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  • Improving faecal sludge dewatering efficiency of unplanted drying bed


    Richard Amankwah Kuffour (MSc.)

    A Thesis submitted to the Department of Civil Engineering, Kwame Nkrumah

    University of Science and Technology


    in partial fulfilment of the requirements for the degree



    Faculty of Civil and Geomatic Engineering

    College of Engineering

    March, 2010

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    I, Richard Amankwah Kuffour hereby declare that this submission is my own work

    towards the PhD and that, to the best of my knowledge, it contains no material

    previously published by another person nor material which has been accepted for the

    award of any other degree of the university, except where due acknowledgement has

    been made in the text,

    Richard Amankwah Kuffour ----------------------- -------------------------

    (20034866) Signature Date

    Certified by:

    Prof. Mrs. Esi Awuah -------------------------- -----------------------

    (Principal Supervisor) Signature Date

    Dr F. O. K. Anyemedu -------------------------- -----------------------

    (Second Supervisor) Signature Date

    Certified by:

    Prof. M. Salifu -------------------------- -----------------------

    Head of Department Signature Date

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    I dedicate this work to the Lord Almighty God.

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    To God be the glory for the opportunity and strength given me to successfully go

    through this research.

    My sincerest gratitude goes to my supervisors Professor Mrs. Esi Awuah and Dr. F. O.

    K. Anyemedu, of the Civil Engineering Department of KNUST who encouraged me

    to undertake this project and whole-heartedly offered constructive suggestions and

    criticisms and directed the research patterns of the project.

    To Martin Strauss (formerly of SANDEC), Kone Doulaye, Agnes, Caterina and all

    SANDEC team of EAWAG in Switzerland, I say a big thank you. But for your

    guidance encouragement and sponsorship, this research would not have been


    To Dr. O. O. Cofie, Benard Keraita, Amoah Philip, all of IWMI, not forgetting

    Kwame, Maxwell, Lesley and all IWMI staff, I thank you for offering me free office

    with computer and internet, library together with your advice and guidance.

    To all my friends, Noah Adamptey, Daniel Sarpong, Oworae Alex, Sharon Quarshie

    and others, God richly bless you for your countless contributions, encouragements and

    assistances. To my colleagues at College of Agriculture, Asante – Mampong campus,

    University of Education, Dr. Harrison Dapaah, Vincent, Eben, just to mention a few,

    for your explanations, editing, data analyses, encouragement, prayers and support.

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    Mr. Bruce, Mr. Antwi, Mr. Botchwey and Kingsford all technicians of the Civil

    Engineering Environmental Laboratory for their immense assistances, not forgetting

    Mr. Gilbert Fiadzo of soils laboratory, for your explanations and assistance in the

    analyses of the sand.

    To my brothers, sisters, nephews and nieces, I extend my sincere thanks for your

    prayers, assistances and contributions in diverse ways. Finally, I wish to express my

    heartfelt gratitude to my wife, Joyce, my children, Isaac, Festus, Nancy and Blessing,

    for their cooperation, patience, support and prayers.

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    This thesis examines how to improve on faecal sludge dewatering efficiency of

    unplanted filter beds to produce biosolids and less concentrated filtrate. The

    experiment was conducted on bench scale drying beds constructed at the Kwame

    Nkrumah University of Science and Technology campus. Local sand was graded into

    particle size ranges of 0.1- 0.5 mm, 0.5 - 1.0 mm and 1.0 - 1.7 mm, with uniformity

    coefficients of 2.422, 1.727 and 2.029, respectively. They were represented as filter

    media, FM1, FM2 and FM3 respectively. Faecal sludge, consisting of public toilet

    sludge and septage collected from cesspit emptiers discharging at Dompoase treatment

    ponds in Kumasi, Ghana, were mixed in the ratio of 1:1 by volume, and was

    dewatered using the three different filter media, FM1, FM2 and FM3 to determine

    which one was most efficient. The Solid Loading Rate (SLR) of the faecal sludge was

    varied in the phase two whereby public toilet sludge and septage, mixed in the ratio

    of 1:1, 1:2 and 1:3 by volume representing SLR1, SLR2 and SLR3 respectively. These

    were dewatered on filter medium one (FM1) which was selected in phase one. Six

    cycles of dewatering were run for each of the phases. Percolate volume was measured

    every 24 hours. The total solids (TS) of the faecal sludge used for dewatering varied in

    every cycle so the TS was kept constant at 36.64 g/l as SLR1 and 26.93 g/l as SLR3

    in all the cycles in the phase three to determine the effect of constant TS on the

    dewatering process. Further improvement of the dewatering was investigated by

    mixing sawdust with faecal sludge in phase four. Different percentages of sawdust,

