solution finder

27 April 2018

Mechanical Dewatering

Author/Compiled by
Beat Stauffer (seecon international gmbh)
Executive Summary

Mechanical dewatering is normally associated with large wastewater treatment plants and is used to separate sludge (residual sludge from wastewater treatment plants or faecal sludge from on-site sanitation) into a liquid and a solid part. The principal methods are belt filter presses, centrifuges and chamber filter presses. These techniques are usually sophisticated and rarely cost-efficient for smaller systems to be implemented on community level. The process does not treat the sludge, it only separates solid from liquid parts. Both solid and liquid parts still contain pathogens and pollutants.

Advantages
Little operator skill or attention is required during the (automated) process
High operational capacity
These processes can be completely automated
Reduces volume of sludge and saves storage and transport costs
Disadvantages
This is an expensive high-tech solution for dewatering sludge and only used in large scale treatment plants
Needs expert design
Operation and maintenance needs qualified workers
Constant power supply needed
Both dewatered sludge and effluents are still infectious, and further treatment is necessary
In Out

Faecal Sludge (from wastewater treatment plants or on-site sanitation systems)

Blackwater, Organic Solid Waste, Compost/Biosolids

Factsheet Block Title
Introduction
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Dewatering removes water from sludge (residual sludge from wastewater treatment plants such as activated sludge systems, or faecal sludge from on-site sanitation such as septic tanks or pit latrines) making it easier to handle and less expensive to transport, to compost (see also small or large scale composting), to incinerate (see also small or large scale incineration) or to dispose of in a landfill. Dewatering can be done either with the help of presses or centrifuges (mechanical dewatering), filtration (e.g. in filter bags) or physical (and biological) processes in drying beds. Mechanical dewatering and filtration often require the use of chemicals (coagulants and flocculants) to enhance the process.

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Treatment Process and Basic Design Principles
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Belt Filter Press

 A belt press and the produced dewatered sludge cake. Source: JOHNSONSCREENS (2011)

A belt press and the produced dewatered sludge cake. Source: JOHNSONSCREENS (2011) 

Modern belt presses are based on a combination of chemical conditioning, gravity drainage and mechanical pressure in a continuous feed system to dewater sludge. The sludge is squeezed between tensioned serpentine belts and a series of decreasing diameter rollers (to increase the pressure) to remove moisture and create a dewatered sludge cake (JOHNSONSCREENS 2011).


Chamber Filter Press

 A chamber filter press for mechanical dewatering of sludge. Source: MICRONICS (n.y.)

A chamber filter press for mechanical dewatering of sludge. Source: MICRONICS (n.y.) 

Chamber filter presses use filters to separate the liquid (filtrate) from the sludge and collect the solids (filter cake) in chambers. A number of such chambers for the collection of the solids are arranged in a plate pack. The filter cloth is supported by the plates of the plate pack. The sludge (optionally mixed with coagulant) is pumped into the chambers and against the filter media. A moving plate (the tail plate) clamps the filter plate pack. The filtrate passes through the filter cloth and exits through the filtrate ports of the filter plate. The filter cake collects on the surface of the filter medium within the chambers until the chamber is full. The cake may be washed or blown dry if necessary. There are manual, semi-automatic and automatic chamber filter presses available.


Centrifuge

Adapted from EPA (2000)

 The schematic design of a centrifuge thickening and dewatering system. Source: EPA (2000)

The schematic design of a centrifuge thickening and dewatering system. Source: EPA (2000) 

Centrifuges operate as continuous feed units, which remove solids by a scroll conveyor and discharge liquid over the weir. The bowl is conical-shaped which helps lift solids out of the liquid allowing them to dry on an inclined surface before being discharged. The picture shows a typical centrifuge thickening and dewatering system.


