Trickling Filter – SolidsContact Process

Pilot Plant Study

Report No WSAA 1

August 1989




The tricklingfilter 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for sewage treatment is generally regarded as a high stability,low complexity, cheap operating/maintenance, but largely outdated, process(although it is gaining renewed popularity with the advent of plastic media).The older plants cannot usually meet today’s more stringent effluent licenceconditions.


The tricklingfilter solids contact process (TF/SC) was discovered in the USA to lead to amarked improvement in the quality of trickling filter effluent (at a much lowercost than full plant upgrading). It involves a short period of aeration, of thetrickling 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 theperformance of the solids contact process. The pilot plant consisted of anaeration tank and clarifier to enable a direct comparison of the TF/SC processwith the existing trickling filter plant secondary effluent. The pilot plantwas operated with and without sludge return to the aeration stage. It was foundthat aeration of the trickling filter underflow (no sludge recycle) only led toa significant improvement over the TF plant effluent when aeration tankdetention times were greater than 2 hours. However, the settleability of thesolids showed great improvement with aeration, even at detention times wellbelow 2 hours.


For the trueTF/SC process (with sludge recycle) results from the trial show that there wasa significant improvement over the existing TF plant effluent for low aerationtank 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 usingthe solids contact process. Half of the samples analysed showed a tertiarystandard of sewage treatment (less than 15 mg/L NFR) was achieved using thetricking filter solids contact process.


The process alsoshowed that it had the ability to nitrify (i.e. convert ammonia to oxidizednitrogen forms), although for substantial removal of nitrogen a denitrificationfacility needs to be installed.


Despite thefrequent changes in operating conditions applied to the pilot plant and presenceof filamentous organisms in the return sludge, the TF/SC process consistentlyproduced a better effluent than the existing plant.


The SewageTreatment Optimisation Model computer program was used to check thecapabilities of a low detention time aeration process, and validated the pilotplant results.


Overall, theTF/SC process showed itself to be a reliable, easy to operate, and inexpensivemethod of upgrading trickling filter plants. In addition, it is recommendedthat, for new sewage treatment plants, consideration be given to implementing aTF/SC plant to take advantage of the lower capital expenditure (and operatingcosts) as well as the inherent reliability of the trickling filter itself.


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