A by-product of biomass plants, wood ash is used as an amendment in the initial mix and to control composting odors.
WOOD ash generated by biomass plants which utilize wood-fired boilers to produce electricity represents a significant portion of the organic residuals generated in northern New England. Successful reuse options are available to wood ash generators, such as in agricultural land application programs and topsoil manufacturing. Over the past decade, however, there has been major growth in use of wood ash as a bulking agent and compost amendment for biosolids composting facilities.
Wood ash helps to achieve a targeted solids content of 40 to 45 percent for an initial compost blend. It also adds porosity to biosolids, which reduces the bulk density, increases friability and, most importantly, improves oxygen transfer in the composting substrate. Benefits to the finished compost include the addition of beneficial macro and micronutrients, including phosphorous, potassium, magnesium, calcium, copper, and zinc, and color enhancement, giving it a darker appearance.
The most significant advantage to using wood ash as a biosolids compost amendment is its ability to reduce odors during the composting process and in the finished product. Much of the carbon contained in wood ash, especially material from boilers which do not achieve complete combustion, exists in a form similar to that found in activated carbon filters. Carbon in this form has an extremely high surface area to volume ratio and, as such, is very efficient in adsorbing volatile organics generated during the composting process. Research has documented the efficacy of activated carbon filters in the removal of organic sulfur-containing compounds, such as dimethyl disulfide and methyl mercaptan, which are believed to be responsible for many of the nuisance odors generated during biosolids composting.
Tables 1 and 2 list the ranges for those chemical and physical parameters of wood ash which are relevant to biosolids composting. The analyses are 1995 data for three northern New England wood-fired biomass boilers which generate wood ash that is utilized at biosolids composting facilities.
EXPERIENCE IN NEW ENGLAND
Many smaller municipalities in northern New England have found that aerated static pile (ASP) composting is a cost-effective and environmentally sound option for biosolids management. Historically, wood chips have been used to add porosity to biosolids for increased oxygen transfer.
In the mid-1980s, several municipalities performed trials utilizing wood ash as an alternative to wood chips as a compost amendment. West Warwick, Rhode Island was one of the first. The most significant result of these trials was a remarkable reduction in odor when using wood ash. Other benefits evident in the trials were that less wood ash was necessary for bulking biosolids relative to wood chips, no screening of the final compost was necessary, temperature requirements were easier to meet, and the final product looked and smelled better than a wood chip/biosolids compost. Following these trials, many municipalities began using wood ash as a replacement for, or in combination with, wood chips.
The biosolids composting facility in Billerica, Massachusetts is an example of a site using wood ash as a compost amendment. In 1990, the town had just completed a facility expansion and upgrade and had initiated an ASP biosolids composting operation on the grounds of the wastewater treatment facility (WWTF). The WWTF initially used wood chips as its only bulking agent. Soon after start-up, neighbors in the vicinity of the facility began to complain of excessive odors. Attempts to mitigate the odor by using chemical masking agents on the compost piles and by using potassium permanganate in the sludge holding tanks were not enough to reduce neighbor dissatisfaction. Consequently, the composting operation was temporarily shut down in April,1992.
Arthur Malcuit, the plant operator, worked with the engineering firm of Fay, Spofford & Thorndike, Inc. to assess potential odor control measures, including process changes, scrubbers and biofilters. They also visited facilities that were using wood ash as a compost amendment, including Dover and Milford, New Hampshire and Scarborough, Maine. Because the use of wood ash presented almost no capital investment in comparison to any of the other alternatives, the Billerica WWTF decided to conduct a pilot study utilizing it as a bulking agent and for odor control. Approval for the pilot study was obtained from the Massachusetts Department of Environmental Protection (DEP).
The pilot study began in November, 1992 and continued into the spring of 1993. Odor complaints dropped off. Malcuit talked with the neighborhood group about continuing into the summer when warmer weather might potentially induce more odors. The group agreed to have the trial continue, as long as they had some control over deciding whether the facility would continue operations if odors occurred. The summer trial also was successful. After the trial, the WWTF obtained regulatory approval to utilize wood ash on a regular basis. Since that time, there have been no significant odor complaints. If there were, says Malcuit, "the neighbors still have control."
