Solving Odor Control Needs while Meeting Indoor Endotoxin Limits and Optimizing Energy Used for Ventilation at the Avon WWTP

Controlling odors and managing health risks associated with possible exposure to Endotoxins has always been a high priority for ERWSD. The Avon Wastewater Treatment Facility (AWWTF) has eliminated solids handling by discharging biosolids to the downstream Edwards facility, and is in the process of expanding the secondary facility, such that energy usage for heating and ventilating as it relates to odor control and meeting ventilation guidelines for the upcoming Nutrient Upgrade Project is an important element of the design. Design of the project is complete and construction has begun. Details of the design will be presented in this paper, where odor control has been integrated into the Nutrient Upgrade Project. Historically, comprehensive odor studies were conducted during each major process undertaking at Avon, including in 2104 before the plant shut down the Autothermal Thermophilic Aerobic Digestion (ATAD) process in 2016 and in 2018 during the planning stages of the plant expansion. The facility used an extensive odor control system in 2014 including three stages of chemical scrubber treatment for ATAD exhaust air and an Ozone oxidation process for the Headworks and Primary treatment processes. For the 2018 study, after ATAD was eliminated, ERWSD's goal was to meet a new standard of 'as near to zero community odors' as possible. The facility plan called for construction of a building over the aeration basins instead of keeping the flat basin covers, which significantly increases ventilation and exhaust air flow rates, therefore higher levels of odor treatment would be required to meet the goals. The ERWSD requirements were to provide full redundancy and multiple stages of odor treatment. This paper will present data, analysis of alternatives and the methodology used to meet these objectives. The design includes replacement of the ozone treatment system, which was found to be ineffective in treating odors (50% removal efficiencies) with a multiple stage carbon treatment train and the demolition of scrubbers that had reached the end of their useful life. Ozone was injected into the Headworks Building with 12 Air Changes per Hour (ACH) of outside air and exhaust from the primary and grit tank covers were also ducted to the Oxidizer chamber. Strobic Air fans will be used for the discharge of air from the Aeration Basin Building, providing dilution and a high emission profile. Modeling used in the project is presented below, showing improvements in community odors each time changes were made or planned for the facility. These are peak odor contours (Dilutions to Threshold -DT) for the 2014 plant, the 2018 plant and for the future expanded plant, in that order, showing improvement at each stage. Total plant Odor Emission Rates (OER) for each stage are as follows, with the increase in odor emissions for the expanded plant being the result of exhausting 33,000 cubic feet per minute (cfm) from the aeration building instead of a much lower exhaust rate from the existing flat covers: ERWSD has probably done more research and testing on Endotoxins at their plants, starting in the early 1990's and in the secondary clarifier buildings than any other municipality in the U.S. and has established limits on Endotoxin exposure for the buildings where workers are present. Endotoxins are lipopolysaccharides which are found in gram negative bacteria (some of which could be pathogenic) and exist in the outer membrane of the cell wall. These substances can cause respiratory or flu like symptoms so ERWSD keeps workers safe by providing sufficient ventilation to keep endotoxins below 90 Endotoxin Units (EU) per cubic meter (m3) for an eight (8) hour exposure based on 'no-observed-effect' health risk assessments performed in Europe. This paper will present monitoring data collected by ERWSD on Endotoxin levels inside buildings and from point sources. In this case, three (3) ACH's) was selected for the Clarifier Building to dilute Endotoxin concentrations with outside air below the 90 EU/m3 guideline At one point in this study, consideration was given to providing some make up air to the aeration basin from the unclassified clarifier building to save energy, but the consensus was to be safe and not use air that may contain some level of Endotoxins as make up air to another area. Odor control at wastewater plants is closely related to the heat and ventilation systems in the same areas. National Fire Protection Association (NFPA) 820, titled, 'Standard for Fire Protection in Wastewater and Collection Facilities' calls for a variety of fire safety systems, including guidance on how to mitigate hazards by ventilating spaces where fire risks are present. Areas upstream of the Avon WWTP primaries are ventilated per NFPA 820 with 100% outside air (OA) to mitigate explosion risks and are considered National Electric Code (NEC) Class I Division 2 areas, which require properly rated electrical equipment. However, areas downstream of the primary treatment basins are not required to have fire risk mitigation by NFPA 820 requirements. While not required to be ventilated by NFPA 820 to mitigate explosion risks, the areas downstream of the primary basins that can be entered by plant staff will be ventilated at rates to reduce risks associated with indoor air quality, such as limiting exposure to Endotoxins, mitigating buildup of corrosive fumes and meeting odor control goals. Ventilation can have a large impact on energy usage because OA may need to be heated or cooled to protect personnel and equipment inside the building. In Avon, a Colorado mountain town, the winter conditions can be extreme, requiring heating of the OA to prevent freeze and cold-temperature risks. There were a variety of ways to address the energy usage associated with ventilation at Avon including: --Heat the supply air with natural gas or electricity -- Heat the supply air with an existing wastewater-effluent-source heat pump system -- Air transfer ventilation using heated air from areas that do not have air quality risks, -- Energy recovery systems that transfer heat (but not contaminants) between exhaust and supply air This paper explores the various options at the Avon WWTP and how the design team settled on OA ventilation with supply/exhaust energy recovery for post-primary areas. The discussion will also describe how the use of supply/exhaust energy recovery resulted in a variance to IBC required building envelope insulation.