A prominent food processing company in regional New South Wales is reaching its demanding targets for purifying wastewater before discharge into the sewer, without having to invest in expensive biological processing.

The company’s innovative dissolved air flotation (DAF) system for treating wastewater was designed and successfully installed by CST Wastewater Solutions.

The treatment plant reduces the high levels of Suspended Solids (SS) and Biological Oxygen Demand (BOD) in the wastewater sufficiently to allow discharge into the local authority’s sewerage system without the requirement for costly microbial after-treatment.

Generally the achieved Suspended Solids are less than 300 mg/l and the BOD approaches the limits of 600 mg/l, he said.

Besides the food industry, other typical applications for the CST Wastewater Solutions DAF system include dairy and cheese factories, the paper industry, oil industry, wineries, tanneries, textile industry, large laundries, chemical industry, refineries, sludge thickening, and primary and secondary clarification in municipal and industrial biological wastewater plants.

The NSW food company project required on-site chemical testing and evaluation, followed by the installation of a complete pre-treatment system. The treatment plant uses CST’s latest design of an internally-fed rotary screen, followed by a proprietary high-performance DAF.

The DAF system’s designer, Aqua Engineering, has supplied more than 400 wastewater treatment units over a 30 year period.

“Aqua’s DAF flotation system has been developed in line with the most advanced solids/liquid separation techniques,” Mr Bambridge said. “It features a low volume circular flotation cell, coupled with a high efficiency air-dissolving reactor, to achieve 95 per cent saturation efficiency.”

The dissolved air flotation process introduces micro bubbles into the wastewater and with the aid of chemicals removes suspended matter, reducing the pollutant levels in the wastewater. The bubbles released by the DAF process carry the suspended matter to the surface of the flotation cell, where the matter is removed by a rotating scoop or surface skimmer.

The DAF system uses a unique double-injector air dispersal method that results in an exceptionally efficient separation of solids from liquids. The method works by combining hydraulic distribution and filtration in a single system.

The double-injector system provides air saturation of up to 90 per cent. The first stage of air dissolving, up to a theoretical maximum saturation of 50 per cent, occurs in the high turbulence, high intensity mixing zone of the vessel’s central column.

The second stage occurs in the larger volume of the vessel, where longer air-liquid contact provides a large exchange with very fine bubbles and a high exchange surface.

Raw wastewater, saturated with air, is then introduced into the main body of the flotation tank through a series of openings located around the circumference of the lower part of the central column.

Solids are rapidly carried to the surface by the micro bubbles and are evenly distributed as sludge on the surface of the flotation tank in a uniform hydrodynamic condition.

The sludge float is then removed by a rotating adjustable scroll scoop or by a surface skimmer, without disturbing the sludge blanket.

Key characteristics of the DAF system from CST Wastewater Solutions include:

  • high saturation efficiency > 90 per cent
  • complete dissolving of injected air
  • automatic control of the injected air
  • automatic control of the sludge blanket
  • flexible operation: recycle, partial flow or full flow
  • small footprint

DAF models available from CST Wastewater Solutions range in size from 1.48 – 15.40 metres in diameter and 1.00 – 2.6 metres in height.