A Utility's Journey to Advanced Primary Treatment: From Pilot Testing to Full-Scale Implementation of Pile Cloth Media Filter

INTRODUCTION
Four Rivers Sanitation Authority (FRSA) in Rockford, Illinois, has long prioritized energy efficiency and sustainability, guided by its commitment to innovative technologies and operational excellence. Serving over 277,000 residents across 285 square km (110 square miles), FRSA operates a wastewater treatment facility with a permitted design average flow of 151,416 m3/d (40 MGD) and design maximum month flow of 302,833 m3/d (80 MGD)1.

As part of a facility planning initiative launc...hed in 2018, FRSA sought to upgrade its infrastructure, reduce energy consumption, and enhance resource recovery. The Authority's collaboration with the manufacturer and consulting engineer led to the adoption of Advanced Primary Cloth Media Filters. This advanced primary treatment solution was chosen for its ability to increase solids removal efficiency, reduce secondary treatment aeration energy demand, and boost biogas production, aligning with FRSA's sustainability goals.

PILOT TESTING — Understanding Pile Cloth Media Filters Potential
FRSA hosted a pilot study to evaluate cloth media filter performance compared to traditional primary clarifiers (Figure 1). The pilot, conducted using a Cloth Media Filter system, tested hydraulic loading rates (HLR) of 7.9— 9.8 m/h (3.25—4.0 gpm/ft2) and solid loading rates (SLR) of 43.9— 48.8 Kg/m2/day (9—10 lbs. TSS/ft2/day). Key results included:
- TSS Removal: 87.5%
- BOD Removal: 64.2%
- Effluent TSS: 26 mg/L

These results demonstrated pile cloth media filter's superior performance, offering significant reductions in solids and organics compared to existing clarifiers. By diverting more carbon to anaerobic digesters, primary filter increased biogas production and reduced downstream aeration energy demands.

FULL-SCALE IMPLEMENTATION — Scaling Up Sustainability
Based on pilot success, FRSA implemented two (2) 24-disk Pile Cloth Media Filter units, designed to handle an average flow of 56,781 m3/d (15 MGD) and a peak flow of 113,562 m3/d (30 MGD)2. The new filtration systems were strategically designed to be installed within the footprint of the aging primary clarifiers. The internal walls of the existing clarifiers were demolished, leaving the clarifier floors intact to serve as a foundation for constructing the filter units. Due to the hydraulic conditions at the facility, the design incorporates low lift pumps to transfer influent from an influent wet well to the filter's influent channel. The cloth media filter system's design allowed for splitting influent between the filters and remaining primary clarifiers. Backwash water and settled solids from the filters were routed to a dedicated gravity thickener before anaerobic digestion. The plant flow scheme is shown in Figure 2 and the aerial view of the new primary filtration building is shown in Figure 5.

Operational since mid-2024, the primary filter system has demonstrated a 20% higher removal efficiency for TSS and BOD compared to the existing primary clarifier, as detailed in Table 1 and illustrated in Figures 3 and 4.

Key points:
- The reduced flow rate to the Primary Clarifiers has improved their TSS removal by approximately 5%.
- Additional 1,134 Kg/d (2,500 ppd) average increase in primary sludge to digestion, from the 30% of influent flow to Primary Filters compared to all treated in the Primary Clarifiers.
- The backwash and settled solids waste from the primary filters are processed through a dedicated gravity thickener, achieving an average of 5.29% total solids (TS) in the underflow while maintaining a blanket depth of 0.36 — 0.46 m (1.2—1.5 ft). In comparison, the gravity thickener for the primary clarifiers produces an average of 5.07% TS with a higher blanket depth of 1.3 — 1.5 m (4.3—5 ft). Notably, the solids concentration from the gravity thickener for the primary filters is sufficiently high to eliminate the need for a secondary thickening step, such as a gravity belt thickener, allowing the solids to be sent directly to the digesters.
- The increased primary sludge contributes to a 10% boost in enhanced biogas production, generating significant energy to offset operational energy costs.
- The reduced loading to the activated sludge system may lower aeration demands, thereby decreasing secondary treatment energy consumption following the blower upgrades in the upcoming project.

RELEVANCE
The FRSA primary filtration project exemplifies the transformative potential of advanced primary treatment technologies in meeting today's wastewater challenges. By integrating pile cloth media filters into its treatment process, FRSA has achieved significant operational efficiencies, enhanced resource recovery, and made strides toward its sustainability goals. This case study demonstrates the importance of collaboration between utilities, consulting engineers, and technology providers in advancing innovative solutions, providing a model for other facilities seeking to upgrade infrastructure while maximizing sustainability and efficiency.