Characterising and Removing Dissolved Organic Nitrogen and Colour from Wastewater Containing Melanoidin

Jason Ian Dwyer (2008). Characterising and Removing Dissolved Organic Nitrogen and Colour from Wastewater Containing Melanoidin PhD Thesis, Advanced Water Management Centre, The University of Queensland.

       
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Author Jason Ian Dwyer
Thesis Title Characterising and Removing Dissolved Organic Nitrogen and Colour from Wastewater Containing Melanoidin
School, Centre or Institute Advanced Water Management Centre
Institution The University of Queensland
Publication date 2008-04
Thesis type PhD Thesis
Supervisor Paul Lant
Subjects 290000 Engineering and Technology
Formatted abstract Melanoidins are negatively charged, highly coloured, nitrogenous, humic organic compounds.
They are refractory to biodegradation, causing elevated effluent colour and dissolved organic
nitrogen (DON) when present in the influent to sewage treatment plants (STP). DON has become
an increasingly important problem, as effluent nitrogen specifications have become tighter, and with
the growing focus on re-using STP effluent.
Several natural organic matter (NOM) identification methods were used to identify melanoidin and
verify its removal by advanced treatment methods. Specifically, molecular weight fractionation
with surrogate measurements of dissolved organic nitrogen, dissolved organic carbon (DOC),
specific ultraviolet absorbance (SUVA) and fluorescence excitation-emission matrices (EEM) were
used. Three melanoidin-based streams were studied:
• Synthetic melanoidin solution;
• STP effluent containing melanoidin as a result of spent molasses in the influent; and
• STP effluent containing melanoidin produced on-site during the thermal hydrolysis of
process biosolids.
No studies have investigated the removal of the organic nitrogen associated with melanoidin, and its
relationship with colour removal, using advanced treatment methods. This was a major focus of this
thesis. In this work, an advanced oxidation process was first investigated as a method to degrade
the DON and colour associated with melanoidin. Ultraviolet / Hydrogen Peroxide (UV/H2O2)
oxidation was capable of removing 99% of the colour associated with melanoidin, while total DOC
and DON removal were only 50% and 25% respectively. Molecular weight fractionation results
indicated that bond cleavage was the cause of colour reduction in a synthetic melanoidin solution.
Reduction in colour in the largest molecular weight fraction (>10 kDa), which contained the
melanoidin, also saw a significant decrease in DON and DOC in that fraction, with an increase in
smaller molecular weight DON and DOC. Fluorescence EEM indicated the reduction in the
structural complexity and aromaticity of the melanoidin molecule during hydroxyl radical
oxidation. Evidence of some mineralisation of melanoidin transpired with an overall reduction in
organic carbon and organic nitrogen and the formation of ammonia.
Aerobic batch bioassay experiments were performed after various periods of chemical oxidation by
the hydroxyl radical. The results showed that increased Advanced Oxidation Process (AOP) time
increased the concentrations of smaller molecular weight organic carbon and nitrogen formed. All
of the small organics formed were biodegradable, while the residual larger organics remained nonbiodegradable.
Total mineralisation of organic nitrogen was almost doubled using a combination of
chemical and biological degradation instead of chemical oxidation only. With sufficient hydroxyl
radical oxidation time to fully degrade the colour associated with a synthetic melanoidin solution,
48% of the DON was mineralised. With subsequent aerobic biodegradation, this increased to 86%
DON removal.
Alum coagulation was also investigated as a method to remove melanoidin from wastewater. Alum
coagulation jar tests showed a significant reduction in colour and DON from wastewater when
melanoidin was present. Fluorescence EEMs showed that the melanoidin humic-like compounds
were more readily removed by alum coagulation than other organics. A shift in humic peak
location indicated a decrease in aromaticity and structural complexity of the residual compounds
after coagulation.
The link between the operating temperature and the colour formed during the thermal hydrolysis of
biosolids is not well established. Work in this thesis identified melanoidin to be responsible for the
colour formed during the thermal hydrolysis of biosolids (waste activated sludge). Reduction of the
operating temperature of a full-scale thermal hydrolysis process from 165°C to 140°C caused a
significant reduction in the melanoidin-associated colour produced. Bio-methane potential
bioassays identified that the corresponding loss in methane production potential was insignificant,
for a significant reduction in colour.
This is the first research that has focused specifically on characterising and removing melanoidin
when present in sewage treatment plant effluent. Although this work is specific to melanoidin, it
provides a contribution to the characterisation and removal of other refractory organic compounds
present in STP effluent with similar properties to melanoidin, specifically, high molecular weight
nitrogenous, cyclic-based coloured and humic-acid like organic compounds.

 
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Created: Fri, 03 Oct 2008, 13:27:56 EST by Catherine Crawford