Anaerobic co-digestion of municipal primary sludge and whey : a dissertation submitted in partial fulfilment of the requirements for the Masters degree in Environmental Engineering at Massey University, Palmerston North, New Zealand
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2010
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Massey University
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Abstract
The aim of this research was to investigate the feasibility of co-digestion of municipal
primary sludge and whey by anaerobic CSTR (Continuous Stirred Tank Reactor), as well
as the factors that affect the performance of the co-digestion reactors.
Before studying the co-digestion process, a semi-continuous whey digestion experiment
was conducted to analyze the feasibility of anaerobic digestion of whey along with pH
control. The results obtained from the study indicated that supplement of nutrients, trace
elements as well as heavy metals was necessary to maintain the anaerobic whey digestion
system.
To investigate the co-digestion of primary sludge and whey process, the effects of pH,
OLR (Organic Loading Rate), HRT (Hydraulic retention time) as well as the COD
(Chemical Oxygen Demand) loading ratio of primary sludge to whey on the performance
of the reactors were studied. The results of the co-digestion experiments demonstrated
that it was feasible to co-digest primary sludge and whey without nutrient, trace element
and heavy metal supplement. The TCOD (Total Chemical Oxygen Demand) removal
efficiency and the biogas production of the co-digestion system increased with the
increase of OLR. At same OLR, digestion of the mixture of primary sludge and whey
with higher whey content achieved higher biogas production and TCOD removal
efficiency. The anaerobic co-digestion of primary sludge and whey process performed
successfully at OLR of 5.8 ± 0.1g COD/l.d without pH control when the COD loading
ratio of primary sludge to whey was approximately 70:30, due to the fact that the primary
sludge may serve as buffering reagent. By adding sodium bicarbonate (NaHCO3) to
maintain the pH at 6.9 ± 0.1, the OLR of the co-digestion reactor could reach 8.1 ± 0.1 g
COD/l.d at HRT of 20 days. Moreover, by co-digestion of primary sludge and whey
solution, the reactor could be operated successfully at HRT of 10 days and at OLR of 7.6
± 0.1 g COD/l.d with COD loading ratio of primary sludge to whey of 53 : 47. The biogas
production (3.2 ± 0.1 l/d) was 1.5 l/d higher than digestion of the same amount of primary
sludge alone (1.7 ± 0.1 l/d).
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Wastewater treatment