Denitrification of high strength landfill leachate using garden refuse compost and pine bark.
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Date
2009
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Abstract
Landfill leachate, a toxic by-product formed through the decomposition of organic
matter, is harmful to the environment and human health. After nitrification, the
concentration of nitrate in discharged leachate may still present a potential threat to
the environment. Further denitrification is required to reduce the high concentrations of
nitrates in the nitrified effluents to below discharge limits. The eThekwini Municipality is
currently nitrifying leachate from the Mariannhill Landfill site in a Sequencing Batch
Reactor plant. After closure of the landfill (expected in 2012) the effluents from the
plant will not comply with discharge limits, requiring an ad-hoc treatment.
Denitrification, the conversion of nitrates to nitrogen gas, occurs in the presence of a
carbon source in an anaerobic environment. Expensive methods are currently
employed worldwide; however these tend not to be a viable solution for developing
countries.
This investigation aims at identifying an efficient, cost effective, feasible alternative to
expensive easily biodegradable carbonaceous materials such as methanol, which
promotes the use of natural organic sources such as pine bark and garden refuse.
These organic substrates contain relatively high amounts of carbon and are readily
available in the major eThekwini landfills.
The suitability of these substrates as carbon sources for denitrification, were assessed
using characterisation tests, small scale batch tests and larger scale columns. The
preliminary stage of the research was to comprehensively characterise the substrates
through conventional testing done on both the solid substrates and their eluates. The
batch tests were conducted at 3 nitrate concentration levels: 100, 500 and 2000 mg/ℓ.
A synthetic nitrate solution was used to simulate the treated Mariannhill Landfill site
leachate. Substrates selected for large scale experiments in columns were, the fresh
pine bark, the fresh Commercial Garden Refuse (CGR) and immature Commercial
Garden Refuse (CGR) compost. Two nitrate concentrations (500 and 2000 mg/ℓ) at
two different flow rates were used for the column campaign. Finally durability tests
were conducted on previously used substrates of pine bark and immature compost to
determine the period for which the substrates could be used as a means for
denitrification before replacement was necessary.
The characterisation tests indicated that the fresh materials had higher carbon to
nitrogen ratios than the composted substrates. The CGR RAW substrate had the
highest carbon to nitrogen ratio of 90.19 and although the ph value of 5.45 falls just
outside the optimum range for denitrification of 6 – 8, it was expected that this would
be the best performing substrate.
All the batch tests showed positive results, with regard to achieving full denitrification
with a 100% removal occurring in 5 of the 6 substrates, at all the different nitrate
concentrations. The only substrate not to achieve full denitrification was the pine bark.
The best performing substrate was the CGR RAW which achieved full denitrification at
the highest nitrate concentration of 2000 mg/ℓ between 9 – 12 days.
The column tests reflected promising results at C˳= 500 mg/ℓ during experiment 1,
with all 3 achieving full denitrification. Once again the CGR RAW substrate columns
reflected the best results. The column at 500 mg/ℓ displayed a HRT of 8.06 days was
required whereas the higher concentration of 2000 mg/ℓ required a HRT of 8.40 days.
During experiment 2, the CGR RAW substrate column at 500 mg/ℓ was the only one to
achieve 100% nitrate removal. A HRT time required for full denitrification is less than
3.54 days.
Further studies need to be done at different flow rates and concentrations to ensure
that the reactor is robust and flexible to deal with the change in quality of the leachates
during the life of the landfill. Lower concentrations need to be investigated to determine
whether the substrates are suitable for all ranges of nitrates and leachates. The use of
a combination of substrates as well as different levels of maturity is also required to
determine the ideal material for their implementation in a full-scale reactor in the future.
Larger scale reactors and different reactor configurations need to be investigated.
Description
M. Sc. Eng. University of KwaZulu-Natal, Durban 2009.
Keywords
Denitrification., Fills (Earthwork), Leachate., Compost., Organic gardening., Theses--Civil engineering., Landfill leachate.