Community Digesters: Opportunities, Challenges and Strategies

In the U.S., there are about 1,500 digesters at sewage plants and 300 on farms. Success of farm and community digesters is mixed with a success rate of 60 to 80 percent. There are myriad factors that undermine the use of digesters.
By Surya Pidaparti | June 27, 2014

Nearly 40 percent of food produced in the U.S. is discarded. More than 36 million tons were sent to landfills in 2012. Landfilled waste is believed to be a major source of greenhouse gas emissions, and generates leachate that is hard to treat. A patchwork of regulations and initiatives has been implemented to divert the material from landfills to beneficial uses, the most common of which is composting, but only five percent is being diverted for that purpose. Another promising use is anaerobic digestion to generate biogas.

In the U.S., there are about 1,500 digesters at sewage plants and 300 on farms. Success of farm and community digesters is mixed with a success rate of 60 to 80 percent. There are myriad factors that undermine the use of digesters. They include poor characterization of feedstock, lack of adequate construction and operating capital, and lack of process, application and operation understanding, among others.

Digestion requires a proper balance of macro and micro nutrients, as well as an adequate supply of carbon to balance out oxygen and its surrogates. In addition, the feedstock has to be processed to a certain particle size to establish adequate substrate-to-microorganism contact. The processing system is feedstock dependent, and is a major component of digester costs. Feed materials often contain remnants of packaging, discarded prescriptions, inert materials and residual antibiotics. These contaminants can be toxic to the process and cause failures or suboptimal process. Frequently, digester operators collect any and all feedstocks that they can get. The variability in feedstock composition seriously impacts the viability of the digester. The reliability of the feedstock supply must be insured through long-term contracts.

The underlying process in anaerobic digestion involves several steps and sets of organisms working in concert. Each of the groups of organisms in the system needs very specific conditions, including pH and temperature. And much like humans, critical nutrients and micronutrients comprise the critical enzymes in the organisms. The feed must have these elements in the correct proportions for the process to function. These organisms are the earliest forms of life on the earth that formed in the absence of oxygen. Oxygen and its analogues, antibiotics, fungicides are poisonous and can cause the process to fail.

With traditional digester efficiencies, sale of the gas as a replacement for fossil fuels does not provide an adequate financial return. Consequently, digesters are heavily reliant on regulatory requirements, governmental funding and tipping fees for viability. A digester project can be profitable and feasible, however, if the conversion efficiencies are increased, the gas or power could be sold as a premium fuel and the digestate sold as a fertilizer.

After removal of carbon dioxide and other contaminants, biogas and its derivative, biomethane, are classified as a renewable fuel under the Energy Policy Act of 2005. This law and its corollaries require fuel marketers and petrochemical companies to blend renewable fuels into all fuel sold in the U.S. and create a marketplace for renewables. Under this scenario, it is possible to secure long-term purchase agreements from obligated parties for the biogas or biomethane, with the latter fetching a far better price that is several multiples of price for the heat value alone.

Digestate contains nitrogen, phosphorus and potassium, as well as micronutrients in a form that can be easily absorbed by plants. Digestate also contains significant organic matter and microbial populations that enhance soil productivity. For this reason, there is a strong interest from the organic farm sector in using digestate as a fertilizer.

A multifaceted approach is needed to overcome anaerobic digestion challenges. First, the solution must identify the potential feedstock and assess the potential for production of biomethane and fertilizer. The feedstock must be characterized to address the nutrient supplementation and processing needed, and the approach must identify and size a solution best suited to the feedstock. A financial model with potential revenues and costs must also be developed. Once a cost-effective solution is determined, it must be tested in a continuous mode in local conditions to validate effectiveness. The operation will identify changes necessary. 

Novus Energy LLC’s BioCatalytic system has been successfully deployed as a scale mobile plant to several industrial sites with biodiesel, food processing and chemical residues. The company is building a renewable energy plant in Boardman, Ore., that will convert agricultural biomass residues into 2.9 million cubic feet of biomethane, 700 gallons of 6-2-6 fertilizer and 250,000 gallons of clean water daily. Biomethane is being purchased by BP to fuel the natural gas bus fleet of San Diego Metro Transit. The fertilizer is being sold to organic farmers, and the water is being used to clean up discharges from the local industries.

Author: Surya Pidaparti
Vice President, NBC Systems, Novus Energy LLC