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Many utility providers face growing problems with the disposal of the wastewater sludge (residual) that are created as a part of the wastewater treatment process. Still other utility providers are looking to additional methods for converting the residuals into fertilizer/soil conditioner with higher economic and social value. The new technology presented in this paper provides a composting method to address the disposal and/or use of wastewater residuals.

Composting of wastewater residual is a bio-thermal aerobic process that decomposes the organic portion of the residuals. During composting the heat generated by the decomposition of the organic portion of the residuals reduces the moisture content of the residual, stabilizes the residual and renders the residual harmless transforming it into a usable biosolid. The higher the organic content of the residual the greater the quantity of heat that will be generated during the composting process. This greater heat release results in a higher temperature that causes additional moisture to be evaporated. Since raw residual (from primary clarifiers and secondary clarifiers) contains more organic than digested residual it is reasonable to prepare compost from dewatered raw residual. By utilizing raw residual the digesters, pipe, pumps, electrical power, personnel, etc. normally utilized in the digesting process can be reduce or eliminated.

A problem with composting raw residual is the higher intensity odor that is released as compared to odor released by digested residual. This is due to the higher organic content of the raw residual. Experiments show that the organics lose their odor when their pH is raised from the typical of 5.5 – 6.5 to a pH of 10.0 – 10.5. The pH of the raw residual can be raised by the addition of quick lime. While raising the pH, the addition of quick lime also causes additional heat to be released at the beginning of the composting process. This release of heat shortens the time span of the mesophilic phase (25 to 40 degrees Celsius) and drives the process to the thermophilic phase (55 to 65 degrees Celsius) quicker resulting in an overall reduction in the time required for the composting process.

This method of composting requires quick lime to be mixed with the dewatered raw residual. Once the quick lime and residual are thoroughly mixed a bulking agent (sawdust, peat, woodchips, bark, hydrolyzed liquin, etc.) and a portion of recycled compost is added. This mixture is then formed into piles and allowed to compost until a temperature of 55 to 65 degrees Celsius has been maintained for 7 to 11 days.

By maintaining the recommendations presented in this paper a Class “A” biosolid can be produced. This Class “A” biosolid provides the utility operator the maximum flexibility for its disposal or use.
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Document Type: Research Article

Publication date: 2001-01-01

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