Factsheet Block Title
Cost Considerations
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These systems are mostly high-tech and require expert design, which increases costs. As the techniques are usually sophisticated they are rarely cost-efficient for smaller systems to be implemented on community level (HEEB et al. 2008). The most cost effective method of sludge dewatering depends on many factors including plant size, the cost of further processing, end-use (e.g. if the thermal heat can be recovered by incinerating the dried sludge), disposal, odour control requirements, and space limitations (EPA 2000).

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Operation and Maintenance
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These technical systems require a high degree of operator supervision, and operator training. The mechanical systems such as filters, belts, tensioning systems or bearings have to be maintained correctly to guarantee proper functioning. The performance of primary treatment systems prior to mechanical stage is also a critical determinant of performance. Primary treatment commonly includes the removal of gross solids, and slurry thickening. Routine use of flocculants is essential, normally selected at installation using advice and the results of on-site testing from polymer chemical suppliers (FSA 2002).

The liquid part needs to be treated: it is either circulated back into the wastewater treatment plant from where the sludge comes from (e.g. activated sludge or treated in vertical flow or horizontal flow constructed wetlands). The solid part has to be burnt in a small or large scale incineration plant or discharged in a landfill; it may also be a composted (see also small or large scale composting facility).

Factsheet Block Title
Health Aspects
Factsheet Block Body

This process does not serve the treatment of wastewater; it only separates solids from liquids. Both solid and liquid parts still contain pathogens and pollutants (HEEB et al. 2008) and have to be treated (see Operation and Maintenance).

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At a Glance
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Working Principle

Sludge is separated in a solid and a liquid part with the help of a physical process such as filtration, squeezing, centrifugal separation or compaction.

Capacity/Adequacy

Usually applied in large scale and sophisticated wastewater treatment plants. Not used on neighbourhood/ community level.

Performance

High

Costs

High

Self-help Compatibility

Low (rather high-tech).

O&M

Mechanical parts need periodical inspection.

Reliability

Technique does not work in case of power failure.

Main strength

Reduces volume of sludge and saves storage and transport costs.

Main weakness

This is an expensive high-tech solution for dewatering sludge.

Applicability

Mechanical dewatering is mostly applied for the treatment of residual sludge in large-scale centralised wastewater treatment plants. Dewatering of sludge reduces transport loads and sludge with low moisture content is easier to handle. It is also required prior to landfilling to reduce leachate production, before composting or incineration of dewatered sludge.

Library References

Centrifuge Thickening and Dewatering

This document describes the centrifugal thickening and dewatering process. It is a high-speed process that uses the force from rapid rotation of a cylindrical bowl to separate wastewater solids from liquid.

EPA (2000): Centrifuge Thickening and Dewatering. Washington D.C.: United States Environmental Protection Agency (EPA) URL [Accessed: 24.11.2011]

Ecosan Curriculum 2.3

HEEB, J. JENSSEN, P. GNANAKAN CONRADIN, K. (2008): Ecosan Curriculum 2.3. Switzerland, India and Norway: seecon international, International Ecological Engineering Society, Norwegian University of Life Sciences, ACTS Bangalore URL [Accessed: 17.05.2012]

Belt Press

This flyer describes the most important aspects and provides pictures of the Johnson belt filter press systems.

JOHNSONSCREENS (2011): Belt Press. New Brighton: Johnson Screen Inc. URL [Accessed: 24.11.2011]
Further Readings

Centrifuge Thickening and Dewatering

This document describes the centrifugal thickening and dewatering process. It is a high-speed process that uses the force from rapid rotation of a cylindrical bowl to separate wastewater solids from liquid.

EPA (2000): Centrifuge Thickening and Dewatering. Washington D.C.: United States Environmental Protection Agency (EPA) URL [Accessed: 24.11.2011]

Faecal Sludge Management.

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).

ECOSAN CLUB (2012): Faecal Sludge Management.. (= Sustainable Sanitation Practice , 13 ). Vienna: Ecosan Club URL [Accessed: 16.10.2012]

Ecosan Curriculum 2.3

HEEB, J. JENSSEN, P. GNANAKAN CONRADIN, K. (2008): Ecosan Curriculum 2.3. Switzerland, India and Norway: seecon international, International Ecological Engineering Society, Norwegian University of Life Sciences, ACTS Bangalore URL [Accessed: 17.05.2012]

Belt Press

This flyer describes the most important aspects and provides pictures of the Johnson belt filter press systems.

