Solid wastes refer to a variety of solid residues produced in the process of paper and pulp making. The solid wastes include five types, such as barks, sawdust, bagasse pith, the straw of preparation workshop; fiber, fiber fines and packing of paper machine room; green mud and white mud of alkali recovery workshop; ash of power boiler; the sludge of wastewater treatment system. Excepting for the sludge, other four kinds of waste are easy to treat.
In general, the number of solid wastes is large, and the wastes are usually highly centralized, improper treatment has bad influence on the environment. If the wastes are packed too much at the outfall, dike breach will push the wastes into water resource, the water pollution is disastrous. Some pulp mills transport the waste solid to landfills, but these landfills are only used to receive the waste and lack of design and management, which produces the secondary pollution.
Solid waste of materials preparation workshop
For wood pulp making process, the solid wastes are mainly sawdust and barks. 95% of solid wastes are mostly used as the fuel of bark boiler.
For the straw pulp processing, the solid wastes include turfgrass clippings and dust. Generally, these wastes are burnt intensively, wasting manpower and material resources, and pollute the environment. In recent years, wheat straw waste boiler is used to deal with the solid wastes of the straw pulp mill.
The solid wastes of bagasse pulp mill are the bagasse pith. As the secondary product of pulp mill or paper mill, the bagasse pith is basically used as fuel.
Solid wastes of paper machine room
The solid wastes of paper machine room include promoter, packings, and fiber fines, they are discharged with white water. The most of large-scale paper mills have better treatment of white water, so the discharge value of solid wastes is very small. Some small-scale pulp mills directly discharge the solid wastes and wastewater to the river, which is harmful to the environment.
Solid wastes of alkali recovery workshop
Wood pulp mill generally calcines the white mud and send back to the pulping process, so the white mud can’t be solid wastes. In the non-wood fiber pulping process, black liquor contains large content of silicon, which is easy to result in scale on the black liquor evaporator. Huge scaling will stop the alkaline recovery system. The black liquor has high content of silicon and viscosity, and the transportation and concentration of black liquor will be affected.
The burning of lime sludge will increase the silicon content and produce the silicon interference. To avoid the silicon interference, the straw pulp mills usually don’t set the lime recovery. Besides, alkaline recovery causticizing produces much green mud, about 10kg/t, these green muds are backfilled.
Ash of power boiler
The boiler ash includes fly ash, slag, they are discharged from the power boiler, the quantity and feature are decided by the fuel and technology. In the real situation, fly ash and slag are commonly used as the raw material of cement plant, also used as fertilizer.
The sludge of wastewater treatment system
The sludge has different types, the first class sludge is produced by the mechanical clarifying treatment, the first class sludge is related to the paper quality, the area takes about 2% of whole pulp yield; the biological sludge is produced by the biological treatment; chemical sludge is produced by the wastewater treatment of coating; deinking sludge is produced by the deinking waste water treatment.
The wastewater contains a variety of materials, so does the sludge produced by the wastewater. The fibrous sludge contains fibers, fragments of fiber, barks, fillings, stone, sand and etc. The biological sludge is mostly made up of dead or live microbial cells. The deinking sludge contains floccules. Due to the low concentration and massive size of sludge, it must be dehydrated, dried and burned before landfilling or comprehensive utilization.
Burning
The burning is the common way to deal with the sludge of wastewater. The large-scale sludge treatment equipment includes sludge thickening, mechanical dehydration and sludge incineration (suitable sludge drying for improving burning efficiency.). Burning is the final disposal measure, it destroys organics, kills all pathogen, furthest reduce the area of sludge, and recovery in the form of inorganic. What’s more, the burning makes the sludge easy to landfill and reduce the landfill cost.
Landfilling
Landfilling is a necessary method of sludge or other solid wastes treatment. If there is enough field to utilize near the paper and pulp mills, the dehydrated sludge, and other solid wastes can be directly sent into landfilling, save the cost of burning and management. But the design and management of landfilling are vital for whole processing.
- Before building the landfill, there are some factors should be considered, including environment fitness, geological condition, groundwater condition, the influence produced by the wastewater, required equipment and etc.
- The layer for solid wastes is generally 1-1.5 meter deep, and cover with a layer of earth or gravel. The new layer can’t be added until the last sludge blanket is compressed and moisture is leached. The greening should start after the landfilling.
- Leaching water and runoff water discharged from landfill should be collected and send to the water treatment plants.
Other treatment of solid wastes
- As the soil amendment. The sludge and ash discharged from the paper and pulp mills can be used as soil amendment, mainly apply for agricultural land and forest land, it’s common in the world, especially in America.
- Back to the pulp processing. From the view of technology, the sludge back to the pulp processing is possible. But it is also possible to damage the quality and operation. This treatment method should be analyzed in specific case.
- Animal feed production. The activated sludge of wastewater treatment can be applied to animal feed production, but this method is still in the experimental stage.
- Building materials production. The sludge can be applied into brickmaking. The dry sludge and sludge incinerator ash both can be used to make bricks. The strength of sludge brick is similar to red brick’s.
