As more wastewater treatment facilities upgrade to tertiary treatment for pathogen removal and trace pollutant reduction, the options for tertiary filtration expand. The use of cloth-disk media filtration (disk filters) for tertiary filtration is becoming more common because of the effluent quality, lower backwash rates, small footprint, and ability to meet California Department of Health Services Title 22 criteria for the unrestricted reuse of treated effluent. Two manufacturers currently have Title 22 approval, the Aqua Aerobic AquaDisk and the Kruger Hydrotech Discfilter.
Relative performance of disk filter technology can vary depending on the site-specific characteristics of the secondary treated wastewater used as the influent to the disk filters. Pilot testing of the two Title 22 approved disk filter manufacturers was performed concurrently to determine the filter best suited for the specific conditions at the City of Merced Wastewater Treatment Plant (WWTP). Side-by-side piloting included several logistical challenges, but allowed the use of the same procedures, on the same source water, and a direct comparison of testing results. The suitability of each filter technology at Merced was determined based on quantification of the filter operating characteristics and evaluation of filter reliability and performance based on varying influent quality, loading rate, and chemical dose rates. This testing then allowed for a more specific determination of design criteria for the filters.
The City of Merced is in the process of upgrading the Merced WWTP to meet more stringent discharge requirements. These upgrades are occurring concurrently with a plant expansion from 10 million gallons per day (mgd) to 15 mgd. Upgrades include complying with the California Department of Health Services (DHS) Title 22, Chapter 4, of the California Code of Regulations (Title 22). Within the State of California, compliance with Title 22 criteria is required for the unrestricted reuse of treated wastewater.
The principal objective of the filtration pilot study was to evaluate the performance of the two manufacturers of cloth medium filtration for the WWTP secondary effluent with specific reference to Title 22 wastewater reuse criteria. This objective included evaluation of the filter reliability and performance as affected by variations in filter influent quality, loading rate, and chemical addition.
Two disk filter manufacturers currently have Title 22 approval from the California DHS, Aqua Aerobic AquaDisk and the Kruger Hydrotech Discfilter. Both filtration units are gravity flowthrough systems. However, the specific operating process and filtration cloth is different. Piloting was performed because there exists potential site-specific construction requirements as well as the potential for site-specific performance. In addition, the disk filter technology is a relatively new technology and both manufactures have had Title 22 approval for less than 5 years. The specific characteristics of each of the disk filters are described below.