Optimizing Aerated Lagoons, or “How to teach an old dog a few new tricks”

John Christiansen, MS, PE1 Matt Livingston, MS2

1.Novozymes Biological, Inc., Houston,TX

2.Novozymes Biologicals, Inc., Salem, VA

Abstract Wastewater oxidation ponds are one of the oldest forms of treatment. As we enter the 21st century, they are still prevalent as treatment devices for many industries. But we now expect them to meet more stringent criteria such as 95% BOD removal, nutrient control, algae control measurement and control of air emissions. Many new and innovative techniques are available for engineers and operators to use in upgrading an old concept. There are several areas of investigation where we can look to upgrade ponds. The first area is assessment, where we can measure residence time, sludge depth, and air emissions with simple and inexpensive technology. Assessment is important because we can define how to measure success of improvements. We must do more than look at effluent DO and pH. The next area of improvement is aeration. Energy costs impact ponds as well. We have new and innovative ways to oxygenate and mix wastewater. We can use computer technology to measure their effectiveness and even control aeration and mixing. The next area we use to improve performance is sludge control. We can measure sludge buildup with sonar, remove it with dredging and use new enzyme and polymer technology to dewater it. Modeling is now possible with user-friendly pond simulation programs such as BIOWIN. We take all the wastewater BOD, COD and chemical data and load it into models. Then we can simulate various improvements and see the performance change and operating cost. Finally, we can improve biochemical performance with bioaugmentation, real time nutrient measurement, and kinetic simulations. We can promote bacteria growth and check them with DNA techniques. We can also encourage or discourage algae. This paper provides examples of the technology available in each category starting with assessment and providing descriptions sources, benefits and disadvantages of each option. We also present two short case studies where pond BOD improved, emissions decreased and operating costs were controlled. Introduction Sewage lagoons are one of the oldest forms of wastewater treatment. The term lagoon grew when man made innovations were used to upgrade natural ponds. Even in the 21st century, we depend on lagoons More than 8000 of the 20000+ municipal NPDES systems in the USA use some form of lagoon as part of treatment. When PL 92-500 (The first Clean Water Act) was passed in 1972, the majority of US systems were lagoons that removed 70-80% BOD. Now we ask them to remove 90-95% of BOD and reduce air emissions of toxic and odorous chemicals. Achieving improved effluent quality from lagoons Facultative sewage oxidation ponds were the earliest form of wastewater lagoon. The first upgrade was the addition of aeration in some form to create the aerated lagoon. After the standardization of aeration to drive BOD removal, relatively slow change occurred in lagoon design theory. However, with the introduction of modern computer modeling there are now many options open to the designer and operation of an aerated lagoon. Lagoons obey laws of physics like all systems. If we understand them, we can use new technology to tweak their performance. If we upgrade our monitoring methods, we can also have more control than we do when we use lagoon effluent as the point of control. The Critical to Effluent Quality (CTEQ) control points for a lagoon system are:

• Basin Configuration • Hydraulic Retention Time and Mean Cell Residence Time

• Aeration types and performance factors • Kinetic rate improvement • Solids Control • Monitoring • Role of algae in effluent quality

Lagoon Kinetic Theory Aerated Lagoons are designed based on first order kinetics or a mathematical relationship where BOD removal proceeds at a rate based on the initial BOD concentration: Se= So / (1 + K*HRT), where: Se = Effluent BOD5 soluble, mg/L So = Influent BOD5, mg/L K=first order kinetic coefficient (assumes constant biomass concentration in the lagoon), 1/day HRT=hydraulic retention time for the biomass particles, days The total effluent BOD5 is the sum of Se plus any BOD from effluent TSS, so we must control effluent TSS to keep that portion of BOD low as well. If we look at the K factor a little closer, we see it depends on several environmental factors

• K at any temperature= K20 C * 1.045 (Operating Temperature-20C) • If the biomass or VSS concentration varies, then K is proportional to active biomass, or

