New Process Upgrades, Improves Lagoon System Treatment
The population of Johnstown, CO, located about 50 miles north of Denver, has been exploding over the past decade.
The population of Johnstown, CO, located about 50 miles north of Denver, has been exploding over the past decade. In fact, the population has increased over 22% since 2000 alone. Due to this incredible growth, the city was forced to upgrade its wastewater treatment plant (WWTP) to accommodate the new residents. Because of its small footprint, lower cost, and ability to nitrify in cold weather, the AnoxKaldnes LagoonGuard process was chosen for the upgrade.
The wastewater treatment plant originally consisted of a series of three lagoons built in the 1950s and was last upgraded in 1980. Since new, stricter effluent ammonia discharge limits were enacted, combined with the growing population, this prior upgrade could not provide adequate treatment. Also, the plant was running at temperatures well below 7°C (45°F) in the winter months, making it virtually impossible to nitrify in the lagoons during these months.
Each MBBR is filled to 26% of its empty reactor volume with K1 plastic biomedia, providing nearly 160,000 m2 of surface area for specialized bacterial growth.
The city had limited options: the current lagoon system could be abandoned and a new activated sludge plant could be built in its place, or the existing lagoons could be upgraded by adding the LagoonGuard process, which was estimated to cost 50% less than a new activated sludge plant. The latter option was chosen and the new plant has been in operation since June 2004.
The LagoonGuard is a specific application of the patented Moving Bed® Biofilm Reactor (MBBR) process. Its configuration places the MBBR within the series of existing lagoons to aid in nitrification or organic removal. The MBBR technology has been in use over the last 16 years by the Norwegian company Kaldnes Miljoteknologi AS (KMT) in conjunction with the Norwegian Research Institute, SINTEF. In 2002, KMT joined with the Swedish research company, Anox, which has further strengthened the research and development of the technology. Though beginning in Europe, this technology is now available worldwide, distributed in North America by AnoxKaldnes. The company currently has 22 installations in North America and over 400 installations worldwide.
The MBBR operates as a standalone treatment process with no return activated sludge from a secondary clarifier, thereby minimizing operator maintenance and time. The MBBR process uses polyethylene carrier elements which are suspended and mixed using custom designed medium bubble aeration systems or mechanical mixers, depending on the process application of organic removal, nitrification or denitrification. Due to the constant mixing, there is no dead or unused space. Because the media is kept within each basin by custom designed sieves, the bacteria have a chance to mature into specialized “workers” within each separate basin, consuming whatever food is available: organics, ammonia, or nitrates. The media also are constantly sloughing the sieves, creating a process with no head loss or need for backwashing.
Due to the increased surface area for bacterial growth created by the media, it is possible to multiply treatment capacity with minimal footprint. Additional capacity can also be added by simply adding more media. The K1 type biomedia, used in the upgrade at Johnstown, provides 500 m2 of surface area per every m3 of media, allowing for increased treatment capacity within a compact area.
The LagoonGuard upgrade was relatively simple. Two trains, each with two MBBRs in series, were constructed in below ground rectangular concrete basins between Lagoons 2 and 3. Each basin is 30 ft x 30 ft, with a volume of 10,800 ft3. A dissolved air flotation (DAF) device was also added at the end of the treatment process after Lagoon 3. The new flow scheme consisted of two lagoons for BOD treatment, followed by the LagoonGuard for nitrification, a preliminary settling lagoon and a DAF.
The plant currently has a capacity of 0.75 mgd. Each MBBR is filled to 26% of its empty reactor volume with K1 plastic biomedia, providing nearly 160,000 m2 of surface area for specialized bacterial growth. At this fill fraction, the MBBR is designed to treat an average BOD load of 194 lb/day and an average NH3-N load of 126 lb/day.
Future city expansion is estimated to require a 1.5 mgd plant in coming years, which has already been incorporated into the design at Johnstown. Each MBBR will eventually be filled to 52% of its empty reactor volume with plastic biomedia to accommodate the future flow. The increased flow will require the MBBR to treat an average BOD load of 800 lb/day and an average NH3-N load of 275 lb/day. In other words, the wastewater treatment plant can more than double its capacity without increasing its footprint.
The treatment plant is operated in two separate modes during summer and winter. In the summer mode, the process flow is as described above, with influent wastewater passing through two lagoons, the LagoonGuard, a third lagoon, and a DAF. As temperatures approach less than 7°C (45°F) in fall/winter months, one of the pretreatment lagoons is bypassed, which helps prevent temperatures from dropping below the design temperature of 4.5°C (40°F). As cold water temperatures can hinder or even halt the nitrification process, this customized winter configuration is essential for consistent treatment.
Although temperatures did frequently dip below the design temperature of 4.5°C (40°F) during the winter, the MBBR produced consistent effluent, allowing the Johnstown WWTP to meet its permit requirements for both BOD and NH3-N in all months throughout the winter. Although from December to mid-January the average influent water temperature was 4.2°C, the LagoonGuard removed an average of over 40% of influent NH3-N. In fact, the effluent NH3-N decreased by 35%, 42%, and 58% in December, January, and February, when compared with the same months in the winter prior to the installation.
As influent wastewater temperature increased in February 2005, the system removed, on average, over 85% of influent NH3-N at an average surface area loading rate of 0.23g NH3-N/m2-day. Prior to the MBBR installation, nitrification would not accelerate until late March.
The system has demonstrated its resiliency during its first year in operation. When the winter mode was first enacted, pH levels in the influent wastewater to the MBBR increased dramatically. As pH values reached well over 9.00 pH units for nearly a month, nitrification was hindered at first, but the MBBR quickly recovered. pH levels began to decline in early December, when the MBBR still removed over 40% of influent NH3-N. This recovery occurred while temperatures were, on average, below the design temperature of 4.5°C (40°F).
Plant effluent BOD concentrations have also declined since the installation. Effluent BOD has been, on average, less than 1 mg/L, making it easy for the plant to meet its permit requirements month after month. As in the past, January was guilty of the highest effluent concentration. However, January 2005 the effluent concentration at the plant was just 2.4 mg/L, as compared with 18 mg/L at the same time the year before, an over 85% decline in BOD discharge during the most difficult month of the year.
Online since June 2004, the upgraded plant at Johnstown has managed to meet its permit limits every month, an unheard of consistency prior to the installation. Even while temperatures were consistently below 4.5°C (40°F), the LagoonGuard removed over 40% of the NH3-N load during the winter; over 85% of the NH3-N load in the summer. BOD effluent has been reduced to an average concentration of less than 1 mg/L. The system’s compact and flexible design used the existing facilities, minimizing cost and footprint.
About the Author
Ivy Cormier is the Senior Pilot Technician for AnoxKaldnes Inc. Cormier holds a B.S. in Environmental Science. She is particularly skilled in the area of biological wastewater treatment and has conducted numerous treatment studies in various venues throughout North America. Cormier may be contacted at 401-270-3898; email: email@example.com