March 28, 2005 | General

Evaluating Benefits of Class A Biosolids Cake Production

BioCycle March 2005, Vol. 46, No. 3, p. 41
A national survey of both small and large-scale wastewater treatment facilities shows varied approaches to reaching markets with superior methods that take the “biosolids out of the biosolids.” Part I
Sally Brown

FOR the vast majority of cases, the difference between Class A and Class B biosolids in terms of quality is solely based on pathogen concentrations. For exceptional quality materials, both Class A and B biosolids are required to meet the same standards with regard to metal concentrations. For pathogens however, Class B biosolids must be treated to meet PSRP (process to significantly reduce pathogens) standards whereas Class A materials must meet PFRP (process to further reduce pathogens) standards. This means that Class A biosolids have no detectible pathogens when they leave the treatment plant. According to the EPA40CFR Part 503 regulations, Class A biosolids can be used without restrictions.
Increasingly, biosolids program managers are feeling pressure to go to Class A production. Reasons include: Class A biosolids are expected to be viewed as safer by the general public leading to greater acceptance of beneficial use of biosolids; Use of Class A material is not restricted and no site specific permits or plans are required, suggesting that more beneficial end use options are available, potentially at lower cost than traditional beneficial uses for Class B material; and Local ordinances or bans that essentially only allow Class A material to be land applied.
There are several well-tested methods to achieve Class A pathogen reduction standards. Perhaps the best known is composting. Raw sludge or Class B biosolids cake can be processed to produce compost that is similar in appearance, nutrient value and soil conditioning capacity to commercially available composts. Many municipalities compost a portion or all of their biosolids and are able to recover a portion of the production costs through retail and bulk sales of their product. This is true for both small and large scale publicly owned treatment works (POTWs). Examples include the City of Coeur D’Alene, Idaho, Denver Metro, Colorado, and Philadelphia, Pennsylvania.
However, composting is not always a panacea for a biosolids program. Composting facilities can be difficult to site and existing facilities can be forced to shut down due to odor and other complaints from commercial and residential neighbors. One example of this is the Montgomery County (MD) Regional Composting Facility in metropolitan Washington, D.C. that had been operated by Washington Suburban Sanitary Commission using biosolids from the Washington, D.C. Blue Plains treatment plant as a feedstock. Despite sophisticated odor control systems that were used in combination with a negatively aerated static pile system and high commercial demand for the finished compost, the facility was forced to shut down as a result of complaints from neighbors. Additionally, compost can be more expensive to produce in comparison to a Class B land application program. For example, within the King County, Washington biosolids program, the cost of transporting Class B cake to the farms in Eastern Washington is approximately $30/wet ton while the cost of composting is $37/wet ton. The City of Portland, Oregon mothballed its compost facility as costs were three times that of land application of Class B biosolids.
There are other technologies that achieve Class A pathogen reduction requirements which are very capital intensive to build. One approach is pelletization. Pelletized biosolids achieve Class A pathogen reduction through the high temperatures reached in the drying process. This is a high cost system, as a result of both capital costs and energy requirements for daily operation. Production costs can be partially offset through sale of the pellets. For example, the City of Milwaukee, Wisconsin markets Milorganite nationally to retail customers. New York City also pelletizes a portion of its biosolids and these are distributed to citrus growers in Florida. There are some concerns with developing a market for pellets for two reasons: rewetting pellets can result in a highly unpleasant odor and there is no preexisting market for this type of garden product as there is for compost. Milorganite has been produced for over 80 years and is recognized as a name product. However, a new pelletized biosolids product would not have a recognized niche to fall into.
For both composting and pelletization, the final product bears little to no resemblance to biosolids as we know it. They can both be marketed by product name and public concern relating to their feedstocks is generally minimal. They are both higher cost options and may not be viable for POTWs with effective Class B programs and no severe economic pressure to change them. There are other technologies that achieve Class A pathogen reduction and also take the “biosolids out of the biosolids” in the process – where the resulting material does not have the moisture content, texture and odor characteristic of lagoon stabilized biosolids or anaerobically digested polymer thickened cake.
