settling_ thickening ponds _ sswm
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Concept Understand your System Planning & Process Tools Implementation Tools Specific Topics Background
Home Implementation tools Wastewater Treatment Hardware Sludge Treatment Settling / Thickening Ponds
Implementation Tools
Introduction
Water Sources
Water Purification
Water Distribution
Water Use
Wastewater Collection
Wastewater
Treatment
Reuse and Recharge
Compiled by:
Overview of potential, modest-cost options for faecal sludge
treatment including pre-treatment of faecal sludge (solid-liquid
separation, left) and secondary treatment options (for thickened
sludge and liquid fraction respectively). Source: MONTANGERO &
STRAUSS (2004)
Settling/Thickening Ponds
Dorothee Spuhler (seecon international
gmbh)
Executive SummaryThickening ponds are designed to separate solid and liquid parts of faecal
sludge in order to process each other separately (e.g. in waste stabilisation
ponds, drying beds or by composting). They are designed as simple ponds in
which faecal sludge is filled and solids settle to the ground. Fresh sludge
(e.g. from latrines or unsewered public toilets) will require long retention
times during which settled sludge will undergo further stabilisation by
anaerobic digestion. Partially digested sludge with little organic matter and
easily settable solids (e.g. from septic tanks, anaerobic digesters etc.) will
require retention times of some hours only and settled sludge is desludged
more frequently. Short-retention-time thickening ponds are often called
settling tanks, while long-retention and stabilisation-thickening ponds are
called sedimentation or anaerobic ponds.
In Out
Faecal Sludge
Compost (for further off-site composting),
Soil, Treated Water, Fertigation Water or
Water requiering further treatment
All wet wastewater collection or treatment units produce sludge of variable consistency.
Generally, such sludge does not have a higher solids content than 2 to 10 % (SASSE & BORDA
1998). Thickening ponds or tanks are designed as a pre-treatment for faecal sludge to
separate the solid from the liquid part of the sludge in order to treat each one separately. The
supernatant liquid can be treated either in waste stabilisation ponds (optionally together with
other wastewater), mounds or just be infiltrated into the ground (e.g. greywater infiltration in
soak pits, or leach fields) depending on the degree of pollution. The thickened sludge occupies
much less volume and can be more easily transported. However, it can still contain pathogens
and needs to be further treated before reuse or disposal. Typically, sludge from thickening
ponds is transformed into biosolids on drying beds or co-composted together with organic solid
waste (co-composting small or large-scale). In some cases, it may also be spread out in thin
layers on the ground.
Sludge from on-site
collection systems such as
latrines or unsewered public
toilets (or activated sludge
systems) has not yet
undergone significant
degradation and still contains
a high amount of organics
(high-strength sludge,
HEINSS et al. 1998). This
“fresh” sludge is treated in
thickening ponds, which
provide a long liquid
retention time (hydraulic
retention time, HRT) of
several days or a few weeks
(so-called sedimentation
ponds), and hence, also allow for anaerobic degradation of organics (STRAUSS &
MONTANGERO 2002).
Related Topics
Sustainable Sanitation
Grasp what sustainablesanitation actually means andhow the water and thenutrient cycle are interlinked:
Sustainable SanitationThe Nutrient CycleThe Water Cycle
Sludge Drying
Alternative systems for sludgedrying are:
Drying BedsMechanical Dewatering
Precedent Steps
Almost every sludge can bedried in drying beds. Typicallythey are combined with:
Septic TanksBiogas SettlerAnaerobic Baffled ReactorAnaerobic Digestion (Small-scale)Anaerobic Digestion (Large-scale)Anaerobic Digestion (OrganicWaste)
Further Steps
The effluent from drying bedscan be used for fertigation,while the sludge may be usedas soil amendement(optionally after a furthercomposting step):
Fertigation(Co-)composting (Small-scale)(Co-)composting (Large-scale)Use of Compost
Search
ABOUT USER MANUAL CREDITS PARTNERS SSWM COURSES LIBRARY GLOSSARY KEY RESOURCES FAQ CONTACT TRAIN THE TRAINERS
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Non-mechanised settling thickening tanks and
sedimentation/anaerobic ponds for solid-liquid separation
(schematic). Source: STRAUSS & MONTANGERO (2002)
Different types of settling tanks. Source: KLINGEL et al. (2002)
Circular primary settling tank at Haran Al-Awamied treatment
plant. Source: MOHAMMED et al. (2009)
Sludge from septic tanks, or anaerobic digesters, biogas settlers, etc. has already been partly
mineralised and is much more easily settable (low-strength, HEINSS et al. 1998). Such “old”
sludge thickened in so-called settling tanks, which provide liquid retention times of a few
hours only, just enough to ensure quiescent settling of settable solids (STRAUSS &
MONTANGERO 2002).
