Bin the Floc, Boost the Rock: Increasing Activated Sludge Wet Weather Treatment Capacity using Densified Activated Sludge

Introduction
The Mangere Wastewater Treatment Facility (WWTF) is a 365 mgd (16 m3/s) hydraulically-rated facility servicing about 1.3 million population equivalents (PE) in Auckland, New Zealand, with plans to add another 250,000 PE. The Mangere WWTF is owned and operated by Watercare Services Limited, which provides drinking, storm, and wastewater services for nearly one-third of all New Zealanders. In addition to its ocean discharge with strict solids, nutrient, and pathogen limits, the WWTF experiences significant and extended hydraulic events due to the combined collection system and the region's propensity for intense and/or extended wet weather events. The management of peak flows, specifically the retention of total suspended solids (TSS) in the secondary process, was identified as a critical facility bottleneck, and planning efforts determined significant capital costs (approximately $160M+ New Zealand Dollars) would be required to construct a new dedicated wet-weather treatment facility using ballasted-floc technology.

To avoid or delay the future ballasted-floc improvements costs and lower carbon emissions associated with construction, Watercare began investigating ways to improve the peak hydraulic capacity in the secondary process. Continuous flow, densified activated sludge (DAS) with selective wasting hydrocyclones (HCs) was identified as a technology approach that may provide the desired secondary hydraulic uplift due to its ability to condition sludge to 'settle like a rock.' However, due to the lack of similar installations, there was uncertainty over the effectiveness of the DAS improvements on Mangere's four-pass, step-feed secondary process. A full-scale DAS trial on about one-tenth of the total facility capacity began operation in the third quarter of 2021 to demonstrate DAS improvement performance at the WWTF. After about 18 months of operation, the trial improvements were successful at reliably lowering the sludge volume index (SVI) from the historical values of 125 to 225 mL/g down to around 75 mL/g or below. This presentation seeks to share design and operational approaches used to gain about 35 percent in additional peak flow treatment capacity within an existing secondary process so that other facilities facing similar challenges may benefit from these efforts.

Liquid Stream Overview
The WWTF's liquid stream consists of influent pumping, preliminary treatment, primary treatment, activated sludge secondary treatment, tertiary filtration, and ultra-violet (UV) disinfection. The secondary process consists of reactor-clarifiers (R/Cs) with circular secondary clarifiers located in the center of the activated sludge reactors. Like many facilities, the secondary process capacity is limited by the settling performance within the clarifiers. The reactors are configured as four-pass, step-feed consisting of eight zones of alternating unaerated and aerated volume. Each of the unaerated zones is fed a portion of primary effluent. Figure 1 displays a layout of three R/Cs, including the DAS trial train (R/C#8). To test the ability to uplift the secondary peak flow capacity, trial DAS improvements on one R/C were developed utilizing reactor mixed-liquor rather than clarifier underflow for the HC feed. Figure 2 contains a photograph of the inDENSE™ HCs installed on temporary scaffolding adjacent to R/C#8.

DAS Trial Results
Once the HC system was installed and commissioned, the volumetric flow and mass splits for the HC equipment were tested, and minor modifications were completed to fine-tune the operation. Watercare operated the DAS upgrades for about one year (until October 2022) before significant improvements in SVI were recorded, but even then, the impact was highly variable with the trial train with higher SVIs than the remaining control basins. Starting in April 2023, SVI values reduced to about 100 mL/g, and over the next three months, SVIs dropped further to between 50 and 100 mL/g, averaging about 75 mL/g. A particle size distribution analysis completed at about 75 mL/g indicated an average granule size of about 100 micron, which is consistent with small DAS granules. Figures 3 and 4 display the DAS particle size results and SVI results from the trial period, respectively. With the improved settling, Watercare realized about a 35 percent uplift in the existing secondary processes' peak flow capacity through a full-scale peak wet-weather demonstration. Throughout the testing period, optimizations were made within the R/Cs to improve DAS performance until the 75 mL/g SVIs were achieved, including:
- Addressed faulty aeration valve actuators
- Increased the dissolved oxygen setpoints in the aerated zones to 1.5 mg/L
- Implemented supplemental clarifier underflow wasting
- Increased HC feed pressure from 30 to 35 psig (2.1 to 2.4 bar)

Next Steps
Given the successes of the DAS trials, Watercare is currently designing plant-wide DAS improvements with HC facilities located on additional R/Cs. Significant hydraulic structure improvements are required to achieve the hydraulic uplift expected with the DAS improvements. Coupled with the hydraulic uplift improvements efforts, new aeration equipment and controls, in-basin equipment, and supporting facilities will be upgraded to improve Mangere's ability to remove nitrogen as well.