Anaerobic Treatment of Restaurant Waste and FOGs

Transcription

1 Anaerobic Treatment of Restaurant Waste and FOGs Rachelle Huber San Diego State University Time period: June 2012-August 2012 Advisor: Tyler Radniecki, Assistant Professor. San Diego State University Page 1

2 Acknowledgements: "This project was supported by Agriculture and Food Research Initiative Competitive Grant no from the USDA National Institute of Food and Agriculture." I would also like to acknowledge Dr. Radniecki for all his help and for the opportunity to work on this project. Effective Summary The Anaerobic Treatment of Restaurant Waste and FOGs summer research resulted in the ability to build an anaerobic digester that could produce methane. The optimum ratio between the thickened waste activated sludge (TWAS) and innoculum as well as the ratio between the primary effluent and substrate (TWAS + innoculum) were found and applied to the rest of the experiments ran. The quantity of oil to add to the digesters for the optimum amount of methane to be produced was identified. The amount of carbon added to the digester and the alkalinity of the digester were quantified to help determine if optimum carbon and alkalinity ratios are being achieved. The presence of oil resulted in a decrease in the ph which resulted in a decrease of methane production. Further tests are being conducted to determine how to prevent the ph from dropping in the presence of oil, including 2-stage digesters. Project Objectives This project will determine the viability of anaerobic treatment of dilute wastewater and storm water when combined with restaurant waste. Anaerobic reduction of organic matter in wastewater and storm water produces biogas (a methane and carbon dioxide mixture). In order to generate enough biogas to be cost-effective, it is often necessary to supplement the waste water with rich organic substrates (e.g. restaurant waste). The hypothesis for this project is that the anaerobic digestion dilute wastewater containing either primary sludge (from an on-site wastewater treatment plant), or fats, oils and greases (FOG) will be more effectively treated and produce more methane than the anaerobic digestion of wastewater alone. To test this hypothesis, the project will start as bench-scale anaerobic digesters. The digesters will contain thickened waste activated sludge (TWAS), innoculum (the anaerobic bacteria that will digest the waste), primary effluent from a local wastewater treatment plant or DI water, and FOGs. Additionally, several anaerobic digester conditions were optimized, including ph, temperature, ionic strength and nutrients. The goal of this project is to maximize methane production while simultaneously maximizing the treatment of wastewater and storm water. San Diego State University Page 2

3 Project Approach: The Anaerobic Treatment of Restaurant Waste and FOG summer project consists of, constructing and optimizing anaerobic digesters to treat storm and wastewater and to generate energy in the form of methane gas. This process is optimized by using FOGs (Fats, Oils, and Grease). To each digester the following is added: TWAS: thickened waste activated sludge Innoculum: bacteria / seed DI water or Primary effluent: this helps to keep the solution mixed Oil: we are using basic vegetable oil (soybean oil), this is added only to treatment digesters not the control digesters We call the mixture of TWAS and inoculum the substrate Digesters were incubated at 30 C and shook at 150 rpm. Anaerobic digestion is a process in which microorganisms break down organic material in the absence of oxygen. Anaerobic digesters may be used to generate biogas (CH 4 and CO 2 ) to use as energy and in the treatment of wastewater. Digestion is caused by a group of microorganisms called methanogens. Methanogens generate methane under anaerobic conditions and are highly sensitive to oxygen and changes in ph. Anaerobic digestion occurs within two ranges, Mesophilic (20-45 C) and Thermophilic ( C). Our digesters operate under mesophilic conditions, which is the more stable of the two conditions Our experimental setup consists of the following The first step is to make a cozie for each of the digesters. Each cozie consists of duct tape and aluminum foil. This is to ensure no light enters the digester and to keep the digester warm when we take biogas measurements and pull samples out to analyze. The second step, constituents are added, which include primary effluent, TWAS, and oil. The third step is to make the digester anaerobic; the digester is bubbled with nitrogen gas for five minutes. Then CO 2 gas is injected into each digester to create a buffer, for when the innoculum is added. The innoculum is added. The digesters are bubbled with nitrogen gas for 2 min, to ensure that they are anaerobic and that there is no oxygen in the head space. Then, parafilm is put over the caps to ensure a proper seal and the bottles are sprayed with ethanol to ensure that any bacteria from the setup are killed. The last step is to but the digesters on the shaker table (@150rpm) and incubate them at 30 C. San Diego State University Page 3

4 Project Outcomes: The purpose of experiment one was to find the optimum ratios between the constituents that make up the digesters. In experiment 1, deionized (DI) water was used. The ratios were found to be a 3:1 ratio of DI water to substrate (TWAS + innoculum) and a 3:1 ratio TWAS to innoculum. This ratio is shown in the graph below. Figure 1: Gas Production from anaerobic digesters containing DI water In Figure 1 the optimum ratios between all the components in the digesters with DI water were found, and this shows the gas production in ml produced over 1000 hours. The digesters in Figure 1; consist of the following (18.75mL Innoc mL TWAS + 75mL DI). San Diego State University Page 4