K=KX specific rate * X, biomass conc. • K is useful at “Steady State” or conditions where we have adequate aeration and DO,

adequate mixing, nutrients, and absence of toxins How do we apply this information to upgrade design and operation? Basin Configuration- If we have a Steady State system, we can get the lowest Se or soluble effluent by creating a series of complete mix or nearly mixed aerated reactors in series. This simulates the effluent from a complete mix reactor. This improved configuration is shown on the right hand side of Figure 1 at the end of the paper. A “Complete Mix” reactor suspends all the biomass through higher power usage. In theory, the mixing is so thorough that a sample taken at any point is equal to that of the effluent. This type of reactor deals well with shocks by diluting them into a well mixed biomass. A “Plug Flow” reactor is one in which the reaction proceeds slowly and the effluent is gradually reduced (think of a pipe model). Plug flow reactors are ideal for nitrification design. We combined these by creating a series of complete mix reactors, where the cells are separated by earthen berms, concrete walls or floating diversion curtains. In this manner, the effluent from one cell is the influent to the next and performance is increased. The influent can also be introduced at more points in the right-hand configuration. This is useful when the wastewater has a strong BOD, i.e. more than 300 mg/L. The operator can measure DO in each cell and use this as the control measure. One CTEQ concern is that this design requires more oxygen in Cell 1 and more aeration may be required. The minimum DO is 1.0 mg/L for lagoons where BOD is the principal concern and 2.0 mg/L minimum where ammonia (NH3) removal or nitrification is desired. Hydraulic Retention Time and Mean Cell Residence Time- The theoretical HRT is the basin volume divided by flow but most aerated lagoons accumulate biomass solids and primary solids and lose HRT. If we can increase HRT in a Steady State reactor, we increase BOD removal so we get a CTEQ benefit. The recommended practice is to measure HRT every 5 years. This is done by a tracer study. A tracer uses a conservative material such as a non-reactive metal, bacterial spore, or a dye, which is introduced into the influent of the basin. Then sample points are used to collect the tracer. There are mathematical methods published by the National Council for Air & Stream Improvement (NCASI, Triangle Park,, NC) which enumerate how tracers should

be applied and what calculations can be made. From the shape of the tracer curve, we can determine the reactor configuration. The HRT is the centroid of the x-axis, as shown in Figure 2. Once we determine the HRT we can see if it can be improved. The choices are limited to:

• Sludge removal • Use of diversion curtains or other baffles to channel the flow. • Positioning of directional mixers to improve circulation in a cell and limit short-circuiting.

The application of these improvement options is site specific. Local factors will determine which is any of these solutions can be adopted. This HRT measurement can be combined with a side scan sonar survey to identify where the sludge has accumulated. Case Study 1: An aerated lagoon in a meat packing plant in the Texas Panhandle had a 1.8 mgd flow, which was processed in an aerated lagoon with a square configuration. The aerated lagoon achieved 80% BOD removal at 25 deg. C operating temperature. The lagoon had been designed for an 8 day HRT, but sludge accumulation had reduced HRT. A tracer study showed a 4.5-day HRT. A new permit required an upgrade to the lagoon. The lagoon was divided into a series of complete mix reactors separated by floating diversion curtains. The existing aeration was relocated to fit the new configuration but the actual numbers of aerators did not change. A second tracer showed HRT to now equal almost 8 days. The effluent was reduced from an average of 47 mg/L BOD before the upgrade to 28 mg/L average in the six months after the upgrade. This was a 40% BOD improvement. This shows the effect of increasing HRT on aerated lagoon performance. Increasing MCRT-Mean Cell Residence Time is the inventory of biomass solids divided by solids lost to wasting (settling for a lagoon) and effluent. In most cases for a lagoon, HRT=MCRT. However, we know K=KX specific rate * X, biomass conc. If we could increase X or biomass concentration we could increase kinetic activity. One mechanical way to due this is an in situ hydraulic baffle or hydraulic clarifier. This is a floating device, which serves as the effluent conduit for a lagoon cell. The biomass must flow upward to exit. As the particles flows up gravity drags it back into the aerated lagoon, raising the biomass concentration. Thus X increases and lagoon MCRT is no longer limited by HRT. As MCRT increases so does BOD and NH3 removal. These in situ hydraulic clarifiers are most beneficial in locations where colder temperatures limit BOD and NH3 removal. They are usually limited to smaller flow rates of less than 200,000 gpd. One version is shown in Figure 3. Aeration Types and Performance-There are many aeration types available to a lagoon designer or operator. They type must be selected based on oxygen transfer and mixing but are also subjected to local concerns such as aeration air emissions, odor emissions, lagoon depth, primary solids, and foaming. The discussion of aeration types can fill several papers. We have limited our discussion to the most common types of aeration and their general benefits and drawbacks, which we show in Table 1 below. We have also included several figures, which illustrate the types of devices available to designers and operators. Operators should have measurable DO in every aerated lagoon cell. If DO is only measured at the effluent there is no guarantee there is aerobic BOD and COD removal upstream. When DO is deficient the bacteria in the biomass may use sulfate (SO4) as an alternate electron acceptor and produce odorous and dangerous Hydrogen Sulfide (H2S) as a by-product. A good designer takes advantage of the right equipment or equipment combination for the situation. Here are some guidelines for designers and operators:

