HRSD's ROCI Project: Identifying and Fast-Tracking Improvements to Meet Solids Process Reliability and Community Needs

The Hampton Roads Sanitation District (HRSD) Atlantic Treatment Plant has undergone significant changes in the last several years, including the addition of new solids processes and the introduction of new flows from a system-wide wastewater treatment consolidation. Current solids processes include thermal hydrolysis pretreatment (THP) of primary and thickened secondary solids, followed by anaerobic digestion (primary digesters have floating covers), centrifuge dewatering, and long-term seasonal storage on covered pads at the site. Biogas is currently stored and then flared (an RNG system is in design). Figure 1 highlights key existing solids and biogas systems. Changes at the plant revealed vulnerabilities, however, with solids process reliability issues and changes in solids characteristics dramatically increasing odor complaints from neighbors near the plant. Figure 2 shows complaint increases between 2016 and 2023. This significant increase in odor complaints -- and uncertainties regarding the reliability and effectiveness of aging odor control systems on the site -- was the driver behind the HRSD Reliability and Odor Control Improvements (ROCI) project. Key objectives of the ROCI project included: - Improve solids process reliability -- the plant's single hopper for transferring cake to THP processing fails routinely, resulting in the need to haul raw cake from the site. This hauling was directly tied to odor complaints. A new THP train is needed to enhance reliability, and new gravity thickeners are also needed to improve primary solids management. - Reduce odors from existing solids processes -- odors from the digester annular space, digester pressure relief valves (PRVs), candlestick flares and biosolids storage pads have been tied to odor complaints. - Upgrade odor control systems -- odor control is needed for planned facilities (such as gravity thickeners) and aging odor control systems need replacement and/or rehabilitation. Given the number of odor complaints, critical first steps for the project were defining the most significant contributors to offsite odors and defining near-term solutions ('early wins') to reduce those impacts while longer-term solutions were under development. Dispersion modeling was performed (using data collected by HRSD staff) to define both the extent of offsite odors and primary contributors to offsite impacts. This effort showed that two of the existing chemical scrubbers were key contributors and, on the solids side, the biosolids pads and flares contributed as well. The digester annular space remained a concern based upon descriptors provided in some odor complaints. A comprehensive evaluation identified both near-term improvements to reduce odors and a series of long-term construction packages designed to address both odor and reliability concerns while expediting project delivery. Near-term Improvements were identified that could be implemented within a year by plant staff or by an on-call contractor to mitigate odors while other improvements were under design. These relatively simple and low-capital projects are highlighted in Table 1. To date, a counteractant vaporization system has been installed on the perimeter of the digesters (see Figure 3) and a similar system has been designed for the biosolids storage pads. The installed system has reduced the odors from the digester and improved the odor character. Curing trials at the biosolids pads show significant promise, reducing the odor and odorant concentrations of digested biosolids significantly over a 24-day period (see Figure 4 for odorant data). The trial focused on a blend of 75% fresh biosolids and 25% cured biosolids, which were placed in a windrow and turned frequently. While curing is expected to improve the quality of stored biosolids and create a potentially marketable product, the odor profile of the solids did increase over the first week if curing, and so a vaporization system for the pad perimeter is planned to address potential odor concerns during peak odor generation. Additionally, ROCI investigations have led to operational improvements that have already advanced project goals: - Operational strategies identified through the project will minimize THP downtime for annual maintenance, eliminating the need for temporary dewatering (saving $0.5M) and minimizing odor potential by reducing the amount of raw cake hauled - A focus on the biosolids pads and conditions that lead to offsite odors has led to modifications in pad management approaches, eliminating activity during meteorological conditions conducive to odor transport offsite - Detailed investigations of a large chemical scrubber onsite resulted in relatively low cost modifications and changed dosing strategies that have reduced odor from that source by 95% Long-term Improvements The alternatives evaluation for the project identified multiple improvements with varying priorities to meet HRSD objectives, with a combined construction cost estimated at $196,200,000. The team determined that a Construction Manager at Risk (CMAR) delivery method would allow for the most flexibility to complete the project while prioritizing the key objectives of reducing odor and improving solids reliability. The CMAR contractor was brought in at the completion of the preliminary engineering report and assisted with grouping the improvements into Work Packages, with the highest priority items covered under an Emergency Declaration by HRSD. This approach significantly expedited the project by facilitating the procurement of critical and/or long lead equipment and allowing for demolition to begin on plant site before finalizing design. Table 2 highlights key improvements, their priorities and their objectives, while Figure 5 shows the schedule for this fast-track project. Meeting the Needs of HRSD ATP and the Community Although much of the project design and construction remains, early wins have already advanced HRSD odor objectives. Dispersion modeling (Figure 6) shows the benefit of even near-term improvements: the area around the plant exceeding ATP's odor target is reduced by over 60%, and the frequency impacts is significantly reduced. With the implementation of long-term improvements, the area of odor impacts above the ATP target is expected to be 90% lower than at the project's start and those impact exceedances are expected to be rare. Moreover, improvements are staged to meet project objectives on an expedited schedule -- meeting HRSD processing and reliability needs -- as well as those of the community -- as soon as possible.