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

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Mars Hill Wastewater Lagoon System - Mars Hill  Maine. Photo Courtesy of Wright-Pierce Engineers.
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AERATED LAGOON TECHNOLOGY

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

Clemson University - 
Clemson, SC 29634-0919 USA
Email: lrich@clemson.edu
Tel. (864) 656-5575; Fax (864) 656-0672

Technical Note Number 3

CONTROL OF ALGAE

 The problem with algae was discussed in Technical Note 2. Algae growing in an aerated lagoon system will increase both the TSS and the CBOD5 of the effluent. In systems treating municipal wastewaters, the effluent TSS and CBOD5 will often be many times that which would occur if the algae had not been present. Effluent values of these parameters inflated by algae offer no clew as to how well the lagoon is removing the influent TSS and CBOD5. Consequently, engineers mistakenly assume that because the effluent TSS and CBOD5 are approaching, or exceeding, the limit, additional treatment capacity is required when in fact the current capacity may be (and probably is) excessive. Since algae are a distinct liability and play no beneficial role in aerated lagoons, a consideration of ways to prevent, or control, algal growth should be of interest to those responsible for the design and operation of these systems. Such consideration is the focus of this technical note.


Hydraulic Retention Time (HRT)

    Retention time is the most influential factor controlling algal growth. In a lagoon basin with a depth of at least 3 m and fitted with mechanical surface aerators that provide a power intensity of about 1.6 W/m3 (8 hp/106 gal of basin volume) or less, algal growth can be expected to occur if the HRT exceeds about 2 d (Fleckseder and Malina 1970; Toms et al. 1975). If, however, the lagoon basin is divided into two or three cells in series by curtain walls, algal growth can be expected to occur only if the total HRT exceeds about 3 d, and 3.6 d. respectively (Rich 1999). Thus, the post fitting of a lagoon basin with curtain walls may reduce effluent algae. At greater aeration power intensities, shading provided by the suspension of settable solids reduce algal growth. At an intensity of 6 W/m3 (30 hp/106 gal), very few algae will grow.


Depth

     As photosynthetic organisms, algae require light to grow. Per unit volume of lagoon basin, the quantity of light energy available for such growth is proportional to the surface area. For a basin with vertical sides, an increase in the depth will decrease the surface area proportionally. However, because of the trapezoidal cross section typical of lagoon basins, an increase in depth does not always decrease the surface area. Figure 1 illustrates the relationship between the two variables for a basin with a volume of 2840 m3 (750,000 gal) and with side slopes of 1 (vertical) 3 (horizontal). For such a basin, an increase in depth will decrease the surface area up to a depth of about 3 or 4m. Beyond which depths the surface area begins to increase.

aerated lagoons

 

Figure 1. 

Figure 1. Surface area vs. depth for a lagoon basin with a volume of 2840 m3 and side slopes of 1(vertical):3(horizontal)

   Lagoon depths of 3 or 4 m will also create a more favorable geometry for mixing with surface aerators. Reduced surface areas will position the mixing zones in closer proximity.

Mixing

    As was discussed above, if a lagoon basin treating a domestic wastewater is fitted with mechanical surface aerators that provide a power intensity of at least 6 W/m3 of basin volume (30 hp/106 gal), the turbidity of suspended solids is sufficient to minimize algal growth. At lower mixing intensities, algae will grow providing the HRT is sufficient. However, all lagoon basins, including those that are used for sedimentation (polishing), should be mixed a level of about 1 W/m3 of basin volume (5 hp/106 gal). Such mixing is beneficial from several points of view. Without mixing thermo stratification will occur, thereby permitting the retention of undisturbed surface layers for relatively long periods of time. Such conditions provide an excellent environment for algae to become established and grow.

     Mixing will also exhaust the carbon dioxide from the system. For wastewaters, such as those from domestic origin in which there is an excess of nitrogen and phosphorus, carbon dioxide can be growth limiting during a portion of the diurnal cycle. During the night hours when light is not available, carbon dioxide accumulates as the result of respiration of the microorganisms in the lagoon. At dawn, when light does become available, the rate of consumption of carbon dioxide through photosynthesis exceeds that of respiration and, as a result, the store of carbon dioxide is depleted and algal growth becomes limited. In other words, the carbon dioxide accumulated during the night hours is stored for use in the daytime hours. Carbon dioxide concentrations as high as 25 mg/L have been observed at night in lagoons (Williford and Middlebrooks 1967). Since at sea level the saturation concentration of carbon dioxide is only about 0.42 mg/L at 20 C, mixing by aeration will remove significant quantities of carbon dioxide from the system during the night hours, thus ensuring that carbon dioxide becomes growth limiting earlier in the day. During the day, when carbon dioxide is growth limiting, aeration does not significantly replace carbon dioxide in the system because the concentration gradient is too low. As will be discussed in later notes, aeration in settling basin is a must, not only because of the mixing that is created, but also, for the maintenance of dissolved oxygen in the water column. Such maintenance reduces feed back of CBOD and nitrogen from the benthal deposits.

Cover

     Cover of any type, artificial or natural, that will prevent light from entering the water column of a lagoon will prevent the growth of algae. Commercially available floating polyester fabrics have been used to shade aerated lagoons. Such shades should not cover the entire lagoon surface, leaving sufficient room for mechanical surface aerators.