Basic Design Principles
Both sedimentation ponds
and settling tanks are
designed based on the
storage volume required for
a desired depth and quantity
of accumulating solids
(STRAUSS & MONTANGERO
2002) in a given time
(depending on the frequency
of desludging). The specific
volume occupied by
separated solids may vary
from 0.02 (thin sludge) up to 0.15 (high-strength sludge) m3/m3 of raw sludge depending on
the type and composition and on the time allowed for solids consolidation and thickening
(HEINSS et al. 1998; STRAUSS & MONTANGERO 2002).
Sedimentation ponds are designed similar to anaerobic ponds in waste stabilisation ponds with
a sufficient volume for sludge accumulation over a long period (KLINGEL et al. 2002). The
anaerobic digestion process, which stabilises the fresh sludge, causes gas production
(MONTANGERO & STRAUSS 2004). When the gas bubbles rise, this can further limit the sludge
settling as solids are also pushed to the surface, but the produced biogas can be collected and
used for heating or cooking (i.e. by covering the pond with a gastight canvas). Sedimentation
ponds can be operated in batch and continuous mode and sludge is removed only once every 8
to 12 month (STRAUSS & MONTANGENRO 2002). Therefore they require generally more space
than simple settling ponds. The organic load of sedimentation or anaerobic ponds vary from
250 to 350 gBOD/m3 per day and the volume of accumulated sludge per incoming solids may
vary between 0.8 to 2 L/Kg TS (KLINGEL et al. 2002).
Settling tanks need not only
sufficient volume for sludge
accumulation, but also
sufficient depth of the liquid
column (> 1.5 m) in order to
allow a fast and good settling
(KLINGEL et al. 2002). The
loading/resting cycle in
settling tanks is much
shorter than in
sedimentation ponds and
sludge is removed about
every 2 to 4 months
(STRAUSS
& MONTANGERO 2002). The tanks should be equipped with baffles to maintain hydraulic
conditions favourable to good settling and to retain floating scum (KLINGEL et al. 2002).
For design, accumulating sludge volumes per incoming solids load of 5 to 9 L/kg TS can be
considered. However, the consistency and concentration of faecal sludge is highly variable
(depending on storage duration, tank emptying technology and pattern, performance of
treatment of collection unit, added matters, temperature, intrusion of ground- or surface
waters, etc. MONTANGERO & STRAUSS 2004) and treatment systems should be designed on a
case-to-case basis (EAWAG/SANDEC 2008).
Whether a sedimentation
pond or a settling tank is
used depends on the
consistency of the sludge
(e.g. fresh vs. old sludge),
the space available and the
means of sludge removal.
When only little space is
available, fresh toilet sludge
may be diluted with more
stabilised sludge in order to
be treatable in settling tanks
(KLINGEL et al. 2002). It is
also important to consider
that the quantities of solids,
which are produced and have
to be removed in settling
ponds, are much smaller than
the mass of solid to be
removed and handled from sedimentation ponds (EAWAG/SANDEC 2008). But frequent sludge
removal can also become expensive for operators.
Desludging (sludge emptying) can be done either after removal of the liquid column and a
period of drying manually by front loaders or mechanically through draw-off pipes without
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removing the supernatant. In-pond drying is more common for sedimentation ponds. In this
case, at least two parallel units need to be provided in order to assure continuous operation
(KLINGEL et al. 2002; STRAUSS & MONTANGERO 2002).
The removal rates, which may be expected in sedimentation ponds and settling tanks are
(STRAUSS & MONTANGERO 2002):
Sedimentation Pond Settling Tank
Suspended Solids 60 % > 95 %
BOD and COD (unfiltered) 30 to 50 % 70 to 95 %
BOD (filtered) 18 % 45 %
Costs Considerations
Pond systems are relatively low cost, as they do not require much maintenance and no energy.
However, large surface areas must be available and they need to be designed and operated by
skilled staff. Depending on the frequency of desludging, emptying may become expensive or
labour intensive.
Operation and Maintenance
The thickened sludge can be removed either mechanically or manually (after removing the
liquid column) every 8 to 12 or 2 to 4 months for sedimentation ponds or settling tanks
respectively (STRAUSS & MONTANGERO 2002).
Maintenance includes regular desludging and cleaning of the discharging areas (to reduce flies
and odours) and grit, sand and solid waste that are discharged along with the sludge must be
removed.
Trained staff for operation and maintenance is required to ensure proper functioning (TILLEY
et al. 2008).
