Concentrated wastewater treatment studies using an anaerobic hybrid reactor
Introduction
Anaerobic filters were first described in 1968 and have been used as an advanced technology for effective treatment of a variety of industrial wastes. Fixed bed anaerobic treatment processes are applicable to wastewaters with low (<1000 mg chemical oxygen demand (COD) l−1) to intermediate (>20 000 mg COD l−1) concentrations. Originally upflow anaerobic filters were 100% packed, but some researchers pointed out that if the amount of support material is decreased, some disadvantages of fully packed anaerobic filters can be overcome [1], [2]. This kind of reactor is called as ‘hybrid’ reactor and it combines a upflow anaerobic sludge blanket/bed (UASB) in the lower part with a filter in the upper part. The hybrid reactor promotes the advantages of the UASB and AF reactors, while minimising their limitations. The performance of hybrid upflow anaerobic filters depends on contact of the wastewater with both the suspended growth in the sludge layer and the attached biofilm in the media matrix.
A fully packed upflow anaerobic filter was compared with a hybrid with 50% or less packing. In the partially packed reactor, increased solids loss was generally experienced as well as reduced efficiency. Some investigator found that the fully packed upflow anaerobic filter gave the highest degree of solids retention and the lowest yield. Some investigator also found that a fully packed upflow filter performed significantly better than a hybrid with only the top 2/3 packed [3].
In this study, treatment of different types of concentrated wastewater was investigated using a laboratory scale anaerobic hybrid model reactor. At the beginning of the study, the model reactor was operated using synthetic wastewater containing molasses, before running with real industrial wastewater. Treatment efficiencies in the model reactor were investigated at different hydraulic retention times (0.5, 1 and 2 days) and organic loading rates (1–10 kg COD m−3 per day) with synthetic wastewater. Then depending on the results obtained in studies carried out with synthetic wastewater, the model reactor was operated using effluent from the Baker's yeast and meat processing industries.
The aim of this work was to investigate the performance of an anaerobic hybrid reactor using different concentrated wastewater with different operational conditions. System performance was evaluated by COD removal efficiency and methane production.
Section snippets
Model reactor
In this study, a standard type laboratory scale hybrid reactor was used. A schematic diagram of the model reactor is given in Fig. 1. This reactor was made of Plexiglas material (25 cm in diameter×115 cm total height×102 cm liquid height) and upper two-third of the reactor was operated as a fixed bed reactor. The support material used in the fixed bed was hose pieces with 3 cm long (specific surface area 225 m2 m−3). The total volume of the reactor was 53.6 l and the volume of liquid was 50 l.
Performance of the reactor
Fig. 2 shows the performance of the hybrid model reactor for synthetic wastewater treatment studies for a period of over 2 years from the start-up (Phase I). Changing the hydraulic retention time and influent COD concentrations, 11 different operational conditions were applied and COD removal efficiencies ranging from 77 to 90% were achieved. The percentage of methane gas of the total biogas was about 58% and the average methane gas yield was 0.32 m3. CH4 gas produced per kg removed COD per day
Conclusions
After obtaining steady-state conditions, organic loading rate was increased from 1 to 10 kg COD m−3 per day and hydraulic retention times were decreased from 2 to 0.5 days, stepwise. About 11 different operational conditions were applied changing these two parameters in a certain program. COD removal efficiencies ranging from 77 to 90% were achieved during the experimental studies. The percentage of methane gas of the total biogas was about 58%. The amount of methane gas produced per kg of COD
Acknowledgements
The authors gratefully acknowledge the Scientific and Technical Research Council of Turkey (TUBITAK), Research Foundation of Dokuz Eylul University, PAKMAYA Baker's Yeast Factory, and TANET Meat Processing Industry.
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