Anaerobic Treatment at Ethanol Facilities

Anaerobic digestion is an effective system to treat waste streams and produce renewable energy for ethanol plants.
By Ryan Johnston | June 10, 2010
Energy use is a primary cost consideration for ethanol producers. It is crucial that a modern ethanol facility find ways to reduce energy costs to minimal levels, increasing the plant's positive net energy balance in order to increase profit margins. For this reason an increasing number of ethanol facilities are installing energy-producing anaerobic digestion systems. Anaerobic digestion is an ideal fit for the ethanol industry, converting waste streams inherent to the ethanol production process into sources of renewable energy, while reducing the carbon footprint of the facility.

Anaerobic technology relies on the conversion of organics into a biogas rich in methane. The concept of anaerobic digestion is not a new one; cultures around the world have harnessed the power of methane gas for centuries. In the late 1800s, for instance, the British town of Exeter and the Indian city of Bombay were drawing gas from local waste sources to use as a fuel for gas street lamps.

Since then, advances in science and technology have led to new, highly-efficient anaerobic processes housed in cost-effective, emission-free digestion systems, which offer many advantages to those producing ethanol. While many people are concerned about odor, modern designs allow for a completely sealed digester, which creates an odorless anaerobic system for a pleasant operating environment and happy neighbors.

Anaerobic Digestion for Ethanol Production
A number of waste streams in ethanol facilities require cleanup. Evaporator condensate streams, dryer/scrubber streams, and any number of other miscellaneous waste streams specific to a given facility must be treated. Aerobic systems are one possible solution, but the drawbacks of this technology significantly reduce their cost-effectiveness. One issue is that they produce a very high amount of sludge which must be constantly conditioned and disposed of, a process which requires energy, time, effort and an investment in specialized equipment. Aerobic systems also require massive amounts of energy to operate and sustain. As energy use is already a primary concern of today's ethanol producer, it makes little economic sense to install a system that requires large amounts of energy to operate, particularly when other, better options exist.

Anaerobic systems are a far more effective solution, and are not susceptible to the problems aerobic systems face. Rather than act as an energy draw, anaerobic systems produce renewable, methane-rich biogas for the facility, often generating more energy than the system requires. Anaerobic systems also convert influent organics almost entirely into methane and CO2, generating very little excess sludge in the process. The high organic load in the waste streams of ethanol facilities makes this water particularly well-suited to an anaerobic process.

For these reasons, high-rate anaerobic treatment systems have become a standard feature in ethanol facilities. In addition, effluent from anaerobic treatment systems can often be recovered and returned to the ethanol production process, lessening the facility's water demands.

Thin Stillage Digestion
Water is no longer the only source of anaerobically-treatable waste in ethanol facilities. In recent years, the demand for renewable, sustainable energy combined with the fluctuation of corn syrup costs in the marketplace has put new pressure on ethanol producers to find new, more cost-effective sources of energy. This market pressure has led to innovation by Biothane and companies like it, opening the door to the anaerobic treatment of thin stillage.

Thin stillage is a natural byproduct of the biomass fermentation process, generated during the centrifuging of whole stillage out of the first distillation column. As a general practice in an ethanol operation, some of this stillage is used as back-set for the facility's cooking steps. But the remainder of this thin stillage has traditionally been delivered to an evaporator, which concentrates the liquid into corn syrup. The corn syrup is then either sold as-is or is added to the distillers grain, which is then dried and sold as distillers dried grains with solubles.

This method of handling thin stillage has two primary drawbacks. First, evaporating thin stillage is energy intensive, and creates yet another high-energy-related cost for the ethanol facility. Second, the price of corn syrup fluctuates. As the cost-effectiveness of thin stillage evaporation relies on a certain minimum price point for corn syrup, evaporation has become an unreliable and often economically unsound treatment method.

Luckily, thin stillage is rich in organics, and can easily be converted anaerobically into a source of renewable, sustainable biogas. For this reason, anaerobic systems can often provide a more reliable, cost-effective process for treating thin stillage. In addition to generating energy, the solids extracted from the thin stillage treatment system's effluent can be sold for use as an effective soil conditioner. As an additional benefit, many states now offer renewable energy credits, awarded to companies that offset their use of the grid with renewable energy they produce themselves.

Of course, anaerobic treatment of this stillage is not always the best solution. Occasionally, energy costs are low enough, and the demand for distillers dried grains with solubles and corn syrup is high enough, that evaporation and resale of these byproducts provides a greater value than the energy savings recognizable through anaerobic treatment. Ethanol producers should take care to work with a reputable, knowledgeable supplier of anaerobic technology to determine whether a particular thin stillage stream is an economically-viable source of energy for a particular facility.

Comingled Digestion
Some facilities go even farther. A fascinating trend has developed recently where ethanol production plants have been built alongside a dairy or beef cattle operation, for the primary purpose of creating a symbiotic, energy-efficient system between the two utilizing anaerobic digestion. These facilities use comingled, constantly stirred anaerobic digesters to mix the ethanol facility's thin stillage with manure collected from the neighboring farm or feedlot. The digester creates vast amounts of energy, so much that its output can satisfy 70-90 percent of the ethanol plant's entire energy demand. This kind of enormous reduction in energy costs leads to rapid payback for the ethanol producer, followed by greatly reduced operating costs. In addition, the plant's dried distillers grains, no longer mixed with corn syrup, remain an excellent food source for the neighboring cattle.

This closed loop system is both elegant and efficient. Animals generate manure, and ethanol production generates thin stillage. The manure and stillage are fed into a comingled anaerobic digester, eliminating the need for either party to dispose of these waste streams by conventional methods. The digester produces biogas which generates enough electricity to satisfy up to 90 percent of the ethanol facility's needs. The ethanol facility, in turn, produces dry grain which is fed back to the cattle, which in turn generate more fuel for the digester. The system produces very little waste, and both the ethanol facility and the farm benefit.

Considerations Before Purchase
Because of its numerous benefits, anaerobic treatment has become a mainstay at ethanol plants all over the country and around the world. Digestion systems can be incorporated in the early stages of facility design, or can be added to existing ethanol plants without complication.

When adding anaerobic treatment, a system should be custom-designed and custom-built for the site from the ground up. There are three important characteristics of any successful anaerobic digestion system: it must be suitable, technically viable, and economically sound. Economically, an anaerobic system should pay for itself over a timeline that makes sense for the facility. The system must be technically viable, designed to handle the biological needs of the facility's influent, at average as well as peak conditions. Finally, and most importantly, the system must be suitable for a particular site. No treatment system, anaerobic or otherwise, is appropriate for every single plant.

Biothane, or another known expert in anaerobic digestion, will study your facility's specific needs and present a plan that ensures your anaerobic systems meet all three of these important criteria. Be sure to talk to people who are operating systems designed by your potential anaerobic technology designer. Verify firsthand that these other facilities are receiving the benefits they were promised from their system, that they've received excellent training, support and post-commissioning service. Capital cost is an important factor, but so is system reliability and long-term support. EP

Ryan Johnston is sales leader digestion technologies at Biothane LLC, a business unit of Veolia Water Solutions & Technologies. Reach him at ryan.johnston@veoliawater.com or (856) 541-3500.