Health Aspects
Faecal sludge is highly pathogenic and should be handled with care. Thickened sludge is also
infectious, although it is easier to handle and less prone to splashing and spraying (TILLEY et
al. 2008). The supernatant liquid can be polluted with microbes as well and needs to be
correctly treated or infiltrated. Open ponds may cause odour and should therefore be installed
away from densely populated areas.
At a Glance
Working Principle
Thickening ponds are simple ponds designed
for the solid-liquid separation of faecal
sludge. There are two types of thickening
ponds: Sedimentation ponds, which require
long retention times and hence also allow for
anaerobic degradation (similar to anaerobic
ponds in WSP); and settling ponds as a
primary treatment of already partly digested
sludge (e.g. from septic tanks or anaerobic
digesters) providing only short retention
times.
Capacity/Adequacy
Centralised; Requires large land-surfaces and
can cause odour (should be installed far from
communities); Is suited for almost any hot or
temperate climate.
Performance
Depends strongly on the local climate (rain,
runoff); Suspended solids can be removed up
to 60 % (sedimentation pond, fresh sludge)
and 95%, respectively (in settling tanks)
from the supernatant and concentrated in
the thickened sludge. Sludge and effluent
are generally still pathogenic and need
further treatment before being disposed or
reused in agriculture.
CostsModerate investment costs and low
operation costs.
Self-help Compatibility
Can be constructed with locally available
material, but requires expert design and
skilled operation.
O&M
Application of sludge, desludging, cleaning
of surrounding and incoming sludge from
solid wastes. Desludging every 2 to 4 or 8 to
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12 months for settling tanks or
sedimentation ponds respectively.
ReliabilityHigh, if the area is kept dry and hot or
temperate
Main strength Low-tech and no requirement of energy.
Main weaknessRequires space and desludging as well as
further treatment of both liquids and solids.
ApplicabilitySedimentation ponds can be used for partly stabilised faecal sludge such as sludge from septic
tanks, anaerobic baffled reactors, large or small scale anaerobic digesters, biogas
settlers, etc. They are not suited for fresh faecal sludge.
Settling tanks can be used as first step in fresh faecal sludge treatment (e.g. from latrines or
unsewered public toilets). Often, they are the first stage in a sequence of stabilisation ponds
(see also anaerobic ponds in waste stabilisation ponds).
The sludge is not hygienic and requires further treatment before reuse or disposal (drying,
composting) and the supernatant liquid needs to be infiltrated or further treated depending the
pollution/pathogen removal performance of the thickening ponds.
Thickening in ponds or tanks generally requires the availability of large areas of land, thus they
are appropriate where there is inexpensive, available space that is far from homes and
businesses.
Thickening ponds are low-cost and can be installed in most hot and temperate climates.
AdvantagesNo energy required
Relatively low cost
Potential for local job creation
Can be built with locally available material
DisadvantagesRequires large space
Requires expert design and skilled operation
Odours and flies can occur
In the case of frequent desludging, front-end loaders maybe required
References
EAWAG/SANDEC (Editor) (2008): Faecal Sludge Management. Lecture Notes. Duebendorf:
Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in
Developing Countries (SANDEC). URL [Accessed: 23.05.2012]. PDF
EAWAG (Editor); SANDEC (Editor) (2008): Fecal Sludge Management. Presentation.
Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and
Sanitation in Developing Countries (SANDEC). URL [Accessed: 18.02.2011].
