce waste disposal costs and offset energy utility expenses. In addition to treating organic wastes, energy crops are being converted into the energy carrier methane ^6,7. As the application of AD technology broadens for the treatment of new substrates and co-substrate mixtures ⁸, so does the demand for a reliable testing methodology at the pilot- and laboratory-scale.

Anaerobic digestion systems have a variety of configurations, including the continuously stirred tank reactor (CSTR), plug flow (PF), and anaerobic sequencing batch reactor (ASBR) configurations ⁹. The CSTR is frequently used in research due to its simplicity in design and operation, but also for its advantages in experimentation. Compared to other configurations, the CSTR provides greater uniformity of system parameters, such as temperature, mixing, chemical concentration, and substrate concentration. Ultimately, when designing a full-scale reactor, the optimum reactor configuration will depend on the character of a given substrate among many other nontechnical considerations. However, all configurations share fundamental design features and operating parameters that render the CSTR appropriate for most preliminary assessments. If researchers and engineers use an influent stream with relatively high concentrations of solids, then lab-scale bioreactor configurations cannot be fed continuously due to plugging problems of lab-scale pumps with solids or settling of solids in tubing. For that scenario with continuous mixing requirements, lab-scale bioreactors are fed periodically and we refer to such configurations as continuously stirred anaerobic digesters (CSADs).

This article presents a general methodology for constructing, inoculating, operating, and monitoring a CSAD system for the purpose of testing the suitability of a given organic substrate for long-term anaerobic digestion. The construction section of this article will cover building the lab-scale reactor system. The inoculation section will explain how to create an anaerobic environment suitable for seeding with an active methanogenic inoculum. The operating section will cover operation, maintenance, and troubleshooting. The monitoring section will introduce testing protocols using standard analyses. The use of these measures is necessary for reliable experimental assessments of substrate suitability for AD. This protocol should provide greater protection against a common mistake made in AD studies, which is to conclude that reactor failure was caused by the substrate in use, when really it was improper user operation ¹⁰.

Introduction

Anaerobic digestion (AD) is a mature technology involving the biologically mediated conversion of complex organic waste substrates into useful biogas with methane as the energy carrier. There are many benefits of anaerobic treatment, including minimal energy and nutrient inputs and reduced biosolids production compared to aerobic treatment ¹⁰. In addition, the versatility of the mixed microbial community inherent to these systems renders a wide variety of organic substrates suitable as feedstocks ^11,12. Indeed, it is due to these benefits that a growing number of applications for AD are being adopted outside of conventional municipal wastewater t....

Protocol

1. Digester Construction

1. Select a digester vessel that contains all the features shown in Fig. 1 (a cone is not necessary), and your desired working volume (typically between 1-10 L). If your digester vessel is not equipped with a heated-water jacket, place the digester in some other temperature-controlled environment, such as a heated water bath or incubation chamber.
2. Secure the vessel in a vertical position in an area with sufficient horizontal bench space for placement of remaining components (Table 2).
3. Construct the vessel lid according to Fig. 2. The ports of the influent and efflu....

Discussion

The anaerobic digestion system presented in this article provides a general introduction and some basic guidelines for the treatment of most substrates in an experimental context. The wide variety of substrate types, digester configurations, operating parameters, and also the unique ecology of the mixed-microbial community underlying these systems precludes outlining hard quantitative metrics, which can be applied universally. Despite all this variability, all anaerobic digestion systems follow a well-characterized ser.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Heated Recirculator	VWR Scientific	13271-063 VWR	For use with a heating jacket reactor system
Variable Speed Electric Lab Stirrer	Cleveland Mixer Co.	(Model 5VB)	This mixer model facilitates mounting with a ring stand
Wet-Type Precision Gas Meter	Ritter Gasmeters	(Model TG-01)	This model needs a minimum flow of (0.1 L/h) and can handle a maximum flow of 30 L/h
Gas Bubbler	Chemglass	(Model AF-0513-20)
Gas Sampling Tube	Chemglass	(Model CG-1808)
Axial Impeller	Lightnin’	R04560-25 Cole-Parmer	Impeller blades with 7.9375 mm bore diameter
Impeller Shaft	Grainger	2EXC9 Grainger	1.83 m stainless steel rod with 7.9375 mm O.D. (needs to be cut to appropriate size)
Cast Iron Support Stands	American Educational Products	(Model 7-G16)	For mixer mounting
Three-Prong Extension Clamp	Talon	21572-803 VWR	For mixer mounting
Regular Clamp Holder	Talon	21572-501 VWR	For mixer mounting
Peristaltic Pump	Masterflex	WU-07523-80 Cole-Parmer	For effluent decanting
L/S Standard Pump Head	Masterflex	EW-07018-21 Cole-Parmer	For effluent decanting -accessory to peristaltic pump
L/S Precision Pump Tubing	Masterflex	EW-06508-18 Cole-Parmer	For effluent decanting - accessory to peristaltic pump
pH Analysis
pH Meter	Thermo Fisher Scientific - Orion	1212000
Total and Volatile Solids Analysis (Standard Methods: 2540-B,E)
Glass Vacuum Dessicator	Kimax	WU-06536-30 Cole-Parmer
Porcelain Evaporating Dishes	VWR	89038-082 VWR
Lab Oven	Thermo Fisher Scientific	(Model 13-246-516GAQ)
Medium Chamber Muffle Furnace	Barnstead/ Thermolyne	F6010 Thermo Scientific
Total Volatile Fatty Acid Analysis (Standard Methods: 5560-C)
Large Capacity Variable Speed Centrifuge	Sigma	WU-17451-00 Cole-Parmer
Laboratory Hot Plate	Thermo Scientific	(Model HP53013A)
Large Condenser	Kemtech America	(Model C150190)
Acetic Acid Reagent [CAS: 64-19-7]	Alfa Aesar	AA33252-AK
Chemical Oxygen Demand (Standard Methods: 5520-C)
COD Block Heater	HACH	(Model DRB-200)
Borosilicate Culture Tubes	Pyrex	(Model 9825-13)
Potassium Dichromate Reagent [CAS: 7778-50-9]	Avantor Performance Materials	3090-01
Mercury II Sulfate Reagent [CAS: 7783-35-9]	Avantor Performance Materials	2640-04
Ferroin Indicator Solution [CAS: 14634-91-4]	Ricca Chemical	R3140000-120C
Ammonium iron(II) sulfate hexahydrate [CAS: 7783-85-9]	Alfa Aesar	13448-36
Gas Composition by Gas Chromatography Analysis
Gas Chromatograph	SRI Instruments	Model 8610C	Must be equipped with a thermal conductibility detector (TCD), using below mentioned column and carrier gas operated at an isothermal temperature of 105 °C
Helium Gas	Airgas	He HP300	To be used as the carrier gas
Packed-Column	Restek	80484-800	To be used for N2, CH4, and CO2 separation