Aerated Pond

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Aerated Pond

Eawag (Swiss Federal Institute of Aquatic Science and Technology), Dorothee Spuhler (seeconinternational gmbh)

An aerated pond is a large, mixed aerobic reactor[no‐ecompendium]

similar to facultative ponds in [665‐waste stabilization pond] systems,

with the difference that natural oxygenation is enhanced[/no‐

ecompendium]. Mechanical aerators provide oxygen and keep the

aerobic organisms suspended and mixed with water to achieve a high

rate of organic degradation.[no‐ecompendium] As natural oxygenation

is enhanced, ponds can be deeper (thus smaller in surface) and are suited also for colder climates compared.There are two types of aerated ponds: common aerated lagoons (enhanced facultative ponds) and completely

mixed aerated ponds are in essence activated sludge systems without sludge. The effluent of aerated ponds

may be reused or used for recharge, but settled sludge requires a further treatment or correct disposal.[/no‐

ecompendium]

In Out

Blackwater, Faecal Sludge, Greywater, Brownwater,

Faeces, Excreta

Sludge, Fertigation Water, Biogas (if anaerobic

pond is covered)

[no‐ecompendium]

Introduction

[/no‐ecompendium]

Increased mixing and aeration from the mechanical units means that the ponds can be deeper and tolerate

much higher organic loads than a maturation [no‐ecompendium]or a facultative [/no‐ecompendium]pond (see

waste stabilisation ponds). The increased aeration allows for increased degradation and increased pathogen

removal. As well, because oxygen is introduced by the mechanical units and not by light‐driven

photosynthesis, the ponds can function in more northern climates. [no‐ecompendium]Mechanical aeration

enhances the treatment efficiency and reduces the required hydraulic retention time (HRT) for aerobic

degradation of organics (ROSE 1999). It also increases pathogen removal because of the favourable effect of

oxygen on solar water disinfection (CURTIS et al. 1992). The smaller area requirement means that it is

appropriate for both rural, and peri‐urban environments (TILLEY et al. 2008). However, the use of aerators

also increases the complexity of the systems and technical material and energy is needed (ARTHUR 1983).

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Schematic view of an artificially aerated facultative lagoon (partially mixed). Source: TILLEY et al. (2014)

There are two types of aerated ponds:

Aerated facultative ponds or lagoons (see also waste stabilisation pond systems), when it is desired to

have a more aerobic system, compacter than normal facultative ponds, or when loads for conventional

facultative ponds are too high.

Completely mixed aerated ponds or lagoons.

Schematic view and picture of a surface complete mix aerator. Source: unknown

Aerators used in partially mixed lagoons are generally placed on the lagoon surface and provide enough

turbulence to satisfy the oxygen demand for aerobic oxidation, but they allow a sludge layer to form at the

bottom of the pond. When energy is readily available, the water may be pumped through flow form cascades

before entering the ponds for aeration. Aerators used in a completely mixed lagoon can be surface mechanicalmixers or subsurface diffusers. They should provide enough energy to maintain the solids in suspension.

Completely mixed aerated lagoons are in essence activated sludge units without sludge return (ARTHUR 1983).

As the sludge remains in suspension, effluents of completely mixed ponds require a post‐treatment in a

sedimentation pond. Just like for WSPs, the effluent of ponds can be reused in agriculture (e.g. irrigation) or

aquaculture (e.g. macrophyte or fish ponds) even though it has generally lower nutrient loads. However, the

sludge which forms at the bottom of the aerated facultative lagoon or the sedimentation ponds (in the case of a completely mixed pond) needs to be dug out regularly. And before it can be reused in agriculture it requires

a further treatment (e.g. through composting (compost chamber or large scale composting, anaerobic

digestion or a constructed wetland).[/no‐ecompendium]

Design Considerations

Influent should be screened and pre‐treated to remove garbage and coarse particles that could interfere with

the aerators. Because the aeration units mix the pond, a subsequent settling tank is required to separate the

effluent from the solids.

