BIOTOWERS

Description

A biotower is used to reduce the BOD value of a liquid effluent. A biotower is an above-ground cylindrical tank or rectangular structure that contains plastic media with a high surface area, such as randomly filled polypropylene shapes or modular blocks of corrugated PVC. Effluent is pumped to the top of the tower and distributed over the surface of the media using rotating distributors, troughs, or nozzles and splash plates. The effluent trickles down over the media, which become coated with microbial films that consume the organic material.

The outer surfaces of the microbial films are highly aerobic, being continuously aerated by the air that passes upwards through the media. Below this the conditions become progressively less aerobic as the film builds up. Eventually the film sloughs off but the plastic medium has a large void volume that ensures unimpeded passage of sloughed biomass, air and liquor.

The treated liquid may be recycled over the biotower before the biological solids are settled out. Biotowers can be arranged in series with inter-stage settlement. Fan ventilation may be incorporated where the biomass must be highly aerobic, for example where nitrification is required.

Application

Biotowers are used in the food and drink and other industries. They can accommodate high hydraulic and BOD loadings and may be employed as a roughing stage for the treatment of high strength industrial wastewaters prior to discharge to sewer or as the secondary or tertiary part of a complete treatment system. Some effluents, such as sugary solutions, may require nutrients such as nitrogen or phosphorus to be added.

In general biotowers should not be overloaded or fly nuisance or odour may result. However, these problems can be avoided through fan ventilation or treated by roofing over and scrubbing where high loadings produce large quantities of sludge a solids contact re-aeration system may be advantageous.

Sizing

With an average effluent and loading rates of about 0·5 kg BOD/m3/day over 90 % of the BOD may be removed. With a loading rate of 2·5 kg BOD/m3/day about 60 % of the BOD may be removed. Combined with solids-contacting and re-aeration about 95 % of the BOD can be removed.

Media used for BOD removal has surface area ratios of 100-240 m2/m3. Nitrification requires the higher surface area media and low loading.

A biotower may be up to 10 m high and requires energy to pump and recirculate wastewater over the medium. A typical industrial biotower with a volume of 1000 m3, and an effluent flow of 25 m3/hour recycled 6 times would use an 11 kW pump.