In many cases, it is possible to achieve Class A pathogen reduction with minor modifications to existing stabilization processes or through less expensive upgrades to existing operations. While these modifications are sufficient to achieve pathogen reduction requirements and produce Class A biosolids, the resulting material is very similar in appearance and smell to conventional Class B cake. The remainder of this article will focus on whether reaching Class A standards without changing the basic characteristics of the end product is sufficient to identify more cost effective beneficial uses as well as greater public acceptance of biosolids cake. The information was provided by biosolids generators who responded to a general survey. It is not meant as an evaluation of different treatment processes, but instead gives a general overview of what benefits Class A pathogen reduction can provide for a biosolids program manager.
Both Tacoma, Washington and Vancouver, British Columbia deliberately decided to produce a Class A cake at the same time their treatment plants were upgraded to secondary treatment. When Tacoma began producing Class A biosolids in 1987, the new product was similar in appearance to the cake initially produced, but had a sharper, more objectionable odor. This odor was later reduced through modifications of the thermophilic digestion process. Similarly, Vancouver changed its existing digestion process as part of a general upgrade to secondary treatment. This involved constructing new thermophilic digesters and upgrading existing digesters to flow-through vessels, preventing short-circuiting and ensuring a Class A product. The material is dewatered using high solids centrifuge and sludge cake piston pumps rather than a belt filter press. The Class A biosolids is different in both moisture content (33 percent versus 15 percent) and odor from the Class B material, although both materials are odorous.
Production of Class A biosolids has dramatically changed the Tacoma program but to date has had little effect on the Vancouver program. A comparison of these two programs illustrates what factors are necessary to fully take advantage of the benefits that producing a Class A product can offer.
Tacoma made the decision to name and market its biosolids very early on. This decision predated achieving Class A pathogen reduction. Initially, the biosolids were marketed straight off the press. According to Dan Thompson, current manager of the Tacoma program, this initial marketing strategy was not successful. Both the odor and the consistency of the biosolids were not consumer friendly. Tacoma developed a more marketable product through mixing biosolids with other materials. A local topsoil dealer had developed a successful product by mixing manure with sand and sawdust. Tacoma used this as a model and through a learning process with its customers, developed the mixture that is now known as Tagro Classic. “Bank soils were initially used in the blend without prescreening but the product was hard to work with,” says Thompson. “Tagro customers had to screen the material to remove rocks prior to using the blend in their gardens. Sawdust was added but the mix still wasn’t quite right. Customers said they wanted a product they could work into the lawn more easily. We then switched to sand, which created a more granular, customer-friendly mix.” (For more details, see “Dual Digestion System Yields Class A Biosolids,” BioCycle, August 2002.)
Tacoma pays a delivery fee (primarily transport costs) for the sand, sawdust and aged wood bark that are used in the Tagro products mix. Tacoma charges between $8 and $30/cubic yard for its products. Using a fairly conservative approach, the net cost to mix, blend and distribute/transport the products is $22/wet ton. (Transport costs are minimal as the vast majority of the users are local.) Demand for the mixed products is seasonal. In the winter, Tacoma has a limited liquid land application program. They are also testing the mixtures to see if stockpiling over the winter months is a viable option.
A primary factor involved in the decision to develop a consumer friendly product – both in appearance and performance – was the desire to develop a local market, namely home gardeners who were putting in or upgrading their lawns. Tagro was designed with this end use in mind. To demonstrate the benefits associated with use of the product, Tacoma established a reputation for its mix by entering plants grown in Tagro in local competitions and establishing a demonstration garden. Instead of advertising in conventional media, the program targeted a more specific audience to publicize its blend, including Master Gardeners. This approach began with an initial prize winning watermelon in the Puyallup Fair in 1992. By 1994, demand for the product had reached a critical mass.
Tacoma also took advantage of the municipal infrastructure to advertise Tagro. Beginning in 1994, information on Tagro was included in utility bills twice a year to coincide with periods of predicted high consumer demand. Tacoma recently supported research at Washington State University to develop a blend using its biosolids cake that would be suitable for container gardening. The resulting blend is being sold by Tacoma as part of a growing line of biosolids-based products. Thanks to this local demand, Tacoma expects to turn a profit from its biosolids program within a few years (factoring in costs and revenues post-dewatering). Public acceptance of biosolids is not an issue with Tagro products. There are occasional questions on the safety of biosolids, mostly revolving around metals. “We occasionally will get questions on pathogens and pharmaceuticals in personal care products,” says Thompson. “The majority of the questions come through the website. We answer each with a personal communication – usually an email. Overall, these questions are dwarfed by the advocates of the Tagro product line.”
