STUDY ON THE EFFECTS OF CHELATING AGENTS ON HYDRAULIC RETENTION TIME, METHANE AND HYDROGEN SULPHIDE GAS YIELDS FROM SOLID WASTES.
STUDY ON THE EFFECTS OF CHELATING AGENTS ON HYDRAULIC RETENTION TIME, METHANE AND HYDROGEN SULPHIDE GAS YIELDS FROM SOLID WASTES.
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Date
2014-10
Authors
CALVIN, NWOKEM NSIDIBEABASI
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Abstract
This research work focused on addition of chelating ligands in order to enhance the
overall digestion process; thereby improving methane gas yield, reducing HRT and
decreasing concentration of H2S produced within digester systems. In view of this,
several experiments and analyses such as metal determination, VFA, and CH4 gas yield
were carried out which involved the use of the Atomic Absorption Spectrophotometer
(AAS), Biogas 5000 Analyser, and other analytical tools. The results from these
analyses showed that, at mesophilic temperature range of 29 – 31oC, four of the
different substrates, GU, BL, CH and CD, recorded maximum CH4 gas yield of 8.30 %,
33.30 %, 84.80 % and 67.60 % respectively at an optimum weight of 150 g. The only
exception observed was with CM substrate which recorded a CH4 gas yield of 75.40 %
at an optimum weight of 250 g. On addition of chelating ligands, EDDA, NTA, EDTA,
DTPA, MESYL, PORPH and PENI to the best three performing substrate digesters
namely, CM, CD and CH, it was observed that depending on the chelating ligand added
and the substrate, an increase in the CH4 yield was recorded which ranged from 1-15 %.
This improvement in CH4 gas yield was due to the increased bioavailability of essential
nutrients on addition of chelating ligands which invariably promoted the growth and
stability of the CH4 producing bacteria; thereby improving CH4 production. Also, the
addition of these chelating ligands to the CM CD and CH digester systems reduced the
HRT, from 50 days to 25 days, 52 days to 30 days and 57 days to 32 days in the CM CD
and CH digester systems respectively. The decrease in HRT was due to the metal
chelate catalysis of the hydrolysis stage of the anaerobic digestion process where the
metal chelate formation served as the driving force in the solvolysis process. The effect
of increasing chelating ligand concentration on production of H2S was also studied.
Also, the results showed a general decrease in H2S cconcentration from 80 % to 30 % on increasing the chelating ligand concentration within the best three performing substrate
digesters namely, CM, CD and CH, from 10 μM to 100 μM. This reduction was due to
the redox reaction occurring between the various metal chelates and H2S, leading to the
formation of elemental Sulphur with the release of H+ ions; thus reducing the
concentration of H2S within the digester systems. Generally, addition of ligands to the
substrate digesters increased production of CH4, reduced HRT and decreased H2S
production greatly.
Description
A DISSERTATION WRITTEN IN THE DEPARTMENT OF CHEMISTRY AND
SUBMITTED TO THE SCHOOL OF POSTGRADUATE STUDIES, AHMADU
BELLO UNIVERSITY, ZARIA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD
OF DOCTOR OF PHILOSOPHY DEGREE IN ANALYTICAL CHEMISTRY
OF
AHMADU BELLO UNIVERSITY, ZARIA
NIGERIA.
Keywords
EFFECTS, CHELATING AGENTS, HYDRAULIC RETENTION, METHANE, HYDROGEN SULPHIDE GAS, SOLID WASTES