Sludge pasteurisation using the Dirk-RDP Rhenitherm process at West of Scotland Water's Girvan WWTW

Girvan Wastewater Treatment Works, located 5 miles south of the famous Turnberry Golf links on the Ayrshire coast, is an existing West of Scotland Water site. The works, which treat a mixture of primary, partial secondary and treated sludges to a population equivalent of 55,000, have recently been upgraded as a result of increasingly strict legislation being enforced by The Scottish Environmental Protection Agency (SEPA). The upgrades to the plant have been based upon the introduction of Sequencing Batch Reactor (SBR) technology and sludge pasteurisation. The objectives behind the changes were based on providing an environmentally safe, pathogen-free sludge biosolids product which is to be recycled on local farms.

For sludge pasteurisation, Dirk's Rhenitherm RDP process was chosen since it could provide the guarantee of enhanced treated biosolids as defined by the Adas Sludge Matrix, and also comply with the HACCP system. HACCP, or the Hazard Analysis Critical Control Point system, is a process control system that identifies where hazards might occur in the food production process and puts into place stringent actions to take to prevent the hazards from occurring. It has been embraced by the Food Retailers in the UK who are, in some ways, the major influence in determining the future of biosolids recycling to agriculture. Spot sampling and temperature probing of sludge piles doesn't meet HACCP requirements and is an unreliable measure of pathogen kill. The Rhenitherm system coupled with Dirk's ISO 9002 programme gives a further level of safety.

In essence, The Rhenitherm system is based upon electrical heating and temperature-controlled pasteurisation that ensures that every particle of sludge meets the time/temperature requirements of advanced treatment. Sludge is kept from re-infection by the addition of low quantities of alkaline admixtures. The system was designed to incorporate the stringent US EPA 503 regulations for advanced treated or "Class A" sludges. The principal steps are shown in Figure 1 and described as follows:

1) The Thermoblender(tm) receives and preheats the sludge 2) Dirk's branded admixture Rhenipal and/or lime is added at a proportional rate to the sludge feed 3) The Thermoblender(tm) mixes the sludge with the Rhenipal (or 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 further enhanced biosolids product

Figure 1. Rhenitherm process flow diagram

The Girvan plant will pasteurise an average 4,000 kg/day sludge cake with a dry solids content of 20 ± 2%. Ambient sludge temperature will be between 7 - 22ºC. In order to cope with potential changes in legislation, the Rhenitherm plant has been designed to heat the sludge at 55ºC for two hours or 70ºC for half an hour. The plant has also been designed to use lime or alternative admixtures. The use of other admixtures is possible since the system only uses them for pH correction. Alternative admixtures, such as Dirk's Rhenipal blend, are generally much cheaper than lime. Additionally, they contain nutrients and soil ameliorants, which may actually have to be added to sludges treated only with lime. The low lime content of Rhenitherm pasteurised biosolids means that
(approx. 50%) less land is required for application than lime-only treated material, so it is less restricted by lime limits to land. Spreading and sampling costs are also reduced when compared with lime only pasteurisation.

The main advantages of the technology over more traditional treatment systems are highlighted in Table 1:
When compared with lime-only treatment processes the Rhenitherm system has very low operating costs that include: processing, raw materials, electricity and disposal costs. Processing costs with Rhenitherm are considerably cheaper since electricity is far more efficient way of providing heat than lime is, and it is at least 4 times cheaper to heat the sludge electrically. Lower admixture addition rates also result in lower disposal and spreading costs. The affect of these differences is demonstrated in Figure 2, which shows how operating expenses change at Girvan over a 20 year period.

Table 1. Comparison of technologies

Figure 2. Comparison in operating costs between lime-only and Rhenitherm processes over a 20 year period at Girvan	Class A sludge product equivalent to enhanced treated biosolids as defined by Adas Sludge Matrix

In conclusion, the Rhenitherm process can be described as:

• Safe technology: design based on strict US EPA 503 regulations for Class A material which guarantees a pathogen-free product
  
• Flexible: systems are designed to comply with latest and future legislation
  
• HACCP compatible: which is the industry standard set by the food industry
  
• Providing a popular product: sludge contains important nutrients and requires 50% less land for application than lime only pasteurisation systems. It also provides a friable product which is easy to store and spread
• Simple to operate: with full automation possible and is also fully enclosed
• Economical: lowest whole life costs when compared with similar technologies due to lower materials, operating and disposal costs than competing systems
  

Rhenitherm controls and equipment

Ailsa Craig	Girvan Waste Treatment Works