Sedimentation/ Thickening Ponds

Sedimentation or thickening ponds are settling ponds that allow sludge to thicken and dewater. The effluent is removed and treated, while the thickened sludge can be further treated in a subsequent technology.

Faecal sludge is not a uniform product and, therefore, its treatment must be specific to the characteristics of the specific sludge. Sludge, which is still rich in organics and has not undergone significant degradation, is difficult to dewater. Conversely, sludge that has undergone significant anaerobic degradation, is more easily dewatered.

In order to be properly dried, fresh sludge rich in organic matter (e.g., latrine or public toilet sludge) must first be stabilized. Allowing the sludge to degrade anaerobically in sedimentation / thickening ponds can do this. The same type of pond can be used to thicken sludge which is already partially stabilized (e.g., originating from Septic Tanks, etc.), although it undergoes less degradation and requires more time to settle. The degradation process may actually hinder the settling of sludge because the gases produced bubble up and re-suspend the solids.

As the sludge settles and digests, the supernatant must be decanted and treated separately. The thickened sludge can then be dried or further composted.

Design Considerations

Two tanks operating in parallel are required; one can be operated, while the other is emptied. To achieve maximum efficiency, loading and resting periods should not exceed 4 to 5 weeks, although much longer cycles are common. When a 4-week loading and 4-week resting cycle is used, total solids (TS) can be increased to 14% (depending on the initial concentration).

Appropriateness

Sedimentation/thickening ponds are appropriate where there is inexpensive, available space located far from homes and businesses; it should be established at the border of the community. The thickened sludge is still infectious, although it is easier to handle and less prone to splashing and spraying.
Trained staff for operation and maintenance is required to ensure proper functioning.
This is a low-cost option that can be installed in most hot and temperate climates. Excessive rain may prevent the sludge from properly settling and thickening. 

Health Aspects/Acceptance

Both the incoming and thickened sludge are pathogenic; therefore, workers should be equipped with proper protection (boots, gloves, and clothing). The thickened sludge is not sanitized and requires further treatment before disposal. Ideally, this technology should be coupled with a subsequent treatment technology if a hygienic product is required.

The ponds may cause a nuisance for nearby residents due to bad odours and the presence of flies. Thus, they should be located sufficiently away from residential areas.

Operation and Maintenance

Maintenance is an important aspect of well-functioning ponds, but it is not intensive. The discharging area must be maintained and kept clean to reduce the potential of disease transmission and nuisance (flies and odours). Solid waste that is discharged along with the sludge must be removed from the screen at the inlet of the ponds.

The thickened sludge must be mechanically removed (with a front end loader or other specialized equipment) after it has sufficiently thickened.

References

Further Readings

  • Cover image of a reference book or miscellany.

    TILLEY, E.; ULRICH, L.; LUETHI, C.; REYMOND, P.; SCHERTENLEIB, R.; ZURBRUEGG, C. (2014): Compendium of Sanitation Systems and Technologies (Arabic). 2nd Revised Edition. Duebendorf, Switzerland: Swiss Federal Institute of Aquatic Science and Technology (Eawag). PDF

    This is the Arabic version of the Compendium of Sanitation Systems and Technologies. The Compendium gives a systematic overview on different sanitation systems and technologies and describes a wide range of available low-cost sanitation technologies.

  • Cover image of a reference book or miscellany.

    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).

  • Cover image of a reference book or miscellany.

    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

    The report sets out to provide guidelines for the preliminary design of faecal sludge treatment schemes comprising solids-liquid separation and stabilisation ponds. The document is based on the results of collaborative field research conducted by the Ghana Water Research Institute and SANDEC on full and pilot-scale faecal sludge (FS) treatment plants located in Accra, Ghana.

  • Cover image of a reference book or miscellany.

    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.

  • Cover image of a reference book or miscellany.

    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.

  • Cover image of a reference book or miscellany.

    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.

Case Studies

Training Material

  • Cover image of a reference book or miscellany.

    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.

  • Cover image of a reference book or miscellany.

    EAWAG (Editor); SANDEC (Editor) (2008): Faecal Sludge Management. Pdf Presentation. Duebendorf: Swiss Federal Institute of Aquatic Science (Eawag), Department of Water and Sanitation in Developing Countries (Sandec). URL [Accessed: 23.05.2012]. PDF

    A presentation about faecal sludge management in developing countries.

  • Cover image of a reference book or miscellany.

    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.

  • Cover image of a reference book or miscellany.

    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.