Municipal wastewater characterization : application of denitrification batch tests.
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Date
1999
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Abstract
The biological treatment of wastewater has evolved significantly from simple single sludge systems
practicing organic carbon removal to ones which now include either nitrification/denitrification (N/DN)
and / or phosphorus (P) removal. The inclusion of more biological processes have increased the
complexity of current wastewater systems which has subsequently led to the development of more
complex mathematical models. The operation of plants can be assessed and improved by the use of
mathematical modelling tools which require accurate input data. Thus, knowledge of the wastewater
characteristics is an important step towards the optimum modelling, design and operation of present and
future plants. However, for these tools to be effective, the input data needs to be accurate which is
dependent on the current methods used to determine them.
Wastewater is a complex substrate consisting of compounds of differing biodegradability. Biokinetically,
these compounds have been divided into readily biodegradable (RBCOD), slowly biodegradable
(SBCOD) and unbiodegradable substrate groups. Compounds with intermediate biodegradability i.e.
compounds which fall between the RBCOD and SBCOD groups, have been termed readily hydrolyzable
organic substrates (RHCOD). The organic matter is discussed in terms of chemical oxygen demand
(COD). The readily biodegradable and readily hydrolyzable COD fractions of wastewater can be
determined by respirometric tests such as the oxygen utilization rate (OUR) and nitrate-N utilization rate
(NUR) tests.
The principal aim of this project was to investigate the NUR test as a tool for wastewater characterization
and to study denitrification kinetics in batch reactors. In addition, an experimental readily biodegradable
substrate, acetate, was used to determine the reliability of the NUR tests. Acetate was also used to
ascertain utilization profiles and rates of a typical readily biodegradable substrate during denitrification.
Biodegradable COD characterizations with enhanced biological phosphorus removal (EBPR) sludges
were also investigated to determine the impact of anoxic phosphorus removal on NUR tests. The results
obtained from the numerous NUR tests added to the undestanding of the NUR test.
Samples from 22 wastewater treatment plants were tested, most of which were located in France. Four
South African plants were also tested. Data obtained from the NUR tests were used to calculate the
RBCOD and RHCOD fractions. The SBCOD, however, could not be determined directly from the 6 h
NUR batch tests. The readily biodegradable COD (RBCOD) fractions ranged between 7 and 25 % of the
total COD concentration of raw wastewater, with majority of those results falling within the 10-20 % (of
the total COD) range. The results also showed that the initial rapid rate associated with readily
biodegradable COD utilization was sometimes followed by a short intermediate phase (i.e. short duration,
2 to 3 h). The intermediate fraction was found to range between 5 and 29 % of the total COD
concentration and was classed as a readily hydrolyzable COD component of raw wastewater since the
magnitude of the RHCOD fraction was too small to be classed as slowly biodegradable COD which
comprises approximately 30 to 60 % of the total COD found in raw wastewaters. The variability of the
RHCOD fractions suggests that this fraction is either very variable or that the NUR test does adequately
or accurately characterize it. Another possibility is that the RHCOD (or second biodegradable fraction)
calculated from the NUR test is a component of the RBCOD of the influent wastewater. In this case, the
bacteria may have used some of the RBCOD directly for energy and accumulated or stored the rest as
part of a survival mechanism which allows them to be more competitive under dynamic operating
conditions. Once the readily biodegradable COD becomes limiting, the bacteria will use the accumulated
or stored compounds. This hypothesis is substantiated by tests done with acetate as substrate.
An intermediate phase was also observed when acetate was the sole substrate. Thus, it was possible with
the 3-phase profiles to calculate a second biodegradable fraction. Results suggest that a significant part of
the added acetate (as COD) was stored and the second phase is in fact an 'apparent or residual' phase
brought about by the consumption of the stored or accumulated acetate products. This is suggested in two
ways: (1) the calculation of the yield coefficient is lower and closer to the 0.5 mg/l values, cited in the
literature, when the COD calculated from phases 1 and 2 are considered, and (2) the acetate mass
balances were found to be approximately 100 % when phases 1 and 2 were used to calculate the amount
of acetate utilized under anoxic conditions.
