Proceedings

The Problem

Sludge disposal becomes more and more expensive as fuel and transportation costs are rising. Long distance hauling of dewatered sludge that still contains 75 – 85 % water makes little economical sense.

If dewatered sludge is dried from 20 %DS to 90 %DS, its mass is reduced to 22 %. Dry sewage sludge has a caloric values that is similar to that of brown coal and can be used as additional energy source that is carbon-dioxide neutral.

High-temperature sludge dryers require heat that is generated from increasingly expensive fossil energy. To prevent dust combustion or explosion, the oxygen concentration within such dryers must be kept minimal and sophisticated safety controls are required.

Solar dryers need no or little additional heat, but have a very large footprint and need much sunshine. Generation of Class A biosolids usually requires addition of chemicals, such as lime. The sludge needs to be turned over many times by mechanical means whereby a dusty product is generated. Extremely long sludge detention times are needed to achieve a product with 90%DS.

Goals and Objectives

Low- to medium-temperature dryers can be operated with waste heat from various sources. Medium-temperature dryers are able to produce Class A biosolids by maintaining the required time-temperature relationship, independent of climatic conditions.

Generally, sludge dryers should be easy to operate and maintain and inherently safe. They should generate a virtually dust-free and dry product that is easy to handle. They should not emit odors.

The Solution

Belt dryers are compact: several open-porous horizontal belts are stacked above each other within an insulated enclosure. Dewatered sludge is distributed on the upper belt as a porous layer that is slowly moved back and forth as it gently drops from belt to belt. Blowers draw warm dryer air upwards through the belts and the sludge. On its way through the dryer the sludge is gradually dried to between 60 and 90 %DS, depending on the selected air temperature and sludge detention time. The air is circulated several times through the dryer whereby it is heated through heat exchangers or an additional gas burner. Less than 10% of ambient air is admitted at the bottom and the same flow of water saturated air is drawn off at the top. The exhaust air is cooled through heat exchangers whereby heat is recovered to pre-heat the incoming ambient air. The exhaust air is then quenched and water is condensed in a scrubber. An additional bio-filter is usually provided for deodorization.

The dryer air temperature in medium-temperature belt dryers can be selected between 80°C and 130°C , depending on the temperature of available heat, e.g. from a co-generation plant.

2.9 – 3.4 MJ heat is required per kg of water evaporation. This is equivalent to 0.8 – 0.95 kWh/kg. The heat demand is little dependent on the drying temperature, but design and dimensioning of the dryer and its heating and heat recovery systems depend on the drying temperature. Design details will be presented in the paper.

Belt dryers offer the following benefits:

• Simple design, easy operation and maintenance;
• Intrinsically safe operation due to modest temperature and virtually no dust generation. Sophisticated safety equipment and inertization with nitrogen gas are not needed;
• Virtually dust-free product that is easy to handle; its dryness can be selected between 60 and 92 %DS.
• Can be operated in a wide temperature range, depending on sludge throughput, desired product dryness and available heat source.

Conclusions

Many low-temperature belt dryers with air temperatures well below 80°C have been installed and operated for a number of years in Europe. Due to the fact that more and more sludge in Europe is incinerated, sludge disinfection to produce Class A biosolids is usually not required there. However, several medium-temperature belt dryers were recently installed for economical reasons: the dryers are smaller and the exhaust air flow is lower.

Data from several reference installations will also be presented in the paper.