Paper mill sludge (PMS) and degraded municipal solid waste (DMSW) blended fired bricks–a review

Attempts should be made to recycle, re–use and utilize waste. Paper manufacturing is a complex industry involving multiple processes where different products are produced and large quantities of waste of primary, biological or de–inking origin are generated, waste water treatment sludge, primary sludge, and secondary sludge among them.1,2 The primary sludge is generated in the largest quantities.3 Authors4 state that the production of 1 ton of paper generates about 30 kg of primary sludge. India produces 10.11 million tons paper per annum which is 2.6% of world’s overall production quantity of paper.5 Furthermore, India is one of the fastest growing pulp and paper market in the world with a growth rate of over 10% per year in per capita consumption.2 Nowadays this sludge is used for energy generation through incineration and anaerobic digestion.6–9 Sludge incineration process is regulated in the Europe and typically uses fluidized bed combustion at ~ 850°C to 1100°C.10 The incineration process, however, generates ash, which must be utilized as well. Researchers found that paper sludge ash can be utilized in concrete manufacturing, where part of cement is replaced by hydrophobic paper sludge ash additive.11 12% of paper sludge ash added to the concrete mix significantly reduces water absorption of concrete without compromising on either density or compressive strength. Authors12–15 argues that paper sludge fired at 700–750°C temperature acquires pozzolanic properties. Such substance can be utilized in cement manufacturing because under high temperature conditions kaolin present in paper sludge turns into metakaolin, which has the properties similar to those of commercial metakaolin.16 Besides, the resulting ash retains more regular properties compared to the bottom ash from the incineration of municipal waste. It is also known that paper sludge can be used in concrete and cement brick manufacturing18,19 as a substitute for certain natural additives.19 Other authors suggest producing wood–paper sludge boards where a certain amount of wood chips is replaced with paper sludge waste.20,21 Unfortunately, such boards have a much lower compressive strength due to weak bondin between wood chip and paper sludge particles. The optimal content of paper sludge in such boards is up to 10%.18 Paper sludge can be also used in clay brick manufacturing.22–25 On the one hand, paper sludge additive reduces the density and thermal conductivity of construction products; on the other hand it impairs their mechanical properties. Some authors26,27 propose to modify clay and paper sludge mix with glass scraps.