RDP process chosen by Norway and Scotland
The RDP sludge pasteurisation process has gained a foothold in Norway and Scotland, following successful application in the US. By Keith Panter of Ebcor.
Traditionally in Scandinavia a high lime dose is used for sludge pasteurisation, but NJA wanted to use less lime because of economics and restrictions on the amount of lime which can be added to land.
Fredrik Gaustad, managing director of NJA said: "The farmers told us they like a small addition of lime in the product but that their consumers had to be very sure that their products were safe." He added: "We chose the RDP method because of its track record in the US for making Class A products with a low lime dose. This process is simple, quick, has a high production rate and gives us a stable end-product."
The plant has been running for two years, receiving about 20 tonnes of untreated sludge cake a day at about 22% dry solids from outlying plants. The sludge is delivered by road tankers into a 40m3 hopper which is placed on digital load cells. Three screw pumps at the bottom of a hopper feed sludge to a conveyor, which transports the sludge to a central unit, the Thermoblender.
Sludge is heated in the ThermoBlender to about 550C using an electrically heated twin-shaft stirrer. This pre-heated sludge is softer and therefore easy to mix with quicklime into a crumbly mixture. Quicklime is delivered to the plant using sealed road tankers and stored prior to mixing in a 50m3 silo. The lime is added at a controlled rate to match the sludge inputs using a volume feeder, to create a mix with 15-20% lime. The exothermic reaction generated by the lime takes the temperature above the minimum 700C required for pasteurisation. The mixture is then passed along a plug flow pasteurisation vessel, which is in effect an oven where the sludge remains above 700C for 30min or more. The temperature is recorded at the beginning and end of the vessel to ensures the process meets Hazard Analysis and Critical Control Points (HACCP).
It is this capacity to demonstrate HACCP which led Ebcor to recommend this technology to UK users as the company believes all biosolids processes will have to meet HACCP in the future, in order to satisfy the British Retail Consortium's (BRC's) requirements for safe sludge. In Norway all sludge products have to be stored for at least a month so they can be tested for quality of metals and bacteria before leaving site. Their regulations call for less than 2,500 coliforms per gram DS product and no salmonella.
In order to meet microbiological standards, the temperature and lime dose must be carefully controlled. Lime-only pasteurisation typically uses 10% or more quicklime per wet tonne. NJA wanted to demonstrate safety at 5% lime and below. They carried out trials in the spring of 1998. Each day a different dose was used. They found a lime dose as low as 2.4% resulted in a safe product.
There was a discussion on this subject at the recent IBC sludge conference. It is generally believed that pH 12 is needed for sludge product storage because of pH decay, but NJA has shown that this is not the case. Per Martin Aakeroy, biologist at NJA, said its product remains pathogen free even with storage for many months. George Dirk from the Dirk Group also pointed out that in trials at Stuttgart-Hohenheim Agricultural University in the 90s it was shown that bacterial re-growth did not occur as long as initial pH was above 10.5 using Rhenipal - a blend of PFA, quicklime and other materials. Ebcor and Dirk are sponsoring further trials in the UK to confirm these results. There is tremendous potential for admixture cost savings if a lower pH can be used safely with a consequent increase in land available for the product, as a high lime content often limits options for land application.
Mr Gaustad said that with a lime-only approach their cost would have been about £50 per dry tonne. With the reduction in lime the cost per dry tonne including lime and electricity is about £20-£28, depending on the dry solids of the sludge. Further savings have been made by complete automation of the plant. The sludge hopper and lime silo are fitted with load cells which allow accurate dosing of lime and fine control of pasteurisation temperature.
Automation is allowing NJA to import more sludge from other communities, so input is going up by about 15% per year. The plant can process 0.9 dry tonnes per hour so there is potential to increase production well above the current rate of 1,200 dry tonnes per year. Mr Gaustad is hoping for a dry sludge production rate of 2,200t/yr, above which the plant would be more than cost-effective. At present the plant is only working a seven-hour day, although with existing staff this could easily be raised to ten.
Orkney & Lewis
North of Scotland Water (NoSW) was the first water authority in the UK to adopt a Class A biosolids strategy in response to the need to allay fears about the agricultural use of sludge. Because it is granular the product is said to be particularly suited to grassland and is commonly used for pasture in the US. This, along with a guarantee of a final product free from pathogens, was one of the deciding factors that led to the adoption of the RDP process by NoSW for its wastewater projects in the Orkney Islands and Lewis in the Hebrides.
Providing sludge management for the islands represented a special challenge. Contractors for the wastewater projects Christiani Nielsen Ltd (CNL) were instructed to select a process based on lime pasteurisation. CNL's engineering manager Dennis Colbeck said: "Selecting a process that could guarantee a Class A product at all times was one of NoSW's top priorities as they are totally reliant on grassland. We visited and reviewed various processes including the Norwegian plant with staff from NoSW.
"Following our review the RDP system was the only one we felt comfortable would give a reliable Class A product, which would be suitable for use on grassland. It had the added advantage that only a small amount of lime would have to be transported to the islands."
RDP is best suited to small to medium communities with untreated sludge. In countries that have adopted a Class A or pathogen-free approach, lime pasteurisation has had a rapid growth. There are around 50 RDP-type plants in the US and also several in Europe, Australia and New Zealand.