• Lagoons, which are greater than 10’ deep, are candidates for diffused aeration systems. These are high efficiency and have lower air emissions and foaming than surface aeration. But they can be more expensive than mechanical aeration

• The most efficient diffusers, or fine bubble membrane units may clog or fail due to industrial solvents or just from poor maintenance. Test the diffusers you wish to use in the candidate wastewater for material compatibility.

• The dominant form of aeration for lagoons is a high-speed mechanical unit. These surface aerators have a low capital cost, but may cause foam or affect air emissions. In the petrochemical industry, designers choose the “wand” type aerator.

• Mechanical performance and reliability become long-term concerns, which may be as great as oxygen transfer.

• Control requires regular analyses using the DO measurement and the DOUR test to understand aeration use rates

• Systems, which are controlled automatically from a probe in the system, must have multiple probes or manual backups in case of equipment performance issues.

Table 1-Aeration Equipment available for Aerated Lagoons

AERATION TYPES

DESCRIPTION BENEFITS DRAWBACKS

Slow speed mechanical aerators (See Figure 4)

First used in 1940s, this used a transmission to slow the rotation of a motor to 50-100 rpm to aerate and mix wastewater

Rugged and durable machines that run many years after installation

Air emissions and odors can be stripped by splashing, less common after introduction of high speeds

High Speed Surface Aerators (Figure 5)

Direct connected motor rives a boat type prop at high speed to aerate and mix water

Durable machines, generally less cost that slow speed units

Air emissions and foaming unless covered

Coarse and Fine Bubble Diffusers (Figure 6)

A shore based blower introduces air to subsurface diffusers

Very effective on deeper basins (>10’), with high OTE using fine bubble units. Aeration with improved foam

Less effective in shallow basins, fine bubble units must be tested and maintained to control clogging

Wand Type Aerators (Figure 7)

A motor at the surface drives a prop below surface. Air is introduced via a tube Venturi

Excellent mixers which aerated below the surface reducing emission potential

Slightly less O2 transfer reported that surface units, Designer must protect bottom of basin

Pure Oxygen Supplement Systems

A shore based O2 system adds pure O2 through a pump or diffuser

Excellent “on demand” systems, may be economical on a location by location basis

There are safety concerns with pure oxygen, which must be addressed.

Case Study 2: A paper mill in the SE US had a stream, which included methanol, a compound of air emission concern. The paper mill had a large wastewater system but the stream had to be separated an introduced in a subsurface diffuser. The mill had to measure the amount of methanol that was biodegraded versus than which was air stripped to comply with the USEPA Cluster rule. The mill reconfigured an aerated lagoon system to receive the wastewater. The lagoon was divided into a series of 4 complete-mix and 1 settling reactors using floating diversion baffles. The existing high-speed surface aeration units were modified to have large fiberglass covers (see Figure 5) which limited the direct contact of air with the sprayed water surface. Studies showed this technique would reduce emissions 50% but decrease oxygen transfer only 10%. The mill also added effluent recirculation, a 400’ long subsurface diffuser, and established mixing and biomass control. Nutrients and commercial bacteria supplements were added when any decay in performance began. During the three year, biological methanol removal increased from 90% to 97%, as shown in Figure 8.

