Trickling Filters – Solids Contact Process: Pilot Plant Studies
This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.
Report no. UWRAA 01
The trickling filter process for sewage treatment is generally regarded as a high stability, low complexity, cheap operating/maintenance, but largely outdated, process (although it is gaining renewed popularity with the advent of plastic media).The older plants cannot usually meet today’s more stringent effluent licence conditions.
The trickling filter solids contact process (TF/SC) was discovered in the USA to lead to a marked improvement in the quality of trickling filter effluent (at a much lowercost than full plant upgrading). It involves a short period of aeration, of the trickling filter underflow together with sludge recycle, prior to final sedimentation.
At Richmond STD, an old trickling filter plant, a pilot plant was set up to check the performance of the solids contact process. The pilot plant consisted of an aeration tank and clarifier to enable a direct comparison of the TF/SC process with the existing trickling filter plant secondary effluent. The pilot plant was operated with and without sludge return to the aeration stage. It was found that aeration of the trickling filter underflow (no sludge recycle) only led to a significant improvement over the TF plant effluent when aeration tank detention times were greater than 2 hours. However, the settle ability of the solids showed great improvement with aeration, even at detention times well below 2 hours.
For the true TF/SC process (with sludge recycle) results from the trial show that there was a significant improvement over the existing TF plant effluent for low aeration tank detention times (typically less than 1.3 hours). Humus effluent averaged 33 mg/L NFR, while the pilot plant averaged 21 mg/L – a 36% improvement using the solids contact process. Half of the samples analysed showed a tertiary standard of sewage treatment (less than 15 mg/L NFR) was achieved using the tricking filter solids contact process.
The process also showed that it had the ability to nitrify (i.e. convert ammonia to oxidized nitrogen forms), although for substantial removal of nitrogen a denitrification facility needs to be installed.
Despite the frequent changes in operating conditions applied to the pilot plant and presence of filamentous organisms in the return sludge, the TF/SC process consistently produced a better effluent than the existing plant.
The Sewage Treatment Optimisation Model computer program was used to check the capabilities of a low detention time aeration process, and validated the pilot plant results.
Overall, the TF/SC process showed itself to be a reliable, easy to operate, and inexpensive method of upgrading trickling filter plants. In addition, it is recommended that, for new sewage treatment plants, consideration be given to implementing a TF/SC plant to take advantage of the lower capital expenditure (and operating costs) as well as the inherent reliability of the trickling filter itself.