Nitrogen Transformations and Removal Mechanisms in Algal and J

148 Pages·2009·7.02 MB·English

Omar Zimmo
UNESCO-IHE
Institute for Water Education
Nitrogen Transformations and
Removal Mechanisms in Algal and
Duckweed Waste Stabilisation Ponds
Ammonia volatilisatlon Duckweed uptake Denitrification
• WW
1
Organic nitrogen Ammonia nitrogen Nitrate nitrogen
J
Sediment

NITROGEN TRANSFORMATIONS AND REMOVAL MECHANISMS IN
ALGAL AND DUCKWEED WASTE STABILISATION PONDS

Nitrogen Transformations and
Removal Mechanisms in Algal and
Duckweed Waste Stabilisation Ponds
DISSERTATION
Subtnitted in fulfilment of the requirements of
the Academie Board of Wageningen University and the Academie Board
of the International Institute for Infrastmctural, Hydraulic and
Environmental Engineering for the Degree of DOCTOR
to be defended in public
on Tuesday, 25 March 2003 at 15:00 h in Delft, The Netherlands
by
Omar Zimmo
born in Gaza, Palestine
This dissertation has been approved by the promoter and co-promoter:
Prof.dr. H.J. Gijzen, IHE Delft, The Netherlands
Dr.ir. N.P. van der Steen, IHE Delft, The Netherlands
Members of the Awarding Committee:
Prof dr. D.D. Mara, University of Leeds, United Kingdom
Prof dr.ir. A.J.M. Stams, Wageningen University, The Netherlands
Prof dr. P. Denny, IHE Delft, The Netherlands
Dr. R. Al-Sa'ed, Birzeit University, Palestine
Copyright © 2003 Swets & Zeidinger B.V., Lisse, The Netherlands
All rights reserved. No part of this publication or the information contained herein may
be reproduced, stored in a retrieval system, or transmitted in any form or by any means,
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permission from the publishers.
Although all care is taken to ensure the integrity and quality of this publication and the
information herein, no responsibility is assumed by the publishers nor the author for any
damage to property or persons as a result of operation or use of this publication and/or
the information contained herein.
Published by: A.A. Balkema Publishers, a member of Swets & Zeitlinger Publishers
www.balkema.ni and www.szp.swets.nl
ISBN 90 5809 576 2 (Swets & Zeitlinger Publishers)
ISBN 90 5808 811 1 (Wageningen University)
Dedicated to
my Mother,
my Wife, Suha,
children, Basil, lyad and Tala
God bless them all

ABSTRACT
This thesis describes the results of a comparative study of the performance of algae-
based ponds (ABPs) and duckweed-based ponds (DBPs) for wastewater treatment, with
emphasis on nitrogen transformations and removal mechanisms.
Batch experiments simulating algae and duckweed {Lemna gibbd) stabilisation ponds
for domestic wastewater treatment were conducted to quantify the importance of
various nitrogen removal mechanisms under controlled conditions of pH and DO. N-
removal in both systems by different mechanisms are more dependent on the pH
variations than on the oxygen variations. Significantly higher N-removal efficiency in
the duckweed system (26-33%) than in the algae system (14-24%o) was found at lower
pH range of 5 to 7. At high pH values of 7 to9, the increase in N-removal by
sedimentation and volatilisation in the algae system and the decrease in N-uptake by
duckweed in the duckweed system resulted in significantly higher N-removal
efficiency in the algae system (45-60%) than the duckweed system (38-41%).
Further research was carried out in pilot-scale algae-based and duckweed-based
systems that consisting of 4 similar ponds in series for each system, fed with
wastewater with hydraulic retention time of 7 days in each pond.
Tracer experiments showed that the hydraulic characteristics were similar in both ABPs
and DBPs. Actual retention times were observed to be higher than the theoretical
retention times due to the spurious tracer curves resulting in negative dead spaces. This
suggests that the higher density of LiCl solution compared to pond wastewater density,
probably caused LiCl to pass onto the bottom of the pond and slowly dissolved into
pond water where it leached out at a much longer period and thus giving spurious tracer
curves. The hydraulic behaviour of the ponds was neither plug-flow nor completely
mixed, but rather showed a dispersed flow. A tracer experiment in larger scale algae
and duckweed-based ponds in Colombia showed lesser short-circuiting and more plug
flow conditions in the duckweed pond than the algae pond. For larger surface areas the
presence of duckweed cover improved the hydraulic performance of pond. The better
hydraulic behaviour of the pond with duckweed cover may be explained by reduced
wind-induced short circuiting and reduced mixing caused by the absence of algae
biomass acfivities.
Higher BOD and TSS removal efficiencies were achieved in DBPs compared to ABPs.
In both systems, the removal of BOD and TSS did not differ significanfly during the
different seasons of warm and co ld weather. Total-P was more effectively reduced in
DBPs than in ABPs, irrespective of the season due to phosphoms uptake of duckweed
and subsequent removal from the system via harvesting.
Increase in organic loading resulted in an increase in BOD and TSS removal rates in
both ABPs and DBPs. Phosphorous removal was similar during the two experimental
periods in ABPs. DBPs behaved likewise. Removal of FC in ABPs was higher than in
DBPs. FC removal in the ABPs and in the DBPs was significantly higher during low
organic loading period compared to high organic loading period. FC removal in ABPs
during the low and high organic loading periods was 3.8 and 3.4 log units, respectively.
Corresponding values for DBPs were significantly lower at 2.2 and 1.8 log units.
Lower FC removal was found during the cold period in both systems.
During the low organic loading period at warm temperature, nitrogen removal
efficiency was higher in ABPs (80%) than in DBPs (55%) despite the fact that
approximately one third of the influent nitrogen to the DBPs is removed via duckweed
harvesting. Lower N-removal in both systems was obtained during cold season but
similar removal was found during periods of low and high organic loading.
Quantification of N-fluxes in both systems showed that the major fluxes of nitrogen in
the ABPs were sedimentation (33-40%) and denitrification (14-24%). Sedimentation
and denitrification in DBPs were of equal importance except during the warm season
and low organic loading operation, when sedimentation was low. In DBPs, 30-33% and
15% of the total nitrogen was recovered into biomass and removed from the system via
duckweed harvesting during the summer and winter period, respectively. During the
high organic loading period, nitrogen recovered via duckweed was 19%. Ammonia
volatilisation in both treatment systems was found to be a minor N-removal mechanism
responsible for less than 1.1 % of total influent nitrogen. Nitrification and
denitrification occurred in the aerobic water phase and anaerobic sediment,
respectively. Higher DO concentrations in ABPs, especially during the warm season,
favoured higher nitrification in ABPs as compared to DBPs. Predictive models for
nitrogen removal in ABPs and DBPs were proposed. They presented a good reflection
of nitrogen fluxes on overall nitrogen balance under the prevailing experimental
conditions. Validation of the models with reported data from literature gave poor
results for shallower ponds, while better agreement was obtained using data for deeper
ponds. Further elaboration and validation of the model to accommodate pond design
and environmental parameters that dictate pond performance is required.