Diffusers Here, Diffusers There, Diffusers Everywhere! Which Is Right For Me?

AECOM was engaged by Eastern Municipal Water District (EMWD) to conduct a diffuser replacement study evaluating disc, tube, and panel membrane diffusers to aid EMWD in selecting the appropriate technology to replace the existing diffusers at four of the District's regional water reclamation facilities (RWRF). As part of this effort, AECOM completed a market study to solicit general and performance information from diffuser manufacturers. Each manufacturer was asked to provide information on any of their products considered suitable for installation at all four facilities. Only technologies with more than five installations and five years of uninterrupted service were considered. Three disc, three tube, and two panel diffusers were submitted for this study by the manufacturers.
The following EMWD evaluation criteria were used to compare each diffuser type during the study:
1. Operating Pressure and Air Flow (Flux) Range
2. Compatibility with District's Existing Investment in Blower Systems
3. Compatibility with District's Aeration Basin Designs
4. Maintenance Requirements
5. Market Durability
6. Materials of Construction
7. Ability to Service with Non-Proprietary Parts
8. Ability to Competitively Procure Initial Installation and Subsequent Replacements
9. Facilitating the Cleaning of Aeration Basins
10. Customizable Configurations to Create Zones with Valved Drop Legs for Air Supply
11. Post Installation Manufacturer Support
12. Standard Oxygen Transfer Efficiency (SOTE)
Design Flux Evaluation The typical operating flux provided by each manufacturer, measured as airflow per surface area of diffuser are shown in Table 2 below as normalized to airflow as standard cubic feet per minute (SCFM) per area. For comparison purposes a typical flux of 3.5 scfm per square foot for disc diffusers and 1 scfm per square foot for tube and panel diffusers was used, consistent with the information provided by each manufacturer. While some minimum fluxes provided by the manufacturers are listed as zero, it is important to note the minimum flux for operating diffusers must be designed around mixing requirements. When comparing diffusers for the same application, diffuser energy consumption differences are primarily attributed to differences in the headloss and SOTE for each diffuser. Figure 2 outlines the manufacturer provided information regarding the clean water headloss through the diffusers as headloss in dynamic wet pressure (DWP) versus flux. In installed applications, as membrane fouling occurs, the headloss across the membranes increases. Minimizing headloss is an important aspect of reducing energy costs for the aeration system operation. Additionally, operating at a lower flux also reduces the stress on the membrane and therefore may increase the diffuser life. The manufacturers flux data was used to evaluate compatibility of each diffuser at the four EMWD facilities.
SOTE Evaluation In addition to variations in flux, diffusers will also vary in terms of SOTE. The manufacturers provided SOTE curves for each diffuser product showing the SOTE as percent per foot depth as a function of the diffuser flux. Figure 3 below shows the compiled data from each manufacturer. The information provided was used to estimate the clean water SOTE for each product. Note the testing conditions, including submergence depth and diffuser density, will impact the estimated SOTE. This information is used for preliminary comparison purposes only.
O&M Evaluation Each diffuser manufacturer was asked to provide typical operation and maintenance requirements for the diffuser products. Routine maintenance is essential in preserving the design operation and performance for the diffusers. Typical maintenance recommended by manufacturers include physical cleaning, chemical cleaning, and flexing. While table 4 outlines the manufacturers recommended O&M for each product, the warranty clauses for each product may dictate the type and frequency of cleaning required. The frequency of cleaning may also vary due to wastewater characteristics and other facility issues such as poor screening or reduced grit removal capacity. The materials offered by each manufacturer for membranes, piping, holders, and mounting systems vary. Each manufacturer provided assembly drawings and details for their proposed diffuser technologies. Important items to consider in selecting material include UV resistance for piping, fouling potential for membranes, and corrosion potential for metallic components of the system. The standard materials offered by each manufacturer are outlined in Table 5.
Market Durability Market durability was measured in terms of number of U.S. wastewater installations for each product, which are shown comparatively in Figure 4. All diffuser technologies evaluated have been proven for use in the industry and have a record of significant historical implementation.
Interchangeability EMWD's highest ranked selection criteria is the interchangeability of membranes after experiencing maintenance difficulties with their existing diffusers. A summary of the membrane interchangeability is outlined in Table 6.
Findings
Using the data collected from vendors, AECOM completed a market study evaluation for each diffuser technology. The selection criteria was weighted based on importance to EMWD and each diffuser was assigned a score from 1-10 for each criteria, one being the worst performing and 10 being the best performing. The market study evaluation results are outlined in Table 7 below. Based on the preliminary results of the Market Study, the four highest scoring diffusers are the Disc Diffusers 1-3 and Tube Diffuser 3. This part of study highlights the differences between diffuser types and diffuser products within each type of diffuser. The market study evaluation characterized the differences associated with the diffuser technologies and identified viable options for EMWD. The next phase of this project, to be presented in a future paper, is to complete a life-cycle cost evaluation including staged modifications and expansions at each plant, and a preliminary construction phasing program. The life-cycle cost evaluation will identify the recommended diffusers for use at the EMWD facilities. This work is applicable to any entity with aeration diffusers as it is important to understand the differences in products that may lead to a difference in performance.