http://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/sludge-treatment/settling-thickening-ponhttp://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/semi-centralised-wastewater-treatment-16http://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/processes/anaerobic-digestion-generalhttp://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/sludge-treatment/co-composting-large-scahttp://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/site-storage-and-treatments/composting-thttp://www.sswm.info/category/implementation-tools/reuse-and-recharge/hardware/reuse-blackwater-and-greywater-agricultu-1http://www.sswm.info/category/implementation-tools/reuse-and-recharge/hardware/reuse-blackwater-and-greywater-agricultu-0http://www.sswm.info/category/implementation-tools/reuse-and-recharge/hardware/reuse-blackwater-and-greywater-agricultu-2http://www.sswm.info/category/implementation-tools/reuse-and-recharge#Reuse%20of%20Blackwater%20and%20Greywater%20in%20Agriculturehttp://www.sswm.info/category/implementation-tools/water-purification/hardware/point-use-water-treatment/sedimentationhttp://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/semi-centralised-wastewater-treatments-3http://www.sswm.info/category/implementation-tools/wastewater-treatment/hardware/semi-centralised-wastewater-treatments/w


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The pond should be built to a depth of 2 to 5 m and should have a detention time of 3 to 20 days, depending on

the treatment target.

To prevent leaching, the pond should have a liner. This can be made from clay, asphalt, compacted earth, or

any other impervious material. A protective berm should be built around the pond, using the fill that is

excavated, to protect it from runoff and erosion.[no‐ecompendium]

Aerated facultative pond/lagoon

The design of an aerated facultative pond is very similar to that of a facultative pond, with an aerobic zoneclose to the surface and a deeper, anaerobic zone. But there are no requirements in term of surface area as

the process is independent of photosynthesis. The two main design criteria are HRT and depth. The HRT should

be adopted in order to allow a satisfactory removal of BOD (biological oxygen demand) and is usually 4 to 10

days (VON SPRELING 2005) for organic loads of 20 to 30 g BOD/m3.day (SASSE & BORDA 1998). The depth of

the pond should be planned keeping in mind the compatibility with the aeration system and the need of an

aerobic layer of approximately 2 meters to oxidise the gases from the anaerobic decomposition of the bottom

sludge. The amount of oxygen to be supplied by the aerators for the aerobic degradation/stabilisation of the

organic matter should normally be equal to the total ultimate influent BOD (VON SPRELING 2005). However,

such lagoons are generally designed using empirical methods: a HRT of 4 to 5 days results in 70 to 90% BOD5

removal in a partially mixed aerated lagoon by power requirements of 4 W/m3 (ARTHUR 1983).

Completely mixed aerated pond

Completely mixed aerated lagoons are essentially aerobic. The aerators serve not only to guarantee the

oxygenation of the medium, but also to maintain the suspended solids (biomass) dispersed in the liquid

medium. These systems are also called flow‐through lagoons or CSTR (completely‐stirred tank reactor)

lagoons. Aerated ponds act similarly to aeration tanks in activated sludge processes. The main difference is

that solids are not recirculated. Biomass and solids from the raw sewage are maintained together in

suspension. This enhances the contact between bacteria contained in the biomass (responsible for the

degradation) and the raw sludge to be degraded. Hence, the efficiency of completely aerobic ponds increases

in comparison to partially mixed ponds and allows a reduction in volume. Land requirements for this system

are the smallest within ponds systems. The typical HRT of a completely mixed aerated lagoon is in the order of

2 to 4 days. This time is enough for an efficient removal of the suspended solids (VON SPRELING 2005). A HRTof 4 days, resulting in 70 to 90% BOD5 removal, requires about 20 W/m3 of energy (ARTHUR 1983). Aerated

ponds have removal capabilities similar to facultative lagoons, except that nitrification of ammonia‐nitrogen

can be nearly completed in warm seasons, while cold weather will halt that process (U.S.EPA 2002). As the

quality of effluent from these ponds is not satisfactory for direct discharge into the environment, completely

mixed aerated lagoons should be followed by settling ponds. These may be either several short HRT ponds (i.e.

