How many cows does it take to support an anaerobic digester? The answer: a lot. Large farms have implemented anaerobic digestion (AD) systems in recent years, but little interest has been paid to small- and medium-sized farms until now.

This article provides a review of AD technology and the state of the industry in the United States, Wisconsin and the rest of the Upper Midwest. Pros and cons of AD systems and the use of the biogas with a specific focus on small- to medium-sized dairy farms will be reviewed along with items to consider when planning an AD system. In addition, an overview of vendors that provide AD systems for small- and medium-sized farms is included.

As of November 2007, the number of AD systems in the U.S. totaled 112 systems with Wisconsin and California leading the way. The growth of the industry is accelerating. In 2000, there was less than 50 million kilowatt-hours per year (kWh/yr) produced, while in 2007, over 200 million kWh/yr was produced.

In Wisconsin and the Upper Midwest, individual farms, primarily dairies, have been exploring opportunities to develop biogas projects on their own or with private developers. In total, approximately 25 systems were operating by November 2007, ranging in size from 150 kW to over 2 MWs of electricity production or producing renewable natural gas.

The feedstocks for these plants include manure and other off-farm organics. Most of the systems are mesophilic plug-flow systems with the next largest type of system being completely mixed AD systems which are common in Europe.

A study was completed in Wisconsin that showed the major challenges for AD systems were working with the local utility, securing financing and inadequate labor. However, the implementation of Renewable Portfolio Standards and funding programs has begun to alleviate some of these concerns.

As an example, Focus on Energy, a program in Wisconsin designed to assist farmers with energy efficiency and renewable energy provides cost sharing for farms to complete feasibility studies and AD equipment capital purchases. Other states have put in programs such as guaranteed loans, tax credits and other exemptions to facilitate the growth and development of renewable energy.

The average size dairy farm of the 22,000 farms in the Upper Midwest is approximately 90 cows. Due to the number of farms, there is a great opportunity for small- to medium-sized dairy farms to install AD systems. The motivating factors for small- and medium-sized farms to consider AD systems include odor reduction, creation of a bedding supply, environment, manure management and electricity generation. This market segment has been largely under-serviced by AD vendors; however, several vendors have begun to focus on AD systems for small- and medium-sized farms.

The reason that there has been so much interest in these farms is that manure acts as an ideal base substrate for anaerobic digestion and with the development of funding programs and the inclusion of off-farm organics, they have begun to become feasible for the small- to medium-sized dairy farms.

However, manure-only AD systems are unlikely to provide reasonable economic paybacks. Therefore, farms and private developers must look to co-digest manure with the addition of off-farm organic material. Digestable organic materials include waste products of the food processing industry such as brewers grain, bakery waste, sweet corn waste, fats/oils/greases, whey, etc. Figure 1* shows the potential biogas yields from various off-farm organic feedstocks.

Co-digestion provides a sustainable solution for recycling waste materials and solves problems for the waste management sector. In addition, co-digestion of manure and other organic material will enhance the electricity/biogas production for smaller farms and provide an additional source of revenue through tipping fees paid to the farm for handling the material.

Vendors active in providing AD systems for small- and medium-sized farm include: Genesys Biogas, Bulrush Clean Energy (EEC Energy), Avatar and Andigen. For example, Genesys Biogas out of Ottawa, Canada provides a complete mixed, mesophilic AD system comprised of a poured-in-place concrete structure which is buried to the extent possible and insulated to prevent heat loss and allow for gravity flow. Genesys has three different sizes of biogas systems located on 33 farms in Europe and three farms in Canada. They have been recently active in Vermont and New York with five systems in the planning stages.

Two example systems are Pinehedge Farms and Terryland Farms in Ontario, Canada. Pinehedge has a 500 cubic foot digester with a 100 kW combined heat and power (CHP) unit with steam generation. The dairy operation milks 70 cows and produces yogurt on the farm. Terryland Farms has a 1,000 cubic foot AD system with a 180 kW CHP unit. The farm milks 120 cows and is co-digesting thin stillage from an ethanol plant with manure.

Dairies that are considering AD systems need to ensure they build a manure system that makes sense for their farm. In order to install an AD system, the farm must have a willing purchaser for the electricity or renewable natural gas. Once a market for electricity or natural gas is established, the farm should complete a feasibility study to review the capital costs, operating costs, revenue analysis and avoided costs for an AD system on the farm.

An accurate feasibility is critical to help the farmer develop their business goals, consider their options, develop contracts and review the equity, grants and debt for the potential project prior to proceeding with installation of an AD system. ANM

References omitted but are available upon request at editor@progressivedairy.com

Figure omitted but is available upon request to editor@progressivedairy.com.