Wastewater treatment facility upgrade uses combination of technologies to solve nutrient removal challenges

Following the 2003 upgrade, the plant operations staff was able to operate the facility to achieve an annual effluent objective of 3 mg/L TN by maintaining both reactors and both clarifiers in service. However, current flows were only about half of the rated plant capacity and the operations staff was concerned that this level of performance could not be sustained when flow increased to the rated plant capacity. Several concerns were identified for why the facility could not maintain this level of nitrogen removal performance at the rated capacity, including:

• Influent wastewater strength was greater than original design values, overloading the aeration system during higher loading periods. In particular, measured influent maximum month BOD5 concentration was 276 mg/L compared to an original design value of 225 mg/L.

• Peak to average flow ratios were greater than original design values, impacting clarifier operation and exceeding filter capacity during high flow periods. For example, peak influent flow recorded at the plant was over 6 times greater than the annual average, compared to an original design peak flow of just 3 times the plant annual average flow.

• Influent temperatures were lower during the winter than original design conditions, requiring longer sludge ages in the system that were not possible to maintain given clarifier capacity limitations. As discussed later in the paper, influent wastewater temperature during a pilot study conducted at the facility dropped to 9.1 °C, compared to the original design minimum temperature of 12 °C.

New regulations from the Maryland Department of the Environment (MDE) required that all ‘major’ facilities (those with greater than 1.9 Megaliter per Day (MLD) capacity) upgrade to meet new effluent nutrient removal permit limits of 4.0 mg/L annual average TN and 0.3 mg/L annual average Total Phosphorus (TP). The NERAWWTP was one of the facilities impacted by this regulatory change. At the same time, facility planning was underway to expand the facility to 17.0 ML/d to accommodate planned growth in the service area. In association with the new permit limits, grant funding was available to Cecil County from MDE under the Chesapeake Bay Watershed Restoration Act to fund the required improvements.

In order to meet these needs GHD undertook a pilot study to establish the performance capabilities of the existing process and a technology alternatives evaluation was completed.

In order to achieve the new nutrient removal permit requirements (annual average) of 4.0 mg/L TN and 0.3 mg/L TP, the facility would have to maintain nitrification year-round and maintain close to the annual average limits even during the cold winter months with a goal of making up any overages during the warmer summer months. Because expected performance even during the best months will likely be on the order of 3.0 mg/L TN, the new effluent limits left very little tolerance for higher TN concentrations during the winter.

Under supervision from state regulators, a pilot study was conducted in the winter of 2010/2011 to demonstrate if the facility could achieve the new nutrient removal requirements during cold weather operating conditions under the design operating conditions. Because current plant flows were almost exactly half of the plant design capacity, design flow operating conditions could be simulated simply by taking one existing reactor and one secondary clarifier out of service and running the entire current plant flow through the remaining reactor and clarifier.

The pilot study commenced on November 1, 2010 and continued for six months through May 5, 2011. The average effluent flow measured during the pilot study was 3.78 MLD or almost exactly half of the facility design flow. Wastewater temperature during the pilot study averaged 12.9 °C with a low of 9.1 °C. The pilot study demonstrated that the plant was unable to maintain nitrification and denitrification to degree required to meet the TN limit when temperatures dropped below 12 °C.

The existing sand filters located inside the existing Control Building were found to be undersized for the plant's existing and design flow rates. The existing filter performance also suffered from mechanical and controls limitations. Even when the filters were operating properly, the County had to bypass or divert all or part of the flow around the effluent filters due to high flow events many times during the pilot study.

The pilot study results indicate that both reactors need to be on line at design capacity during winter operations to achieve permit limits. Currently, each reactor had only one mechanical aerator. Should this aerator go down for maintenance, the entire reactor would need to be immediately removed from service. Such an event happened earlier in 2010 when the mechanical aerator in one of the reactors failed due to an oil pump failure in the main gearbox and the unit had to be taken out of service for 6 weeks, along with the corresponding reactor. Since plant flows were only about half the design flow at that time, the plant was able to switch to the other reactors while the repairs were made. However, if the plant were operating at capacity, adequate SRT for nitrification and denitrification could not have been maintained under most temperature conditions and effluent violations would have resulted. GHD recommended that a redundant mechanical aerator be provided in each existing oxidation ditch such that plant operations can be maintained in the event of a mechanical aerator failure.

