Rhenitherm Sludge pasteurisation by DIRK and RDP

For more than 15 years DIRK has been supplying products and services to help the water and wastewater industry. Rhenitherm is a new process that combines the successful Rhenipal sludge stabilisation method with the RDP EnVessel supplemental heat pasteurisation system. The result is a guaranteed Class A sludge product which displays environmental benefits compared with other treatment processes. The system is tried, tested and very reliable and proven in numerous full-scale applications in the USA, Europe and Australasia. A typical unit is shown in Figure 1.

	The process may be described as follows:
1	The Thermoblender™ receives and preheats the sludge.
2	Rhenipal and/or lime is added at a proportional rate to the sludge feed
3	The Thermoblender™ mixes the sludge with the Rhenipal (lime) and heats the contents to required temperature.
4	The heated sludge/Rhenipal/lime mixture discharges to the Pasteurisation vessel.
5	The Pasteurisation vessel stores the treated material for the required time period maintaining the proper temperature to destroy pathogens and provide the Class A product.

Figure 2 shows the overall view of a 1.5 dry tonnes per hour operation and detail of the heating system is shown in Figure 3.

This technology has many environmental and cost benefits compared to traditional lime only heat pasteurisation. Lime is expensive and uses large quantities of energy in manufacture - much more energy than it gives up as heat. The Rhenitherm method is based on electrical heating and temperature-controlled pasteurisation that ensures that every particle of sludge meets the time/temperature requirements of advanced treatment. The technology has significant advantages over more expensive processes as demonstrated in Table 1.

Table 1 Comparison of technologies

Color	Digested Cake	Dried Products	Rhenitherm
Safe - meets code of practice	If pre-pasteurised	Yes if time/temperature criteria are demonstrated	Yes, HACCP* compliant
Ease of Storage	Stockpiles tend to rewet and can be smelly	Stockpiles can catch fire Bags are a nuisance	Yes, because of a granular consistency
Popular Product	Seen as source of phosphate/trace elements	Yes, except for dust and slow release of nutrient. Low capital costs, low lime systems are available	Yes, farmers like added incentive of free lime
Simple to Operate	Yes	Definitely not, requires constant attendance	Very simple and robust, wet scrubbing for odour
Economic	High initial capital costs	Very high initial cost and continuing operating cost	Low capital costs, low lime systems are available

* Hazard Analysis and Critical Control Point

What makes Rhenitherm an attractive alternative?

It is safe - Rhenitherm technology provides a reliable and safe Class A product, compliant with strict US 503 regulations, and countless tests have shown consistent pathogen-free product. The unique design of the process means that every particle in the sludge can meet current and future time/temperature requirements. This advanced treatment is fully compatible with the Hazard Analysis and Critical Control Point (HACCP) system. HACCP is a process control system (used by food industry) that identifies where hazards might occur in a process and puts into place stringent actions to prevent the hazards from occurring by corrective action. This is due to continual monitoring of all process parameters to guarantee a safe product. Spot sampling and temperature probing of sludge piles doesn’t meet HACCP requirements and is an unreliable measure of pathogen kill. The pH of the product can be easily adjusted to ensure that the product does not reinfect with pathogens. The RDP system coupled with our ISO 9002 programme gives a further level of safety.

It provides a popular product - Rhenipal stabilisation generates a consistent granular “crumbly” sludge making it easy to spread and store. It also provides nutrients required by the farming industry. It’s low lime content means that (approx. 50 %) less land is required for application so it is less restricted by lime limits to land. Spreading and sampling costs are reduced compared with lime only pasteurisation.

Technology is simple to operate - It is a fully enclosed system with odour control. Unlike drying technologies, Rhenitherm doesn’t require skilled and full-time operators or complex maintenance. The process is very reliable and robust consisting of slow moving machinery, therefore little operator attendance is required and full automation is an option.

Economics - The Rhenitherm process has very low capital and operating costs. The Rhenipal product is considerably cheaper than lime and in this process the liming equivalence is about a third of lime only pasteurisation. It is at least 4 times cheaper to heat the sludge electrically than with lime. The design of the Rhenitherm process means that operating costs are also significantly lower than other advanced treatments. The process has consistently lower whole life cycle costs than other advanced treatments. In conclusion, the Rhenitherm process provides:

• Lowest whole life cycle costs

• Guaranteed HACCP compliant Class A biosolids product

• Proven advanced treatment

• A highly desirable product

• Simple operation

We keep our customers green, but we don't cost them the earth.