Research project: Evaluation of a reticulated foam bioreactor

'MERDER' is the trade name for a new type of biological treatment process which at first sight appears to be a type of rotating biological contactor (RBC, also referred to as a biodisc ). MERDER, however, differs from the conventional biodisc in that it employs compressible reticulated foam elements for the biomass support. These biomass filled foam wedges are mounted in a drum structure which is driven in a power efficient manner using a combination of electric and pneumatic drives. The design of the drum is such that the foam wedges can be subjected to a squeezing action as part of their rotational cycle; this maintains an open matrix structure within the reticulated foam and enhances the flow of wastewater and atmospheric gases. The overall effect is that extremely high biomass concentrations can be maintained in an aerobic state and presented in an optimal way to the wastewater under treatment. The mechanism of treatment of the wastewater relies on the action of aerobic heterotrophic bacteria and hence is the same as is found within the activated sludge process, biological filters, RBC’s, biological aerated filters (BAF’s) and all other aerobic biological treatments. What makes MERDER novel is the very high biomass concentrations that can be maintained per unit volume; this is made possible by using a foam biomass support which can have a surface area as great as 1000m2/m3 ; this is coupled with induced mechanical ventilation by squeezing of the foam.

Summary results

1. MERDER was presented as an advanced biological process for the treatment of wastewater. We would confirm that MERDER meets those features that are usually regarded as possessed by advanced systems, in that it is:
   - Compact
   - Operates at a high organic loading rate
   - Appears to be relatively energy efficient (although precise measurements were not taken).
2. It was capable of treating waste from the Mayglothling site to the same degree of purity as the RBCs installed on that site despite the loading to the MERDER being at least 20 times greater than that fed to the RBCs.
3. A reticulated polyether foam with a pore size of 20 PPI gave the best overall performance. Work still remains to be done in optimizing the foam and particularly the degree of ‘squeeze’ best suited to the properties of the foam under loaded biomass conditions
4. The basic design of the foam squeezing system proved to be mechanically reliable.
5. The system played an important role in ensuring that the oxygen transfer capacity of the plant was met. The work showed that adjustment of the rate of squeeze could meet the increasing oxygen demand as loading increased. There appeared to be still spare capacity in the system even at the highest loading rate used.
6. High suspended solids in the wastewater caused many operating difficulties to the feed and flow measurement systems. It is reassuring to note that despite these unusually high levels of solids they did not appear to be detrimental to the foam.
7. MERDER can achieve nitrification at low loadings and produce a final effluent with a low ammonia concentration. At higher loadings it may be necessary to use multistage MERDERS if nitrification is required.
8. The waste being treated during the trial period was in itself difficult to treat by biological means. MERDER performed well under these circumstances but further trials on domestic sewage are recommended.