Aerated Lagoons and Their Designs

Fig1: First cell of an aerated lagoon with floating aerator, Porch Creek Reservation, Alabama

Aerated lagoons are the lagoons or treatment system which receive oxygen from either mechanical or diffused air systems. This contrasts with stabilization ponds which obtain oxygen from photosynthesis and surface re-aeration. It is believed that aerated lagoon technology began as an attempt to remedy the problems of over loaded stabilization ponds. Initially, aeration was added to the lagoon in order to reduce the incoming BOD to a level at which the lagoon could meet its discharge permit.
Most newly designed aerated lagoons are multi-celled. As a rule most of the aeration is applied in the first cell, with the later cells serving as sedimentation/polishing ponds. Aerated lagoons have been classified by some by the amount of mixing provided. If enough energy is provided to keep all solids in suspension, they have been termed a complete mix.  If energy is provided only to provide sufficient oxygen to oxidize BOD entering the lagoon, and no attempt is made to keep solids in suspension, it is termed a partial mix lagoon. Complete mix lagoons have more in common with the activated sludge process than other lagoon technologies. The retention times are typically shorter and performance is better than the latter, though energy costs are higher for the complete mix systems.

Common Modification

Fig 2: Aerated first cell, Roatan Honduras, under construction

If pollution of the ground water from lagoon seepage is a concern, some type of impervious lagoon liner should be installed. Vinyl, concrete and compacted clay are common liner materials. Significant nitrification can occur if a portion of the secondary solids are recycled to the first cell. This increases the overall solids retention time which allows nitrifying bacteria to flourish.

Technology Status

Fig 3: Settling Cell with baffles, Roatan

While not as widely utilized as stabilization ponds such as facultative lagoons, aerated lagoons have been in use for many years. They are a fully demonstrated technology.  The system we will be discussing was developed by Linvil G. Rich, Alumni Professor Emeritus, Department of Environmental Engineering and Science, Clemson University, Clemson, South, Carolina. He calls this system a dual-power, multicellular (DPMC) aerated lagoon system. Many systems using this design have been built, and perform as designed. This design will yield better performance, with a shorter detention time than other aerated lagoon designs currently in use.

Applications

Aerated lagoons have been used to treat domestic and industrial, low to medium strength, wastewaters. Use of this system is advantageous where land is available and affordable.  Operation and maintenance costs are low as compared to the activated sludge process, but higher than the O&M cost for stabilization ponds such as facultative lagoons. The chief advantage this system has over the latter, is a smaller footprint due to the three to five days of detention required in the system. It will require one fourth to one third the land area as a suitably designed facultative lagoon. The DPMC system will use 1/3 to ¼ the energy an activated sludge plant, such as an oxidation ditch, will use.

Performance

The DPMC lagoon system will consistently meet effluent limits of 25 mg/L for CBOD5 and 30 mg/L for TSS. Other aerated lagoon systems do not typically perform as well. This is usually due to excessive detention time which leads to the proliferation of algae. In aerated lagoon systems practically all of the effluent TSS and CBOD5 is caused by algae that grows in the lagoon. The DPMC lagoon system is designed such that algal growth is minimized.

Residuals

Fig 4: Sludge accumulation in a DPMC lagoon, from Rich

Solids in the system will have to be removed at least once every 10 to 20 years.  Most of the solids will accumulate in the cells following the complete mix, first cell with the first settling pond receiving the largest accumulation. Figure 1 illustrates the sludge accumulation depths in the four cells of a DPMC lagoon system located in Berkeley County, South Carolina after 16 years of operation.

Nutrients

Significant phosphorus removal can occur in a DPMC lagoon system as can be seen in the data below from such a system in North Myrtle Beach, South Carolina though 50% removal is a more likely occurrence.  Some nitrification can occur during the summer time with these systems, but it is unpredictable and can not be counted on.  Little nitrogen removal occurs.

Toxics

Volatile toxics will be removed and incidental removal of other toxics can be expected to be similar to the activated sludge process.

Pathogen removal

Without disinfection, only a 3 to 4 log reduction in bacteria can be expected.  A 2 to 3 log reduction in viruses is possible when compared with other processes which have data.  Helminth  and protozoan removal should be about 95-99%.  If the effluent is going to be reused without disinfection, the use of a maturation pond of 10 to 20 days detention could reduce the pathogens to acceptable levels depending on the type of reuse contemplated.

