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Environmental Technology

ISSN: 0959-3330 (Print) 1479-487X (Online) Journal homepage: http://www.tandfonline.com/loi/tent20

Enhancing faecal sludge dewaterability and end-use by conditioning with sawdust and charcoaldust

Swaib Semiyaga, Mackay A. E. Okure, Charles B. Niwagaba, Philip M. Nyenje& Frank Kansiime

To cite this article: Swaib Semiyaga, Mackay A. E. Okure, Charles B. Niwagaba, Philip M. Nyenje& Frank Kansiime (2017): Enhancing faecal sludge dewaterability and end-use by conditioning withsawdust and charcoal dust, Environmental Technology, DOI: 10.1080/09593330.2017.1300191

To link to this article: http://dx.doi.org/10.1080/09593330.2017.1300191

Accepted author version posted online: 01Mar 2017.Published online: 16 Mar 2017.

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Enhancing faecal sludge dewaterability and end-use by conditioning withsawdust and charcoal dustSwaib Semiyagaa, Mackay A. E. Okureb, Charles B. Niwagabaa, Philip M. Nyenjea and Frank Kansiimec

aDepartment of Civil and Environmental Engineering, College of Engineering, Design, Art and Technology, Makerere University, Kampala,Uganda; bDepartment of Mechanical Engineering, College of Engineering, Design, Art and Technology, Makerere University, Kampala, Uganda;cDepartment of Environmental Management, College of Agricultural and Environmental Sciences, Makerere University, Kampala, Uganda

ABSTRACTFaecal sludge (FS) treatment in urban slums of low-income countries of sub-Saharan Africa is pooror non-existent. FS contains over 90% water and therefore dewatering it within slums decreasestransport costs, facilitates local treatment and end-use. This study was designed to enhance thedewatering efficiency of FS, using two locally available physical conditioners (sawdust andcharcoal dust), each applied at dosages of 0%, 25%, 50%, 75%, 100% and 125% TS. Theoptimum dosage for both conditioners occurred at 50% and 75% for cake moisture content andcapillary suction time, respectively. The dewatering rate improved by 14.3% and 15.8%, whereasdewatering extent (% cake solids) improved by 22.9% and 35.7%, for sawdust and charcoal dust,respectively. The dewatering in FS conditioned with sawdust and charcoal dust was mainlygoverned by absorption and permeation (porosity), respectively. The FS calorific value improved(from 11.4 MJ kg1) by 42% and 49% with 50% TS dosage of sawdust and charcoal dust,respectively. The FS structure also became porous after dewatering which hastens thesubsequent drying and/or composting processes. Due to comparable performance indewatering, sawdust or charcoal dust, whichever is locally available, is recommended to treat FSin low-income urban slum settlements.

ARTICLE HISTORYReceived 23 November 2016Accepted 22 February 2017

KEYWORDSCalorific value; charcoal dust;dewatering; sawdust; urbanslums

1. Introduction

Sanitation needs of over 80% of the urban population insub-Saharan Africa (SSA) countries are met by use ofsome form of on-site sanitation technology, such asseptic tanks and pit latrines [1,2]. Consequently, pit latrinesare the dominant excreta disposal technologies in urbanslums (densely populated areas in cities, inhabited bythe poor) of low-income SSA countries [3,4]. When thesepits are full, large quantities of semi-solid slurry materialknown as faecal sludge (FS) is generated, often mixedwith non-faecal materials (solid wastes) that are depositedin the pits [2]. The users and/or owners face challenges,among which are the high costs of emptying and sub-sequent transportation of FS to treatment facilitiesoutside of the slums due to high density of housingunits and long haulage distances to treatment facilities[5]. A number of slum dwellers resort to emptying FS inthe living environment, which is not only a high risk topublic health, but also environmental pollution. Suchcosts and risks can be minimised by adoption of decentra-lised treatment of FS within urban slums [6]. However,dewatering (solidliquid separation) forms a crucial part

in decentralised treatment of FS since it contains >90%water [7]. Dewatering reduces solid fraction volumes andsubsequently the costs of transportation and handling [8].