The pilot study demonstrated that despite the historically good performance at the facility, the performance could not be expected to be sustained once flows increased to the design capacity and that upgrades to improve the following were required:

• Longer SRTs to maintain complete nitrification under cold weather conditions

• Additional denitrification capacity to reduce effluent nitrate to target 1.0 mg/L

• Increase in effluent filtration capacity to avoid filter bypasses

• Installation of a standby aerator in each reactor

During facility planning, the GHD team focused the alternatives evaluation on those which could be accommodated within the existing property limits. Three alternatives were considered:

• Alternative 1: Double the secondary capacity by building two more reactors and two more clarifiers and replace the existing filters with deep bed denitrification filters.

• Alternative 2: Replace the secondary clarifiers and filters with a membrane filtration system (MFS) and modify the aeration system to convert the oxidation ditch system into a membrane bioreactor (MBR).

• Alternative 3: Convert the oxidation system to a magnetite ballasted reactors system, build two more secondary clarifiers, and decommission the existing filters.

The lowest initial cost alternative to meet the effluent nutrient removal limits at the current design capacity was found to be to Alternative 1. Under this alternative, the plan was to add a second mechanical aerator to each Carrousel^® reactor (for redundancy), convert the post-anoxic zones of the Carrousel^® reactors to operate as aerobic switch zones during cold weather conditions (for longer aerobic SRT to maintain nitrification), and construct new deep bed denitrification filters with a supplemental carbon feed system (for additional denitrification of effluent nitrates from the reactors).

However, the lowest capital and lifecycle cost alternative for expanding the plant to the future 17.0 ML/d capacity was determined to be Alternative 2. This is because the MFS would allow the target mixed liquor concentration in the Carrousel^® to be doubled, effectively doubling the SRT and treatment capacity of the existing units and avoiding the need to construct additional Carousel units on the constrained site in the future to expand capacity. Based on this analysis, the County decided to proceed with Alternative 2. Grant funding for the project from MDE was limited to the capital cost that would have been required to implement the lower initial cost Alternative 1, with the balance of the project funding financed through low interest state revolving fund loans.

Design criteria for both phases for the retrofitted Carrousel^® reactors and MFS are shown in Table 1.

Following the planning phase, GHD began the implementation process by developing procurement documents for pre-selection of the MFS equipment. The team comprehensively evaluated proposals from three hollow-fiber membrane manufacturers considering cost and technical approach in the scoring evaluation. The outcome was selection of GE Zenon's LEAP MBR system. Selecting the membrane equipment prior to detailed design allowed the team to work with the manufacturer to focus on the specific requirements of the LEAP system during the design.

During construction, one existing Carrousel^® reactor was taken out of service for modifications, while the remaining original unit provided wastewater treatment. Similar to what happened during the pilot study, when temperatures dropped below 12 °C during construction, nitrification in the remaining original Carrousel^® reactor was lost for several months. Functional testing for the first retrofitted Carrousel^® reactor and new MFS was conducted in Fall 2015, with startup of the new system on November 17, 2015. Once the new system was in operation, modifications were made to the secondary clarifiers and the second Carrousel^® reactor, which was placed into service on April 6, 2016.

Initial operational data for the new system is shown in Table 2. MLSS concentration was 5,000 mg/L at startup of the initial reactor in November and was gradually increased to the design concentration of 8,000 mg/L by April.

As indicated by the data, full nitrification was maintained in a single retrofitted Carrousel^® reactor during the initial winter of operation. The data also indicates nearly complete denitrification. Although a supplemental carbon system was included with the new design for the purpose of improving denitrification in the post-anoxic zone, no supplemental carbon was added at the facility during the initial period of operation.

Thanks to Wastewater Division Chief Jeff Coale at Cecil County for overseeing the pilot study and initial operation of the new system. Jeff provided the operational data for this paper.