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FOG – Figuring Out Good Solution to Fats, Oils and Grease in Advanced Wastewater Treatment Systems

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The Miami-Dade Water and Sewer Department (MDWASD) is implementing a major upgrade program at the South District WWTP in response to capacity upgrade requirements, regulatory mandates and commitments to optimize water resources. In the recent past, MDWASD initiated implementation of one of the largest High Level Disinfection Programs in the country (285-mgd peak hour). In general, wastewater treatment facilities deal with a large number of operating and maintenance concerns, not the least of which is Figuring Out Good solutions to Fats, Oils and Grease (FOG) problems. The MDWASD SDWWTP has unique FOG problems including typical clogging and aesthetics concerns as well as the presence of very fine and dispersed FOG in its secondary effluent. The chemical composition and characteristics of FOG result in an inherent difficulty with its treatment and its impacts to WWTP's, especially for effluent polishing (filtration and chlorination) and advanced treatment with membranes. Based upon significant concerns for impacts of Fats, Oils and Grease levels on the existing secondary WWTP and concerns for clogging of 30 planned, large deep bed filters with a capacity rating of 285 mgd, an intensive field sampling, testing and evaluation program for FOG sources and treatment options was completed. Results of this evaluation were presented at WEFTEC 2005, and are hereinafter referred to as “FOG Part 1”. Under FOG Part 1, FOG clogging of main WWTP pipelines, and existing continuous backwash filters for effluent reuse onsite resulted in a detailed sampling and analytical program that reviewed and characterized FOG sources. FOG characterization in the septage, leachate and influent sources to the WWTP were addressed. Based upon this characterization, influent sources and clarifier scum removal were identified as key areas of focus in the WWTP. Related improvements to these key areas including the addition of a Septage Receiving and Treatment System at the front end of the WWTP, improvements to FOG removal at the secondary clarifiers, direct removal or vacuum removal of FOG from the wastewater flow stream, additional separation and concentration of FOG that is removed from the wastewater flow, and the addition of a select residuals management system for mixing, handling and disposal of FOG along with other residuals (grit, screenings, etc) were recommended at this secondary plant in preparation for the addition of an HLD system. These improvements are currently under construction with a planned completion of around fall of 2011.

Concurrent with the HLD and FOG upgrades, MDWASD is implementing one of the country's largest water reclamation facilities – 21-mgd treatment, delivery of 18.6 mgd of finished product water, used for augmentation of Miami-Dade County's drinking water supply through groundwater recharge. Thus, potential impacts of FOG on advanced treatment membrane processes- microfiltration (MF) and reverse osmosis (RO) needed to be investigated due to potentially significant impacts to capital costs, operation and maintenance and downtime that could limit capability for reliably providing the necessary quantities of water to augment drinking water supplies in this region.

An advanced treatment pilot system was constructed and tested using secondary effluent at the SDWWTP. The pilot system consisted of deep bed filters and high level disinfection (HLD) with chlorine solution, plus advanced wastewater treatment processes including MF, RO, ion exchange (IX) and ultraviolet disinfection with advanced oxidation (UV-A). The pilot was utilized to demonstrate treatment capability of the proposed process units, define design criteria, and better understand operational and maintenance issues and potential concerns such as FOG.

At the time of the pilot testing, the majority of the upgrades for improved FOG removal from the secondary process were still under construction, and therefore, the reclamation pilot was tested under “worst-case” conditions from a FOG perspective. FOG influent and effluent data were collected and analyzed. The pilot deep bed filter system removed FOG from the secondary process - FOG levels of roughly 1 to 11 mg/l were reduced to roughly 1 to 3 mg/l. Also, visual observations indicate that this removal of FOG at the filters did not impact the filter media. However, due to the nature of the fine FOG particles at SDWWTP, FOG downstream of the filters still needed to be addressed from the perspective of potential accumulation on the membranes. Regarding potential impacts of FOG on the membranes, specific flux plotted versus run time for the RO membranes shows relatively stable specific flux values, with gradual decline over a duration of about 3 months. A cleaning cycle then was implemented for collection of “cleanability” data. Subsequent to cleaning, the data generally show an increase in specific flux to roughly the initial specific flux rate of the membranes which supports that FOG did not accumulate on the membranes. Similarly, Permeate Conductivity Versus Run Time and showing Differential Pressure Versus Run Time indicate relatively stable data over roughly 3 months. In general, pilot testing supports that FOG, which has been a concern for the South District Secondary WWTP, is not anticipated to impact the advanced treatment processes for production of reclaimed water that will supplement the Miami-Dade County drinking water supply through groundwater recharging.
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Keywords: FOG; FOG recycling; FOG removal; O&M problems; advanced wastewater treatment; fats; filter clogging; membrane fouling; grease; oil; septage; water supply augmentation

Document Type: Research Article

Publication date: 2010-01-01

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