Thesis - faculty.ait.ac.th - Asian Institute of Technology

nitrification-denitrification process, an external carbon source in the form of leachate,
methanol etc. could be necessary. An external carbon source could further increase the
operational costs. Chemical treatment by coagulation, flocculation and precipitation
eliminates the increased chemical costs making this option realistic for ammonia removal.
Thus, ammonia stripping seems to be the most viable option. While ammonia stripping can
be conducted in packed towers with efficiencies up to 95 %, the intention of this study is to
merely reduce leachate with total nitrogen content of 1,800-2,000 mg/L to a level below
toxicity for further biological treatment, for which a conventional ammonia stripping
would be sufficient. Thus, by maintaining an optimal operating condition by controlling,
air flowrate and pH, an ideal condition based on ammonia removal, ammonia toxicity and
mixing power efficiency could be obtained using a conventional stirred tank.
One of the main disadvantages of ammonia stripping is the high cost associated with
pH adjusters. The choice of pH adjuster is also crucial in design and rendering the process
cost effective, a reduction in the amount of adjuster can be brought about by pre-aerating
the leachate. The synthetic leachate used in this study has an average pH of 8.5 ±0.5. Since,
this is already in the alkaline range; the amount of buffer added to raise pH for ammonia
stripping is not significant.
2.13.3 Need for Membrane Bioreactors
If the pre-treatment of ammonia stripping fails, this would lead to shock loading in
the biological system, making it difficult for the floc to settle down. This problem can be
solved by the adoption of a membrane process to replace the clarifier in a normal activated
sludge process since the membranes can retain total solids until the sludge recovers from
shock loading of ammonia.
Purification of leachate by membrane processes aids in preventing further
contamination of groundwater resources and surface water. However, in selecting a
treatment option, or a combination of treatment operations, the economic feasibility and
affordability of the technology should also be considered. In this regard, membrane
filtration has proven to be a justifiable and economic solution in most cases, even when the
overall costs for the purification are compared with other approaches for leachate treatment
(Peters, 1997).
By coupling of a membrane with the activated sludge reactor, a membrane bioreactor
emerges as a logical treatment option. The reduced operational costs associated with
immersed membrane bioreactors proves advantageous in its application and therefore
preferred in the present study.
The use of a MBR allows the HRT to be reduced from 1 to 10 days (Qasim and
Chiang, 1994) to less than 24 h. This reduction is drastic and viable in terms of operation
costs and effectiveness. Reduction in SRT from conventional activated sludge SRT of 15
to 60 d has the advantage of reducing air requirements in the MBR. Maintaining a lower
MLSS is advantageous since lower the sludge produced, the greater is the effectiveness of
aeration. This approach effectively reduces the aeration requirements and a smaller SRT
and HRT reduces the required reactor volume and thus the capital cost.
53