5 Figure 2: Gas produced from an anaerobic digester consisting of DI water with Oil In Figure 2 oil was added to the optimum ratios with DI water from Figure 1. Figure 2 shows the gas production in ml over 250 hours. The digesters in Figure 2 consist of the following; 18.75mL Innoc mL TWAS mL Oil +75mL DI Experiment 2, refer to Fig 2, was the first time that oil was added to the digesters and biogas production ceased within the first three days. Since Experiment 2 ceased to produce biogas so quickly with the addition of oil, the purpose of Experiment 3 was to trouble shoot Experiment 2. The addition of primary effluent kept the ph more stable and gas production length was extended from 3 to10 days. The primary effluent acted as a buffer, and held the ph steadier when FOGs were present. Primary effluent was used instead of DI water for the rest of the experiments. The following graph, refer to Fig 3, shows the gas production with primary effluent at optimum conditions. For Experiment 3 the ph was measured weekly to see how far it declined, how fast it declined, and how the decrease in ph affected the gas production in the digesters. In comparing the ph graph and gas production graph it is clear that as ph decreases so does the amount of gas produced. It is known that methanogens are very sensitive to ph change, and this may be the cause of the decrease in biogas production observed as the ph decreased. San Diego State University Page 5

6 Figure 3: Gas produced from primary effluent mixed with oil Figure 3 is the optimum ratio of all the components with primary effluent. The digesters in Figure 3 consist of the following constituents (18.75mL Innoc mL TWAS mL Oil +75mL Primary). San Diego State University Page 6

7 Figure 4: ph measurements from a digester consisting of primary effluent and oil Figure 4, shows how ph decreases throughout the course of the experiment. The digesters in Figure 4 consist of the following constituents (18.75mL Innoc mL TWAS mL Oil +75mL Primary). San Diego State University Page 7

8 Figure 5: Gas production comparison between anaerobic digesters with DI water and primary effluent Figure 5 shows the comparison in gas production between the DI water and primary effluent. In figure 5 there is no significant increase in the gas production from the anaerobic digesters that contain DI water vs. the anaerobic digesters that contain primary effluent in the absence of oil. The digesters in Figure 5 consist of the following constituents; Set 6 (18.75mL Innoc mL TWAS + 0mL Oil + 75mL DI) vs. Set 3 (18.75mL Innoc mL TWAS + 0mL Oil +75mL Primary). San Diego State University Page 8

9 Figure 6: Gas production comparison between an anaerobic digester containing DI water and oil vs. and anaerobic digester containing Primary effluent and oil In figure 6 when oil is added to the anaerobic digesters, there is a significant difference in the biogas production, between the DI water and primary effluent. In figure 6 the digesters contain the following constituents, set 1: (18.75 ml Innoc mL TWAS mL Oil +75mL DI) vs. set 3: (18.75mL Innoc mL TWAS mL Oil +75mL Primary). In comparing figure 5 and figure 6, the difference is the addition of oil to the digesters. In figure 5, without the addition of oil gas production is steady and increase and the ph of the digesters is stable. In figure 6, oil is added to the digesters and there is a significant difference in the amount of biogas production. As seen in figure 6, the digesters from set 1, stop producing biogas around 75 hours, where as the bottles in set three don t level off until about 150 hours. The difference in the digesters is the addition of primary effluent. Primary effluent has higher alkalinity than DI water and helps to hold the ph of the digesters stable. As seen in figure 4, when oil is added to the digester, the ph of the digester drops. Methanogens are able to break down the organic matter in the absence of oxygen, but are sensitive to ph and oxygen changes. At the same time methanogens are working there are also acidogens and acetogens working to produce acid. When the ph drops low enough, the methanogens stop working and the acidogens and acetogens keep producing acid. This stops the production of biogas being produced. San Diego State University Page 9

10 Figure 7: Gas production at 35 C In figure 7; it shows that an increase in the temperature that the digesters are incubated at does not increase the amount of biogas produced. In figure 7 the digesters contain the following constituents, (18.75 ml Innoc ml TWAS ml Oil + 75 ml Primary). The digesters in figure 7 are from Experiment, they were incubated at 35 C as opposed to 30 C. This experiment was run to see if an increase in the temperature would affect the ph and/or the gas production of the digesters. This experiment can be compared to the experiment graphs above figure 5 and 6. When Experiment 5 was compared to Experiment 3 there was not much difference in the amount of biogas produced or the stability of the ph. Therefore, it showed that increasing the temperature of the digesters from 30 C to 35 C was not necessary for optimum production of biogas and it would not help to keep the ph stable. Conclusion: The optimum methane production comes from a digester that contains the following constituents; 4.69 ml of TWAS, ml of Innoculum, 75 ml of primary effluent, and 1.56 ml of FOG at a constant temperature of 30 C. The summer ended with a problem that has been present from the beginning, the ph dropping to a point where it affected the amount of biogas produced and then ended up killing the digesters. The future of this project is to find a way to keep the ph stable throughout an entire experiment. A proposed idea is a two phase digester; where the oil is digested in phase 1, and then the effluent from phase one is digested further in phase 2. The setup of the digester is as follows phase 1, an anaerobic digester with FOG will run for two weeks. Phase 2, the anaerobic digesters from phase 1will be spun down and the effluent transferred to a new anaerobic digester, with new TWAS and innoculum. San Diego State University Page 10

11 San Diego State University Page 11