This biofilter was installed in 2001 and treats 12,500 cfm of air coming from a sludge liming and drying operation. The footprint of the biofilter is 90’ by 36’ (see photo). The air is first treated in an acid scrubber (which is now run without the acid), and then in the biofilter. Such air streams are known to contain high ammonia and some reduced sulphur compounds and are usually very odorous and difficult to treat. The acid scrubber was designed into the plant because of the high ammonia in the air stream. While the scrubber is essential for proper humidification, Ambio was of the opinion that the acid was not required and the operator now runs the unit with the acid turned off—in fact the original acid tank is still full and unused.
The main challenge to the biofilter in the facility is that the liming and drying is not a continuous process, and runs only about 3 days per week. We built into the design enough flexibility to keep the biofilter warm and moist when the liming and drying was not in operation by using a VFD to allow for some air to go to the system (~500 cfm) during these times to keep the bed warm and aerobic, and using our secondary humidification system (network of soaker hoses buried in bed) to keep the bed moist.
The bed was changed after 6.5 years of operation.
The biofilter was independently tested for odour removal efficiency by Lehder Environmental. The results indicated an average odour removal of greater than 96.8%, and 26 odour units/m3 on top of the bed.