Materials Science

Waste Embraced 2.0: A Novel Study of the Effects of Optimized Chemical Precipitation on Biogas Production and Resource Recovery from Municipal Landfill Leachate and Wastewater Centrate

ABSTRACT

This research aimed to engineer a novel process to utilize Landfill Leachate and Wastewater Centrate, two highly prevalent forms of waste around the world, to recover valuable nutrients and to increase methane gas production. Leachate is a dangerous waste product that forms from rainwater in landfills around the world. Without proper application, leachate can contaminate groundwater, posing health risks. However, in several countries, leachate is proving to be a viable source of methane gas production to generate fuel. Currently, raw leachate is fed to anaerobic digesters in order to produce methane, but resources are not recovered. Leachate contains ammonia, phosphorus, and heavy metals, which can be repurposed as sustainable fertilizers. The efficacy of resource recovery via chemical precipitation was tested at five different pH levels ranging from 8 to 10 with either Magnesium Chloride or Calcium Chloride. After determining the optimal pH and salt for precipitation, two batch anaerobic digesters were built. One was fed raw leachate/centrate, while the other was fed precipitated leachate/centrate. The methane gas production was monitored over the course of 4 weeks. It was found that a pH level of 9 with Magnesium Chloride was the optimal pH level and salt for resource recovery. More than 76% of phosphorus was recovered, which is 25% higher than the industry standard. Anaerobic digestion with a precipitated mixture of leachate and centrate produced over 40% methane composition, compared to only 15% methane composition through anaerobic digestion with raw leachate/centrate. The findings, which have a worldwide application, allow leachate and centrate to be recycled in a highly sustainable manner, increasing methane gas production for use as fuel while also recovering profitable materials.

INTRODUCTION

As our world continues to deplete non-renewable energy sources at rapidly alarming rates, the search for alternative energy sources is on the rise. As of 2016, Renewable Energy accounted for about 11% of the United State’s Energy consumption, with waste products constituting 50% of this. [1]

In this research, waste products are utilized as energy sources and for resource recovery. My past research focused on using waste cooking oil from municipal university restaurants as well as biomass from Wastewater Treatment Plants in order to optimize methane gas production via anaerobic digestion and a genomic analysis. Methane Gas Production was optimized to 70%, 15% more than the industry standard. This proved that waste products were a viable source of biogas production, and further experimentation with different waste products would be necessary.

 

Landfill Leachate is the percolation of rainfall in landfills in every climate across the globe. In my home state of Utah, over 43,000 gallons of leachate are pumped out from landfills every day. [2] Leachate contains valuable materials such as ammonia, phosphorus, and heavy metals, which can be repurposed as sustainable fertilizers. However, Leachate also contains toxic waste, such as electronic waste, pesticides, and medicines. Leachate may sometimes contaminate ground and surface water, posing numerous health risks. [3] There have been reports of leachate contamination in countries around the world in a range of climates.

 

Wastewater Centrate is an abundant, nutrient-rich effluent removed from thickened sludge in Wastewater Treatment Plants. After substantial percolation, Centrate is disposed through dumping in streams and rivers, which causes dangerous pollution. Because of this, it is crucial that we engineer safe processes to beneficially repurpose Landfill Leachate and Wastewater Centrate.

 

Currently, Landfill Leachate and Wastewater Centrate are commonly used for dust control and in some cases, for biogas production. However, resource recovery has not been widely implemented with Leachate and Centrate. In this research, both Resource Recovery and Biogas Production via Anaerobic Digestion from Leachate and Centrate are combined. Objectives included discovering the optimal process for Resource Recovery and analyzing whether removing these resources affects methane production. This project coalesces Energy Research with Environmental Engineering to address numerous global issues, including resource waste, leachate and centrate pollution, and alternative energy demands.

 

METHODS

I conducted all methods independently throughout my experimentation.

Resource Recovery through Chemical Precipitation

Discovered the optimal pH level and salt for Phosphorus/Ammonia Recovery from Leachate and Centrate. Three Erlenmeyer Flasks filled with 100 mL of a raw 50/50% mixture of Leachate and Centrate were precipitated with either Magnesium Chloride Hexahydrate (MgCl2 · 6H2O) or Calcium Chloride Hexahydrate (CaCl2 · 6H2O) at one of five pH’s: 8, 8.5, 9, 9.5, 10. I used these two salts because they are cost-effective and readily available in most labs. Alkaline pH’s were chosen because acidic environments would adversely affect methanogenic growth. [4] One beaker served as a control. Sodium Hydroxide and Sulfuric Acid were used to increase and decrease pH. The mixtures were stirred at 400rpm for 45 minutes. Precipitates settled to bottom of flask. Method repeated three times for accuracy.

