Keeping Granules Happy In A Salty Environment: Aerobic Granular Sludge Testing At Sand Island WWTP

The Sand Island Wastewater Treatment Plant (SIWWTP) provides removal of five-day biochemical oxygen demand (BOD5) and total suspended solids (TSS) through advanced primary treatment. The City and County of Honolulu (City) is in the process of upgrading SIWWTP to meet secondary treatment water quality standards (30 mgTSS/L, 30 mgBOD/L) using a hybrid secondary treatment approach of membrane bioreactor (currently under construction) and aerobic granular sludge (AGS) technologies.

SIWWTP is one of the few places in the world that will use AGS with a low-strength, high-salinity wastewater, providing insight to other facilities. SIWWTP influent wastewater is unique compared to conventional wastewater, such that chloride concentration ranges from 3,000 to 5,000 mgCl-/L, BOD is ~160 mgBOD/L, TSS is ~140 mgSS/L, and COD is ~350 mgCOD/L. The high-salinity wastewater impacts go beyond AGS performance and stability; it also affects instrumentation reliability which can be challenging when using a treatment technology that is reliant on online instrumentation control.

Technology testing (Figure 1) of AGS is underway to evaluate AGS performance using SIWWTP influent wastewater with respect to maintaining sufficient granulation conditions, stable effluent quality, and representative online measurements. Results from the technology testing will be used to inform full-scale design.

AGS technology testing began March 2024 and is anticipated to end March 2025. Testing is being completed in three phases: Phase 1 acclimate the granules to SIWWTP influent wastewater, Phase 2 compare influent sources of primary influent (PI) vs. primary effluent (PE) at various load conditions and evaluate alternative aeration control strategies, and Phase 3 seed with conventional mixed liquor (ML) to evaluate the rate of granule formation.

Phase 1 occurred over 3 months during which side-by-side AGS reactors were seeded with ML from the Aqua-Aerobics Systems Inc. demonstration facility and were gradually introduced to SIWWTP PI. The reactors adequately acclimated and stable performance was reached with the key performance indicators (target values) met, presented in Table 1.

Phase 2 occurred over 4 months during which influent wastewater sources were modified; one reactor was fed with PI and the other PE, to evaluate if effluent performance and granulation is impacted with a low influent carbon source (PE, average annual conditions). Various load conditions including average annual, maximum month, maximum day, and wet weather flow were also tested. Similar performance was observed between the two reactors for the duration of Phase 2 with respect to effluent quality and sludge characteristics shown in Table 1 and Figure 2. The results of the Phase 2 influent comparison and load testing provided the City with confidence to design the AGS at full-scale with primary clarification upstream at a variety of influent loading conditions. Maintaining the use of primary clarification will reduce the AGS aeration demand and allow the City to benefit from a high gas production rate with primary sludge.

During Phase 2 alternative aeration cycle control setpoints were also tested to evaluate the impacts of limiting nitrification while maintaining performance. Nutrient removal is not required at SIWWTP so there is flexibility in aeration control. The results for pH control were favorable, offering a reduction in aeration time. For full-scale, this finding may result in potential cost savings (i.e. operating the blowers for a shorter duration). Effluent nitrogen and phosphorus from the various tests of Phase 2 are shown in Figure 2.

#The AGS reactors were drained at the end of Phase 2 and reseeded with ML from a nearby facility in October 2024. Phase 3 is evaluating the speed at which granules form and if the influent source (PI vs. PE) impacts the rate of formation. The current status of Phase 3 is shown in Figure 3.

Throughout the testing program, the performance of the online instrumentation was monitored to understand the potential impacts of the high-salinity wastewater on analytical equipment. The AGS cycle schedule depends on sensor readings and the accuracy of online instrumentation is integral to maintaining process performance. The sensors provided with the technology testing unit are representative of the typical sensors recommended for full-scale, such as pH, NH3, DO, ORP. The sensors require routine maintenance; cleaning, validation, and calibration and some have interferences with salinity and potassium. Findings from sensor interferences and maintenance frequency will be used in instrumentation recommendations during full-scale design.

AGS technology testing at SIWWTP provides the opportunity to test side-by-side reactors with different influent strengths (PI, PE) and unique wastewater. This presentation will provide an overview of the results, including the impacts of using AGS in a low-strength high salinity wastewater, performance using different aeration control strategies, lessons learned on online instrumentation, and design implications.