BEING GOOD STEWARDS: IMPROVING EFFLUENT QUALITY ON A BARRIER ISLAND Brett T. Messner, PE, Tetra Tech, Inc., 201 E Pine St, Suite 1000, Orlando, FL 32801 Brett.Messner@tetratech.com, Ph: 239-851-1225 Fred J. Mittl, PE, Tetra Tech, Inc., Fort Myers, FL Kevin M. Friedman, Tetra Tech, Inc., Orlando, FL 1.0 Executive Summary The City of Sanibel s Donax Water Reclamation Facility (WRF) has a permitted design capacity of 2.375 million gallons per day (MGD) on a maximum monthly average daily flow (MMADF) basis. The facility produces effluent that meets its regulatory requirement of Florida Department of Environmental Protection (FDEP) criteria for public access reuse; it does not have a surface water discharge. The City is considering treatment improvements / alternatives to: Extend the useful life of the facility. Improve effluent quality to reduce nutrient loadings. Current biological treatment at the Donax WRF consists of three (3) biological treatment process units, WWTP Nos. 1, 2, and 3. All three biological treatment units are supplied from a common influent structure and discharge through a common effluent structure. A process flow schematic of the major treatment process units for the current facility is presented in Figure 1-1. Primary treatment and disinfection were not considered in this analysis and are not shown. In wastewater treatment unit operations and processes are grouped together to provide treatment to meet required effluent quality standards. Effluent quality requirements for key regulatory parameters for three (3) different effluent quality standards are identified in Table 1-1. Figure 1-1 Existing Block Diagram 1
Table 1-1 Key Effluent Quality Parameters Parameter Public Access Reuse (Permit Limit) Advanced Waste Treatment Numeric Nutrient Criteria BOD 30 mg/l 5.0 mg/l -- TSS 5.0 mg/l 5.0 mg/l -- NITROGEN* 12.0 mg/l (NO3) 3.0 mg/l (TN) < 1 mg/l (TN) PHOSPHORUS* REPORT 1.0 mg/l < 1 mg/l * Effluent quality standards for numeric nutrient criteria are dependent upon receiving body of water as set forth in FAC 62-302.532. The Donax WRF currently provides a level of treatment to meet public access reuse effluent quality. The capital construction and operating costs to provide treatment to meet numeric nutrient criteria (FAC 62-302.532) is an order of magnitude greater than the capital and operating costs for treatment that meets advanced waste treatment criteria. For this reason, and because improved treatment is not required to meet the facility s regulatory requirements, treatment alternatives to meet numeric nutrient criteria were not considered. Treatment goals for the improvement alternatives considered ranged between public access reuse and advanced waste treatment (AWT) standards and will focus on nitrogen reduction. All of the treatment improvements / alternatives considered address the areas of the facility that require attention to extend the useful life of the facility as identified in the recently completed Operation and Maintenance Performance Report (OMPR). Nitrogen reduction will vary with the treatment alternatives considered. The following treatment alternatives were considered: Baseline Upgrade Treatment Process Unit 1 from conventional activated sludge treatment consisting of aeration and a clarifier to a Modified Ludzack-Ettinger (MLE) process with clarifier. Alternative 1 Upgrade Treatment Process Unit 1 from conventional activated sludge treatment consisting of aeration and a clarifier to a Modified Ludzack-Ettinger (MLE) process with clarifier. Rehabilitate the existing steel flow equalization tank. Add new deep bed denitrification gravity filters. Alternative 2 Convert Treatment Process Unit 1 from conventional activated sludge treatment to a flow equalization tank. Upgrade Treatment Process Units 2 and 3 from a MLE process to a four-stage Bardenpho process. Add new membrane biological reactor consisting of bioreactor and microfiltration. The goals for effluent nitrogen and phosphorus concentrations for the treatment alternatives considered in the analysis are presented in Table 1-2. Table 1-2 Treatment Improvement Alternatives Effluent Nutrient Limits Parameter Baseline Alternative 1 Alternative 2 Total Nitrogen (TN) 10 mg/l 5 mg/l 3 mg/l Total Phosphorus (TP) 1 mg/l 1 mg/l 1 mg/l 2
The alternatives analysis focused on the biological treatment components of the Donax WRF. Although not discussed in detail, the following are common to all alternatives considered: Improvements to address areas requiring attention identified in the OMPR. No improvements to the existing pretreatment structure were anticipated. It has since been determined that new headworks screens will be required to assist with the MBR process. Disinfection was not considered in this analysis. No changes to the odor control systems are anticipated. No changes to the residuals handling facilities are anticipated. It was assumed that the existing electrical service and electrical facilities and the existing standby power systems will be able to support the improvements. It has since been determined that new VFD s and a generator will be required. Paving, grading, and drainage improvements were not considered in this evaluation. It was assumed that the existing stormwater system has sufficient capacity to support the improvements. Phosphorus removal will continue to be achieved with the addition of alum. The alternatives analysis was developed for conceptual level planning and decision making purposes. Once an alternative was selected, the alternative was presented to City Council for approval and design was begun. 