    50%, 100%, 150% and 0% (control) by weight of the TS of the faecal sludge, with TS

    of 26.93 g/l which was selected in phase two, were mixed with the faecal sludge to

    determine the effect of different quantities of sawdust as physical conditioner on the

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    dewatering of faecal sludge. The dried biosolids obtained from the different phases of

    dewatering were analysed for plant nutrients and heavy metals to determine their

    agricultural potential. The dewatering on different filter media, FM1, FM2, and FM3

    showed average dewatering times of 9.8, 9.9 and 9.1 days respectively without

    significant differences (p=0.212). However the percolate quality showed significant

    differences between the different filter media in the removal of TS, TVS, SS, COD,

    DCOD and NH3-N with FM1 having the highest removal efficiency for each

    parameter. Accumulation of organic matter in the top 10 cm of the filter bed indicated

    that FM1 was least likely to clog since it had the least quantity of organic matter in the

    sand. It also produced the largest quantity of organic matter and thus had the potential

    to generate the most biosolids. The faecal sludge of SLR1, SLR2 and SLR3 dewatered

    significantly at different average dewatering times of 7.2, 4.8 and 3.8 days

    respectively. Removal efficiencies at the different solid loading rates, though very

    high for TS, SS, TVS, COD, DCOD, NH3-N, did not show any significant difference.

    Organic matter build up in the top 10 cm of the filter bed was least in SLR3, hence

    least likely to clog the filter bed. Again, SLR3, SLR2 and SLR1 showed the potential

    for annual generation of biosolids at 438, 421 and 379 (kg/m 2 /year) respectively.

    Therefore SLR3 of faecal sludge was recommended for dewatering on the selected

    filter bed.

    When TS of the faecal sludge for dewatering was kept constant and the number of

    cycles was increased to eight, FM1, FM2 and FM3, dewatering FS of SLR1 at

    constant TS of 36.64 g/l improved their dewatering time to 8.9, 8.8 and 8.7 days

    respectively while SLR3 with constant TS of 26.93 g/l, dewatering on FM1 had the

    shortest dewatering time of 4.4 days. SLR3 was significantly most efficient in

    removing TS, SS, TVS, COD and EC. Organic matter accumulation rate in the top

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    10cm of filter bed was least for SLR3 followed by FM1, FM2 and FM3. The

    percentage organic matter of the biosolid were 69, 68, 62 and 59, leading to

    estimation of annual organic matter production of 334, 193, 202 and 177 kg

    TVS/m 2 /year for SLR3, FM1, FM2 and FM3 respectively.

    The sawdust-faecal sludge mixture of 50%, 100%, 150% and 0% TS of faecal sludge

    dewatered at 5.3, 4.9, 3.9 and 5.6 days respectively with 150% being fastest to

    dewater. The 150% sawdust-faecal sludge treatment was most efficient with respect to

    removal of contaminant loads like TVS, SS, COD and NH3-N, but was least in TS and

    EC removal. The 150% sawdust-faecal sludge mixture showed the least organic

    matter accumulation rate in the top 10cm of the filter media, followed by the 100%,

    50% and the 0% treatments. The percentage TVS of the biosolids produced were 70.1,

    77.3, 80.6 and 66.3 for 50%, 100%, 150% and 0% sawdust-faecal sludge mixtures,

    leading to annual organic matter estimation of 359, 567, 987 and 225 (kg

    TVS/m 2 /year) respectively.

    The agricultural potential of the biosolids analysed showed that, dried biosolids from

    the filter beds had percentage carbon (C) ranging between 28% and 43.5% while

    percentage nitrogen (N) in the same samples ranged between 1.82% and 3.53%

    leading to C/N ratio, ranging between 8.7 and 23.9. The percentage phosphorus (P) in

    the same samples ranged between 1.73% and 3.69% while the percentage potassium

    (K) values were within the range of 0.66% and 1.67%. The maximum concentration of


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