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Date: September 17, 1999 - page 3 / 9





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As per our letter of proposal dated March 2, 1999, Earth Tech has been requested to evaluate options available to the Prescott Valley Wastewater Treatment Plant (WWTP) for biosolids beneficial reuse. Earth Tech was also to prepare cost estimates for construction, operation, and maintenance of the biosolids technologies. It was necessary for Prescott Valley to undertake an evaluation of biosolids technologies, in order to plan for the current and future growth of the area. The following study provides that evaluation.


The WWTP in the past has utilized sand drying beds for the dewatering and stabilization of biosolids. The existing sand beds, however, have been problematic to date. Drainage through the sand beds has been poor, due to plugging of the media and/or crushed underdrains, causing a backlog of solids at the WWTP. This ahs manifested in process and aesthetic/housekeeping issues at the WWTP. To immediately rectify this problem, the WWTP employed the use of a belt filter press to dewater and remove the solids from the system at a more accelerated rate. This system, unfortunately, does not provide any form of stabilization and, as such, may necessitate the continued disposal of biosolids at a local land ill.

Stabilization of solids is dictated by federal, Environmental Protection Agency, regulations (40 CFR Part 503 – Standards for the Use or Disposal of Sewage Sludge). These regulations dictate the classification of biosolids based on the various pollutant concentrations, vector attraction, and pathogenic organism populations contained in the biosolids. Class A biosolids are considered to be of the highest beneficial reuse value with little danger to humans or animals. Class B is of a lower quality, with the main difference being the quantity of fecal coliform organisms contained in a specific volume of the biosolids. Biosolids classified below a Class B, are not suitable for beneficial reuse. Until recently, Prescott Valley’s biosolids were beneficially reused as a Class B product, through a contract with Pima Gro. The return to that practice is made part of this report.

Earth Tech has evaluated a number of processes that would achieve Prescott Valley’s objective of obtaining a Class A biosolid and would fit into the specific operational needs of the WWTP. Capital and operations and maintenance costs for this analysis were based on a maximum month flow and concentrations of 3.0 million gallons per day (mdg) and 250 milligrams per liter (mg/l) BOD and TSS, respectively. These values follow the full capacity of the existing phase II system (daily average flow and concentrations of 2.5 mgd and 200 mg/l, respectively). It is assumed that the capacity of the system will be reached within the next 5 to 10 years at the projected growth rate of the community. By utilizing these flow and loadings, the analysis/design will provide a system that will meet Prescott Valley’s growth rate and ensure proper biosolids management well into the future.

Earth Tech evaluated the following biosolids technologies:

  • Quick Dry Filter Beds

  • Belt Filter Presses

  • On-Site Solids Holding/Contract Hauling & Beneficial Reuse

  • Agitated Air Drying and Curing

  • Autothermophilic Aerobic Digestion

  • Aerobic Digestion

  • Land Application

  • Biosolids Enrichment and Recycling

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