Raw sewage sludge consists to approximately 95 percent of water. In Austria, 7 million tons are produced every year, 38 percent of this amount are incinerated, 25 percent landfilled, 17 percent are used in agriculture or composted and 20 percent are disposed in some other way. However, due to the increasing load of sewage sludge by drug residues, hormones and other pollutants, its use in agriculture is already becoming problematic.
The new sewage sludge ordinance - coming into force in 2004 - stays abreast of this situation, but will at the same aggravate the disposal problem. Sewage sludge incineration is thus becoming increasingly important and requires the optimisation of - so far - cost-intensive sludge drying facilities.
The development of a fully automatic solar sewage sludge drying system - a joint development by the University Stuttgart-Hohenheim and the German company Thermo System - constitutes a technological quantum leap. The plant, which recently started operations in Bramberg, Oberpinzgau (National Park Hohe Tauern), will for the first time in Austria produce a low-cost granulated residual material, which is perfectly suited for incineration.
The planners of the first solar sewage sludge drying facility in Austria - the experts of the Vienna company ACAT - did not have to venture out for this purpose. After all, the world patent has already proved its practical feasibility in Germany for about five years (plant Fuessen for 70,000 PE; Renquishausen: sludge drying without pre-dewatering at an altitude of 900 m!).
"Moles" in a hothouse
ACAT sales manager Ewald Kern outlines the functioning of the environmentally sound system for up to 200,000 inhabitants: "All kinds of sludge, i.e. liquid, pre-drained or decayed, can be dried down to 90 percent DM. This is done in hothouse-like drying chambers made from non-corroding solid constructions and translucent glass elements. The solar drying unit operates at a low temperature level with solar radiation being absorbed by the dark sludge. A sophisticated aeration system exhausts the humid and warm air, which is free from harmful emissions and practically odour-free."
The even distribution of the sludge on the drainage floor (which can be up to 40 cm high depending on the water content) is effected by the technically most interesting part of the new system. The "Electric Mole" - about 4 square metres large and weighing 300 kg - tills and mixes the sludge. This process follows a semi-randomised pattern (curve track), which achieves a mixing probability of 99 percent.
Thus, by the end of the day, every section has been mixed and aerated at least 12 times. Supersonic sensors control the Mole and keep it within the pre-set operation zone (red bands). Thus, the drying chamber can be divided in two parts, which allows for continuous feeding via conveyor belt. The sludge is considered to be dry after about 20 days (Fuessen), and 40 days respectively (Renquishausen: altitude and no pre-dewatering).
Same caloric value as brown coal
The advantages of fully automatic solar sewage sludge drying are obvious. Reduced transport expenses and less storage volume will alleviate sludge disposal problems as of 2004. Compared to conventional high energy drying methods, solar power plus a minimum of personnel expenditure result in a low-cost (add-on) fuel with a caloric value similar to brown coal (about EUR 36.34 to 50.87/t of sludge after mechanical pre-dewatering).
In Germany this fuel is already used in cement works. As to the use of sewage sludge in agriculture, the new method entails a considerable reduction of pathogens, a higher concentration of nutrients and also a better nutrient absorbing capacity in plants.
State-of-the-art
Bramberg is not only the location of Austria's first solar sewage sludge drying facility, but also represents the state-of-the-art of this technology. Designed for 25,000 PE, mechanical pre-dewatering and subsequent solar drying are spatially combined. Since incineration of the dried sludge is not planned (National Park), it is mixed with insufficiently dewatered sludge and finally composted. The efficiency the test run has shown since its start-up in June has not only convinced the manager of the plant, Walter Freiberger, of its value, but has meanwhile also turned the plant into a mecca for interested experts from at home and abroad.
"Sludge grubbers" with the Thermo System/ACAT technology seem to have a promising future, and the recent quality award of the GWT (Guetegemeinschaft Wassertechnik) will promote the technology even more. ACAT managing director Manfred Zabl: "We are certainly convinced that our system is the most economical solution for 90 percent of all sewage sludge treatment plants up to 200,000 PE in Austria!" (Source: aqua press Int. 6/2001)
Mag. Christof Hahn
Information & Contact:
ACAT - Applied Chemicals Handels GesmbH
Wolfgang-Pauli-Gasse 3
A–1140 Vienna
Ing. Ewald Kern
Phone: +43 1 979 34 73-32
Fax: +43 1 979 34 73-55
