Investigation of the Performance of an Anaerobic Membrane Bioreactor in the Treatment of Mixed Municipal Sludge Under Ambient, Mesophilic and Thermophilic Operating Conditions
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Date
2016-02-18
Authors
Pileggi, Vincenzo
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Anaerobic membrane bioreactors (AnMBRs) may provide a sustainable treatment technology
for the digestion of mixed municipal sludge based on their ability to achieve elevated
volatile solids (VS) conversion and a net positive energy balance. However, AnMBRs may
have throughput limitations particularly when operated at high solids inventories due to
membrane fouling. This study characterized the anaerobic digestion bioprocess and the
membrane performance under various operating conditions and identified the foulant mode
using the classical ‘Hermia-Field’ blocking models while operating the system under low
crossflow and low transmembrane pressures. Using the in-vitro yeast estrogen screen (YES)
bioassay the sludge and permeate quality was assessed for its estrogenic potential. The
permeate was analysed for a group of environmentally relevant trace organic compounds.
The study used a side-stream pilot scale AnMBR with two parallel negative tubular membranes
and a flow through anaerobic digester (AD) as a control. Each reactor was fed in
parallel from a common mixed sludge source. The tubular ultrafiltration membranes were
polyvinylidene difluoride based with a nominal pore size of 20 nm, operated at a crossflow
of 1–1.2 m/s and a transmembrane pressure of 34–54 kPa. Four operating conditions
that included different SRT:HRT ratios under ambient (25 C), mesophilic (35 C) and
thermophilic (55 C) temperatures were investigated.
The main AnMBR advantage over the AD is the ability to decouple and independently
control the system SRT and HRT resulting in increased throughput, at lower HRT, while
maintaining a low food to microorganisms ratio by increasing the SRT. This operational
strategy was used and under mesophilic conditions, the AnMBR showed a 54 and 64 %
volatile solids conversion at SRT:HRT of 30:15 and 21:7 days, with loading rates of 2.1 ±
0.4 and 3.7 ± 0.9 kg COD/m3 · d, respectively. Under ambient and thermophilic operating
conditions, the SRT/HRT ratio was adjusted considering the system kinetics and the
AnMBR showed a 49 and 55 % volatile solids conversion at SRT/HRT of 40:8 and 22:7
days, with loading rates of 3.5 ± 0.4 and 3.4 ± 0.8 kg COD/m3 · d, respectively. Under all
the operating conditions the AnMBR was operated at more than double the loading rate
and showed an improvement of 13–30 % increase in volatile solids conversion, compared
to the AD. A comparison of the energy balance between the AnMBR and AD showed
a net positive energy balance for the AnMBR when operated at mesophilic and ambient
temperatures but not during thermophilic operation. The AD proved sustainable under
ambient operation only.
The membrane performance showed a median sustainable flux of 6–7 ± 2 LMH which was
maintained through inter-permeation cycle rest and clean-in-place (CIP) strategy. The
frequency of the CIP increased with an increase in SRT, TS and during ambient operation
due to an increase in viscosity of the mixed liquor. The dominant fouling mode was found
to be cake fouling under all operating conditions and primarily of a reversible type. The
permeability decline was assessed and found to be affected primarily by TS, SRT/HRT
ratio and extracellular polymeric substances (EPS) protein to polysaccharide ratio.
The estrogenicity of the feed sludge was found to persist in anaerobically treated mixed
municipal sludges and the AnMBR permeate. This was corroborated by the permeate
chemical analysis which found a significant contribution from a small number of estrogens.
Description
Keywords
sludge, treatment, anaerobic digestion, AnMBR, mesophilic, ambient, thermophilic, estrogenicity