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

The overall goal of this procedure is to demonstrate the construction inoculation, daily operation and proper monitoring of a laboratory scale. Continuously stirred anaerobic digester for the purpose of assessing the suitability of a given organic substrate for long-term anaerobic digestion treatment. This is accomplished by first constructing an anaerobic digester system.

The second step is to inoculate the digester with active metogenic biomass by establishing a suitable anaerobic environment within the bioreactor system. Then the proper operation of the digester system is described including effluent, decanting, feeding, and troubleshooting. The final step is to introduce a monitoring strategy to assess digester performance and stability using standard analysis conventionally used in the field of anaerobic biotechnology.

Ultimately, the results show the effectiveness of anaerobic digestion in treating an organic substrate for energy production and organic strength reduction through a well controlled, regularly monitored laboratory scale anaerobic digestion system. Many people that are new to AOB digestion process will find it challenging because you have to maintain a oxygen free environment for the the strict anaerobic biomass. And also users will find the startup period a little bit slow and tedious because it takes a long time for enough methanogens to grow to develop a a biomass concentration that's able to handle a certain organic loading rate.

But it should be known that if one follows, the protocol outlined in this article assiduously that these problems can be avoided. And finally, after stable digestion is achieved, you can start experimenting with these systems. But this can also cause problems for experimenters because it's we're working with complex biological systems that are very sensitive to external or environmental perturbations.

Digestion technology can exhibit extraordinary stability and predictability when operated with a consistency and regularity and method. This is something that we hope to convey in this video. To begin construction of a digester, select a vessel that contains the features shown here with the desired working volume.

If the vessel is not equipped with a heated water jacket, place it in a temperature controlled environment such as a heated water bath or incubation chamber. Secure the vessel in a vertical position in an area with sufficient horizontal bench space for placement of the remaining components. Construct the vessel lid as shown here.

The ports of the effluent and effluent tubes should be wide enough to prevent clogging the tubes inside. The bioreactor should be of sufficient length to stay submerged in the digester medium during decanting while extending out from the top of the lid. To allow attachment of tubing, the impeller sheathing should extend as far as possible into the digester medium.

Apply silicone-based vacuum grease to the contact surface of the lid and clamp it to the top of the digester vessel. Using a ring stand and clamps, secure the variable speed mixer parallel to the digester vertical axis, then affix the impeller shaft. Because of mixer motor vibrations and movement, it is important to use an independent and free moving stand.

Connect a section of flexible tubing to both the in fluent and effluent tubes, and then connect another section of tubing to the gas port to be used as the gas line tubing should be tighter than shown to prevent leaks. The next step involves connecting the biogas line to each of the various components and placing them on the shelving above the digestor. First, connect the gas sampling port, which should be positions near the reactor headspace.

Next, connect the foam trap, which should be at least 25%The volume of the reactor, it should contain two ports, one for the biogas inlet line, and another for the biogas outlet line. The inlet tube should extend to a greater depth than the outlet tube. Construct the hydrogen sulfide scrubber using a long glass tube with an inner diameter of two centimeters.

Pack it with steel wool to provide enough surface area for stripping, but not so tightly that biogas flow is blocked. Add a biogas inlet and outlet port on either end of the tube. Finally, make a gas reservoir out of any collapsible airtight material such as a thick rubber balloon or air ball with a volume exceeding twice the targeted feed volume.

Then add the bubbler and gas meter if the system will be temperature controlled by a circulating water heater. Place the unit above the liquid level of the heating jacket and use flexible tubing to connect it to the heating jacket. Set the heater to the appropriate temperature for mesophilic or thermophilic digestion.

Perform a leak test of the system. Start by filling the digester tank with water. Then use gas to slightly pressurize the in fluent line to a pressure of less than five PSI now connect the anaerobic gas to the in fluent tube.

Next clamp, the biogas line and effluent lines to check for leaks using soapy water around the reactor lid, and then remove the biogas line clamp to test for leaks for the entire gas handling system. Turn on the impeller and heating element and let them run overnight to ensure that the mixer and heater can sustain continuous operation to inoculate the digester. If necessary, use water that was flushed with an anaerobic gas to dilute the inoculum to a volume that will fill the digester.

