Achieving Pathogen Stabilization Using Vermicomposting
Within the last decade, implementation of state and federal regulations and other local codes have changed biosolids processing in Florida. Previously, biosolids stabilization varied greatly among the state’s 3,500 to 4,000 wastewater treatment facilities. Public and privately owned wastewater treatment facilities were required to stabilize their biosolids to a minimum Class C standard for land application, with most facilities using aerobic or anaerobic digestion. Requirements for septage solids stabilization, prior to publication of the current rules and regulations, were minimal at best. While record keeping was more organized than for septage stabilization, it was still insufficient.
With implementation of the new rules, these facilities were required to stabilize to a Class B standard. Class C was no longer satisfactory for land application. For most small facilities, this was impossible to achieve without prohibitive retrofitting and expansion, as they usually generated Class C biosolids.
Consequently, it became incumbent upon government to explore alternative methods. The Orange County (Florida)
Environmental Protection Division (OCEPD) undertook research for the potential use of earthworms as an alternative human-pathogen (pathogen) stabilization method for biosolids. Research revealed studies suggesting that vermicomposting may be effective in stabilizing pathogens. In some cases, precomposting of biosolids was done to eliminate pathogens. OCEPD staff felt, however, that the earthworms would eliminate pathogens during the vermicomposting process, making the precomposting step unnecessary. Thus the “nonthermal windrow vermicomposting” method was developed.
In 1995, a partnership was formed between OCEPD, American Earthworm Company and the city of Ocoee, Florida. The goal was to develop a Class A pathogen reduction method that is both cost-effective and meets all criteria for public health and safety set forth by the governing agencies. However, the U.S. Environmental Protection Agency (EPA) had not established standards in this area. After communicating the goals of the project and its potential benefits, the EPA developed criteria by which this project and future research could be applied to public health and safety.
Initially, a pilot study was conducted to evaluate vermiculture’s effectiveness with biosolids on a small scale. The pilot study demonstrated a noticeable reduction in the four pathogen indicators: fecal coliform, Salmonella sp., enteric virus and helminth ova in the biosolids. The next step was to begin a full-scale operation to define the project’s operational feasibility. The EPA issued a two-year experimental permit in March, 1997, with project oversight for EPA performed by the Florida Department of Environmental Protection (DEP). Standard operating procedures would be developed from the information gathered throughout the full-scale operation.
In order for vermicomposting to be considered by the EPA as an alternative methodology for Class A pathogen stabilization, the project needed to demonstrate a three to four fold reduction of pathogen indicators that had been spiked into the biosolids in the test and control plots. The EPA office in Cincinnati set those parameters using the reasoning that if vermiculture can demonstrate a three to fourfold reduction in residuals that have abnormally high number of pathogen indicators, than it could be assumed that it would reduce pathogen content to the requirements of Part 503 in residuals containing normal numbers of pathogens. (EPA’s Class A pathogen reduction requirement is as follows: The density of fecal coliform in the biosolids must be less than 1,000 most probable numbers (MPN)/gram total solids (dry-weight basis) or the density of Salmonella sp. bacteria in the biosolids must be less than three MPN/four grams of total solids (dry weight-basis)).
To test whether the three-to fourfold reduction could be accomplished, a portion of the full-scale operation was utilized to house the experimental plots. A structure was constructed to protect these plots from adverse weather. Biosolids (15 to 20 percent solids) were land applied into two rows approximately 6 m long by 1.5 m wide by 20 cm deep, utilizing approximately 1,361 kg of biosolids each. One row was designated the test and the second was the control. These two rows were inoculated with a minimum 105 spike of three of the four pathogen indicators: fecal coliform, Salmonella sp. and enteric virus.
