Introduction
Anaerobic digestion (AD) of dairy manure and other organic residuals produces biogas, a source of renewable power and fuel, while also benefitting the environment by capturing and converting methane, destroying pathogens and odors, and stabilizing organic matter (Informa Economics 2013; EPA 2008). Unfortunately, costs can be sizable, with capital costs estimated to be $2,000 per cow in 2011 for combined heat and power (CHP) projects in Washington State (David Paul Rosen and Associates 2011). While revenue generated from the sale of electricity from biogas can offset these costs, the value of electricity has decreased in many parts of the US. In some places, it has been valued so low that electricity sales do not even cover operation and maintenance costs (Costa and Voell 2012). These lower electrical revenues have threatened profitability for both existing and planned dairy digester projects (Novak 2012; Coppedge et al. 2012), leading AD project developers to focus on increasing value from all of the other digester revenue streams (Gorrie 2014). These can include tipping fees (as well as increased biogas or power) from accepting off-farm organics to be digested, various environmental credits, and sales from the digested fibrous solids or its value-added products. This publication describes the composition and separation process for fibrous solids that result from the digestion of dairy manures. It also reviews both current and future potential uses of fiber.