Sludge Treatment and Disposal Technology (5)

21. Are There Any Regulations to be Followed for the Agricultural Use of Sludge?

The agricultural use of sludge must comply with a premise: only the sludge that has been thermally dried, bio-composted, or chemically stabilized can be used. The drying plant or composting plant must conduct routine testing on the sludge. In addition to ensuring that the agricultural products must have basic fertilizer efficiency, they must also keep complete records of all test results and application purposes. At the same time, users should apply quantitatively under the guidance of experts and regularly sample and test the soil to ensure the application effect and that the soil will not undergo reverse changes. Similarly, for agricultural use of sludge, complete records of grown crops and heavy metal content must be kept for comparison.

22. Can Sludge be Incinerated?

Sludge has a certain calorific value, but it is far lower than coal. First of all, the sludge must be dried or semi-dried, and a small amount of auxiliary fuel must be added during ignition, and then it can achieve spontaneous combustion.

Advanced heat exchange system can rely on the heat energy generated by sludge incineration for drying, and its heat quantity can meet most or even all of the drying needs. Sludge that has not been dried or semi-dried will be difficult to ignite due to excessive moisture, and its heat balance is negative, which means that fuel must be added to maintain incineration.

23. What Are the Problems with Sludge Landfill?

Direct landfilling of dewatered sludge cake is a serious waste of resources. In addition, it may also cause many problems for the landfill site:

(1) The landfill site is generally covered with soil layer by layer to ensure better space utilization. The high water content and high viscosity of sludge often cause the rolling machinery to slip or even get stuck in it, which brings difficulties to landfill operations.

(2) The rheological properties of sludge make the landfill body easy to deform and slide, becoming an artificial "swamp", which poses a great safety hazard to the landfill site.

(3) The high water content of sludge greatly increases the amount of leachate in the landfill site. Due to the small size of the sludge, it often blocks the leachate collection system and drainage pipes, increases the load of the garbage dam, and brings difficulties to the safety and management of the landfill site. The cost of cleaning the collection system is extremely expensive.

(4) The space of the landfill site is limited and cannot accommodate so much sludge.

24. Are There Any Restrictions on the Water and Organic Matter Content of Sludge Landfill?

Considering that sludge is a resource, some countries have begun to restrict the direct landfill of sludge. The rising landfill costs have triggered market demand for deep sludge treatment, sludge reduction and resource utilization.

These restrictions include:

(1) The water content of sludge landfill must be less than 40%.

(2) The organic matter content must be less than 30%.

25. What Are the Other Options for Comprehensive Utilization of Sludge?

In addition to land use, incineration, and landfill, other resource utilization options for the final disposal of sludge are still being explored. These include:

(1) Drying to make cement.

(2) Drying to make bricks.

(3) Drying to make roads.

(4) Drying to use as landfill cover.

(5) Semi-drying to make fuel oil through catalytic cracking.