To raise the solids content of Billerica's compost pile to approximately 40 percent, it was calculated that the wood ash to biosolids ratio should be between 2-3:1. Initially, the solids content of the compost mixtures was analyzed on a regular basis to ensure that it was optimum, but after becoming familiar with the mixes, the operators were able to achieve target solids contents by feel. One rule of thumb the operators developed for achieving a good mix was to form a ball of the mixed wood ash and biosolids and throw it against a concrete wall. Based on how well or how poorly the material adhered to the wall, the operators could determine whether the mix was too wet or too dry.
Since 1992, Billerica's compost recipe has been modified occasionally, but the use of ash has become standard. Initially, wood ash alone was used. Then, because Malcuit did not want to be dependent on only one amendment, he experimented with recipe changes. For a year, finished compost was blended with the ash. Then sawdust was used since there was generally more of it available at lower cost. Currently, biosolids are amended with a mixture of equal amounts of wood ash, sawdust and finished compost, although in the warm summer months, when finished compost is in high demand and the potential for odor problems increases, the amendment blend includes less finished compost and more wood ash.
POTENTIAL DRAWBACKS, VARIABILITY IN PROPERTIES
Although wood ash does work to mitigate some of the difficulties presented by the biosolids composting process, if not managed properly, the wood ash itself can offer handling challenges. Because of its high pH and relatively small particle size, wood ash is both corrosive and abrasive. Use of wood ash requires more frequent routine maintenance of composting equipment, and care must be taken to minimize airborne wood ash from accumulating on the grounds and machinery at compost facilities. It is recommended that ash be stored under cover if at all possible. At Billerica, separate enclosed bins store wood ash, sawdust and the prepared ash/sawdust mixture. In some cases, it may be necessary to wet down exposed piles of wood ash to keep them from emitting dust.
The physical and chemical characteristics of wood ash vary from boiler to boiler, and not all wood ash from wood-fired boilers has the characteristics necessary to be a successful biosolids composting amendment. One that is most effective as an odor control agent contains abundant carbon -- evidence of incomplete combustion of fuel. Over the past several years, efforts to increase boiler efficiency at biomass plants have significantly altered the characteristics of wood ash generated in northern New England. With increased efficiencies, the carbon content of many ashes has been reduced. Loss on ignition (LOI) is an analytical parameter which is often used to determine the effective odor-controlling ability of a particular wood ash. In general, the LOI of northern New England wood ash, and the particle size, have decreased over the past decade. At the same time, the pH and calcium carbonate equivalence has increased. A wood ash that contains high amounts of carbon, and will be most effective in controlling odors, will have a high LOI relative to today's average.
Changes in wood ash characteristics have not brought about a decrease in the use of wood ash as a composting amendment, but it has required ongoing adjustments of biosolids and amendment ratios in order to maintain optimum mixes for the composting process. For instance, in cases where a high LOI wood ash has historically been utilized as the only amendment in biosolids composting, some operations have had success in mixing a lower LOI wood ash with other wood by-products - most commonly sawdust - to achieve a blend which continues to offer many of the same benefits as a high LOI wood ash. Billerica, on the other hand, continues to use only high carbon, high LOI ash in its ash and sawdust amendment mixture. Table 3 provides a summary of how some composting facilities are utilizing wood ash.
Wood ash produced at northern New England's biomass electrical generators will continue to evolve. Burner efficiency is not the only factor affecting the wood ash market. For instance, the largest electricity provider in New Hampshire has, in recent years, attempted to purchase and close down some of the wood-fired biomass plants, a move that would reduce supplies of this valuable compost amendment. And, in 1996, New Hampshire began a pilot program involving competition among providers, a change that is likely to impact the economics of the small wood-fired biomass plants and the ash generated.
[Author Affiliation]
Andrew Carpenter is a Project Manager for White Mountain Resource Management, Inc. (RMI) in Belfast, Maine. Ned Beecher is a Project Assistant with RMI in Ashland, New Hampshire. RMI recycles about 21,000 wet tons/year of wood ash from six biomass plants in northern New England.
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