Liquid Spectrophotometry

Liquid Spectrophotometry was used to determine the Phosphorus and Ammonia concentrations in my initial and final samples of Leachate and Centrate for each pH for each salt. This allowed me to see how much phosphorus and ammonia were precipitated out for each pH and salt. Calibration Curves were determined prior to experimentation. Phosphorus Wavelength of 880 nm and Ammonia Wavelength of 655nm were used. Beer’s Law was used to determine concentration from the absorbance. Repeated three times for each pH for accuracy.

Inductively Coupled Plasma Mass Spectrometry (ICPMS)

ICPMS was used to determine how much metal was removed each time the mixture was precipitated and to display how varying pH levels affects heavy metal recovery. The metal concentrations measured were of Zinc, Nickel, Copper, Aluminum, Lead, Sodium, Magnesium, Potassium, and Calcium. ICPMS was conducted for the pH levels of 8.5, 9, 9.5 and 10.

Anaerobic Digestion: Design and Maintenance for Methane Gas Production

I independently designed and built two basic identical Anaerobic Batch Digesters. Each digester contained 600 mL of raw anaerobic seed, along with a feed of 300 mL of a 50/50 mixture of leachate and centrate. A 50/50 mixture was chosen to address disposal issues. One digester was fed precipitated feed, and another digester was fed raw feed. This allowed me to determine whether removing ammonia and phosphorus affected methane production. Methane production was monitored through water displacement over a course of three weeks. Digesters were incubated at 37ºF, an optimal mesophilic temperature.

Gas Chromatography (GC): Agilent Technologies

Gas Chromatography was used to measure the Methane Gas composition produced in each digester. This enabled me to analyze how the methane gas production changed as time passed and how it varied between the two digesters. 100µL of gas was injected into the GC. Percentages of Methane, Carbon Dioxide, and Nitrogen Air composition were given from the GC. The GC was measured every day for 4 weeks.

RESULTS AND DISCUSSION

As determined by the Chemical Precipitation, the optimal pH level and salt for Resource Recovery is a pH level of 9 with Magnesium Chloride Hexahydrate for both Ammonia and Phosphorus. This may be due to the fact that phosphorus-based compounds form at a higher pH level than the original pH level, which in this case was 8.2. [5]

Based on Liquid Spectrophotometry results, on average for the three trials at a pH level of 9, over 76% of Phosphorus was recovered from the mixture of leachate and centrate, whereas only 27% of Ammonia was removed. Ammonia is soluble in most compounds, which may explain why Chemical Precipitation is not the most effective form of recovery. Precipitation with Magnesium Chloride Hexahydrate removed 13% more phosphorus and 14% more Ammonia than precipitation with Calcium Chloride Hexahydrate.

From the Anaerobic Digestion phase of the experimentation, Gas Chromatography results showed that the digester fed a precipitated mixture of leachate/centrate had a 40% methane gas percentage compared to just 13% for the digester fed raw leachate and centrate. Within the 4 weeks that the digesters were run, the precipitated-feed digester produced almost 200 mL of methane gas compared to only 30 mL of methane gas from the raw-feed digester. This may be due to the fact that the precipitated mixture has less Ammonia, heavy metals, and phosphorus. Ammonia and Heavy Metals such as Copper are natural inhibitors to methanogenic growth. [6]

As shown by the ICPMS, after precipitation at a pH level of 9, heavy metal concentration decreases for every metal, proving that Chemical Precipitation is effective at removing heavy metals. These results prove that by removing valuable nutrients from Landfill Leachate and Centrate, biogas production can be increased by 25%.

CONCLUSION

The data from this experimentation yielded promising results and a substantial viability for further application. In times like these, it is important to consider sustainability and environmental impacts when discussing potential alternative energy sources. Landfill Leachate and Wastewater Centrate are both heavily abundant resources that are adversely impacting our environment. Utilizing them in ways such as Resource Recovery and Biogas Production is a sustainable process of recycling and reusing. We can not only reap the benefits of the removal of profitable materials that can be repurposed as sustainable fertilizers, but also of biogas production. Chemical Precipitation of Leachate and Centrate resulted in Phosphorus Recovery competitive with leading industry standards. It was determined that by removing valuable resources such as Ammonia and Phosphorus, methane gas production can be increased substantially from 15% to 40%. Leachate and Centrate are currently not being utilized to their full potential, and if further research is conducted, findings may serve to prove that both waste products may revolutionize the field of Alternative Energy and Environmental Engineering. My research is solely preliminary, with the hopes that this topic is researched into further and implemented correctly. This project addresses alternative energy demands, Landfill Leachate and Wastewater Centrate Overabundance and Contamination, and Resource Waste/Pollution. A cost-benefit analysis should be performed in order to weigh benefits against costs, as well as future research on Methane production optimization. This research can be performed with other waste products as well, and eventually an optimized framework for utilizing waste products as sources of energy may be engineered. In this time and age when the discovery of alternative energy sources is vital, this project may ultimately revolutionize the world of renewable energy and create a safer, more sustainable world.