HEINSS, U.; LARMIE, S.A.; STRAUSS, M. (1998): Solids Separation and Pond Systems for the
Treatment of Faecal Sludges in the Tropics . Lessons Learnt and Recommendations for
Preliminary Design. . Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG),
Department of Water and Sanitation in Developing Countries (SANDEC). URL [Accessed:
12.04.2010]. PDF
KLINGEL, F. ; MONTANGERO, A. ; KONE, M.; STRAUSS, M. (2002): Fecal Sludge Management in
Developing Countries - A Planning Manual. Duebendorf: Swiss Federal Institute for
Environmental Science (EAWAG). URL [Accessed: 23.06.2010]. PDF
MOHAMED, A. ; KLINGEL, F.; BRACKEN, P.; WERNER, C. (2009): Effluent reuse from
constructed wetland system Haran Al-Awamied, Syria. Eschborn: Sustainable Sanitation
Alliance (SuSanA) . URL [Accessed: 26.01.2011]. PDF
MONTANGERO, A.; STRAUSS, M. (2004): Faecal Sludge Treatment. Duebendorf: Swiss Federal
Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing
Countries (SANDEC). URL [Accessed: 10.06.2010]. PDF
STRAUSS, M.; MONTANGERO, A. (2002): FS Management – Review of Practices, Problems and
Initiatives. London and Duebendorf: DFID Project R8056, Capacity Building for Effective
Decentralised Wastewater Management, Swiss Federal Institute of Aquatic Science (EAWAG),
Department of Water and Sanitation in Developing Countries (SANDEC). URL [Accessed:
24.05.2012]. PDF
STRAUSS, M.; DRESCHER, S.; ZURBRUEGG, C.; MONTANGERO, A.; OLUFUNKE, C.; DRECHSEL,
P. (2003): Co-composting of Faecal Sludge and Municipal Organic Waste. Swiss Federal
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Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing
Countries (SANDEC) and International Water Management Institute (IWMI). URL [Accessed:
23.06.2010]. PDF
STRAUSS, M.; BARREIRO, W.C.; STEINER, M.; MENSAH, A.; JEULAND, M.; BOLOMEY, S.;
MONTANGERO, A.; KONE, D. (2003): Urban Excreta Management - Situation, Challenges, and
Promising Solutions. In: IWA Asia-Pacific Regional Conference Bangkok, Thailand. URL
[Accessed: 23.06.2010]. 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. URL [Accessed: 15.02.2010]. PDF
See document in FRENCH
UNEP (Editor); Murdoch University (Editor) (2004): Environmentally sound technologies in
wastewater treatment for the implementation of the UNEP/GPA "Guidelines on Municipal
Wastewater Management". The Hague: United Nations Environment Programme Global
Programme of Action (UNEP/GPA), Coordination Office. PDF
SASSE, L. ; BORDA (Editor) (1998): DEWATS . Decentralised Wastewater Treatment in
Developing Countries. Bremen: Bremen Overseas Research and Development Association
(BORDA). PDF
Further Readings
ECOSAN CLUB (Editor) (2012): Faecal Sludge Management. Vienna:
Ecosan Club. URL [Accessed: 16.10.2012]. PDF
This issue presents studies from different regions (Bangladesh,
Cameroon, Burkina Faso) that mainly show the non-existence of
faecal sludge management. Additionally, the last paper describes a
new technological solution (LaDePa) for producing hygienically safe
organic fertiliser from sludge from ventilated improved pit toilets
(VIPs).
KONE, D.; STRAUSS, M. (2004): Low-cost Options for Treating
Faecal Sludges (FS) in Developing Countries - Challenges and
Performance. Duebendorf: Water and Sanitation in Developing
Countries (SANDEC), Swiss Federal Institute for Environmental
Science (EAWAG). URL [Accessed: 23.06.2010]. PDF
This article analyses and discusses the performances of low-cost
technology for treating faecal sludges in developing countries. It
shows that where septic tanks are the predominant type of on-site
sanitation installations, septage is the only or predominant type of
faecal sludge generated. It also shows that constructed wetlands,
settling tanks/ponds, or unplanted drying beds might prove
suitable as a pre-treatment.
MONTANGERO, A.; STRAUSS, M. (2004): Faecal Sludge Treatment.
Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG),
Department of Water and Sanitation in Developing Countries
(SANDEC). URL [Accessed: 10.06.2010]. PDF
This document reviews current practices of faecal sludge
management and treatment.
STRAUSS, M.; DRESCHER, S.; ZURBRUEGG, C.; MONTANGERO, A.;
OLUFUNKE, C.; DRECHSEL, P. (2003): Co-composting of Faecal
Sludge and Municipal Organic Waste. Swiss Federal Institute of
Aquatic Science (EAWAG), Department of Water and Sanitation in
Developing Countries (SANDEC) and International Water
Management Institute (IWMI). URL [Accessed: 23.06.2010]. PDF
The document gives an overview on the combined composting of
(faecal) sludges and organic solid waste based on a pilot project in
Kumasi, Ghana. Results of the investigation should help the city’s
waste management department to develop its biosolids
management strategy and enable the project team to develop
guidelines for planners and engineers on the option of co-
composting.
STRAUSS, M.; LARMIE, S.A.; HEINSS, U. (1997): Treatment of
sludges from on-site sanitation — Low-cost options. In: Water
Science and Technology 6, 129-136. URL [Accessed: 23.06.2010].
Published in 1997, this article gives an overview on current
literature-based knowledge regarding faecal sludge treatment
along with results and conclusions from field research.
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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. URL [Accessed:
15.02.2010]. PDF
This compendium gives a systematic overview on different
sanitation systems and technologies and describes a wide range of
available low-cost sanitation technologies.
