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

that extensive loss of nitrogen (up to 70%) could be secured. DOC less than 20 mg/L
suggested that the heterotrophic denitrification could be excluded.
The nitrification rate reached 3-4 g NH4-N/m 2 .d at a pH of 7 to 7.3 in the first two of
three RBC compartments. It was said that an increasing partial pressure of oxygen and
increasing ammonium concentration had favoured nitrogen removal over ammonium
oxidation. The reduction of nitrite in the aerobic biofilm layer close to the surface might
have been therefore coupled with ammonium oxidation, and probably took place in the
deeper or temporarily anoxic layer of the biofilm. Henderson, et al. (1997) also found that
RBC could be effective in treating the methanogenic landfill leachate.
Anaerobic Treatment
The most common anaerobic treatment is the methanogenic degradation where the
organic matter is completely degraded to mainly methane and carbon dioxide. Anaerobic
degradation as suggested by Kylefors (1997) follows a sequence where the interaction of
several different microorganisms performing hydrolysis, fermentation, acetogenesis and
methanogenesis is required. Anaerobic processes are generally carried out in attached film
reactors. These reactors are insensitive to variations in loading, can retain biological solids
irrespective of the waste flow and maintain a sufficiently high solids concentration over an
extended period. It has been reported that removal efficiencies in anaerobic filters are
higher than anaerobic digesters maintained at the same hydraulic retention time (Pohland
and Kim, 1999).
The main advantages of anaerobic treatment over aerobic treatment are:
1. The energy requirement is lower since no oxygen is required, thus reducing the
operational cost.
2. Since only 10 to 15 % of organic matter is transformed into biomass:
• Low sludge production making the sludge disposal unproblematic.
• Low nutrient supplement requirement, which is beneficial for leachate
treatment which is nutrient deficient.
• Biogas production (85-90 %) favouring the energy balance.
• Possibility to treat leachate with high organic material concentration without
dilution as required by the aerobic process, thus reducing the space
requirements, the size of the plant and capital cost.
3. Anaerobic microorganisms seldom enter endogenous phase, which is important for
the treatment of leachate with variable volume and strength.
4. Anaerobic sludge is highly mineralized than aerobic sludge, which increases its value
as a fertilizer if toxic metals are removed.
5. Anaerobic sludge tends to settle more easily than aerobic sludge, where addition of
flocculants is required.
The main drawbacks of anaerobic systems are:
1. Working temperature above 30 °C is required for efficient kinetics.
2. Complexity of start-up period and the need for strict control of operating conditions.
3. The apparently lower performance of anaerobic methods in elimination of heavy
metals when compared with aerobic treatment.
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