Remove water from the anaerobic digester before inoculation. Use anaerobic gas to flush the empty digester system for several minutes by connecting it to the feeding tube, clamping the effluent line and taping the space between the mixer, axle, and sheath. To prevent loss of anaerobic gas, make sure to flush out the gas reservoir.

Next, connect a funnel to the feeding tube and add the inoculum. Making sure to mix it periodically to ensure uniformity. Reconnect the anaerobic gas to the feeding tube.

Turn on the mixer and flush the digester contents for at least 15 minutes. However, mixing is not necessary. Then disconnect the gas clamp, the feed tube, and unclamp the gas reservoir.

The digester is now in operation and should run for a couple of days before feeding. After calculating a conservative initial organic loading rate or OLR gradually increase the OLR until a target value is reached. Prepare the feed mixture and store it at four degrees Celsius until feeding time.

Decant the effluent from the digester by connecting the effluent tubing to a pump and remove a volume equal to the volume of feed to be added. Store the effluent at four degrees Celsius for later analysis. To add the feed, connect a funnel to the feed tube and pour in the feed, making sure that the solids get carried in with the bulk fluid.

Monitor the system by frequently checking the digester and its components during operation. Paying particular attention to the mixing and heating systems insufficient mixing will cause an abrupt decrease in the concentration of effluent solids. Periodically check that the oil or water in the gas meter is at the appropriate levels and replace the steel wool in the hydrogen sulfide trap as it will become black and glossy.

From reacting with hydrogen sulfide and forming iron sulfide. Finally perform the analysis referred to in the written protocol and verify that the values consistently fall within the specified optimum range. Successful inoculation of the digester is marked by the production of biogas within several days.

The methane to carbon dioxide ratio of the biogas will increase during the acclimatization period as more metagenic biomass is recruited. The slow growth of methanogens compared to acid ens makes long acclimatization periods and gradual operational changes necessary. Here we demonstrate the dynamic response of a digester when a high OLR is introduced too early in the startup phase.

In this example, there was insufficient meth biomass to remove the volatile fatty acids or VFAs evolved from the substrate degradation step acid agenesis. This led to an accumulation of VFAs and subsequently a reduction in the pH. To rectify this situation, the OLR was reduced to limit the production of VFAs by ENS and to allow greater anogen recruitment before returning to the higher OLR.

The digesters then exhibited stable digestion for three hydraulic retention periods. Stable digestion or pseudo steady state conditions can be assumed when the measured parameters such as the biogas production rates total VFA concentrations volatile solids concentrations and pH levels are consistently maintained within 10%of their average values for a minimum time period of one HRT or 25 days. The significance is revealed in this figure, which shows the prolonged response of the CSTR system to a perturbation caused by insufficient mixing.

The lack of proper mixing allowed the solids to settle in the reactor, which meant fewer solids were removed. During effluent decanting, their accumulation resulted in higher effluent solids concentrations. After sufficient mixing was restored, it took approximately one HRT to return the digester to a normal effluent solids concentration.

An anaerobic digester is a biological system, thus it will exhibit some internal variability and performance. This variability must be quantified before the experimenter can discern the specific effects caused by experimental perturbations imposed on the system. Three HRT periods are required before an experimental change is made to the reactor system.

Because this is generally considered an adequate period of time to assume stable concentrations of chemical species in the mixed liquor, by the end of this interval, the experimenter should be able to construct a reliable baseline for each measured parameter. This baseline serves as a basis of comparison for future experimentation Following this procedure. Other methods like the biochemical methane potential assay, the meth genetic activity test and the enteric toxicity assay can be performed to answer questions like, what is the maximum methane yield that can be achieved through a given substrate?

What is the metabolic activity of the meth biomass in the reactor system? And also, what is the extent and duration to which a substrate related toxicity is affecting the anaerobic biomass? Performing these types of analyses will allow the operator to better characterize the substrate and also better assess the suitability of IC treatment for handling these types of organic wastes.

Thank you.