December 14, 2006 | General

Rotary Digester Operational Realities

BioCycle December 2006, Vol. 47, No. 12, p. 53
Annual meeting of in-vessel composting facility operators included representatives from seven MSW composting plants that utilize the rotary digester technology. Lessons learned regarding equipment wear, inspections and critical maintenance are summarized.
Robert L. Spencer

MANY compost facility design engineers believe a rotary digester that processes municipal solid waste, source separated or mixed, followed by a trommel screen, offers an advantage over other technologies in separating inorganic material from compostable organics. This is due to a combination of physical, biological, and chemical degradation that takes place inside the digester drum. A significant reduction in particle size occurs, depending on detention time, which ranges from six hours to 10 days.
Typical feedstocks include paper, cardboard, food, biosolids, animal manure and yard trimmings, which are transformed into raw compost in two to three days. Upon discharge, material passes through a screen opening of one to two inches. At the same time, plastic bags, metal, wood and some portion of glass, are not reduced to particle sizes less than one to two inches, and can be screened from the organic fraction after discharge from the digester. Further screening is usually required to remove the remaining glass and residual inorganic materials in order to produce a compost product suitable for commercial and residential landscaping purposes.
Operational challenges with rotary digesters (drums) were discussed at the Third Annual In-Vessel Users Group meeting held at Nantucket Island’s (Massachusetts) Waste Options composting facility on October 3-4, 2006. Representatives from seven MSW composting plants that utilize rotary digesters were present at the meeting, and most acknowledged that they have had operational problems related to cracks in the structural steel shells of their digesters. The two-day meeting had a session on digester inspection and maintenance. (Other sessions included odor control, compost marketing and a tour of the Nantucket solid waste facility, which includes the compost plant, MRF, C&D recycling facility and landfill.) As a former rotary digester facility manager, the author attended the session to participate in this important exchange of information and prepare this Compost Operators Forum.
A tally of the total number of large-scale rotary digester composting units in operation in North America includes 27 vessels as of October 2006, with more than 100 units scattered across the planet. Approximately 20 of the North American vessels were originally Bedminster Bioconversion’s design, produced by various manufacturers. The two other major designers include A-C Equipment Services, which manufactured a number of the original Bedminster digesters, as well as replacement digesters at some of those plants, and Conporec, Inc. which has one digester operating in Sorel-Tracy, Quebec, one in Delaware County, New York and approximately 20 worldwide. Most of these digesters are designed to process 25 to 100 tons/day of municipal solid waste and biosolids. There also are an undetermined number of smaller composting rotary digesters operating in North America, primarily in agricultural applications.
For most of the compost plants, it is very expensive to shut down a rotary composting digester for repair since the MSW and biosolids must be diverted to other disposal facilities, thus losing revenue, and sometimes paying a higher disposal cost. The most expensive problems are cracks in the shell because they require corrective welding, a temporary patch, or replacement of the shell section. The other expensive repair is failure of a pinion or gear, which often have a long lead-time for manufacturing a replacement.
Even though rotary drum technology has been successfully utilized in cement and lime kilns for more than 100 years, there is a significant difference in the design and operation of a cement kiln compared to a composting digester. Both are made with carbon steel plate, however kilns are lined with a refractory material, which is replaced every 12 to 18 months. The purpose of the metal shell in a kiln is to hold the refractory material in place considering that the steel shell would melt at the high temperatures used in kilns.
Most of the MSW composting digesters in North America, to date, are constructed of carbon steel, without liners. To protect the steel shells from abrasion, most designs incorporate some type of “wear bars” that are welded longitudinally to the inside of the shell, trapping a layer of compost that provides a physical barrier to the abrasive action of the MSW tumbling through the shell “24-7.” Unless such wear bars are regularly inspected, maintained and replaced, the metal shell will be subjected to significant abrasive forces. A number of facilities that did not maintain wear bars on the discharge end walls of the digester have found these walls have worn thin due to a combination of abrasion and corrosion, requiring replacement.
Chemical corrosion is a significant contributor to composting drum deterioration, according to Bill Hanke, Vice President of Engineering for A-C Equipment Services, a manufacturer of cement kilns and composting drums. “We don’t typically see cement kiln failures after just five years as we have with some compost drums,” something he attributes to the extremely corrosive nature of MSW and biosolids. “I have been involved with failure evaluation at a number of composting facilities, and we have determined that it is a combination of corrosion and abrasion that has caused the problems with the drums.” Cement kilns do not have the acidic environment since high temperatures drive off most corrosive chemical compounds.
He adds that rotary vessel dryers and equipment commonly used in the aggregate industry are often constructed with an even lighter gauge plate than compost drums, but due to the “extremely corrosive nature of the product” there is more wear on the compost vessel shells. “Building drums out of stainless steel is an option for some industries. But it can double or triple the cost of a drum considering that stainless steel costs five or six times more than carbon steel,” Hanke explains. Another option, albeit more expensive, would be to make the shells thicker, using 1-1/4-inch steel plate instead of 1-inch plate. Conversely, recognizing the limited life of the shells, they could be built even thinner and replaced more frequently.
A-C Equipment Services lists the following primary factors affecting the life expectancy of a rotary drum: Product density; Product distribution inside drum; Product load during shutdowns and during start-up; Corrosion/erosion due to product; Ambient temperatures at startup; and Total number of drum revolutions.