Kinetic Rate Improvement As previously indicated, there several ways to improve kinetic rate, K. We discussed increasing biomass using hydraulic in pond clarifiers to separate TSS. Another well-published method is to add surface area for reactions using plastic or synthetic medias. Medias have been used in industrial aerated lagoons since 1970, when RinglaceTM was introduced. Today there are several second-generation medias. Many publish their BOD and NH3 removal rates for designers. Figures 9 & 10 show two types employed by the author in industrial aerated lagoons. The first is a plastic media, called BioreefTM, attached to a floatation baffle, which is strung across an aerated lagoon like a floating diversion curtain. A media hangs down from the float to the lagoon bottom. This media, which is made of UV resistant HDPE, has more than 100 SF/CF. The wastewater must flow through the media to reach effluent so contact is guaranteed. The bacteria colonize the high surface area and increase biomass, which increases K for the lagoon. The second media pictured is a frame holding a textile material called a WebitatTM. This media also has a very high surface area for bacteria to colonize. The WebitatTM has a diffuser located below the media, which comes periodically to encourage uniform slough of accumulated biomass from the media. These and other medias have been successfully employed to improve BOD removal and to maintain NH3 removal during cold weather. The designer must consider local factors in selecting the material for his project. One factor is contact. There must be sufficient aeration and mixing for the biomass to grow on the media, but to avoid accumulation of thick anaerobic growth. Baffles may be used in the case of some medias to force flow through the media area in the lagoon. Finally, the media will incrementally improve the performance of a lagoon but cannot substitute for HRT or aeration. The kinetic rate in industrial applications is affected by nutrient balances. Some wastewaters, such as pulp and paper, may be deficient in nitrogen and phosphorus. When a nutrient is deficient, the biomass’ kinetic rate leaves Steady State and may be biomass limited. Control of nutrients is required. Nutrient sources are the process carryover nutrients, sludge benthal feedback, and the nutrients added by the operator. We find the best practice is to add nutrients based on estimates of these sources and a BOD based ratio formula. However, a critical nutrient monitoring should take place at the end of the last aerated cell. We recommend these analyses:

• Ammonia analyses using either a specific ion electrode, a reliable spectrophotometric method or ion chromatography

• Nitrate analyses to detect the conversion of excess ammonia to nitrate using biological nitrification

• Soluble phosphate using spectrophotometric methods • Nutrient discharge limits have made it more difficult for operators to set minimum nutrient

residuals. Where the permits allow we recommend 0.5 mg/L minimums of ammonia (NH4

+) and ortho phosphate (o-PO4), since these are the chemical species used by bacteria for growth and cell production

Bioaugmentation or addition of cells to an aerated lagoon is practiced whenever the lagoon leaves Steady State conditions. In most cases where influent is non-toxic, operators can control aerated lagoons to limit the need for outside biomass. In the cases of toxins, the biomass defaults to Haldane kinetics or a lower kinetic rate based on the biomass, which can metabolize the toxin. There are many anecdotal cases studies and publications for successful use of Bioaugmentation, but almost all share the common theme of upset response. The operator controls his need for seed alternatives after startup by his monitoring and control programs. One special area is nitrification. Some aerated lagoons nitrify their waste. This process depends on slow-growing strict aerobic cultures. Literature describes circumstances where successful Bioaugmentation using nitrifying cultures worked because it boosted nitrifying organisms in a low HRT and low MCRT system. Nitrifiers are often added to lagoons in practice during colder temperatures. Here are some key CTEQ points for Bioaugmentation:

• The key monitoring mechanism that can uncover the health of existing lagoon biomass is using a microscope to identify the quality of the floc formation and higher life forms, as

discussed later in this paper. This can be a trigger to use Bioaugmentation or a confirmation that there is no need.

• The more operators achieve Steady State conditions with nutrient and aeration control the less the need for Bioaugmentation

• Using inexpensive Bioaugmentation, or hauling municipal sludge, has potential downsides:

o Many bacteria will not acclimate to the lagoon and pass through to effluent as TSS

o Municipal sludge may carry filaments and slimes that will establish themselves in the lagoon they are added to

o The pathogens and virus associated with municipal waste may transfer with the sludge and increase the safety precautions for industrial lagoons

o Bioaugmentation is often used as a method to increase lagoon activity during cold-water temperatures where there are few other options for the operator.

o In the pulp & paper industry, the best Bioaugmentation results were achieved with the use of a small seeding bioreactor or PAD. These are automated batch reactor, some with media, used to grow the seed cultures on plant effluent prior to addition to the influent end of a lagoon. A PAD is shown in Figure 11.

