Aerial view of the Warren, Maine lagoon system. Photo courtesy of Woodard and Curran.

Lagoon Systems In Maine 

Systems In Maine

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Mars Hill Wastewater Lagoon System - Mars Hill  Maine. Photo Courtesy of Wright-Pierce Engineers.
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Wastewater Engineering




Linvil G. Rich
Alumni Professor Emeritus
Department of Environmental
Engineering and Science

Clemson University - 
Clemson, SC 29634-0919 USA
Tel. (864) 656-5575; Fax (864) 656-0672

Technical Note Number 7


   As was discussed in Technical Note 6, during warm summer months, some nitrification generally occurs in most aerated lagoons. However, such nitrification is usually unpredictable and cannot be depended upon to meet effluent limits, especially during the winter months. Therefore, for aerated lagoons to be considered as viable processes for nitrification, the lagoon process must be modified so that the solids age is uncoupled from the hydraulic retention time (HTR). This can be accomplished either through sedimentation in clarifiers with solids recycle or through the retention of the solids in the aeration basin by use of sequencing batch reactor (SBR) technology. The latter approach has been used in single basin, continuous-feed, intermittent discharge (CFID) treatment systems for many years both in Australia and the United States. Their performance has been well documented (Goronszy 1979, Arora et al., 1985, Deeney et al. 1991). Although generally successful, some of the CFID systems have had major operational problems as the result of short-circuiting and/or sludge bulking. However, such problems can be minimized, or even eliminated, by modifying the design to deal directly with conditions that promote the problems. The mixed-liquor recycle (MLR) lagoon system incorporates such modifications. The MLR nitrification system, which is not proprietary, is illustrated in Fig. 1. The system consists of two earthen basins in series – a reactor basin for ammonia and CBOD5 removal and a sludge basin for solids stabilization and storage. The overall configuration and size of such a system are discussed in the following paragraphs. Design details are to be found in Rich (1999).



   The reactor basin is divided into two cells by a floating curtain wall or a hard wall. The first cell is aerated continuously, whereas the second cell is aerated intermittently in a controlled cycle that includes sedimentation and supernatant decant. Mixed liquor is recycled from the second cell to a manhole just upstream from the headworks of the plant. There it is mixed with the incoming sewage. The mixture flows through the headworks and into the first cell in a continuous stream. The two cell configuration eliminates short circuiting through the reactor basin as well as modulates the peaks of the diurnal loading pattern. Furthermore, by designing for nitrification to occur in the second cell, the system separates the oxygen demand of nitrifiers from that of the heterotrophs creating more favorable conditions for nitrifiers. The mixed liquor recycle promotes the contact of the biomass with the soluble CBOD5 in the sewage, thus reducing the tendency for filamentous bulking.

    The nitrification process is controlled through the continuous wasting of the mixed liquor from the reactor basin to the sludge basin. Such control can be accomplished by diverting a portion of the recycle flow. The flow rate of the diverted mixed liquor will determine the solids retention time at which the process is operated. The rate can be controlled simply by adjusting a hand valve inserted in the piping for the diverted flow.

     In addition to aeration equipment and mixers to ensure solids suspension, equipment required includes a programmable logic controller (PLC), liquid level sensors, a low-head recycle pump, and a decant device. The latter can be as sophisticated as a decanter designed for sequencing batch reactor technology, or as simple as pumps or fixed pipes with automatic valves.

   The design retention time in the reactor basin should be such that sufficient volume is provided to dilute peak organic and ammonia loads, yet low enough to reduce the power requirements for solids suspension. Therefore, it is recommended that the retention time at design flow rate, HRTD, be a function of the ratio of the initial flow rate, QI, and the design flow rate, QD. If QI/QD > 0.5, then HRTD should be 2d. If QI/QD < 0.5, then HRTD should be 1d.


        The sludge basin is designed for solids stabilization by benthal processes and multiyear sludge storage. Furthermore, the basin serves as an effluent balance tank to attenuate the intermittent decants from the reactor basin. Two floating curtain walls are used to divide the basin into three cells, each with a hydraulic retention time of about one day at the design flow rate. All cells are aerated, but not at a rate that interferes with sedimentation. Aeration is required in the sludge basin to prevent ammonia feed back from the bottom solids, and to eliminate dead spaces in the water column where algae can become established and grow. If QI/QD > 0.5, then the HRTD should be 3d. If QI/QD < 0.5, then the HRTD should be 2d.


        Although the MLR nitrification system must still be considered innovative, two such systems are currently in operation and a third is under design. The two systems in operation are located close to Liberty, SC. The Cramer lagoon initially was a dual-power, multicellular aerated lagoon system consisting of a separate reactor basin followed by a sludge basin divided into three cells in series. The upgrade consisted of dividing the reactor basin into two cells in series with a hard wall, fitting both cells with aerators and mixers, providing a decant pump and line to the sludge basin, and a mixed liquor recycle pump and a line to a manhole above the headworks of the plant. A programmable logic controller (PLC) was installed to control aerators, mixers, and pumps. At the present, discharge from the second cell of the reactor is on a six-hour cycle. The Cramer system is permitted at 157,000 gal/d. Cost of the upgrade was $312,000. Photographs of the Cramer lagoon system are shown in Fig. 2.

  Initially, the Roper lagoon was a single-basin, facultative aerated lagoon. The upgrade consisted of dividing the existing basin into two cells in series with a floating curtain wall, fitting both cells with aerators and mixers, a decant pump and line to a new sludge basin, and a recycle pump and line to a manhole above an enlarged headworks. The upgrade included also a new three-cell sludge basin, with aerators, and an enlarged chlorination-dechlorination facility. A PLC was installed for control of the aerators, mixers and pumps. The Roper system is permitted at 500,000 gal/d. Cost of the upgrade was $1,100,000.

 tech7.jpg (11873 bytes)


Figure 1. 
Mixed-liquor recycle (MLR) 
nitrification system


Reactor basin with first cell in foreground

Closer view with decant and mixed-liquor recycle 
pumps in left background


Second cell of reactor basin in foreground with headworks in far background


Three-cell sludge basin with chlorination facilities in far background

Figure 2.
Photographs of the Cramer mixed-liquor nitrification system at Liberty, SC.



Arora, M. L. et al. (1985). “Technology evaluation of sequencing batch reactors.” J. WPCF, 57(8),867-875.

Deeney, K. et al. (1991). “Implementation of sequencing batch reactor technologies in the United States.” 64th Annual Conf., WPCF, Toronto, Canada.

Goronszy, M. C. (1979). “Intermittent operation of the extended aeration process for small systems.” J. WPCF., 51(2),274-287.

Rich, L. G. (1999). High-Performance Aerated Lagoon Systems. American Academy of Environmental Engineers.Annapolis, MD.


Technical Note 1 Effluent BOD5 - A Misleading Parameter For the Performance of Aerated Lagoons Treating Municipal Waste
Technical Note 2 Aerated Lagoon Effluents
Technical Note 3 Control of Algae
Technical Note 4 Nitrites and Their Impact on Effluent Chlorination
Technical Note 5 Aerated Lagoons for Secondary Effluent
Technical Note 6

Nitrification in Aerated Lagoons With Intermittent Sand Filters

Technical Note 7

Mixed Liquor Recycle (MLR) Lagoon Nitrification System

Technical Note 8 Facultative Lagoons - A Different Technology
Technical Note 9 Sludge Accumulation in High Performance Aerated Lagoon Systems
Technical Note 10

Ammonia Feed Back in the Sludge of a CFID Nitirification System



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