REFERENCES

1. U.S Energy Information Administration – EIA – Independent Statistics and Analysis.” Renewable & Alternative Fuels – U.S. http://www.eia.gov/renewable/ (retrieved July 28, 2017).

2. Adams, Andrew. “Modern marvel keeps ‘garbage juice’ in check at Salt Lake County Landfill.” DeseretNews.com. Deseret News, 16 June 2016.

3. “How Contaminants Leach Out of Landfill Waste and Into Leachate.” The Leachate Expert Website. N.p., 24 Mar. 2012. Web<http://leachate.co.uk/main/leachate-chemistry-testing/how-contaminants-leach-out-of-landfill-waste-and-into-leachate/>

4. “Chapter 4 – Methane production.” Chapter 4 – Methane production. Agriculture and Consumer Protection, n.d.

5. Prater, J. (2014). Improved Production of Magnesium Ammonium Phosphate (Struvite) from Landfill Leachate. University of Wisconsin-Stevens Point

6. Sossa, Katherine, Manuel Alarcón, Estrella Aspé, and Homero Urrutia. “Effect of ammonia on the methanogenic activity of methylaminotrophic methane producing Archaea enriched biofilm.” Anaerobe 10.1 (2004): 13-18. Web.Hill, Joshua S. “Renewable Energy Demand Among Fortune 500 & 100 Companies Growing Quickly.” CleanTechnica. N.p., 07 Dec. 2016.

Other References:

Raghab, Safaa M., Ahmed M. El Meguid, and Hala Hejazi. “Treatment of leachate from municipal solid waste landfill.” Treatment of leachate from municipal solid waste landfill.

“Utah Department of Environmental Quality: Hazardous Waste Landfills.” Utah Department of Environmental Quality Rules and Regulations. Utah Department of Environmental Quality, n.d. Web. <http://www.deq.utah.gov/businesses/C/cleanharbors/docs/grassy/2012/10Oct/mod6.pd>.

Berghezan, Ovidiu. “Wastewater Treatment.” Sewage Treatment: Wastewater Treatment – Public Works – Region of Peel. N.p., n.d. Web.

“What is Leachate? Landfill Leachates Defined and Explained.” The Leachate Expert Website. N.p., n.d. Web. 10 Aug. 2016. <http://leachate.co.uk/main/what-is-leachate/>.

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“Biogas from waste | Biogas CHP | Biogas cogeneration.” Clarke Energy. N.p., 19 Jan. 2017. Web.

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“LEACHATE, EVAPORATION, AND DECONTAMINATION WASTE MANAGEMENT PLAN.” Utah Department of Environmental Quality. Energy Solutions LLC., 7 Aug. 2014. Web.

Khanal, Samir Kumar. Anaerobic biotechnology for bioenergy production: principles and applications. Ames, IA: Wiley-Blackwell, 2008. Print.

“Basic Information about Landfill Gas.” EPA. Environmental Protection Agency, 18 Oct. 2016.

“Benefits of Landfill Gas Energy Projects.” EPA. Environmental Protection Agency, 20 Sept. 2016.

“Landfill Gas and Biogas.” Landfill Gas and Biogas – Energy Explained, Your Guide To Understanding Energy – Energy Information Administration. N.p., 11 Jan. 2017.

Rahman, Mukhlesur, Mohamed Mood, Umer Rashid, and Chang Ra. “Production of slow release crystal fertilizer from wastewaters through struvite crystallization – A review.”

Keller, Jurg. “Resource Recovery from Wastewater – Opportunities and Achievements.” University of Queensland. N.p., 17 Aug. 2012. Web.

Bird, Amanda R. “Evaluation of the Feasibility of Struvite Precipitation from Domestic Wastewater as an Alternative Phosphorus Fertilizer Resource.” USF Scholarship: a digital repository @ Gleeson Library | Geschke Center. University of San Francisco, 22 May 2015. Web.

“Phosphorus Solutions.” Phosphorus Solutions. Massachusetts Institute of Technology, 2013. Web. 26 Jan. 2017. <http://web.mit.edu/12.000/www/m2016/finalwebsite/ solutions/phosphorus.html>.

Acknowledgements:

University of Utah Civil and Environmental Engineering Department

Utah Department of Environmental Quality (DEQ) (195 N 1950 W, Salt Lake City, UT 84116)

South Valley Sewer District (1253 Jordan Basin Ln, Bluffdale, UT 84605)

Central Valley Landfill (7213 W 1300 S, Magna, UT 84044)

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