Proceedings

Population growth and more stringent effluent quality requirements together have propelled the production of sludge ever upward.  In many cases, the sludge resulting from the phosphorus removal process with ferric products has proved to be very difficult to settle.  Consequently, large numbers of digesters often operate under a hydraulically overloaded condition due to the high water content of the sludge and gas production per unit volume is significantly reduced.   The process energy balance becomes less favourable not to mention a possible breach of the code of practice for the agricultural recycling of sewage sludge.   In such circumstances, gravity belt thickening (GBT) is the most apparent process solution.  However, GBT comes with a high whole life cost and in many cases it can only remove one third of the water content.  Inverted Phase Fermentation (IPF), a recently developed alternative, promises to deliver an improved water removal capability at a reduced whole life cost.

IPF is a sludge thickening process that operates on the basis of flotation with nascent carbon dioxide. The application of IPF should allow the digestion process to be intensified with organic loading rates up to 4.9 kg VS/m³d without significant risk of foaming.   No major modifications of existing digestion assets are required.  IPF also provides improved pathogen destruction and a free carbon source for Biological Nutrient Removal (BNR) applications.

This case study was based on one of United Utilities (UU) digestion sites where a sludge quality improvement programme had been planned. The study focussed on three options including the UU asset standard design (GBT followed by EEH); and two innovative options (IPF followed by EEH and an integrated IPF/EEH design).   The discussion will cover the whole life cost as well as the benefits and risks of each option.

Keywords: Digestion Intensification; Existing Assets; Case Study; Inverted Phase Fermentation; Enzymic Hydrolysis; Sludge thickening; Viscosity.