     Natural cover can be provided by surface-growing plants such as duckweed. Duckweed, if kept from the effluent by inserting surface baffles in front of the effluent weir, is very effective toward reducing algae in the lagoon. Furthermore, experience in South Carolina has shown that for aerated lagoons, it is not necessary to periodically harvest the duckweed, nor does the duckweed appear to result in significant accumulations in the bottom of the lagoon. Floating grids placed across the lagoon surface have been used to ensure surface coverage. However, several aerated lagoons covered with duckweed have operated successfully without grids. Regardless of the type of cover used, provision must be made for aerating the lagoon. Otherwise, the lagoon will become anaerobic.

Intermittent Discharge

     Algae respond to the diurnal variation in light by moving vertically through the water column. King et al. (1970) found that during the afternoon hours, the particulate COD at 8 inches below the surface of a facultative lagoon was about four times that at the same depth during the night hours. Such vertical migration suggests that effluent quality might be improved if the daily flow is released only during the night, or from two different depths over the diurnal cycle.

Chlorination

     Several studies have shown that chlorination will kill algae. The focus of most of these studies has been on the impact that algae have on the chlorine demand of plant effluents. In these studies, the chlorine doses used have been large (5-20 mg/L) and the contact periods short (15 min to 2h), conditions under which algae are killed and lyse. At least two authoritative studies, however, have shown that much lower chlorine doses (2-4 mg/L) over much longer contact periods (>10h) will impair algal growth (Echelberger et al. 1971; Kott 1971). This suggests that by continually adding chlorine in a relatively low dose in a aerated lagoon settling basin, effluent algae reduction would occur as a result of a lower growth rate.

Copper Sulfate

     Copper sulfate has long been used by waterworks personnel to control algal growth in reservoirs. Some waterworks personnel use a standard dose of 1 mg/L of copper sulfate which is sufficient to kill most types of algae. However, care must be taken to protect fish in the receiving stream. Trout, which appear to be the most sensitive of the fish, do not tolerate copper sulfate in concentrations greater than about 0.14 mg/L (Steel and McGhee 1979). It has been reported that the combination of chlorination and copper sulfate has given excellent results (Courchene et al. 1975).

Water Soluble Dyes

     Certain non-toxic, organic water soluble dyes that blocks out the specific light rays utilized in photosynthesis are used for killing algae. Some of these dyes, which leave the water a natural teal blue, have been used to kill algae in sewage lagoons. Such dyes are marketed commercially.

Effluent Treatment

     During the late 1960's and early 1970's, much research was conducted on the removal of algae in the effluents of lagoon. At least three review papers describes the scope of such research (Kothandaraman and Evans 1972; Middlebrooks et al. 1974; Parker 1975). A wide range of wastewater treatment processes were investigated in the hope that effluent treatment would be economically feasible. With a single exception, it appears that none of the processes are at the present considered to be feasible, especially for the treatment of aerated lagoon effluents. The exception is intermittent sand filtration, which is used primarily to achieve nitrification, the removal of algae being an added benefit. The performance of intermittent sand filtration in the treatment of aerated lagoon effluents will be discussed in a future technical note. Rapid sand filtration has two disadvantages. The removal of some algal species is marginal, and there is always the problem of what to do with the back-wash water. If the back-wash is simply recycled to the lagoon, algae accumulates in the lagoon, causing more frequent back wash.

REFERENCES
 

Courchene, J. E. and Chapman, J. D. (1975). "Algae control in Northwest Reservoirs". J. AWWA, 67(3), 127.

Echelberger, W. F. et al. (1971). "Disinfection of algal laden waters." J. San. Engr. Div., ASCE, 97(5), 721-730.

Fleckseder, H. R. and Malina, J. F. (1970). "Performance of the aerated lagoon process." Technical Report CRWR-71, Center for Research in Water Resources, University of Texas, Austin, TX.

King, D. L. et al. (1970). "Effect of lagoon effluents on a receiving stream." 2nd Symposium for Water Treatment Lagoons, Kansas City, MO, June 23-25.

Kothandaraman, V. and Evans, R. L. (1972). Removal of Algae from Waste Stabilization Pond Effluents - A State of the Art. Circular 108, Illinois State Water Survey, Urbana, IL.

Kott, Y. (1971). "Chlorination dynamics in wastewater effluents." J. San. Engr. Div., ASCE, 97(5), 647-659.

Middlebrooks, E. J. et al. (1974). Evaluation of Techniques for Algae Removal from Wastewater Stabilization Ponds. PRJEW115-1, Utah Water Research Lab., Utah State Univ., Logan, Utah.

Parker, D. S. (1975). Performance of Alternative Algae Removal Systems. Seminar on Ponds as a Wastewater Treatment Alternative, Univ. of Texas, Austin, TX, July22-24, 1975.

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

Steel, E. W. and McGhee, T. J. (1979). Water Supply and Sewage, McGraw-Hill, New York, NY.

Toms, I. P. et al. (1975). "Observations on the performance of polishing lagoons at a large regional works." Water Pollution Control, 74, 383-401.

Williford, H. K. and Middlebrooks, E. J. (1967). "Performance of field-scale facultative wastewater treatment lagoons." J. WPCF, 39, 2008-2019.

 

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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