Vermicomposting with Spent Coffee Grounds

By G. Roekle, C. Schilling, E. Meisel, B. Thomas, and B. Jorgensen
Saginaw Valley State University, University Center, Michigan

   When cheap oil and natural gas became widely available in the 20^th century, this stimulated the so-called green revolution – the creation of inexpensive chemical fertilizers, pesticides, and herbicides, which in turn guaranteed record American crop yields continuing to the present. Food has never been so plentiful.
   It turns out natural gas is one of the most significant raw materials in this regard: in the late 20^th century, natural gas prices remained low and stable, stimulating the mass production of cheap nitrogen-based fertilizer, the backbone of today’s production agriculture. Cheap oil played an equally important role, as it is needed to manufacture transportation fuels and most agricultural chemicals.
   Bottom line: American production agriculture critically depends on low and stable prices of oil and natural gas. Its no surprise such prices have been volatile and rising in the past decade.   In the case of natural gas, volatile and rising prices have led to record increases in nitrogen fertilizer prices. Ask any American farmer how this cost alone has recently reduced farm profits. The International Fertilizer Industry Association recently published a worldwide fertilizer trade flow analysis showing a growing trend that the vast majority of the supply of U.S. nitrogen fertilizer is for the first time imported from overseas (www.icispricing.com).   We believe better methods are needed to manufacture cheap nitrogen fertilizer without natural gas as a raw material - especially methods that promote local manufacturing in large and small communities alike.
   The benefits of the green revolution came at a price – depleted nutrients in top soils and decreased top soil depth, increased soil compaction, and decreased water quality, to name a few. Furthermore, several leading economists are expressing concern that, at the rapid rate the world population is now growing, global food demand will sooner or later exceed supply (National Geographic June 2009). Thus, a major challenge in producing sufficient food while reducing negative economic and environmental impacts is to find cost points where profitability and sustainability intersect. We believe one such cost point may be vermicomposting, a process using worms to convert organic wastes into cheap, nitrogen-based soil amendments.
   Since 2007, researchers at Saginaw Valley State University in Michigan have been vermicomposting blends of preconsumer waste lettuce, shredded office paper, and spent coffee grounds. These materials were respectively supplied by Aramark Corporation (which manages the university cafeteria), the university library, and a Starbucks café located on campus. Recently, an interdisciplinary team concluded a 12-week study, which tracked the effects of coffee concentration on vermicompost quality. We discovered that the age-old practice of adding spent coffee grounds to soil does much more than give plants a caffeine boost.
   Spent coffee grounds were chosen because of their zero cost and abundant availability – currently they are part of the university waste stream. Spent coffee grounds are also rich in nitrogen compounds, including proteins, amino acids, and related natural chemicals. This suggests the possibility of recycling an abundant waste stream, potentially reducing the need for natural gas as a raw material to produce nitrogen-based soil amendments.   Our experiments show that the addition of spent coffee grounds increases the concentration of available nitrogen in vermicompost.
   It turns out other universities have reported interesting methods to recycle coffee grounds. For example, the University of Nevada produced biodiesel using vegetable oil extracted from coffee grounds (The Economist, March 5, 2009). The University of Malaya studied vermicomposting of manure blended with coffee grounds and kitchen waste (Bioresource Technology 100:1027 2009). The University of Glamorgan in Wales reported production of methane by anaerobic digestion of coffee grounds in wastewater flowing from an instant coffee manufacturing plant (Water Research 30: 371 1996).
   In our study, we chose to examine the effect of coffee concentration on the quality of vermicompost produced. We began with a literature review, suggesting that vermicomposting is viable under a wide range of conditions, using many different feedstocks. However, the review suggested that certain critical parameters are needed to provide optimum results – most importantly, good aeration, the pH (between 6 and 8), temperature (60 to 85 F), moisture concentration (~ 70%), and the carbon-to-nitrogen ratio (C:N) (between 20:1 and 40:1, with 30:1 as optimal).
   With these optimum conditions in mind, we designed 3 feedstocks: (1) a control feedstock containing no coffee, (2) one with a low coffee concentration, and (3) another with a high coffee concentration.
   To design the recipe for each feedstock, we used a convenient compost mix calculator developed by John Longfellow, Recycling Coordinator of Klickitat County, Washington. Available on the internet (http://klickitatcounty.org/solidwaste), this calculator was developed with the assistance of Dr. Thomas Richard of Cornell University (now at Penn State University) and the Natural Resource, Agriculture, and Engineering Service (http://www.nraes.org).
