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  • ANAEROBIC DIGESTION OF HIGH-STRENGTH ORTOXIC ORGANIC EFFLUENTS

    A Survey of Anaerobic Digesters in the KwaZulu-Natal Region to Assess theirAvailability for the Treatment of High-Strength or Toxic Organic Effluents

    Joanne Sacks

    BSc (Hons)

    Submitted in fulfilment of the academic requirements for the degree of

    Master of Science in Engineering

    in the

    Department of Chemical Engineering,

    University of Natal, Durban

    December 1997

  • Abstract

    The objective of this investigation was to assess the potential for the treatment of high-strength or toxic organiceffluents in available anaerobic digester capacity, in the KwaZulu-Natal province. A strategy was developed forthe simultaneous assessment of available digester capacity and effluent degradability evaluation prior to loadinginto a full-scale digester.

    A number of under-utilised and under-performing anaerobic digesters were identified. Collation of physical andoperating data facilitated the evaluation of the performance efficiency of each digester. Available capacity wasassessed in terms of hydraulic load and organic load. Suggestions for remedial action were made. Industriesproducing high-strength or toxic organic effluents were identified and the effluent compositions detailed.

    A laboratory-scale (serum bottle) test protocol was developed for the evaluation of the anaerobic degradabilityand potential toxicity of a substrate. The batch tests provided information on the volumes and concentrations ofan effluent that could be treated effectively, thereby preventing digester failure due to overloading or theloading of an inhibitory substrate. Material and energy balances provided an indication of the efficiency of thedigestion process within the serum bottles. This method was illustrated by the degradation of a known substrate,glucose.

    A detailed evaluation of the anaerobic digesters at the Umbilo Sewage Purification Works verified theavailability of digestion capacity. The hydraulic load to the entire works was 15 % (v/v) below its designcapacity and the flowrates to the anaerobic digesters were low which indicated available hydraulic capacity. Theorganic load to the anaerobic digesters was low. The digesters were well mixed and heated to 36 1 C,therefore they had the ability to accept an organic load of ca. 3 kg VS/m3.d The digesters were only fed anaverage of 1.12 kg VS/m3.d ; there was available organic capacity. The operation of the digesters was healthyand they could accept a greater load in the form of industrial effluents. The average HRT was calculated atca. 17.29 d. The residence time distribution test showed that there was no dead volume and that the mixing wasefficient as the digester was found to be comparable to an ideal completely stirred tank reactor (CSTR).

    The serum bottle screening test was applied to assess the anaerobic degradability of a textile size effluent.Investigation of the individual components of the size solution identified those that were potentially inhibitory,and the concentrations at which they would inhibit the anaerobic biomass. These included: PVA, which becameinhibitory at concentrations > 30 g/l; Plystran, at concentrations > 10 g/l; acrylic, at concentrations > 4 g/l; andbiocide which became inhibitory at concentrations > 0.5 mg/l and toxic at a concentration of 50 mg/l.Investigation of the synthetic size solution showed that it was degraded by the anaerobic digester biomassalthough the tests suggested that the system could become overloaded at high size concentrations. This verifiedthe importance of the screening tests prior to loading into a digester to prevent digester failure. The results wereapplied for the prediction of treatment in a full-scale digester. The ability of anaerobic microorganisms toacclimate to inhibitory substrates was demonstrated by the enrichment tests. The biomass was acclimated to theinhibitory components of the size solution. The acclimated biomass was able to degrade the substrate at aconcentration that had previously been inhibitory, the lag period was reduced and the degradation rateincreased. This investigation identified and verified the potential for treatment of high-strength or toxic organiceffluents in available anaerobic digester capacity in the KwaZulu-Natal region.

    ii

  • Preface

    I, Joanne Sacks, declare that unless indicated, this dissertation is my own work and that it has not beensubmitted, in whole or in part, for a degree at another University or Institution.

    ............................................................

    Joanne Sacks

    December 1997

    iii

  • Acknowledgements

    I would like to express my deepest thanks to the following people for their help and contribution:

    Prof. Chris Buckley for all of his encouragement, advice and friendship but, most importantly, for teaching me

    to consult the tables!

    Prof. Eric Senior for his continued help and encouragement.

    Prof. Hamanth Kasan for his interest and advice.

    The Water Research Commission for the funding of the project.

    The Foundation for Research Development (FRD) for financial assistance.

    Members of the WRC Steering Committee for their valuable contributions.

    The KwaZulu-Natal Water Research Network for collaboration and the use of equipment during the project.

    Dr Bill Ross for his commitment, inspiration and advice.

    Mr Chris Brouckaert for all his assistance with the RTD study. Dr

    David Stuckey, of Imperial College, London, for his interest.

    Prof. Alberto Rozzi, of Politecnico di Milano, for his input and advice.

    Dhiraj Maharaj for all of his help with the reactors!

    The Workshop staff of the Department of Chemical Engineering for building the reactors and assisting with

    equipment maintenance.

    Local authorities for their assistance in obtaining information on local digesters and effluents.

    The staff at Umbilo Sewage Purification Works for their assistance.

    My fellow postgraduate students for friendship, support and many good memories!

    Connel Bell for all his help, encouragement and support. I couldn't have done it without you!

    My parents and family for their support, encouragement and interest in everything I have done.

    .

    iv

  • Glossary

    Acclimation The adaptation of a microbial community to degrade a previously recalcitrant compound through prior exposure to that compound.

    Adaptation A change in the microbial community that increases the rate of transformation of a test compound as a result of prior exposure to that test compound.

    Agro-industry Industry based on the processing of agricultural products.

    Anaerobe A microorganism capable of growing or metabolising in the absence of free oxygen. These microorganisms may be facultative or obligate; the latter will perish in the

    presence of free oxygen.

    Anaerobic digestion/ biodegradation The microbial degradation of an organic compound in the absence of oxygen. It is effected by anaerobic bacteria which degrade the compound in a step-wise process yielding organic acids, carbon dioxide and hydrogen and,

    ultimately, methane and carbon dioxide.

    Anoxic An environment where oxygen is present in the form of chemical compounds such as nitrate or sulphate.

    Archaebacteria An evolutionary distinct group of prokaryotes, including the methanogenic, extremely halophilic and

    sulphur-dependent bacteria.

    Batch culture A closed culture environment in which conditions are continuously changing according to the metabolic state of the microbial culture.

    Biodegradable A property which allows the microbial decomposition of an organic compound to inorganic molecules.

    Biorefractory A property which renders a compound resistant to biological degradation.

    v

  • Biogas The gas produced, principally methane and carbon dioxide, by the action of anaerobic microorganisms on organic compounds.

    Catabolism The degradation of complex organic molecules into simpler compounds with the release of energy.

    Chemical oxygen demand (COD) A measure of the total amount of organicmaterial in a waste stream.

    Co-disposal The calculated and monitored treatment of industrial and commercial liquid and solid wastes by interaction with biodegradable wastes in a controlled landfill.

    Degrade Break down into simpler substances by bacterial action.

    Dewatering The removal of water from a sludge.

    Eco-efficiency The delivery of competitively priced goods and services that satisfy human needs and quality of life, while progressively reducing ecological impacts and resources intensity throughout the life cycle to a level in line with the Earth's carrying capacity.

    Effluent A stream flowing from a sewage tank or industrial process.

    Facultative anaerobe An organism capable of either aerobic or anaerobic growth.

    Feed schedule Pre-determined schedule which sets out the digester feed programme.

    Grit Heavy mineral matter associated with wastewater e.g.. sand.

    Hazardous waste An inorganic or organic element or compound that, because of its toxicological, phy

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