Sustainable Sludge Management

The following sections are included:

• About the Authors
• Contents
• List of Figures
• List of Tables

Converting sludge into construction materials is deemed to be a sustainable approach to alleviating sludge disposal problems and conserving natural resources. The problems of disposal and depleting raw material resources will be drastically reduced if sludge can be put to large-scale economic applications. This review collates the state-of-the-art development in recycling sludge into major construction materials with in-depth discussion on properties of the products such as bricks, aggregates, and cement replacement materials, their processing technology and technical suitability for construction applications, and leaching assessment of toxic chemicals from the products. Challenges and prospects of sludge recycling are also outlined.

Sludge is an odious, semisolid residual that resembles thick soft mud produced from the solid–liquid separation processes in wastewater treatment. It is usually very inconsistent in its composition and most often unmanageable. Contaminants removed from the wastewater are concentrated in the sludge, which pose serious problem to the health and environment if it is not properly managed. On the other hand, sludge could become a valuable and renewable resource with proper treatment and prudent management. Sludge usually undergoes a series of treatment steps involving thickening, stabilization, conditioning, dewatering, and biological or chemical degradation prior to final disposal. Figure 2.1 outlines typical process alternatives and flow for sludge processing and final disposal or reuse.

Bricks are generally made from clay and used in construction for aesthetic, economical and durability purposes. Brick-making materials are required to behave plastically when moistened while retaining its intended shape. Raw bricks ought to have sufficient “green strength” for handling after drying and should not crack or shrink excessively on drying or during firing.

Aggregate is used in concrete as economic filler providing concrete with better dimensional stability and wear resistance. Aggregates made up 75% of concrete volume hence provide a great potential for large quantities of sludge application. However, sludge aggregates should not react adversely with other constituents of the mix, increase the cement requirement or have any adverse effects on the concrete properties, and should preferably provide beneficial value to the concrete.

Cement is the important binding agent that enables the formation of concrete. Cement material may consist of three following components; the hydraulic component, pozzolanic component and the inert component. The hydraulic component reacts with water forming cement gel that hardens and binds things together. The pozzolanic material is mainly composed of silicium and alumina oxides and reacts with lime to form a precipitate that provides the binding property. Inert materials that do not react with the cement materials can be used as fillers to improve the strength of cement while reducing the overall cost.

Road pavements consist of several layers, namely subgrade, subbase, base and surface layer consisting of the wearing course and sometimes an extra binder course. Road pavement can be constructed of a variety of materials or their combinations: gravel or aggregate, stone, bitumen, concrete or improved soils. The choice of materials and thickness of the pavement layers depends mainly on the projected traffic frequency. The other factors are available budgets, road location and the availability of local materials.

Ceramic roof tiles are designed mainly to keep out rain, and are traditionally made from locally available materials such as terracotta or clay. Of the many advantages of clay roofing tile, its durability is probably the biggest one. By firing the clay roof tiles at high temperatures, this pure natural product can last for over 100 years. Another advantage is that it can be designed in a variety of shapes, sizes, and colors, adding more character and interest to the appearance of roof. It also has reflective properties such as fire-resistance and UV-resistance, which help to increase the efficiency of heating and cooling systems. This type of material is not susceptible to mold or rot, and does not shrink and expand with temperature like wood.

Leaching is the removal of materials from the waste, usually solids such as sludge products, by dissolution, and the released constituents would then be transferred through the leachate to the surrounding environment. The effects of the constituents on living organisms, humans in particular, were determined by the concentration (Conner, 1990). Therefore, water quality standard is specified in concentration and these standards, especially drinking water standards, are usually adopted as the basis for leaching limits. Thus the rate of leaching is customarily measured and expressed in terms of concentration of the constituent in the leachant.

Sludge disposal is in a dynamic period of rapid change in line with the expansion of wastewater treatment plant construction and increasingly strict disposal requirements arising from industrial growth and stringent environmental regulation. With increasing population and economic growth, it can no longer rely entirely on expanding the environmental infrastructure to cope with ever-increasing amounts of sludge, nor can it entirely rely on enforcement against pollution from sludge disposal. In a number of highly urbanized cities, sludge disposal by landfilling may no longer be appropriate owing to land scarcity and increasingly stringent environmental control regulations.

The review of various studies on the application of sludge material in the production of building and construction materials found that it is possible to incorporate sludge into the production of construction materials while meeting the basic property requirements. Sludge material can be used in both original and ash form. The sludge ash material is usually low in plasticity and rather inert in property, while the original sludge provides a better plasticity and cohesion. It was also observed that different sludge ash may exhibit dissimilar properties. Sludge ash in the form of loose powder has a finely dispersed and porous nature while sludge ash present in the solid mass form may have a denser and impervious property.

The following sections are included:

• References
• Index