The approach in Vancouver has followed similar lines but is much more tentative, resulting in little change to their program as a result of producing Class A biosolids. As a result of producing Class A cake, Vancouver is now able to apply its material to ranch land. Initial attempts have been made to develop a mixture that is more consumer friendly. Several blends have been identified, however, there has been no effort to market them. This is due in part to concerns about contaminants such as dioxins in the biosolids. Concern also has been expressed within the program about a range of newly identified organic compounds that may or may not be present in the biosolids and may or may not prove to be of environmental concern. When asked if perception of biosolids has changed as a result of producing Class A material, Ken Lee, the Senior Project Manager, said that for some people within the wastewater treatment staff, production of Class A was a source of pride.
The Tagro example has been used as a model for other municipalities in their marketing of Class A biosolids. Some cities attempting to build a program based on the Tagro model include Washington, D.C., Madison, Wisconsin and Everett, Washington. Of these, the City of Everett is furthest along in the process.
Everett has been producing a Class A biosolids through lagoon stabilization since 1998. “We have two separate processes here, lagoon and mechanical,” says Chris Chesson, manager of the Everett program. “The lagoon system consists of two 15 acre facultative ponds, a 135 acre oxidation pond and a 30 acre polishing pond. Digestion is achieved in the second of the facultative ponds. This is the pond that we harvest biosolids from every other year. All waste sludge from the mechanical process goes into the first of the two facultative ponds and eventually winds up in the second pond.” Chesson adds that VAR (Vector Attraction Reduction) and metals limits are met right out of the pond. Pathogen reduction is verified with batch sampling. “This Class A option will probably not be available to us much longer,” he notes, because of state requirements.
Everett began to develop a consumer friendly product by mixing biosolids with wood ash. This improves the texture of the biosolids, raises the pH and reduces odors. In addition, Everett has been attempting to remove impurities such as plastics, using a two-fold approach. Chesson is working with treatment plant operators to develop methods to improve screening to remove plastics during the initial phase of wastewater treatment.
According to Chesson, the rationale for the approach is as follows “We are now trying to develop a product and markets for that product rather than trying to find a place to put something. Quality characteristics are a big part of what you are looking for. With an improved quality we can assure that we can market the product close to home and improve public acceptance. Actually public acceptance, it seems like a nonissue.”
For Everett, the decision to develop a marketable Class A product was made with economics as the primary consideration. Before being able to market Class A material, Everett was forced to transport its biosolids over the mountains to forest and agricultural sites at a cost of $25 to $30/wet town. Now, biosolids are all used on the West side of the Cascades at a cost of less than $10/wet ton. Uses for the biosolids include fertilization for poplars at a City-owned plantation and as soil and cover material for the municipal landfill. “Application to the poplars is a very inexpensive option for us,” says Chesson, adding, “this summer, we will apply to other timber lands (Douglas Fir) the City owns.” The WWTP is also working with the Parks Department to develop a blend that would be suitable for a range of its projects. The thought here is that use of the biosolids within the municipal infrastructure will lead to greater visibility, acceptance and demand for the product by homeowners within the municipality.
For both Everett and Tacoma, working within the City infrastructure appears to be an integral part of their marketing strategy. There is an existing framework – utility bills – to target potential homeowners. In addition, within the municipality, there is an existing demand for the product. Potential uses in-house include public parks, landfills and other public works projects.
If the ties between the wastewater treatment program and the municipality are close enough, it may be possible to identify a need for Class B biosolids internally as well. In Whistler, British Columbia, the treatment plant produces either a Class A or Class B biosolids, depending on the flow rate into the treatment works where the solids are stabilized using an ATAD (advanced thermophilic anaerobic digestion) process. According to Jerry Chesuck, there are uses within the municipality for both materials – Class A cake for local landscaping, and Class B biosolids as cover material for the local landfill. This is an example of a sufficiently small and well-integrated municipal structure to make use of both materials economically similar.
Sally Brown is a Research Assistant Professor, Soil Remediation, in the College of Forest Resources, University of Washington. This article is based on a paper presented at the 2004 Northwest Biosolids Management Associations “Biofest” conference (

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