Performance 

Phase separation is essentially a side effect of the fermentation process and is significantly affected by the process temperature.   The optimum process temperature is to be found in the range of 30°C to 42°C.   At either extreme of temperature the bacterial activity become too sluggish for the process to be useful.  As the temperature increases the viscosity of the medium is reduced leading to an increase in the rate of separation.    IPF is suitable for use with sludge streams of initial solid concentration of 1% to 5% DS.  The graph below shows variations in the separated phases with the initial sludge concentration.  The final solid concentrations in all the separated phases have been found to be within a very narrow range of values.  The solid phase would typically have a solid content of 7% to 11% DS; and the liquid phase would typically have a solid content of about 0.35% DS. 

Table 1 shows a set of typical operating parameters for an  IPF digestion process. 

 GHG reduction relative to conventional digestion = 0.65 tCO₂e per tonne digested (estimate).

Application

IPF would substantially cut the amount of chemicals used in conventional sludge-thickening, and as a result could be a significant cost-saver. Most companies use a chemical, polyelectrolyte, as a coagulant to thicken sludge, combined with Gravity Belt Thickener (GBT) machines – but it’s an expensive and relatively inefficient solution.

With our IPF project we’re looking at a different way of doing things. The process heats sludge to body temperature, before being allowed to settle for a couple of days. The sludge in the IPF tank separates into layers using the natural carbon dioxide produced from the fermentation process. These bubbles of gas rise to the surface and the organic material in the sludge sticks to those gas bubbles. What is left is a thick layer of sludge on top, and a watery layer of liquor underneath. This simple process produces thicker sludge – 10 per cent dry solids, as opposed to the six or seven you get with a GBT. When the volumes involved are hundreds of tonnes of sludge every day, those are significant numbers.

Our IPF process also creates more gas. More gas means that we can create more of our own electricity using our combined heat and power engines.

There’s another great spin-off: the watery layer mentioned above is much thinner and rich in volatile fatty acids. These acids are ideal for the biological nutrient removal that could become a key part of wastewater treatment in the future, and reduce our dependence on chemicals. With increasing pressure on companies like us to reduce phosphates, we can use these volatile fatty acids as a carbon source to aid the process of nutrient removal from the sewage. Currently the only way to get rid of these phosphates is by using large amounts of ferric chloride. Like poly, ferric chloride is an expensive chemical.

So with IPF, we’re reducing our reliance on chemicals, while cutting our power consumption – a sustainable solution that’s good for the environment and our customers’ bills.

 Ó 2009 United Utilities PLC. 

IPF was a joint development with Cranfield University through the Knowledge Transfer Partnership.