Development and study of dissolved gas flotation for biomass recovery after anaerobic treatment

On site industrial waste treatment is gaining popularity because of increased charges by the private utilities. Anaerobic treatment at breweries, distilleries, dairies, food processors and other industrial production facilities is a cost effective solution to liquid waste disposal. This research investigates the design of one of the most popular anaerobic technology systems the Contact Stirred Tank Reactor (the CSTR). This reactor is completely mixed by mechanical stirring and/or biogas recycle. Conventional CSTRs use a gravity settlement clarifier for biomass recovery and clarification of final effluent. The biomass however, is warm and gassing and is difficult to settle. On occasions this causes major solids losses from the clarifier and ultimately process instability.

The research reported here based on data from full scale plant indicates that shock increases in organic load and consequent gas production is one factor leading to biomass losses. The work described also implicates filamentous bacteria as interfering with settlement. An observation not commonly investigated or reported.

Long term (10 years data) operating data from an operational CSTR at the Wrexham Lager Brewery Company (WLBC) treating brewing effluent were presented. Detailed results are included on chemical oxygen demand, loadings, removal rates, suspended solids in effluents, metals and filamentous biomass counts. This data included an analysis of the effects of changes in organic load, filamentous growth and trace nutrients as factors affecting settlement. Organic loads and filaments were shown to be important parameters influencing settlement efficiency.

To overcome the potential settlement problems this research also investigated laboratory scale Dissolved Air Flotation (DAF) as an alternative solids recycle technology.

Comparisons were made between a laboratory scale CSTR plant, under well controlled laboratory (5 litre) scale, with an identical digester using DAF. These rigs were run for 5 years and their performance assessed and compared to the full scale WLBC plant. The performance results were comparable whilst flotation of the biomass was more successful for solids recovery. A daily injection of supersaturated water containing the air to float the anaerobic biomass did not toxify or inhibit the process.

Fundamental experiments on bubble size, air to solids ratio, and redox recovery were also reported. Bubble size was shown to be the most important influence on flotation performance.