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Title: Process development for co-digestion of toxic effluents : development of screening procedures
Authors: Dlamini, Sithembile
Keywords: Sewage--Purification--Anaerobic treatment;Sewage sludge digestion;Factory and trade waste--Purification
Issue Date: 2009
Abstract: The primary objective of this project was to establish a screening protocol which could be used
to access high strength/toxic effluent for toxicity and degradability prior to being disposed in
wastewater treatment works.
The serum bottle method (materials and method section) is simple, makes use of small glass vials
(125 mℓ-volume were used in this research) which do not require any stirring nor feeding device
or other engineered tool: a serum bottle is sealed immediately after all components are poured
inside and thereafter conducted in a batch mode and occasionally shaken to ensure adequate
homogenisation of the components. The only variables which are regularly measured are the
volume of biogas produced and gas composition. The two assays, originally developed by
Owen et al. (1979) to address the toxicity and the biodegradability have been combined in a
single test called AAT, Anaerobic Activity Test, which enables one to assess simultaneously the
inhibitory effect on the methanogenic biomass and the biodegradability of the test material as
well as the ability of the biomass to adapt to the test material and therefore to overcome the
initial inhibition.
The screening protocol is illustrated in Annexure A. The protocol consists of a sequence of
assays which employ the serum bottle methodology. A first step of the procedure is aimed at
rapidly estimating whether the effluent is potentially toxic to the methanogenic biomass and in
what concentration. The second step is a more extensive screening, aimed at precisely
characterising the toxicity of the effluent, the extent of biodegradation that can be achieved, as
well as at establishing whether a potential for adaptation of the biomass exists upon exposure. If
the sample passes the screening stage, the same serum bottle method will be used to conduct a
series of batch co-digestion experiments aimed at evaluating a convenient volumetric ratio
between the test material and the readily biodegradable substrate. Finally, a laboratory-scale codigestion
trial could simulate the full-scale process, thus enabling the selection of appropriate
operating conditions for the start-up of the full-scale implementation.
This the protocol has been used to assess the amenability to be anaerobically (co)digested of four
industrial effluents, i.e. size and distillery effluents which are classified as high strength and
scour and synthetic dye effluents classified as toxic. From the biodegradability and toxicity
assays the following conclusions were drawn. The size and distillery effluent were found to be
degradable at 32 g COD/ℓ and 16 g COD /ℓ concentrations respectively. Concentrations higher
than these stipulated above were found inhibitory. Scour effluent was found to be recalcitrant at
all concentration tested and synthetic dye was 100 % degradable at 0.12 g COD/ℓ and lower and
highly inhibitory at concentration higher than 1.1 g COD/ℓ.
Co-digestion experiment using serum bottle AAT method were undertaken between effluents i.e.
size + distillery, size + scour, distillery + synthetic dye in an attempt to verify whether the
digestion performance benefits from simultaneous presence of the two substrates. The volumetric
ratios between the effluents were 1:1, 1:2, 2:1. The presence of two mixtures in the case of size
and distillery had better methane production compared to individual substrate i.e. size or
distillery separate. The mixture with volumetric flow rate ratio of 2:1 (size: distillery) was
preferable in terms of process performance as it had highest COD removal compared to the other
mixtures /ratios and individual substrates. The mixture of size and scour (2:1) had highest
degradation percentage compared to other ratios but not high enough to qualify as degradable
(less than 50 %). The mixture of distillery and synthetic dye had the same pattern with ratio of
2:1 giving the best COD conversion. The pattern than can be drawn from the degradability of
mixtures is: the degradability of mixtures increase with the increasing amount of the most
biodegradable compound/effluent in the mixture.
Serum bottle results provided the detailed information regarding the safe operating parameters
which should be used during the starting point for the larger scale investigation i.e. lab-scale
investigations. The lab scale investigations were conducted primarily to validate screening and
monitor how the digestion progresses and also to provide data for future project i.e. pilot plant
investigation. Other effluents i.e. scour and synthetic dye and their co-digestion mixture were
excluded from the lab-scale investigations since they were found to be non- biodegradable i.e.
their COD conversion was less the 50 % in the screening protocol. Due to time constrains and
other technical difficulties in the laboratory, the co-digestion of size and distillery mixture trials
we not conducted on the laboratory scale.
Laboratory-scale digestion trials showed that the best organic loading rate for distillery effluent
in terms of reactor performance and stability was 1.0g COD/ℓ with efficiency of about 45 %, and
for size was 2.0g COD/ℓ with an efficiency of 40 %. The efficiencies obtained in both effluents
trials could be greatly improved by acclimation; however these results showed that the digestion
of these effluents on the bigger scale is possible.
Description: Submitted in partial fulfillment of academic requirements for the degree of Masters of Technology: Department of Chemical Engineering, Durban University of Technology, 2009.
Appears in Collections:Theses and dissertations (Engineering and Built Environment)

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