Design

Essentially, the DPMC system consists of four cells in series. For municipal wastewater treatment in the southeastern United States, the system will have, at design flow, a total HRT of 4.5 to 5 days, and a depth of at least 3 m. The first cell (HRT = 1.5 – 2 d) is aerated at 6 W/m3 of volume (30 hp/mgal), a level that will 1) maintain all solids in suspension, and 2) provide oxygen sufficient for the conversion of the influent CBOD to carbon dioxide and biomass.

The following three cells, each with a HRT of approximately 1 d, serve the functions of sedimentation, solids stabilization, and sludge storage. Each cell is aerated at 1 W/m3 of volume (5 hp/mgal), a level that permits the settle-able solids to settle, but, is sufficient to maintain a thin aerobic layer at the top of the solids deposit. The aerobic layer reduces feed-back of nitrogen and CBOD to the water column, and maintains a stable deposit. Aeration also reduces the dead-space volume of the cells.

Since the control of algal growth is crucial in the reduction of effluent suspended solids, careful attention is paid to factors influencing such growth.  In tropical areas, detention times for this system would be 3 to 4 days ore less due to the higher water temperatures.

Control of algae

Fig 5: Conceptual sketch of influence of hydraulic retention time (HRT) on lagoon effluent CBOD5

Hydraulic retention time: Figure 5 illustrates in a conceptual way how algal growth can be minimized through control of the hydraulic retention time (HTR). There is a minimum HTR (point a) required to reduce the influent CBOD5 to an acceptable level. There is also an HRT (point b) beyond which algae become established and grow. The key to the design of a system that will produce an effluent with minimal algae is to design a system where the effective HRT falls between points a and b, preferably close to point b considering the sludge storage function of the system. Also, it must be kept in mind that the effective HRT should be based on a consideration of the initial flow rate as well as the design rate. It is just as important for the system to perform well the day that it goes into operation as it is at the end of its design life.

Fig 6: Surface area vs. depth for a lagoon basin with a volume of 2840 m3 and side slopes of 3:1

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 6 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.  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.

First Cell Design

The DPMC system design procedure provided a more rational approach to aerated lagoon design than the empirical approach, and its use has resulted in improved performance of this type of process.  In the mid 90’s, this design was modified for the following reason.  Hydrolysis of the biodegradable particulate matter is the rate limiting step in the process.  Additionally, in order to take advantage of parameters and relationships developed by The International Association on Water Pollution Research, this approach was defined in terms of COD rather than CBOD5. If it is assumed that hydrolysis is rate limiting, a steady-state mass balance across a completely mixed system yields the following two equations for the analysis of the size of the first cell.

XS / XB,H = KX(1+bh(V/Q))/{(V/Q)(kH- bh)-1     ……………..(1)
where
XS=concentration of biodegradable particulates, mg/l COD
XB,H= concentration of active heterotrophic biomass, mg/l COD
KX= half-saturation constant, mg COD/mg biomass COD
bh =specific decay rate, d-1
Q= flow rate m3/d
V=volume of cell m3
kH=maximum specific hydrolysis rate, d-1

XB,H = YH(X50+S50)/[1+(1-0.92 YH) bh(V/Q)]     ……………..(2)
where
YH= growth yield, mg biomass COD/mg COD oxidized
X50=influent concentration of biodegradable particulates, mg/l COD
S50= influent soluble COD, mg/l

Fig 7: Detention time vs COD @20degree-centigrade

Parameters
kH=3(1.103)T-20
bh =0.62(1.120)T-20
KX=0.03(1.116)T-20
YH=0.67

For municipal waster waters:
X50=247 mg/l
S50=83 mg/l

By combining equations 1 and 2 and solving forXS versus detention time a plot as shown in figure 7 can be developed.
This plot was developed for a waste-water temperature of 20oC.

Keeping in mind that the preceding equations were developed for steady-state conditions which don’t exist in the municipal waste-water world, a detention time for the first cell should be selected from the portion of the curve falling below the steep rise.  For the above curve, a detention time of 0.75 to1.25 days should be considered.