   The main advantage of this calculator is that it predicts the C:N ratio for a wide variety of feedstock blends. We used this calculator to create a C:N ratio of 30:1 in three feedstocks: (1) the control was a blend of 66.5 wt% lettuce, 33.5 wt% shredded paper, and no coffee grounds; (2) a blend of 45.4 wt% lettuce, 32.1 wt% paper, and a low concentration (22.6 wt%) of spent coffee grounds.; and (3) a blend of 34.3 wt% lettuce, 31.4 wt% paper, and a high concentration (34.2 wt%) of coffee grounds. Lettuce is well known as a dominant nitrogen source, and white photocopier paper is well known to be a dominant carbon source. Coffee contains both carbon and nitrogen.
   In our experiment, 3 plastic vermicomposting bins (so-called Worm Wigwams^®), each measuring 3 feet in diameter by 3 feet tall, were set up in our university greenhouse. Each bin was initially loaded with a six-inch layer consisting of a wet mixture of shredded newspaper, peat moss, coconut coir, and 10 pounds of red wiggler worms (Eisenia fetida). The Acme Worm Farm of Tucson, Arizona provided the bins and worms.
   Over a time period of 12 weeks, we added a total of 133 pounds of biomass to each bin. Each bin was fed twice weekly. Feedstocks (1), (2), and (3) were separately maintained in each bin throughout the course of the experiment. Each week, measurements of pH, moisture concentration, and soil temperature were recorded to maintain worm and compost system health.
   We observed no significant change in pH among all three bins; the average pH was 6.5, well within the safe range of 6 to 8 recommended by our literature review. It turns out pH 6.5 is optimum to maximize nutrient uptake for a wide range of plant species. Soil temperatures were kept between 65 and 83 F, within the ideal range of 60 to 85 F recommended by the literature review.
   Two independent soil analyses were conducted, one by Servi-Tech Laboratories in Hastings, Nebraska (www.servi-techinc.com), and the second by Growers Mineral Solutions of Milan, Ohio (www.growersmineral.com). Both analyses concluded that the N-P-K concentrations in all three vermicomposts were at or above optimum levels for healthy plant growth. The addition of spent coffee grounds to bins (2) and (3) increased the levels of available nitrogen in the final compost. Carbon to nitrogen ratios were approximately 20:1 with slight decreases observed in the coffee containing bins.
   Concentrations of calcium, magnesium, iron, and sulfur were at optimum levels.
   In addition, the cation exchange capacity averaged 17.1 meq/100 grams, which is well within the optimum range of 1 to 40.
   We observed that addition of coffee grounds reduced the degree of compaction of vermicompost in bins (2) and (3), based on strictly qualitative observations. Our literature review suggests that this improves soil aeration, which can stimulate worm reproduction and reduce insect infestation. Earlier, we mentioned the University of Malaya study on vermicomposting blends of manure, kitchen waste and spent coffee grounds.   In that study, it was reported that the addition of coffee significantly reduced the degree of insect infestation during vermicomposting. That study suggests the possibility that some of the 800 molecular species in coffee may be responsible for this effect.
   In summary, we believe our experimental results could give vermicompost, compost, and organic fertilizer producers a reason to take a look at spent coffee as an amendment in their current processing methods. Detailed results of these experiments can be found at the website of the Saginaw Valley State University Green Cardinal Initiative (www.greencardinal.org).
   In our experiments we have not measured the mass of vermicompost produced, as we are currently continuing the same experiments for a longer duration, and we prefer not to disturb the three bins in the process. Based on our literature review, potential yields of approximately 60% are typical in many vermicomposting systems. However, no published report that we’re aware of specifies yields when spent coffee grounds are used as feedstock. Assuming that similar yields can be achieved with spent coffee grounds, we speculate that producers of coffee waste could potentially profit from recycling such waste by vermicomposting. There is plenty of waste available – globally, 7 million tons of coffee are consumed each year, according to the U.S. Department of Agriculture (The Economist, March 5, 2009). Historically, coffee is the second most traded primary commodity after crude oil.
   It is our estimation that vermicomposting with spent coffee grounds can be scaled up for a variety of profitable uses, including direct application for farm fields, greenhouses, lawns and turf grass, nursery plants, and other soil-related industries. Moreover, we believe this technology could offset the use of natural gas as a critical raw material to produce nitrogen fertilizer.   Our hope is that this environmentally sustainable technology can be widely adopted on a local level, creating jobs in large and small communities alike.

G. Roekle is a student of biochemistry and technical writing. C. Schilling, E. Meisel, B. Thomas and B. Jorgensen are professors of Mechanical Engineering, Chemistry, Sociology, and English, respectively. The authors wish to thank The Saginaw Valley State University Foundation, the Saginaw Valley State University Greenhouse Program, and the U.S. Department of Labor for funding this study.