2 day), requiring frequent de‐sludging (VON SPRELING 2005); or a single 10‐day facultative pond with

sufficient depth to allow long‐term sludge storage. Aerators should be positioned carefully to avoid dead areas

where solids are able to settle out already in the aerated pond. Small aerators rather than fewer large ones

provide more evenly spread mixing, and rounded pond corners also help in avoiding dead areas (ARTHUR

1983).

[/no‐ecompendium]

[ecomp‐appropriateness]

Health Aspects/Acceptance

The pond is a large expanse of pathogenic wastewater; [no‐ecompendium]health hazards can be caused by the

aerosol effect releasing pathogens into the air (ARTHUR 1983); [/no‐ecompendium]care must be taken to

ensure that no one comes in contact with or goes into the water. The aeration units can be dangerous to

humans and animals. Fences, signage, or other measures should be taken to prevent entry into the area.

[no‐ecompendium]A mechanically aerated pond can efficiently handle effluents with a high concentrationand can reduce pathogen levels significantly. But electricity service must be uninterrupted and replacement

parts available to prevent extended downtimes that may cause the pond to turn anaerobic (TILLEY et al.

2008). [/no‐ecompendium]

[no‐ecompendium]

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Costs Considerations

Investment costs are moderate to high, but expert design is required. Due to the large energy consumption for

mixing and aeration, operation and maintenance is expensive. The aeration devices also increase the

complexity of the unit and thus the vulnerability for technical failure (due to lack of replacement/spare parts

or engineering skills). In consequence, the costs are generally higher than for WSP systems, depending on the

local context and the availability of electricity. In some cases, solar‐power driven aeration systems may be

worth considering.[/no‐ecompendium]

Operation and Maintenance

Permanent, skilled staff is required to maintain and repair aeration machinery and the pond must be desludged

every 2 to 5 years.[no‐ecompendium]and the sludge needs to be either post‐treated (e.g. [1182]‐anaerobicallydigested], composted, [1868compos‐incinerated]) or correctly disposed (TILLEY et al. 2008). The influents

need also to be screened or settled as any large object could damage the aeration system. The pond itself must

also be fenced off so no coarse objects can be thrown in.[/no‐ecompendium]

Care should be taken to ensure that the pond is not used as a garbage dump, especially considering the damage

that could result to the aeration equipment.[no‐ecompendium]

At a Glance

Working Principle

Aerated ponds or lagoons are similar to facultative ponds, with the difference that natural

oxygenation is enhanced by mechanical air injection. As the oxygenation does not dependon algae activity and photosynthesis, ponds can be deeper (thus smaller in surface) and are

also suited for colder climates. Mechanical aeration enhances the treatment efficiency

due to more intense bacterial aerobic degradation and increases pathogen removal.

Capacity/Adequacy

Adapted for almost all wastewater (also industrial) in rural or urban areas. However,

electricity supply needs to be continuous and a foul odour might be a problem in urban

areas.

Performance

Aerated facultative ponds: 70 to 90 % BOD; HRT: 4 to 10 day

Completely mixed aerated ponds: 70 to 90 % BOD; HRT: 2 to 4 day; high phosphorus,nitrogen and ammonia removal.

Costs

Investment costs depend on the availability of mechanical aerator systems. However,

operation and maintenance costs are high due to the large energy consumption of aeration

and the need for skilled staff for operation and maintenance.

Self‐help

Compatibility

Planning and construction supervision carried out by technical experts; community labour

contribution during construction possible; permanent skilled staff required for operation.

O&M

Permanent skilled staffs are required to repair and maintain aeration machinery; Sludge

needs to be dug out every 2 to 5 years and either further treated (e.g. anaerobic digestion,

composting) or correctly disposed.

ReliabilityHigh reliability regarding treatment efficiency and shock loadings; however, the system

does not work at all in case of power failure.Main strength

High treatment efficiency for relatively small area requirement and (compared to

activated sludge) lower technology requirements.

Main weakness Requires permanent energy and skilled staff.[/no‐ecompendium]

Applicability

A mechanically aerated pond can efficiently handle concentrated influent and significantly reduce pathogen

levels. It is especially important that electricity service is uninterrupted and that replacement parts are

available to prevent extended downtimes that may cause the pond to turn anaerobic.