Aeration Requirements

Aeration in the first, complete mixed cell, is provided for two purposes, solids suspension and oxygen demand.  Power required for suspension of solids is a function of several factors including the concentration and nature of the solids, basin size and geometry, and the type of aeration equipment used.  An equation to calculate the required power for mechanical surface aerators, the most common aeration equipment used in this type of system is:

P=0.004X + 5   ……………….(3)
where P=power level, W/m3and X=total suspended solids, mg/l.  The use of this equation is limited to suspensions of 2000 mg/l or less.

Fig 8: Complete mix basin, N. Myrtle Beach, SC

Typically, the TSS in a basin treating domestic wastewater is 180 to 250 mg/l.  For a TSS of 250 mg/l the equation predicts the need for 6 W/m3 (30 hp/mg) to maintain a suspension.

 A conservative estimate of the maximum oxygen demand is given by the following equation:

Ro2=4.16 x 10-5rQSo   ………….(4)
where Ro2=maximum oxygen demand rate, kg/h; r=ratio of the maximum rate to the average; and So=influent BOD5, mg/l.  The value of r can assumed to be 1.5.

Power required can be calculated by the following equation:

P=103 (Ro2/NV)   …………………..(5)
where P=power level, W/m3; N=expected aerator performance, kg O2/kWh.  For preliminary calculations, V= basin volume, m3.   N can be taken as 1.25 for preliminary purposes.

Sedimentation Basins

The following three cells, each with a HRT of approximately 1 d, serve the functions of sedimentation, solids stabilization, and sludge storage. Each cell is aerated at 1 W/m3 of volume (5 hp/mgal), a level that permits the settleable solids to settle, but, is sufficient to maintain a thin aerobic layer at the top of the solids deposit. The aerobic layer reduces feed-back of nitrogen and CBOD to the water column, and maintains a stable deposit. Aeration also reduces the dead-space volume of the cells.

Fig 9: Settling Basin, Porch Creek Reservation

Since the control of algal growth is crucial in the reduction of effluent suspended solids, careful attention is paid to factors influencing such growth. The turbidity created in the first cell by maintaining all settable solids in suspension reduces light in the water column to the extent that very little algal growth occurs in that cell. The focus of concern, therefore, centers on factors in the remaining three cells. Those factors include HRT, multicellular configuration, surface area, and mixing.

The following equation can be utilized to size the settling basins such that excessive algal growth is not a problem:

(V/Q)f=[1-(XAN/ XAO)-1/n]uA   ……………………..(6)
where (V/Q)f =hydraulic detention time in each cell, days; XAN and XAO=algal concentrations in influent to first cell and effluent from last cell, mg/l; uA=net algal specific growth rate, d-1; and n=number of settling cells.  For uA use 0.48 d-1, the highest reported growth rate in any pond system.  As long as the ratio XAN/ XAO is less than 25, the corresponding hydraulic detention times should not result in significant algal growth.

Aeration Requirements:
The oxygen demand rate can be estimated with the following equation:

Ro2=4.16 x 10-5AB   …………………….(7)
where Ro2= oxygen demand rate, kg/h; A = area of sludge-water inter face, m2; and B=unit rate of benthal oxygen demand, g/m2d.  From benthal studies conducted by Rich, 80 g/m2d is the highest value to be expected at temperatures in excess of 16o C for B.

References

1. Rich, Linvil Gene, High-Performance Aerated Lagoon Systems, American Academy of Environmental Engineers (December 1, 1998)
2. Rich, Linvil Gene, Technical Notes 1-7, Clemson University, 1993-95
3. U.S. EPA, 1983. Design Manual -Municipal Wastewater Stabilization Ponds, EPA-625/1-83-015, US IPA CERI,Cincinnati, OH.
4. Rich, L.G. (1996). “Modification of Design Approach to Aerated Lagoons.” Journal of Environmental Engineering. ASCE, 122. 149-153.
5. EPA (1980). Innovative and Alternative Technology Assessment Manual, 430/9-78-009, CD-53. U.S. Environmental Protection Agency, Washington, D.C.
6. Bowden, Mike; Henry, Bruce, USEPA Region 4, Low-Tech Alternative to Activated Sludge Promises Big Savings (1997)