Monitoring the Aerated Lagoon Most operators monitor conventional parameters such as influent and effluent pH, temperature, DO, BOD, TSS, and COD or TOC as a surrogate for BOD. However, industrial operators or operators meeting a water quality permit find it useful to add other sampling locations and parameters:

• Determine what the organic soluble content of the aerated lagoon is with COD, total and filtered

• Intermediate DO measurements at the cells in the aerated lagoon • Nutrient analyses and nutrient balance at the end of the aerated zone. Nutrient analyses

can be automated. Reliable ammonia analyzers are available from HACH, Orion, Danfoss, and ChemStat

• Dissolved Oxygen Uptake Rates at the end of the Aerated Zone • Monitoring of the biomass microbiology. For a lagoon we centrifuge 10-20mLs of sample

at 100 g for 5 minutes, pour off all but 1 mL, elutriate it and prepare a wet mount slide. o We use a system described by Steve Leach called FFHB o F=Floc, we desire a large diameter floc (see Figure 12) o F=Filaments are enumerated in density of the floc per Jenkins, et al. o H=The higher life forms are indentified and enumerated o B=Bulk water is examined for free cells and debris

• A numerical scoring system has been developed to allow graphical tracking of the lagoon CTEQ parameter

Role of algae in effluent quality Algae and non-bacterial organism play an interesting role in lagoon operation. They are a “blessing” in that they add DO and some times remove trace organics. However, they are often a “curse” as they can contribute to high effluent pH and TSS. We observe that algae plays a role in long HRT “Hold and Release” ponds where BOD is minimal and nutrients accumulate. Shallow ponds have more algae than deep ones. Several NPDES permit holders have petitioned USEPA to eliminate algal cells from TSS in permit considerations but have failed. Some permit violations are due to algae. There are no easy strategies to control algae. The best is to have some aeration in effluent basins and to control excess nitrogen and phosphorous. We also recommend sampling and identifying the algae to target a treatment to a specific type and species. But though chorination, UV-peroxide disinfection, flocculation, oxidation, filtering, sonicators, chlorine, and bioaugmentation have been tried there are no easy ways to remove algae. We suggest further research be funded to get better and more effective solutions.

Predicting what the Aerated lagoons will do Modeling has been used on activated sludge for several years. We use two models in aerated lagoon control:

• BioWinTM -from EnviroSim Associates Ltd. This is a modeling program, which can be adapted to an aerated lagoon. It does air emissions estimates as well as biodegradation. Calibration is provided by the operator using their monitoring parameters. BioWinTM is available as a licensed program. Self-training is possible but the owner provides training for fee services as well.

• NCASI has an EXCEL based “Aerated Stabilization Basin Model Version 4 (SASBV4), which is available for purchase at www.ncasi.org

Both are reliable predictors of effluent soluble BOD and can predict total BOD is the user develops a BOD/TSS correlation. Each model requires 300-600 data points to be useful and one must invest 30-50 man-hours to set it up. Once done the model gives an owner operator a good predictor for effluent quality. Some industrial users use an EPA model WATER9 to estimate emissions for compliance report. This uses a math model where stripping runs at a second order function and biodegradation runs at first order so it can over predict stripper. The user goes through several iterations to gain a useful emissions model. This model is often used to estimate the magnitude of odor emissions as well. Conclusion Aerated lagoons are “old dogs” which still “do tricks” and improve their performance well into the 21st century. We can still get more out of them in wastewater treatment by using technologies such as:

• Configuration-The use of complete mix reactors in series has improved BOD removal in aerated lagoons

• HRT and MCRT-We need to measure it every few years and improve it with diversion partitions or dredging (or both)

• Aeration-There are many choices and many considerations beyond how much oxygen on e needs. This is where a consultant can pay dividends. However, the maintenance issues with aeration are as important as the products claimed transfer.

• There are several tools in the tool box to improve the kinetic rate: o Use operator knowledge to keep systems at “Steady Sate” by balancing nutrients

and having enough DO o Use Bioaugmentation when your own biomass is damaged o Use monitoring to measure elements that are critical to effluent quality (CTEQ) o Do not forget the microscope as a tool

• Plastic medias that are suspended in the lagoon and hydraulic insert clarifiers offer methods to avoid the limitations of lagoon biomass

• Systems can be modeled for greater control and prediction if one is willing to spend the time.

