Biogas Production Potential From Mixed Retail Wastes

I was studying Master of Environmental Engineering and Pollution Control at Griffith University when I completed this task. During my master studies, I gained much interest on wastewater treatment especially on the importance of nitrogen and its flow in waste streams. Therefore, worked as a volunteer with my supervisor, and performed a number of experiments on wastewater. This project was assigned to me to assess methane production potential from different retail wastes. Since retail wastes such as fruit, vegetable, shopping centre waste, and restaurant waste are usually end up in landfill, where an extended time period is required for their degradation. Therefore, I had to investigate the overall methane production potential from these wastes in order to provide recommendations about their further treatment. I completed this project at Griffith University, which has demonstrated as one of the leading research universities in Australia, and ranked 19. The university offers a large number of quality engineering courses on bachelor, master and doctorate levels including a range of professional studies.

While studying various courses on waste management and treatment, I found that there is a large scope of doing research and development work on a range of solids, semi-solids, and liquid waste streams. I had good experience in literature review, which allowed me to have a better control on investigating and searching various aspects of environmental engineering. I found that there is a limited information available on treating certain type of trade or retail wastes that could have large methane production potential. I discussed the idea with my supervisor, and I got approval without any issue as there was little cost involved in performing this type of experiment. I reviewed the literature on digesting the waste anaerobically, and found that a batch experiment could provide sufficient amount of information in terms of methane production. The degradation of retail waste via anaerobic digestion could help in investigating further option to effectively handle, manage, and treat them with high energy outputs. However, the retail waste are of varying quality due to the production practices at each shop, and it could difficult to provide a standard mechanism to treat them. Nevertheless, assessment for methane production could help in re-producing the method for varying qualities of retail wastes.

Since this was a small project, I worked alone on this project. My responsibilities were to analyse the samples, prepare experimental plan, and execute the experiment. Below is my position in the project.

I completed all required inductions and training which were mandatory to execute the experiment. The induction were mostly on the safe use of instruments and laboratory procedures. It was a good experience to me to understand how different procedures work in a laboratory. Though I already had few laboratory induction, but I preferred to repeat some of them including few more. I completed the annual fire safety, chemical safety, handling spills, working the wastewater, and risk assessments on conducting laboratory experiments. I also learned operation manuals where required to enable myself confident in operating them along with motivation to work safely. I had to use the laboratory for the measurement of total solids, volatile solids, and chemical oxygen demand. I used standard methods to measure these three parameters.

I contacted a nearby shopping centre, and discussed about this preliminary study, and collected the fruit, vegetable, restaurant waste, and general shopping centre wastes. I stored the sample in the laboratory fridge to avoid degradation. Following this, I blended each sample in a blender to homogenise them. I labelled the samples correctly, and separated certain amounts to perform total and volatile solids analysis. I also measured chemical oxygen demand of each sample. The purpose of measuring solids and chemical oxygen demand was to establish the experimental setup accurately. I did the analysis in triplicates to ensure accuracy of the data.

I consulted few books on anaerobic digestion processes with a focus on solid to semi-solids substrates, and found that food based waste have generally higher amount of volatile fatty acids that could potentially inhibit the degradation process. The inhibition occurs due to long chain fatty acids, which are difficult to convert into acetic acid to produce methane. Therefore, I needed to add minor amounts of each waste for the degradation process to avoid possible inhibitions or process failure.

I also needed some cultured microorganisms which could utilise the substrates as food. I collected the anaerobically digested sludge from my laboratory, which was cultivated with time at mesophilic temperature. I analysed the sludge as microorganisms for total and volatile solids. Following this I used the volatile solids data to prepare the microorganism to food ratio, and the mass needed to add in the anaerobic reactor. I used Microsoft Excel for data calculation and processing. I maintain a microorganism to food ratio of four, which could have one fourth of substrate as food. I used volatile solids data as the organic matter to calculate the ratio.

Following measuring the extent of organic matter present in each waste, I calculated the amount of microorganisms and food required make a balanced mixture of one litre. I had few anaerobic reactors of 1.5 L capacity, I contacted the relevant person to use them for this experiment, and he permitted me to use them. Getting approval to use someone else’s apparatus is counted towards professionalism, which I did during this task. Table below shows the amount of each sample added including their g-VS added in each reactor.

I presented the experimental plan to my supervisor, she re-checked my calculations, advised to round the data due to the high amounts to be added. I received acknowledgement on understanding the scope of work, and I felt quite motivated, and satisfied on my learning and improving skills. In one of the advices from my supervisor, I added water as a balance to maintain the one litre working mass. This was done to avoid any variation in the biogas production in the reactor head-space.

Following calculating the amount of each sample required to maintain the anaerobic process, I weighed nearly exact amounts of each sample, and added into the respective reactor. I checked pH of each solution, adjusted to neutral with either sodium hydroxide or hydrochloric acid. I knew that pH adjustment is quite important to execute a successful anaerobic digestion, and therefore, I adjusted it accordingly. I setup the reactors in three replicates to ensure the data is accurate, and I can calculated the standard deviation in terms of error.

As I know that batch experiment is only conducted with the addition of microorganism, I needed to prepare a reference reactor as well to subtract the balance methane, this would allow to calculate methane production only from the substrate. Following filling each reactor with the required amounts of samples, I adjusted the pH, purged nitrogen gas to remove any oxygen in the head-space, and closed the lid. I used a gasket with the lid to ensure that there is no leakage. I connected the gas outlet with the water filled U-shape measuring cylinder. The measuring cylinder had an over-flow outlet to remove water from the cylinder.

I recorded the water displacement every day, filled the cylinder to the marked level, and analysed the biogas sample for individual contents. I found that there was a lot of nitrogen at start of the experiment, I highlighted this concern in my meeting, but received a reasonable answer from my supervisor. I came to know that the nitrogen purged in the head-space does not participate in the anaerobic digestion, and is released with the initial biogas production. This was confirmed to me later as methane contents were increased. The corresponding nitrogen was reduced, and remain negligible as trace gas.

I calculated specific methane yield form the reference reactor, and the multiplied with the amount of volatile solids of digested sludge added in the test reactors. Following this, I subtract the methane production from assay’s methane production, and divide the answer with the amount of volatile solids of substrate added in the reactor. This final answer was the specific methane yield from the substrate only. Following this, I applied the Ideal Gas Law to compensate the methane yield with respect to the temperature. I divide the yield with the calculated factor to get the final values.

I also calculated standard deviation in the triplicates followed by the average values. I recorded and processed the data on daily basis, and continued for 25 days. I plotted the mean methane values of each substrate with time using Microsoft Excel, and compared the three wastes in same plot to evaluate the total methane produced along with the comparison. I also applied standard deviation to the data points to see whether all samples have similar output in terms of preparation and performance. I found that only fruit and vegetable samples gave a bit high error, but it was almost in the acceptance range.

I performed whole experimental work at the university laboratories, and complied with all regulations. I used genuine guidelines and procedure to execute and complete the project, which give me huge experience in many aspect of environmental engineering. I sued licensed software under the subscription of the university. I organised the data in effective way, which could be interpreted in efficient manner. I also labelled the experiment correctly, which would not conflict with the existing or on-going experiments.

Summary

I found that there is a large potential in retail wastes in a bio-energy perspective. However, there is limited information available to considerably perform the complete analyses. This project enhanced my skills to a great extent where I could reproduce my work efficiently in other platforms as well. I worked with my other colleagues efficiently, professionally, and with organisation. All of the members working around me were quite happy including my supervisor. As I am already diverse, I felt myself fit to work any environment, and feel to be best fit in the environmental engineering field.