JOHNSONSCREENS (2011): Belt Press. New Brighton: Johnson Screen Inc. URL [Accessed: 24.11.2011]

Filter Presses

Read more about chamber filter presses and their applications, benefits and options in this paper.

PARKSON (2008): Filter Presses. Fort Lauderdale: Parkson Corporation URL [Accessed: 21.05.2012]

A Directory of Environmentally Sound Technologies for the Integrated Management of Solid, Liquid and Hazardous Waste for SIDS in the Caribbean Region

This directory is part of UNEP collaboration with SIDS on the implementation of the Waste Management chapter of the Barbados Programme of Action. It focuses primarily on proven sound environmental technologies for solid, liquid and hazardous waste management plus those currently successfully being used in SIDS within the Caribbean Region.

UNEP (2004): A Directory of Environmentally Sound Technologies for the Integrated Management of Solid, Liquid and Hazardous Waste for SIDS in the Caribbean Region. Nairobi: United Nations Environment Programme Global Programme of Action (UNEP-GPA)) and Caribbean Environmental Health Institute (CEHI)

Sludge Treatment Technologies

This document describes several steps of sludge treatment technologies such as sludge stabilisation, dewatering and incineration.

VIGNESWARAN, S. KANDASAMY, J. (n.y): Sludge Treatment Technologies. Paris: UNESCO-EOLSS (Encyclopedia Of Life Support Systems) URL [Accessed: 24.11.2011]

Full-Chain Sanitation Services That Last

This paper sets out a framework for the delivery of non-sewered sanitation services that last, are accessible to all and are at scale. The framework is based on IRC International Water and Sanitation’s (IRC) experience and lessons learnt from its engagement in non-sewered sanitation service at scale.

VERHAGEN, J. CARRASCO, M. (2013): Full-Chain Sanitation Services That Last. Non-Sewered Sanitation Services. The Hague: International Water and Sanitation Center (IRC) URL [Accessed: 07.08.2013]

Sludge Treatment and Disposal

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.

ANDREOLI, C.V. ; SPERLING, M. von ; FERNANDES, F. (2007): Sludge Treatment and Disposal. (= Biological Wastewater Treatment Series , 6 ). London: International Water Association (IWA) Publishing URL [Accessed: 01.11.2013]
Case Studies

Impact of Sludge Conditioning on Mechanical Dewatering and Convective Drying

Management of sludge produced within wastewater treatment plants has become a key issue. After thickening, the removal of remaining water using mechanical dewatering and/or thermal drying is essential before any type of valorisation. Polymers are usually employed in the conditioning step in order to promote particle aggregation, making the dewatering easier. In this work the impact of the polymer dosage and nature on dewatering and convective drying of sludge is studied.

BAZZAOUI, R. FRAIKIN, L. GROSLAMBERT, S. SALMON, T. CRINE, M. LEONARD, A. (2011): Impact of Sludge Conditioning on Mechanical Dewatering and Convective Drying. Palma: European Drying Conference – EuroDrying 2011 URL [Accessed: 24.11.2011]

Opportunities in Fecal Sludge Management for Cities in Developing Countries: Experiences from the Philippines

In July 2012, a team from RTI International deployed to the Philippines to evaluate four FSM programs with the goal of reporting on best practices and lessons learned. The four cases—Dumaguete City, San Fernando City, Maynilad Water for the west zone of metro Manila, and Manila Water from the east zone of metro Manila—were chosen to highlight their different approaches to implementing FSM.

ROBBINS, D. STRANDE, L. DOCZI, J. (2012): Opportunities in Fecal Sludge Management for Cities in Developing Countries: Experiences from the Philippines. North Carolina: RTI International URL [Accessed: 15.01.2013]

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