2.0 Existing Facility The following is a brief overview of the existing facility to provide the background necessary to evaluate the proposed treatment alternatives. The Donax WRF was acquired by the City of Sanibel in August 1991. Treatment Process Unit 1 was constructed in 1995, prior to the build-out of the Sanibel centralized wastewater collection and treatment system. The City of Sanibel subsequently undertook a series of projects to extend the centralized sewer service to all of its citizens. In order to provide the necessary treatment capacity, the City added Treatment Process Units 2 and 3 to the Donax WRF in 2003. 2.1 Description of Existing Facilities The Donax WRF includes three biological treatment process units. All three biological treatment units are supplied from a common influent structure and discharge through a common effluent structure. Current treatment at the Donax WRF consists of the following: Master pump station. Pre-treatment Mechanical bar screen, a manual bar screen, a vortex grit removal system, an influent flow splitter, and a side-line flow equalization tank. Treatment Process Unit 1 A conventional activated sludge domestic wastewater treatment consisting of an aeration basin and a secondary clarifier. 3
Treatment Process Units 2 and 3 Modified Ludzack-Ettinger domestic wastewater treatment systems each consisting of an anoxic basin, an aerobic basin, and a secondary clarifier. Filtration Sand filtration consisting of five (5) deep-bed continuous backwash filters and an alum feed system. Disinfection Chlorine disinfection consisting of two (2) chlorine contact basins and a sodium hypochlorite feed system. Effluent Pumping. Effluent Storage Treated wastewater is stored on-site in a 1.5 MG reuse storage tank. Substandard Storage Substandard wastewater is stored on-site in a 1.8 MG substandard tank. Residuals Handling Two (2) rotary drum thickeners and two (2) aerobic digesters. Odor Control Three (3) separate odor control systems to address odors at pre-treatment, treatment process unit 1 and residuals handling, and treatment process units 2 and 3. 3.0 Basis of Design The purpose of this section is to provide the basis of design for the treatment alternatives considered for the Donax WRF improvements. This section includes a summary of the permitted effluent limits and reliability requirements, the influent wastewater flows and loadings, and the treatment goals for the Donax WRF. 3.1 Effluent Limits and Reliability Requirements The Donax WRF has a permitted design capacity of 2.375 MGD on an MMADF basis. A portion of treated effluent from the plant is disposed of via spray irrigation in public access areas. Florida Administrative Code (FAC) Chapter 62-610 sets forth effluent limits and plant component reliability requirements for treatment facilities that produce public access effluent. Table 3-1 summarizes treatment and reliability requirements set forth under FAC Chapter 62-610 which are applicable to the facility. In addition to the effluent limits set forth in FAC Chapter 62-610, FAC Chapter 62-600 establishes design criteria for "High Level" disinfection. These criteria are presented below: 1. For a reclaimed water or effluent containing 1,000 fecal coliforms, or less, per 100 ml before disinfection, the product of the total chlorine residual used for design (expressed in mg/l) and the contact time at peak hourly flow (expressed in minutes) shall be at least 25. 2. For a reclaimed water or effluent containing greater than 1,000 and up to and including 10,000 fecal coliforms per 100 ml before disinfection, the product of the total chlorine residual used for design (expressed in mg/l) and the contact time at peak hourly flow (expressed in minutes) shall be at least 40. 3. For a reclaimed water or effluent containing greater than 10,000 fecal coliforms per 100 ml before disinfection, the product of the total chlorine residual used for design (expressed in mg/l) and the contact time at peak hourly flow (expressed in minutes) shall be at least 120. 4
As shown in Table 3-1, "Class I Reliability" is required for reuse systems utilizing irrigation in public access areas. In addition to spray irrigation of effluent, deep well injection is used as an alternate means of effluent disposal. Effluent limits for discharge to deep well injection are set in FAC Chapter 62-600 and in general are less stringent than the effluent limits for spray irrigation. Table 3-1. Effluent Limits and Reliability Requirements for Spray Irrigation in Public Access Areas Parameter FAC 62-610 Requirement Biochemical Oxygen Demand (CBOD5) 20 mg/l Total Suspended Solids (TSS) 5 mg/l Fecal Coliform Non-detectable in 75% of daily samples and less than 25/100 ml at all times Disinfection Level High Level Reliability Required Class I* *As defined by USEPA MCD-05 The facility is required to provide continuous on-line monitoring for turbidity and chlorine residual before application, and a standard operating protocol designed to ensure high level disinfection must be met before reclaimed water is released to the system. Wet weather storage is also required by 62-610, F.A.C. when discharging to a reclaimed water irrigation system. The minimum FDEP accepted volume of wet weather storage is 3 days average daily flow when there is no alternative reuse or disposal system. The City has access to an alternate disposal system, the Island Water Association (IWA) deep injection well. Given the ability to discharge through the IWA deep injection well and the provision of reuse and substandard storage on-site, the City is not required to provide wet weather storage at this time. Chapter 62-640, Domestic Wastewater Residuals, F.A.C. regulates the management and disposal of domestic wastewater residuals, encourages the recycling of domestic wastewater residuals, and establishes minimum requirements for the treatment and disposal of domestic wastewater residuals. Currently, waste activated sludge is thickened by rotary drum thickeners and stabilized by aerobic digestion to Class B standards. 3.2 Level of Treatment The City would like to improve effluent quality to reduce nutrient loadings. The highest level of treatment of the improvements considered will be consistent with advanced waste treatment (AWT) standards for surface water discharge as set forth in FS 403.086 (4) prior to the implementation of numeric nutrient criteria (NNC) water quality based effluent limitations. The effluent limits for the treatment alternatives are presented in Table 3-2. 5