In the end, the greatest control measure is an operator who takes pride in his or her lagoon and dedicates themselves to a good effluent! References

1. A Review of Procedures for Conducting Conservative Tracer Studies in the Hydraulic Characterization of Effluent Treatment Basins, NCASI Technical Bulletin 408, 1983.

2. “The Efficacy of Employing Bacillus globigii as a Particulate Tracer in Aquatic Systems”, C. Lange et al, Proceedings of WEFTEC 2000, Anaheim, Ca.

3. V. L Snoeyink, D. Jenkins, Water Chemistry, John Wiley & Sons, New York, NY, 1980 4. C.P.L. Grady, Jr, et al., Biological Wastewater Treatment, 2nd Edition, Marcel Dekker, Inc.,

New York, N.Y., 1999. 5. G. Tchobanoglous, et al., Metcalf Eddy, Wastewater Engineering; Treatment, Disposal,

and Reuse, 3rd Edition, McGraw-Hill, New York, N.Y., 1991.

6. M.T. Madigan, et al., Brock’s Biology of Microorganisms, 9th Edition, Prentice-Hall, Saddle River, N.J., 2000.

7. Teh Fu Yen, Environmental Chemistry, Prentice-Hall, Saddle River, N.J., 1999. 8. Rittman, B.E. (1997). Closure of: How input active biomass affects sludge age and

process stability. J. Env. Eng., ASCE, 123 (1), 103. 9. Rittman, B.E. and Whiteman, R. (1994). Bioaugmentation comes of age. Water Quality

Int., 1, 22-26. 10. Hultman et al. (1998). Seeding of nitrifying bacteria to the activated sludge process - role

of separate nitrification of supernatant from dewatering of digested sludge. Poster presented at the 19th Biennal Conference of the International Association on Water Quality (IAWQ), Vancouver, Canada. Hydrogen Sulfide, PEL/IDLH, current, http://www.cdc.gov/niosh/idlh/7783064.html

11. “Chemical Analyses of Water & Wastes”, EPA-600/4-79-020, USEPA, Washington, DC 12. Stumm, W., ed. (1987), Aquatic Surface Chemistry, Chemical Processes at the Particle-

Water Interface, Wiley Interscience, NY, NY. 13. APHA/WEF/AWWA (1998), Standard Methods for the Examination of Water and

Wastewater, 20th ed., APHA, Washington, DC. Figures

Figure 1-Changing a aerated lagoon to a series of complete mix reactors

Figure 2-Tracer curve from an aerated lagoon—Courtesy Novozymes

CompleteMix Reactor

PlugFlowReactor

4X

X

X

X

A Seriesof CompleteMix ReactorssimulatesPlug Flow

“Tapered”Aerationis used.

Effluent Effluent

InfluentInfluent

Influent

Effluent

Concentration Curve-ASB Tracer Study

0

50

100

150

200

250

300

350

400

0 50 100 150 200

Time (hr)

cfu/

mL

Average

Known HRT=48 hoursMeasured HRT 43 hoursRecovery 78%Two CSTRS in series

Figure 4 Slow Speed Aeration Unit (Fort Polk, Leesville, LA)

Figure 5-High speed aerators at a paper mill, some have been covered to control foam and air emissions-Courtesy Aqua Aerobics Systems

Figure 3-The pictures show the Atlas-IS, one version of a hydraulic clarifier

Courtesy EDI

Figure 6-Diffused aeration (with blow out on left, fine bubble diffuser (top),

coarse bubble diffuser (bottom)-courtesy EDI Figure 7-A Wand type of subsurface mixing aerator, which uses a Venturi for oxygen transfer. These types of aerators are preferred for lagoons where air emissions are a concern

Figure 8 Improved Methanol removal in a paper mill Aerated lagoon, 2003-2006

Biological Methanol Removal

80%85%90%95%

100%

June 2003-May 2005

May 2005-April 2006

April 2006onward

Figure 9-Layout of a BioreefTM media curtain courtesy of EDI

Figure 10-Example of a WebitatTM media installed in an aeration tank which can be adapted to a lagoon

Figure 11-A PAD reactor used in Bioaugmentation at a mill lagoon- Courtesy Novozymes Small (<150 μm) dispersed floc (pin floc). Weak and open floc.

Medium (150-500 μm) size floc that is mostly dense and compact.

Medium to large sized (>500 μm) floc that is dense, round, and compact

Figure 12-Floc is scored for its diameter and density