The test row was then seeded with E. foetida at a 1:1.5 earthworm biomass to biosolids ratio. This ratio represented the earthworms’ feeding rate for a 24 hour period. Earthworms were provided 1,361 kg of biosolids for a 14-day feeding period. Samples of both rows were collected from random locations and analyzed throughout the project. The helminth ova portion of the experimental project was conducted at a separate time due to difficulty in acquiring the helminth ova eggs. Biosolids (15 to 20 percent solids) were land applied into two rows approximately 2.3 m long by 1.5 m wide by 23 cm deep. One row was designated as the test and the other the control. The test row was then seeded with E. foetida similar to the three pathogen tests. Florida peat, the substrate in which the earthworms were held, was spread across the test row, adding approximately 15 cm to the depth. Earthworms were provided 531 kg of biosolids for feeding for a seven-day period. Samples of both rows were randomly collected and analyzed throughout the project.
ACHIEVING REQUIRED REDUCTION
Analytical results showed that all of the pathogen indicators in the test row had a greater reduction than in the control row. EPA’s required three- to fourfold reduction was achieved in all of the pathogen indicators within 144 hours. Fecal coliform, Salmonella sp. and enteric virus achieved the EPA goal in 24 hours, 72 hours and 72 hours, respectively. The helminth ova achieved this goal within 144 hours. The helminth ova reduction times were slightly elevated compared to the previous test with the three pathogen indicators. OCEPD staff felt this occurred because of the addition of peat (substrate in which the earthworms were held) when the earthworms were added. The earthworms remained in the peat and did not immediately migrate to the biosolids. Therefore, slower reductions may have occurred because the earthworms already had a food source in the peat.
The initial baseline analysis for fecal coliforms in the test row was an average 8.5 billion MPN/one gram; the control row average was 8.3 billion MPN/one g. After just 24 hours, the test row samples showed an average six-fold reduction (98.70 percent) of fecal coliforms; the control row samples had a less than one-fold average reduction (20 percent). Samples collected every 24 hours for 14 days showed that reductions continued in both the test and control rows. However, the reductions in the test row were much greater and quicker than those of the control. This is due to the vermicomposting process, whereas reductions in the control row can be attributed to the natural die-off of the organisms. This is true for all of the pathogen indicators in the control row.
The initial baseline analysis for Salmonella sp. in the test row was an average 4.6 billion cells/25 ml; the control row average was 5.2 billion cells/25 ml. Samples for the Salmonella sp. analysis were collected at 72 hours and 144 hours. After 72 hours, the test row samples showed an average 13-fold reduction (99.99 percent); the control samples showed an average three-fold reduction (93.18 percent).
The initial baseline analysis for enteric virus in the test row was an average 197,000 plaque forming units (PFU)/four grams; the control row average was 173,000 PFU/four g. Samples collected for enteric virus analysis were collected at 72 hours and 144 hours. After 72 hours, the test row samples showed an average six-fold reduction (98.92 percent); the control row only had an average one-fold reduction (53.8 percent).
Viability tests done by a research team at Tulane University (headed by Dr. Robert Reimers) indicated that the helminth ova spike in the test row was 826,000 viable eggs (Ascaris sp.); the control row spike was 841,000 viable eggs (Ascaris sp.). Samples collected for helminth ova analysis were collected at 72 hours and 144 hours. After 72 hours the test row samples showed a less than one-fold average reduction (47.5 percent); the control row samples showed no reduction. After 144 hours, the test row samples showed an average six-fold reduction (98.87 percent); the control row samples showed a one-fold reduction (74.24 percent).
These results show that EPA’s required pathogen reduction in the indicator organisms was obtained, suggesting that vermicomposting can be used as an alternative method for stabilization of Class A biosolids. After pathogen stabilization has been achieved, the castings would need to be air dried to 75 percent solids to meet vector attraction reduction requirements. Drying can be done by windrowing or through a mechanical process. The latter takes one to two days (or less); however, caution should be used to prevent the destruction of the beneficial bacteria developed during vermicomposting.
All supporting documentation from this project will be submitted to the US EPA for consideration as an alternative “Class A Pathogen Stabilization Methodology.”