Graduation Date

Summer 2019

Document Type

Project

Program

Master of Science degree with a major in Environmental Systems, option Environmental Resources Engineering

Committee Chair Name

Brad Finney

Committee Chair Affiliation

HSU Faculty or Staff

Second Committee Member Name

Robert Gearheart

Second Committee Member Affiliation

Community Member or Outside Professional

Third Committee Member Name

Eileen Cashman

Third Committee Member Affiliation

HSU Faculty or Staff

Subject Categories

Environmental Resources Engineering

Abstract

Oxidation ponds (OPs) are a natural wastewater treatment system that have been used internationally for their low maintenance, cost, and technology characteristics. The City of Arcata has relied on the performance of their two-oxidation ponds for the past 50 years to meet National Pollutant Discharge Elimination System (NPDES) discharge permit requirements. However, during specific times of the year and after decades of operation and minimal maintenance, these oxidation ponds serve as a source of solids and nitrogen that create stress for downstream components of the system. As bio solids have accumulated, the hydraulics of the OPs have deteriorated and the effects of stored pollutants such as ammonia and suspended solids have started acting as an internal load that the downstream system has to remove. As a result, the Arcata Wastewater Treatment Facility (AWTF) is exploring options for bio solids remediation during its facility upgrade.

The objective of this project was to characterize the accumulated bio solids for OP1 in three different ways. First, a spatial analysis and inventory was done using a field sample survey and ArcGIS to determine the bio solid volume, mass accumulation, and its spatial distribution within OP1. Second, the composition of the bio solids was explored using standard analytical procedures to determine what and how much of regulated pollutants are deposited in OP1. Lastly, the treatability of the bio solids was tested using a series of simple aeration experiments that were operated with roughly 6-ft3 of a solids/water slurry that mirrored the conditions in OP1. Aeration was tested because it was the primary treatment option considered by the AWTF.

The results of the characterization process revealed that nearly 40% of the OP1 volume is occupied by wet bio solids with the most impacted areas being located at the inlet and outlet borders of the pond. The mass of these stored solids (approximately 11,416,000 lbs.) is roughly equivalent to the solids load that the AWTF would receive over more than eight years of operation. These stored solids contain significant masses of nitrogen (325,000 lbs.: estimated to more than one-year worth of AWTF influent nitrogen mass) and other high oxygen demanding substances. Over 1.5 million lbs. of dissolved oxygen would be required under a best-case scenario to aerobically treat this mass of nitrogen alone.

The results of these characterizations support the need for solids remediation of some form in OP1. The accumulated solids are occupying volume that could be used for operational storage and the integration of stored pollutants into the bulk water volume is creating a strain for the treatment systems downstream of OP1. Aeration that both provides oxygen and keeps the bio solids suspended is recommended along with the need for the development of best operating procedures should aeration equipment be installed.

Citation Style

ASCE

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