Technologies for pretreatment of waste activated sludges (WAS) prior to digestion are of
increasing interest to wastewater treatment utilities because of their promise for improving
sludge digestibility and reducing the mass of biosolids remaining after digestion. While there
has been considerable study of pretreatment processes, a common approach to describing the
impact of pretreatments on sludge biodegradability has not been developed. The overall
objective of this study was to develop protocols that can be employed to characterize the impact
of pretreatment processes on WAS digestion.
Sonication and ozonation were employed as models of physical and chemical
pretreatment technologies respectively. A range of physical, chemical and biological responses
were evaluated to assess the impact of pretreatment on WAS properties as well as digestibility.
WAS samples that were generated over a range of solids residence times (SRTs) under
controlled operating conditions were employed to facilitate an assessment of the interaction
between pretreatment and WAS properties on digestibility.
The VS, COD and soluble TKN responses indicated that a significant fraction of the
WAS solids were solublized by sonication and ozonation, however, it appeared that the types of
materials which were solublized was affected by the SRT at which the WAS was generated and
the level of pretreatment. The results indicated that the impact of pretreatment on
biodegradability of WAS was not described by solublization values exclusively without
considering the SRT of the sludge and the level and type of pretreatment. A higher level of
proteinaceous materials was preferentially solublized as the result of pretreatment. Respirometry
revealed that both sonication and ozonation substantially reduced the viable heterotrophs in the
sludge and modestly increased the readily biodegradable fraction of COD. The ultimate yields of
CH4 and NH4 in BMP tests and VFAs in BAP tests revealed that pretreatment marginally
increased the ultimate digestibility of the sludges. Only a high dose of ozonation substantially
increased the digestibility of the 15 day SRT sludge. However, both sonication and ozonation
substantially increased the rate of hydrolysis which is typically the rate limiting process in WAS
digestion.
The BMP test was not a useful test to evaluate the rate of methane generation due to
inhibition of methanogens in the early days of BMP test for pretreated sludges. The comparison
between VFA and ammonia responses in day 10 of BAP test and ultimate values of these
responses after 60 days in BMP test revealed linear relationships between these responses.
According to these relationships, a set of models were introduced in this study. The models can
be employed to predict the ultimate methane and ammonia generation using soluble COD, VFA
or ammonia responses in day 10 of BAP tests. The BAP test was determined to be a shorter test
(10 days) than the BMP (55 to 60 days) test and could provide information on the rates of
hydrolysis and acidification/ammonification processes. Characterization of biodegradable and
non-biodegradable material in WAS samples was conducted using a simplified ADM1 model.
The characterization also revealed that proteins are a substantial fraction of biodegradable
materials. The estimated ammonia, VFA and methane values from the stoichiometric model
were similar to the corresponding values from the experiments. This supported the validity of the
simplified model for all sludges employed in this study.
| Identifer | oai:union.ndltd.org:WATERLOO/oai:uwspace.uwaterloo.ca:10012/5023 |
| Date | January 2010 |
| Creators | Kianmehr, Peiman |
| Source Sets | University of Waterloo Electronic Theses Repository |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
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