Evolving changes in biosolids management programs and industry demands prompted Siemens Water Technologies to conduct Mechanically Enhanced Biodrying (MEB) Pilot Studies as a new application for the existing IPS Composting System. The Studies determined the ability of the automated, agitated bin technology to achieve 65 percent solids concentration (35 percent moisture) in biosolids by using only the finished dried product as the amendment. Summer and winter studies focused on variables such as feedstock properties, turning frequency, bin retention time, and process aeration cycles.
The goal of 65 percent solids was achieved when the dewatered cake (Sludge) was at least 20 percent solids concentration with the volatile solids content of at least 60 percent and the combined in-feed mix (Test Mix) was 40 percent solids concentration or greater. Overall, the solids increased an average of approximately one percent per day and as much as two percent per day when minimum in-feed conditions were met.
The study addressed industry requests for a versatile end product that could be used for fuel or fertiliser and created by using less energy than traditional drying technologies. It also addressed a biosolids composting challenge when wood waste and other carbon rich amendments are in short supply.
Keywords: Biodrying, Biosolids, IPS Composting System, Sewage Sludge, Sludge Drying
Biodrying is a biological process in which organic materials produce self-generated heat that evaporates water and thereby reduces the overall weight and volume of the feedstock. Mechanically Enhanced Biodrying (MEB) incorporates physical processes such as forced aeration and turning of the materials to expedite moisture evaporation during biodrying. In these studies, Siemens Water Technologies (Siemens) sought to apply the mechanical advantages of its IPS Composting System (IPS) to the biodrying principles and develop an engineered protocol to achieve the desired results.
Biodrying is on the spectrum between drying and composting. While many principles of composting and biodrying can be similar, there are significant differences with respect to wastewater Sludge or biosolids. Biosolids composting requires a relatively high C:N ratio, high porosity, moisture maintenance, and a long process retention time to produce a marketable end product. Shredded wood waste or other cellulose bulking agents are used, moisture may be added during the composting stage, and a curing phase follows active composting. In biodrying, the goals usually are to stabilise, decrease both volume and moisture content, and retain the calorific value of the feedstock for end use as a fertiliser or a fuel. Therefore, woody amendments and moisture are not added during the biodrying phase and the retention time is shorter since no curing is required.
While biodrying has been used and promoted for on-farm manure management, only limited references document biodrying being applied as an engineered system to dry sewage sludge by Collick, et al. (2007), Choi, et al. (2001), VanBlarcom, et al. (2004) and Wright (2002). A rudimentary practice of biodrying occurs where sludge is removed from drying beds and lagoons, formed into piles or windrows, allowed to self-heat and eventually dry to various extents. However, this drying method tends not to be an engineered or mechanically enhanced process.