The primary location for cracks in shells is where internal baffles are welded to the shell. Such baffles are included in the design in order to meter the waste product down the vessel, as well as retain some compost behind the baffle as inoculant for newer, fresher waste. After years of operation the baffles, if not maintained, will fail from a combination of corrosion and abrasion. Radial cracks will form in the baffles and ultimately progress to the shell. If the baffle is welded to the shell, the cracks will propagate into the shell plate and become longitudinal cracks. Occasionally the longitudinal cracks will change directions and propagate around the shell.
In addition, the baffles, manufactured from stainless steel and welded to a carbon steel shell, create a galvanic corrosion cell due to two different metals in a corrosive environment. This reaction can cause the stainless/carbon steel weld to corrode at a faster rate than either of the parent materials. The baffles are often destroyed by the physical process, leaving only a retaining ring around the shell interior, resulting in faster flow rates of the waste through the digesters.
A variety of changes are being tried, including eliminating the baffles entirely, or installing only one baffle just before the discharge end of the drum. One facility has been using oak wood to protect baffles, replacing them periodically. Another design incorporates a stainless steel sleeve with the baffle so that the baffle is not welded directly to the shell. One facility has abandoned all baffles, and simply uses the digesters as mixing devices with shorter retention times, relying more on the composting process after the digester.
Regular maintenance is essential to maximizing the life of a rotary digester used for composting. John Vitas, President of A-C Equipment Services, emphasizes that owners of rotary digesters need to spend sufficient money on maintenance since repair and replacement ultimately cost much more. “Municipalities think it’s too expensive to pay for regular, experienced maintenance but in the long run it will be less expensive,” he says.
He cites an example of a facility that did not replace its worn out thrust roller, a $12,000 expense. As a result, the gear guard was damaged, a $25,000 repair. But that repair was not made, and the gear itself was damaged – a $100,000 repair if it fails.
One of the keys to maintaining the digester is proper lubrication of the gears and bearings. The type of lubricant selected is often a critical parameter, with synthetic lubricants having some advantages where high temperatures are experienced in carrying or thrust roller bearings. For ring gears and pinions, asphaltic-based lubricants are usually best. “Again, it may seem costly to replace lubricants, particularly with more expensive synthetic compounds, but the cost of not maintaining proper lubrication is much higher,” adds Vitas
A number of digester liners are being tried to address the shell corrosion problem. One is a urethane coating inside the vessel, instead of wear bars. Another is a light gauge stainless steel liner with an abrasion resistant material coating, stud-welded to the interior of the shell.
Conporec, Inc. recently installed a rotary digester at the new MSW composting facility in Delaware County, New York (see “Composting Mixed MSW And Biosolids To Extend Landfill Life,” November 2006). According to Claude Marmen of Conporec, to prevent shell cracking problems experienced at other plants, the Delaware County digester thickness was increased to three-quarters inch, with 1.5 inches under the tires. He explains that this change was made based on increasing the assumed bulk density of the waste from 32 lbs/cubic foot to 50 lbs/cubic foot (includes a safety factor), with the digester 75 percent filled and material at a 53 percent moisture content. Based on finite limit analysis conducted with those assumptions, the design thickness of the shell was increased.
On the operating side of the equation, both Marmen and Vitas emphasize the importance of operating the drums to maintain pH above 4.5 to 5.0 in the digester. This is accomplished primarily by maintaining an aerobic composting environment through use of blowers that push air through the digesters counter to the flow of the waste products. A number of facilities, for a variety of reasons, have stopped using the air blowers, and in the opinion of both of these manufacturers, created a more acidic, and therefore more corrosive environment.
Most of the Bedminster and A-C Equipment Service digesters are installed outside, and thus are insulated on the exterior with a 2-inch thick layer of polyurethane insulation to help maintain biological activity during cold weather. Although the insulation works well, this layer becomes problematic since it prevents visual inspection of the shell to identify small cracks that can most likely be repaired by welding. It was not until bulging bubbles of leaking liquids built up under the insulation that some of the facilities detected cracks, making repairs much more costly.
Conporec’s facilities in Quebec and New York have the rotary digester inside a building, and are not insulated. According to Marmen, this allows for early detection of cracks, and repair.
Both companies recommend ultrasonic testing of the shell on a periodic basis to detect cracks that are not yet visible from the outside. Regular ultrasonic testing also will detect reductions in shell thickness so that the rate of corrosion can be monitored. Other methods of crack detection include dye penetrants, X-ray, and magnetic particle inspection.
Bob Spencer is a Contributing Editor to BioCycle. He has managed several MSW/biosolids cocomposting facilities utilizing the rotary digester technology.
OPERATORS of facilities with rotary digesters discussed a variety of wear and tear issues at the Third Annual In-Vessel Users Group Meeting on Nantucket Island, Massachusetts in October. The accompanying article delves into a few key areas, as well as describes important maintenance steps. Other issues identified by meeting attendees regarding rotary digester and associated drive gears include: Interior wear bars, in many cases, did not stay in place and were discharged from the digesters. That resulted in a loss of the compost coating on the shell, exposing the shell to greater physical abrasion. Sacrificial anodes welded to the interior of the digester did not stay in place and were discharged from the digester, thus losing some of the corrosion protection benefits. Thrust rollers wear out and allow the digesters to move up and down their longitudinal axis. Excessive movement allows the gear to contact the gear guard and damage it. Thrust and plain tires that are welded to the shell break loose. Gear guards wear out. Lubricant leaks associated with the gear drive system. Digester thrust adjustment not properly maintained.

Sign up