Aerated ponds can be used in both rural and peri‐urban environments. They are most appropriate for regions

with large areas of inexpensive land located away from homes and businesses. Aerated lagoons can function in

a larger range of climates than [665‐Waste Stabilization Ponds] and the area requirement is smaller compared

to a maturation pond.[no‐ecompendium] But they are also more complex from a technical and operational

point of view. [/no‐ecompendium]

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Advantages

Resistant to organic and hydraulic shock loads

High reduction of BOD and pathogens

No real problems with insects or odours if designed and maintained correctly

Can treat high loads

Less land required than for simple pond systems (e.g. WSP)

The treated water can be reused or discharged if a secondary maturation/settling pond follows the

aerated lagoon/completely mixed aerated pond

Disadvantages

Requires a large land area

High energy consumption, a constant source of electricity is required

High capital and operating costs depending on the price of land and of electricity

Requires operation and maintenance by skilled personnel

Not all parts and materials may be locally available

Requires expert design and construction [no‐ecompendium] supervision [/no‐ecompendium]Sludge and possibly effluent require further treatment and/or appropriate discharge

References

ARTHUR, J.P. (1983): Notes in the Design and Operation of Waste Stabilization Ponds in Warm Climates of Developing Countries . (=World Bank Technical Paper, 7). Washington: The World Bank. PDF

CRITES, R.; TCHOBANOGLOUS, G. (1998): Small and Decentralized Wastewater Management Systems. New York: The McGraw‐HillCompanies Inc.

CURTIS, T.P.; MARA, D.D.; SILVA, S.A. (1992): Influence of pH, Oxygen, and Humic Substances on Ability of Sunlight to Damage Faecal

Coliforms in Waste Stabilization Pond Water. In: Applied and Environmental Microbiology 58, 1335‐1343. URL [Accessed: 02.04.2010].PDF

NATURGERECHTE TECHNOLOGIEN, BAU‐ UND WIRTSCHAFTSBERATUNG (TBW) GmbH (Editor) (2001): Decentralised WastewaterTreatment Methods for Developing Countries. GTZ and GATE. PDF

SASSE, L. ; BORDA (Editor) (1998): DEWATS. Decentralised Wastewater Treatment in Developing Countries. Bremen: Bremen OverseasResearch and Development Association (BORDA). PDF

SPERLING, M. von (2005): Part Three: Stabilization Ponds. In: SPERLING, M. von; LEMOS CHERNICHARO, C.A. de (2005): BiologicalWastewater Treatment in Warm Climate Regions Volume 1. London, 495‐646. URL [Accessed: 16.02.2011].

TCHOBANOGLOUS, G.; BURTON, F. L.; STENSEL, H. D.; METCALF & EDDY Inc. (Editor) (2003): Wastewater Engineering, Treatment andReuse. (= Fourth Edition). New York: McGraw‐Hill Companies, Inc.. PDF

TILLEY, E.; ULRICH, L.; LUETHI, C.; REYMOND, P.; ZURBRUEGG, C. (2014): Compendium of Sanitation Systems and Technologies. 2nd

Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). URL [Accessed:28.07.2014]. PDF

TILLEY, E.; LUETHI, C.; MOREL, A.; ZURBRUEGG, C.; SCHERTENLEIB, R. (2008): Compendium of Sanitation Systems and Technologies.Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (EAWAG) and Water Supply and SanitationCollaborative Council (WSSCC). URL [Accessed: 15.02.2010]. PDF

See document in FRENCH

U.S.EPA (Editor) (1980): Onsite Wastewater Treatment Systems Manual. (= EPA 625/1‐80, 12). United States Environmental ProtectionAgency, Office of Water Office of Research and Development. PDF

U.S.EPA (Editor) (2002): Onsite Wastewater Treatment Systems Manual Technology Fact Sheet 5. Fixed‐Film Processes. In: U.S.EPA(Editor) (1980): Onsite Wastewater Treatment Systems Manual. , 008. PDF

For further readings, case studies, awareness raising material, training material, important weblinks orthe related powerpoint presentation, see www.sswm.info/taxonomy/term/
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