Table 3-2. Treatment Improvement Alternatives - Effluent Quality Parameter Baseline Alternative 1 Alternative 2 Biochemical Oxygen Demand (CBOD5) 5 mg/l 5 mg/l 5 mg/l Total Suspended Solids (TSS) 5 mg/l 5 mg/l 5 mg/l Total Nitrogen (TN) 10 mg/l 5 mg/l 3 mg/l Total Phosphorus (TP) 1 mg/l 1 mg/l 1 mg/l Since the treatment improvements are not mandated by a regulatory requirement, advanced waste treatment standards were selected to achieve the highest level of treatment at a reasonable cost. All options considered provide greater treatment with improved effluent quality, but not all options produce effluent that consistently meets advanced waste treatment standards. To achieve the desired level of treatment it was determined that Alternative 2 (conversion to a four-stage Bardenpho process followed by a new membrane biological reactor) was the direction the City would like to move forward. 4.0 Conceptual Design The existing biological treatment unit will be converted to a flow equalization tank, and the existing flow equalization tank will be demolished to provide space for the MBR with microfiltration and second (post) anoxic tank. The addition of the MBR with microfiltration will allow the existing secondary clarifiers to be demolished, which will provide additional process treatment volume in the existing biological treatment process units. The existing biological treatment process units will be modified to provide pre-anoxic and aeration volumes upstream of the new post anoxic tank. The MBR will be an integrated bioreactor and will follow the post anoxic tank. The MBR tank will serve as the reaeration tank. The modification of the existing process tanks and the addition of the second anoxic and MBR tanks will result in a 4-stage biological treatment process with microfiltration. The MBR concept combines the bioreactor and microfiltration into a single process, thereby replacing secondary clarification and effluent filtration. By eliminating the secondary clarification process, mixed liquor suspended solids (MLSS) concentration is not controlled by the clarifier solids loading limitations. The biological process can operate at higher MLSS concentrations than conventional activated sludge processes increasing the solids retention time for a given process volume. Effluent (permeate) is pulled through the membranes by vacuum pumps while solids remain in the reactor. The MBR will be an internal (integrated) membrane configuration with the membranes immersed in the activated sludge reaeration tank. An internal recycle returns activated sludge from the reaeration tank to the first anoxic zone. Activated sludge can be wasted at a controlled rate / time from the internal recycle flow. Membrane fouling is controlled with the use of aeration, backwash, and maintenance cleaning. The aeration operates continuously to provide a mechanical scouring. Filtration is interrupted on a periodic basis, typically every 30 minutes, and the membranes are backwashed with permeate for 30 to 45 seconds. The system remains online while backwashing. On a less frequent basis the 6
membranes cleaned in-situ with a strong chlorine solution or citric acid. Continued membrane fouling over time will cause pressure drop across the membranes to increase. At a maximum pressure drop the membranes are removed from operation for a recovery cleaning. Depending on the amount of carbon available in the post anoxic zone supplemental carbon may be required to facilitate denitrification to achieve the desired level of nitrogen removal. It is not anticipated that supplemental carbon will be required. However, if supplemental carbon is required, a portion of the raw influent can be fed to the post anoxic zone to provide a source of carbon. Currently, the facility has the ability to remove phosphorus by precipitation with the chemical addition of aluminum phosphate (alum). This alternative may provide the opportunity for biological phosphorus removal with the addition of an anaerobic zone upstream of the first anoxic zone resulting in 5-stage process. For the purposes of this analysis it was assumed that the current method, precipitation of phosphorus with the chemical addition of alum, will be continued. 5.0 Conclusion Based on the space available at the existing WWTP, as well as the City s effluent quality goals of advanced waste treatment standards, it became clear that MBR was the best option for the City of Sanibel. Although the MBR option included the highest construction cost, the reduction of oxygen transfer and the additional maintenance requirement due to membrane fouling, it allowed for more forgiveness to fluctuations in flow, eliminated the requirement for supplemental carbon and RAS pumping, allowed for a longer SRT to be achieved and provided for a greater flexibility for future expansion. 7