FS from lined pits (cement-mortar sealed containmentpits to prevent liquid loss), used by over 75% of slumpopulation in Kampala, Uganda [4], has a low dewateringextent (low percent dry solids in dewatered cake) [9].Chemical conditioners and plant extracts have beenused to improve the dewatering rate (rate at whichwater filters through the FS sample) [10], but no improve-ment in dewatering extent is reported. Physical condi-tioners such as coal fines, char, sawdust, bagasse, ricehusks, rice bran and wheat dregs have been reportedto improve dewatering extent of sewage sludge. Theyenhance the mechanical strength of sludge through for-mation of a rigid lattice structure to improve per-meability within sludge solids, hence allowing water toeasily flow out of the porous sludge structure [8]. Also,physical conditioners have a low moisture content com-pared to FS, which leads to reduction in cake moisturethrough absorption. They are generally more attractivefrom the economic and environmental points of view,

2017 Informa UK Limited, trading as Taylor & Francis Group

CONTACT Swaib Semiyaga semiyaga@gmail.com Department of Civil and Environmental Engineering, College of Engineering, Design, Art and Tech-nology, Makerere University, P.O. Box 7062, Kampala, Uganda

ENVIRONMENTAL TECHNOLOGY, 2017http://dx.doi.org/10.1080/09593330.2017.1300191


since they are often waste materials from domestic andagricultural activities or cheap process by-products,therefore cost effective when applied in dewateringenhancement. Additionally, some of these physical con-ditioners are carbon-based, hence preferred if the dewa-tered sludge is to be used for energy recovery purposes[8,11]. Diener et al. [12] reported energy recovery from FSas a viable venture that can incentivise FS managementto bear its own cost.

In this study, sawdust and charcoal dust were investi-gated as suitable physical conditioners for improvingdewaterability and also improve energy potential ofthe resulting FS from urban slums. Sawdust and charcoaldust from the Bwaise slum (Kampala) were used becausethey are biodegradable and locally available at low or nocost. Since a large number of slum dwellers depend onwood charcoal as their major cooking fuel, there is pro-duction of high volumes of charcoal dust (a waste fromwood charcoal). The other physical conditioners suchas coffee, palm, ground nuts and rice husks, as well asbagasse were not considered because they are costlyto transport since they are generated in areas withlarge-scale agricultural activities [13]. For example,these conditioners are obtained at long distances inrelation to Kampala slums, where a lot of FS that needslocalised handling to reduce haulage costs is generated.In addition, the current competition for these wastes asfuel sources to generate heat in a number of industriesand agricultural production (e.g. poultry and piggeryfarming; some on a commercial scale) has made themexpensive [12]. Also, application of sawdust and charcoaldust conditioners maintains potential utilisation value ofthe resulting FS cake after dewatering. Sawdust andcharcoal dusts possess calorific values of about 20 and28 MJ kg1, respectively [12], but FS has a slightly lowercalorific value in the range of 1217 MJ kg1 [7,14],which is expected to be improved by sawdust and char-coal dust conditioning. The improvement in energycontent could be a driver for sustained end-use, henceimproved sanitation through management of not onlyFS, but also the sawdust and charcoal dust wastes fromthe surrounding environment of urban slums.

Over 70% of the urban households in a number oflow-income countries of SSA rely on wood charcoal (car-bonised wood product) as the main source of cookingfuel [15]. The population growth in SSA is likely to raisethe demand of wood charcoal, yet about 520% of thecharcoal volume is wasted in the form of charcoal dust,formed either during parking, transportation and/orstorage [16]. Some charcoal dust is currently mixedwith binders to make fuel briquettes, while the restends up in municipal solid waste [17]. Also, sawdust isa globally abundant organic waste from timber sawmills.

In fact, the wood loss in the form of sawdust is estimatedat 1820% of the log volume in some SSA countries suchas Uganda [18]. Part of sawdust is currently used toprovide energy at household and industrial scale [12]as well as bedding layer in rearing of chicken, w


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