Your news item “Solar magic: Greenhouse turns wastewater into organic fertilizer” (August) was interesting and made some very optimistic claims about potential savings.
I have to ask, where’s the magic? I remember some decades ago working with a utility to compost biosolids and generate organic fertilizer. A major operational concern was achieving and maintaining a minimum core temperature to kill potential pathogenic bacteria and viruses. The solar greenhouse appears to create a warm environment to air-dry the biosolids, hoping to “somehow reduce the pathogens to below detectable levels.” So, where is the reduction in pathogens?
The article says the city is waiting for test results from the Mississippi Department of Environmental Quality. I myself would like to see those results.
, senior civil engineer
CJ Roberts Inc. (San Diego)
The August story focused on the “what” of this project. To find out the “how,” we went back and asked David Gardner, superintendent of the Natchez (Miss.) Water Works, to provide some more detail.
In this process, the pathogens are reduced by starving them of water, subjecting them to ultraviolet light, and exposing them to naturally occurring large temperature swings. It is very hot during the day, when the sun is shining, and much cooler at night, Gardner said.
When each batch of biosolids has completed processing in the solar dryer, it will be tested for pollutant limits, Class A pathogen reduction, and vector attraction reduction. Each batch also will be tested for total nitrogen and alkalinity for labeling purposes. One solar dryer, or each subdivided portion thereof, constitutes one batch.
Gardner explained that the regulatory mechanisms at work to consider this material Exceptional Quality Class A biosolids are Alternative 4 for pathogen destruction, Option 7 for vector attraction reduction, and metals testing under the U.S. Environmental Protection Agency Part 503 regulation.
Alternative 4, called “Biosolids Treated in Unknown Process,” requires three criteria to be met:
- Either the density of fecal coliform must be less than 1000 MPN per gram of total solids (dry-weight basis) or the density of Salmonella sp. must be less than 3 MPN per 4 g of total solids (dry-weight basis).
- The density of enteric viruses must be less than 1 PFU per 4 g of total solids (dry-weight basis).
- The density of viable helminth ova in the biosolids must be less than 1 per 4 g of total solids (dry-weight basis).
Option 7, called “Moisture Reduction of Biosolids Containing No Unstabilized Solids,” requires that the biosolids do not contain unstabilized solids generated during primary treatment and solids content is at least 75%.
The metals content of biosolids also will be tested to ensure that it does not exceed the limits listed in Table 3 of 40 CFR 503.13.
Natchez was scheduled to submit a composite sample from three separate batches of solids to a certified laboratory on Sept. 26, according to Gardner. The results were not available at press time.
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