The results obtained with sodium acetate as a readily biodegradable substrate were used to formulate
several conclusions on acetate utilization during denitrification. Firstly, from acetate mass balances it was
found that acetate may be used exclusively for denitrification (100 % acetate was accounted for). In this
case, the sludge contains a significant proportion of denitrifiers and little or no polyphosphate
accumulating organisms. This observation was made only when non-EBPR (enhanced biological
phosphorus removal) sludges were used. Secondly, acetate mass balances which were found to be < 100
% suggest that acetate could be used for denitrification and the production of storage products like
polyhydroxyalkanoates (PHA's). These sludges probably contained a higher proportion of polyphosphate
accumulating organisms which competed for the available acetate in the bulk liquid. This observation
was made for both EBPR and non-EBPR sludges. Thirdly, acetate could be used for denitrification by
denitrifiers and for polyhydroxyalkanoate synthesis by denitrifying polyphosphate accumulating
organisms. The stored PHA's in the denitrifying polyphosphate accumulating organisms are subsequently
utilized during denitrification. This secondary utilization is manifested in the second denitrification phase
and is supported by the observation of phosphorus uptake. These results showed that wastewaters high in
volatile fatty acids (VFA's) were also subject to denitrifying polyphosphate accumulating organism
activity even though the sludge was sampled from non enhanced biological phosphorus removal systems
(non EBPR).
Several of the NOx profiles revealed either 2 or 3 rates due to the control of the substrate to biomass ratio
(S/X: :<_0.1 mgO2 / mgO2). Majority of the samples (i.e. 85%) tested produced initial maximum specific
denitrification rates (k1) between 3 and 6 mgN/gVSS.h. The intermediate denitrification rate (k2) was
found to vary between 2 and 3 mgN/gVSS.h. Denitrification rates (k3) obtained from utilization of
influent and. endogenous slowly biodegradable COD (SBCOD) varied between 1.0 and 1.5 mgN/gVSS.h.
This latter rate is significantly higher than the endogenous denitrification rates cited in the literature. One
of the reasons for these higher rates could be be linked to the the reuse of stored or accumulated products
by the microorganisms.
In addition, a comparative study on RBCOD determination of wastewaters with enhanced biological
phosphorus removal and non-EBPR sludges. It was found that the RBCOD values derived by NUR tests
with EBPR sludge were consistently lower (4 to 5 %) than those with non-EBPR sludge. Thus, the NUR
tests with EBPR sludge resulted in a 4 to 5 % underestimation of the RBCOD fraction of raw
wastewaters. This loss in RBCOD to polyphosphate accumulating organisms appears to be linked to the
influent raw wastewater acetate concentration.
These tests showed that the RBCOD fraction could be adequately characterized using the NUR method.
The accuracy of the tests appears to be compromised when enhanced biological phosphorus removal
sludges are used in the NUR tests. Moreover, it was found that non-EBPR sludges can also consume
some of the acetate that is present in the system for the production and replenishment of storage
compounds. Fortunately, for the wastewaters tested, the acetate component of the RBCOD fraction was
small and therefore, did not significantly affect the results. Mechanisms such as substrate accumulation
and storage may also impact on substrate removal and hence, the determination of the readily
biodegradable COD concentration of municipal wastewaters. Thus, while the results showed that the
NUR is a useful characterization tool for wastewaters, it will continue to be a more tedious
characterization tool than the oxygen utilization rate test, until a suitable nitrate/nitrite electrode is
developed to automate the test.
Description
Thesis (Ph.D.)-University of Natal, Durban, 1999.
Keywords
Sewage--Purification--Nitrogen removal., Sewage--Purification--Phosphate removal., Sewage--Purification--Activated sludge process., Sewage sludge., Theses--Chemical engineering.