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The BFBR is, to our knowledge, the first high-rate reactor capable of treating complex wastewater, achieving complete methanisation.

Thiruvananthapuram , Sept. 16

THE Wastewater Technology Programme at the Regional Research Laboratory-Thiruvananthapuram (RRL-T) is developing a `buoyant filter bioreactor' (BFBR) in an attempt to enhance the loading rate and treatment efficiency of complex wastewater in anaerobic reactors.

The BFBR is able to decouple biomass and insoluble chemical oxygen demand (COD) retention time from hydraulic retention time by means of a granular filter bed made of buoyant polystyrene beads, according to Dr Ajit Haridas, who heads the Environment Technology Programme at RRL-T.

The development of modern high-rate anaerobic wastewater treatment reactors, like the Upflow Anaerobic Sludge Blanket (UASB), fixed-film and fluidised bed reactors, have made anaerobic digestion the most competitive treatment technology for high and medium strength biodegradable wastewaters.

However, all modern high-rate anaerobic reactor systems have severely reduced capacity for treating wastewater with insoluble and complex organic substrates, usually termed `complex wastewater'. Such effluents are discharged by several industries including dairies, slaughterhouses, palm oil mills, food and fruit processing plants.

Although of low-strength, municipal sewage is also a complex wastewater because a substantial fraction of its COD is present in insoluble form.

There are several reasons why the current high-rate anaerobic reactor designs are effective only when wastewater COD is present either in soluble or rapidly solubisable form. The primary concept used in high-rate anaerobic reactors is the decoupling of biomass retention time from hydraulic residence time.

Various reactor designs differ by the method by which the hydraulic retention time is decoupled from biomass retention time. It is assumed implicitly in many of the anaerobic reactor designs that the rate-limiting step of the anaerobic mineralisation process is the conversion of volatile fatty acids to methane.

The assumption is valid for many simple soluble organic substrates, whereas for complex effluents, the rate limiting steps are most often hydrolysis and solubisation of insoluble substrates. Therefore, insoluble substrates should be retained in the reactor long enough for hydrolysis and solubisation. In other words, a complex effluent high-rate anaerobic reactor requires the decoupling of insoluble COD retention time from hydraulic retention.

Further, traditional high-rate anaerobic reactors have not explored fully concepts such as substrate to biomass mass-transfer and gas-liquid mass transfer with regard its to the potential for process intensification. Mass-transfer aspects assume greater significance in the treatment of complex wastewater, because the reactor has to facilitate contact of insoluble substrates, with insoluble biocatalysts, while the intermediates and products are in gaseous and liquid form.

The anaerobic treatment of fat and lipid rich complex wastewater in high-rate anaerobic reactors is known to be problematic. Even when COD removal is achieved, complete methanisation is not usually obtained in current commercial state-of-art reactors.

"The BFBR is, to our knowledge, the first high-rate reactor capable of treating complex, lipid rich wastewater, achieving complete methanisation and producing high quality effluent", says Dr Haridas, who presented a paper at the just concluded national conference on biological treatment of wastewater and waste air (BTWWA) at RRL-T.

The paper, titled, `The Buoyant Filter Bioreactor - A high-grade anaerobic reactor for complex wastewater', was co-authored by Mr S. Suresh, Ms K. R. Chitra and Mr V. B. Manilal of the Environment Technology Programme.

Unlike other reactor designs, the BFBR decouples both solids retention time and mixing from hydraulic retention time, and provides more intensive mixing conditions in the reaction zone.

If the reactor is unfed for a period of several days, the solids solubisation activity is lost and scum accumulates on the liquor surface on restart.

It takes two to three weeks for the BFBR to regain steady-state performance i.e., complete conversion activity. Before reaching the steady state, a reduction in the quantity of accumulated scum in the reactor is observed. The scum reduction stage is confirmed by the yield of methane exceeding theoretical yield calculated from the COD loading and removal rate.

Unlike in UASB reactors, granular sludge is formed in the BFBR without either plug-flow conditions or the washout of poor settling biomass. The reactor has capacity to treat effluent containing high fat content at a loading rate of 2.6 g fat COD/l.day.

The fat COD loading rate can be a useful tool in digester sizing for the treatment of particular wastes.

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