See document in FRENCH
ANDREOLI, C.V. (Editor); SPERLING, M. von (Editor); FERNANDES,
F. (Editor) (2007): Sludge Treatment and Disposal. London:
International Water Association (IWA) Publishing. URL [Accessed:
01.11.2013]. PDF
Sludge Treatment and Disposal is the sixth volume in the series
Biological Wastewater Treatment. The book covers in a clear and
informative way the sludge characteristics, production, treatment
(thickening, dewatering, stabilisation, pathogens removal) and
disposal (land application for agricultural purposes, sanitary
landfills, landfarming and other methods). Environmental and
public health issues are also fully described.
Case Studies
HEINSS, U.; LARMIE, S.A.; STRAUSS, M. (1994): SOS - Management
of Sludges from On-Site Sanitation. Characteristics of Faecal
Sludges and their Solids-Liquid Separation. Duebendorf and Accra:
Swiss Federal Institute of Aquatic Science (EAWAG). PDF
This document gives an overview on the characteristics of different
sludges as well as monitoring results and recommendations for
design of solid-liquid separation. It is based on a field report.
MOHAMED, A. ; KLINGEL, F.; BRACKEN, P.; WERNER, C. (2009):
Effluent reuse from constructed wetland system Haran Al-Awamied,
Syria. Eschborn: Sustainable Sanitation Alliance (SuSanA) . URL
[Accessed: 26.01.2011]. PDF
In the village of Haran Al-Awamied a gravity sewer system already
existed and waste water was collected for irrigation without any
treatment. GTZ and MHC (Syrian Ministry of Housing and
Construction) initiated a project for a new ecological treatment
plant (settling tank and a vertical flow CW).
Awareness Raising Material
STRAUSS, M.; BARREIRO, W.C.; STEINER, M.; MENSAH, A.;
JEULAND, M.; BOLOMEY, S.; MONTANGERO, A.; KONE, D. (2003):
Urban Excreta Management - Situation, Challenges, and Promising
Solutions. In: IWA Asia-Pacific Regional Conference Bangkok,
Thailand. URL [Accessed: 23.06.2010]. PDF
The objective of this paper is to render planners, decision makers,
and consultants aware that faecal sludge management (FSM)
should form an integral part of the urban development planning
process. For this, three illustrative cases are presented, based on
which an array of measures or tools, as well as
institutional/regulatory, financial/economic, and technical aspects
are discussed.
Training Material
EAWAG/SANDEC (Editor) (2008): Faecal Sludge Management.
Lecture Notes. Duebendorf: Swiss Federal Institute of Aquatic
Science (EAWAG), Department of Water and Sanitation in
Developing Countries (SANDEC). URL [Accessed: 23.05.2012]. PDF
This module pays special attention to the haulage, treatment and
reuse or disposal of faecal sludge. It covers both technical and non-
technical (socio-cultural, economic, political etc.) aspects and
provides practical information on design, financing and planning of
faecal sludge treatment plants.
EAWAG (Editor); SANDEC (Editor) (2008): Fecal Sludge
Management. Presentation. Duebendorf: Swiss Federal Institute of
Aquatic Science (EAWAG), Department of Water and Sanitation in
Developing Countries (SANDEC). URL [Accessed: 18.02.2011].
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KLINGEL, F. ; MONTANGERO, A. ; KONE, M.; STRAUSS, M. (2002):
Fecal Sludge Management in Developing Countries - A Planning
Manual. Duebendorf: Swiss Federal Institute for Environmental
Science (EAWAG). URL [Accessed: 23.06.2010]. PDF
This manual is a first approach to provide guidance on strategic
planning of faecal sludge management. The study took place in the
City of Nam Dinh, in Vietnam. The main principles for strategic
sanitation planning have been adopted from the guide “Strategic
Planning for Municipal Planning” from GHK Research and Training
Ltd.
STRAUSS, M.; MONTANGERO, A. (2002): FS Management – Review
of Practices, Problems and Initiatives. London and Duebendorf:
DFID Project R8056, Capacity Building for Effective Decentralised
Wastewater Management, Swiss Federal Institute of Aquatic
Science (EAWAG), Department of Water and Sanitation in
Developing Countries (SANDEC). URL [Accessed: 24.05.2012]. PDF
A study on management and institutional aspects regarding the
challenges and possible improvements in managing faecal sludge.
Important Weblinkshttp://www.eawag.ch/ [Accessed: 05.05.2010]
http://www.unep.or.jp/ [Accessed: 23.06.2010]
Link to the online version of the “International Source Book On Environmentally Sound
Technologies for Wastewater and Stormwater Management” from the United Nations
Environmental Programme. This section is about drying beds.